Thermal analysis and FT-IR studies of adsorbed poly(ethylene-stat-vinyl acetate) on silica

Thermal Analysis and FT-IR Studies of Adsorbed Poly(ethylene-stat-vinyl

acetate) on Silica

Madhubhashini Maddumaarachchi, Frank D. Blum

Department of Chemistry, Oklahoma State University, Stillwater, Oklahoma 74078

Correspondence to: F. D. Blum (E-mail: [email protected])

Received 22 January 2014; revised 11 March 2014; accepted 12 March 2014; published online 29 March 2014

DOI: 10.1002/polb.23476

ABSTRACT: Adsorbed poly(ethylene-stat-vinyl acetate) (PEVAc)

on fumed silica was studied using temperature-modulated dif-

ferential scanning calorimetry (TMDSC) and FT-IR spectros-

copy. The properties of the copolymers were compared with

poly(vinyl acetate) (PVAc) and low density polyethylene (LDPE)

as references. TMDSC analysis of the copolymer-silica samples

in the glass transition region was complicated for the copoly-

mers because of the ethylene crystallinity. Nevertheless, exam-

ination of the glass transition region for small adsorbed

amounts of these copolymers indicated the presence of tightly-

and loosely-bound polymer segments, similar to other poly-

mers which have an attraction to silica. Compared with bulk

polymers with the same composition, the tightly-bound poly-

mers showed an increased glass transition temperature (Tg)

and a loosely-bound fraction with a lower Tg than bulk. FT-IR

spectra of the surface copolymers indicated that the fraction of

bound carbonyls (p) increased as the fraction of vinyl acetate

in the copolymers decreased, consistent with the notion that

the carbonyls from vinyl acetate preferentially find their way to

the silica surface. Spectra from samples with different

adsorbed amounts of polymer were used to obtain the amount

of bound polymer (Mb) and the ratio of molar absorption coef-

ficients of bound carbonyls to free carbonyls (X). The copoly-

mers had very large p values (up to 0.8) at small adsorbed

amounts and dependent on the composition of the polymer.

However, an analysis of the bound fractions, based on only the

vinyl acetate groups, superimposed the data, suggesting that

the ethylene units simply dilute the vinyl acetate groups in the

surface polymer. The sample with the smallest fraction of vinyl

acetate did not show this behavior and may be considered to

be “carbonyl poor.” VC 2014 Wiley Periodicals, Inc. J. Polym.

Sci., Part B: Polym. Phys. 2014, 52, 727–736

KEYWORDS: adsorption; bound fraction; copolymer; FTIR; silica;

TMDSC

INTRODUCTION Copolymers are a very important class of mac-romolecules with variable properties due to the presence oftwo different monomers in the polymer. The properties ofcopolymers are determined by the composition of the mono-mers, as well as their distribution in the primary structure.1 Byvarying the fraction of comonomers in the copolymer, proper-ties such as chemical and heat resistance, strength, toughness,and flexibility can be altered.1,2

Polymers adsorbed on surfaces such as silica have receivedmuch attention due to their potential applications in variousfields such as coatings, flame-retardant materials, opticaldevices, and tribology.3–5 The interaction of polymers withsurfaces leads to changes in the physical and mechanicalproperties of those polymers.6 Investigating the chemistryand physics of the polymer-surface interface is important forthe development of polymer films with desirable propertiesfor specific applications. Interfacial polymer behavior has

been studied for a large number of polymers including thosethat bind strongly7–10 or weakly11,12 to the surface.

Studies of the adsorption of random copolymers on surfacessuch as silica are interesting because of the different typesand strengths of the interaction arising from the differentmonomers in the copolymer. Compositional distribution,distribution of chain length, chain blockiness, and self-association are the main factors that determine the adsorp-tion properties of random copolymers.6 There are severalstudies on the adsorption of copolymers on surfaces in theliterature.13–29 Interfacial interactions of copolymers haveclassified copolymers into two broad categories; those inwhich both monomers interact strongly with the surface23

and those in which one monomer interacts with the surfacepreferentially.16

Poly(ethylene-stat-vinyl acetate) (PEVAc) is a statistical ran-dom copolymer used in various applications such as wire

Additional Supporting Information may be found in the online version of this article.

VC 2014 Wiley Periodicals, Inc.

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and cable insulation, packaging film, adhesives, paper coat-ings, and carpet backing.30 The variation of properties, whichoccur by changing the percentages of ethylene and vinyl ace-tate, makes it useful in different fields. The properties of thispolymer can be further altered by adsorbing them on to asurface such as silica allowing expanding the usage of thispolymer in various applications. There have been severalstudies on the adsorption of PEVAc on silica in litera-ture.13,15–17,22,25–27 It has been shown that nonpolar, or atleast pseudo nonpolar polymers will not adsorb or weaklyadsorb on polar surfaces such as silica.11 When weaklyadsorbed, these are easily washed off.6,11 For a copolymercontains comonomer with carbonyl groups would adsorb onsilica surface preferentially compared with the other como-nomer. Therefore, PEVAc can be categorized as a copolymerin which the vinyl acetate groups interact with the silica sur-face preferentially over the ethylene groups.13,16,17,25,27

Although there have been several studies on the adsorbedPEVAc, there is a lack of knowledge on how the vinyl acetategroups arrange themselves to adsorb when there is a differ-ent amount of vinyl acetate groups in the copolymer. Therealso seems to have been no reports to date on how theadsorption affects the glass transition temperature of PEVAccopolymers.

