Role of Nanoclay Shape and Surface Characteristics on the Morphology and Thermal Properties of...

Role of Nanoclay Shape and Surface Characteristics on theMorphology and Thermal Properties of Polystyrene NanocompositesSynthesized via Emulsion PolymerizationNagi Greesh,† Suprakas Sinha Ray,*,†,‡ and Jayita Bandyopadhyay†

†DST/CSIR National Centre for Nanostructured Materials, Council for Scientific and Industrial Research, Pretoria 0001, South Africa‡Department of Applied Chemistry, University of Johannesburg, Doornforntein 2028, Johannesburg, South Africa

ABSTRACT: This work evaluates the role of the surface properties and shape of clay type on the morphology, thermal, andthermo-mechanical properties of the polystyrene (PS)/clay nanocomposites prepared via free-radical emulsion polymerization.Attapulgite clay (ATT) with a needle-like morphology and montmorillonite clay (MMT) with a platelet-like structure were usedin this study. The dispersed behavior of the clay particles in the PS matrix was studied using X-ray diffraction and transmissionelectron microscopy. Only intercalated structures were obtained with ATT at all of the clay loadings, whereas semiexfoliatedstructures were observed with MMT in the low clay loading. All of the composites obtained were found to be more thermallystable than neat PS. However, the composites prepared with the MMT exhibited greater thermal stability relative to thoseprepared using the ATT at similar clay loading. Furthermore, the composites prepared with MMT exhibited higher storagemoduli than those prepared using ATT.

■ INTRODUCTION

Over the past two decades, clay-containing polymer nano-composites (PNCs) have emerged as attractive engineeringmaterials due to their unique properties and broad ranges ofapplications.1−4 However, these properties are strongly depend-ent on the way the clay particles are dispersed in the polymermatrix.5−8 PNCs can be prepared using various processes, forexample, template synthesis, melt intercalation, and in situintercalative polymerization.2−9 In situ intercalative polymer-ization has been successfully performed by polymerization insuspension,10 in solution,10 in bulk,10,11 in dispersion,12 inemulsion,13,14 and in miniemulsion.14−16 Emulsion polymer-ization has been successfully performed to prepare PNCs withvarious polymer matrices, including polystyrene (PS),11,17,18

poly(methyl methacrylate),19,20 poly(styrene-co-methyl meth-acrylate),21 poly(n-butyl acrylate-co-methylmethacrylate)22 andpoly(n-butyl acrylate-co-styrene).9 However, clays are naturallyhydrophilic, making them poorly suited for mixing andinteracting with most polymers and monomers.2,5 Moreover,the stacks of clay platelets are held tightly together byelectrostatic forces, which further hinder the penetration ofmonomers or polymers into the clay galleries. Hydrated cationspresent at the surface of clay platelets can be replaced byorganic cations, thus making the clay more organophilic andcompatible with nonpolar species, including synthetic polymermatrices.2,6 For this reason, organic modifiers are often used torender the silicate layers hydrophobic and to facilitate thepenetration of monomers and/or polymers.6,23,24 This can beachieved by the replacement of inorganic cations on the surfaceof the clay galleries by cationic surfactants, a method known asthe ion-exchange reaction.25 Most published research in toPNCs has been focused on platelet-like clay minerals, such asmontmorillonite (MMT).26−31 However, other types of claycan also be used depending on the precise properties requiredfrom the product.

Attapulgite clay (ATT) is a kind of needle-shaped silicate.The particles are 20 nm in diameter, and the length can be fromseveral hundred nanometers to several micrometers.32,33

Recently, there have been several studies describing thepreparation of PNCs using ATT.34−37 For example, Shen etal.37 successfully synthesized polyamide-6 (PA6)/ATT nano-composites via in situ polymerization, in which ATT wasmodified with cetyltrimethylammonium bromide (CTAB) andtoluene-2,4-diisocyanate (TDI). The microscopic resultsshowed that the ATT particles were well-dispersed in thePA6 matrix on a nanometer scale and formed an “exfoliated”morphology. PMMA/ATT nanocomposites were also preparedvia soapless seeded emulsion polymerization. ATT needles withlong length/diameter ratios were encapsulated by the two-polymer shell to form a bead-string shape, and the ATT needleswith short length/diameter ratios were encapsulated by the twopolymer shell formed the core−shell particles.35This investigation focuses on the preparation of PS-clay

nanocomposites (PSNCs) by in situ free-radical polymerizationin emulsion, with a particular emphasis on the role of the shapeand surface properties of the clay on the morphology andthermal properties of the synthesized PSNCs. PS is atechnologically important polymer, and it has has been widelystudied as a matrix in PNCs because it is a model amorphouspolymer. In recent time, a significant number of articlesdescribed the preparation of clay-containing nanocomposites ofPS using different conditions.38

The clays used herein were chosen for their needle-like andplatelet-like structures to investigate how the shape of clays mayplay a major role in the delamination process occurring

Received: July 31, 2013Revised: September 12, 2013Accepted: October 24, 2013Published: October 24, 2013

Article

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© 2013 American Chemical Society 16220 dx.doi.org/10.1021/ie4024929 | Ind. Eng. Chem. Res. 2013, 52, 16220−16231

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throughout in situ free-radical polymerization. Therefore, thisstudy reports for the first time, the impact of the clay surfacecharacteristics and shapes on both the morphology and thermaland thermomechanical properties of PSNCs prepared byemulsion polymerization. This will allow for the preparationof PNC materials with tailored properties for specificapplications. Prior to polymerization the ATT and MMTsurfaces were modified with 2-(dimethylamino)ethyl meth-acrylate (DMAEM). The copolymerization of styrene andDMAEM has been reported.39,40 The copolymerization ofstyrene and the reactive monomer (DMAEM) between the claygalleries and clay surfaces constitutes the driving force for clayexfoliation and gives rise to the formation of a nanocompositematerial comprising a polymer matrix with a dispersed phase ofclay particles.

