Polystyrene / Montmorillonite Nanocomposites : Study of theMorphology and Effects of Sonication Time on Thermal Stability

Polymer nanocomposites of polystyrenematrix containing 10%wt of organo-montmorillonite (organo-MMT)were prepared using the solution method with sonication times of 0.5, 1, 1.5, and 2 hours. Cetyltrimethylammonium bromide (CTAB) is used to modify the montmorillonite clay aer saturating its surface with Na ions. Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) were used to characterize the montmorillonite before and aer modi�cation by CTAB. e prepared nanocomposites were characterized using the same analysismethods.ese results con�rm the intercalation of PS in the interlamellar spaces of organo-MMTwith a very small quantity of exfoliation of the silicate layers within the PS matrix of all samples at all studied times of sonication. e thermal stability of the nanocomposites was measured using thermogravimetric analysis (TGA). e results show clear improvement, and the effects of sonication time are noted.


Introduction
Recently, nanocomposites, such as polymer-layered silicate nanocomposites, have become effective alternatives to conventional polymer composites in many applications.ese systems may be classi�ed according to the dispersion of clay in the polymer.In general, two types of polymer-layered clay nanocomposites can be obtained, intercalated, and exfoliated, and the latter are known as delaminated nanocomposites.Nanocomposites are de�ned as composites in which the dispersed particles are in the nanometer range in at least one dimension [1,2].e thickness of the layer in layered clay is approximately 1 nm, and the lateral dimensions of these layers may vary from 300 ′ Å to several microns or even larger depending on the particular silicate.e layers organize to form stacks with a regular van der Waals gap, which is called the interlayer [3].Intercalates are obtained when a single polymer chain is located between silicate layers in a manner that increases layer spacing while attractive forces between the layers maintain regularly spaced stacks.Exfoliates are obtained when the layer spacing is sufficiently increased to overcome the interactions between the layers, which are randomly dispersed in a continuous polymer matrix [4].e preparation method of nanocomposites can be classi�ed into three categories according to the starting materials, and processing techniques: intercalation polymerization, which includes the intercalation of one or more suitable monomers and subsequent polymerization [5][6][7][8]; polymer intercalation by the solution method [9][10][11]; and polymer intercalation by the melt method [12].However, melt intercalation is advantageous because of the absence of the need for organic solvents, which makes this method environmentally benign, and because of the ability to adopt processing techniques such as extrusion and injection molding, the solution method gives good control on the homogeneity of the constituents, which helps to understand the intercalation process and nanocomposite morphology.It also leads to a better understanding of the structure and dynamics of the intercalated polymers in these nanocomposites, which can provide molecular insight and lead to the design of materials with desired properties [13].erefore, in this work, modi�cation of local montmorillonite clay to use as �ller will be achieved by the solution method and will be used to prepare PS/organo-MMT nanocomposites.e effect of sonication time, which is performed during the preparation process, on the thermal stability of the PS/organo-MMT nanocomposites will be studied.e morphology of the clay and nanocomposites will be characterized using FTIR, XRD, and TEM.

Washing and Saturation of Montmorillonite.
To prepare the MMT suspension, 31 mL of distilled water was added to 150 g of MMT and shaken for 24 h.Aer, the suspension was allowed to sit for 30 min.Distilled water was added to the separated MMT suspension, and the mixture was shaken for 48 h.e mixture was allowed to sit for 24 h.Aer, the MMT suspension was separated and saturated by shaking with 0.5 mol/L of NaCl solution for 24 h.is step was repeated �ve times.e sediment was washed with distilled water to remove excess salt.is was con�rmed by a negative chloride ion test using AgNO 3 .e saturated montmorillonite was dried at 105 ∘ C and grounded.e montmorillonite was stored in well-stoppered bottles in desiccators over CaCl 2 .e samples of Na-saturated montmorillonite were labeled with Na-MMT.

