Study of Superficial Properties of Some Polymers and Oxides.

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Advanced Powder Technol., Vol. 14, No. 5, pp. 547– 558 (2003)Ó VSP and Society of Powder Technology, Japan 2003.Also available online - www.vsppub.com

Original paper

Study of supercial properties of some polymers

and oxides

TAYSSIR HAMIEH 1;2;¤, JOUMANA TOUFAILY 1;3

and MOHAMED-BAKERFADLALLAH1

1 Laboratoire de Chimie Analytique, Matériaux, Surfaces et Interfaces (CHAMSI),

Département de Chimie, Faculté des Sciences, Section 1, Université Libanaise, Hadeth, Mont-Liban, Beyrouth, Liban

2 Institut de Chimie des Surfaces et Interfaces (ICSI-CNRS), UPR 9096, 15 Rue Jean Starcky, BP 2488, 68057 Mulhouse Cedex, France

3 Laboratoire de Matériaux Minéraux (UPRES-A-7016, CNRS), ENSCMu, 3 Rue Alfred Werner,68093 Mulhouse Cedex, France

Received 16 October 2002; accepted 25 December 2002

Abstract—This paper is devoted to the determination and the quantication of some physico-chemical properties of solid surfaces. The new equations and models which we proposed were usedto determine the acid– base supercial characteristics of solid substrates like oxides and/ or polymeradsorbed on oxides. We also determined the second-order transition temperatures of poly(methylmethacrylate) (at various tacticities) adsorbed on alumina or silica, by studying the evolution of thesurface energy (or the enthalpy variations of adsorption) of polymers as a function of the temperature(or the inverse of the temperature). The technique used was inverse gas chromatography at innitedilution. The acid– base constants were calculated for many solids: Monogal, MgO, ZnO and fourdifferent carbon bers. On the other hand, we proposed a new relationship to describe the acid–baseproperties of solids, and added a new parameter K reecting the amphoteric character of the solidand correcting the classical relationship previously used to obtain the acid– base constants of solidsurfaces.

Keywords: Inverse gas chromatography; retention volume; adsorption; MgO; ZnO; carbon bers;poly(methyl methacrylate); transition temperatures; tacticity; surface energy; acid– base constants.

NOMENCLATURE

a surface area of molecule (Å2)



548 T. Hamieh et al.

C constant depending on the reference state of adsorption (kJ/ mol)D ow rate (cm3 /min)

DN donor number of electronsj correction factor taking into account the compression of a gas

K amphoteric constant (J/mol)KA acidic constant (J/ mol)KD basic constant (J/ mol) N Avogadro’s numberP pressure (mmHg)R ideal gas constant (R D 8:314 J/ K /mol)t 0 zero retention reference time (min)t R retention time of a probe (min)

T absolute temperature (K)T ̄ ¯-transition temperature (±C)T g glass transition temperature (±C)T l¡l liquid– liquid transition temperature (±C)V n net retention volume (cm3)

Greek

° s surface energy of the solid (mJ/ m2)

° dl dispersive component of the surface energy of the probe (liquid) (mJ/m2)° ds dispersive component of the surface energy of the solid (mJ/m2)1G0 free enthalpy of adsorption (kJ/mol)1Gd free enthalpy of adsorption corresponding to dispersive interactions

(kJ/mol)1Gsp specic free enthalpy (kJ/mol)1H sp specic enthalpy (kJ / mol)

1. INTRODUCTION

In this paper we use theoretical results and models previously obtained, andexperimentally conrmed, to determine the surface area, a, of organic moleculesadsorbed on solid substrates. The models used were those elaborated in previousstudies: the geometrical model, cylindrical molecular model, liquid density model,BET method, Kiselev results and other models using the transposition from three-

to two-dimensional spaces. Parameter a was necessary to determine the dispersivecomponent of the surface energy of solids ° ds and the specic interactions andid b i f lid b ( l id ) Th h i d



Supercial properties of some polymers and oxides 549

methacrylate) (PMMA)/alumina, PMMA/silica. In particular, we determined thetransition temperatures of these polymers, even when they were adsorbed on someoxides in small quantities.