■ EXPERIMENTAL DETAILS

Materials. Stabilized styrene monomer was obtained fromAldrich. The stabilizers were removed from these monomers bywashing three times with a 3% potassium hydroxide solution,Sodium dodecylbenzenesulfonate (98%) (SDBS) and theinitiator 4,4-azobis (4-cyanovaleric acid) (98%) (AVC) wereobtained from Fluka and were used as received. DMAEM wasobtained from Aldrich and was used as received.ATT was supplied from Ecca Holdings (South Africa) as fine

powders with a CEC of 17 meq/100 g clay. The ATT needlesare of different lengths and are complex magnesium (Mg)aluminum (Al) silicates with an open-channel structure, whichform elongated (needle-shaped) crystals. The chemical formulais Mg1.5Al0.5Si4O10(OH)·4H2O. The actual composition variesbecause of the partial replacement of Mg and Al by iron (Fe)and other elements.32 ATT consists of double silica tetrahedralchains linked together by octahedral oxygen and hydroxylgroups containing Al and Mg ions in a chain-like invertedstructure. The inverted tetrahedral occurs regularly and formschannels throughout the structure.33 Parts a−c of Figure 1respectively show the crystal structure and the transmissionelectron microscopy (TEM) image of ATT.The other type of clay used in this particular study is MMT,

which has a platelet-like structure and is also supplied by EccaHoldings (South Africa). It is a South African MMT and

commercially known as EFD. The EFD used in this study has aCEC of 65 meq/100 g.The chemical formula of MMT is Mx(Al4‑xMgx)Si8O20(OH)4

[M, monovalent cation; x, degree of isomorphous substitution(between 0.5 and 1.3)]. Parts d−f of Figure 1 respectively showthe crystal structure and the TEM image of EFD. Thecharacteristic parameters of ATT and EFD characterized in ourlaboratory are summarized in Table 1. The Fourier transforminfrared spectroscopy (FTIR) results of ATT and EFD aresummarized in Table 2.

Clay Modification. The modification process of clay byDMAEM is illustrated in Scheme 1. A quantity of ATT (2 g)was dispersed in 150 mL of deionized water. The suspensionwas stirred at room temperature until aggregates were nolonger observed. The DMAEM (200 mol % relative to the CECof ATT, 0.106 g) was dissolved in 50 mL of deionized water,and the resulting solution was slowly added to the claysuspension. The pH of the resulting suspension was 8.5. Toconvert DMAEM to a zwitterionic form, the pH was adjustedto the desired value (i.e., <6) by adding an aqueous solution ofHCl (0.1 M), and the reaction mixture was stirred for 24 h atroom temperature. The obtained modified clay (OATT) wasfiltered, thoroughly washed with acidified water, and dried for 3days at 45 °C in a vacuum oven. The modified EFD (OEFD)was obtained in a similar way, except for the quantity ofDMAEM used (0.408 g).

Polymerization. The OATT and OEFD were utilized asseed particles in emulsion polymerization to obtain PS with

Figure 1. Crystal structure, surface morphology (SEM image), and nanoscale structure (TEM image) of ATT (a, b, c) and EFD (d, e, f).

Table 1. Physical Properties of ATT and EFD Clays

physical characteristics ATT EFD

density (g·cc−1) (from pycnometer) 2.26 2.36total pore volume (cc·g−1) (from pycnometer) 0.56 0.58specific surface area (m2·g−1) (from BET) 181.28 34.53pore volumea (cc·g−1) (from BET) 0.22 0.09pore sizeb (nm) (from BET) 8.83 18.73modulus (E′) at 30 °C (GPa)c 10.06 10.08

aBJH adsorption cumulative volume of pores between 1.7 and3000000 nm diameter. bBJH adsorption average pore diameter.cDynamic mechanical analyzer using pocket material sample holder.