Modi�cation of Montmorillonite
. e organo-MMT was prepared using the ion exchange method.A cetyltrimethylammonium bromide (CTAB) solution was used to modify Na-MMT using a concentration of CTAB that was twice that of the cation exchange capacity (CEC = 80 meq/100 g) of the montmorillonite.Ten grams of Na-MMT was dispersed in 1000 mL of deionized water, and the dispersion was vigorously stirred overnight.Approximately 7.3 g of CTAB was dissolved in 100 mL of deionized water and was slowly added to the Na-MMT suspension.e mixture was magnetically stirred for 24 h at room temperature [14].Organo-MMT particles were separated by centrifugation at ambient temperature and repeatedly washed to remove excess Br − .e product was dried in a vacuum oven at 80 ∘ C. e sample was labeled as organo-MMT.

PS/MMT Nanocomposites.
Ten percent of the weight of organo-MMT was added to 20 mL of toluene.e suspension was stirred magnetically for 1 h and sonicated for 1 h before PS (2 g) was added.e resulting mixture was magnetically stirred for 1 h and further sonicated for 0.5, 1, 1.5, and 2 h to study the effect of sonication time.e mixture was cast into petri dishes.e thin �lms, which were obtained, were removed from the glass plates aer 24 h at room temperature, and the solvent was evaporated.is procedure was performed at room temperature and ambient pressure.e nanocomposites obtained were labeled as PS/organo-MMT.A virgin polystyrene sample was prepared for comparison by mixing it with toluene using a magnetic stirrer.is sample was labeled as PS.

Characterization and Measurements.
Particle size distribution measurements of Na-B and MB suspensions were performed using Malvern Zetasizer, ZS, ZEN3500 (UK).Aqueous suspensions were prepared as follows.0.1 g of clay sample was dispersed in distilled water.e suspension was kept in ultrasonic for 5 minutes in order to break powders' agglomerates resulting �ne, colloidal particles dispersed in water.e structures of Na-MMT, organo-MMT powder, and the prepared nanocomposite �lms were investigated using Fourier-transform infrared (FTIR) spectroscopy (FI-IR Spectrometer 1000, Perkin Elmer).e structure of nanocomposites were monitored using X-ray diffraction (XRD), RIGAKU Ultima-IV diffractometer (Japan), and intensity data were collected in the 2 range of 0 ∘ -25 ∘ at a step of 0.02 ∘ and a 2 s count time using Cu-K radiation.e surface morphology of the prepared organoclays was examined by using a JEOL JSM-6360LV scanning electron microscope.Small amounts of the dried powders (approximately 0.01 g) were placed on sticky carbon tape on standard Al mounts, then sputter coated with a thin conductive layer of gold.
Transmission electron microscopy (TEM) images were recorded to investigate the morphology and inner structure of the PS/organo-MMT nanocomposite samples.TEM images were recorded on a JEOL JSM-6060LV transmission electron microscope.TEM is at an accelerating voltage of 100 kV.For TEM observation, small drops of dilute suspensions of 0.1 g of the clays (Na-B and MB) in 5 cm 3 of doubly distilled water were placed on Cu mesh grids which had been coated with a thin carbon �lm.e grids were air-dried then brie�y placed in a 60 ∘ C oven to ensure complete drying prior to insertion into the instrument.e samples of nanocomposites �lm were microtomed at room temperature with a diamond knife using a Reichert utramicrotome in order to obtain 80 nm thick sections.Microtomed sections were transferred from water onto 200 mesh copper grids and used without staining.ermodegradation of the PS/organo-MMT nanocomposites were determined using thermo gravimetric analysis (TGA) with a Perkin-Elmer analyzer.ermograms, using approximately 10 mg of sample, were recorded from 25 ∘ C to 800 ∘ C at a heating rate of 10 ∘ C min −1 under nitrogen �ow.

