CHAPTER 3 EFFECT OF COLLOIDAL SILICA DISPERSIONS ON THE PROPERTIES OF PDMS-COLLOIDAL SILICA COMPOSITES

CHAPTER 3 EFFECT OF COLLOIDAL SILICA DISPERSIONS ON THE PROPERTIES OF PDMS-COLLOIDAL SILICA COMPOSITES 3.1 INTRODUCTION The study on PDMS-CS composites prepared with the use of CS of varying particle sizes indicated that the proper choice of particle size and the dispersion medium with which CS particles are dispersed into PDMS matrix, are critical to obtain PDMS-CS composite with improved optical and mechanical properties. As the addition of codispersing aid, along with aqueous dispersion of CS is found to have a significant effect, it is important to understand the effect of dispersion of using different dispersing aids. Along this line, various commercially available dispersions of CS are identified and used for preparing the PDMS-CS composites. The detailed characteristics of various dispersions of CS as procured from Nissan chemical company are described in Table 3.1. ORGANOSILICASOL Particle Size (nm) SiO 2 (wt %) H 2 O % Viscosity (mpa.s.) Specific Gravity ph Solvent (Boiling Point C) DMAC-ST 10-15 20-21 < 3.0 < 10 1.06-1.09 4-6 N,N-Dimethyl acetamide EG-ST 10-15 20-21 < 2.0 < 100 1.22-1.26 3-5 Ethylene glycol IPA-ST 10-15 30-31 < 1.0 < 15 0.96-1.02 2-4 Isopropanol IPA-ST-L 40-50 30-31 < 1.0 < 15 0.96-1.02 2-4 Isopropanol IPA-ST-UP 9-15/40-100 15-16 < 1.0 < 20 0.85-0.90 2-5 Isopropanol IPA-ST-ZL 70-100* 30-31 < 1.0 < 15 0.96-1.02 2-4 Isopropanol MA-ST-M 20-30 40-41 < 3.0 < 5 1.07-1.12 2-4 Methanol MEK-ST 10-15 30-31 < 0.5 < 5 0.98-1.02 NA Methyl ethyl keytone MEK-ST-L 40-50 30-31 < 0.5 < 5 0.98-1.02 NA Methyl ethyl keytone MEK-ST-UP 9-15/40-100 20-21 < 0.5 < 10 0.90-0.95 NA Methyl ethyl keytone MIBK-ST 10-15 30-31 < 0.3 < 5 0.98-1.02 NA Methyl isobutyl ketone MT-ST 10-15 30-31 < 2.0 < 5 0.98-1.02 2-4 Methanol NPC-ST-30 10-15 30-31 < 1.5 < 25 1.10-1.14 3-5 Ethylene glycol mono-n-propyl ether PMA-ST 10-15 30-31 < 0.4 < 10 1.14-1.20 NA Propylene glycol mono-methyl ether acetate Table 3.1 Summary of characteristics of commercially available CS dispersions. Many commercial silica sol manufacturers produce various different aqueous and organic sols (section 2.1 Chapter 2) with varying particle sizes and shapes of CS. The broad spectrum of sols with different particle size and ph may not be available with a single company and there are only very few manufacturers who deal with wide range of 1

CS dispersions. Among, different commercial suppliers the Nissan chemical company is found to have a wide range of organo sols with different sizes, shapes and ph 1. On the basis of the boiling point of the dispersion mediums, availability of CS with similar particle size and the solubility parameters, the three dispersion media are selected for detailed investigation in the present study. The CS dispersions of similar particle size are characterized to understand the surface characteristics of CS using FTIR, TEM, and elemental analysis. The PDMS-CS composites prepared with varying loadings of CS derived from these dispersions are studied for thermal, rheological and mechanical properties. The viscoelastic, tensile properties, and morphology of fractured tensile samples of the resultant PDMS-CS composites are studied and compared with PDMS-CS composites obtained with the use of aqueous dispersion of CS. The results of the present study are discussed in terms of particle-particle / polymer-particle interactions which vary with the use of different dispersing media. 3.2 EXPERIMENTAL 3.2.1 Materials The dispersions of CS (SNOWTEX and ORGANOSILICASOL ) with average particle size of ~ 15 nm in water, methanol, isopropanol and methylethylketone (MEK) media were received from Nissan Chemicals, USA. Details of the CS used in the present study are summarized in Table 3.2. Various materials Sample Name Trade Name Particle Size (nm) SiO 2 (wt %) Viscosity (mpa.s.) Specific Gravity CS-Water STO 10-15 20-21 <10 1.12-1.14 CS-MeOH MT-ST 10-15 30-31 < 5 0.98-1.02 CS-IPA IPA-ST 10-15 30-31 < 15 0.96-1.02 CS-MEK MEK-ST 10-15 30-31 < 5 0.98-1.02 Table 3.2 Particle size, viscosity and specific gravity data of various CS dispersions used in the present study. used for making cross-linked PDMS composites such as vinyl end capped PDMS with the average molecular weight of ~ 65000 µ, hydride functionalized PDMS cross-linker with molecular weight of ~2800 µ, the hydrosilylation catalyst (chloroplatinic acid) and 2

