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Chapter 5

Polyvinylidene Fluoride and its Blend: Miscibility and

Crystallization Studies by FTIR Spectroscopy

5.1 INTRODUCTION

The material having electric polarization vector, exhibits three different types of

phenomena such as piezoelectricity, pyroelectricity, and ferroelectricity. The word

piezoelectric has its roots in the Greek word “piezo” which means ‘to stress or to press’.

“Piezoelectricity” is the phenomenon defined as the generation of electric charge when

the material is subjected to mechanical pressure. “Inverse piezoelectricity” is the reverse

effect of “piezoelectricity” and results in change in shape when the material is subjected

to an external electric field. These two effects are the basis of piezoelectric phenomenon.

Pyroelectricity is derived from the Greek word “pyr” means ‘fire’, is the ability of certain

materials to generate a temporary voltage when they are heated or cooled (i.e. in response

to temperature change). The amazing characteristic of certain materials that possess a

spontaneous electric polarization, which could be reversed by the application of an

external electric field, is termed as “ferroelectricity”.

Polyvinylidene Fluoride (PVDF) is the first ferroelectric material in the history of

polymers (Kawai 1969). Because of its outstanding properties such as flexibility, low

weight and thermal conductivity, high heat as well as corrosion resistance, PVDF could

be used as an insulator for some kinds of electrical wires. Since PVDF has good

mechanical strength and inertness, it can be fabricated into pipelines in chemical plants,

used as membranes, etc. Presence of electric polarization in PVDF makes it suitable for

use in many electronic applications such as sensors, actuators, nonvolatile memory

devices, transducers, etc.

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5.1.1 HISTORICAL BACKGROUND

Jacques and Pierre Curie brothers proposed the first idea of piezoelectricity (Ikeda

1990) when they presented their discovery to the French Academy of Science on August

2, 1880.

“The crystals that have one or more axes with dissimilar ends… possess a

particular physical property of giving rise to two electric poles of opposite

signs at the extremities of theses axes…. by subjecting them to variation in

pressure along their hemihedral axes.” (Ikeda 1990)

Their studies mainly focused on the conversion of mechanical energy into electrical

energy with improved efficiency in materials like Rochelle salt, quartz and tourmaline.

Despite the idea of piezoelectricity a long back, ‘this’ effect remained a subject of

scientific curiosity rather than a practical application. It all changed in 1916 when a

Frenchman named Paul Langevin devised the first major application by developing an

ultrasonic submarine detector based on the ‘inverse piezoelectric effect’ (Gautschi 2002).

Later, multichannel telephones had been developed using quartz crystals as wave filters

by the Bell Telephone Laboratories (Taylor et al 1985). In 1930’s, Barium Titanate

(BaTiO3) was used as the first synthetic piezoelectric material in microphones (Pauliat et

al 1991) and its piezoelectric capabilities were comparable to Rochelle salt (Pauliat et al

1991). In 1968, the availability of synthetic quartz crystals helped in reducing the

dependency on the natural crystals (Taylor et al 1985). Kawai (1969) discovered a strong

piezoelectric effect in PVDF by applying an electric field in 1969. He demonstrated that

poled thin films exhibited a large piezoelectric coefficient. In 1971, Bergman et al (1971)

and Glass et al (1971) discovered pyroelectricity when that PVDF films were polarized.

While studying the pyroelectric and nonlinear optical properties of PVDF, Bergman et al

(1971) and Glass et al (Glass et al 1971) discovered the ferroelectric properties in the

early 1970s.

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5.1.2 POLYVINYLIDENE FLUORIDE

PVDF has a very simple chemical formula, -CH2-CF2- intermediate between

polyethylene (PE), -CH2-CH2- and polytetrafluoroethylene (PTFE), -CF2-CF2-. The

solubility is less than 0.02/100 g of water at room temperature. It is a semi-crystalline

polymer (~50 % crystalline), having its glass transition temperature at about -35 ºC. Due

to the rotation of molecular chains about single bond, PVDF exists in three different

molecular conformations, TG+TG

-, TTTT, and TTTG

+TTTG

- as shown in Fig. 5.1 where

T refers to trans (when the substituents are at 180 oC) and G refers to guache (when the

substituents are at ±60 oC). When these conformations made into the unit cell, PVDF

exhibits in four different crystal forms (Lovinger 1983). Another two types of crystalline

modifications have been additionally proposed. The polymorphism and the properties of

PVDF were reviewed by Lovinger (Lovinger 1983). The four different crystalline phases

are α, β, γ and δ (a polar form of α) (Lovinger 1983). The electrical properties

(piezoelectric) and mechanical properties are varied depending on the molecular

conformation and the chain packing within the unit cells (Kepler and Anderson 1992;

Nalwa 1995). By using appropriate external conditions, these crystalline modifications

can be transformed to each other reversibly or irreversibly. However, the transition

mechanism is complicated but could be understood qualitatively on the basis of trans-

gauche conformational exchange (Nalwa 1995).

(α) (β) (γ)

Fig. 5.1: Molecular structure of four crystalline modifications of PVDF (Tashiro et al

1971).

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In PVDF, the ‘head’ and ‘tail’ of the monomer unit is CH2 and CF2, respectively.

Most of the polymer chain sequences are head-to-tail configuration. However, defects

like ‘head to head’ or ‘tail to tail’ do occur. The internal stresses that form in the polymer

matrix can be reduced by such defects (Kepler and Anderson 1992). The occurrence of

such defect upto 10 % of the total polymer in PVDF reduces the average dipole moment

by 6 – 10 % (Lovinger 1983). Approximately 3 - 10 % of PVDF polymer is associated

with this type of defect (Lovinger 1983). However, PVDF is free from tacticity due to the

absence of asymmetric carbon atoms in the polymer (Lovinger 1983).

5.1.2.1 CRYSTAL PHASES OF PVDF

As stated earlier, PVDF exhibits different crystal modifications, designated as α-

(TG+TG

-), β- (TTTT), γ- (TTTG

+TTTG

-), δ- and ε-phases (Lovinger 1983; Lovinger

1982; Chen and Frank 1984). The common polymorphs such as α-, β- and γ-phases are

obtained from usual solution or melt processing with favorable conditions. The δ- and ε-

phases are formed through solid phase transformation of α- and γ-phases, respectively,

under special processing conditions (Tashiro et al 1981; Takahashi et al 1982). Electric

poling of α-phase results in δ-phase, which is a polar analog of the α-phase (Buchmann et

al 1980). The ε-phase is an anti-polar analog of the γ-phase (Lovinger 1982). The

crystallization and interconversion routes among all the aforementioned crystal forms are

shown in Fig. 5.2.

(a) β-phase (Form I)

Form I has an orthorhombic structure and belongs to the space group C2v with

lattice constants, a = 8.58 Å, b = 4.91 Å, and c = 2.56 Å (Hasegawa 1972). This phase is

a highly polar and it is the ferroelectric phase of PVDF. The β-crystalline phase which is

all trans (TTTT) planar zig-zag conformation, has permanent electric dipoles on the C –

F bonds as shown in Fig. 5.1. As a result, it shows net dipole moment value. This phase

could be obtained by mechanical drawing (uniaxial or biaxial) of the α-phase (~300 –

400%) (Stefanou 1979). After drawing, PVDF assumes 95% of the β-phase and 5% of the

α-phase (Kupferberg 1988). Form I can also be obtained by casting from high polar

hexamethylphosphorictriamide solvent (Tashiro et al 1981; Kobayashi et al 1975).

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Fig. 5.2: Crystallization and interconversions of the polymorphic phases of PVDF (T =

temperature; P = pressure; E = electric field; MCB = monochlorobenzene) (Nalwa 1995).

(b) α-phase (Form II)

Form II is monoclinic and belongs to space group C2h with lattice dimensions a =

4.96 Å, b = 9.64 Å, and c = 4.62 Å (Kobayashi et al 1975). This form is non-polar and is

electrically inactive phase. This phase has trans-gauche-trans-gauche (TGTG)

conformation and has zero dipole moment since the orientation of dipoles are in opposite

and cancels each other as represented in Fig. 5.1. This phase is produced when PVDF is

crystallized from the melt (Nalwa 1995).

(c) γ-phase (Form III)

Form III being monoclinic has lattice dimensions of a = 4.96 Å, b = 9.58 Å, and c

= 9.23 Å and belongs to space group Cs (Takahashi and Tadokoro 1980). However, the

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structure of this form was reported as an orthorhombic with unit cell dimensions (a =

0.497 nm, b = 0.966 nm, and c = 0.918 nm) by Weinhold et al (1980), whereas Lovienger

(1981) confirmed it as a monoclinic with dimensions of unit cell, a = 4.96 Å, b = 9.67 Å

and c = 9.20 Å, determined by electron diffraction. The investigation carried out by

Lovienger (1981) supports the results given by Takahashi and Tadokoro (1980) that form

III has monoclinic structure. The structures such as Orthorhombic vs. Monoclinic of γ-

phase were analyzed by Weinhold et al (1985) via different treatments. It has been

proven that monoclinic modification with regular chain packing is resulted from

crystallization of the γ-phase at a high temperature (Takahashi and Tadokoro 1980;

Weinhold et al 1982; Takahashi et al 1981), while crystallization at low temperature

yields the statistically disordered orthorhombic modification of the γ-phase (Weinhold et

al 1982; Weinhold et al 1979). Form III on prolonged exposure to these high

temperatures induces an orthorhombic to monoclinic transition. The γ-phase has

TTTG+TTTG

- configuration and has dipole moment greater than 0 as displayed in Fig.

5.1. One of the most typical methods of preparing form III is to cast from DMAc or DMF

solution at ca. 60 oC (Kobayashi et al 1975). Form III could be obtained from isothermal

crystallization of the molten sample at a temperature just below the melting point (Prest

et al 1975) as well as under high-pressure and high-temperature crystallization conditions

(Doll and Lando 1970).

(d) δ-phase (Form IV)

The δ-phase is the polar analog of the α-phase (Weinhold et al 1984). It has an

orthorhombic structure with the lattice dimensions of a = 4.96 Å, b = 9.64 Å, and c = 4.62

Å, similar to the α-phase (Bachmann et al 1981; Bachmann et al 1980). In the δ-phase,

there are two chains in this unit cell (Bachmann et al 1980) and every second chain has

undergone a 180 o rotation, resulting in the dipole moments parallel to each other,

therefore, making the phase polar (Lovinger 1983). It is obtained by the application of a

high electric field (corona poling at approximately 2 MV/cm) to the α-phase (Bachmann

et al 1980). Thus, the δ-phase does exhibit piezoelectric activity because of net

dipolemoment unlike α-phase.

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5.1.2.2 PROPERTIES OF PVDF

Since PVDF is a semi-crystalline polymer, it shows both the amorphous (Tg of

about -35 oC) and crystalline nature (50-60 % crystalline). Upon stretching mechanically

and then poling, it exhibits piezoelectric properties. Being ferroelectric polymeric nature

upon poling, it reveals both piezoelectric and pyroelectric properties. It is chemically

stable and possesses high elastic modulus. It is resistance to organic solvents. PVDF is

used as a dielectric material because of its high permittivity and dielectric strength and

low dissipation factor. It is highly rigid, resistant to deformation, heat & combustion,

ageing, abrasion and non-toxic in nature.

5.1.2.3 APPLICATIONS OF PVDF

The combination of ferroelectric and mechanical properties of PVDF, it finds in

many promising applications such as speakers, head phones, microphones, infrared

detectors, transducers, etc. Since PVDF exhibits both piezoelectric and pyroelectric

properties upon poling, it is used in sensor and battery applications. Because of

combination of flexibility, low weight, low thermal conductivity, high chemical corrosion

resistance and heat resistance, it is employed as insulation to electrical wires. Since it is

chemically inert, it is used as coating materials, pipes, etc. It finds not only in

applications of lithium ion batteries as a standard binder material used in the production

of composite electrodes but also in membranes, monofilament fishing lines, etc.

5.1.3 LITERATURE SURVEY

5.1.3.1 SPIN COATING

The β-phase which has a planar zig-zag molecular structure in PVDF and its

trifluoroethylene (TrFE) copolymer known for thin film based electronic applications

such as non-volatile organic memory (Reece et al 2003) renders especially large

piezoelectric and pyroelectric coefficients (Lovinger 1982). Thus, in order to carry out

ferroelectric applications, it is mandatory to control their crystal structure to ensure the

formation of β-phase in huge quantity.

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PVDF exhibits many types of molecular orientations, resulting in different crystal

modifications, which get changed depending upon the preparation conditions of the

sample. There are many methods to induce β-phase in PVDF by uniaxial stretching of

melt extruded PVDF cast films at different cast roll temperatures (Mhalgi et al 2007; Du

et al 2007), by poling under high electric field (Luongo 1972), by mechanical stretching

(Lando et al 1966), annealing or crystallization at high temperatures and at high pressure

(Weinhold et al 1980; Takahashi and Tadokoro 1980; Lovinger 1981; Lovinger 1982;

Matsushige and Takemura 1978), quenching from melt (Lovinger 1981; Lovinger 1982;

Matsushige and Takemura 1978; Hsu, and Geil 1984; Yang and Chen 1987; Oka and

Koizumi 1985; Song et al 1990), by the addition of metal salts (Yoon et al 2008), by the

addition of nanoclay and carbon nanotubes (Lund et al 2011), etc. However, the

aforementioned methods were useful in producing films of micro-thickness, which could

not be utilized for nano-electronic applications. Besides, PVDF films produced either by

spin coating or by solution casting results in four different crystalline forms depending

upon the solvent evaporation rate, temperature and polarity of the solvent used (Kepler

and Anderson 1978; Kobayashi et al 1975; Cortilli and Zerbi 1967). The effect of

temperature on the crystallization of α-, β- and γ-phases of PVDF from DMAc solution

depends upon the evaporation temperature of the solvent. The all trans form (β-phase) is

formed below 70 oC, while at higher temperature, α-phase is commenced and becomes

predominant above 110 oC (Gregorio and Cestari 1994). Gregorio et al (2008)

investigated the effect of solvent type and the temperature on the formation of α- and β-

phases from solution-cast of PVDF by using DMF, NMP and hexamethylphosphoramide

(HMPA) solvents. Low evaporation rates result mainly in the trans-planar β-phase, high

rates predominantly in the trans-gauche α-phase and intermediate rates in a mixture of

these two phases, regardless of the solvent and temperature used. Since the rate of

evaporation of the solvent is mainly related to temperature, PVDF films can be obtained

predominantly either in one of these phases or a mixture of these by an adequate choice

of the evaporation temperature range for a given solvent. Ultrathin films of PVDF

containing β-crystalline phase have been prepared by heat-controlled spin coating set-up

(Ramasundaram et al 2008). The sample was prepared at different spin coating

temperatures from 10 to 70 oC and it was found that at elevated temperatures (40, 50, 60

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and 70 oC), PVDF is crystallized into β-phase, while at near-ambient conditions (20 and

30 oC), α-phase is formed.

5.1.3.2 MISCIBILITY AND CRYSTALLIZATION BEHAVIOR OF PVDF AND ITS

BLENDS

PVDF and its copolymers with trifluoroethylene (TrFE), hexafluoropropylene,

chlorotrifluoroethylene, etc. have been studied by many research groups in the past

(Tashiro et al 1984; Kim et al 1989). Among them, PVDF-TrFE (72/28) copolymer has

many advantages over PVDF such as spontaneous polarization of C-F dipoles even after

crystallization from melt, solubility in low-boiling solvents such as acetone, MEK, etc.

However, its higher cost and complicated synthetic procedures has led the focus back to

PVDF and its blends (Kim et al 1995). Compared to the synthesis of PVDF copolymers,

the physical blending of PVDF with other polymers is an easier, less time consuming and

cheapest method to obtain new material combinations, provided the blend properties are

superior to those of the copolymers. By changing the blend composition and combining

different components, the properties of PVDF can be tailored within a shorter period of

time, thereby resulting in lower investment costs for designing new materials.