In this paper, we determined and quantied the acidic (KA) and basic (KD )

supercial characteristics of some solid substrates. A new model was proposedby adding a new constant (K/ to take into account the amphoteric character of the solid. Results obtained improved the understanding and the knowledge ofsurface properties of some solid substrates, and gave a new contribution to thecharacterization of acid–base properties of solid surfaces.

2. PRINCIPLE AND METHODS OF IGC

IGC at innite dilution [1] was used to characterize the surface characteristics of various oxides and polymers. Probes of known properties are injected in the columncontaining the solid. The retention times of these probes, measured at innitedilution, allow us to determine the interactions between the organic molecules andthe solid in the absence of interactions between the probe molecules themselves.Measurements were carried out with a DELSI GC 121 FB chromatograph equippedwith a ame ionization detector of high sensitivity. The retention data were obtained

with a stainless steel column of length 30 cm and internal diameter 2 mm packedwith 1–2 g of polymer powders. The net retention volume V n was calculated from:

V n D jD.t R ¡ t 0/; (1)

where t R is the retention time of the probe, t 0 is the zero retention reference timemeasured with a non-adsorbing probe such as methane, D is the ow rate and j isa correction factor taking into account the compression of the gas.

The free energy of adsorption 1G0

of n

-alkanes is given by:1G0

D RT ln V n C C D 2 Na¡

° dl ° ds

¢1=2; (2)

where R is the ideal gas constant, T is the absolute temperature, C is a constantdepending on the reference state of adsorption, N is Avogadro’s number and a isthe surface area of one molecule adsorbed on the solid. ° dl and ° ds are the dispersivecomponents, respectively, of the surface energy of the liquid (probe) and the solid.

Equation (2) allowed us to obtain, from the slope of the straight line, the value of

the dispersive component of the surface energy of the solid ° ds [2, 3]. We plottedthe ariation of RT ln V a a f nction of 2Na

q ° d of n alkane for different olid




2.1. Determination of specic interactions

The various solid substrates were analyzed by IGC at innite dilution by usingthe same principle previously detailed. It consists of the determination of the netretention volume V n of the probes (of known properties) injected in the column that

contains the solid to be characterized. The free enthalpy 1G0 of adsorption of theprobes on the solid was given by (2). Papirer et al. [5–7] have obtained a straightline when plotting RT ln V n against ln P 0, where P 0 is the vapor pressure of theprobes. For a homologous series of n-alkanes, they wrote:

RT ln V n D A ln P 0 C B; (3)

where A and B are constants depending of the nature of the solid substrate.If polar probes are injected into the column, specic interactions given by 1Gsp

are established between these probes and the solid surface. The specic interactions1Gsp of the polar molecules with the solid substrates can be used to determine theacidic and basic constants by using the donor ( DN ) and acceptor ( AN ) numbers of polar molecules [8].

Plotting 1Gsp of the polar molecules as a function of the temperature, we candeduce the specic enthalpy (1H sp) and the specic entropy (1S sp) from:

1GspD 1H sp

¡ T 1S sp; (4)

or by dividing by T :

1Gsp=T D 1H sp=T ¡ 1S sp: (5)

We can also plot the variations of (1Gsp =T ) as a function of (1=T ) for all polarmolecules, and then deduce 1H sp and the specic entropy 1S sp of different polarmolecules adsorbed on the solid substrates.

2.2. Determination of acid – base constants of a solid

To determine the specic interactions with the solid substrates, we used polarmolecules that are characterized by their donor ( DN ) and acceptor ( AN ) numbers of electrons [8], and deduced 1H sp of the various polar molecules from (4). We candetermine the acidic constant KA and basic constant KD of the solid substrate usingthe following and classical relationship:

¡1H spD .KA DN C KD AN /; (6)

or by dividing by AN :.¡1H sp=AN/ D KA.DN=AN/ C KD ; (7)




previous equations cannot be used to determine KA and KD . In fact, (7) has to becorrected. A new model will be detailed in Section 4.