In this article, we report the behavior of adsorbed PEVAccopolymers on silica in the glass transition region, examineand compare the adsorption characteristics of PEVAc copoly-mers containing different percentages of vinyl acetate. Inparticular, this work involves the use of temperature-modulated differential scanning calorimetry (TMDSC) whichgives increased sensitivity and resolution than standard DSCdue to the sinusoidal modulated heating ramp that yields anincreased instantaneous heating rate. The improved sensitiv-ity and resolution allow us to identify overlapping thermaltransitions and transitions associate with polymers in verythin films.31 In addition, using FT-IR, we determine if thecarbonyl groups of the vinyl acetate arrange themselves toadsorb directly to the silica surface.

EXPERIMENTAL

MaterialsPEVAc copolymers with 18, 28, 33, 40, and 70% vinyl ace-tate, by mass, and poly(vinyl acetate) (PVAc) homopolymerwere purchased from Scientific Polymer Products, Inc(Ontario, NY) and used as received. The molecular mass dis-tributions for the copolymers were obtained with gel perme-ation chromatography (GPC) in tetrahydrofuran (THF),except for the PEVAc 18% VAc polymer that was not solublein THF. Since Mark-Houwink coefficients were only knownfor PEVAc (28% VAc),32 the weight average molecular massfor this polymer was determined to be 55 kDa. From theGPC traces, all of the copolymers tested had similar maximain the GPC curves. In addition, fractions at different elutionvolumes for each copolymer were obtained from the GPCruns and tested for monomer composition with FT-IR. Themonomer composition in each of the copolymers tested did

not vary with elution volume, so that it can be concludedthat the composition of each of the copolymers was inde-pendent of molecular mass. Low density polyethylene (LDPE1122) was received from Chevron Phillips Chemical (Bartles-ville, OK) and used as received. Cab-O-Sil M-5P fumed silica(surface area of 200 m2/g) was obtained from Cabot Corp.(Tuscola, IL) and dried at 200 �C at least 24 h before use.The toluene used to prepare polymer-silica samples wasfrom Pharmco-aaper (Brookfield, CT) and used as received.

A series of PEVAc copolymer and PVAc homopolymer solu-tions with various concentrations (2 to 40 mg/mL) was pre-pared in toluene. Cab-O-Sil M-5P fumed silica (300 mg) wasadded to each polymer solution. Test tubes containing thepolymer and silica were shaken in a mechanical shaker for48 h. After shaking, the solutions were centrifuged at 1500rpm until a clear supernatant was observed. The supernatantliquid was removed and the resulting gel-like material waswashed three times, each with 7 mL of toluene.

A small amount of the translucent gel (polymers on silica)after third washing was placed on an NaCl plate and allowedto dry under air followed by placement of the plate undervacuum at 60 �C for about 3 h. FT-IR spectra were taken intransmission mode using a Varian 800 FT-IR Scimitar seriesFT-IR with a resolution of 4 cm21. Peak fitting of the car-bonyl resonance was done using OriginPro 8.6 software (Ori-ginLab, Northampton, MA). After subtracting the baseline, aGaussian line shape was used to fit the free carbonylresonances and Gaussian-Lorentzian mixture was used to fitthe bound carbonyl resonances.

After a third washing, the remainder of gel-like samples weredried by passing air through the sample using a Pasteur pip-ette and followed by drying under vacuum at 60 �C for 48 h.The adsorbed amount of the polymer was determined bythermogravimetric analysis (TGA) using a TA Instrumentsmodel 2950 Thermogravimetric Analyzer (TA Instruments,New Castle, DE) with a heating rate of 20 �C/min.

Temperature-modulated differential scanning calorimetric(TMDSC) analysis of the bulk polymers and polymer-silicacomposites was carried out using a Q2000 DSC (TA Instru-ments, New Castle, DE). The heating and cooling scans wererun from 280 to 120 �C, with a scan rate of 2.5 �C/min, amodulation amplitude of 61.0 �C, and a period of 60 s.