Industrial & Engineering Chemistry Research Article

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ATT (PS/ATT PNCs) and PS with EFD (PS/EFD PNCs).The polymerizations were conducted in a 250 mL three-neckedround-bottom flask equipped with a baffle stirrer, a refluxcondenser, a nitrogen inlet, and a rubber septum, at 85 °C.Using a typical procedure, OATT (1−10 wt % relative to 10.0 gmonomer) was suspended in water and the mixture was stirredfor 2 h. Styrene (10.0 g) was added to the dispersion of theorganoclay, and the resulting mixture was stirred for 30 min.SDBS (0.4 g) was added and the stirring was continued for anadditional 1 h. The initiator 4,4-azobis (4-cyanovaleric acid)(0.04 g) was added, and the mixture was stirred at roomtemperature for 1 h, under a nitrogen atmosphere. The flaskcontaining the mixture was placed in an oil bath at 85 °C, andthe mixture was stirred for 24 h. The reactor was then cooledby immersion in a water bath for 10 min. Small quantities of thedispersion were regularly sampled throughout the polymer-ization and dried in a vacuum oven at 100 °C for 48 h to obtaina powder sample for characterization. The preparation of PNCswas performed in a similar manner using different OEFDconcentrations. A typical polymerization formulation isdepicted in Table 3. A controlled experiment without anyclay was also conducted. For various characterizations finelygrounded power samples of neat PS and composites were used.Characterizations. X-ray diffraction (XRD) analyses (2θ =

1−40°) were performed by an X’Pert PRO diffractometer fromPANalytical under the reflection mode. Finely grounded driedpowder samples were used for XRD study. The beam was CuKα (α = 0.154 nm) operated at 45 kV and 40 mA. The scan

speed and exposure time were 0.109419°/s and 6 min 30 s,respectively.FT-IR was used to qualitatively prove that the organic

modifiers had interacted with the clay minerals. FT-IRexperiments of different powder samples were performedusing a Perkin-Elmer Spectrum 100 spectrometer in thediamond crystal 7 mode to collect the spectra of all the clays.The spectral study was extended over the range of 4000−1500cm−1.TEM images were obtained from a JEOL 2100 instrument at

an accelerating voltage of 200 kV. Prior to analysis, the driedpowder PSNC samples were embedded in epoxy resin andcured for 24 h at 60 °C. The embedded samples were cut intosections of a nominal thickness of approximately 100 nm usingan ultramicrotome with a glass knife on a Leica ultramicrotomeat room temperature. The sections were transferred from waterat room temperature onto a 300-mesh copper grid. TEManalysis was also conducted on the latex to observe the particlesmorphology on a nanometer scale. The samples were preparedby diluting the latex in water. The diluted samples weremounted on copper grids for TEM analysis.The thermal stability of the organoclays, neat PS, and PNCs

was determined by a TGA Q500 (TA Instruments)thermogravimetric analyzer using a heating rate of 10 °Cunder nitrogen atmosphere, from room temperature to 900 °C.The analyzed samples were approximately 3.5 mg.The thermomechanical properties of dried powder of neat PS

and various PNC samples were examined with the use of adynamic mechanical analyzer (Perkin-Elmer DMA8000) in asingle cantilever bending mode with a pocket material. Pocketmaterial is a special type of sample holder for the powdersample. To find out the variation of flexural storage modulus(E′), loss modulus (E″), and tan δ (E″/E′) values as a functionof temperature, the temperature sweep experiments werecarried out at a constant strain amplitude of 0.05%, with amultifrequency option (1 Hz). The heating rate was 2 °C/minfor the temperature range room temperature to 180 °C.

Table 2. Determination of Radicals Present in Clays on the Basis of FTIR Results

wave number (cm−1) radical ATT EFD

3630.6 OH group of molecule of water crystallization present present3426.4 OH stretching present present1637.5 δ(H2O) deformation present present1423.6 carbonate impurity present absent915.3 OH deformation attached to Fe3+,, Al3+, Mg2+ present present877.8 carbonate impurity present absent800 vibration of Si−O−Si in the formed free silica absent present

Scheme 1. The Modification of Clays by DMAEM

Table 3. Formulations Used for the Preparation of Clay-Containing Composites of PS

OATT OEFD

styrene(g)

wt % relativeto monomer

actualmass(g)

wt % relativeto monomer

actualmass(g)

SDBS(g)

AVC(g)

10.0 1 0.1 1 0.1 0.4 0.0410.0 3 0.3 3 0.3 0.4 0.0410.0 5 0.5 5 0.5 0.4 0.0410.0 7 0.7 7 0.7 0.4 0.04



■ RESULTS AND DISCUSSION

Modification of Clays. The FT-IR spectra of neat EFD andOEFD were recorded and are shown in Figure 2A. Figure 2Bshows the FT-IR spectra of pure ATT and OATT. The FT-IRspectrum of EFD shows absorption bands at 3432, 3630, and1642 cm−1, corresponding to the OH stretching of the claysilicate layers. The appearance of new bands in the FT-IRspectrum of the OEFD indicates the presence of DMAEM inthe clay. The band at 2975 cm−1 is related to the C−Hstretching of DEMAEM. Another new band in the FT-IRspectrum of OEFD can be seen at 1719 cm−1, corresponding tothe CO of DEMAEM. An absorbance band at 2926 cm−1

ascribed to the C−H of the DEMAEM was also observed in theFT-IR spectrum of the OATT (refer to Figure 2B). Other newbands in the FT-IR spectrum of OATT at 1711 cm−1,correspond to the CO stretching of DEMAEM. The FT-IRresults indicated that the DEMAEM was assembled onto thesurfaces of the ATT.To gain quantitative insight into the extent to which the

grafting process occurred, TGA was used to measure thequantity of DMAEM molecules that were chemically anchoredon the clays after extensive washing. Parts A and B of Figure 3show the TGA curves before and after the grafting of theDMAEM molecules onto EFD and ATT, respectively.