3. MATERIALS AND SOLVENTSMany solid substrates and chemical products were used and analyzed by IGC:

² ®- Alumina. The ®-alumina (®-Al2O3/ used in this work was obtained by theBayer method from Sigma-Aldrich (St Quentin Fallavier, France) in powder formwith particle sizes between 0 and 200 ¹m. It contains small quantities of Na2O(about 200 p.p.m. relative to Al2O3) and of other mineral impurities (SiO2:810 p.p.m. and CaO: 450 p.p.m.). It exhibits a specic surface area equal to1.5 m2 /g and a density equal to 3.97 m2 /g.

² Silica. The silica, Aerosil 130 (A130), was obtained from Degussa (Frankfurt,Germany) in powder form. The silica analyzed here had a very high specicsurface area, 105 m2 /g (determined by the BET method). Its density was2.2 m2 /g.

² PMMA. Three types of PMMA were used: atactic PMMA (a), isotactic PMMA(i) and syndiotactic PMMA (s), all synthesized by anionic polymerization andpurchased from Polymer Source (Montreal, Canada).

² ZnO. Zinc oxide powder from Sigma-Aldrich (St Quentin Fallavier, France) withhigh purity (>99%) exhibiting a specic area equal to 4 m2 /g.

² MgO. Magnesium oxide powde from Cerac-Neyco (Paris, France) with a 99.95%purity having a specic surface area equal to 2 m2 /g.

² Monogal. Monogal is an industrial product from Cerac-Neyco (Paris, France)that contains zinc oxide in a majority, 0.3% aluminium and 0.2% free oxygen. Itsspecic surface area is about 0.16 m2 /g.

² Carbon bers. Four high strength polyacrylonitrile (PAN)-based carbon bers

from Socar (Pau, France) corresponding to different stages of manufacturingwere been used in this study: untreated ber (UF), untreated ber having receiveda proprietary surface treatment through electrolytic oxidation, designated theoxidised ber (OF), two oxidized bers having received a supplementary sizingtreatment: the rst one coated by an epoxy-polyester polymer denoted CF1 andthe second coated by an epoxy polymer denoted CF2. The diameter of the beris about 300 ¹m.

² Model organic molecules. Classical organic molecules, characterized by their

donor and acceptor numbers [8], were used in this study. In this study weused the corrected acceptor number AN0 given by Riddle and Fowkes [9] who




Table 1.

Values of donor ( DN ) and acceptor ( AN ) numbers of various solvents

Solvent DN AN

CCl4 0 2.3

CHCl3 0 18.7CH2Cl2 3 13.5THF 50 1.9Toluene 9.75 3.3Ether 48 4.9

Table 1 gives donor and acceptor numbers of solvents used in this study. The newcorrected donor and acceptor numbers are also noted respectively DN and AN .

4. EXPERIMENTAL RESULTS

4.1. Determination of the transition temperatures of polymers by using the various

models of the surface areas of organic molecules

In this section, we use IGC at innite dilution to determine the second-ordertransition temperatures of PMMA adsorbed (or not) on alumina or silica. Three

types of PMMA were used: atactic (a), syndiotactic (s) and isotactatic (i). To obtainqualitative results concerning the various polymers used in this study, we applied thedifferent models giving the surface areas of the various molecules. We determinedthe variation of ° ds of the solid as a function of the temperature and deduced someproperties concerning the transition phenomena in PMMA, especially when thepolymer is adsorbed on solid substrates like alumina or silica.

In this study we analyzed the variations of the dispersive component of the surfaceenergy of ®-alumina and silica as a function of temperature for all the models

of molecular areas, and showed that the variation of ° d

s of ®-alumina or silicais monotonous and decreases with increasing temperature. These curves do notshow any transition phenomena. However, the variations of ° ds of PMMA arenot monotonous as a function of the temperature T . In Fig. 1, we observed threetemperature maxima that indicate the various transition temperatures of the polymer.

On the other hand, we plotted in Figs 2 and 3 the variation of ° ds when PMMAwas adsorbed on silica and alumina, respectively.