the inhibitor (ethynylcyclohexanol (ECH)) were received from Momentive Performance Materials, Inc. Leverkusen, Germany used as such without further purification. 3.2.2 Preparation of PDMS-colloidal silica composites Using similar procedure described in section 2.3.3 of Chapter 2, PDMS composites of various loadings (40 wt %, 30 wt %, 20 wt % and 10 wt %) of colloidal silica dispersions were prepared. The loading levels along with sample code of the composites are provided in Table 3.3. Table 3.3 Sample codes and loading levels (in wt %) of CS in PDMS-CS composites. 3.3 RESULTS & DISCUSSION 3.3.1 Analysis of commercial CS dispersions Prior to the preparation of PDMS-CS composites, the colloidal silica dispersions are analyzed to check the actual particle size distribution using TEM analysis of ~1 % 3

diluted CS dispersions (Figure 3.1). It is evident that all commercial CS dispersions used in the present study contain CS particles with size of 15 ± 5 nm. Figure 3.1 TEM micrographs of different dispersions of CS used in the present study. FTIR analyses of dried samples (at 110 C for 8 hour) obtained from different CS dispersions are shown in Figure 3.2. All the samples showed the vibration absorption peaks between 950-1100 cm-1 and ~ 785 cm-1. While the peaks in the range of 950-1100 cm-1 are assigned to Si-O-Si asymmetric stretching modes, the peak at 785 cm-1 can be assigned to both symmetric Si-O-Si stretching and bending vibrations. The broad peak at 3000-3800 cm -1 is assigned to OH stretching vibrations of Si-OH and residual H 2 O. The peak at 950 cm -1 is assigned for residual Si-OH bending vibrations. Among different CS dispersions studied, CS in MEK dispersion showed an additional peak at ~2900 cm -1, which is typically assigned to C-H stretching vibration absorption. Figure 3.2 FTIR spectra of dried samples of CS derived from different dispersion of CS The CH percentages as depicted in Table 3.4, indicate that the CS derived from CS-MEK possess higher percentage carbon and hydrogen when compared to those 4

derived from CS-IPA and CS-MeOH. The CS derived from CS-water appeared to have least percentage of carbon and hydrogen. The observed carbon content in the dried samples possibly indicates the surface modification of CS in CS-MEK and also possible esterification or non-covalent binding of alcoholic solvents on to CS in CS-IPA and CS- MeOH 2. CS-Water CS-MeOH CS-IPA CS-MEK % Carbon 0.03 0.32 0.78 1.86 % Hydrogen 0.273 0.41 0.53 0.67 Table 3.4: The carbon and hydrogen percentages of dried powders of CS derived from different CS dispersions. Further, the pre-dried samples derived from different CS dispersions were analyzed using TGA data as shown in Figure 3.3. The thermal stability and degradation pattern Figure 3.3 TGA of dried samples of CS derived from CS-Water, CS-IPA, CS-MeOH and CS-MEK indicate different degree of decomposition with different CS. Wherein, CS-IPA and CS- MEK shows relatively high percentage of weight loss between 200 C to 500 C, when 5