RESULTS

The reversing heat flow curves of bulk LDPE, PEVAc, andPVAc are shown in Figure 1. The melting temperatures ofthe LDPE homopolymer and PEVAc copolymers are clearlyseen in the reversing heat flow curves. In order to observethe glass transition of the bulk polymers more clearly, thederivatives of the reversing heat flow rates of the bulk poly-mers are plotted in Figure 2. The Tgs of the bulk polymerswere estimated from the peak maxima in the derivative ofTMDSC curves. All the copolymers showed Tgs in the rangeof 240 to 210 �C, which was consistent with literature

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values.33–36 The Tgs of the copolymers containing 40%, 33%,and 28% vinyl acetate were fairly similar. There was a slightdecrease in the Tg of copolymer containing 18% vinyl acetateand a significant increase for the copolymer containing 70%vinyl acetate. Copolymers with smaller vinyl acetate contents(lower than 40% VAc) showed broader glass transitions thanthe completely amorphous copolymer (PEVAc 70% VAc) andPVAc homopolymer. The melting temperature range of thecopolymers was also very broad and extended up to around100 �C for copolymers with very small vinyl acetate con-tents. Nevertheless, there was a significant increase in theTm with decreasing vinyl acetate contents, which was similarto the previous reports.36–39 The PEVAc (70% VAc) did notshow any melting transition and is considered to beamorphous.

TMDSC thermograms for bulk and adsorbed PEVAc on silicahaving different vinyl acetate contents at different adsorbedamounts are shown in Figure 3. Two separate transitions inthe region from -40 to 20

�C of the thermograms were found

for the adsorbed PEVAc (70% VAc). The transition in theregion from 240 to 210 �C was similar to that of the bulkpolymer and assigned to “loosely-bound” polymers.10 Sur-prisingly, the Tg for the loosely-bound polymer was less thanthat for the bulk polymer. The transitions in the region from0 to 20 �C was due to “tightly-bound”10 polymers which hadhigher Tgs than those found in the bulk copolymers. It isclear that the relative intensity of the loosely-bound transi-tions increased as the adsorbed amounts increased. TMDSCthermograms of adsorbed PEVAc on silica other than PEVAc(70% VAc) showed complex thermal behavior. In the thermo-grams for small adsorbed amounts that did not show muchcrystallinity; the transition just after 0 �C was likely due tothe glass transition of the tightly-bound polymer.

The TMDSC regions for the melting of bulk and adsorbed sam-ples of PEVAc (18% VAc) copolymer are shown in Figure 4.

This sample has the highest percentage of ethylene among thecopolymers used in this work. The peak temperatures for themelting of more highly ordered crystals and the areas underthose transitions were smaller for adsorbed samples of PEVAc(18% VAc) copolymer than for bulk. Further, the peak tempera-ture for the melting of more highly ordered crystals movedtowards the bulk copolymer with increasing adsorbed amounts.Due to the complex nature of the transition, proper quantifica-tion of the heat of fusion was difficult. Nevertheless, it can stillbe observed that the heat of fusion increased with increasingadsorbed amounts. A similar observation was made for theadsorbed samples of other PEVAc copolymers which showedcrystallinity as well.

The expanded carbonyl stretching regions of the FT-IR spec-tra of adsorbed PEVAc and adsorbed PVAc on Cab-O-Sil areshown in Figure 5. All of these homopolymer and copolymersamples included had relatively small, but similar, adsorbedamounts (ca. 0.85 mg/m2). There were two overlappingresonances for adsorbed PEVAc and PVAc due to the freeand bound carbonyls. The peaks centered at about 1740 and1710 cm21 were for free carbonyls and carbonyls bound tothe silica surface, respectively. This observation was consist-ent with literature results for adsorbed PVAc8,9,17,25 andadsorbed PEVAc.17,25,27 The peak centered around 1630cm21 was attributed to water adsorbed on silica surface.40,41

The relative intensity of the peak around 1630 cm21

increased in intensity with decreases in the adsorbedamount and decreases in vinyl acetate content. This peakcould be eliminated by prolonged heating at about 70 �Cunder vacuum. Comparison of carbonyl resonances forPEVAc copolymers with PVAc homopolymer showed that theintensities of the bound carbonyl resonances relative tothose for the free carbonyls were larger for PEVAc copoly-mers than the homopolymers at similar adsorbed amounts.

FIGURE 1 Reversing heat flow rates for bulk LDPE, PEVAc, and

PVAc showing the glass (Tg) and melting (Tm) transitions. The

scale and position on the vertical axis are shifted to distinguish

the different curves. The curves are in the order as shown in

the legend. [Color figure can be viewed in the online issue,

which is available at wileyonlinelibrary.com.]

FIGURE 2 Derivatives of the reversing heat flow rates of bulk

LDPE, PEVAc, and PVAc highlighting the glass transitions as

peaks. The scale and position on the vertical axis are shifted to

distinguish the different curves. The curves are in the same

order as the legend. [Color figure can be viewed in the online

issue, which is available at wileyonlinelibrary.com.]



http://wileyonlinelibrary.com


The FT-IR spectra for bulk and adsorbed PEVAc with 33%vinyl acetate are shown in Figure 6. The relative intensitiesof the free carbonyls increased with increases in theadsorbed amounts of the polymers. A similar observationwas made for the other PEVAc copolymer composites aswell. Their spectra are shown in the Supporting Information.