Size Distribution of Montmorillonite Suspensions.
Particle size distribution analysis (PSD) is a measurement designed to determine information about the size and range of a set of particles in a representative material.Particle size was determined using dynamic light scattering measurements.Figures 1 and 2 show the size distribution histogram of Na-MMT and organo-MMT suspensions.e Zetasizer soware uses algorithms to extract the decay rates for a number of size classes to produce a size distribution.e -axis shows a distribution of size classes, whereas the -axis shows the relative intensity of the scattered light.e particle size distribution of the Na-MMT suspension had a wide distribution (three peaks); therefore, its distribution is very heterogeneous.e particle size distribution for Na-MMT was 88.2% of the particles were 748.5 nm, 6.4% were 105.4 nm, and 5.4% were 426.3 nm.e organo-MMT suspension shows one peak with a narrow size distribution in the histogram, which may be related to one type of size distribution.is result revealed the particle size of organo-MMT to be highly monodispersed, indicating that they have the same size and shape with an average diameter of 1298 nm, which was obtained with 100% intensity.e increase in the size was attributed to an aggregation of particles because of an interaction of the positively charged CTAB molecules with the negatively charged clay particles.e average size and intensity are presented in Table 3.

Infrared Absorption Spectra
3.2.1.Infrared Absorption Spectra of Montmorillonite.e infrared absorption spectra (FT-IR) of Na-MMT and organo-MMT were recorded in the region from 400-4000 cm −1 .e full FT-IR spectra of the samples are presented in Figure 3.For the Na-MMT spectrum, the absorption band at 3624 cm −1 is because of the stretching vibrations of structural OH groups coordinated to Al-Al pairs, the complex broadband at approximately 1032 cm −1 corresponds to Si-O For the organo-MMT spectrum, there are several absorption bands that appeared at different positions.e asymmetric stretching vibration of the structural OH groups of Na-MMT at 3626 cm −1 , the symmetric stretching vibration of the hydroxyl groups at 3430 cm −1 , and the bending inplane vibration of H-O-H at 1656 cm −1 correspond to the structural changes from hydrophilic to hydrophobic characteristics [18,19].e stretching band of the OH groups at 3624 cm −1 (Na-MMT) shied to 3626 cm −1 in the organo-MMT sample.is slight shi towards the smaller frequencies implies the replacement of H 2 O molecules from the galleries when CTAB is adsorbed.Similarly, the bending in-plane vibrations of the OH groups are characterized by a broad band at 1639 cm −1 .is band shied to 1656 cm −1 in the FT-IR spectrum of the organo-MMT sample, which indicates an intercalation of surfactant molecules between the silica layers in montmorillonite.e broad band observed at 3447 cm −1 corresponds to the stretching vibrations of the structural and free OH groups.is band shied to 3430 cm −1 in organo-MMT.e sharp peaks that appear at 2922 and 2822 cm −1 are characteristics of CTAB, and these peaks are indicative of intermolecular attractions between adjacent alkyl chains of CTAB in Na-MMT galleries [20,21].A small peak appeared at 915 cm −1 and belonged to the in-plane stretching vibrations of the Si-O bonds of the tetrahedral silica layers that shied to 911 cm −1 .All of these changes refer to the organophilic modi�cation of Na-MMT by CTAB in the prepared sample of organo-MMT [19,[22][23][24][25][26].Table 4 shows the assignments of Na-MMT and organo-MMT.absorptions bands that are attributable to PS and organo-MMT.All PS bands appeared in these spectra with slight shiing.New bands that appeared in the regions of 3625, 1027, and 466 cm −1 were attributed to OH stretching of structural hydroxyl groups (Al-OH), Si-O stretching, and bending vibrations of organo-MMT and were indicating the existence of organo-MMT in the PS matrix where the polymer chain was inserted between the layers of the organo-MMT by secondary valence forces.