The weight loss of pure EFD between 20 and 100 °Ccorresponds to the removal of the water coordinated with thecation from the interlayer.41 The difference between the weightloss of the unmodified and the modified clay confirmed theincorporation of DMAEM onto the EFD; therefore, theamount of DMAEM loaded into the EFD was determinedfrom the difference between the residual weight difference ofOEFD and pristine clay measured at 700 °C using eq 1.42

Amount of DMAEM grafted into EFD:

· =−

− −

−

WW M

DMAEM (mequiv g ) 10( )

(100 )1 3 200 700

200 700 (1)

where M (g·mol−1) is the molecular weight of the DMAEMmolecules incorporated into clay. Only 42.3 meq/100 g ofDMAEM was incorporated onto EFD.The TGA results of pure ATT and OATT are shown in

Figure 3B, which reveal a three-stage change during heating.The first stage, at a temperature of approximately 100 °Ccorresponds to the loss of moisture, which may exist in theATT powder as the free water.43 The second stage occurs atapproximately 200 °C when the zeolitic tube is destroyed,coinciding with the loss of hygroscopic water and zeoliticwater.43 The third stage, beyond 450 °C, occurs when thehydroxyl group is gradually reduced. The total weight loss ofpure ATT is close to 13.87%, whereas the total weight loss of

Figure 2. FT-IR spectrum of (A) pure EFD and OEFD, (B) pure ATT and OATT.

Figure 3. Thermal gravimetric curves of (A) pure EFD and OEFD and (B) pure ATT and OATT.



OATT is close to 14.27%, indicating that only a few DMAEMmolecules were attached to ATT surface. This observation wasexpected due to low CEC values of ATT or may be due to theneedle-like structure. Therefore, only a small percentage of theorganic molecules can be ion-exchanged with the cationic ions.Although the unusual surface area of ATT compared to EFD33

could lead to the adsorption of larger amount of DMAEMbonded to the surface, the amount of DMAEM loaded intoEFD was found to be greater than that of ATT. ATT couldadsorb DMAEM by the formation of ion−dipole interactions orhydrogen bonds, but these bonds are relatively weak andreversible.44 Desorption of a fraction of the DMAEM moleculesmay have occurred during the washing process, and only asmall amount was ion-exchanged.The changes in the interlayer distance (d-spacing) before and

after the modification process were monitored using XRDmeasurements. The XRD patterns of the clay materials beforeand after grafting are shown in parts a and b of Figure 4.Figure 4A shows that the XRD peak of the modified clay

shifted to low 2θ values, indicating that there are slightincreases in the d-spacing of the modified clay relative to that ofthe pure EFD. Such an increase in the d-spacing of the OEFDrelative to the pure EFD confirmed that the insertion ofDMAEM occurs between clay platelets and not only on theexternal surface of the EFD. Because of the short chains lengthof DMAEM, d-spacing underwent only slight increases.A typical diffraction peak of pure ATT at 8.42°, which

corresponded to a basal spacing of 10.49 Å, was evidentaccording to Figure 4B. After organification with DMAEM, thispeak was at 8.38°, corresponding to a basal spacing of 10.55 Åas shown in Figure 4B. Therefore, on the basis of XRD andTGA results, we can conclude that a small amount of DMAEMis adsorbed on the surface of ATT during the cation-exchangeprocess without destroying the crystalline structure, and thedistance between the clay needles remains the same even withan excess amount of modifiers. Lei et al.45 also modified ATTwith hexadecyltrimethyl ammonium bromide and found thatthere was no significant change in the distance between needlesobserved by XRD.Polymerization and Latex Characterization. The

morphologies of ATT based hybrid latexes are shown inFigure 5. The particle size distribution was fairly narrow, whichis an indication that little to no secondary particle nucleationoccurred during the polymerization process. This was expectedin our system because the clay layers could form a physical

barrier and make it difficult for species, such as growingradicals, to be transported toward the particle surface prior tothe exit into the water phase. However, as clay loadingincreased, a few small particles began to appear. Theappearance of these secondary particles was attributed toinitiator derived chains emanating from a small amount of AVCthat dissolved in the aqueous phase. A TEM image of the PSlatexes prepared in the presence of 1 wt % ATT is shown inFigure 5B. The clay particles were found to be separated fromthe polymer latexes and dispersed in the water phase; this is nota surprising result due to the high hydrophilicity of ATTneedles caused by the loss of DMAEM during the modificationprocess as confirmed by TGA. The morphologies of theparticles prepared in the present study were compared withthose obtained by other researchers. Liu et al.34 preparedPMMA/ATT composites via soapless seeded emulsionpolymerization; ATT was modified with CTAB. They foundthat ATT needles with longer length/diameter ratios were

Figure 4. XRD patterns of the following: (A) pure EFD and OEFD, and (B) pure ATT, and OATT.

