8.1. ABSTRACT - Shodhgangashodhganga.inflibnet.ac.in/bitstream/10603/13586/16/16_chapter 8.pdf ·...

Polymer Composites Based on Cellulosics Nanomaterials Chapter 8

Vilas Karande 189

8.1. ABSTRACT

Nanowhiskers obtained from microcrystalline cellulose (MCC) are of huge interest

due to their good mechanical strength. The present study deals with preparation of

cellulose nanowhiskers (CNW) from microcrystalline cellulose using high pressure

homogenizer. Microcrystalline cellulose (MCC) was prepared by acid hydrolysis of

short staple cotton fibers using hydrochloric acid. To achieve better homogenization,

MCC was passed through homogenizer repeatedly till 15 passes; and after every 3

passes was characterized using scanning electron microscopy (SEM), atomic force

microscopy (AFM), X-ray diffractometer (XRD) and viscometer. From SEM it was

found that the average diameter of MCC decreased significantly from 5-10 μm to

about 60 nm (also confirmed by AFM) after 15 passes. Force distance (FD) curve

analysis demonstrated that the Young’s modulus of CNW was about 452 MPa.

Prepared CNW was used as a reinforcing agent in chitosan. Tensile strength and

Young’s of the composite increased by 33.3% and 52.3% respectively, whereas %

elongation at break decreased by 69.6%, at 3% loading of CNW. Enthalpy of melting

increased with increase in concentration of CNW in Chitosan but no significant

change was observed in the melting temperature. CNW was found to have uniformly

distributed at 3% concentration, above which it started aggregating as depicted by

SEM.


Vilas Karande 190

8.2. INTRODUCTION

Biocomposites are composite materials comprising one or more phase(s) derived from

a biological origin. Bionanocomposites are plastics from renewable raw material

reinforced with either Nanocellulose extracted from pulp fibers or some other

nanomaterials. Bionanocomposites have established themselves as a promising class

of hybrid materials derived from natural and synthetic biodegradable polymers and

organic/inorganic fillers (Hule et al, 2007). Attempts to improve the properties of

materials through the addition of reinforcing fillers, either inorganic or organic, are

not new. For years, synthetic polymer composites have been developed and applied in

various industrial fields, domestic equipment, the automotive industry, and even in

aerospace. However, these synthetic materials come from limited, non-renewable

sources, and are not easily decomposed (Lu et al, 2009 and Tokiwa et al, 2009) by

microorganism present in nature. In addition, global warming, caused in part by

carbon dioxide released by the combustion of fossil fuels, has become an increasingly

important problem and the disposal of items made of petroleum-based plastics such as

fast-food utensils, packaging containers, and carrier bags also creates a serious

environmental problem (Wool and Sun, 2005). As we begin the 21st century, there is

an increased awareness that non renewable resources becoming scarce and

dependence on renewable resources is growing (Rowell et al, 2008). A growing

environmental awareness all over the world and the pressures to use evermore

“greener” technologies (Bismarck et al, 2008) have encouraged researchers and

industrialists to consider natural plant fibers as an alternative reinforcing agent or

filler to produce composite materials known as biobased composites (Thakur et al,

2010). Natural fiber composites are also claimed to offer environmental advantages

such as reduced dependence on non-renewable energy/material sources, lower

pollutant emissions, lower greenhouse gas emissions, enhanced energy recovery etc.

(Joshi et al, 2004) There are various types of the natural fibers and are classed

according to their source; plants, animals or minerals (Eichhorn et al, 2001).

By far the most abundant are the wood fibers (Siqueira et al, 2010) from trees;

however other fiber types are emerging. A common choice for reinforcement of bio-

based composites is cellulose. It is well known that cellulose fiber networks–as in the

case of paper – provides good mechanical properties because of the degree of

hydrogen bonding obtained between the fibers in the network. The greater the


Vilas Karande 191

hydrogen bonding, the stronger the composite material and nature of hydrogen

bonding depends on the size of the cellulose fibers or whiskers (figure 8.1)

Figure 8.1. Comparison of hydrogen bonded cellulose network in nano (A) and micro

(B) size.