PVDF is reported to form miscible blends with polyvinyl fluoride (Gupta et al

2006), polyvinyl acetate (Kim et al 1997), polyvinyl pyrrolidone (Chen and Hong 2002),

poly-3-hydroxybutyrate (Liu and Jungnickel 2003), poly-1,4-butylene adipate (Kim and

Kyu 2003), etc. Based on the miscibility at molecular level, polymer blends are classified

into immiscible (or incompatible), partially miscible and miscible. Quite a few reports

have focused on the miscibility of PVDF with acrylic (amorphous) polymers such as

poly(methyl acrylate) and poly(ethyl acrylate) (Bernstein et al 1977), poly(methyl

methacrylate) (Nishi and Wang 1975; Leonard et al 1983; Leonard 1985; Leonard et al

1988; Wahrmund et al 1978; Coleman et al 1977) and poly(ethyl methacrylate)

(Bernstein et al 1977; Kwei et al 1976). The miscibility of these blends is assigned to the

interaction between -CF2 groups in PVDF and carbonyl groups of the blend component

(Alfonso et al 1989; Benedetti et al 1998; Kim et al 1995; Elashmawi and Hakeem 2008;

Li and Woo 2008).

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The miscibility of i- and s-PMMA with PVDF by DSC and FTIR

microspectroscopy was investigated by Benedetti et al (1988). The polymer blend films

were prepared by using THF solvent for different compositions for both the types of

blends (PVDF/i-PMMA and PVDF/s-PMMA). The interactions between the C=O groups

of PMMA and CF2 groups of PVDF observed for PVDF/i-PMMA is greater than that for

PVDF/s-PMMA from major shift of the carbonyl band in FTIR microspectroscopy data.

Both the blends show the decline in melting point as well as decrease in ΔHm from DSC

data indicating that the miscibility is due to interactions between the C=O groups of

PMMA and CF2 groups of PVDF and the decrease is well pronounced for PVDF/i-

PMMA in comparison to that of PVDF/s-PMMA. Leonard et al (1998) also examined the

crystallization behavior of the blend (PVDF/PMMA) containing different tactility of

PMMA for a wide range of composition. Blends of PVDF/i-PMMA and PVDF/s-PMMA

with varying wt. fraction of PVDF ranging from 0 to 1 were prepared using DMAc

solvent. FTIR-TS spectra for the blends which, were quenched from the melt and

annealed above Tg, were measured. The formation of α- and β-phases was noticed on

both sides of the composition range and intermediate compositions, respectively.

However, i-PMMA could get crystallized upon annealing from ca. 0.2 wt. fraction of

PVDF, whereas s-PMMA is from ca. 0.4 wt. fraction of PVDF. As the cooling rate was

increased, the crystallization temperature is decreased and the decrease was detected

more for higher content of PMMA in the blend leading to the reduction of crystallization

rate. At high and medium crystallization temperatures, only α-phase is produced whereas

at lower crystallization temperatures, β-phase is formed.

Kim et al (1995) had confirmed the miscibility between PVDF and PMMA by

carrying out Factor Analysis for the different compositions of the blends ranging from

95:5 to 20:80 (PVDF:PMMA) of FTIR spectra measured at 180 oC depending on the

number of factors (four factors) on the basis of the Malinowski's IND criterion

(Malinowski 1977). This indicates that PVDF is miscible with PMMA in the molten state

due to specific interactions in the melt state. These specific interactions due to hydrogen

bonding between the carbonyl and CF2 groups is further confirmed by the shift of

carbonyl band of PMMA in the blend to lower frequencies by FTIR spectra. With

increasing PMMA concentration, the β-phase content increases up to 15-20 wt.-% of

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PMMA and then it decreases after 20 wt.-% of PMMA for the samples quenched below

55 °C.

Miscibility and morphological studies were investigated for PVDF/PMMA blends

prepared by dissolving in THF with varying compositions (PVDF/PMMA 100/0, 80/20,

60/40, 50/50, 60/40, 80/20 and 0/100) by solution cast (Elashmawi and Hakeem 2008).

The melting point depression for the blends by DTA reveals the miscibility of PVDF and

PMMA. XRD data indicated that as the concentration of PMMA was increased in the

blends, the amorphousness is also elevated while the FTIR study shows the shift of the

carbonyl band of the blend to lower frequency indicating the existence of the interaction

between the blend. SEM picture of the blends exhibited that the surface morphology of

the PVDF is changed due to the presence of PMMA. Presence of PMMA in the blends

leads to the formation of spherulitic structure and which could be observed up to 60 wt.-

% of PMMA in the blend. However, for 80 wt.-% of PMMA in the blend has distinct

longitudinal shape rather than spherulitic structure.

Li and Woo (2008) investigated the miscibility and the strength of interaction

between PVDF and polyesters such as poly(trimethylene adipate) (PTA) and

poly(pentamethylene adipate) (PPA) by FTIR, DSC and optical microscopic (OM)

studies. The interaction between the C=O groups of polyesters and the CF2 of PVDF

leading to miscibility is found to be weak from FTIR study and low value of interaction

parameter (χ = – 0.13) from melting point depression by DSC data. The miscibility

between PVDF and the polyesters could also be proved by the formation of coarseness

and/or ring-band spacing by OM study. With increasing polyester content, the increase in

coarseness and ring-band space could be observed. The miscibility and crystallization

behavior of the blends of PVDF and ethylene-co-vinyl acetate (20/80) copolymer

(EVAc80) had been studied using a differential scanning calorimeter and a polarizing

microscope equipped with a heating stage. From the melting point depression and the

FTIR analysis of samples quenched from the melt to various temperatures, the increase in

content of β-phase with increasing amount of blended EVA80 along with lower

quenching temperature was observed and indicated the miscibility of EVAc80 in the

blend (Lee and Kim 2007).

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The blend PVDF/PMMA (70/30) designated as MF0.7 had been prepared in three

different solvents (a) in methylethylketone (MEK), (b) THF and (C) only DMF and (d)

THF/DMF with different mass ratios: 9/1, 8/2 and 5/5 and their crystallization behavior

had been investigated by Ma et al (2008). From FTIR and WAXD data, it was proved

that the films casted using DMF yields predominantly the β-phase with the highest

crystallinity of PVDF while those from MEK and THF exhibit a mixture of β- and α-

phases with the lowest crystallinity of PVDF. Since morphology depended on the solvent,

SEM of the MF0.7 film prepared by using either MEK or THF, shows radially oriented

lamellae with the un-dissolved PVDF particles on the top surface formed at the

air/solution interface, while perfect spherulite is produced using DMF solvent. Film

prepared by using THF/DMF (5/5) shows more of β-phase with very less γ-phase without

α-phase.

Phase separation behavior, crystallization morphology and crystallization kinetics

in PVDF/PBA blends had been studied by using electric filed (Lee et al 2008). The blend

showed LCST behavior and the homogeneity is obtained between the melting point of

PVDF and the LCST. Lee et al (2009) analyzed the effect of electric field on phase

separation behavior and crystallization kinetics in PVDF/PMMA blends (Lee et al 2009).

By using time-resolved 2-D laser light scattering in presence or absence of an electric

field, PVDF/PMMA showed UCST-type phase separation experimentally. In the

temperature range above the UCST (150 oC ≤ T ≤158

oC) where non-polar α-crystals are

dominantly formed, an electric field reduces the overall crystallization rate in neat PVDF

whereas, it is increased the crystallization rate in the PVDF/PMMA blends. Nucleation

rate of γ-crystals under an electric field is faster at lower crystallization rate. With

increasing PMMA content and at higher crystallization temperatures, γ -crystals are

formed at the expense of α-crystals. Liquid-liquid phase separation and melt

crystallization appears simultaneously when the blends were quenched from 220 oC to a

temperature below UCST, which is lower than the melting point.

Hyperbranched polymer (HBP) blends with linear polymers are receiving

attentions of a few researchers only for the last two decades (Kim and Webster 1992;

Massa et al 1995; Nunez et al 2000; Jang et al 2000; Mulkern and Tan 2000; Wen et al

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2000; He et al 2012; Yao et al 2005; Okrasa et al 2005; Monticelli et al 2005). HBP was

blended with polystyrene (PS) either by solution or by melt in different weight

percentages. The melt viscosity of 5 wt.-% of HBP with PS blend was lowered and the

presence of HBP has improved the thermal stability of PS. The flexural modulus of the

blend and PS were found to be the same (Kim and Webseter 1992). Phase behavior of

blends of both hydroxy-terminated (OH-HBP) and acetoxy-terminated hyperbranched

polyesters (AcO-HBP) with a variety of linear polymers was carried out. OH-HBP is

miscible with linear polymer due to hydrogen bonding interaction. The AcO-HBP

exhibits less miscibility than those with hydroxyl end groups, which is consistent with the

absence of strong interactions in this case. Blending of all-aromatic HBPs with

polycarbonate resulted in a greater amount of hardness and elevated heat-deflection

temperatures while comparing to that of unmodified polymers (Massa et al 1995).

Nunez et al (2000) have studied the rheological properties of different generations

of HBPs in 1-methyl-2-pyrrolidinone (NMP) solvent and their blends with poly(2-

hydroxyethyl methacrylate) (PHEMA) polymer. The viscosity was reduced for the blends

of different generation of HBPs. Different generation blends have exhibited similar

viscosities. Reduction in viscosity was noticed to a larger extent when linear polymer was

replaced by HBPs due to decrease in the number of physical entanglements between

linear and HBP in the blend.

The rheological properties of polymer blends prepared by melt blending of a

nonreactive polystyrene (PS) with HBP-G4 represented as PS/HBP and two reactive

styrene maleic anhydride (SMA) resins with different maleic anhydride (MA) content

with HBP as designated as SMA/HBP with different weight percentages of HBP was

studied by Mulkern and Tan (2000). HBP added were found to be a good lubricant and

processing aids in the blends because of reduction in rheological properties of the blends

even by the addition of 2 wt.-% of HBP. However, after 2 wt.-% addition of HBP in

SMA/HBP blends, HBP has produced successively lesser reductions in melt viscosity.

SEM, DSC and FTIR study revealed that HBP forms immiscible blend with these PS and

SMA polymer in the blends (PS/10 wt.-% HBP and SMA/10 wt.-% HBP).

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Solution and melt blends have been prepared by mixing polyamide 6 (PA6) with

HB aromatic polyamides (aramids). The miscibility of solution and melt blends from

viscosity data and DSC was found to be good (Wen et al 2000). The polymer electrolytes

blend prepared using HBP, polyethylene oxide (PEO) with LiN(CF3SO2)2 salt by the

solvent casting technique exhibits improved lithium ionic conductivity and

lithium/electrolyte interfacial performance when compared to that of polymer electrolyte.

Ionic conductivity is enhanced because of reduction in crystallization but the mechanical

strength of PEO is decreased due to the presence of HBP.

The crystallization behaviors of PET in HBP/PET blends were studied by in situ

FTIR spectroscopy and differential scanning calorimetry. From FTIR study, the shift of

C=O band to lower wavenumber due to increased dipole-dipole interaction was observed.

The terminal groups such as hydroxyl (OH), acetate (Ac) and bezoate (Bz) of HBPs and

the composition of the HBPs had much influence on the crystallization behaviors of PET.

Similarly, the crystallization behavior of PET by linear polyester (LPE) is also studied

and compared with that of HBPs and found that it decreased the crystallinity of PET with

increase of LPE content in LPE/PET blend, Unlike HBP-(OH) and HBP-(Ac), HBP-(Bz)

increased the crystallization rate of PET. The maximum crystallization rate was found for

12 wt % HBP-(Bz) composition. HBP-(Bz) acting as a nucleating agent in the blends

with PET was confirmed from both FTIR and DSC studies (Jang et al 2000).

A series of HBPs (including HBP-1, HBP-2, HBP-3 and HBP-4) wrapped

CNTs/PVDF composite films prepared by solution coagulation was designated as S3, S4,

S5 and S6, respectively. These films have the same mass ratio of 1:3:1000 = HBP: CNTs:

PVDF. Neat PVDF (S1) and CNTs containing PVDF (S2) films were also prepared and

considered for comparison. β-phase is increased from S1 to S4 and then decreased,

indicating that the increase in β-phase is due to the presence of HBP in the composite

film while comparing to that of S1 and S2 films (He et al 2011).

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5.1.4 OBJECTIVE OF THE CHAPTER

So far in the literature, innumerous articles dealing with PVDF blended with

linear polymers have been reported. In the present study, blending of PVDF with

hyperbranched polyester (HBP) is studied. The objective of this chapter is as follows:

(a) To study the miscibility of HBP with PVDF by using FTIR and DSC

studies.

(b) To study the crystal modifications of PVDF at different spin coating

temperatures (30, 40, 50, 60, 70 and 80 oC) using heat-controlled spin

coating set-up for different substrates such as KBr Windows, ITO and

Gold-coated glass substrates using a mixture of DMF and acetone as

solvent.

(c) To compare the effect of annealing (from 30 to 200oC) upon PVDF and

PVDF/HBP (90/10 w/w) blend ultrathin films using FTIR-GIRAS and to

extract the pure-component spectra as well as to determine the percentage

of ferroelectric content by using Factor analysis (FA) as a function of

different annealing temperatures for the annealed samples.

(d) To report the qualitative and quantitative analysis of the effect of heating-

cooling on PVDF and PVDF/HBP (90/10 w/w) blend ultrathin films using

FTIR-TS and FTIR-GIRAS for as-cast (AC) and annealed samples (AN,

annealed at 130 oC).

5.2 EXPERIMENTAL

5.2.1 MATERIALS

PVDF powder (Mw = 60 000, Polysciences Inc., U.S.A), acetone, DMF and KBr

disks (for TS) (Sigma-Aldrich, U.S.A) were used as received. ITO- and Gold-coated

glass substrates (for FTIR-GIRAS) purchased from SD Tech., Korea were cleaned using

a 3-step process by immersing in soap solution, acetone-deionized water mixture and

isopropanol-deionized water mixture in a series for 2 h each. The cleaned substrates were

stored in a desiccator for further use.

121

5.2.2 SYNTHESIS OF HYPERBRANCHED POLYESTER

Third generation hyperbranched polyester (HBP) synthesized by using

pentaerythritol as a core and DMPA as a monomer is used as a blend component in this

chapter. The synthesis and characterization of it are reported in Chapter 4.

5.2.3 SAMPLE PREPARATION

PVDF (2 wt.-%) was dissolved in acetone:DMF (80:20 v/v) solvent mixture and

kept for stirring at 50 oC in a water bath for one week to get homogenous transparent

solution. Blends of PVDF with 5, 10, 15, 20, 30 and 50 wt.-% of HBP were prepared by

the aforesaid method. The prepared blend concentration of each stock solution is 2 wt.-%.

The prepared stock solutions of all the blends as well as of PVDF solution were filtered

using micro-filter and then used for various following studies.

5.2.3.1 MISCIBILITY STUDIES BY FTIR

The above stock solutions of both PVDF and the blends were solution-cast on

KBr pellets and dried at room temperature for one day to remove the maximum amount

of solvent. They were kept in vacuum oven at 50 oC for 5 days to remove the solvent

mixture completely. One set of the samples were used for measuring FTIR-TS at room

temperature and the second set of samples were melted at 210 oC for 20 min. in a

vacuum oven and then quenched to –5 oC using a bath containing a mixture of ice and

sodium chloride salt. Now the absorption spectra (FTIR-TS) for all the melt-quenched

(MQ) samples were recorded at room temperature to study the miscibility of the blend.

Third set of samples were utilized to determine the miscibility by measuring absorption

spectra at 40 oC using in-situ FTIR.

5.2.3.2 MISCIBILITY STUDY BY DSC AND CRYSTALLIZATION BEHAVIOR BY

FTIR AND XRD

Small amount of the PVDF and the blend solutions (2 wt.-%) were poured into a

glass beaker and kept at room temperature for several days to remove the maximum

amount of the solvent. Later, they were kept at vacuum oven at 50 oC for 5 days for

122

further removal of the solvent completely. The films were peeled off from the beaker and

were used for DSC, FTIR and XRD studies.