Figure 5. TEM images of (A, B) PS-nanocomposites with 1 wt % ATTat low and high magnification, respectively, and (C, D) PS-nanocomposites with 5 wt % ATT at low and high magnification,respectively.



encapsulated by the two polymer shell to form a beaded stringshape, whereas the ATT needles with shorter length/diameterratios were well-encapsulated by the two-polymer shell andformed the core−shell particles. However, ATT has a largerlength/diameter ratio relative to PS particles, which have anaverage particle size of approximately 90−85 nm, as seen inTEM images. Therefore, the monomer-encapsulated ATT latexclaimed by Liu et al.34 was not established in our system.Furthermore, the general purpose of the pretreatment of theATT needles with surfactants was to introduce the organicchains onto the ATT needle surfaces so they had more affinityfor the monomers, thereby facilitating encapsulation. However,according to the TGA result in Figure 3B, the amount ofDMAEM adsorbed by ATT was insufficient to convert thesurface to hydrophobic, and the copolymerization of styreneand DMAEM took place away from the ATT surface.Therefore, the dispersion of ATT needles throughout thewater phase is more preferable.PS latex prepared by emulsion polymerization using different

concentrations of platelet-like structure clay (EFD) wascharacterized using TEM as can be seen in Figure 6. The

PS/EFD PNCs exhibited predominately armored particles,where EFD platelet stakes were located at the surface of the PSparticles as indicated by the arrowhead in Figure 6B,D. A partialsurface modification of the EFD surface yielded organoclayswith a moderate degree of hydrophobicity. The resultingamphiphilic character of the modified EFD constitutes a drivingforce to its adsorption onto the polymer particles as they form.In OEFD, the clay layers remained slightly hydrophilic, whereasthe DMAEM brushes at the edges were hydrophobic andanchored the PS particles, leaving the negatively charged ionsfacing the polar dispersion medium.The thermodynamically driven adsorption of a partially

modified clay at the interface of multiphasic systems is

controlled by its relative affinity for the immiscible phasepresent, as described by Pickering.46 Furthermore, the presenceof a reactive monomer (i.e., DMAEM) on the surface of theEFD facilitated the formation of this type of particlemorphology by copolymerization with the styrene monomer.The increase in OEFD to the polymerization reaction did notshow any effect on the particles sizes and morphology as can beseen in Figure 6C,D. Because of a faceting effect caused by therigidity of the EFD platelets, the colloid particles are notperfectly spherical. On the TEM images, most of the particlesare not isolated but rather tend to assemble together. Thiscould be because DMAEM molecules are distributed on bothsides of the clay surface, and two colloid particles share thesame clay layers.Figure 7 shows the statistical average particle size distribution

of the different latexes as determined by dynamic light scattring(DLS). For the PS/ATT PNCs, the average particle size of thefinal dispersion, as measured by DLS, decreased with increasedEFD loadings; however, the average particle size was notaffected by increased EFD loadings. A possible reason is thatOFED has an amphiphilic character, with unmodified hydro-philic areas and modified hydrophobic patches, hence favoringthe ability of partially modified EFD to locate at the surface ofthe PS particles, and restrict the diffusion of monomers and theinitiator to the growing particles, resulting in smaller particles.The particle size determined by DLS analysis was furtherconfirmed by TEM (refer to Figures 5 and 6). The particle sizesof the PNC latexes were found to be slightly smaller whenmeasured by TEM than when determined by DLS. This isattributed to the different modes of analysis, although the sametrends were obtained with both methods.

Nanocomposites Characterization. The XRD patterns ofthe PS/ATT PNCs synthesized using different OATT loadings(1−7 wt %) are shown in Figure 8. All of the synthesized PS/ATT PNCs had diffraction peaks at the same position asOATT and pure ATT, indicating that the interaction of PSchains and OATT during polymerization occurred only on thesurface of the ATT nanoparticles without intercalating into therod-shaped silicate galleries, and the distance between ATTneedles remained the same after the polymerization.Dry films of the two different PNC materials were prepared

by removing the dispersion phase, drying the obtained powderat 120 °C for 24 h and microtoming a 100 nm thin film. Parts Aand B of Figure 9 show the TEM images of the PS/ATT PNCscontaining 5 wt % OATT at low and high magnifications,respectively. From Figure 9A,B it noticed that needle-likestructured ATT with an average single fiber diameter ofapproximately 20 nm and length between 0.32 and 0.64 μm wasnot well dispersed as single needles in the PS matrix. However,they appeared as stacks of needles in the polymer matrix.A suitable range of CEC for clays that can be intercalated is

60−120 meq/100 g.41 However, the CEC of ATT is only 17−20 meq/100 g29 which may be the reason for the unsuccessfuldispersion of the ATT needles in the PS matrix. The TGAresults illustrated in Figure 3B confirmed that there were only asmall number of DMAEM molecules attached to the ATTsurface, which were not enough to expand the ATT needlesand remain hydrophilic. Thus, the hydrophilicity of the ATTneedles limited the migration and the extent to which thehydrophobic monomers (i.e., styrene) could polymerizebetween the ATT needles. Even though a limited quantity ofthe monomers entered between the ATT needles, they couldonly partially copolymerize with the DMAEM molecules,

Figure 6. TEM images of (A, B) PS-nanocomposites with 1 wt % EFDat low and high magnification, respectively, and (C, D) PS-nanocomposites with 5 wt % EFD at low and high magnification,respectively.



limiting the interfacial adhesion between the polymer matrixand clay leading to the formation of this morphology.The structures of the PS/EFD PNCs containing different