Renewable biomaterials offer a broad range of commodities, including forests, crops,

and farm and marine animals; all of which have many uses. The idea of using plant-

based fiber as reinforcement has been around since the beginning of human

civilization. The use of straw-reinforced clay for making stronger bricks is one of the

earliest composites referred to in the literature (Prassad, 1992). Plenty of examples

can be found of plant-based fibers being used for reinforcement of petrobased

thermoplastic polymers such as polypropylene (PP), polyethylene (PE), nylon and

polyvinylchloride (PVC). Plant fibers have also been used to reinforce biodegradable

polymers such as cellulose ester, polyhydroxybutyrate (PHB), polyester amide,

polylactic acid (PLA) and starch derivatives and blends (Peijs, 2000). These types of

composites can be used for packaging (Kumar et al, 2010, Bucci et al, 2005,

Savenkova et al, 2000, and Sorrentino et al, 2007) agriculture (Dilara et al, 2000 and

Dave et al, 1999), and biomedical (Kamel S., 2007 and Kim et al, 2005) applications.

Microcrystalline cellulose (MCC), bundles of crystallites, has attracted attention as

the starting material for preparing biocomposites (Mathew et al, 2005). The source of

raw material and manufacturing process decisively influence the characteristics of the

MCC (Douglas et al, 2008).

In the present work, cellulose nanowhiskers (CNW) were prepared using high

pressure homogenizer, in which combination of shear, cavitational and impact forces

acted on MCC. CNW prepared by the homogenization process were characterized by

Scanning electron microscopy (SEM), Atomic force microscopy (AFM), X-ray


Vilas Karande 192

diffraction (XRD) and viscometer. The cellulose nanowhiskers produced by above

method were used as reinforcing agent in chitosan to enhance its performance. The

chitosan/CNW nanocomposites were prepared using solution casting process and

characterized for mechanical, morphological, thermal, optical and barrier properties.

8.3. EXPERMINTAL WORK

8.3.1. Materials and Methods

Short staple cotton fibers were used as the starting material for preparing MCC was

procured from Fem Cotton Pvt. Ltd., Rajkot, India. Sodium hydroxide and hydrogen

peroxide were purchased from Thermo Fisher Scientific India Pvt. Ltd., Mumbai,

India. Sodium silicate (meta) nonhydrate and hydrochloric acid (11.6N) were supplied

by S. D. Fine-Chem Ltd., Mumbai, India. Nonyl phenol ethylene oxide (wetting

agent) was supplied by Amrutlal Industrial products, Mumbai, India. All chemicals

were used as supplied without any modification or further purification.

8.3.2. Process Flow Diagram and Experimental Work:

Figure 8.2 depicts the typical route followed to synthesize CNW and its application in

chitosan nanocomposites. The bleached cotton fibers were used as the raw material

for production of MCC. MCC was produced by acid hydrolysis using 4N HCl

followed by cooling, neutralization and drying. The dried MCC was used as the

starting material in homogenization process to obtain CNW. Initially, MCC was

dispersed in distilled water (10 g MCC in 1l water) and passed through the

homogenizer at a pressure of 35000 psi. Uniform nanowhiskers were obtained by

repeated passing of solution through the homogenizer (max 15 passes) and after every

3 passes were characterized for settling, degree of polymerization, AFM and SEM.

The prepared CNW were used as a reinforcing agent in chitosan to improve its

performance properties. The chitosan/CNW composites were prepared by solution

casting process. The composites were characterized for mechanical, thermal, barrier,

optical and morphological properties.