compared to CS-MeOH and CS-water indicating possible surface functionalization. These results support the elemental analysis. 3.3.2 Effect of dispersion media on thermal and rheological properties of uncured PDMS-CS composites 3.3.2.1 Thermal characteristics Thermogravimetric analysis (TGA) and differential thermogravimetric analysis (DTGA) curves for the PDMS-CS composites prepared with the use of different dispersion of CS are shown in Figure. 3.4. It is known that the thermal degradation of pure PDMS in inert atmosphere occurs by depolymerization over the range 400-650 C (Section 2.3.2.1of Chapter 2). The incorporation of CS in the PDMS matrix resulted in increased on-set degradation temperature (Td) and the char residue at elevated temperatures (> 650 C). This behavior is related to a change in the thermal degradation mechanism of the PDMS in the presence of CS. The adsorption of thermal energy and barrier effect of the CS particles on the diffusion of volatiles may have shifted the shift of the main weight loss processes to higher temperatures, as compared to the unfilled PDMS Error! Bookmark not defined.. Among the different PDMS-CS composites, the samples prepared with the use of water or methanol dispersion of CS was found to have higher Td (60 C), when compared to those prepared with the use of MEK and IPA dispersions of CS. The observed trend in degradation behavior suggests that different extent of interactions between CS and PDMS, assisted by both the dispersion medium and the surface functionalities. Both the dispersion medium and the surface functionalization can lead to varying degree of filler-filler and polymer-filler interactions, due to the non-covalent interaction of silica with dispersion medium, the solubility parameter of the solvent with PDMS and the compatibility of PDMS with surface functionalities of CS. The char residue is found to vary slightly for different PDMS composites and such variations can be rationalized to a difference in the dispersion of silica in the chosen samples. 6

Figure 3.4 TGA curves of neat PDMS and PDMS-CS composites 7

3.3.2.2 Rheological characteristics Figure 3.5 (a, b, c and d) represents the change in the viscosities of uncured PDMS-CS composites with varying loading of CS dispersed with the use of different dispersion media. The incorporation of CS fillers into PDMS matrix resulted in enhancement of the viscosity, as expected Error! Bookmark not defined.. But, the magnitude of increase in viscosity is found to depend on filler loadings and the dispersion medium with which CS is dispersed within the PDMS matrix. A linear increase in viscosity with increasing loading of CS is evident for PDMS-CS composites prepared with the use of water and methanol dispersions of CS. But a significant increase of viscosity at higher loadings of CS is evident for the composites prepared from MEK and IPA dispersions of CS. Figure 3.5 Viscosity PDMS-CS composites prepared from different dispersions Further, the composites are analyzed for their viscoelastic behavior by studying modulus and damping factors with varying applied frequencies. Storage modulus (G ) gives the information on the elastic nature of the material and it is representative of storage of energy during deformation. The loss modulus (G ) gives the information on 8

viscous nature of the material and is representative of the energy dissipation during the flow. Figure 3.6 shows the comparative information on the G and G of PDMS-CS Figure 3.6 Effect of dispersing media on the storage and loss modulus versus frequency. composites prepared with the use of different dispersions of CS. Both storage and loss modulus are found to increase with increasing filler content, as compared to neat PDMS. In PDMS-CS-Water composites, the G increases with increasing filler content except in case of 20 % loading. The G increases with filler loading but the increase is not very significant even at higher (20-40 wt %) loading and they show frequency dependence for all filler loadings. It is clear from the Figure 3.7 that, in the lower frequency range, damping factor is high and it decreases with increase in frequency and reaches a plateau for all the composites. In general, the damping factor is found to increase with increased filler loading in the composites, indicating a relatively facile, 9

dissipation of the energy in PDMS-CS composites as compared to pure PDMS. In the case of CS-MeOH composites, irrespective of filler loading G is found to be higher Figure 3.7 Damping factor of PDMS-CS composites. than G indicating a dominance of viscous nature of the composites. However, in CS- MeOH composites, there is a significant increase in storage modulus for the composites with 30 wt % and 40 wt % of CS and it could be due to increase in elastic regions though the viscous behavior of the material dominated. The damping factor is very much frequency dependent in the lower frequency range, in the cases of CS-MeO, CS-IPA and CS-MEK based composites up to 30 wt % of filler loading. In comparison, CS-MeOH 40 and CS-IPA 40 composites show a significantly less frequency dependence. A decrease of both G and G is evident for CS-MEK 40 composites. Distinct cross over frequencies are observed at 8.9 and 14 rad/sec for CS-MEK 30 and CS-MEK 40 composites, respectively, indicating more viscous nature of these composites at higher frequencies. As there is a significant variation of G and G at 30 wt % loading of CS, a comparison is made for all PDMS-CS composites to understand the effect of the 10