OEFD loadings were characterized using XRD, and the resultsare shown in Figure 10. The final morphology of thenanocomposites prepared using EFD is influenced by theOEFD content.At 1 and 3 wt % EFD loadings, no peak in the XRD patterns

was distinguishable, due either to the absence of periodicallystacked MMT layers (i.e., an exfoliated structure was obtained)or to the detection limit of the measuring system. Thedelamination of EFD platelets within the polymer matrix at 1

wt % EFD loading was confirmed by TEM (refer to Figure 11A,where the silicate layers appear as dark lines that are distributedas groups in the PS matrix. At high magnification TEM, seen inFigure 11B, the silicate layers are partially separated from eachother with some loss of order.At 7 wt % OEFD loadings, an XRD diffraction peak appeared

at 2θ = 5.54° as seen in Figure 10, showing intercalatedstructures. The XRD and TEM data show that the nano-composite structure is strongly dependent on the clay/polymerratio. These results are consistent with those of previousresearchers who obtained exfoliated structures in emulsionpolymerization at low clay loading and intercalated structures athigh clay loading.17,22,30,47

At relatively high clay concentrations, such as 7 wt %, thedistance between the clay layers is comparable to, or even

Figure 7. DLS size distribution graph of PS/ATT PNCs with 1 and 5 wt % and PS/EFD PNCs with 1 and 5 wt % loadings.

Figure 8. XRD patterns of PS/ATT PNCs prepared at differentOATT loadings.

Figure 9. TEM images of PS/ATT PNCs microtomed samplesprepared at 5% ATT: low magnification (A), high magnification (B).



smaller than, the size of a single layer of clay. This increased thefrictional interactions between the OEFD layers and restrictedthe movements of the EFD platelets, inhibiting their separationand giving a mixed morphology with a number of intercalatedstructures. The methacrylic polymerizable group of DMAEMmakes it a good candidate to readily copolymerize with thestyrene monomer. As a consequence, the preparation of PS/EFD PNCs led to a semiexfoliated nanocomposites structuredue to the extensive motion of the clay platelets throughout thefree-radical polymerization process.The molecular weights and polydispersities of the nano-

composites are shown in Table 4. The samples were preparedin THF and filtered several times through a 0.45 μm pore sizenylon filter prior analysis to remove any trace of clay. Themolecular weights shown in Table 4 are actually that ofunbound polymer. The acrylate group on the DMAEM actuallycopolymerized with the polystyrene in these experiments,

because washing the polymer with THF and filtration wouldnot remove just the clay, it would also remove any polymer thatwas chemically bound to the DMAEM groups. Therefore, thePS molecular weight reported here is actually the molecularweight of unbound polymer that grew between clay particlesand did not react with the DMAEM or clay during theexperiment.In the general case, the synthesized polymers had a high

molecular weight with a relatively broad polydispersity. This iscommonly observed for polymers prepared by free-radicalpolymerization in emulsion, as termination by recombination isunlikely because theoretically only one radical is present in theaggregates at a time.48 This has been verified in an earlier workperformed by Wang et al.10 who synthesized polystyrene−claynanocomposites via different polymerization methods (bulk,suspension, solution, and emulsion polymerization) andshowed that the highest molecular weight was observed foremulsion polymerization, followed by suspension, bulk, andsolution polymerization.In the present case, the molecular weight underwent a slight

decrease due to the presence of inorganic clay particles in thepolymerization system, which impeded the diffusion of theinitiator and monomers, as reported elsewhere.11 Clay loadinghad a significant effect on the molecular weight of the polymermatrix in the nanocomposites. As a general trend, the molecularweight of PNCs decreased slightly as the clay concentrationincreased, although this effect only became obvious for 7 wt %clay loading. Choi et al.48 observed a similar effect of clay onthe molecular weights of the polymer matrix in PNCs. Theseauthors found that the molecular weights of the nanocompositematerials were 60−90% lower in the presence of clay than in itsabsence. They interpreted their results as follows: clay acts asan ‘‘additional micelle’’ allowing for polymerization to occur inmore micelles and consequently decreases the molecularweight.10,49 Furthermore, the presence of clay particles in apolymerization system can hinder the growth of polymerchains, which also leads to a decrease in the molecular weight asthe clay concentration increases.50 The obtained molecularweights of the PNCs were affected by clay types. The PS in PS/ATT PNCs had a lower molecular weight than PS in PS/EFDPNCs. The ATT has a larger surface area than EFD (refer toTable 1), thus most of the initiator radicals adsorb onto theATT surface and increases the chance of termination anddecreases the initiator efficiency.

Thermal Stability. The thermal stability of the PS/EFDPNCs, PS/ATT PNCs, and the pure PS were studied by TGA.Parts A and B of Figure 12 show the TGA curves for the PS/EFD PNCs and PS/ATT PNCs, respectively. Table 5

Figure 10. XRD patterns of PS/EFD PNCs prepared at differentOEFD loadings.

Figure 11. TEM images of PS/EFD PNCs prepared with differentOEFD loadings: (A, B) 1 wt % at low and high magnifications, (C, D)5% at low and high magnifications.

Table 4. Molecular Weight and Polydispersities of PNCsMatrices

sampleclay content(wt %) Mn × 103 Mw × 103 (g·mol−1) PDI = Mw/Mn

PS/EFDPNCs

1 292 758 2.583 294 711 2.415 239 549 2.297 122 303 2.04

PS/ATTPNCs

1 304 657 2.163 273 757 2.765 148 270 1.827 101 291 2.88

Neat PS 0 506 1056 2.08



summarizes the temperatures at various mass loss (in wt %)values during the thermal decomposition.