Vilas Karande 193

Figure 8.2. Process Flow Diagram for Preparation of CNW and CNW/Chitosan

Nanocomposite

8.4. CHARACTERIZATION

8.4.1. Characterization of cellulose nanowhiskers

8.4.1.1. Settling Study

Settling of the suspension depends upon the particle size. Particle size was expected to

decrease with increase in no of passes of MCC through the homogenizer. For this

study, 50 ml of homogenized suspension after every 3 passes was added to a

Cotton fibers

Kier boiling and

Bleaching

Drying at 40°C

Hydrolysis with

HCl

Neutralization,

Filtration and Drying

Homogenization

Centrifugation and

Lyophilization

Characterization

(Settling study, SEM,

AFM, XRD, DP)

Application in

Chitosan by Solution

Casting

Characterization(Mechanic

al, XRD, Thermal, Optical,

Barrier and Morphological

Properties)


Vilas Karande 194

measuring cylinder, opening of which was closed by using parafilm. The suspensions

were allowed to settle for 24 hrs and then their settling height was recorded.

8.4.1.2. Scanning Electron Microscopy (SEM)

A Phillips® scanning electron microscope operated at 15 kV was used to take

micrographs of MCC samples before and after homogenization. A drop of diluted

(1ml homogenized sample diluted in 100 ml deionized water) sample was taken on a

mica sheet and was dried at room temperature. Dried samples were then vacuum

sputtered with gold-palladium mixture to improve their conductivity.

8.4.1.3. Atomic Force Microscopy (AFM)

A scanning probe microscope (Veeco®) was used to characterize the surface

morphology of CNW. AFM height images of the MCC samples after homogenization

were taken in a tapping mode with silicon cantilever at a frequency of 253Hz.

The force distance analysis was also carried out in order to evaluate the strength of a

single CNW. Imaging was done in a contact mode using silicon nitride tip at different

magnifications. 0.5µm magnification was selected for the measurement of modulus of

elasticity. Before starting the analysis, spring constant was measured by calibrating

the tip. After completion of the calibration force distance analysis was carried out by

indenting the tip on the sample surface with the help of point spectroscopy. The

voltage data obtained by point spectroscopy was analyzed using SPIP (scanning probe

image processor) software which converts the voltage into force-distance relation.

There are different models18-19

such as DMT and Sneddon which describes about the

force distance curve analysis. The DMT model assumes that the probe has some

radius of curvature i.e. spherical in nature at end where as Sneddon model assumes

that the probe has conical shape. In the present study DMT theory has been used for

the analysis. From this we can get Youngs modulus which gives an idea about the

mechanical strength of the sample.

8.4.1.4. X-ray diffraction (XRD) analysis

XRD measurements were performed on a Rigaku® Wide Angle X-ray diffractometer

(WAXD). The XRD of CNW and chitosan-CNW nanocomposites were measured in a


Vilas Karande 195

2θ range from 4° to 40°. Crystallinity of the samples was measured by using the

equation 8.1.

% Crystallinity ={[Ic-Ia]/Ic} x 100………………………………………………..(8.1)

Ic is the peak intensity of crystal plane at 2θ=22 which is also called as principle peak

and Ia is the peak intensity of amorphous phase at 2θ=18.

8.4.1.5. Degree of polymerization (DP)

Degree of polymerization was measured with the help of Ubbelohde viscometer. 50

mg of sample was dissolved in 100 ml of cupra ammonium hydroxide. The mixture

was vigorously shaked and kept in a water bath at 25°C. Viscosity was measured only

when the sample got completely dissolved in the solvent. The viscosity was

calculated from the efflux time of the cellulose solution and pure solvent. Degree of

polymerization was calculated from equation 8.2 and 8.3,

DP = [{(2000*ηspec)/(c*(1+0.29* ηspec)}].................................................................(8.2)

ηspec = [(η/η0)-]…………………………………………………………………….(8.3)

Where, ηspec is specific viscosity, η/η0 is relative viscosity, c is concentration (g/l), t0 is

the efflux time of the solvent and t is the efflux time of the sample.

8.4.2. Characterization of the Chitosan/CNW Nanocomposites

8.4.2.1. Mechanical Properties

The tensile strength and percent elongation at break of the films was determined using

Universal Testing Machine (LR-50K, LLOYD instrument, UK) using 500N load cell

in accordance to ASTM D 882.