X-Ray Diffraction of Montmorillonite
. XRD analysis provides information on changes in the interlayer spacing of the clay layers.e formation of an intercalated structure should result in a decrease in the 2 value, which would indicate an increase in  spacing, whereas the formation of an exfoliated structure usually results in the complete loss of registry between the clay layers such that no peak can be observed in the XRD trace [4].us, the Na-MMT and organo-MMT samples were studied by WAXD measurements in the range of 2 = (1-10) ∘ .Figure 5 shows the XRD patterns of Na-MMT and organo-MMT.Upon intercalation of CTAB, the basal spacing increased as expected.e XRD pattern of Na-MMT and the characteristic peak (001) re�ections of montmorillonite appeared at 2  1 ∘ , which corresponds to 12.4 Å, and this peak in organo-MMT was observed at a lower angle compared to Na-MMT.e  spacing increased from 12.4 (2  1 ∘ ) for Na-MMT to 19.6 Å (2   ∘ ) for organo-MMT.is increase in  spacing provides evidence to support the exchange of the interlayer sodium by CTA + ions.Figure 6 demonstrates the probable mechanism of adsorbing CTAB molecules onto the montmorillonite surface layers as well as the relationship between CTAB adsorption and the structure of the adsorption layer in�uenced by the charge distribution of the clay surface.Initially, the clay saturated by a concentration of CTAB less than a CEC of 0.7, CTAB adsorption occurs by cation exchange (Figure 6(a)) [27], which causes nonuniform interlayer swelling.As the CTAB concentration approaches the CEC (0.8-1.0 CEC) of clay, the selective intercalation of CTAB develops along the silicate layers (Figure 6(b)).When the CTAB concentration increases beyond the CEC (>1.0 CEC) of the clay, CTAB adsorption may predominantly occur through hydrophobic bonding, which increases interlayer spacing to ∼40 (Figures 6(c) and 6(d)) [27].e result, which was obtained for organo-MMT, con�rms the probable mechanism where the  spacing of the (001) peak shis directly from 12.4 Å (2  1 ∘ ) in the Na-MMT to 19.6 Å (2   ∘ ) in the organo-MMT (see Table 6).
T 7: e XRD data of organo-MMT and PS/MMT nanocomposite spectra.

X-Ray Diffraction of Nanocomposites. X-ray diffraction
(XRD) measurements were also applied to analyze the structural characteristics of PS and PS nanocomposites, which were prepared at different sonication times.Figure 7 shows the XRD patterns of PS and PS/organo-MMT nanocomposites, and Table 7 shows the 2 and -spacing values.For PS, only a broad amorphous peak, which appeared within the range of 2 = (18-22) ∘ , can be observed.
For PS/organo-MMT nanocomposites, it is clear from �gure that some diffraction peaks of organo-MMT and the broad peak of PS appeared in the spectra for all nanocomposite samples.e diffraction peak of montmorillonite, which appears at 2   ∘ in the organo-MMT spectrum, appeared at a smaller angle with increasing  spacing for all PS/MMT nanocomposite spectra, and the angle was approximately the same for all samples (see Table 7).e increase in  spacing indicates that intercalated nanocomposites were formed.It was also noticed from the �gure that there are sharp peaks that clearly appeared in the region of amorphous PS with increasing sonication times.is may indicate that the crystalline structure of the polymer was positively affected.An optimal sonication time of 1 h was found to produce nanocomposites with maximum peak intensity.e sonication time for 0.5 hour is not enough for the order and regularity of polymer chains between the interlayers of MB.At sonication time of 1 h resulted in region with crystalline structure (Figure 7).When the time of sonication increases more than an hour this may be lead to entering the polymer chains between the clay layers with random orientation due to the instability of the structure of clays.However, the effect of sonication on the clay polymers nanocomposites still needs further studies to identify the regularity of polymer chains between the clay layers.

Transmission Electron Microscopy (TEM).
Transmission electron microscopy (TEM) provides an actual image of the clay layers, which permits the identi�cation of the morphology of the nanocomposites [4].us, TEM was another method to observe evidence for the formation of the intercalated structure of the prepared nanocomposites.Figure 8 shows the TEM images of PS/organo-MMT at 1 h of sonication time at a variety of magni�cations.As shown in these images, the clay platelets were still stacked on each other (see the darkest areas).In addition, the clay layers have expanded with varied interlayer spacing, such as 2.15, 2.56, and 2.6 nm, whereas little individual clay layers were well distributed in the polymer matrix.Finally, from these images, it can be determined that intercalated structures and a very small amount of exfoliated structures were formed, which con�rms and agrees with the XRD results.

