Paper

Synthesis and Characterizationof Hybrid NanocompositesComprising Poly(vinyl Alcohol)and Colloidal Silica

MOUSUMI DE SARKAR, PAROMITA DEBRubber Technology Center, Indian Institute of Technology, Kharagpur 721 302, India

Received: July 2, 2007Accepted: February 20, 2009

ABSTRACT: Organic–inorganic hybrid composite films were developed usingpoly(vinyl alcohol) (PVA) and an aqueous dispersion of colloidal silica of initialparticle size of 15–30 nm. The hybrid films, prepared with varied proportion ofcolloidal silica (10–90 phr), were found to be transparent, indicating the nanoleveldispersion of the inorganic component over the polymer. Morphological studiesfurther revealed no significant agglomeration of the silica domains embeddedinto the polymer matrix. A depression in glass transition temperature of PVA isobserved with increasing proportion of silica. The degree of crystallinity alsoshowed a decreasing trend with increasing amount of silica. However, thecomposite films demonstrated superior mechanical performances, higherresistances to dissolution in boiling water, and lower permeability compared withvirgin PVA, owing to the better interaction between PVA and silica as well as thereinforcing action of nanosilica particles in the polymer matrix. C© 2009 WileyPeriodicals, Inc. Adv Polym Techn 27: 152–162, 2008; Published online in WileyInterScience (www.interscience.wiley.com). DOI 10.1002/adv.20129

KEY WORDS: Colloidal silica, Morphology, Nanocomposites,Organic–inorganic hybrid, Structure–property relations

Correspondence to: Mousumi De Sarkar; e-mail: [email protected].

Advances in Polymer Technology, Vol. 27, No. 3, 152–162 (2008)C© 2009 Wiley Periodicals, Inc.

HYBRID NANOCOMPOSITES OF POLY(VINYL ALCOHOL) AND COLLOIDAL SILICA

Introduction

O rganic–inorganic hybrid composites have at-tracted considerable attention recently be-

cause of their distinctive physical and chemicalproperties.1 The hybrid composites inherit charac-teristics such as flexibility, easy processabilty, andlightness from the organic components, whereasproperties such as strength, dimensional sta-bility, chemical inertness, and thermal stabil-ity are bestowed to them from the inorganicscaffolds.1−3 Because of their extraordinary perfor-mances, organic–inorganic hybrid composites findwidespread applications in diverse high-technologyfields such as optics, electronics, sensors, and lasertechnologies.1 Hybrid composites consisting of awide variety of inorganic and organic componentsdeveloped so far are mostly prepared through sol–gel techniques at ambient temperature, in which thestarting materials are in solution phase.4−7 The ma-jor challenge associated with the sol–gel technique isthe sensitivity of the size of the in situ generated in-organic scaffolds on the reaction conditions such astemperature, pH, type of solvent used, and rate of re-moval of solvent.8 Agglomerations of the inorganiccomponents are often evidenced, particularly attheir higher concentrations, thereby preventing ho-mogeneity and nanolevel dispersion and resulting inthe loss of optical transparency and deterioration ofmechanical performance of the composites.9 More-over, the sol–gel process often leads to significantshrinkage, causing internal stress and generation ofmicrovoids.10 The use of preformed nanosized col-loidal silica has proven to be successful while devel-oping hybrid composites.11−16 The approach of us-ing colloidal silica provides the advantage of precisecontrol on the size distribution of the inorganic do-mains and thus results in their better dispersion overthe organic matrix, thereby giving rise to improvedproperties. A thorough literature survey reveals thatthe hybrid composites composed of nanosized col-loidal silica were mostly synthesized through in situpolymerization with acrylated monomers.15,16 Theuse of colloidal nanosilica with preformed polymerfor developing hybrid composites has not yet beenexplored extensively. The reports published so farare limited to the hybrid composites containing acry-late polymers or epoxies and colloidal silica.17,18