dispersion media at PDMS-CS 30 composites (Figure 3.8). CS-MEK showed significantly high viscosities compared to that of CS-Water, CS-MeOH and CS-IPA Figure 3.8 Comparison of viscoelastic properties of PDMS-CS 30 composites. composites. Similarly, G and G values observed for CS-MEK are significantly high by one to two orders of magnitude, compared to other three PDMS-CS composites. G and G values appear to follow the following trend; CS-MEK >> CS-IPA > CS-MeOH > CS water. A reverse trend is evident for damping factor. The significantly high damping factor for CS-water could be related to loosely bound clusters which are also evidenced from TEM. CS-MEK and CS-IPA composites appear to follow the frequency independence unlike CS-MeOH and CS-Water composites, indicating a formation of three- dimensional network or structured sols, in cases of CS-MEK and CS-IPA. This is further strengthened using the power law relation (G α ω 2 and G α ω) as shown in the Table 3.5. The power coefficient (n) value for PDMS is 1.17 from G and 0.96 from G. The n value is found to be further reduced for composites with CS-MEK (0.25 from G, 0.44 from G ) and CS IPA (0.88 from G, 0.86 from G ). The reduction 11

in power coefficient (n) values follows the trend; CS-water > CS-MeOH > CS-IPA > CS- MEK. This could be related to pseudo-solid like behavior due to incomplete relaxation of molecular chains. Sample Name n (G') n (G'') PDMS 1.17 0.96 CS-Water 30 1.46 0.97 CS-MeOH 30 1.25 0.92 CS-IPA 30 0.88 0.86 CS-MEK 30 0.25 0.44 Table 3.5: Power coefficients of PDMS-CS 30 composites. It is clear that there could be three different interactions possible in these PDMS- CS composites, i) solvation phenomena due to the hydrogen bonding interaction ii) solubility parameter of dispersion medium with PDMS (lower the difference higher the interaction) and iii) compatibility of PDMS with the surface functionalities of CS particles. It is evident from the compositional analysis that CS-IPA and CS-MEK dispersions are pre-functionalized, whereas CS-water and CS-MeOH are nonfunctionalized. Given the polar nature of the water (solubility parameter- 47.9, polarity index- 10.2 and dielectric constant- 80.10) molecules, they can get organized at silica interface via non covalent interaction such as hydrogen bonding with silanol groups present at the CS surface. This would lead to the formation of a solvation layer surrounding the CS, resulting in the formation of clusters through filler-filler interactions, when dispersed in PDMS matrix, both due to their hydrophilic surface and due to its immiscibility with non-polar PDMS (solubility parameter-15.1) matrix 3. In the case of methanol (solubility parameter- 29.6, polarity index- 5.1 and dielectric constant- 32.70) dispersion of CS, the surface silanol groups are known to form weak reversible ester linkages leading relatively more hydrophobic CS particles. These methanol linkages are relatively weak that can be easily removed during the application of vacuum. Hence, while removing MeOH from the composite mixture, the liberated silanol groups can form hydrogen bonding with adjacent silanols leading to aggregates and agglomerates. When a dispersion medium with low hydrogen bonding capability (MEK) is used, the solvation layer around each particle is precluded due to the low affinity between the dispersion 12