5.2.3.3 EFFECT OF SPIN COATING TEMPERATURE ON DIFFERENT

SUBSTRATES

The prepared 2 wt.-% PVDF solution was spin coated (1500 rpm for 30 s; step

speed 500 rpm) on KBr Window, ITO and gold-coated glass substrates at varying spin

coating temperatures (from 30 to 80 oC) using a heat-controlled spin coating (HCSC) set-

up as shown in Fig. 5.3 (Figure 2 of the reference by Ramasundram et al 2008). Prior to

spin coating, ITO substrates were cleaned by immersing in soap-deionized water mixture,

filtered deionized water, filtered acetone and filtered isopropyl alcohol and water (50:50)

mixture in a series for 2 h each. After cleaning the ITO substrates with nitrogen gun, they

were kept in a vacuum oven at 140 oC for overnight. The as cast samples (AC) of KBr

windows, ITO and Gold-coated glass substrates were prepared by spin coating at various

temperatures (30, 40, 50, 60, 70 and 80 oC). They were subjected to different thermal

treatments: (a) annealing at 130 oC (AN130) in a vacuum oven for 12 h and then

quenched immediately to room temperature and (b) melted at 200 oC for 3 h and then

slowly cooled to room temperature (MSC).

Fig. 5.3: Heat-controlled spin coating set-up.

123

5.2.3.4 EFFECT OF ANNEALING AND HEATING-COOLING STUDIES

The ultrathin PVDF and the blend (PVDF/HBP 90/10) films were prepared by

adopting the aforementioned method by spin coating the PVDF and the blend

(PVDF/HBP 90/10) solutions at 40 oC for annealing studies by GIRAS technique (ITO

substrate) as well as heating-cooling studies for both TS and GIRAS techniques

(mentioned in 5.2.3). For annealing studies, the ultrathin film samples of both PVDF and

its blend were subjected to various annealing temperature from room temperature (30,

AC sample) to melt annealing temperature (200 oC) by keeping the spin coated samples

in a vacuum oven at a particular annealing temperature for 3 h and then the samples were

slowly cooled to room temperature. For heating-cooling studies, the as-cast (AC) samples

were prepared by spin coating PVDF and its blend (PVDF/HBP 90/10) solutions on KBr

window and ITO substrate at 40 oC. The annealed samples (AN) for heating-cooling

studies of both PVDF and its blend (PVDF/HBP 90/10) were obtained by annealing the

AC sample at 130 oC in a vacuum oven for 3 h and then slowly cooled to room

temperature.

5.2.4 CHARACTERIZATION TECHNIQUES

5.2.4.1 FTIR

Unpolarized FTIR-TS and FTIR-GIRAS (at an incident angle of ca. 85 oC from

normal to the surface) spectra were recorded using Bruker 66V FTIR spectrophotometer

at a resolution of 2 cm-1

with 5 min. scans at room temperature for the solution-cast as

well as spin coated samples. To determine the miscibility of the blends, the FTIR-TS

absorption spectra of solution cast samples were recorded at a preset defined temperature

(40 and 210 oC) by mounting KBr pellets on a custom made heating cell. Heating–

cooling studies for both TS and GIRAS (at an incident angle of ca. 85 oC from normal to

the surface) were carried out by mounting the sample in a custom made heating cell. The

spectra were recorded in the temperature range from 30 to 210 oC, kept at 210

oC for 10

min. to remove the thermal history and then the sample was cooled to 30 oC at 1

oC/min.

All the raw IR spectra were smoothened by Savitsky-Golay smooth using 15 degrees of

smooth, base-line corrected, normalized and then used for further analysis.

124

5.2.4.2 MEASUREMENT OF MELTING POINT BY DSC

On a Mettler-Toledo DSC-1, each sample (PVDF and its blend) was heated at a

heating rate of 10 oC/min. from room temperature to 210

oC at which the sample was

melted for 10 min. and then quenched to -80 oC at 100

oC/min. The same sample was

again re-heated to 210 oC/min. at a heating rate of 10

oC/min. to get melting endotherms.

5.2.4.3 XRD

X–ray diffraction patterns were obtained using powder X-ray diffraction (XRD)

(D8 Advance, Bruker) with CuKα radiation in the 2θ range, 3–80 o at room temperature

using Lynx Eye detector (silicon strip detector technology) for PVDF and its blend films

as well as for powdered HBP.

5.2.5 DIFFERENCE BETWEEN TRANSMISSION (TS) AND GRAZING

INCIDENCE REFLECTION ABSORPTION SPECTROSCOPY (GIRAS).

The infrared absorption intensity, I is proportional to the square of the inner

product between the transition dipole, ' and the electric field vector, E as shown below:

I 'E)2 = (')

2 E

2cos

2 )

where is an angle between 'and E. When the infrared beam is incident along the

normal to the sample film and the transmission absorption spectra (TS) are measured, the

electric field, E of the infrared beam interacts with the transition dipoles, ' included

within the film plane according to eqn. (1). Once the infrared beam strikes the metal

surface with high incidence angle (ca. 88 o from the normal to the surface), the electric

field pointing in the direction of the normal is enhanced remarkably and thus only the

vibrational modes with the transition dipoles normal to the film surface can be observed

selectively as shown in Figs. 5.4 (Nalwa 1995) and 5.5 (Prabu et al 2006). The spectra

thus measured are called reflection-absorption spectra (RAS). By combining these two

methods of TS and RAS, we can deduce a concrete image of dipole orientation in the

film (Nalwa 1995).

125

(a) Transmission (b) Reflection-Absorption

Fig. 5.4: Schematic representation of the infrared spectral measurements by (a)

Transmission (TS) and (b) Reflection-absorption (RAS) modes.

Fig. 5.5: Schematic diagram of GIRAS showing the predominant p-polarized component

light (out-of-plane) and reduced s-polarized component light (in-plane) reflected from

surface at high incidence angles.

5.3 RESULTS AND DISCUSSIONS

The vibrational bands assigned to specific conformations given in Table 5.1 are

used to interpret the structural changes in PVDF.

126

Fig. 5.6 represents the FTIR spectra of PVDF and HBP solution-casted on KBr

pellet in the region of 1800-600 cm-1

measured at room temperature. The band at 1733

cm-1

represents the carbonyl band of HBP. The FTIR spectrum of PVDF as represented in

Fig. 5.6 shows a number of absorption bands (1274 and 840 cm-1

) which are sensitive

towards the changes in ferroelectric crystalline phase. The characteristic bands of trans-

zigzag conformation: for example, the stronger absorption bands at 1274 and 840 cm-1

(A1,

║ b

) associated with sCF2 stretching vibrations for the trans sequence longer

than TTTT and sCF2 + sCC for the trans sequence longer than TTT, respectively. The

1176 and 881 cm-1

absorption bands (B2,

║ a

) are associated with asCF2 and rCH2

modes, respectively. The 1411 and 1074 cm-1

(B1,

║ c

) absorption bands are assigned

to CH2 + asCC whose transition moment is along the chain orientation direction (Prabu

et al 2006; Prabu et al 2009).

Fig. 5.6: FTIR spectrum of solution-cast PVDF and HBP.

127

Table 5.1: Vibrational bands chosen for analyzing PVDF

† Vibrational modes: υas, antisymmetric stretching; υs, symmetric stretching; δ, bending;

ω, wagging; r, rocking; t, twisting.

Wave

number

(cm-1

)

Symmetry

species Assignments

†

Chain

conformation Ref.

3022 B2 υas (CH2) Tashiro and Kobayashi

1994; Nalwa 1995

2977 A1 υs(CH2) Tashiro and Kobayashi

1994; Nalwa 1995

1431 A1 δ (CH2) Nalwa 1995; Kim et al

1989

1402 B1 ω (CH2), υas

(CC)

Nalwa 1995; Reynolds

et al 1989

1273 A1 υs (CF2), υs (CC),

δ (CCC) tm (m > 4)

Tashiro and Kobayashi

1994; Nalwa 1995

1177 B2 υas (CF2), r

(CF2), r (CH2)

Tashiro and Kobayashi

1994; Nalwa 1995;

Kim et al 1989

1074 B1 υas (CC), ω

(CH2), ω (CF2)

Tashiro and Kobayashi

1994; Nalwa 1995

980 A2 t (CH2) Tashiro and Kobayashi

1994; Nalwa 1995

885 B2 r (CH2), υas

(CF2), r (CF2) t

Tashiro and Kobayashi

1994; Nalwa 1995

848 A1 υs (CF2), υs (CC) tm (m > 3)

Tashiro and Kobayashi

1994; Nalwa 1995;

Kim et al 1993

802 - r (CH2) tttg Reynolds et al 1989

612 - δ (CF2), δ(CCC) tg Reynolds et al 1989

508 A1 δ (CF2) Nalwa 1995; Kim et al

1993

471 B1 ω (CF2) Tashiro and Kobayashi

1994; Nalwa 1995

128

5.3.1 MISCIBILITY STUDIES

5.3.1.1 SPECIFIC INTERACTIONS BETWEEN PVDF AND HBP

FTIR is a good analytical technique to study the polymer blend miscibility by

observing the changes in the carbonyl peaks (Cowie 1989; Walsh 1989). The specific

interaction between the C=O and CF (or CH2) groups is one of the major contributors to

miscibility of PVDF and HBP in the blend by forming either H-bonding between C=O

and CF (or CH2) / and or dipole-dipole interaction (Kim et al 1994; Kim et al 1995;

Tanaka and Nishi 1986; Kim et al 1997; Kim et al 1993; Kim and Kyu 1999; Penning

and Manley 1996). This type of interaction such as H-bonding in the polymer blends

could be identified by IR spectroscopy by verifying in terms of the frequency shift of the

stretching vibration of C=O to lower frequencies or the formation of new absorption

bands .

Fig. 5.7(a) shows the TS spectra of PVDF and PVDF/HBP (100/0, 95/5, 90/10,

85/15, 80/20, 70/30 and 50/50) blend solution-cast samples measured at 210 oC. By

simple comparison of raw IR data from Fig. 5.7(a), it is very difficult to find a significant

frequency shift or formation of new band in the blend spectrum attributable to the

specific interaction. Thus, the nature of specific interactions in the melt blend could be

detected from the difference spectra, which could be obtained by completely removing

the contribution of HBP (amorphous polymer) in the blend by using the reference band of

HBP at 1047 cm-1

(Kim et al 1995; Kim et al 1997). The difference spectra as shown in

Fig. 5.7(b) for different blend ratios PVDF/HBP (80/20, 70/30 and 50/50) was obtained

by subtracting HBP from PVDF/HBP (80/20, 70/30 and 50/50) blends using the

reference band of HBP at 1047 cm-1

. A new peak is observed clearly at 1712, 1704 and

1712 cm-1

for the PVDF/HBP (80/20), PVDF/HBP (70/30) and PVDF/HBP (50/50)

blends, respectively. This indicates that the carbonyl band of HBP at 1733 cm-1

is shifted

to a lower frequency due to specific interaction between the C=O groups of HBP and the

CH2 or CF2 groups of PVDF in the melt state. Indeed, the specific interaction between

HBP and PVDF could be confirmed by noticing the absorption peaks of carbonyl groups

of blends for the solution-cast samples measured at room temperature as well as at 40 oC

as shown in Fig. 5.8. From Fig. 5.8(a), it could be obviously seen that there is no

129

frequency shift for all the blends except for PVDF/HBP (50/50) blend, but the peaks are

broadened while skewing towards the lower frequencies, which implies that amorphous

HBP could be miscible with the amorphous portion of PVDF at room temperature at a

relatively lower extent. It is well established that the extent of frequency shift in the C=O

stretching band to lower frequencies is proportional to the number fraction of the C=O

groups participating in the specific interaction with CF2 or CH2 group of PVDF as in the

case of P(VDF/TrFE)/PBA blend (Kim and Kyu 1999). Moreover, the magnitude of the

frequency shift is useful in evaluating the level of specific interaction between the

polymers. By examining the changes in the peak position and the line shape of the C=O

stretching band of HBP at various blend compositions, the specific interaction in the

blend can be assessed. The frequency shift of the stretching vibration of C=O band to

lower frequencies, which is caused by the reduction in the force constant of C=O

associated with H-bonding or dipole-dipole interaction (Kim and Kyu 1999) could be

noticed from Fig. 5.8(b). The shift of C=O peak is small (Δν = 3 cm-1

) with increasing

PVDF concentration as represented in Fig. 5.8(b), but it is definitely discernible at 40 oC

for all the blends and for (PVDF/HBP 50/50) blend measured at room temperature. This

broadening and shifting of C=O band indicate that both HBP and PVDF are intimately

mixed due to either relatively weak H-bonding and/or dipole-dipole interaction (Kim and

Kyu 1999; Penning and Manley 1996). Fig. 5.9 shows the IR absorbance

Fig. 5.7: FTIR spectra of solution-cast PVDF and PVDF/HBP blends with different ratios

measured at (a) 210 oC and (b) Difference spectra obtained by subtracting HBP from

PVDF/HBP blend of different ratios.

Wavenumbers (cm-1

)

60080010001200140016001800

80/20

70/30

50/50

PVDF/HBP

1712

1704

1712

(b)

Wavenumbers (cm-1

)

60080010001200140016001800

(a)

100/0

95/5

90/10

85/15

80/20

70/30

50/50

PVDF/HBP

130

Fig. 5.8: Absorbance IR spectra of solution-cast blends and HBP measured at (a) RT

(room temperature) and (b) 40 oC.

Fig. 5.9: IR absorbance spectra of solution-cast HBP and blends recorded at room

temperature: (a) solution-cast HBP and the blends melt-quenched at – 5 oC and (b) HBP

and blend (PVDF/HBP 90/10) melt-quenched at – 5 oC.

spectra of HBP and its blend samples quenched at -5 oC from the melt and measured at

room temperature. The shift of C=O frequency for all the melt quenched blends to lower

frequency indicate that both HBP and PVDF are intimately mixed due to the formation of

H-bonding between C=O group of HBP and CF2 or CH2 groups of PVDF as in the case

of PVDF/PVAc (Kim et al 1997) and PVDF/PMMA (Kim et al 1995).

Wavenumbers (cm-1

)

1650170017501800

HBP

MQ 95/5(PVDF/HBP)

MQ 90/10(PVDF/HBP)

MQ 85/15(PVDF/HBP)

MQ 70/30(PVDF/HBP)

MQ 50/50(PVDF/HBP)

(a)

Wavenumbers (cm-1

)

1650170017501800

HBP

MQ 90/10(PVDF/HBP)

(b)

Wavenumbers (cm-1

)

16601680170017201740176017801800

HBP

95/5 (PVDF/HBP)

90/10 (PVDF/HBP)

85/15 (PVDF/HBP)

80/20 (PVDF/HBP)

70/30 (PVDF/HBP)

50/50 (PVDF/HBP)

(a) RT

Wavenumbers (cm-1

)

16601680170017201740176017801800

HBP

95/5 (PVDF/HBP)

90/10 (PVDF/HBP)

85/15 (PVDF/HBP)

80/20 (PVDF/HBP)

70/30 (PVDF/HBP)

50/50 (PVDF/HBP)

(b) 40 oC

131

5.3.1.2 THERMAL ANALYSIS

DSC is another technique to detect the miscibility of the components in the blend

by noticing either the melting point (Tm) or the glass transition temperature behavior (Tg)

(Cowie 1989; Walsh 1989). Appearance of single glass transition temperature as well as

melting point depression of the blend samples indicates the miscibility of the blend

components. Fig. 5.10 shows the melting endotherms of all the blends and neat PVDF.

The depression in melting point as represented in Fig. 5.11 shows that both PVDF and

HBP are miscible at molecular level. The melting point depression phenomenon is found

to be explicable in terms of thermodynamic mixing of a crystalline polymer with an

amorphous polymer. The decrease in melting point is obviously observed with increase in

the content of HBP in the blend. The melting point is declined from 167 oC to 163

oC

with the addition of HBP from 0 to 50 wt.-% of HBP while the enthalpy of melting is

decreased from 36.67 to 28.28 J/g as shown in Fig. 5.11. The melting peak is not

disappeared even with the addition of 50 wt.-% of HBP unlike the addition of PMMA to

P(VDF/TrFE) where, the melting peak disappeared completely at 40 wt.-% of PMMA

addition (Faria and Moreira 1999) indicating that the blend still be crystalline in nature.