8.4.2.2. Differential Scanning Calorimeter (DSC)

DSC was used to measure thermal transitions of the chitosan/CNW nanocomposite

films. The test was performed using Q100 DSC (TA Instruments) equipment, fitted

with an nitrogen-based cooling system. The samples were weighed in aluminium pans

whereas an empty pan was used as the reference pan. All the measurements were

performed in the temperature range from -40 to 200ºC at a heating rate of 10ºC/min.


Vilas Karande 196

8.4.2.3. X-ray Diffraction (XRD) Analysis

X-ray Diffraction (XRD) patterns were obtained using a Rigaku Miniflex X-ray

diffractometer using Cu target and having X-ray wavelength of 1.54 A through 4 to

40° angle.

8.4.2.4. Optical Properties

The light transmittance of the chitosan and chitosan/CNW films having thickness of

about 70 µm was measured using an ultraviolet–visible (UV–Vis) spectroscope (UV-

160A, Shimadzu, Japan) in a wavelength range from 200–800 nm.

8.4.2.5. Water Vapour Transmission Rate (WVTR)

Water Vapour Transmission Rate (WVTR) of the films was determined

gravimetrically in accordance to ASTM E96. The composite films were cut into

circles of 90 mm diameter and then were sealed on the permeation cells, containing

calcium chloride, using paraffin wax. The permeation cells were placed in a

desiccator in which RH was maintained at 71%. The water transferred through the

film gets absorbed by the desiccant which is determined from the weight of the

permeation cell. Each permeation cell was weighed at an interval of 24 hrs. The

WVTR was expressed in g/h.m2 per day.

8.4.2.6. Morphological Properties (SEM)

The morphology of the nanocomposite films was observed under a scanning electron

microscope (SEM). SEM analysis was carried out using Philips® XL30 (Netherland)

Scanning Electron Microscope. Samples were fractured under liquid nitrogen to avoid

any disturbance to the molecular structure. The specimens were then coated with gold

and palladium using sputter coater before imaging.

8.5. RESULTS AND DISCUSSIONS

8.5.1. Characterization of cellulose Nanowhiskers

8.5.1.1. Settling study

In order to understand the effect of number of passes on the size of the obtained

whiskers, settling study was performed. Settling depends on the particle size of the

sample. Bigger size particles settles faster compared to the smaller size particles.


Vilas Karande 197

Figure 8.3. Settling study of homogenized cellulose suspension after 3, 6, 9, 12 and

15 passes

Figure 8.3 shows the settling study of the MCC before and after (3, 6, 9, 12, 15

passes) homogenization. It was observed that as the number of passes increased,

stability of the cellulose suspensions also increased. This can be attributed to the size

reduction taking place which enhanced the surface area of the whiskers. Settling was

observed up to 9 passes but after 12 the suspension was stable which indicates that all

MCC is converted in to CNW.

8.5.1.2. Scanning Electron Microscopy Analysis

Surface morphology of the CNW was done using SEM. Figure 8.4 shows

micrographs of MCC and homogenized MCC after 3 and 15 passes.


Vilas Karande 198

Figure 8.4. Scanning electron micrographs of (a) MCC (2000X), (b) MCC (15000X),

(c) whiskers after 3 passes and (d) CNW after 15 passes

It was observed that the initial diameter of the MCC was about 6 µm however got

reduced to 50 nm after 15 passes in homogenizer. Even after 3 passes MCC was

converted into CNW but the amount produced was very less and also lacked

uniformity. After 15 passes MCC got completely converted in to CNW with uniform

size distribution. This can be due to the continuous exposure of MCC to the severe

forces like shearing, cavitational and impact which acted on it during the

homogenization process.

8.5.1.3. Atomic Force Microscopy Analysis

Surface morphology and strength of CNW was analyzed using atomic force

microscopy. Figure 8.5 depicts the AFM image of the homogenized microcrystalline

cellulose (MCC) after 15 passes.