TGA curves show a single mass loss step. Furthermore, allthe PNCs were found to be thermally more stable than the neatPS prepared under similar conditions. The improvement in thethermal stability of the polymers filled with nanoclay isattributed to the formation of a clay char that acts as a masstransport barrier and thermal insulator between the bulkpolymer and the surface where the combustion occurs.2,51

Hindered diffusion of the volatile decomposition products

within the nanocomposite materials has also been attributed tothe presence of delaminated clay sheets.2

In addition, restricted thermal motion of polymer chainslocalized inside the clay galleries also promotes the enhance-ment of the thermal stability of the PNCs.52 From Figure 12Aand Table 5, the extent to which the thermal stability wasimproved did not correlate linearly with the EFD loading. Thisis because the extraordinary thermal stability in PNCs is notonly due to clay loading but is also dependent on other factors,such as the degree of interaction between the polymer matrix,the degree of dispersion of the clay nanofiller and the overallmorphology.2,51 For example, PS/EFD PNCs with 3 wt % EFDshowed a large increase in the degradation onset temperature(i.e., ±45 °C) relative to the neat PS. Additionally, thetemperature at which 50% degradation occurred increased asmuch as 50 °C. However, the PS/EFD PNCs with 7 wt % EFDloading also showed an improvement in thermal stability but toa lesser extent. The extent of thermal stability was directlycorrelated with the structure of the nanocomposites obtained.The homogeneous dispersion of clay in the polymer matrix inthe partially exfoliated structures provided a greater interfacialarea between the polymer chains and clay platelets, allowing forgreater thermal stability than intercalated nanocomposites.Figure 12B and Table 5 show that the PS/ATT PNCs alsoexhibited improvement in thermal stability compared to theneat PS, although the ATT needles were not well-dispersedthroughout PS matrix. The presence of several stacked ATTneedles might provide a more efficient thermal barrier between

Figure 12. Thermogravimetric thermograms of (A) PS/EFD PNCs and (B) PS/ATT PNCs, prepared at different clay loadings.

Table 5. TGA Data of the PS/EFD PNCs and PS/ATTPNCs at Various Clay Loadings

sampleclay content(wt %) T10% (°C)a T50% (°C)b

theoretical claypercentage

1 326 377 1.43 356 410 5.5

PS/EFDPNCs

5 326 383 6.3

7 349 398 7.81 322 372 1.13 325 365 2.8

PS/ATTPNCs

5 327 378 4.4

7 330 378 6.4neat PS 0 312 363 0aTemperature for 10% weight loss. bTemperature for 50% weight loss.

Figure 13. Storage modulus as a function of the temperature of (A) pure PS and PS/EFD PNCs with different EFD loadings, and (B) pure PS andPS/ATT PNCs with different ATT loadings.



the decomposing zone and the polymer matrix. Table 5 showsthat only a limited improvement of the thermal degradationonset temperature was observed when needle-like clay was usedcompared to PNCs prepared with platelet-like clay. In addition,the temperature at which 50% degradation occurred increasedby only 10−15 °C for the PS/ATT PNCs but increased by 40−50 °C for the PS/EFD PNCs relative to the neat polymer. Thedifferent temperature increases indicate that the thermalstability was strongly affected by the clay types; the platelet-like clay indeed had a higher contact surface relative to theneedle-like clay. More PS chains were localized between EFDplatelets and restricted the thermal motion of the PS chains andimproved the decomposition temperature. Furthermore, thehomogeneous dispersion of EFD platelets in the polymermatrix in the partially exfoliated structures provided a greaterinterfacial area between the PS chains and the EFD platelets,and a more efficient thermal barrier between the pyrolysis zoneand the polymer chains was created.Thermo-mechanical Properties. Dynamic mechanical

analysis was performed on the PNC samples to evaluate theeffect of clay type and loading on the thermo-mechanicalproperties and glass transition temperature (Tg). Parts A and Bof Figure 13 show the variation in the storage modulus withtemperature for the pure PS and PS/EFD PNCs and pure PSand PS/ATT PNCs, respectively.All of the synthesized nanocomposites exhibited a higher

storage modulus relative to the neat PS. This is due to theinteraction between the polymer and silicate layers at theinterface, hindering the mobility of the polymer segments at ornear the interface and leading to an increase in the storagemodulus at temperatures below the Tg.