In this study, poly(vinyl alcohol) (PVA) is utilizedas the organic component, and the colloidal silica inthe size range of 15–30 nm is used as the inorganic

moiety. The interest in combining PVA and colloidalsilica is essentially boosted by the possibility of hy-drogen bond formation among themselves involv-ing the hydroxyl groups present in both the moi-eties, thereby essentially eliminating the need for acoupling agent. Strong interaction between the or-ganic and inorganic phases is expected to improvehomogeneity and molecular-level mixing. Reportsare available depicting interaction between surfacehydroxyl groups of SiO2 and PVA.19−21 Potential ap-plications of PVA–silica hybrid films may includemembranes for pervaporation, packaging, immobi-lizations of enzymes for sensors, etc. This researchcontribution is primarily directed to capture the in-fluence of colloidal silica nanoparticles on the prop-erties of PVA/silica hybrid composites. The morpho-logical and structural features as well as the ther-mal and mechanical characteristics of the hybridcomposite films have been studied using differentinstrumental techniques. The performances of thecomposite films have been evaluated on the basis oftheir resistance toward dissolution in boiling wateras well as in terms of the extent of permeability ofoxygen gas through them.

Experimental

MATERIALS USED

The organic component, PVA (98 mole% hy-drolyzed grade), used in this study, having degreeof polymerization of 1800, is procured from LobaChemie Pvt. Ltd., (Mumbai, India). The inorganiccomponent, colloidal silica, supplied by SterlingChemicals (Gujarat, India), is an aqueous dispersioncontaining 30 wt% of silica of the average particlesize ranging from 15 to 30 nm, with pH 10. Both PVAand colloidal silica utilized throughout this work areused as received from the suppliers without furtherpurification.

PREPARATION OF PVA–COLLOIDALSILICA HYBRID COMPOSITES

A 5 wt% homogeneous solution of PVA was firstprepared by dissolving PVA powder in boiling waterand then continuously stirring for an hour. Requisiteamount of aqueous dispersion of colloidal silica wasadded to the PVA solution and continuously stirredfor another hour at 27 ± 2◦C to obtain a perfectlyhomogeneous solution. The amount of aqueous

Advances in Polymer Technology DOI 10.1002/adv 153


TABLE IComposition of Hybrid Composites

Amount of Silica (wt%)Sample Aqueous ColloidalDesignation PVA (phr) Silica Dispersions (phr) Calculated Amount (Wc) Actual Amounta (Wa)

P0 100 0 0 0P10 100 10 3 2.9P20 100 20 6 5.8P30 100 30 9 8.7P40 100 40 12 11.4P50 100 50 15 14.1P60 100 60 18 16.5P70 100 70 21 18.3P80 100 80 24 20.6P90 100 90 27 21.8

aAs obtained from the residue at 700◦C in an air atmosphere through TGA.

dispersion of colloidal silica was varied from 0 to90 phr (parts per hundred parts resin) with respectto PVA. The compositions of all the reaction mixturesare presented in Table I. The reaction mixtures con-taining PVA and colloidal silica solution were thencasted onto thoroughly cleaned stainless steel platesand left undisturbed at room temperature for 48 hto remove water. The resultant films were then keptat 50◦C for further drying for 24 hours to eliminatethe residual moisture. Thicknesses of the resultantdried films, measured with a high-precision thick-ness gauge, were about 0.05 to 0.07 mm. Since PVAis substantially hygroscopic in nature, the films werestored inside desiccators filled with anhydrous silicagels before proceeding for characterizations.

CHARACTERIZATION OF THE HYBRIDCOMPOSITES

The transmittance of the hybrid composite filmswas quantified with a UV–visible spectrophotometer(UV 1600 Shimadzu) in the wavelength range of 200to 800 cm−1 at room temperature.

The size, shape, and distribution of silica par-ticles over the polymer matrix were precisely de-termined with a transmission electron microscope(Model C12, Philips). Very dilute solutions of theprecursor mixtures of the hybrid composites werecasted directly on copper grids of 300-mesh size andallowed to dry completely. With the resultant verythin films (<0.5 μm thickness) of the composites,transmission electron microscopic (TEM) studieswere performed. The acceleration voltage used was80 kV, and magnification was 13,500×. A scan-

ning electron microscope (JOEL SEM 5800) with anintegrated energy-dispersive spectroscopic (EDAX)system was used to map the location of siliconatoms in the hybrid films at a magnification of2000×.