Vilas Karande 199

Figure 8.5. AFM height image of CNW obtained after 15 passes in the

homogenization process

Figure 8.5 depicts the AFM height image of CNW obtained after 15 passes in the

homogenization process. AFM height image of the CNW, obtained after 15 passes,

was taken in a tapping mode at a frequency of 253Hz using silicon nitride cantilever.

It can be observed from the height image that the diameter of CNW was about 40 nm.

The force distance analysis was also carried out in order to evaluate the strength of a

single CNW. The Youngs modulus was found to be about 452.39 MPa.


Vilas Karande 200


Figure 8.6. X-ray crystallograph of the MCC before and after (3, 6, 9, 12 and 15

passes) homogenization

Figure 8.6 illustrates X-ray crystallographs of the MCC before and after (3, 6, 9, 12

and 15 passes) homogenization. Crystallinity of MCC before homogenization was

found to be 93.23% whereas after 3, 6, 9, 12 and 15 passes in homogenizer was found

to be 90.71, 90.39, 90.37, 90.34 and 89.84% respectively. Homogenization process

did not appreciably influence the crystallinity of the cellulose whiskers. Mechanical

forces exerted in homogenization process were unable to disturb the structural

arrangement of the cellulose even after 15 passes due to its high crystallinity.


Vilas Karande 201

8.5.1.5. Degree of Polymerization

Table 8.1. Degree of polymerization of the homogenized MCC after 3, 6, 9, 12 and

15 passes

No. of Passes Degree of Polymerization

0 1528.17±26.74

3 1262.31±26.97

6 1032.76±27.15

9 1013.56±27.15

12 936.66±27.2

15 917.39±27.24

Degree of polymerization (DP) is the number of repeating units present in a polymer

chain. Mechanical stresses generated due to shearing, cavitational and impact forces

immensely influenced the chain scission process of cellulose and hence it’s DP.

Homogenization resulted in significant reduction of DP of the cellulose. Table 8.1

signifies that the DP decreased by 17.4, 32.4, 33.7, 38.7 and 39.9% for 3, 6, 9, 12 and

15 passes respectively in the homogenization process.

8.5.2. Characterization of the Chitosan/CNW Nanocomposites

8.5.2.1. Mechanical properties

Table 8.2. Mechanical Properties of the Chitosan/CNW Composite

Sample Tensile Strength

(MPa)

Youngs Modulus

(MPa)

Elongation

at Break (%)

Control Chitosan 27.44±2.63 572.83±21.37 43.91±0.97

1%CNW Chitosan 33.73±2.09 1201.6±44.7 36.81±2.18

3%CNW Chitosan 41.12±1.46 1360.22±63.2 13.35±1.92

5%CNW Chitosan 37.87±1.77 1059.62±81.62 16.73±6.82

Table 8.2 depicts the mechanical properties like of the tensile strength, Youngs

modulus and percentage elongation at break of control chitosan and chitosan/CNW


Vilas Karande 202

nanocomposites. Tensile strength and Youngs modulus increased up to 3%

concentration of CNW in chitosan, above which they started decreasing. Tensile

strength & Youngs modulus were found to have increased by 33.3 and 52.3%

respectively, whereas, percentage elongation at break reduced down by 69.6%. As the

concentration of CNW increased above 3% it might have started forming aggregates

resulting in more number of stress concentration points.


Figure 8.7. X-ray crystallographs of the control cellulose (CNW), control chitosan

and chitosan/CNW Nanocomposites

Figure 8.7 indicates the X-ray crystallographs of the control cellulose (CNW), control

chitosan and chitosan/CNW nanocomposite. It was observed that as the concentration

of the CNW increased, the crystallinity of the nanocomposite also increased.

Crystallinity of the control chitosan was 2.05% but increased to 5.9, 6.3 and 7.1%

after incorporation of 1, 3 and 5% of CNW in chitosan respectively. Increase in the

crystallinity of the sample was attributed to incorporation highly crystalline cellulose

nanowhiskers (CNW).