Scanning Electron Microscopy (SEM)
3.5.1.SEM of Montmorillonite.To determine the shape, size, and morphology of the particles, SEM was used.In Figure 9, micrographs of powder of raw MMT, Na-MMT, and organo-MMT samples are presented.It is clear that particles of all samples are irregularly shaped and contain many edges with different sizes.It is known that these factors play an important role in the interaction between �ller, matrix, and interfacial adhesions.For the morphology of the samples, the images showed that the original Na-MMT had massive and   curved plates.Compared to the morphology of Na-MMT, the montmorillonite clay, which was modi�ed with CT��, showed signi�cant changes in morphology, and there were a lot of aggregated particles, and the plates became �at.

SEM of Nanocomposites.
To observe the dispersion of the particles, the nanocomposites aer 1 h of sonication were examined using SEM, and the images are presented in Figure 10.Incorporation of 10% organo-MMT with polystyrene  matrix resulted in a region with spherulite texture (lamella structure).is texture could be related to the crystallinity of the polymer.Spherulite-looking crystals with beautiful impinged boundaries were observed in PS/organo-MMT aer 1 h of sonication time compared to PS. Good dispersion of this percentage of MMT throughout the PS matrix under sonication could be led to a nucleation effect and increase the percentage of crystallinity.is result strongly agrees with the XRD results.

ermogravimetric Analysis (TGA)
3.6.1.TGA of Montmorillonite.Weight loss because of the formation of volatile products aer degradation at high temperature was monitored as a function of temperature.e data, which were obtained using TGA analysis, included the temperature at which 10% degradation occurred, which is a measure of the onset temperature of degradation [4].Figure 11 shows the TG curves as well as the corresponding derivative DTG curves of Na-MMT and organo-MMT.For Na-MMT, thermal decomposition occurred in two steps.For organo-MMT, the thermal decomposition took place in three steps.e �rst step of the TG graph of Na-MMT up to 110 ∘ C was related to the dehydration of water molecules adsorbed in pores and between the silicate layers during heating, such that the �rst endothermic peak appeared at  onset 66 ∘ C in the DTG graph, which is where the mass loss accounted for 13% at 110 ∘ C. For organo-MMT, the dehydration peak appeared smaller in the DTG graph, and the corresponding mass loss percentages was 2%.
e second step of the TG curve of organo-MMT began at approximately 200-216 ∘ C and ended at approximately 300-350 ∘ C, and the maximum mass loss rates occurred at  onset 229 ∘ C. At the end of this stage, the mass loss percentage of organo-MMT was 13%.ese losses were attributed to the thermal degradation of the alkyl tails (-CH 2 ) and ammonium heads (-N(CH 3 ) 3 ) [18].In the third step of TG, which went up to 600 ∘ C, thermal degradations lowered down, and decomposition was attributed to the remaining alkyl chains.ese �ndings verify the successful bonding of CTA� to the montmorillonite surface and the replacement of the OH groups [28][29][30][31].e residues at 600 and 450 ∘ C, which were roughly 12% Na-MMT and 45% organo-MMT, respectively, were also distinctive, which explains the permanent structural modi�cations of the clay, although they retained their inorganic character.
3.6.2.TGA of Nanocomposites.e thermal stability of the nanocomposites and virgin PS were studied by TGA. Figure 12 and Table 8 show the TG mass loss curves and corresponding derivative curves (DTG) of PS and a series of PS/organo-MMT nanocomposites,which were prepared at  various sonication times (0, 0.5, 1, 1.5, and 2 h).For the PS sample, the curve shows a single degradation step, which is the decomposition of the polymer backbone, and an onset temperature of 388.07 ∘ C. e thermal decomposition of all PS/organo-MMT samples exhibit a single degradation step of weight loss from 450 to 500 ∘ C. All samples exhibit an increase in the onset temperature of degradation relative to virgin PS, which indicates an enhancement in thermal stability.From the DTG curve, the maximum rate of change of the curve (DTG) of PS/organo-MMT nanocomposites appeared at 457, 460, 453, and 457 ∘ C for 0.5, 1, 1.5, and 2 h of sonication time, respectively.e improvement in thermal stability of PS/organo-MMT nanocompositesis explained as follows.First, formation of organo-MMT, which acts as a mass transport barrier and insulator between the polymer and super�cial zone where polymer decomposition takes place [32,33].Second, it can be attributed to restricted thermal motions of the polymer localized in the galleries [34].ird, the increase in the crystallinity of PS, which was suggested from the XRD and SEM results.However, it is clear that there is little difference between the  onset readings of the nanocomposites at different sonication times.is indicates that the difference in sonication time does not play an important role on the thermal decomposition of nanocomposites.