The wide-angle x-ray diffraction studies of thehybrid composite films were carried out with a PW1710 x-ray diffractometer. Nickel-filtered Cu Kα ra-diation with λ = 1.54 A was used as the x-ray source.The x-ray generator was run at 40 kV and 20 mA.Measurements were performed at 2θ values between10◦ and 60◦, with a scanning speed of 2◦/min. Thesamples of almost equal thickness and area were ex-posed. In the wide-angle x-ray diffraction plots, thecrystalline and amorphous segments were separatedby curve fitting by nonlinear least-square methodunder the assumption that the intensity peak pro-files could be approximated by the Lorentzian func-tion. The degree of crystallinity (xc) was calculatedusing the following equation22:

xc(%) =(

Ic

Ic + Ia

)× 100 (1)

where Ic is the area under crystalline peak and Ia thearea under amorphous peak.

The interplanar distance (d), the interchain dis-tance (r ), and the size of the crystallites were deter-mined by the following equations22:

d = λ

2 sin θ(2)

r = 1.22 × d (3)

154 Advances in Polymer Technology DOI 10.1002/adv


Dynamic mechanical thermal analysis was carriedout with a DMTA IV analyzer (Rheometric Scien-tific) under tension mode at a frequency of 1 Hzwith a heating rate of 2◦C/min to record the storagemodulus, loss modulus, and loss tangent within thetemperature range of 0◦C to 150◦C. The glass tran-sition temperatures of the composites correspond tothe maxima of the loss tangent (tan δ) peaks or theinflation points in tan δ plot.

Thermogravimetric analysis (TGA) was per-formed with a Pyris Diamond TG/DTA thermo-gravimeter in both nitrogen and air atmospheres.For the TGA studies, approximately 20 mg of thesample was placed in the platinum pan and heatedfrom room temperature to 700◦C at a heating rateof 10◦C/min. The residual weights of the samplesfound at 700◦C in an air atmosphere were consid-ered to be the exact amount of silica incorporated inthe hybrid composites.

The mechanical performance of the compositefilms was studied through the testing of tensile prop-erties as well as hardness. For the tensile testing,dumbbell-shaped specimens were first punched outfrom the prepared composite films. Tensile prop-erties of the specimens were performed with aUTM tensiometer, Honsfield, at room temperature(27 ± 2◦C) at a strain rate of 500 mm/min follow-ing the ASTM D412 method. The values of ultimatetensile strength and elongation at break reportedhere were based on the average of at least threemeasurements from each specimen. Hardness of theprepared hybrid films was determined by static in-dentation tests with a Shore-D durometer. Hardnessvalues of five specimens were tested for each com-position and the average readings were noted.

Boiling water resistance of the composite filmswas measured in terms of the time taken by thefilms to get completely dissolved (without any vis-ible trace) in boiling water. For this experiment,small pieces of approximately identical dimensionsand weights were cut from the hybrid compos-ite films. Each piece was then immersed indi-vidually into test tubes containing boiling water.The dissolution time taken for each specimen wasrecorded. The values of the dissolution time reportedhere are based on the average of five consecutiveexperiments.

Permeability tests were carried out with hybridcomposite films with thickness of about 0.06 mm.Oxygen transmission rates (OTRs) were evaluatedat room temperature (27◦C) according to the ASTMD1434 method.

Results and Discussion

SILICA CONTENT OF THEHYBRID COMPOSITES

The aqueous dispersion of colloidal silica solu-tion used in this study contains 30 wt% of silica.Therefore, the amount of silica practically added forthe preparation of the hybrid samples can easily becalculated from the initial loading of aqueous dis-persion of colloidal silica solution and is displayedin Table I. The actual amount of silica retained in thehybrid composites is determined from the amountof residue obtained through TGA, carried out in anair atmosphere at 700◦C. The actual amount of silicaretained in the composite films is also given in thetable. It is apparent from the table that the actual sil-ica content of the composite films is less than that oftheir theoretically added amount, which is probablyassociated to their loss during the preparation andprocessing of the composite films.