medium and the CS surface. However, the significance of this effect can potentially vary when surface is already treated with a coupling agent. The observed difference in properties in cases of composites containing CS-IPA and CS-MEK can be correlated to above interactions. In the case of CS-IPA, both surface functionalization and higher solubility parameter of IPA (solubility parameter- 23.14) (Table 3.6 ) can lead an improved dispersion of PDMS matrix. The relatively more non-polar nature, closer solubility parameter of MEK (solubility parameter- 17.6) and pre-surface treatment of CS in CS-MEK leads to relatively more structured sols, which in turn results in high viscose materials with a space-filling morphology. Sample Name Dispersion solvent Boiling point ( C) Polarity index (P') Solubility parameter Dielectric constant CS-Water Water 97 10.2 47.9 80.1 CS MeOH Methanol 64.1 5.1 29.6 32.7 CS-IPA Isopropanol 82.4 3.9 23.14 19.92 CS-MEK Methyl ethyl ketone 80 4.7 17.6 18.51 Table 3.6: Physical characteristics of different CS dispersion solvents. 3.3.3 Effect of particle size on structure and morphological properties of cured composites 3.3.3.1 FTIR characterization of PDMS composites To understand the possible interactions of surface hydroxyl groups of hydrophilic colloidal silica with PDMS backbone including H-bonding interaction between surface OH functionalities with oxygen of Si-O-Si linkages of PDMS, the cured sheets of PDMS composites containing colloidal silica dispersed using different media (CS-water, CS- MeOH, CS-IPA and CS-MEK) are analyzed using FTIR spectra in transmission mode as shown in the Figure-3.9. The FTIR spectrum of neat PDMS is also incorporated in the figure for comparison. The vibration absorption peaks observed at 2962 cm -1 are ascribed to the symmetric and asymmetric vibrations of C-H stretching of CH 3 of PDMS. All the samples showed the vibration absorption peaks between 950-1100 cm-1 and ~ 785 cm-1. While the peaks in the range of 950-1100 cm-1 are assigned to Si-O-Si asymmetric stretching modes, the peak at 785 cm-1 can be assigned to both symmetric Si-O-Si 13

stretching and bending vibrations. The absorption peak observed at 540 cm 1 is usually ascribed to the backbone vibrations of siloxane chains when compared to the neat PDMS. The vibration absorption peaks in the region of ~ 900 to 1100 cm -1 are found to be more intense and also broader for PDMS-colloidal silica composites 4-5. The enhancement of peak intensity and the associated peak broadening are found to increase with decreasing polarity of dispersion in the order CS-MEK > CS-IPA > CS-MeOH > CS-H 2 O, despite all composites contained similar loadings of CS (40 wt %). Figure 3.9 FTIR spectra of PDMS-CS composites with 40 wt % loaded silica. The observed increase in the intensity of peak at 1100 cm -1 could be attributed to the more Si-O linkages formed due to the reaction of surface Si-OH with functionalization agents in cases of CS-IPA and CS-MEK as indicated by the % CH analysis (Table 3.2) and TGA data. The increased intensity of the peak at 1100 cm -1 observed for CS-water and CS-MeOH composites as compared to PDMS can be attributed to H-bonding interactions of OH groups of CS on the Si-O-Si stretching vibrations. A similar peak broadening is reported in cases where there is a change in the fluctuation of Si-O-Si linkages Error! Bookmark not defined.. The observed variation in the splitting pattern of the peak has also been probably rationalized to the strained Si-O-Si linkage due to its interaction (disorder-induced coupling) with surface OH groups of the hydrophilic colloidal silica Error! Bookmark not defined.-error! Bookmark not defined.. Further, the area under the curve in the region of 3049-3652 cm -1 which is assigned to OH stretching vibration, after normalizing with respect to the peak at 2962 cm 1, assigned to CH stretching vibrations of methyl group, is found to decrease with increasing polarity of solvents as shown in the Figure 14

3.9. This indicate the presence of more surface hydroxyl groups on the CS particles in the composites that are prepared with use of polar dispersing media. 3.3.3.2 Powder XRD of PDMS composites Figure 3.10 shows the powder XRD patterns observed for cured PDMS composites containing 40 wt % of CS, prepared with the use of different dispersing media. The XRD pattern observed for neat cured PDMS is also shown in the figure for comparison. As discussed earlier, the broad diffraction peaks (at 2θ ~ 12 and a broad hump at 2θ ~ 23 ) observed for neat cured PDMS is attributed to the presence Figure 3.10 Powder X-ray diffraction patterns for cured PDMS composites along with neat PDMS. of a few crystalline phases, dispersed in the mostly amorphous PDMS matrix. Such crystalline phases are formed due to the ordered packing of small segments of PDMS chains, as reported in the earlier in the literature Error! Bookmark not defined.,6. In comparison, no significant change in the diffraction pattern is evident for PDMS-colloidal silica composite, irrespective of the medium with which the colloidal silica is dispersed into PDMS. A relatively lower intensity of PDMS-colloidal silica composites when compared to that observed for neat PDMS indicate that the packing of PDMS chain is affected due to their interaction with CS particles. Though, varying level of interactions between the PDMS and CS particles, depending on the dispersion medium with which CS particles are dispersed with the PDMS evident from other analyses, such variations are not distinctly evident from the present XRD data. 15