Other experiments were performs when PMMA was adsorbed on ®-alumina or

silica by varying the tacticity of the polymer. The study of the variation of thedispersive component of the surface energy of a-, i- and s-PMMAs adsorbed onl i ( ili ) f i f h l h d h i




Figure 1. Variation of the dispersive component of the surface energy of atactic PMMA as a function

of the temperature T using the different models for the molecule surface areas.




temperature of ¯-relaxation, T g .D T 2 / the glass transition temperature and T l¡l

.D T 3/ the liquid– liquid transition temperature or order-disorder transition.Table 2 shows that three transition temperatures can be observed on a- and

s-PMMA, but only two transitions on i-PMMA. This observation is consistent with

the conclusions drawn by other workers [11, 12] who showed that the ¯-relaxationand the glass transition are partially merged for the highly isotactic isomer. It isworth noting that the glass transition temperature increases when the PMMA isadsorbed on a reactive polar surface such as alumina as compared to the T g valuerecorded for the same polymer in its bulk phase. Moreover, this increase of T g

Figure 3. Variation of the dispersive component of the surface energy of atactic PMMA (5 g/ l) a

dsorbed on alumina as a function of the temperature T using the different models for the moleculesurface areas.

Table 2.

Values of transition temperatures of the various PMMA

Samples T ̄ (±C) T g (±C) T l¡l (±C )

a-PMMA 60 110 160a-PMMA/SiO2 60 115 160

a-PMMA/ ®-Al2O3 60 120 170i-PMMA — 60 120i PMMA/SiO 100 140




ranges from 5±C for the s-PMMA to 50±C for the i-PMMA. The same behavior isobserved for the liquid–liquid transition temperature.

The results previously obtained were conrmed by using another method whenwe plotted the evolution of RT ln V n D f .1=T / for the adsorption of n-alkanes on

silica, PMMA, PMMA/silica and PMMA/alumina.We then prove how IGC can be used to determine the surface characteristics of some polymers like PMMA at various tacticities, in particular, their surface energiesand transition temperatures.

4.2. Study of specic interactions and acid–base constants: development of a new

model

We previously used IGC at innite dilution [1, 13] to determine the acid–baseproperties of PMMA, silica, alumina, PMMA/alumina and PMMA/silica systemsby varying the tacticity of the polymer and the recovery fraction of PMMA adsorbedon oxides.

Equation (5) proved sufciently accurate for many solid substrates as polymersand especially for PMMA in the bulk phase or in adsorbed form. However, whenother solid substrates were studied, such as MgO, ZnO, Monogal and four differentcarbon bers, we proved that (5) was not veried in such cases. As examples, we

give the results obtained with previous solid substrates by applying (6). Table 3gives the values of KA, KD and the linear regression coefcient for the varioussolids. Table 3 clearly shows that the linear regression coefcient r is very poor,particularly for MgO and ZnO. Thus, (5) is not correct and we must nd anothermodel that describes more accurately the experimental results obtained by IGC.That is why we develop a new model below.

By studying various metallic oxides (MgO, ZnO, etc.) and carbon bers, wecorrected the relationship (5) and proposed a new relationship by adding a third

parameter K reecting the amphoteric character of the oxide according to:.¡1H sp/ D KA DN C KD AN ¡ K DNAN : (8)

Table 3.

Values of KA and KD of MgO, ZnO and Monogal

Oxide KA KD Regression(J/mol) (J/mol) coefcient r

Monogal 180 ¡772 0.776MgO 52 ¡49 0.113Z O 111 575 0 270




By dividing by AN , we obtain:

¡1H sp

AN D KA

DN

AN C KD ¡ K ¢ DN : (9)

Equation (9) can be symbolically written as:

X1 D KD C KAX2 ¡ K X3; (10)

where

X1 D ¡1H sp

AN ; X2 D

DN

AN ; X3 D DN and K D K.KA; KD /;

X1, X2 and X3 are known for every polar molecule, whereas KD; KA and K are theunknowns of the problem.