OPTICAL TRANSPARENCY OF THEHYBRID COMPOSITES

The visual inspections of the hybrid films ofthickness of about 0.06 mm reveal that the opti-cal transparency is retained even with the signifi-cantly high loading of silica of ∼22 wt% (P90). Itis well known that any particle with size greaterthan 100 nm diffracts light.3 Since the compositefilms prepared here show reasonably good opticaltransparency, it can be logically concluded that theinorganic domains present in the composites areof less than 100-nm size and therefore the com-posites developed here can be technically termed“nanocomposites.” Since initial size of the colloidalsilica added was in the range of 15–30 nm, it canalso be inferred that agglomeration of the silicawas not pronounced in those composites. Had therebeen significant agglomeration of the colloidal silica,the transparency would have been much hinderedand the films would have appeared to be translu-cent/nearly opaque.

To quantify the optical transparency of the PVA–colloidal silica composite films, UV–visible spec-troscopic studies were performed. Figure 1 showsthe percentage transmittance of the composite filmsprepared with various loading of colloidal silicaagainst wavelength as observed through UV–visiblespectroscopy. From the figure, it is evident that



150 300 450 600 750 9000

25

50

75

100T

ran

smit

tan

ce (

%)

Wavelength (cm )

P0 (0%)P30 (8.7%)P50 (14.1%)P70 (18.3%)P90 (21.8%)

P0

P90

FIGURE 1. UV–visible spectroscopic results ofPVA–colloidal silica hybrid composite films with variousamounts of silica. (The values in the parenthesis signifythe actual silica loading in wt%.)

transmittance drops marginally with increasing col-loidal silica loading. The slight decrease in opticaltransparencies of the composites with high loadingof silica may be attributed to the alteration of refrac-tive indices of the polymer films with embeddedsilica. However, all the composite films show rea-sonably high transparency of more than 70% in thevisible wavelength range irrespective of their col-loidal silica content.

MORPHOLOGY OF THEHYBRID COMPOSITES

Figure 2 shows the representative TEM micro-graph of the PVA–colloidal silica composite film con-

600 nm

FIGURE 2. Transmission electron micrograph ofPVA–colloidal silica hybrid composite (P80) with20.6 wt% of silica.

taining about 21 wt% of silica (P80). The black spotsin the TEM micrograph indicate the position of silicadomains over the continuous polymer matrix. It isapparent from the TEM micrograph that silica do-mains are dispersed well over the PVA matrix. Theaverage size of the silica domains was subsequentlymeasured through standard image analysis softwareand was found to be in the range of 20–25 nm, whichis about the same as that of the colloidal dispersionof nanosilica particles initially used to develop thePVA-based hybrid composite.

The distribution of silica over the polymer ma-trix in the hybrid composites was further elucidatedby the EDAX silica mapping technique. The EDAXmapping of the composites with two different sil-ica contents of ∼14 wt% (P50) and ∼21 wt% (P80)are shown in Figs. 3a and 3b, respectively. The loca-tion of silica is designated here as white dots, overthe polymer matrix (the dark background). Fromthe figures, it is apparent that the silica particles areuniformly distributed over the matrix, without anyevidence of agglomeration even with high loadingof silica. As is obvious, the number of silica parti-cles per unit area is more in P80 than in P50 (Fig. 3bvs. 3a). In the hybrid composites, the homogeneousdistribution of colloidal silica in the PVA matrix is

(a)

(b)

2 μm

2 μm

FIGURE 3. EDAX silicon mappings for hybridcomposites: (a) P50 and (b) P80, comprising 14.1 and20.6 wt% of silica, respectively.



probably due to the plausible intermolecular hydro-gen bonding between the two components involv-ing hydroxyl groups present in both the moieties.The formation of intramolecular hydrogen bondingamong the colloidal silica particles themselves lead-ing to their aggregations is improbable owing to theelectrically charged layers over the silica nanoparti-cles in the colloidal suspension.

X-RAY DIFFRACTION STUDIES OF THEHYBRID COMPOSITES

The x-ray diffraction results of the hybrid com-posite films with different amounts of silica areshown in Fig. 4. Pure PVA (P0) displays a strongdiffraction peak at 2θ = 19.35◦, corresponding to 101reflection.23 From the figure, it is evident that theintensity of this crystalline peak of PVA decreasesin the composites upon the introduction of colloidalsilica. The degree of crystallinity, interplanar spac-ing, and interchain distances of the PVA segmentsin the composite films are calculated from the x-raydiffraction results and are given in Table II. It is ev-ident from the table that the degree of crystallinitymonotonically decreases, whereas interplanar andinterchain distances increase with increase in silicacontent.