(a) 1st heating

Temperature (oC)

130 140 150 160 170 180

En

do

PVDF/HBP

50/50

70/30

80/20

85/15

90/10

95/5

100/0

(b) 2nd

heating

Temperature (oC)

130 140 150 160 170 180

PVDF/HBP

100/095/590/10

85/1580/20

70/3050/50

Fig. 5.10: Melting endotherms of various PVDF/HBP blends and PVDF for (a) first

heating and (b) second heating.

132

Fig. 5.11: (a) Enthalpy of melting and Tm and (b) Tg for all the PVDF/HBP blends from

second heating cycle by DSC.

5.3.1.3 CRYSTALLIZATION BEHAVIOR OF PVDF AND ITS BLEND OF

VARYING THICKNESS SOLUTION CAST SAMPLES

In order to comprehend the crystallization behavior of PVDF and its blends, XRD

and FTIR-TS measurements were carried out and the obtained data for the

aforementioned experiments are shown in Fig. 5.12. Thick film ( 1mm) samples were

used for XRD measurement. From XRD data as shown in Fig. 5.12(a), it could be noticed

that the peaks at 18.2, 26.6, and 38.6 o correspond to alpha crystal with (0 2 0), (0 2

1) and (0 0 2) reflections, respectively (Park et al 2005; Gregorio and Ueno 1999; Wang

et al 2011). The β-phase can be identified from the peak at 20.2 o with (1 1 0)

diffraction plane (Gergorio and Ueno 1999). The halo with peaks for HBP appears at

11.8 and 17.5 o. The halo with peak corresponding to 17.8

o shows only in 50 wt.-% of

HBP in the blend sample indicating that some amorphous portion could be clearly seen

only for 50 wt.-% of HBP in the blend.

FTIR data of micron thickness ( 100 µm) samples are shown in Fig. 5.12(b). The

peak at 1733 cm-1

indicates the presence of carbonyl group of ester in HBP. As the

content of HBP is increased from 5 to 50 wt.-% in the blend, the carbonyl band of ester

moiety in HBP is shifted (to lower frequency) from 1733 to 1724 cm-1

. This indicates that

specific interaction between C=O group of HBP and CH2 or CF2 of PVDF is enhanced

due to increased content of presence of H-bonding (Kim and Kyu 1999; Penning and

Manley 1996). The absorption intensity of it is enhanced with the increase in the

Wt% of HBP

0 20 40 60 80 100

Tg (

oC

)

-40

-20

0

20

40

60

(b)

Wt % of PVDF

50 60 70 80 90 100

H

m,

J/g

of

PV

DF

24

26

28

30

32

34

36

38

Tm

(oC

)

162

163

164

165

166

167

168(a) 2

nd heating

133

concentration of HBP present in the blend. The bands at 1274 (Kang et al 2008; Li et al

2002; Kim et al 1995) and 1234 cm-1

(Ma et al 2008; Li et al 2002; Kim et al 1995)

correspond to β- and γ-phases, respectively, for both PVDF and its blend samples. The

absorption intensity of all trans band (β-phase) is greater for PVDF sample than that of

its blend samples. However, absorption intensity of this phase does not drastically decline

even with increased weight percentage of HBP in the blend compositions. While

increasing the addition of HBP to PVDF, the absorption band intensity for γ-phase is

raised well. The absence of bands at 975, 853, 795, 763 and 614 cm-1

indicate the absence

of α-phase (Ma et al 2008; Li et al 2002; Kim et al 1995).

Fig. 5.12: (a) X-ray diffractograms of PVDF and its blend very thick cast (AC) films

measured at room temperature and (b) FTIR transmission spectra of PVDF and its blends

solution-cast (AC) thin films measured at room temperature.

5.3.2 EFFECT OF SPIN COATING TEMPERATURE ON PVDF ULTRATHIN

FILMS

Fig. 5.13 shows the FTIR-transmission spectra of PVDF ultrathin films measured

at varying temperature treatment conditions: as-cast (AC), annealed at 130 oC (AN130)

and melted at 200 oC followed by slow cooling to room temperature (MSC) as a function

of different spin coating temperatures (30, 40, 50, 60, 70 and 80 oC). The AC samples

were annealed at 130 oC for 12 h under vacuum and then quenched rapidly to room

temperature for AN130 samples. The samples were further melted at 200 oC for 3 h under

vacuum and then slowly cooled to room temperature for MSC samples. Using previously

reported data as reference (Kang et al 2008; Li et al 2002 ; Kim et al 1995), the β-phase

peak for AC, AN130 and MSC samples were identified at 1277, 1276 and 1274 cm-1

,

(a)

2(degree)

10 20 30 40 50 60 70 80

100/0

95/5

90/10

85/15

80/20

70/30

50/50

0/100

PVDF/HBP

Wavenumbers (cm-1

)

60080010001200140016001800

(b)

100/0

95/5

90/10

85/15

80/20

70/30

50/50

PVDF/HBP

134

respectively. For AC and AN130 samples, the absorption intensity of all trans form

decreased while increasing spin coating temperature due to the fast evaporation rate of

the solvent. The abrupt decrease in β-phase absorption intensity was noticed from 60 oC

onwards. Compared to AC and AN130 samples, the β-phase absorption intensity of MSC

samples declined drastically irrespective of varying spin coating temperatures owing to

the conversion of significant amount of β-crystal into -crystal (615 cm-1

). The presence

of -crystalline phase is noticed at 1235, 1234 and 1232 cm-1

for AC, AN130 and MSC

samples, respectively. These results indicate the favorable formation of β-phase at lower

spin coating temperatures (30 to 50 oC) rather than at higher spin coating temperature (60

to 80 oC) aided by slow evaporation rate of the solvent. Regardless of the solvent used,

the β-crystalline form with longer trans sequences (>4) are formed more predominantly

with reduced evaporation rate of the solvent (Nalwa 1995). The data obtained from the

present study is consistent with the report given by Nalwa (1995).

Fig. 5.13: FTIR-transmission spectra of PVDF ultrathin films spin coated on KBr

substrate at different spin coating temperatures in the region 1500-1000 cm-1

for (a) AC,

(b) AN130, (c) MSC, and (d) MSC in the region of 1000-450 cm-1

.

135

AC samples upon annealing (AN130) transformed the non-equilibrium -crystals

into equilibrium state, which resulted in increased -phase absorption intensity for

AN130 samples compared to AC samples. Conversion of some amorphous phase into

ferroelectric crystalline phase can also be attributed to the observed increase in -phase

absorbance for annealed samples. At lower spin coating temperature, -peak shows a

hump while at higher spin coating temperature above 50 oC, it emerges as a well-

distinguished peak in the case of AC and AN130 samples. The absorbance intensity of -

phase peaks at 1216, 975, 853, 795, 763 and 614 cm-1

increased with increasing spin

coating temperatures for MSC samples but were conspicuous by their complete absence

in AC and AN130 samples.

Fig. 5.14 depicts the relative amount of β-, γ- and α-crystalline phases as

calculated from the absorbance ratio of β (1277 cm-1

), γ (1234 cm-1

) and α (615 cm-1

)

with respect to internal thickness band (1073 cm-1

) (Yoon et al 2008). AN130 samples

exhibit higher β-phase content in comparison to that of AC and MSC samples. Yoon et al

(2008) reported maximum A1277/A1073 ratio of 2.1 by adding 15 wt.-% of CaCl2 salt in

PVDF whereas in the present study, a significantly higher A1277/A1073 ratio (3.1) was

obtained for 40-AN130 (AN130 sample spin coated at 40 oC) sample without adding any

metal salts. As the spin coating temperature increases, the content of all trans form is

decreased irrespective of thermal treatment conditions as observed from Fig. 5.14(a). The

ordered β-phase formed in AC and AN130 samples were remarkably reduced and

transformed into disordered α-phase for MSC samples, which can be confirmed from the

higher β-phase content in AC and AN130 samples (Fig. 5.14(a)) and higher α-phase

content in MSC samples (Fig. 5.14(c)). The relative γ-phase content for AN130 samples

is lower than that of AC and MSC samples (Fig. 5.14(b)) which implies the conversion of

γ- into β-phase upon annealing.

136

Fig. 5.14: FTIR-TS: Relative amount of (a) β-, (b) γ- and (c) α-crystalline phases of

PVDF ultrathin films subjected to varying thermal treatment conditions as a function of

different spin coating temperatures.

137

FTIR-GIRA spectra of PVDF ultrathin films spin coated on ITO substrate at

different temperatures (30, 40, 50, 60, 70 and 80 oC) is represented in Fig. 5.15 in the

range of 1500-1000 cm-1

and 1000-450 cm-1

as a function of varying thermal treatment

conditions: as-cast (AC), annealed at 130 oC (AN130) and melted at 200

oC followed by

slow cooling to room temperature (MSC). The absorption intensity of β- and γ- phases in

AC samples is varied with increasing spin coating temperatures as seen from Fig. 5.15(a).

The characteristic absorption band for all trans form at 1277 cm-1

(Yoon et al 2008;

Ramasundaram et al 2008; Benz et al 2002; He and Yao 2006; Chen et al 2007; Benz and

Euler 2003) is declined, whereas the absorption band for γ-phase appearing at 1238 cm-1

(Yoon et al 2008; Ramasundaram et al 2008; Li et al 2002; Benz et al 2002; He and Yao

2006; Chen et al 2007; Benz and Euler 2003) is increased with increasing spin coating

temperatures. In the case of 30 oC spin coating temperature, the absorption intensity of β-

phase is higher in comparison to that of the γ-phase, whereas it remains equal for both

phases for the sample spin coated at 40 oC. However, the -phase absorption intensity

increased when compared to that of -phase from the spin coating temperature of 50 oC

onwards. The 1218 cm-1

band corresponding to α-phase (Sakata and Mochizuki 1991) is

formed at higher spin coating temperatures (60 to 80 oC). Though Ramasundaram et al

(2008) reported the presence of α-phase for the PVDF-DMAc sample spin coated at 30

oC, no such peak is observed in the present study, thereby confirming the absence of non-

polar and disordered α-phase in our sample prepared under same spin coating

temperature. Although, the crystalline structure of the AC samples remains unaffected

even after annealing treatment at 130 oC (AN130), the absorption intensity of β-phase

(1280 cm-1

) is enhanced in comparison to that of AC samples as shown in Fig. 5.15(b)

and Fig. 5.15(e). However, the MSC samples exhibit abrupt decrease in absorption

intensity of β-phase at 1290 cm-1

when compared to AC and AN130 samples. The γ-

phase appearing at 1230 cm-1

increased for MSC samples. For AC and AN130 samples,

the α-phase at 614 cm-1

emerges from the spin coating temperature of 60 oC onwards,

whereas it could be observed even at lower spin coating temperatures of 10, 20 and 30 oC

by Ramasundram et al (2008). However, MSC samples show higher absorption intensity

for α-phase which appears at 795, 762 and 615 cm-1

for all the spin coating temperatures

138

as shown in Fig. 5.15(f) compared to much less intense absorbance observed for AC and

AN130 samples and that too at higher spin coating temperatures from 60 oC onwards.

Fig. 5.15: FTIR-GIRA spectra of PVDF ultrathin films spin coated on ITO at different

spin coating temperatures in the region 1500-1000 cm-1

for (a) AC, (b) AN130 and (c)

MSC and in the region 1000-450 cm-1

for (d) AC, (e) AN130 and (f) MSC samples.

139

Fig. 5.16 shows the FTIR-GIRA spectra of PVDF ultrathin films spin coated on

gold-coated glass slides at different spin coating temperatures in the range of 1500-1000

cm-1

and 1000-450 cm-1

for different thermal treatments. For AC and AN130 samples,

the β band appears at 1277 cm-1

whereas the α band emerges well at 1216, 1221 and 1218

cm-1

for 60, 70 and 80 oC spin coating temperatures, respectively, but for MSC samples,

the formation of well-distinguished peak for γ-phase is observed for all the spin

Fig. 5.16: FTIR-GIRA spectra of PVDF ultrathin films spin coated on gold-coated glass

slides at different spin coating temperatures in the region 1500-1000 cm-1

for (a) AC, (b)

AN130 and (C) MSC samples and in the region 1000-450 cm-1

for (d) AC, (e) AN130 and

(f) MSC samples.

140

coating temperatures at 1238 cm-1

. However, there is no well distinguished α-peak for

MSC samples in the region of 1500-1000 cm-1

. The alpha peaks at 975, 797, 763 and 614

cm-1

for MSC samples for all the spin coating temperatures increases its absorption

intensity with increasing spin coating temperatures, whereas for AC and AN130 samples,

alpha band is observed for the spin coating temperature from 60 oC onwards.

Fig. 5.17 represents the quantitative estimation of α-, β- and γ- crystalline phases

for varying thermally treated PVDF ultrathin films spin coated on ITO and gold-coated

substrates as a function of spin coating temperatures. The amount of crystalline phases

were calculated by measuring the absorbance ratio of α- (615 cm-1

), β- (1280 cm-1

) and γ-

(1229 cm-1

) characteristic peaks relative to the internal thickness band at 1073 cm-1

(Yoon et al 2008). From Fig. 5.17(a) and (b), the amount of β-crystalline phase for both

ITO and gold-coated glass slides slightly declined at lower spin coating temperatures (30

and 40 oC) but showed the increasing trend at higher spin coating temperatures (50 to 80

oC) irrespective of varying thermal treatments conditions (AC, AN130 and MSC).

Though AN130 sample spin coated at 60 oC exhibits the largest β-phase content among

the varying spin coating temperature samples, it also shows higher γ- and α-crystalline

phase content at that temperature, which is not favorable for electronic applications.

Instead, AN130 samples spin coated at 30 and 40 oC exhibited considerable amount of β-

phase along with the least γ- and α-crystalline phase content, which is highly favored for

electronic applications. Among the thermal treatment methods, the AN130 samples show

higher β-phase content in comparison to that of AC and MSC samples. The lower β-

crystalline content observed for MSC than that of AC and AN130 samples may be

attributed to relaxation of the molecular chain axis and the resultant crystalline

transformation from the ordered β-phase to the disordered γ- (Fig. 5.17(c) and (d)) as well

as α-phases (Fig. 5.17(e) and (f)).

A1 (1277 cm-1

)/B1 (1411 cm-1

) absorbance ratio of PVDF ultrathin films spin

coated on ITO and GOLD substrates at varying spin coating temperatures (30 to 80 oC)

are shown in Fig. 5.18 as a function of varying thermal treatment conditions (AC, AN130

and MSC). Annealed samples exhibit higher A1/B1 ratio in comparison to that of AC and

MSC samples. Among the AC samples spin coated at varying temperatures, the 30 oC

141

spin coated sample (AC-30) shows greater formation of β-phase due to slow evaporation

rate of the solvent compared to the faster evaporation rate of it at higher spin

Fig. 5.17: FTIR-GIRAS: Relative amount of β-phase for (a) ITO and (b) gold-coated

substrates, γ-phase for (c) ITO and (d) gold-coated substrates and α-phase for (e) ITO and

(f) gold-coated substrates of PVDF ultrathin films spin coated as a function of different

spin coating temperatures for different thermal treatments.

142

coating temperature. This sample upon annealing at 130 oC (30-AN130) showed further

increase in β-phase as depicted in Fig. 5.18.

The data obtained from the present study is compared with that reported by

Ramasundram et al (2008). In their studies, the A1/B1 ratio is enhanced with increasing

spin coating temperatures (30 to 70 oC) for PVDF sample dissolved in DMAc solvent.

They attributed this trend to the increasing formation of ‘edge-on’ lamella crystallites (the

molecular chains which are oriented along the substrate surface and their dipoles being

oriented normal to the substrate) at higher spin coating temperature than in the case of

spin coating at ambient conditions. The results obtained from our study are in

contradiction to their reported data. A closer examination revealed the higher A1/B1 ratio

of 0.88 for our sample spin coated at 30 oC and further annealed at 130

oC (30-AN130)

compared to the A1/B1 ratio of 0.46 as reported in Reference given by Ramasundaram et

al (2008). In addition, Ramasaundaram et al (2008) were able to achieve higher A1/B1

ratio (0.85) only at higher spin coating temperature (60 oC), whereas in the present study,

a much higher A1/B1 ratio of 0.88 even at lower spin coating temperature (30 oC) has

been achieved. This is attributed to the influence of solvent (DMF:acetone mixture) as

well as sample preparation conditions followed in our study which resulted in higher

degree of β-phase formation even for the samples spin coated at ambient temperature

conditions, thereby avoiding the dependence on heat-controlled spin coating setup.