Vilas Karande 203

8.5.2.3. Differential Scanning Calorimetry

Figure 8.8. DSC thermograms of the control Chitosan and Chitosan/CNW

Nanocomposite

Figure 8.8 depicts the DSC thermograms of the control chitosan and chitosan/CNW

nanocomposite. During the analysis the samples were heated from -40 to 210°C at a

heating rate of 10°C/min. Melting points of the control chitosan, 1, 3 and 5% CNW

loaded chitosan were 107.7, 107.1, 111.3 and 106.9°C respectively. Enthalpy of

melting (∆H) for control chitosan, 1, 3 and 5% CNW loaded chitosan were 65.2, 68.6,

70.9 and 62.2 respectively. ∆H might have increased due to the increase in

concentration of highly crystalline CNW in chitosan, which required more energy for

melting.


Vilas Karande 204

8.5.2.4. Optical Properties

Figure 8.9. Effect of CNW concentration on the Transparency of the Chitosan/CNW

Nanocomposite

It was observed from figure 8.9 that control chitosan films were more transparent and

it started reducing as the concentration of CNW increased from 1% to 5%. Addition

of CNW in chitosan increased its crystallinity providing barrier to the transmission of

light, thus increasing haziness of the composite film.

8.5.2.5. Water vapour transmission rate (WVTR)

Figure 8.10. Water vapour transmission rate of chitosan/CNW Nanocomposites


Vilas Karande 205

Figure 8.10 depicts the WVTR of control chitosan and chitosan/CNW

nanocomposites. It was observed that as the concentration of CNW increased, WVTR

decreased up to 3% CNW, above which it started increasing. As compared to control

chitosan WVTR decreased by 18% and 32% for 1 and 3% CNW loaded chitosan

respectively. Addition of CNW in chitosan increased its crystallinity providing barrier

to the transmission of water vapour, thus increasing the barrier property of the

composite film. Increase in WVTR for 5% CNW concentration was due to the

formation of aggregates as evident from SEM micrographs.

8.5.2.6. Scanning electron microscopy analysis

Scanning electron microscopy (SEM) analysis was done in order to understand the

correlation between the dispersion behavior of CNW in to chitosan films and their

performance.

Figure 8.11. SEM micrographs of the chitosan/CNW nanocomposite taken at 15KV

and 10000X. (a) 3% CNW and (b) 5% CNW

Figure 8.11 depicts SEM micrographs of the chitosan/CNW nanocomposite for (a)

3%CNW and (b) 5% CNW loaded chitosan. It was observed that at 3% concentration

of CNW, dispersion was uniform but as the concentration of the CNW increased to

5%, presences of aggregates were observed. Formation of aggregates in 5% CNW

loaded chitosan (as evident from Figure 10 (b)) can be a probable reason for overall

reduction in the properties above 3% CNW concentration.


Vilas Karande 206

8.6. CONCLUSIONS

CNW from MCC were successfully produced using homogenization process. Settling

study showed that the cellulose suspension was stable above 12 passes. SEM

micrographs depicted that some amount of MCC were converted into CNW after 3

passes of homogenization but were not of uniform size, but after 15 passes all

whiskers were very fine and had uniform size disribution. Youngs modulus obtained

from force distance curve using AFM was about 452 MPa. Degree of polymerization

decreased by 39.9% after 15 passes of homogenization. Crystallinity of CNW was

insignificantly affected even after 15 passes in the homogenization process. Tensile

strength and Youngs modulus of the chitosan/CNW nanocomposite improved by 33.3

and 52.3% respectively, whereas % elongation at break decreased by 69.6%.

Crystallinity of the nanocomposite increased from 2% (control chitosan) to about 7%

after incorporation of CNW in chitosan. DSC analysis showed that there was no

significant change in the melting temperature but enthalpy of melting increased with

increase in concentration of CNW. WVTR decreased by 32% for 3% CNW loaded

chitosan. SEM micrographs of the nanocomposites depicted that uniform dispersion

was observed at 3% concentration of CNW, but as the concentration increased CNW

started forming aggregates.

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