It is well known that the crystallinity of PVA arisesfrom the strong hydrogen bonding between the adja-cent polymer chains involving the hydroxyl groups( OH) present in the PVA backbone.3 On the basis

10 20 30 40 50 60

2θ (°)

(a)

(b)

(c)

(d)

(e)

(f)

FIGURE 4. X-ray diffraction results of (a) pure PVA andPVA–colloidal silica hybrid composites: (b) P10, (c) P30,(d) P50, (e) P70, and (f) P90 with 2.9, 8.7, 14.1, 18.3, and21.8 wt% of silica, respectively.

TABLE IIResults from X-Ray Diffraction Studies

Degree of Interplanar InterchainSample Crystallinity (%) Spacing (d, A) spacing (r, A)

P0 55 4.54 5.54P10 53 4.54 5.54P30 50 4.54 5.54P50 44 4.57 5.58P70 39 4.59 5.60P90 35 4.59 5.69

of the x-ray diffraction results obtained here, it canbe concluded that the colloidal silica interacts withthe PVA chains, resulting in an increase in the in-terplanar and interchain spacings, and thereby dis-turbing the crystalline arrangement of PVA in thecomposite films. Moreover, since the colloidal silicagels are amorphous in nature, they do not construc-tively contribute to the crystallinity of the compos-ites. Colloidal silica particles practically act as de-fects in the crystalline arrangement of the PVA. As aresult, the degree of crystallinity decreases linearlywith increase in colloidal silica loading.

DYNAMIC MECHANICAL THERMALANALYSIS OF THE HYBRID COMPOSITES

The influence of silica loading on the storage mod-ulus and loss modulus values of PVA–colloidal silicacomposite films, observed through DMTA at roomtemperature (25◦C), are given in Table III. It can benoted that the storage modulus values increase ap-preciably with increase in colloidal silica loadingcompared with pure PVA. The storage modulus ofvirgin PVA increases from 0.54 × 10−8 MPa to 22.2× 10−8 MPa with 90 phr (∼22 wt%) of silica—anincrease of more than 40 times. As apparent fromthe table, the silica content follows a nonlinear rela-tionship with storage modulus of PVA–silica com-posites. The increment in storage modulus becomessuddenly more pronounced with the addition of50 phr or higher amount of silica. It can be con-cluded that inorganic colloidal silica may be act-ing as physical reinforcement, thereby increasing thestorage modulus of the resultant composites. A rela-tively minor increase in loss modulus values is alsoobserved with increasing amount of silica. The in-fluence of colloidal nanosilica content on the losstangent peak (tan δ) of the composites is shownin Fig. 5. It can be observed that the temperature



TABLE IIIDynamic Mechanical and Mechanical Properties of Hybrid Composites

Dynamic Mechanical Properties Mechanical Properties

Storage Modulus (E′) Loss Modulus (E′′) Glass Transition Tensile Elongation HardnesssSample at 25◦C (×10−8, MPa) at 25◦C (×10−8, MPa) Temperature (Tg, ◦C) Strength (MPa) at Break (%) (Shore D)

P0 0.54 0.10 71 32.9 80 45P10 1.02 0.19 76 34.0 72 51P20 0.93 0.14 71 36.8 70 55P30 1.94 0.31 52 36.3 65 57P40 2.08 0.32 50 35.8 74 60P50 4.55 0.86 38 37.1 80 62P60 6.83 0.86 23 38.4 62 63P70 10.50 1.17 27 45.2 89 65P80 15.31 1.75 25 46.3 84 67P90 22.20 2.44 24 48.1 90 69

corresponding to tan δ peak maxima of the com-posites, representing the glass transition tempera-ture, shifts to lower values upon incorporation ofsilica. As observed in the figure, tan δ peaks werenot well defined for certain composites such as P50and P70. Artifacts, in terms of multiple peaks, areseen with those compositions. For them, the tem-perature corresponding to the most prominent tanδ peak is considered as Tg. The influence of silicacontent on the glass transition temperatures (Tg) ofthe composites films is captured in Table III. Theglass transition temperature of pure PVA has beenfound here to be ∼71◦C. With increasing amount ofcolloidal silica, the glass transition temperature de-creases. The composite films with about 22 wt% of