Expanded spectra in the -OH stretching region for adsorbedPEVAc having similar adsorbed amounts (0.85 mg/m2) andCab-O-Sil M-5P are shown in Figure 7. The sharp resonanceat around 3750 cm21 was due to the isolated -OH on thesilica surface. The second broad peak in these spectra (cen-tered around 3400 cm21) was from hydrogen-bonded sila-nols and adsorbed water. These spectra clearly showed thepresence of isolated -OH resonances for the adsorbed sam-ples of PEVAc (18% VAc) copolymer and their absence forthe adsorbed samples of other copolymers at these adsorbedamounts.

An example of the fitting of the FT-IR resonances in the car-bonyl region of the adsorbed copolymers is shown in Figure 8.The spectrum was taken after the adsorbed water in the sam-ple was removed. A Gaussian line shape was used to fit thefree carbonyl resonances and Gaussian-Lorentzian mixturewas used to fit bound carbonyls using OriginPro 8.6 software(OriginLab, Northampton, MA). The analysis of the peak areasof the bound and free carbonyl resonances upon heating toeliminate adsorbed water revealed that the presence ofadsorbed water did not affect the relative areas of free andbound carbonyl resonances. Additional results and analysis onthe fitting with water resonances are given in the SupportingInformation.

The peak areas of the bound and free carbonyl resonancesof adsorbed PEVAc and PVAc obtained using OriginPro 8.6software were used to fit the data to the model developedby Kulkeratiyut et al. in 2006.42 Based on this model the

FIGURE 3 TMDSC thermograms (derivative mode) for bulk and adsorbed PEVAc on silica at different adsorbed amounts for the

copolymers of different compositions. The adsorbed amounts are expressed in mg polymer/m2 silica. The dashed vertical lines show

the position of the Tgs for the loosely-bound polymer, which were drawn based on the peak maximum of the sample with the highest

adsorbed amount. The thermograms are in the order as shown in the legend. The scale and positions on the vertical axes are shifted

to distinguish the different curves. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]


POLYMER SCIENCE



relative intensities would be expected to fit the followingequation:

Mt ¼ Mb1ðAf=AbÞXMb (1)

where Mt is the total adsorbed amount of the polymer, Mb isthe mass of the polymer segments which are bound to thesilica surface, Af and Ab are the absorbances correspondingto the free carbonyl resonances and bound carbonylresonances, respectively, and X is the ratio of the molarabsorption coefficients of the bound and free carbonylresonances. The model was developed considering each dif-ferent adsorbed sample contained a constant amount ofbound polymer, Mb and beyond Mb, additional polymer willbe loosely bound to the surface. The graphs of the total

adsorbed amounts (Mt) as a function of ratio of the peakareas of the free carbonyl resonances to bound carbonylresonances (Af/Ab) for PVAc homopolymer and PEVAccopolymers are shown in Figure 9. The linearity of the datasuggests a good fit to the model.

DISCUSSION

PEVAc is a statistical random copolymer containing bothcrystalline and amorphous regions due to ethylene and vinylacetate mers, respectively. Although the presence of bothcrystalline and amorphous regions has made the behavior ofPEVAc copolymers complicated, much useful important infor-mation on this copolymer system can still be obtained. Previ-ous studies,33 investigating the effect of vinyl acetate contenton the crystallinity and second order transitions indicated

FIGURE 4 Melting transitions for bulk and adsorbed samples

of PEVAc (18% VAc) copolymer as a function of adsorbed

amount. Adsorbed amounts are expressed in mg polymer/m2

silica. The heat flow rates are normalized to the amount of

polymer adsorbed on the surface. The thermograms are in the

order as shown in the legend. [Color figure can be viewed in

the online issue, which is available at wileyonlinelibrary.com.]

FIGURE 5 Carbonyl stretching regions for adsorbed PEVAc and

adsorbed PVAc on Cab-O-Sil with about 0.85 mg/m2 adsorbed

amount. The scales and positions on the vertical axes are

shifted to distinguish the different curves. [Color figure can be

viewed in the online issue, which is available at wileyonlineli-

brary.com.]

FIGURE 6 Carbonyl stretching regions of the bulk and

adsorbed PEVAc (33% VAc) with different adsorbed amounts.

The adsorbed amounts shown in the legend are expressed in

mg polymer/m2 silica. The spectra are in the order as shown in

the legend and scaled to the intensity of the bound carbonyl

resonances except for the bulk PEVAc in the region for free car-

bonyl resonance. The intensity of the spectrum of bulk PEVAc

has been reduced for clarity. [Color figure can be viewed in the

online issue, which is available at wileyonlinelibrary.com.]