Conclusions
Nanocomposites containing locally produced polystyrene and modi�ed local montmorillonite were prepared.e modi�cation of montmorillonite was achieved using CTAB with concentrations of 2.0 CEC.Preparation of PS/organo-MMT using the solution method by applying different sonication times was tested.e results of FT-IR, XRD, and SEM analyses have shown the following.
(i) e modi�cation of the montmorillonite surface by CTAB is successfully intercalated into the clay gallery.(ii) Using sonication during PS nanocomposite preparation gives good miscibility between organo-MMT and the PS matrix.(iii) e type of nanocomposites of PS/organo-MMT is intercalation.(iv) e improvement in thermal stability of PS/organo-MMT nanocomposites is noted.(v) e difference in the sonication time that was applied during preparation stage is not an important factor on the thermal stability of PS/nanocomposites.
Lastly, the study of a wide range of sonication times during the preparation stage with different materials is recommended to give a clear picture and excellent judgment regarding this factor.

T 3 :F 1 :F 2 :
e particle size distribution of Na-MMT and organo-e particle size distributions of the Na-MMT suspension.e particle size distributions of the organo-MMT suspension.

T 4 :F 3 :
e infrared spectra absorption data of Na-MMT and organo-MMT.Assignments Na-MMT Organo-MMT Shi (cm −1 ) OH stretching of structural hydroxyl groups (Al-OH) 3624 3626 2 OH stretching of water 3447 3430 −17 Asymmetric stretching of the CH 2 (in CTAB) -2922 -Symmetric stretching of the CH 2 (in CTAB) FT-IR spectra of Na-MMT and organo-MMT.stretching, and the 528 and 467 cm −1 bands are related to Al-O-Si, Si-O-Si, and Si-O deformations [15, 16].Adsorbed molecules of water result in a broad band at 3447 cm −1 , which corresponds to the H 2 O-stretching vibrations, and the absorption band of the H 2 O bending vibration was at 1639 cm −1 [17].

F 4 :
FT-IR spectra of PS and PS/MMT nanocomposites at different sonication times.

F 7 :
XRD patterns of organo-MMT, PS, and PS/MMT nanocomposites at different sonication times.

F 10 :F 11 :
S�M images of PS/organo-MMT aer 1 h of sonication time at two magni�cation.TGA curve of Na-MMT and organo-MMT.

F 12 :
TGA curve of virgin PS and its nanocomposites at different sonication times.
Spectra of PS and the PS Nanocomposites.e infrared absorption spectra (FT-IR) of PS and the PS/organo-MMT nanocomposites aer different sonication times were recorded in the region from 400-4000 cm −1 , as shown in Figure4.einfraredspectrum of PS features bands at 3066, 3025, 2922, 2851, 1666-1945, 1491-1599, 1188-1368, 1026, 698-756, and 543 cm −1 , and salient absorption bands and explanations are listed in Table5.e FTIR spectra of the PS/organo-MMT nanocomposite at all applied times of sonication clearly exhibit the characteristic F 5: X-ray diffraction patterns of Na-MMT and organo-MMT.T 5: Positions and assignments of the IR vibration bands of PS.

T 8 :
TG data of PS and PS/organo-MMT nanocomposites at different sonication times.