25 0 25 50 75 100 125 150 175

0.05

0.10

0.15

0.20

0.25

Temperature (oC)

P0 (0%) P50 (14.1%) P70 (18.3%) P90 (21.8%)

FIGURE 5. Variation of tan δ values with silica loading inPVA–colloidal silica hybrid composites. (The values in theparenthesis signify the actual amount of silica in wt%.)

colloidal silica (P90) demonstrate a Tg of 24◦C. It isworth mentioning here that precise and unambigu-ous determination of Tg based on the tan δ peak max-ima was not possible through this study for most ofthe compositions. The error in determining Tg val-ues has been found to be as high as ±6◦C. However,it is certain through the DMTA results that Tg de-creases with increasing silica content in the hybridcomposites. The decrease in Tg upon incorporationof silica in polymer composites was documentedpreviously. Preghenella et al.24 reported reduction inTg with increasing loading of fumed silica in epoxynanocomposites. Sun and coworkers25 also noticedthat a reduction in Tg of the silica-filled compositionsresulted because of the presence of residual moistureand organic materials in the silica surface. In case ofPVA–colloidal silica hybrid composites, the decreasein Tg can also be correlated to the presence of en-trapped moisture in the colloidal silica surface. TheTGA results shown in a later section also provide ev-idence of entrapped water in the hybrid composites.

In addition to entrapped moisture, there could beother factors contributing to the reduction of Tg insilica-filled composites, as well. In pure PVA, intra-and intermolecular hydrogen bonds are present, in-volving the hydroxyl groups in polymer backbone.With increasing amount of colloidal silica, it is likelythat more and more PVA chains become attachedto silica through hydrogen bonds. In other words,the hydrogen bonds involving only PVA chainsare getting replaced by the hydrogen bonds be-tween PVA and colloidal silica, which implies thatthe polymer–polymer interaction initially presentin pure PVA may be changing to polymer–filler(nanosilica) interactions in the composites. It has



been revealed from x-ray diffraction results shownearlier that upon incorporation of colloidal silica,the crystallinity of the PVA chains gets disruptedowing to lesser extent of hydrogen bonds involvingexclusively the PVA chains. The decrease in crys-tallinity in PVA–silica composites upon the additionof silica may lead to relatively easier movement ofpolymer chains, thus resulting in the depression ofglass transition temperature. At this time, the ac-tual cause of depression of glass transition temper-ature upon the addition of silica is debatable. Ex-haustive research is required to reach a coherentconclusion.

MECHANICAL PROPERTIES OF THEHYBRID COMPOSITES

Figure 6 displays the stress–strain curves of thePVA–silica compositions with different levels of sil-ica. It is apparent that the nature of the tensile curvedoes not alter appreciably with changing silica load-ing. Table III elucidates the influence of silica onthe ultimate tensile strength values for the hybridcomposites. An increase in tensile strength has beennoticed with increasing amount of silica. The purePVA film demonstrates the ultimate tensile strengthof 32.9 MPa, whereas the composite film preparedwith 90 phr of colloidal silica dispersion (P90) con-taining about 22 wt% of silica shows tensile strengthof 48.1 MPa. These observations can be explained byconsidering the fact that the colloidal silica may beacting as physical reinforcement in the PVA matrix,causing a decrease in the tensile strength. The elon-

0 20 40 60 80 100

0

10

20

30

40

50

Str

ess

(MP

a)

Strain (%)

P0 (0%) P30 (8.7%) P50 (14.1%) P70 (18.3%) P90 (21.8%)

FIGURE 6. Tensile stress–strain curves ofPVA–colloidal silica hybrid composites. (The values in theparenthesis signify the actual silica loading in wt%.)

gation at break values, on the other hand, does notfollow a definitive trend, as shown in Table III.