Spin coating temperature (oC)

20 30 40 50 60 70 80 90

A1/B

1

0.4

0.5

0.6

0.7

0.8

0.9

AC

AN130

MSC

(a) ITO

Spin coating temperature (oC)

20 30 40 50 60 70 80 90

A1/B

1

0.4

0.5

0.6

0.7

0.8

0.9

1.0AC

AN130

MSC

(b) GOLD

Fig. 5.18: The ratio of A1/B1 of PVDF thin films spin coated at different temperatures for

(a) ITO and (b) GOLD substrates.

143

The A1/B1 ratio for the samples spin coated on (a) ITO and (b) Gold as substrates

as shown in Fig. 5.18 exhibited almost similar values and hence ITO is preferred as a

suitable substrate rather than Gold substrate in electronic applications due to its cheaper

cost than gold.

5.3.3 EFFECT OF ANNEALING STUDIES

Fig. 5.19 represents the FTIR-GIRA spectra for (a) PVDF and (b) PVDF/HBP

(90/10) blend ultrathin films for (i) as-cast (AC), (ii) annealed at 90 oC (AN-90) and (iii)

melt-annealed at 200 oC (MAN-200) samples measured at room temperature. The A1

bands responsible for changes in ferroelectric crystalline phase appear at 1280, 843 cm-1

for PVDF and 1275, 843 cm-1

for the blend samples. Compared to the stronger A1

absorption bands (

║ b

) exhibited by AC and AN-90 samples at 1280 cm-1

(for PVDF)

and 1275 cm-1

(for blend) associated with sCF2 stretching vibrations for the trans

sequence longer than TTTT as well as the 843 cm-1

band (for both PVDF and its blend)

associated with sCF2 + sCC for the trans sequence longer than TTT (Prabu et al 2009),

the aforementioned peaks exhibit reduced absorption intensity in the case of MAN-200

sample. The 1412 and 1073 cm-1

(B1,

║ c

) absorption bands assigned to CH2 + sCC

whose transition moment is along the chain orientation direction along with 491 cm-1

bands (ωCF2) (Prabu et al 2009) show higher peak intensity for MAN-200 in comparison

to that of both AC and AN-90 samples irrespective of the blend composition and this

band is highly sensitive to changes in chain orientation by GIRAS technique (Prabu et al

2006). Unlike P(VDF/TrFE) (72/28) copolymer samples, which exhibited significant

reduction in B1 band intensity for AN samples compared to AC sample (Prabu et al 2006

; Lee et al 2010) resulting in the favorable orientation of polymer chains and C-F dipoles,

not much significant changes were observed in the B1 band peak intensity of AN samples

from the present study. The band at 512 cm-1

(A1) for both samples, associated with δCF2

along with 1199 (PVDF), 1194 (blend) and 888 (both samples) cm-1

bands (B2,

║ a

)

associated with asCF2 and rCH2 modes, respectively, exhibit decrease in absorption

intensity in the case of MAN-200 sample when compared to that of AC and AN-90

samples. The band appeared at 1199 and 1194 cm-1

below melting for PVDF and the

144

Fig. 5.19: FTIR-GIRAS measured at room temperature for (a) PVDF and (b) PVDF/HBP

(90/10) blend ultrathin films for (i) as-cast, (ii) annealed at 90 oC, and (iii) melt-annealed

at 200 oC samples in the range of (a) 1500–1000 cm

-1 and (b) 950–450 cm

-1.

blend samples, respectively, is shifted to lower frequency at 1182 (PVDF) and 1178

(blend) cm-1

after melting. A strong band is observed at 1256 cm-1

for PVDF sample for

AN-160 sample, whereas such band is not formed for the blend sample. The intensity of

the α-peak appearing at 615 cm-1

is increased drastically for MAN-200 sample when

compared with AC and AN-90 samples. In addition to the peak at 615 cm-1

, peaks

corresponding to α-phase also appear at 975, 795 and 763 cm-1

only in the case of MAN-

200 sample for both PVDF and its blend. The α band is clearly seen at 1214 cm-1

only for

AC sample in the case of PVDF sample while it is observed at 1211 cm-1

upto annealing

at 150 oC (from AC to AN-150 sample) for the blend sample. In both cases, the alpha

Wavenumbers (cm-1

)

5006007008009001000

Absorb

ance (

a.u

)

(b) PVDF

(i)

(ii)

(iii) A1

B2

B1

Wavenumbers (cm-1

)

5006007008009001000

Absorb

ance (

a.u

)

(b) Blend

A1

B2

(iii)

(ii)

(i)

B1

(a) Blend

Wavenumbers (cm-1

)

100011001200130014001500

Absorb

ance (

a.u

)

(iii)

(ii)

(i)

B1

A1

B2

B1

(a) PVDF

Wavenumbers (cm-1

)

100011001200130014001500

Absorb

ance (

a.u

)A

1

B1

B1B

2

(i)

(ii)

(iii)

145

band at ca. 1214 cm-1

is not appeared as a well-distinguished peak in the melt annealed

sample. A shoulder at 1240 cm-1

is clearly seen at near melting point for both PVDF

(AN-160, AN-180, AN-200) and the blend (AN-150, AN-160, AN-180) samples. The γ-

band of melt-annealed samples appears at 1227 cm-1

and at 1230 cm-1

for PVDF and the

blend samples, respectively.

Fig. 5.20 represents the quantitative estimation of the α- (615 cm-1

), γ- (1229 cm-1

for PVDF and 1230 cm-1

for blend) and β- (1280 cm-1

for PVDF and 1275 cm-1

for blend)

crystalline phases relative to the 1073 cm-1

band (internal thickness band) (Sikka and

Kaush 1979; Roy et al 2005) for both PVDF and the blend samples. The relative β-

crystalline phase content for the blend sample is higher than that of PVDF sample.

However, reduction in relative beta content was noticed for the annealed samples below

the annealing temperature, 160 oC for the blend samples while for the PVDF samples,

beta content is raised. The sample annealed at 90 oC for PVDF exhibits higher beta

content among all the annealed samples as observed from Fig. 5.20(a). The β-crystalline

phase content is declined drastically for the MAN-200 samples and the decrease is found

to be around six times lesser than that of the AC sample of PVDF and two times for the

blend sample. The relative amount of α- and γ- crystalline phases of PVDF as well as the

blend samples show the same trend. The melt annealed samples of both PVDF

Fig. 5.20: FTIR-GIRAS: Relative amount of crystalline phases as a function of annealing

temperature for (a) PVDF and (b) PVDF/HBP (90/10) blend ultrathin films.

146

and its blend exhibit higher alpha and gamma content in comparison to that of AC and

annealed samples. During annealing, the relative gamma content is decreased to a lesser

extent than that of AC sample for PVDF samples. Hence, annealing leads to the

enhancement of ferroelectric crystalline phase due to partial conversion of -phase into

ferroelectric -crystalline phase in the case of PVDF annealed samples before melting.

The ratio of A1/B1 absorbance of PVDF and its blend samples annealed at varying

temperatures (30 → 200 oC) is depicted in Fig. 5.21. The FTIR-GIRA spectra show a

remarkable difference in chain orientation as well as dipole orientation as a function of

varying annealing temperatures which can be clearly noticed from the ratio of IR

absorption of peaks observed at 1280 and 1275 cm-1

(sCF2, A1, the vibrational transition

moment is parallel to the CF2 dipole, the b-axis of the unit lattice of the β-phase crystal

and normal to the polymer chain axis) for both PVDF and the blend ultrathin films,

respectively, to 1412 cm-1

(ωCH2 coupled with asC–C, B1, the vibrational transition

moment is parallel to the polymer chain axis, the c-axis of unit lattice of the β-phase

crystal and normal to the CF2 dipole) (Prabu et al 2006). For AC sample, the ratio, A1/B1

is 1.21 for PVDF ultrathin films whereas for the blend sample, it is 1.63. The reason for

increase in ratio for the blend is as follows: (i) it may be due to the presence of HBP

being nanometer (2-15 nm) in size (Yan et al 2010) acted as a nanoparticle in the blend

mixture, (ii) the enhancement of β-phase is either the presence of more extended all trans

fractions formed during cold crystallization by interrupting the chain mobility or by the

presence of HBP in the blend nucleates beta, perhaps epitaxially on their surfaces

(Buckley et al 2006) and/or (iii) the existence of H-bonding (Paleo et al 2011; Benz et al

2002; He and Yao 2006; Chen et al 2007) between the carbonyl and/or hydroxyl groups

of HBP and CH2 or CF2 of PVDF in the blend. The absorbance ratio (A1/B1) of the PVDF

annealed samples (T < 160 oC) is almost same for all the annealing temperatures but it is

lower than that of blend annealed at 70 oC whose ratio is 1.62. This is not only in

increasing the CF2 dipoles orientation normal to the substrate as well as to the polymer

chain axis but also some of the polymer chains which are not parallel to the substrate

before annealing, aligns parallel to the substrate after annealing treatment. Thus,

annealing leads to the formation of ‘edge-on’ lamella crystallites (Lee et al 2010) because

147

Annealing Temperature (oC)

20 40 60 80 100 120 140 160 180 200 220

A1(1

275)/

B1(1

412)

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8(b)(a)

Annealing Temperature (oC)

20 40 60 80 100 120 140 160 180 200 220

A1(1

280)/

B1(1

412)

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

of the presence of more number of trans-zigzag chains. However, in the case of blend,

the annealed samples before melting (from 40 to 160 oC) compared to AC sample, the

A1/B1 absorbance ratio is decreased due to either reduction in the H-bonding between the

carbonyl and/or hydroxyl groups of HBP and CH2 or CF2 groups of PVDF or the blend

getting softened upon heating when it was kept in a vacuum oven for annealing leading to

the relaxation of polymer chains which are otherwise in an stretched position induced

during the spin-coating process. The A1/B1 absorbance ratio is decreased for the melt

annealed sample, for both PVDF (0.42) and the blend (1.36) samples in comparison to

that of AC and annealed samples implying that the molecular chains of PVDF are

oriented along normal to the surface plane resulting in increase in B1 absorption intensity

as well as many of the dipoles responsible for ferroelectricity are oriented parallel to the

substrate in the melt state and hence it leads to the formation of mixture of ‘face-on’ and

‘edge-on’ lamella crystallites (Lee et al 2010). However, A1/B1 ratio for the melt annealed

blend sample is higher than that of AN as well as AC samples of PVDF which suggests

that higher β-crystalline content could be obtained even in the melt-annealed blend

sample.

Fig. 5.21: FTIR-GIRAS: A1/B1 absorbance ratios for (a) PVDF and (b) PVDF/HBP

(90/10) blend ultrathin films as a function of different annealing temperatures.

5.3.3.1 FACTOR ANALYSIS - ANNEALED SAMPLES

In this study, nine PVDF samples and eight blend samples previously annealed at

different temperatures (30 to 200 oC) was subjected to Factor Analysis (FA). In general, it

148

is difficult to prepare samples exhibiting only ferroelectric crystalline or amorphous

phases when annealed at a particular temperature. Using FA of the annealed samples

measured at room temperature, it is possible to extract the pure crystalline and amorphous

phase contents albeit with varying content of each phase depending upon the annealing

temperature. Raw GIRA spectra in the frequency range between 1500 to 1000 cm-1

containing 390 data points for PVDF and its blend were used for FA. The spectra were

smoothened, baseline corrected and normalized before applying FA and the FA data were

given in Table 5.2.

Table 5.2: Factor analysis results of GIRAS data of PVDF (9 samples) and

PVDF/HBP (90/10) blend (8 samples) ultrathin film samples annealed at different

temperatures

Components PVDF Blend

Eigenvalue 104

x IND Eigenvalue 104

x IND

1 8.3224 2.3025 8.5654 1.8439

2 0.6105 1.0112 0.3542 1.1066

3 0.0554 0.6176 0.0662 0.6807

4 0.0047 0.7471 0.0071 0.7549

5 0.0035 0.8973 0.0030 0.9826

6 0.0023 0.9755 0.0018 1.4698

7 0.0004 1.9083 0.0010 2.7923

8 0.0003 6.1274 0.0003 -

9 0.0001 -

Each eigenvalue given in Table 5.2 corresponds to the magnitude of each

independent component present in the normalized mixture absorbance matrix. Three

pure-components present in the mixture spectra were indicated by Malinowski’s indicator

function (IND)k to obtain non-zero eigenvalues between the first maxima and the minima

(Malinowski 1977; Malinowski 1980). Higher the magnitude of eigenvalue, larger the

probability of its corresponding component to be present in the mixture spectra and hence

149

from Table 5.2, it is understood that component 1 has the highest magnitude of non-zero

eigenvalue followed by other components with much lesser non-zero eigenvalues. Since

FA can be effectively applied for ‘binary mixtures’ using eqn. (9) as shown by Prabu et al

(2006), only the first two components having the highest non-zero eigenvalues among all

the components present in the mixture spectra were considered. The initial assumption

was that each sample used in this study exists as a mixture of both crystalline and

amorphous phases as their major components. The third component factor may be related

to changes in the chain orientation and its magnitude is assumed negligible or much less

when compared to the changes in the crystalline and amorphous phase contents. As seen

from Table 5.2, the first eigenvalue and its corresponding abstract eigenspectrum as

shown in Fig. 5.22 is found to be significant followed by the second eigenvalue and its

eigenspectrum as represented in Fig. 5.22 and hence both were used for further analysis

in order to get the pure-component spectra.

Fig. 5.22: FTIR-GIRAS: Abstract eigenspectra of (a) PVDF and (b) PVDF/HBP (90/10)

blend ultrathin films of annealed samples using FA.

Fig. 5.23 shows the product spectrum of both PVDF and the blend ultrathin

samples obtained by multiplying the first abstract eigenspectrum and the second

eigenspectrum as shown in Fig. 5.22 of both PVDF and the blend ultrathin samples. The

1276 and 1229 cm-1

bands were used as characteristic bands for minimum and maximum,

Eigen spectrum 1

Wavenumbers (cm-1

)

100011001200130014001500

Absorb

ance (

a.u

)

2

3

(a)

Eigen spectrum 1

Wavenumbers (cm-1

)

100011001200130014001500

Absorb

ance (

a.u

)

2

3

(b)

150

respectively, for the PVDF sample while, the bands at 1274 and 1229 cm-1

as minimum

and maximum, respectively, were used for the blend sample to get pure-components for

both the samples. The pure-component spectra, viz. PuS-1 and PuS-2 are depicted in Fig.

5.24 for annealed samples of PVDF and the blend ultrathin films. In the extracted pure

crystalline spectrum, PuS-1 for both the samples, the bands at 1276, 1199, 1079 cm-1

and

1272, 1199, 1128, 1076 cm-1

(Kobayashi et al 1975; Tashiro et al 1985) exist for PVDF

and the blend samples, respectively, as represented in Fig. 5.24. The 1199, 1079 cm-1

and

1199, 1076 cm-1

bands of pure crystalline bands of PVDF and the blend samples,

respectively, partially overlap with the bands at 1181 (PVDF), 1180 (blend) and 1072

(both) cm-1

in PuS-2 and hence these bands are not considered as pure crystalline bands.

Since the band at 1276 cm-1

of PVDF and at 1272 cm-1

of the blend ultrathin samples,

shows with enhanced absorption intensity signifying that crystallinity has been raised

well but do show some overlapping with the band at 1294 cm-1

in the PuS-2 as shown in

Fig 5.24, this non-overlapping band is not considered for measuring ferroelectric

crystallinity of the sample. The band at 1229 (PVDF) and 1227 cm-1

(blend) is non-

overlapping with the amorphous bands of PuS-2 spectrum (refer to Fig. 5.24) and can be

related to the pure amorphous phase (Cortili and Zerbi 1967; Kobayashi et al 1975).

Apart from this amorphous band, one more non-overlapping band appearing at 1151 and

1150 cm-1

for PVDF and the blend ultrathin samples, respectively, as shown in PuS-2 as

an additional amorphous band was detected (Prabu et al 2009). The non-overlapping

ferroelectric crystalline band is not clearly recognized from FA results using GIRAS due

Fig. 5.23: FTIR-GIRAS: Product spectrum of (a) PVDF and (b) PVDF/HBP (90/10)

blend ultrathin films of annealed samples using FA.