By using N probes, (10) will allow us to write the following equations:N X

iD1

.X1/i D KD N C KA

N XiD1

.X2/i ¡ K

N XiD1

.X3/i ; (11)

N XiD1

.X1X2/i D KD

N XiD1

.X2/i C KA

N XiD1

.X2/2i ¡ K

N XiD1

.X2X3/i ; (12)

N XiD1

.X1X3/i D KD

N XiD1

.X3/i C KA

N XiD1

.X2X3/i ¡ K

N XiD1

.X3/2i : (13)

We obtain a linear system given by (11)–(13) at three unknown numbers: KD,KA and K . The matrix representing this linear application is a symmetrical one, wededuce that the problem (11)–(13) has a unique solution for N > 3. We appliedthis method to calculate the acid–base constants of our solid substrates.

4.3. Experimental results

By studying various oxides and carbon bers (treated or untreated), we observedthat when we used (5), the results obtained by this model are very poor (the linearregression coefcient was, in general, less than 0.6). However, when we appliedour new model given by (8), the results were improved and the linear regressioncoefcient was, in general, more than 0.9. Using the previous data obtained by IGCon the given solids, calculations lead to the results given in Table 4.

Table 4 allows us to compare the acid–base properties of Monogal, MgO and

ZnO — MgO was the more basic oxide. This interesting result was conrmedin a previous study [9] by using the potential technique in aqueous and organic




Table 4.

Values of KA, KD and K of oxides and carbon bers using our model

Oxide or ber KA KD K Regression(J/mol) (J/mol) (J/ mol) coefcient r

Monogal 320 810 90 0.987MgO 270 2270 140 0.892ZnO 830 400 70 0.889UF ber 232 585 4 0.957OF ber 438 2516 64 0.853CF1 ber 555 7500 184 0.911CF2 ber 591 4580 124 0.830

However, the bers CF1 and CF2 have the same acidity strength, but they aremore acidic than the untreated and oxidized bers.

4.4. Physico-chemical relevance of the new model

The main opposing fact to a direct transposition of bulk quantities AN and DN

in (8) to interfacial interactions lies in the discontinuity in the symmetry of thelocal interactions involving the molecules, when transferring these molecules fromthe bulk to a rigid surface. Because of this situation, steric constraints due to theinterfacial structuration of the contacting liquid over a few monolayers towardsthe bulk phase will increase. This local structuration may not allow, because of coordination and steric reasons, the donor and acceptor sites away from the interfaceto participate and, most importantly, to interact as in the bulk phase. Basically, theselocal structuration and related constraints may be dependent on both the nature andstrength of the interaction sites on the solid surface [14].

The empiric acid– base strengths of usual liquids refer, in the concept of Gutmann,to their bulk interactions with arbitrary chosen and exclusively donor or acceptorprobes (SbCl5 and Et3PO). One can therefore expect these liquid molecules, whenconned at the interface with a solid, to have a quite different interaction efciencyfor coordination and steric constraint reasons, but also for the more obvious factthat the interacting species differ from SbCl5 and Et3PO. Therefore, regardless of the coordination and steric effects arising from the shift from bulk interactions to

conned ones at the solid interface, AN and DN cannot simply be transposed sincesurface site – liquid interactions are not symmetric like those involving liquid– SbCl5

li id Et PO A di l th d d f th f b lk titi




5. CONCLUSIONS

IGC allowed us to obtain the dispersive component of the surface energy ° ds forvarious theoretical and geometric models of molecules, using the results givenpreviously. By plotting ° ds as a function of the temperature T , we proved the

presence of three particular temperatures that correspond, respectively, to thetransition temperature relative to ¯-relaxation (T ̄ ), the glass transition temperature(T g), and the liquid – liquid transition temperature or order– disorder transition(T l¡l). Results obtained showed an important effect of the tacticity of PMMA andthe recovery fraction of polymer adsorbed on alumina or silica on the transitiontemperatures.

IGC at innite dilution was also used to characterize the supercial and interfacialproperties of solid substrates as oxides, polymers or carbon bers.

The classical equation generally used to obtain the acidic and basic constantsof solid surfaces was proved not sufcienty accurate, and, in most cases, thelinear regression coefcients obtained were very poor. A new model allowing thecalculation of the acid–base constants was proposed by the introduction of a newparameter K that reects the amphoteric features of solid substrates like oxides orcarbon bers.

REFERENCES

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Study of Superficial Properties of Some Polymers and Oxides.

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