The change in Shore-D hardness of the PVA–colloidal silica composite films with change in silicacontent is also displayed in Table III. It is apparentfrom the data that hardness increases significantlywith increase in silica loading. With the addition ofcolloidal silica to the PVA matrix, hydrogen bondsare probably formed between colloidal silica andPVA chains. Moreover, the inorganic colloidal silicamay impart mechanical reinforcement in the PVAmatrix. As a result, hardness of the composite filmsincreases with increase in silica content.

THERMAL PROPERTIES OF THEHYBRID COMPOSITES

Figure 7a shows the TGA curves of pure PVA (P0)and composite films comprising different levels ofcolloidal silica in N2 atmosphere. It is clear that thetemperature at which the initial weight loss occurredremained almost the same with increasing silica con-tent. However, the amount of residue (as shown inTable IV) at 700◦C increases with increasing silicacontent, suggesting successful incorporation of thesilica moiety into the hybrid materials. The blackcolor of the residues found with the hybrid compos-ites after the TGA runs at 700◦C provides evidenceof some amount of organic moiety being trappedin the silica. Virgin PVA showed a residue of about1.8%, which may be due to its incomplete decompo-sition in an inert atmosphere (N2) or due to a traceof impurity present in the TGA pan.

To understand the decomposition behavior of thehybrid composites lucidly, derivative plots of TGAresults were evaluated. Figure 7b shows the plot ofdifferential TGA of pure PVA, as well as the hybridcomposites. Virgin PVA demonstrates two distinctsteps of thermal decomposition ranging between35◦C–200◦C and 210◦C–380◦C. A not-so-significantstep of decomposition is also observed in pure PVAin the range of 400◦C to 500◦C. On the other hand,the hybrid composites demonstrate three distinctsteps of thermal decomposition. In addition to thetwo similar decomposition steps as observed inpure PVA in the ranges of 35◦C to 200◦C and 210◦Cto 380◦C, the hybrid composites featured prominentdecomposition in the range of 400◦C to 500◦C. Thisdecomposition step, distinctly found in hybrid com-posites, becomes more prominent with increasingloading of colloidal silica. The decomposition peakswere deconvoluted through curve fitting and thearea under each peak was subsequently evaluated



0 100 200 300 400 500 600 700 8000

20

40

60

80

100W

eig

ht

loss

(%

)

Temperature (oC)

P0 P10 P30 P50 P70 P90

P0

P90

(a)

0 150 300 450 600 750

Temperature ( oC )

P0

P10

P30

P50

P70

P90

(b)

FIGURE 7. (a) TGA curves of pure PVA (P0) anddifferent PVA–colloidal silica hybrid composites.(b) Derivation plots of TGA results for pure PVA (P0) anddifferent PVA–colloidal silica hybrid composites.

and their relative proportions are given in Table IV.The first step of decomposition, found in pure PVAand the hybrid composites in the range of 35◦C to200◦C, can be related to the loss of entrapped watermolecules. It is a well-known fact that PVA is a hy-groscopic material. The humidity of the atmospherecauses PVA to absorb some moisture on accountof the hydrophilic nature of the hydroxyl groups.Moreover, the test films were prepared throughcasting from the aqueous solutions. Therefore, the

presence of some amount of entrapped water isinevitable. It is interesting to notice from the tablethat in the temperature range of 35◦C to 200◦C, thedecomposition peak area for virgin PVA is almostidentical with those for composites with differentsilica loadings. Therefore, it can be concludedthat, since the amount of PVA is the same in allthe composites, the extent of first decompositiondue to the elimination of water is comparablefor the entire compositional range. At elevatedtemperature (>100◦C), in virgin PVA, the intra- orinterchain hydrogen bonding can be converted tocovalent bonding with the elimination of water. Thecrosslinking involving the hydroxyl groups of PVAand colloidal silica with the elimination of water isalso possible for the hybrid composites.

The second step between 210◦C and 380◦C signi-fies the decomposition of the main chain PVA moi-ety. It can be noticed that the maxima of the decom-position peaks shift to lower temperatures in thehybrid composites with increasing silica content ascompared with the virgin PVA. The third decompo-sition step observed in the composites in the rangeof 400◦C to 500◦C may involve the decompositionof polymer chains directly associated or entrappedin the colloidal silica systems. In virgin PVA, thisstep is not prominent and may involve breaking of

C O covalent bonds in the PVA network. It isevident from Fig. 7b and Table IV that with the in-crease in silica loading, the third step of decomposi-tion becomes prominent and the peak maxima shiftsto higher temperatures.