Wavenumbers (cm-1

)

100011001200130014001500

Absorb

ance

-0.08

-0.06

-0.04

-0.02

0.00

0.02

0.04

0.06

1276

1229

1411

1180

1425

1072

(a)

1199

1151

Wavenumbers (cm-1

)

100011001200130014001500

Absorb

ance

-0.04

-0.02

0.00

0.02

0.04

1425

1229

1072

1274

1180

(b)

1411

1153

1383

151

Fig. 5.24: FTIR-GIRAS: Extracted pure-component spectra (PuS-1 and 2) of (a) PVDF

and (b) PVDF/HBP (90/10) blend ultrathin films of annealed samples using FA.

to partial overlap of the third component corresponding to the changes in the chain

orientation with the crystalline bands as well as amorphous bands as observed in

P(VDF/TrFE) (72/28) annealed samples by GIRAS technique (Prabu et al 2006). In

GIRAS, a considerable change in absorption intensity of B1 band for the annealed

samples could have resulted in some interference of B1 band over with A1 band during

FA.

However, the degree of crystallinity can be quantitatively calculated using eqn.

(8) as given in Ref. (Prabu et al 2006). Fig. 5.25 portrays the plot of % crystalline phase

content as a function of varying annealing temperatures by using FA. The amount of

crystalline phase remains almost unchanged for the sample annealed at 30 oC and for

higher annealing temperature upto 130 oC and 150

oC for the PVDF and its blend

samples, respectively, unlike in the case of P(VDF/TrFE) (72/28) annealed samples

where it is increased significantly for samples annealed in the Tc range (Prabu et al 2006).

Thus, in PVDF and its blend samples, the absence of drastic increase in the ferroelectric

content for the annealed samples but below melting, indicates the absence of curie

transition behavior as observed in P(VDF/TrFE) (72/28) sample (Prabu et al 2006).

However, the ferroelectric content is dropped significantly for PVDF sample after

annealing at 160 oC and it leveled off at higher annealing temperature while it is

Wavenumbers (cm-1

)

100011001200130014001500

Absorb

ance

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0.18

0.20

0.22PuS-1

PuS-2

1076

1199

1272

1411

1150

1072

1227

1180

1414

1425

(b)

11281

430

1383

1294

Wavenumbers (cm-1

)

100011001200130014001500

Absorb

ance

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0.18

0.20

0.22PuS-1

PuS-2

1079

1199

1276

1411

1151

1072

1229

1181

1432 1

416

1425

(a)

1294

152

decreased for the melt-annealed sample of the blend sample. The content of ferroelectric

crystalline phase for PVDF at 30 oC was calculated to be 71 %, which is huge in quantity

when compared to P(VDF/TrFE) (72/28) for the sample annealed at room temperature by

GIRAS (Prabu et al 2006), but it is lesser than that of blend sample whose value is found

to be 87 %. The increase in all trans fraction content of the blend sample is due to the

presence of HBP. Overall, the as-cast PVDF and its blend sample exhibited much higher

percentage content of ferroelectric crystallinity than the widely used P(VDF/TrFE)

(72/28) annealed samples.

Fig. 5.25: FTIR-GIRAS: The content of crystalline phase for (a) PVDF and (b)

PVDF/HBP (90/10) blend ultrathin films of annealed samples using FA.

5.3.4 EFFECT OF HEATING-COOLING STUDIES

5.3.4.1 FTIR-TS: PHASE TRANSITION AND FACTOR ANALYSIS STUDIES OF

AS-CAST (AC) SAMPLES

After deciding the best annealing temperature (130 oC) for non-volatile random

access memory (NvRAM) applications, the PVDF and its blend samples are subjected to

heating–cooling (30→210→30 oC) studies to comprehend the ferroelectric crystalline

phase change as a function of phase transition temperatures in order to determine the

working temperature range for NvRAM devices. Fig. 5.26 represents the FTIR-

transmission spectra (TS) recorded during heating–cooling (30→210→30 oC) cycle for

AC-PVDF and AC-(PVDF/HBP 90/10) blend ultrathin films as a function of different

(a)

Annealing Temperature (oC)

20 40 60 80 100 120 140 160 180 200 220

% c

onte

nt of cry

sta

llin

e p

hase

0

10

20

30

40

50

60

70

80

90

100

(b)

Annealing Temperature (oC)

20 40 60 80 100 120 140 160 180 200 220

% c

onte

nt of cry

sta

llin

e p

hase

0

10

20

30

40

50

60

70

80

90

100

153

phase transition temperatures. During heating–cooling cycle, both PVDF and its blend

ultrathin film samples undergo phase transition from low-temperature phase to high-

temperature phase. The A1 (1276 cm-1

) band assigned to ferroelectric crystalline phase

and sensitive to sequences of four or more trans form, is decreased sharply in absorption

intensity during heating cycle. The absorption intensity of the B1 band at 1405 cm-1

assigned to chain orientation does not change considerably even when the sample

transformed to high-temperature phase. The reason for this is that the trans-gauche

conformational change is considered to occur without any changes in the chain

orientation as reported by Tashiro and Kobayashi (1989). During phase transition from

heating to cooling, the all trans form associated with A1 band is converted to amorphous

phase (1234 cm-1

) (Cortili and Zerbi 1967; Kobayashi et al 1975).

Fig. 5.26: TS-AC in the range of 1459-1000 cm-1

: (a) PVDF and (b) PVDF/HBP (90/10)

blend ultrathin film samples. Temperature induced changes as a function of heating and

cooling sequence in the order of (i) 30H, (ii) 140H, (iii) 200H, (iv) 140C, (v) 80C and

(vi) 30C.

The spectral features of the high-temperature (above Tm) and low-temperature

phases (below Tm) were investigated by applying FA for the 27 and 23 transmission

spectra recorded during heating-cooling (30→210→30 oC) cycle for AC-PVDF and AC-

blend samples, respectively. The frequency range of 1459–1000 cm-1

containing 358 data

points for both the samples was considered for FA. Table 5.3 gives the FA data for both

the samples from heating–cooling studies. (IND)k between the first maxima and the

minima showed the presence of eight and six components in the mixture spectra for AC-

Wavenumbers (cm-1

)

10001100120013001400

Absorb

ance (

a.u

)

(a)

B1

B2

A1

(vi)

(v)

(iv)

(iii)

(ii)

(i)

1460Wavenumbers (cm

-1)

10001100120013001400

Absorb

ance (

a.u

)(b)

B1

B2

A1

(vi)

(v)

(iv)

(iii)

(ii)

(i)

1460

154

PVDF and AC-blend samples, respectively. The initial assumption was that each sample

from our heating–cooling studies exists with the major component present being either

the low- or high-temperature crystalline phase. The other following components (from

3rd

) may be factors related to the changes in the chain orientation, surface scattering, etc.

and their magnitude is assumed to be negligible or much less when compared to the

changes in the low- or high-temperature phase contents. As shown in Table 5.3, the first

and the second eigenvalues corresponding to their eigenspectra were employed for FA.

Table 5.3: TS: Factor analysis results of PVDF and its blend (PVDF/HBP 90/10)

samples of AC and AN subjected to heating-cooling (30→210→30 oC) cycle

Compo

-nents

PVDF-AC PVDF-AN Blend-AC Blend-AN

Eigenvalue IND x

104 Eigenvalue

IND x

104 Eigenvalue

IND x

104 Eigenvalue

IND x

104

1. 25.0251 0.1669 24.1473 0.1732 21.404 0.2018 24.0928 0.1783

2. 0.7990 0.0785 0.7503 0.0667 0.4767 0.1019 0.7953 0.0696

3. 0.1672 0.0194 0.0942 0.0211 0.1094 0.0329 0.1000 0.0253

4. 0.0057 0.0127 0.0060 0.0122 0.0067 0.0205 0.0078 0.0166

5. 0.0021 0.0077 0.0008 0.0105 0.0022 0.0105 0.0020 0.0131

6. 0.0002 0.0077 0.0005 0.0090 0.0001 0.0105 0.0005 0.0127

7. 0.0001 0.0076 0.0002 0.0087 0.0001 0.0106 0.0003 0.0128

8. 0.0001 0.0074 0.0001 0.0088 0.0000 0.0112 0.0002 0.0131

9. 0.0000 0.0076 0.0000 0.0093 0.0000 0.0116 0.0001 0.0140

10. 0.0000 0.0080 0.0000 0.0098 0.0000 0.0130 0.0001 0.0151

11. 0.0000 0.0086 0.0000 0.0104 0.0000 0.0147 0.0000 0.0166

12. 0.0000 0.0094 0.0000 0.0115 0.0000 0.0167 0.0000 0.0185

13. 0.0000 0.0104 0.0000 0.0129 0.0000 0.0187 0.0000 0.0208

14. 0.0000 0.0116 0.0000 0.0147 0.0000 0.0217 0.0000 0.0221

15. 0.0000 0.0132 0.0000 0.0155 0.0000 0.0236 0.0000 0.0236

16. 0.0000 0.0152 0.0000 0.0169 0.0000 0.0296 0.0000 0.0274

17. 0.0000 0.0161 0.0000 0.0198 0.0000 0.0379 0.0000 0.0323

18. 0.0000 0.0177 0.0000 0.0235 0.0000 0.0509 0.0000 0.0393

19. 0.0000 0.0214 0.0000 0.0293 0.0000 0.0721 0.0000 0.0494

20. 0.0000 0.0263 0.0000 0.0383 0.0000 0.1103 0.0000 0.0643

21. 0.0000 0.0340 0.0000 0.0531 0.0000 0.2247 0.0000 0.0881

22. 0.0000 0.0472 0.0000 0.0780 0.0000 0.7554 0.0000 0.1300

23. 0.0000 0.0700 0.0000 0.1285 0.0000 - 0.0000 0.2134

24. 0.0000 0.1175 0.0000 0.2650

0.0000 0.4382

25. 0.0000 0.2551 0.0000 1.0047

0.0000 1.5517

26. 0.0000 0.9506 0.0000 -

0.0000 -

27. 0.0000 -

155

The abstract eigenspectra and product spectrum of both PVDF and the blend

samples are represented in Fig. 5.27 and in Fig. 5.28, respectively. Product spectrum is

obtained by the product of first abstract eigenspectrum and the second eigenspectrum

from FA for both samples by TS. The minimum at 1275 cm-1

and maximum at 1231 cm-1

were employed for obtaining the pure-component spectra as well as the contents of pure-

components for AC-PVDF sample while for AC-blend sample, the bands at 1231 and

1275 cm-1

as minimum and maximum, respectively, were chosen for acquiring the

aforementioned data.

Fig. 5.27: TS-AC: Abstract eigenspectra for (a) PVDF and (b) PVDF/HBP (90/10) blend

ultrathin film samples from heating-cooling studies using FA.

Fig. 5.28: TS-AC: Product spectrum for (a) PVDF and (b) PVDF/HBP (90/10) blend

ultrathin film samples from heating-cooling studies using FA.

Eigen spectrum 1

Wavenumbers (cm-1

)

10001100120013001400

Absorb

ance (

a.u

)2

3

4

5

6

1460

(b)

Eigen spectrum 1

Wavenumbers (cm-1

)

10001100120013001400

Absorb

ance (

a.u

)

2

3

(a)

4

5

6

1460

7

8

Wavenumbers (cm-1

)

10001100120013001400

Absorb

ance

-0.08

-0.06

-0.04

-0.02

0.00

0.02

0.04

0.06

1275

1231

1407

1171

1430

1073

(a)

1392

1204

1460

Wavenumbers (cm-1

)

10001100120013001400

Absorb

ance

-0.06

-0.04

-0.02

0.00

0.02

0.04

0.06

1231

1275

1407

1430

1074

1392

1171

1460

(b)

156

The extracted pure low-temperature and high-temperature phase spectra as shown

in Fig. 5.29 are designated as PuS-1 and PuS-2, respectively, for both the samples. The

PuS-1 for both the samples describes the narrow and well-defined peaks representing that

they are low-temperature ferroelectric crystalline phases. The broad band indicating the

amorphous phase is depicted as PuS-2 in Fig. 5.29. The bands at 1275, 1173 and 1073

cm-1

in PuS-1 are crystalline bands for both the samples. The presence of these bands in

PuS-1 of Fig. 5.29 is confirmed by the previous reports by Tashiro et al (1985) and Prabu

et al (2006). The bands at 1173 and 1073 cm-1

do partially overlap with high-temperature

phase bands at 1194, 1067 cm-1

for the PVDF sample and 1184, 1052 cm-1

for its blend

sample as could be seen from the Fig. 5.29. The band appearing at 1275 cm-1

for both the

samples is considered for measuring sample crystallinity since it is non-overlapping with

any disordered phase bands (Prabu et al 2006 and Prabu et al 2009). The band at 1231

cm-1

for both the samples as represented in PuS-2 of Fig. 5.29 is non-overlapping with

crystalline bands and regarded as amorphous phase (Cortili and Zerbi 1967; Kobayashi et

al 1975). In addition, one more non-overlapping band is noticed at 1108 for the PVDF

sample and 1117 cm-1

for its blend sample as represented in PuS-2 of Fig. 5.29 are

regarded as amorphous phase (Kim et al 1989; Kim et al 1993; Prabu et al 2009).

Fig. 5.29: TS-AC: Extracted pure-component spectra (PuS-1 and 2) of (a) PVDF and (b)

PVDF/HBP (90/10) blend ultrathin film samples from heating-cooling studies using FA.

Wavenumbers (cm-1

)

10001100120013001400

Absorb

ance

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0.18

0.20PuS-1

PuS-2

1073

1173

1275

1396

1108

1067

1231

1194

1429

1406

1427

(a)

1460

Wavenumbers (cm-1

)

10001100120013001400

Absorb

ance

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0.18PuS-1

PuS-2

1073

1173

1275

1397

1117

1052

1231

1184

1429

1405

1427

(b)

1460

157

(a)

Temperature (oC)

20 40 60 80 100 120 140 160 180 200 220

% c

onte

nt

of

cry

sta

lline p

hase

0

10

20

30

40

50

60

70

80

90

heating

cooling

(b)

Temperature (oC)

20 40 60 80 100 120 140 160 180 200 220

% c

onte

nt of cry

sta

lline p

hase

0

10

20

30

40

50

60

70

80

90

heating

cooling

Fig. 5.30 represents the percentage content of all trans crystalline phase during

heating-cooling studies from FA for (a) AC-PVDF and (b) AC-blend samples by FTIR-

TS studies. The ferroelectric content at 30 oC for both AC-PVDF and AC-blend samples

were found to be 59% and 67%, respectively, during heating cycle, whereas when the

same samples cooled to 30 oC after melting at 210

oC, the ferroelectric contents were

found to be 27% and 25%, respectively. The amount of crystalline content observed after

cooling at 30 oC is almost equivalent to that at near 190

oC during heating cycle for both

the samples. Compared to the crystalline content of P(VDF/TrFE) (72/28) sample

measured at 30 oC after cooling from below Tm (Prabu et al 2006), the PVDF-AC sample

from the present study exhibited slightly higher crystalline content that requires further

attention. In addition, the reversible phase transition between low-temperature ↔ high-

temperature crystalline phases attributed to curie transition temperature (Tc) in Ref.

(Prabu et al 2006) was not observed in the present study using PVDF. At near 170 oC,

both AC-PVDF and AC-blend samples exhibit melting behavior, which may further

result in the loss of polarization through an order-disorder transition. The sample

becomes more amorphous above this temperature and the possible statistical combination

of TG, TG’, T3G, and T3G’ isomers packed in a disordered lattice could be obtained

during this phase transition from ferroelectric crystalline to melt amorphous phase

(Furukawa 1989; Tashiro et al 1984; Tashiro et al 1987; Tahsiro and Kobayashi 1989;

Tahsiro and Kobayashi 1994).

Fig. 5.30: TS-AC: The percentage content of crystalline phase for (a) PVDF and (b)

PVDF/HBP (90/10) blend ultrathin film samples from heating-cooling studies using FA.