3.3.4 Microscopic analyses of PDMS composites The TEM images of 40 wt % CS containing PDMS composites obtained with the use of different dispersion media are shown in Figure 3.11. The analysis of the images suggests the presence of relatively larger aggregates in the case of CS-water and CS MeOH composites. A relatively good dispersion is evident for CS-IPA and CS-MEK based composites. The observed difference in dispersion for CS-water and CS-MeOH as compared to CS-IPA and CS-MEK can be explained based on their surface interaction with PDMS. The poor miscibility of polar dispersion media with such as water and Figure 3.11 TEM micrographs of cured PDMS-CS composites: (a) CS-Water 40 (b) CS-MeOH 40 CS-IPA 40 and (d) CS-MEK 40. (c) methanol with the non-polar PDMS and their sorption efficiency on the colloidal silica during their removal potentially lead to more the filler-filler interactions, resulting in the formation of aggregates. It is well known that the secondary structure such as aggregates and agglomerate formed by the hydrogen bonding between silanol groups on silica particles Error! Bookmark not defined.,error! Bookmark not defined.,error! Bookmark not defined. and that can vary depending on the size and surface chemistry of silica. The observed improvement in dispersion of CS-IPA and CS- MEK composites can be attributed to the relatively nonpolar nature of dispersion medium (IPA and MEK) and their solubility with PDMS leading to better polymer-filler interaction and the possible functionalization of CS as evidenced from % carbon analysis, FTIR data and TGA measurements. 3.3.5 Mechanical properties of PDMS-CS composites The Figures 3.12, 3.13 and 3.14 represent the modulus, tensile strength and elongation at break of PDMS-CS composites, respectively. Figure 3.12 presents normalized modulus of PDMS-CS composites with that of neat PDMS. The normalized modulus of composites with the use of different CS dispersions remained unchanged up 16

to 30 wt % of CS loading, irrespective of the dispersion medium with which CS is dispersed into the PDMS matrix. However, a significantly higher elastic modulus is evident for the composite with 40 wt % CS loading. The normalized modulus is also found to be noticeability different for the composite prepared with the use of different CS dispersions at 40 wt % CS loading, in the following order CS-MEK > CS-IPA > CS- MeOH > CS-water. The Figure 3.13 compares the tensile strength of all PDMS-CS composites normalized to neat PDMS. Irrespective of the type of dispersions of CS, the tensile strength PDMS-CS composites showed increase with increased CS loadings. Figure 3.12. Normalized modulus of PDMS-CS composites with different loadings of CS. The extent of increment, however, varies with the dispersion media. In particular, the PDMS-CS composites with CS-MEK and CS-IPA possessed improved tensile properties, as compared to CS-Water and CS-MeOH based composites. The Figure 3.14 presents the normalized elongation at break (% E) values, obtained with the use of different dispersion media. Though a linear trend is found in the case of CS-water based composites, the composites with CS-IPA showed a higher % elongation values up to 20 wt % loading of CS. However, a further increment in CS loading resulted in lower % E values. The CS-MEK composite showed increased elongation up to 30 wt % of CS loading, beyond which the % E is found to decrease. In general, a greater amount of network chain inhomogenities and higher network strains can prevail in the composites with higher loading of fillers, due to which these composites can have different 17