FIGURE 7 Expanded -OH stretching region for adsorbed PEVAc

(about 0.85 mg/m2 adsorbed amount) and Cab-O-Sil M-5P. The

scales and positions on the vertical axes are adjusted to distin-

guish the different curves. [Color figure can be viewed in the

online issue, which is available at wileyonlinelibrary.com.]








that there was a dilution of the amorphous phase of thecopolymer by the ethylene segments with increased vinylacetate content up to 40%. It appears that, this dilution maykeep the apparent concentration of vinyl acetate segments inthe amorphous phase constant for PEVAc copolymers up to40% vinyl acetate. This effect could account for the consis-tency in Tgs observed for bulk PEVAc copolymers with smallvinyl acetate contents.33,35,36 The PEVAc copolymer isexpected to be totally amorphous when it contains over 45%vinyl acetate,33,43 which explains the significant increase inTg for PEVAc (70% VAc) compared with the other PEVAccopolymers. The differential conformational mobility of theamorphous polymer chains with different proximity to theethylene crystallites resulted in broad glass transitions forthe copolymers containing small amount of vinyl acetate. Onthe other hand, the vinyl acetate fraction in PEVAc wasresponsible for the disruption of the crystallinity of ethyl-ene.30,36–39 It is found that both crystal size and crystalstructure are changed with the amount of vinyl acetate.33,44

Therefore, decreases in the vinyl acetate content in thecopolymer will result in larger crystalline domains withhigher order. More well-defined crystals should result inincreased melting temperatures with a decrease in vinyl ace-tate content, as observed.

On surfaces, polymers usually behave differently than inbulk. Thermal analysis studies on adsorbed polymers withpolar groups (e.g. PMMA, PMA, PVAc, PEO, etc.) on surfacessuch as silica have shown increased glass transition tempera-tures due to interactions between the polymer and surfaceof the substrate.7,10,28,31,45,46 In the current system, theadsorption results from the hydrogen bonding between sur-face silanols of silica and carbonyl groups of vinyl ace-tate.7,26,27 Because some segments are likely stuck to thesurface and some not, it is sometimes useful to interpret thesurface behavior in terms of trains (directly bound to thesurface), loops (away from the surfaces, but ending in trains)and tails.6 When the number of vinyl acetate groups is small,as in a copolymer with a lot of ethylene, one might expectthe vinyl acetate groups to be preferentially located at the

surface, in trains. A relative excess of vinyl acetate segmentsin trains, implies that there will be a relative excess of ethyl-ene segments in loops and tails.

The glass transition regions of the TMDSC thermograms forthe adsorbed copolymers were complex. In the region from240 to 210 �C, the transitions for adsorbed PEVAc copoly-mers on silica, which increased in intensity as more polymerwas added, were due to the glass transition of loosely-boundpolymer in loops and tails. This is most clear for the 70%VAc sample, but evident in the others. It is known that theglass transition temperature of a polymer can be eitherincreased or decreased in nanocomposites.47 Attractive inter-actions of polymers with surfaces result in increased glasstransitions,31 whereas repulsive interactions lead todecreased glass transitions.48 However, decreased Tgs innanocomposites might not only be due to repulsive interac-tions. For example, polymer segments at the air-polymerinterface on PVAc on silica have decreased Tgs, but only for avery small fraction of segments.49 Another possible mecha-nism for lowering of the Tg, which in this case, affects amuch larger fraction of segments for most of the PEVAccopolymers, was the dilution of the loops by ethylene, sincethe loops would be richer in ethylene than the bulk copoly-mers. The ethylene segments will decrease the glass transi-tion temperature of the loosely-bound segments comparedwith those in bulk. This mechanism is plausible because insystems where there was tightly-bound polymer due toattractive polymer-surface interactions, such as PMMA-silica,the loosely-bound polymer had a Tg similar to bulk or aslightly elevated Tg.

10,50

The transition from 5 to 30 �C for adsorbed PEVAc (70%VAc) was for the polymer which was tightly-bound to thesilica surface. The complexity in the thermograms ofadsorbed samples of other PEVAc copolymers was likely dueto the influence of crystallinity. Examination of the glasstransition region for small adsorbed amounts of copolymersindicated that there was an increase in the glass transition

FIGURE 8 Example of the fitting for the carbonyl FT-IR

resonances for adsorbed PEVAc (28% VAc) (0.85 mg/m2) based

on free and bound carbonyls. [Color figure can be viewed in

the online issue, which is available at wileyonlinelibrary.com.]

FIGURE 9 The total adsorbed amounts (Mt) of homopolymer

and copolymers as a function of the ratio of the absorbances

of the free and bound carbonyl resonances (Af/Ab). The lines

represent the linear least square fits of the experimental data

to eq 1. [Color figure can be viewed in the online issue, which

is available at wileyonlinelibrary.com.]


POLYMER SCIENCE




temperature of the adsorbed copolymers compared with thebulk copolymers. Although this transition (0 to 20 �C) wasnot well resolved for adsorbed samples with large adsorbedamounts because of the melting transition, there was a dis-tinguishable peak in the thermograms for tightly-bound seg-ments. However, both the glass transitions for tightly-boundpolymer and loosely-bound polymer were more prominentfor copolymers with higher amounts of vinyl acetate thancopolymers with lower amounts. This can be attributed tothe presence of large amorphous domains in the copolymerswith higher vinyl acetate fractions. Further, the thermogramsindicated that the incorporation of silica had reduced thecrystallinity of the copolymers and also had made the poly-ethylene crystals even more disordered.