158

5.3.4.2 FTIR-TS: PHASE TRANSITION AND FACTOR ANALYSIS STUDIES OF

ANNEALED (AN) SAMPLES

The phase transition behavior of transmission spectra for AN-PVDF and AN-

blend samples is depicted in Fig. 5.31 when the sample is subjected to heating–cooling

(30→210→30 oC) cycle as a function of different phase transition temperatures and they

followed the similar behavior observed for TS-AC of PVDF and its blend samples.

From heating-cooling (30→210→30 oC) cycle for both AN-PVDF and AN-blend

samples, 26 transmission spectra were selected and they were subjected to FA in order to

extract pure low-temperature (ferroelectric) and pure high-temperature (amorphous)

phase spectra and to calculate quantitatively the content of each phase as a function of

phase transition temperatures. For FA, 358 data points for both the samples were taken

into consideration in the frequency range of 1459–1000 cm-1

and the FA data were given

in Table 5.3. Based on (IND)k between the first maxima and the minima, seven and six

factors were obtained for AN-PVDF and AN-blend samples, respectively, and the

corresponding abstract eigenspectra of both samples based on these factors were shown

in Fig. 5.32.

Fig. 5.31: TS-AN: (a) PVDF and (b) PVDF/HBP (90/10) blend ultrathin film samples.

Temperature induced changes as a function of heating and cooling sequence in the order

of (i) 30H, (ii) 140H, (iii) 200H, (iv) 140C, (v) 80C and (vi) 30C.

Wavenumbers (cm-1

)

10001100120013001400

Absorb

ance (

a.u

)

(a)

B1

B2

A1

(vi)

(v)

(iv)

(iii)

(ii)

(i)

1460

Wavenumbers (cm-1

)

10001100120013001400

Absorb

ance (

a.u

)

(b)

B1

B2

A1

(v)

(iv)

(iii)

(ii)

(i)

1460

(vi)

159

The product spectra for both samples by TS from FA data obtained from the first

abstract eigenspectrum and the second eigenspectrum are shown in Fig. 5.33. The

minimum at 1227 cm-1

and maximum at 1276 cm-1

were chosen for AN-PVDF while for

AN-blend sample, the minimum at 1231 cm-1

and maximum at 1276 cm-1

were preferred

for obtaining the pure-component spectra.

Fig. 5.32: TS-AN: Abstract eigenspectra for (a) PVDF and (b) PVDF/HBP (90/10) blend

ultrathin films samples from heating-cooling studies using FA.

Fig. 5.33: TS-AN: Product spectrum of (a) PVDF and (b) PVDF/HBP (90/10) blend

ultrathin film samples from heating-cooling studies using FA.

Wavenumbers (cm-1

)

10001100120013001400

Absorb

ance

-0.06

-0.04

-0.02

0.00

0.02

0.04

0.06

0.08

1276

1392

1430

1073

(a)

1407

1172

1227

1203

1460

Wavenumbers (cm-1

)

10001100120013001400

Absorb

ance

-0.06

-0.04

-0.02

0.00

0.02

0.04

0.06

1231

1276

1407

1430

1073

1393

1172

1460

(b)

1204

Eigen spectrum 1

Wavenumbers (cm-1

)

10001100120013001400

Absorb

ance (

a.u

)

2

3

(a)

4

5

6

7

1460

Eigen spectrum 1

Wavenumbers (cm-1

)

10001100120013001400

Absorb

ance (

a.u

)

2

3

(b)

4

5

6

1460

160

The extracted pure low-temperature (crystalline phase) and high-temperature

phase (amorphous) spectra are depicted in Fig. 5.34 and designated as PuS-1 and PuS-2,

respectively, for both the samples. Low-temperature ferroelectric crystalline phase (PuS-

1) and high-temperature amorphous phase (PuS-2) are shown in Fig. 5.34 for both the

samples. PuS-1 of both the samples exhibits well-defined crystalline bands at 1276, 1173

and 1073 cm-1

and a shoulder appears at 1238 cm-1

for AN-PVDF sample while for AN-

blend, it is present at 1235 cm-1

(Tashiro et al 1985; Prabu et al 2009). The ferroelectric β-

phase appearing at 1276 cm-1

for both the samples is non-overlapping with the

amorphous bands of PuS-2 as shown in Fig 5.34 could be regarded as the pure

ferroelectric crystalline phase and considered for measuring the sample crystallinity. The

bands, at 1173 and 1073 cm-1

do partially overlap with the high-temperature phase bands

at 1195 and 1060 cm-1

, respectively, as seen from the Fig. 5.34. Since the band at 1238

cm-1

is buried under the peak at 1229 cm-1

of high-temperature phase spectrum for AN-

PVDF while for AN-blend, 1235 cm-1

band mixes with the band at 1231 cm-1

, the bands

at 1229 (PVDF) and 1231 (blend) cm-1

are not considered as amorphous bands. The

bands at 1106 and 1110 cm-1

for AN-PVDF and AN-blend, respectively, are non-

overlapping peak and regarded as amorphous phase (Cortili and Zerbi 1967; Kobayashi et

al 1975) as represented in PuS-2 from Fig. 5.34.

Fig. 5.34: TS-AN: Extracted pure-component spectra (PuS-1 and 2) of (a) PVDF and (b)

PVDF/HBP (90/10) blend ultrathin film samples from heating-cooling studies using FA.

Wavenumbers (cm-1

)

10001100120013001400

Absorb

ance

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0.18PuS-1

PuS-2

1073

1175

1276

1397

1110

1052

1231

1194

1429

1406

1427

(b)

1460

1235

Wavenumbers (cm-1

)

10001100120013001400

Absorb

ance

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0.18PuS-1

PuS-2

1073

1173

1276

1394

1106

1060

1229 1195

1429

1406

1427

(a)

1238

1460

161

Fig. 5.35 represents the changes in ferroelectric phase, which was quantitatively

determined by the eqn. (8) as mentioned by Prabu et al (2006) as a function of phase

transition temperature during heating-cooling studies from FA for both the samples

namely, AN-PVDF and AN-blend. The amount of crystalline phase at 30 oC was found to

be 67% and 79% for AN-PVDF and AN-blend, respectively, during heating cycle. The

percentage content of AN-PVDF at 30 oC for heating cycle is higher than that of AN-

P(VDF/TrFE) (72/28) sample (Prabu et al 2006) by TS studies. Hence, the annealed

sample of PVDF could be the best alternative polymer to P(VDF/TrFE) (72/28) for

electronic applications such as non-volatile memory since the cost of PVDF is lesser than

that of P(VDF/TrFE) (72/28). The content of β-phase for AN-PVDF and AN-blend were

found to be 40% and 35%, respectively, when the sample was cooled to 30 oC from melt.

The ordered ferroelectric phase is transformed to disordered amorphous phase indicating

the loss of dipole orientations because of melting at around 160 oC during heating cycle

for both the samples, which is lower than that of AC samples measured by transition

studies as a function of phase transition temperatures. Above 160 oC, the sample turned to

be amorphous phase because of melting. For AN-PVDF sample, around 7% of the

crystalline content was increased during heating at 30 oC and the percentage content

during cooling at 30 oC was also increased around 10% in comparison to that for the AC-

PVDF sample from transmission heating-cooling studies. Thus, annealing

Fig. 5.35: TS-AN: The percentage content of crystalline phase for (a) PVDF and (b)

PVDF/HBP (90/10) blend ultrathin film samples from heating-cooling studies using FA.

Temperature (oC)

20 40 60 80 100 120 140 160 180 200 220

% c

onte

nt

of

cry

sta

llin

e p

hase

0

10

20

30

40

50

60

70

80

90

heating

cooling

(a)

Temperature (oC)

20 40 60 80 100 120 140 160 180 200 220

% c

onte

nt

of

cry

sta

llin

e p

hase

0

10

20

30

40

50

60

70

80

90

heating

cooling

(b)

162

amounts to more number of dipoles to be oriented normal to polymer chain axis and

some of the amorphous phase has been converted into ferroelectric crystalline phase.

5.3.4.3 FTIR-GIRAS: PHASE TRANSITION AND FACTOR ANALYSIS STUDIES

OF AC-CAST (AC) SAMPLES

The phase transition behavior for AC-PVDF and AC-blend samples using GIRAS

technique by heating–cooling (30→210→30 oC) cycle as shown in Fig. 5.36 is similar to

the phase transition behavior exerted by heating–cooling studies of AC and AN samples

of PVDF and its blend by transmission (TS) studies except the fact that since GIRAS is

sensitive to dipole and chain orientation bands, the A1 band (

║ ) associated with the

ferroelectric crystalline phase appearing at 1274 cm-1

(PVDF) and 1272 cm-1

(blend) is

sensitively detected by GIRAS and its absorption intensity is enhanced exclusively,

whereas the B1 band corresponding to chain orientation (

║ ) at 1413 cm-1

for both

PVDF and its blend samples showed reduced absorption intensity. The B2 absorption

intensity band (B2,

║ a

) at 1199 cm-1

for both PVDF and the blend samples, associated

with asCF2 is reduced remarkably for both the samples unlike AC and AN of PVDF and

its blend samples by TS (given earlier) and P(VDF/TrFE) (72/28) sample as reported by

Prabu et al (2006).

Fig. 5.36: GIRAS-AC: (a) PVDF and (b) PVDF/HBP (90/10) blend ultrathin film

samples. Temperature induced changes as a function of heating and cooling sequence in

the order of (i) 30H, (ii) 140H, (iii) 210H, (iv) 125C and (v) 30C.

Wavenumbers (cm-1

)

1200130014001500

Absorb

ance (

a.u

)

(b)

B1

B2

A1

1105

(v)

(iv)

(iii)

(ii)

(i)

Wavenumbers (cm-1

)

11001200130014001500

Absorb

ance (

a.u

) (i)

(ii)

(a)

(iii)

(iv)

(v)

B1

B2

A1

1037

163

The spectral features of the high-temperature and low-temperature phases were

studied by using FA for the 29 and 19 reflection absorption spectra recorded during

heating-cooling (30→210→30 oC) cycle for AC-PVDF and AC-blend samples,

respectively. The frequency range of 1500-1037 and 1500-1105 cm-1

having 341 and 206

data points for both AC-PVDF and AC-blend samples, respectively, were considered for

FA for extracting the pure-component spectra. The FA data were given in Table 5.4 for

both the samples by heating–cooling studies. The number of factors for both the samples

were determined from the (IND)k value and were found to be seven and four for AC-

PVDF and AC-blend, respectively. Eigenspectra and product spectrum of the first

eigenspectrum and the second eigenspectrum are represented in Fig. 5.37 and Fig. 5.38,

respectively, for both the samples.

Fig. 5.37: GIRAS-AC: Abstract eigenspectra for (a) PVDF and (b) PVDF/HBP (90/10)

blend ultrathin film samples from heating-cooling studies using FA.

The bands at 1211 and 1278 cm-1

were employed as minimum and maximum,

respectively, in the product spectrum as shown in Fig. 5.38(a) for AC-PVDF while the

bands at 1211 (minimum) and at 1274 (maximum ) cm-1

for AC-blend were considered

for obtaining the pure-component spectra by FA. Though, the extracted pure-component

spectra as shown in Fig. 5.24 of the annealed samples by GIRAS is not conclusive like

the literature reported by Prabu et al (2006), the heating-cooling studies by GIRAS is

more effective in extracting the pure-component spectra as depicted in Fig. 5.39. The

Eigen spectrum 1

Wavenumbers (cm-1

)

1200130014001500

Abso

rban

ce (

a.u

)

2

3

4

(b)

1105

Eigen spectrum 1

Wavenumbers (cm-1

)

11001200130014001500

Absorb

ance (

a.u

)

2

3

4

5

6

7

1037

(a)

164

PuS-1 for both the samples represents the ferroelectric low-temperature all trans phase

while the PuS-2 is high-temperature amorphous phase. The crystalline bands obtained for

PuS-1 for AC-PVDF are 1378, 1274, 1232, 1184, 1155 and 1074 cm-1

whereas for AC-

blend, the crystalline bands are shifted to lower wavenumber in comparison to that of

AC-PVDF as seen from Fig. 5.39(b). The bands at 1274 and 1155 cm-1

for AC-PVDF

and 1270 and 1151 cm-1

for AC-blend are non-overlapping with the amorphous phase

Table 5.4: GIRAS: Factor analysis results of PVDF and its blend (PVDF/HBP

90/10) samples of AC and AN subjected to heating-cooling (30→210→30 oC) cycle

Comp-

onents

PVDF-AC PVDF-AN Blend-AC Blend-AN

Eigenvalue IND x

104 Eigenvalue

IND x

104 Eigenvalue

IND x

104 Eigenvalue

IND x

104

1. 28.2381 0.1355 22.2253 0.2404 18.5236 0.3498 24.0991 0.2086

2. 0.4345 0.0973 0.4732 0.1684 0.3056 0.2416 0.4826 0.1581

3. 0.3040 0.0286 0.2793 0.0516 0.1554 0.0845 0.3912 0.0449

4. 0.0116 0.0224 0.0128 0.0381 0.0072 0.0722 0.0139 0.0352

5. 0.0036 0.0207 0.0034 0.0348 0.0019 0.0750 0.0024 0.0358

6. 0.0017 0.0205 0.0024 0.0306 0.0015 0.0783 0.0020 0.0366

7. 0.0015 0.0201 0.0011 0.0293 0.0012 0.0818 0.0016 0.0376

8. 0.0009 0.0205 0.0005 0.0302 0.0008 0.0884 0.0011 0.0397

9. 0.0009 0.0204 0.0004 0.0314 0.0006 0.0981 0.0010 0.0420

10. 0.0005 0.0212 0.0002 0.0341 0.0004 0.1118 0.0007 0.0453

11. 0.0004 0.0220 0.0001 0.0379 0.0003 0.1327 0.0006 0.0490

12. 0.0003 0.0232 0.0001 0.0427 0.0002 0.1623 0.0005 0.0538

13. 0.0002 0.0250 0.0001 0.0490 0.0002 0.2077 0.0004 0.0598

14. 0.0002 0.0270 0.0001 0.0565 0.0001 0.2788 0.0003 0.0676

15. 0.0001 0.0297 0.0001 0.0665 0.0001 0.3908 0.0003 0.0773

16. 0.0001 0.0325 0.0000 0.0803 0.0001 0.6459 0.0003 0.0897

17. 0.0001 0.0359 0.0000 0.1032 0.0001 1.2352 0.0002 0.1081

18. 0.0001 0.0404 0.0000 0.1391 0.0000 4.0034 0.0002 0.1342

19. 0.0001 0.0469 0.0000 0.2009 0.0000 - 0.0001 0.1715

20. 0.0000 0.0553 0.0000 0.3224 0.0001 0.2296

21. 0.0000 0.0667 0.0000 0.7058 0.0001 0.3256

22. 0.0000 0.0829 0.0000 2.6991 0.0000 0.5496

23. 0.0000 0.1059 0.0000 - 0.0000 1.1735

24. 0.0000 0.1404 0.0000 4.3646

25. 0.0000 0.2040 0.0000 -

26. 0.0000 0.3251

27. 0.0000 0.6102

28. 0.0000 2.0716

29. 0.0000 -

165

bands. The bands at 1274 (PVDF) and 1270 (blend) cm-1

are considered as ferroelectric

crystalline bands and could be used for measuring the sample crystallinity. Except the

bands at 1218, 1105 and 1257, 1214 cm-1

for AC-PVDF and AC-blend, respectively, all

other bands in PuS-2 as shown in Fig. 5.39 are overlapping with the crystalline bands of

PuS-1 as represented in Fig. 5.39. Hence, the non-overlapping bands (1218 for AC-PVDF

and 1257, 1214 cm-1

for AC-blend) are considered as amorphous phases (Prabu et al

2006; Prabu et al 2009).

Fig. 5.38: GIRAS-AC: Product spectrum of (a) PVDF and (b) PVDF/HBP (90/10) blend

ultrathin film samples from heating-cooling studies using FA.

Fig. 5.39: GIRAS-AC: Extracted pure-component spectra (PuS-1 and 2) of (a) PVDF and

(b) PVDF/HBP (90/10) blend ultrathin film samples from heating-cooling studies using

FA.