6,21,53 It is clear fromFigure 13A that the storage modulus values were affected bythe structure of the PS/EFD PNCs obtained. Semiexfoliatedstructures at low EFD loading (i.e., < 5 wt %) exhibited a higherstorage modulus than the intercalated structures obtained athigher clay loading. The different storage modulus values aredue to the homogeneous dispersion of EFD platelets into thePS matrix and a higher interfacial area, which restricted thechain mobility to a greater extent.From Figure 13B and Table 6 it is clear that the addition of

clay with a needle-like structure (i.e., ATT) enhanced the

storage moduli compared to the neat PS; however, the curvesfor the 3 and 5 wt % nearly overlap with the curve for the 7 wt% ATT composite, indicating that the storage moduli reached amaximum when the concentration of the ATT reached 3 wt %.The storage moduli obtained for the PS/EFD PNCs are greaterthan those for the PS/ATT PNCs; this is due to the differencein the clay shape or the surface properties of both types of clay.As mentioned earlier, the CEC of ATT is in the range of 17−20

meq/100 g;29 therefore, there were few DMAEM moleculesattached to the ATT surface that copolymerize with the styrenemonomer. Thus, only a few PS chains interacted directly withATT surface, and the majority of PS chains were not restrictedby the ATT needles. The shape of the filler particles also had amarked effect on its efficiency as a reinforcing agent of polymersystems. The plate-shaped EFD had a greater stiffening effecton the PS matrix than the needle-like ATT. For the sheet-likestructure, the interaction between PS chains and EFD plateletsat the interface is higher and hinders the mobility of thepolymer segments at or near the interface leading to an increasein the storage modulus. Additionally, the PS/EFD PNCs wereeither semiexfoliated or intercalated. In both cases, there are anumber of PS chains that are confined between clay plateletsand the motion or relaxation of those chain segments becomesdifficult in this position resulting in high storage moduli.Parts A and B of Figure 14 shows the variation of tan δ as a

function of temperature for pure PS, and PS/EFD PNCs andPS/ATT PNCs, respectively. The Tg values of the purepolymer and the PNCs were determined from the onset of therelaxation peak in the DMA tan δ curve. Figure 14A and Table6 show that the glass transition relaxation peaks of the PNCssynthesized with 1 and 3 wt % EFD shifted to a slightly highertemperature relative to that for the standard neat polymer,indicating an increase in Tg. The increase is due to theincorporation of EFD platelets that act as mobility retarders inthe polymer network. The presence of EFD leads to interfacialmaterial that has different properties than the bulk material.However, as the clay loading increased, the peak maximumshifted to significantly lower temperatures, indicating that Tgvalues were not improved by adding more clay but rather by thedispersion state of the EFD platelets in the polymer matrix. Thehighly dispersed clay platelets inhibited the mobility of the PSchains to a greater extent than when the clay was onlyintercalated.The Tg of PNCs was also affected by the addition of needle-

like clay, as can be seen in Figure 14B, but was not linearlycorrelated with the clay loading; there were no significantchanges in Tg for the PNCs prepared at 3 and 5 wt % ATTloadings. This is likely due to agglomerations of clay particles,which prevented any significant enhancement of the mechanicalproperties of the PNCs. At higher ATT loading (7 wt %), theTg shifted to a lower temperature. This shift could be due tobroad molecular weight disruptions of the polymer matrixbecause a polymer with a broad molecular weight distributionmay contain a low molecular weight portion. The lowmolecular weight materials reduce the Tg in a manner similarto plasticizers or lubricants. This plasticizer effect is coupled tobroadening of the tan δ peaks, which is generally attributed to adistribution of the degrees of chain mobility restriction causedby clay filler and the high variety of PS chain lengths.

■ CONCLUSIONSNanocomposites of polystyrene with two different clay typeswere synthesized by in situ free radical polymerization inemulsion. The morphologies of the synthesized PNCs weredependent on the type of clay used. Semiexfoliated structureswere obtained when the clay had the following characteristics:the ability to interact significantly with the organic modifiers viathe ion-exchange reaction, a compatibility with the water/monomer system and a platelet-like structure that provides ahigher polymer chain interface area. An EFD fulfils theserequirements and produces a semiexfoliated structure of clay

Table 6. Storage Modulus (below and above Tg) Data

sample clay content (wt %) G′ × 1010 (Pa) Tg (°C)

pure PS 0 1.26 103PS/EFD PNCs 1 1.89 102

3 2.00 1085 1.77 987 1.78 99

PS/ATT PNCs 1 1.60 1003 1.91 1035 1.86 1037 1.89 93



particles that is well-dispersed in the PS matrix. Clay particleswere not well-dispersed in the polymer matrix when the clayonly fulfilled one or two of the three requirements, as was thecase when a needle-like clay (ATT) was used.All of the synthesized PNCs were thermally more stable than

the neat PS. However, PS/EFD PNCs exhibited significantlyhigher thermal stability than the PS/ATT PNCs at a similarloading. An increase in the storage modulus and the Tg of thePNCs synthesized with platelet-like clay was found andcorrelated to the PNC morphology. The polymerization ofstyrene in the presence of ATT also produced PS compositematerials with higher storage moduli relative to the neat PS.However, the storage modulus values were constant for entireATT loading range.This study highlights the need to further study the role of the

clay type has on the final morphology and properties of thenanocomposites. This will allow for chemical engineering in thenear future to prepare PNC materials with tailored propertiesfor specific applications by fine-tuning the types of the clay.

■ AUTHOR INFORMATION

Corresponding Author*Tel.: +27128412388. Fax: +2712841 2229. E-mail:[email protected].

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

The authors would like to thank the CSIR, DST, and NRF,South Africa for financial support.

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Figure 14. tan δ as function of the temperature of (A) pure PS and PS/EFD PNCs with different EFD loadings and (B) pure PS and PS/ATT PNCswith different ATT loadings.



mailto:[email protected]

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