The binding of PEVAc copolymers to silica through hydrogenbonding can be clearly observed in the FT-IR spectra ofadsorbed polymers. There were two overlapping peaks inthe carbonyl region for all PEVAc copolymer-silica compo-sites regardless of the copolymer composition and theadsorbed amount. The peak centered at about 1740 cm21

corresponded to free carbonyls and the peak centered atabout 1710 cm21 was for carbonyls bound to the silica sur-face. The position of the peak maxima of the free and boundcarbonyl resonances were roughly the same for allcopolymer-silica composites, in spite of the change in thecopolymer composition and the adsorbed amount. The rela-tive intensities of the free carbonyl resonances (Fig. 6)increased with the amount of the polymer adsorbed for allPEVAc copolymer-silica composites regardless of the copoly-mer compositions. Beyond a certain adsorbed amount, theadditional carbonyls behave as free carbonyls, increasing rel-ative intensity of free carbonyl resonance. This behavior wasconsistent with TMDSC results as well.

According to the model developed by Kulkeratiyut et al.,42

the plot of the total adsorbed amount of a particular poly-mer, as a function of the ratio of the intensities of free andbound carbonyl resonances of that polymer (Fig. 9), can beused to obtain the amount of bound polymer (Mb) and theratio of the molar absorption coefficients of bound and free

carbonyl resonance (X). The amount of bound polymer (Mb)was obtained from the intercept of the line, whereas theratio of the molar absorption coefficients of the bound andfree carbonyl resonances (X) can be obtained by slopedivided by intercept. Table 1 shows the Mb and X values forPVAc homopolymer and PEVAc copolymers. The Mb and Xvalues for PVAc were very close to the values which wereobtained for a similar system, PMMA.51

From the results in Table 1, insight can be gained on thestructure of the adsorbed copolymer. Mb for a particularpolymer represents the mass of the polymer segments of thebound carbonyls.42 The value of Mb is a hypothetical mass ofthe polymer with carbonyls covering the silica surface. For acopolymer in which one monomer interacts with the surfacepreferentially, more total polymer segments will be requiredto cover the same amount of silica surface than required bya homopolymer. To a first approximation, this is simply adilution effect. For PEVAc copolymers, vinyl acetate segmentsadsorb on the silica surface preferentially compared withethylene. Therefore, the larger values of Mb for copolymersare indicative of requiring more PEVAc polymer segments tocover the silica surface than PVAc alone. PEVAc copolymerscontaining small amounts of vinyl acetate were expected toform larger loops at the polymer silica interface than PEVAccopolymers containing larger amount of vinyl acetate.16 Inother words, if the conformation of the copolymer changedto get the same number of silanols bound for each copoly-mer, the mass of the polymer responsible for the bound frac-tion should increase with increasing ethylene content. Thiscan be clearly seen when changing the copolymer from 70%vinyl acetate (Mb 5 0.170 6 0.027) to 40% vinyl acetate(Mb 5 0.346 6 0.021). However, it seemed that the Mb val-ues for copolymers with smaller vinyl acetate percentages(lower than 40%) were fairly similar. The mass of the poly-mer responsible for the bound carbonyls was found to bemore or less the same irrespective to the fraction of thevinyl acetate in the copolymers when there was a smallamount of vinyl acetate (lower than 40%) present in thecopolymer. This may have been a consequence of conforma-tional difficulties of the carbonyl segments making their wayto the surface.

The relative amount of bound polymer can also be estimatedwith respect to only the mass of vinyl acetate present, Mb

0 (5Mb 3 f), which can be obtained by multiplying the Mb valueswith the mass fraction of the vinyl acetate in the copolymer, f.The Mb

0 values for PVAc homopolymer and PEVAc copolymersare also listed in Table 1. The values of Mb

0 for PEVAc copoly-mers and PVAc homopolymer were more or less the samewithin the error, except for PEVAc having 18% VAc. Therefore,the larger values of Mb compared with Mb

0 can be explainedby a dilution effect caused by ethylene for most of the PEVAccopolymers as the Mb includes both mers.

An examination of FT-IR resonances for isolated silanols inPEVAc-silica samples indicated the presence of an IR reso-nance for isolated silanols (about 3750 cm21) for most of

TABLE 1 Effective Amount of Bound Polymer (Mb) and Ratio of

Molar Absorption Coefficients of Bound and Free Carbonyl

Resonances (X) for PVAc Homopolymer and PEVAc

Copolymers

Polymer Mb (mg/m2)a Mb0 (mg/m2)b X

PVAc 0.119 6 0.024 0.119 6 0.024 7.6 6 1.6

PEVAc (70%VAc) 0.170 6 0.027 0.119 6 0.019 6.7 6 1.1

PEVAc (40%VAc) 0.346 6 0.021 0.138 6 0.009 6.6 6 0.4

PEVAc (33%VAc) 0.390 6 0.070 0.129 6 0.023 7.6 6 1.4

PEVAc (28%VAc) 0.376 6 0.048 0.105 6 0.013 9.3 6 1.3

PEVAc (18%VAc) 0.374 6 0.039 0.067 6 0.007 8.7 6 1.0

a The Mb value represents the effective mass and should not be taken

as a layer thickness.b The Mb

0 value is based on the adsorbed vinyl acetate alone (see text).



the adsorbed amounts (up to ca. 2 mg/m2) for PEVAc (18%VAc)-silica. The spectra in the Figure 7 clearly showed thepresence of isolated -OH resonances for the adsorbed sam-ples of around 0.85 mg/m2 of PEVAc (18% VAc) copolymerand the absence of that resonance for the adsorbed samplesof other copolymers having larger amounts of vinyl acetate.This observation shows that a large amount of ethylene lim-its the adsorption of vinyl acetate on the silica surface, eitherdue steric or chain confirmation effects, causing a smallervalue of Mb

0 for PEVAc (18% VAc) than the other copoly-mers. This polymer may be considered carbonyl poor withrespect to adsorption on silica.