Wavenumbers (cm-1

)

1200130014001500

Absorb

ance

-0.06

-0.04

-0.02

0.00

0.02

0.04

0.06

1211

1274

1400

1417

11781230

1253

1105

1151

(b)

Wavenumbers (cm-1

)

11001200130014001500

Absorb

ance

-0.08

-0.06

-0.04

-0.02

0.00

0.02

0.04

0.06

0.08

14

00

12

78

10

74

12

53

11

82

14

17

12

30

1037

12

11

(a)

Wavenumbers (cm-1

)

1200130014001500

Absorb

ance

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0.18

0.20PuS-1

PuS-2

1178

1232

1270

1401

1113

1120

1257

1214

1378

1415

1428

(b)

1105

1151

1116

Wavenumbers (cm-1

)

11001200130014001500

Abso

rba

nce

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0.18

0.20PuS-1

PuS-2

1074

1232

1274

1401

1105

1056

1253

1218

1184

1415

1430

1037

1155

1378

(a)

166

The quantitative estimation of ferroelectric crystalline phase content using FA for

AC-PVDF and AC-blend samples from heating-cooling cycle measured using GIRAS is

shown in Fig. 5.40. The ferroelectric content of AC-blend at 30 oC during heating was

found to be 74%, which is higher than that of AC-PVDF (72%) and AN-P(VDF/TrFE)

(72/28) (Prabu et al 2006) samples. The ferroelectric content after melting at 210 oC for

both AC-PVDF and AC-blend samples were found to be 13% and 51%, respectively. The

ferroelectric contents for AC-PVDF and AC-blend samples after cooling to 30 oC from

melt were found to be 59% and 80%, respectively. The AC sample exhibits a reversible

crystalline phase transition with increased ferroelectric crystalline content in the case of

blend sample due to “non-isothermal annealed effect” (Choi et al 2008). The reason for

enhancing the ferroelectric content in the blend is owing to the presence of HBP in the

blend mixture. The reversible phase transition between the low-temperature and high-

temperature phase detected in the range of 130-150 oC for both the samples implies the

existence of polarization at higher temperature range than that observed from other earlier

studies using P(VDF/TrFE) (72/28) sample (Prabu et al 2006).

Fig. 5.40: GIRAS-AC: The percentage content of crystalline phase for (a) PVDF and (b)

PVDF/HBP (90/10) blend ultrathin film samples from heating-cooling studies using FA.

5.3.4.4 FTIR-GIRAS: PHASE TRANSITION AND FACTOR ANALYSIS STUDIES

OF ANNEALED (AN) SAMPLES

Fig. 5.41 depicts the grazing incidence reflection absorption spectra of AN-PVDF

and AN-blend samples obtained during heating-cooling cycle (30→210→30 oC) from

Temperature (oC)

20 40 60 80 100 120 140 160 180 200 220

% c

on

ten

t of

cry

sta

llin

e p

hase

0

10

20

30

40

50

60

70

80

90

heating

cooling

(b)(a)

Temperature (oC)

20 40 60 80 100 120 140 160 180 200 220

% c

onte

nt of cry

sta

llin

e p

hase

0

10

20

30

40

50

60

70

80

90

heating

cooling

167

GIRAS technique. The phase transition behavior of these samples is consistent with that

of AC-PVDF and AC-blend by heating-cooling studies using GIRAS technique.

Fig. 5.41: GIRAS-AN: (a) PVDF and (b) PVDF/HBP (90/10) blend ultrathin film

samples. Temperature induced changes as a function of heating and cooling sequence in

the order of (i) 30H, (ii) 140H, (iii) 210H, (iv) 125C and (v) 30C (PVDF) and 45C

(Blend).

In order to determine the percentage content of ferroelectric crystalline as well as

the working temperature for making nano device for electronic application, 23 and 25

spectra of both AN-PVDF and AN-blend samples, respectively, selected from a series of

spectra recorded during heating-cooling cycle (30→210→30 oC) were employed for FA.

For extracting the pure phases, the frequency range of 1500–1000 cm-1

containing 260

data points for both the samples were considered for FA. The FA data were given in

Table 5.4 for both the samples from heating–cooling studies. The number of eigenspectra

as shown in Fig. 5.42 is determined from the number of factors from (IND)k value as

given in the Table 5.4 and were found to be seven and four for AN-PVDF and AN-blend

samples, respectively.

The product spectrum, which is obtained by the product of the first eigenspectrum

and the second eigenspectrum, is depicted in Fig. 5.43 for both the samples. The

minimum at 1278 and the maximum at 1228 cm-1

for AN-PVDF sample, while the

minimum at 1253 and the maximum at 1274 cm-1

for AN-blend sample as shown in Fig.

Wavenumbers (cm-1

)

100011001200130014001500

Absorb

ance (

a.u

)

(b)

B1

B2

A1

(v)

(iv)

(iii)

(ii)

(i)

Wavenumbers (cm-1

)

100011001200130014001500

Absorb

ance (

a.u

)

(i)

(ii)

(a)

(iii)

(iv)

(v)

B1

B2A

1

168

5.43 were employed for obtaining the pure-component spectra by FA. The extracted

pure-component spectra designated as PuS-1 and PuS-2 as shown in Fig. 5.44 represent

the low-temperature and high-temperature phases, respectively. The crystalline bands that

were extracted by FA for PuS-1 of AN-PVDF sample as represented in Fig. 5.44 are

1278, 1199, 1083 and 1054 cm-1

whereas for AN-blend sample, they are identified as

1270, 1232, 1182, 1153 and 1074 cm-1

. Since, the crystalline band at 1278 cm-1

is

overlapped with the band at 1297 cm-1

of high-temperature phase spectrum, it is not

considered for measuring sample crystallinity in the case of AN-PVDF sample. This

Fig. 5.42: GIRAS-AN: Abstract eigenspectra for (a) PVDF and (b) PVDF/HBP (90/10)

blend ultrathin film samples from heating-cooling studies using FA.

Fig. 5.43: GIRAS-AN: Product spectrum of (a) PVDF and (b) PVDF/HBP (90/10) blend

ultrathin film samples from heating-cooling studies using FA.

Wavenumbers (cm-1

)

100011001200130014001500

Absorb

ance

-0.08

-0.06

-0.04

-0.02

0.00

0.02

0.04

0.06

0.08

1253

1274

1400

1417

1074

1180

(b)

1211

Wavenumbers (cm-1

)

100011001200130014001500

Absorb

ance

-0.10

-0.08

-0.06

-0.04

-0.02

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

1436

1228

1068

1278

1149

1417

1199

1407

(a)

Eigen spectrum 1

Wavenumbers (cm-1

)

100011001200130014001500

Absorb

ance (

a.u

)

2

3

4

5

6

7

(a)

Eigen spectrum 1

Wavenumbers (cm-1

)

100011001200130014001500

Absorb

ance (

a.u

)

2

3

4

(b)

169

is due to increase in the change orientation absorption band intensity (B1) may have

resulted in some interference of B1 sensitive band over A1 and B2 sensitive bands during

FA, since FA can be used for binary mixtures containing only two factors, the presence

of a third component probably sensitive to chain orientation may have resulted in the

partially overlapped crystalline and amorphous bands (Prabu et al 2006) as observed in

annealed samples by GIRAS. All the crystalline bands except 1232 and 1074 cm-1

of

PuS-1 of PVDF-AN sample are non-overlapping bands with the amorphous bands of

PuS-2 as shown in Fig. 5.44. Though the crystalline bands, 1270, 1182 and 1153 cm-1

as

seen from PuS-1 are non-overlapping, the band at 1270 cm-1

is considered for measuring

the ferroelectric crystallinity of the sample whereas, the non-overlapping bands at 1228

and 1149 cm-1

are considered as amorphous phases of PuS-2 spectrum as shown in Fig.

5.44(a) for AN-PVDF sample. In the case of AN-blend sample, the bands obtained for

PuS-1 are 1380, 1270, 1232, 1182, 1153 and 1074 cm-1

as noticed from Fig. 5.44(b). The

non-overlapping bands at 1270, 1182 and 1153 cm-1

are regarded as low-temperature

phases while the band at 1255 cm-1

is considered as high-temperature amorphous phase

since it is not overlapping with any low-temperature bands (Prabu et al 2006).

Fig. 5.44: GIRAS-AN: Extracted pure-component spectra (PuS-1 and 2) of (a) PVDF and

(b) PVDF/HBP (90/10) blend ultrathin film samples from heating-cooling studies using

FA.

The quantitative variation in ferroelectric phase as a function of phase transition

temperature is shown in Fig. 5.45 for AN-PVDF and AN-blend samples. The

ferroelectric content of AN-blend at 30 oC during heating is higher than that of AC-blend

and AN-PVDF by GIRAS and computed to be 85%, which is higher than that of AN-

Wavenumbers (cm-1

)

100011001200130014001500

Absorb

ance

0.00

0.04

0.08

0.12

0.16

0.20

0.24 PuS-1

PuS-2

1054

1199

1278

1070

1228

1149

1083

1434

1405

1403

(a)

1297

1105

Wavenumbers (cm-1

)

100011001200130014001500

Absorb

ance

0.00

0.04

0.08

0.12

0.16

0.20 PuS-1

PuS-2

10741182

1270

1401

1070

1255

1232

1415

1427

(b)

1153

1380

170

P(VDF/TrFE) (72/28) sample (Prabu et al 2006). In the previous study reported by

Sathiyanathan et al (2012), the spin-coated AN-P(VDF-TrFE)(72/28) sample exhibited an

irreversible phase transition occurring at around 120 and 115 oC during heating and

cooling, respectively. The crystalline phase that initially formed due to the spin-coating

action was destroyed at melt state resulting in reduced ferroelectric crystalline phase

content when cooled to room temperature from melt (Sathiyanathan et al 2012).

However, in the present study, the AN-blend sample exhibits a reversible phase transition

between the low-temperature and high-temperature phases occurring at near 135 and 120

oC during heating and cooling, respectively. The higher amount of β-phase and the

unique reversible hysteresis loop between ordered and disordered phases observed even

after cooled from melt exhibited by AN-blend sample makes it suitable for use in the

fabrication of electronic devices such as non-volatile memory, sensors, etc with higher

operating temperature range. Such reversible crystalline phase behavior is not observed in

the case of AN-PVDF, which implies the importance of the role played by HBP as a

suitable blend component for improving the ferroelectric crystalline phase in PVDF.

Fig. 5.45: GIRAS-AN: The percentage content of crystalline phase for (a) PVDF and (b)

PVDF/HBP (90/10) blend ultrathin film samples from heating-cooling studies using FA.

Temperature (oC)

20 40 60 80 100 120 140 160 180 200 220

% c

onte

nt

of

cry

sta

llin

e p

hase

0

10

20

30

40

50

60

70

80

90

heating

cooling

(a) (b)

Temperature (oC)

20 40 60 80 100 120 140 160 180 200 220

% c

onte

nt

of

cry

sta

llin

e p

hase

0

10

20

30

40

50

60

70

80

90

heating

cooling

171

5.4 CONCLUSIONS

In this chapter, the miscibility of HBP with PVDF in different ratios (2 wt.-%)

was confirmed from melting point depression and shift of the carbonyl band of the blend

to lower frequency by DSC and FTIR studies, respectively. XRD data of the solution cast

PVDF/HBP blends show that the beta crystalline phase is not completely reduced. From

DSC study, it was noticed that the melting peak is not completely disappeared indicating

that even with the addition of 50 wt.-% of HBP to PVDF, the blend (PVDF/HBP) sample

exhibits crystalline characteristics.

2 wt.-% PVDF solution was spin cast on different substrates such as KBr, ITO

and Gold-coated glass slides as a function of varying spin coating temperatures (30 to 80

oC). From effect of spin coating studies, it was inferred that maximum β-phase with less

α- and γ-phases could be obtained when the PVDF was prepared at 40 oC and both ITO

and GOLD substrates attained equal amount of beta crystalline phase fraction from A1/B1

ratio plot. Since both the substrates have been used for real time applications in electronic

fields, the cost of Gold-coated glass slides is higher than that of ITO substrates and ITO

was chosen for determining the amount of all trans crystalline phase for annealing and

heating-cooling studies in order to fabricate the device.

During annealing, A1 (trans sequence longer than TTTT ) band responsible for

changes in ferroelectric crystalline phase appears at ca. 1280 cm-1

for both PVDF and the

PVDF/HBP (90/10) blend samples exhibits higher absorption intensity below melting

whereas, after melting the absorption intensity of this band is reduced because of

transformation into amorphous phase from ferroelectric phase. Thus, annealing leads to

the formation of ‘face-on’ lamella crystallites because of more number of trans zigzag

chains in comparison to that of AC and MAN samples. In addition to that, annealing

studies revealed that the PVDF/HBP (90/10) blend has higher amount of ferroelectric

crystalline phase in comparison to that of PVDF from FA study quantitatively when both

the blend and PVDF samples were subjected to different annealing temperatures (30 to

200 oC) and pure-component spectra were extracted by FA from different annealed

samples. The increase in the beta content in the blend is (i) due to the presence of HBP

172

being nanometer (2-15 nm) in size acted as a nanoparticle in the blend mixture, (ii) the

enhancement of β-phase is either the presence of more extended all trans fractions

formed during cold crystallization by interrupting the chain mobility or by the presence

of HBP in the blend nucleates beta, perhaps epitaxially on their surfaces and/or (iii) the

existence of H-bonding between the carbonyl and/or hydroxyl groups of HBP and CH2 or

CF2 of PVDF in the blend. A1/B1 ratio obtained from GIRAS data for annealing of

PVDF sample, the sample annealed at 130 oC shows that more number of the all trans

form appearing at 1280 cm-1

is normal to the polymer chain axis. Hence, 130 oC was

chosen as annealing temperature for heating-cooling studies by FTIR-TS as well as

FTIR-GIRAS techniques in order to find out the operating temperature for the device for

non-volatile memory applications.

Heating-cooling (30→210→30 oC) studies were carried out for PVDF and the

PVDF/HBP (90/10) blend ultrathin samples for AC and annealed sample at 130 oC (AN)

for comparison. Pure high-temperature and low-temperature spectra as well as

quantification of percentage of ferroelectric content were extracted from FA for both AC

and AN samples of both PVDF and its blend. It was found that the AN sample exhibits

higher ferroelectric content in comparison to that of AC sample of both PVDF and its

blend by FA. The AC sample exhibited a reversible crystalline phase transition with

increased ferroelectric crystalline content in the case of blend sample due to “non-

isothermal annealed effect.” The reason for enhancing the ferroelectric content in the

blend is owing to the presence of HBP in the blend mixture. The reversible phase

transition between the low-temperature and high-temperature phase detected in the range

of 130-150 oC for both the samples implies the existence of polarization at higher

temperature range than that observed from other earlier studies using P(VDF/TrFE)

(72/28). The AN-blend sample exhibits a reversible phase transition between the low-

temperature and high-temperature phases occurring at near 135 and 120 oC during

heating and cooling, respectively. The higher amount of β-phase and the unique

reversible hysteresis loop between ordered and disordered phases observed even after

cooled from melt exhibited by AN-blend sample makes it suitable for use in the

fabrication of electronic devices such as non-volatile memory, sensors, etc with higher

173

operating temperature range. Such reversible crystalline phase behavior is not observed in

the case of AN-PVDF.

Among FTIR-TS and GIRAS techniques used in this study for evaluating the

changes in ferroelectric crystallinity, chain and dipole orientations as a function of (a)

annealing and (b) heating-cooling studies, FTIR-GIRAS data is more preferred than using

FTIR-TS data for the following reasons:

(i) FTIR-TS using KBr as the measurement substrate is not preferred due to the fact

that the substrates used for memory devices (Gold or ITO coated glass slides) are

not IR transparent. On the other hand, the substrate used for measuring FTIR-

GIRAS (Gold or ITO coated glass slides) is similar to that used in the fabrication

of electronic devices and hence the data obtained from GIRAS is more reliable

than that obtained from TS technique.

(ii) Due to the inherent measuring parameters like variable angle of incidence (ca.

80~88o from the normal to the surface) and polarization of the incident i.r. beam

(into s- and p- polarization) upon reflectance from the surface, GIRAS data can be

effectively used in characterizing the changes in C-C and C-F dipoles.