elongation at break values. From the present study, it is evident that the dispersing medium has profound influence on the amount and the nature of inhomogenities and network strains. The increased modulus, tensile strength and %E values in case of CS- IPA and CS-MEK based composites can be rationalized to the better dispersion of CS- IPA, CS-MEK in PDMS facilitated by the improved polymer filler interaction derived from the partial surface modification of CS-MEK and CS-IPA. The optimal loading of CS to achieve optimal reinforcement varies with the type of dispersing media as evidenced from the present study. The stress-strain curves obtained for 30 wt % loading of CS obtained with different dispersion media are also analyzed (Figure 3.15) to understand reinforcement behavior that exist in the PDMS-CS composition. Figure 3.13. Normalized tensile strength of PDMS-CS composites at different loading of CS. The stress-strain curves show pronounced increase of stress followed by nearly linear region, with the increase of strain. All PDM-CS composites shows higher stress values compared to neat PDMS indicating a mechanical reinforcement. A relatively higher stress-strain value observed for CS-IPA and CS MEK based composites indicates a better reinforcing ability of CS in these cases. To get more insights on PDMS-CS interfacial interactions, the tensile fractured PDMS-composites with 30 wt % CS are examined under the SEM. From the SEM image analysis (Figure 3.16 a & b), it can be seen that the CS-Water and CS-MeOH reinforced composites exhibit mostly push out fractures with many agglomerates indicating a poor filler-pdms interaction in these 18

cases. Whereas, both CS-MEK and CS-IPA based composites (Figure 3.16 c & d) show a wavy shear banding of the matrix, indicating considerable energy absorption in these Figure 3.14. Normalized elongation at break of PDMS-CS composite with different loading of CS. Figure 3.15. Tensile stress strain behavior of 30 wt % CS loaded PDMS-CS composites. specimens during tensile test. The fractographs also show an improved interaction between the CS and PDMS, as indicated by uniform distribution of CS-MEK and CS-IPA within the PDMS matrix. Figure 3.17 and 3.18 presents normalized tear strength and hardness data of PDMS-CS composites along with those of neat PDMS respectively. Both tear strength and hardness data suggests that both the parameters increase with increasing loading of CS, due to the increased ability of elastomer to dissipate the strain energy near the tip of growing cracks. The extent of increment is relatively significant in 19

cases of the composites prepared from the IPA and MEK dispersions of CS, indicating a better interfacial interaction of CS with PDMS, resulting from more hydrophobic nature of dispersion medium and the possible functionality on the surface of CS. Figure 3.16. SEM micrographs of tensile fracture surface of composites: (a) PDMS-CS-water composite (b) PDMS-CS-MeOH composite (c) PDMS-CS-IPA composite (d) PDMS-CS-MEK composite. Further, the improved CS-PDMS interactions could also enhance the ability to deflect or arrest crack growth resulting in improved tear strength. The lower tear strength values observed for the composites made with the use of CS-water and CS-MeOH dispersions can be attributed to their higher surface energies resulting in higher filler-filler interaction, as indicated by morphological and rheological results. 20

Figure 3.17. Normalized shore A hardness values of PDMS-CS composites with different loading of CS. Figure 3.18. Normalized tear strength of PDMS-CS composites with different loadings of CS. 21

3.4 CONCLUSIONS In the present study, PDMS-CS composites containing varying loading of colloidal silica are prepared with the use of different CS dispersions and studied for their thermal, rheological and mechanical properties. Both the filler-filler and the filler-polymer interactions are evident in these composites and the extent of these interactions is found to vary with the surface modification and the dispersion medium with which CS is dispersed into the PDMS matrix. An enhanced filler-filler interaction is evident in the composites prepared from the CS-water and CS-MeOH thereby resultant PDMS composites possessed lower mechanical properties. The composites prepared with the use of CS-IPA and CS-MEK, resulted in a better dispersion of CS in PDMS, leading to the PDMS composites with improved mechanical properties. The result of this study underlines the importance of choosing an optimal CS dispersion while preparing composites. Typically, the CS particles are suitably surface modified in order to obtain a stable dispersion. Hence, the observed variation of properties of PDMS-CS composites with the use of different dispersions could arise from both the nature of dispersion media (polar or non-polar) and the nature/extent of surface modifications. The analysis of thermal, rheological, mechanical and spectral data strongly suggests that these properties can be altered by varying dispersion mediums with which CS is dispersed and loadings of CS. These results thus showed that it is possible to adjust both the parameters to prepare suitable composite materials with improved physical characteristics. The influence of surface modification of CS on the properties of resulting PDMS-CS composites need to be understood thoroughly. The following chapter (chapter four) provides a detailed account on the effect of different surface treatments of CS on the properties of PDMS-CS composites. 22

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