BOILING WATER RESISTANCE OF THEHYBRID COMPOSITES

Figure 8 captures the influence of colloidalsilica on the resistance to dissolution of the hybridcomposites in boiling water. The tendency of thehybrid composite samples comprising PVA andcolloidal silica to get dissolved in boiling waterdecreases with the increasing amount of silica, asshown in Fig. 8. It has been observed that purePVA films dissolve almost instantly in boilingwater. However, in composites, the resistance tosolvation in boiling water increases with the gradualincrease in silica loading. Composite films withabout 22% silica remain undissolved even after 2 h.It is interesting to notice that with the compositescontaining more than 14 wt% of silica (P50), theresistance to solvation in boiling water increasesquite drastically, which is in line with the trendobserved in the storage modulus values (Table III).



TABLE IVResults from the Derivatives of Thermogravimetric Analysis

First Decomposition Step Second Decomposition Step Third Decomposition Step

Peak Maxima Area Under Peak Area Under Peak Area Under Residuea atSample (◦C) the Peak (a.u.) Maxima (◦C) the Peak (a.u.) Maxima (◦C) the Peak (a.u.) 700 ◦C (wt%)

P0 92 10.0 302 84.9 421 5.1 1.8P10 92 9.6 298 78.7 430 11.7 8.5P30 102 9.9 292 73.9 432 16.2 15.1P50 112 10.1 291 70.8 432 19.1 16.9P70 122 9.6 282 69.2 434 21.2 21.8P90 122 9.5 272 63.4 436 27.1 27.5

aIn nitrogen atmosphere.

0 5 10 15 20 250

40

80

120

160

200

Dis

solu

tio

n t

ime

(min

)

Actual silica content (wt%)

FIGURE 8. Changes in boiling water resistance ofPVA–colloidal silica hybrid composites with silica content.

PERMEABILITY STUDIES WITHPVA–COLLOIDAL SILICAHYBRID COMPOSITES

The change in oxygen transmission rates (OTRs)of the composite films upon the incorporation ofcolloidal silica is shown in Fig. 9. There is an ap-preciable reduction in OTR observed through thecomposite films with increase in silica loading. PurePVA demonstrates OTR of 2.3 × 10−6 cm3/(m2 day),whereas P90 with about 22 wt% of silica shows OTRof 0.32 × 10−6 cm3/(m2 day)—a reduction of aboutseven times. The reduction in OTR may be a con-sequence of more obstacles created by the silica do-mains over the polymer matrix, causing difficultyfor the gas molecules to penetrate and pass throughthe composite films.

0 5 10 15 20 250.0

0.5

1.0

1.5

2.0

2.5

3.0

OT

R

1 ×0

6(

cm3

m2

day

)

Actual silica content (wt%)

FIGURE 9. Changes in oxygen transmission rates(OTRs) through PVA–colloidal silica hybrid compositefilms with silica content.

Conclusion

PVA and colloidal silica hybrid composite filmsdemonstrated high optical transparencies, indicat-ing nanolevel dispersion of inorganic silica moietiesover the PVA matrix. Microstructure analysis re-vealed that the hybrid composites were capable ofsustaining high silica loading without agglomera-tion. TEM studies showed that the dimension of thesilica domains lie within the range of 20–25 nm, irre-spective of the loading of silica. EDAX map of the hy-brid composites showed homogeneous nanometer-level dispersion of silica particles, even with theirhigh loading. The hybrid composites showed sub-stantial mechanical reinforcements, however, at the



expense of crystallinity, which showed a decreas-ing trend with increasing silica loading. The ten-sile strength and hardness values increased withincreasing amounts of colloidal silica. Hybrid com-posites demonstrated better resistance to solvationin boiling water and lesser permeability than thoseof virgin PVA. On the basis of the experimental ob-servations, it can be inferred that the significant in-teraction took place at the interfaces of PVA andinorganic silica moieties, probably involving the hy-drogen bonds.

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