Our previous studies on PMMA42 and other methacrylatepolymers with different side chains,51 where smallest sidechain, PMMA, showed a relatively large Mb averaging 0.16mg/m2. Other polymers with longer side chains yielded Mb

values less than 0.1 mg/m2 except for poly(lauryl methacry-late), for which Mb was much larger, likely because it is quiterubbery at room temperature, even when bound.51 It isappropriate to compare these values with the Mb

0 values forthe VAc units alone in the copolymers. These Mb

0 valueswere fairly close to those for PMMA and other methacrylatepolymers, except for the 18% VAc copolymer, which hasquite a lot of ethylene. The side chain in the VAc mers makesthe carbonyls bind similar to that for PMMA.

The bound fraction of carbonyls (p), the fraction of carbonylgroups in contact with silica surface can be obtained fromequation (2) which can be derived from the model describedearlier.42,51

p5Ab=ðAb1AfXÞ (2)

Figure 10 shows the variation of the bound fraction of car-bonyls for adsorbed PVAc homopolymer and PEVAc copoly-mers as a function of total adsorbed amount. As previouslymentioned, the model contains a fixed number of bound seg-ments, Mb and when additional polymer is added it contrib-

utes only to free carbonyls, which accounts for the curvatureof p with Mt, that is, only the fraction of bound segmentsdecreases, not the number. The bound fractions of carbonylsfor all the PEVAc copolymers were larger than the boundfraction of carbonyls for PVAc homopolymer at any givenadsorbed amount. Since the number of carbonyl groups persegment of polymer was smaller for PEVAc copolymers com-pared with the homopolymer, at a given adsorbed amountthere will be larger fraction of bound carbonyls in thecopolymers than the homopolymers. Theoretically, the frac-tion of bound carbonyls for PEVAc copolymers having differ-ent vinyl acetate contents should increase with decreasingvinyl acetate contents in the copolymer. This can be seenwhen changing the copolymer from PEVAc (70% VAc) toPEVAc (40% VAc) (Fig. 10). However, as shown in the Figure10, there is no significant variation in the bound fraction ofcarbonyls for PEVAc copolymers having small vinyl acetateamounts (lower than 40% VAc). This may also be due to therestricted adsorption of carbonyls to the silica surface by thepresence of ethylene segments. We believe that these FT-IRresults also correlate with the thermal analysis results whichsuggested a constant composition in the amorphous phasefor the copolymers having vinyl acetate content up to40%.33

It is also interesting to consider the behavior of the adsorbedcarbonyls in the copolymers to be based not on the totalamount of polymer segments, but on only the vinyl acetatecontent of the adsorbed polymer, Mt

0 5 (Mt 3 f). This hasthe effect of shifting the data in Figure 10 to the left basedon the vinyl acetate content. When this is done, as shown inFigure 11, all the copolymers, except the copolymer with avery small amount of vinyl acetate (PEVAc (18% VAc))behave similarly. The single curve for the copolymers exceptfor PEVAc (18% VAc) was drawn using a constant value ofMb0 considering each different adsorbed sample contains a

constant amount of vinyl acetate groups bound to the

FIGURE 10 Plot of bound fraction of carbonyls (p) of PVAc and

PEVAc adsorbed on silica as a function of total adsorbed

amount (Mt). The smooth curves were obtained from the

model which was based on p 5 Mb/Mt.42,51 [Color figure can

be viewed in the online issue, which is available at wileyonline-

library.com.]

FIGURE 11 Plot of bound fraction of carbonyls (p) of PVAc and

PEVAc adsorbed on silica as a function of adsorbed amount

with respect to only vinyl acetate content (Mt0). The smooth

curves were obtained from the model which was based on p 5

Mb0/Mt

0.42,51 [Color figure can be viewed in the online issue,

which is available at wileyonlinelibrary.com.]


POLYMER SCIENCE





surface. For the PEVAc copolymer containing 18% vinyl ace-tate, the large amount of ethylene makes it difficult forenough vinyl acetate groups to go to the silica surface.Therefore, there were not enough carbonyl groups to coverthe silica surface and this leads to the diverse behavior ofthe PEVAc copolymer containing 18% vinyl acetate than theother copolymers and homopolymer.

The estimation of the bound fraction of carbonyls, p, shown inFigures 10 and 11, for the adsorbed PEVAc copolymers with 40%or less VAc, showed very large fractions of bound carbonyls (ca.0.8) at small adsorbed amounts (ca. 0.5 mg/m2). Typically, fumedsilica contains two to four -OH groups per nm2.52–54 It has beenfound that fumed silica contains approximately equal numberof hydrogen-bonded silanols and nonhydrogen-bonded sila-nols and almost all the nonhydrogen-bonded silanols are sus-ceptible for adsorption or reaction.54 On the other hand, Liuand Maciel have found that about 32% of the silanols infumed silica are resistant to D2O exchange and there areinterparticle silanols which have favorable arrangement tohydrogen bond to each other, but are not accessible by D2Omolecules.53 Therefore, if we assume that there were aroundtwo -OH groups per nm2 of surface area of Cab-O-Sil, onlyaround 1.4 -OH groups per nm2 would be available for theadsorption. For adsorbed PEVAc with 40% vinyl acetate, hav-ing about 0.8 bound fraction for adsorbed amount of aboutMt 5 0.5 mg/m2 gives only about one C5O moleculeadsorbed per nm2 of silica. This value is slightly less thanthe number of -OH groups which are susceptible to adsorp-tion and, therefore, this large value for the fraction of boundcarbonyls for PEVAc was very reasonable.

The ratio obtained for molar absorption coefficients of boundcarbonyls to free carbonyls (X) for PEVAc copolymers andPVAc homopolymer were in the range of 6 to 9 (Table 1).Within the error estimates of the results, there was no signif-icant difference for X between PEVAc copolymers as well asbetween PVAc homopolymer and PEVAc copolymers. Thisindicated that the X values did not vary much with the pres-ence of ethylene segments. According to the literature,PMMA and other similar polymers had yielded relativelysimilar values for X obtained using the same type of experi-ment with the same model.42,51 Although the values obtainedfor X were relatively large, these larger values were reasona-ble and somehow due to the interaction with the surface.For example, if X �1, at smaller adsorbed amounts (ca. 0.5mg/m2) for bulk PVAc, the bound fraction of carbonylswould be about 0.7 as opposed to our estimate of 0.2, whichgives a larger number of carbonyls bonded with silanols (ca.2.3 carbonyls per nm2) than the amount of silanols available(ca.1.4 -OH groups per nm2). It would also result in an unre-alistically flat adsorbed polymer. For the copolymers, smallerX values would lead to virtually all of the carbonyls beingbound at small adsorbed amounts, except for PEVAc (70%VAc). Even though, the conformation of the copolymerchanges to get maximum adsorption on the surface, a boundfraction of 1, where all the carbonyls adsorbed is very unre-alistic due the presence of the other comonomer. Therefore,

the larger X values obtained in this work are more realisticto describe the nature of the interface between polymer andsilica.

CONCLUSIONS

In this study, TMDSC and FT-IR spectroscopy were used toinvestigate adsorption characteristics of PEVAc copolymers.TMDSC thermograms of adsorbed PEVAc copolymers onsilica showed two separate transitions in the glass transitionregion. The transition in the region of 240 to 210 �C wasdue to the PEVAc copolymer segments which were loosely-bound to the silica surface. The glass transition for thePEVAc copolymer segments which were tightly-bound to thesurface was in the region of 0 to 30 �C. This observationwas more pronounced for the PEVAc copolymers with higheramounts of vinyl acetate. However, the crystallinity due tothe ethylene fraction of the copolymers had made the glasstransition region of adsorbed PEVAc copolymers complex.The surface adsorption also reduced the crystallinity of thesurface polymer based on the melting temperatures andenthalpies of the more highly ordered crystals.

FT-IR studies of the adsorbed PEVAc copolymers clearlyrevealed the presence of bound polymer segments. Analysis ofthe peak intensities of free and bound carbonyl resonanceswith the adsorbed amounts of the polymers indicated that theamount of polymer that was bound (Mb) to the silica surfacewas higher for PEVAc copolymers than the PVAc homopoly-mer. This can be explained by the dilution effect caused byethylene. The results showed that although the copolymersbehaved differently from the homopolymers, when they wereconsidered in terms of both ethylene and vinyl acetate presentin the copolymer structure, they behaved similarly when theywere considered in terms of only vinyl acetate content. Themolar absorption coefficients ratios of bound carbonyls tofree carbonyls (X) obtained for PVAc and PEVAc copolymerswere found to be more or less the same indicating the inde-pendence of the ratio of molar absorption coefficients ofbound and free carbonyls from the ethylene content. Further,the study revealed that much larger bound fractions of car-bonyls for PEVAc copolymers than PVAc homopolymer indi-cating conformational changes of the copolymer structure toget maximum adsorption to the silica surface.

ACKNOWLEDGMENTS

The financial support of the National Science Foundation(US) under Grant 1005606 is acknowledged. We also thankPaul DesLauriers of the ChevronPhillips Corp., Bartlesville,OK, for the donation of the homo-PE sample.

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Documents

Thermal analysis and FT-IR studies of adsorbed poly(ethylene-stat-vinyl acetate) on silica