NOVEL IN SITU SILICA/POLYDIMETHYLSILOXANE NANOCOMPOSITES: FACILE ONE-POT SYNTHESIS AND CHARACTERIZATION

MATERIALS

Octamethylcyclotetrasiloxane [(CH₃)₂SiO]₄) (D₄), supplied by Momentive Performance Materials, Bangalore, India has a boiling point = 175 °C; η = 1.396; d = 0.955 (purity > 99%) (GC) which was freshly distilled before use. Tetraethylorthosilicate (TEOS), purchased from Fluka (purity > 98%), boiling point = 163–167 °C; d = 0.933) was used as received. Dibutyltin dilaurate (DBTDL) was received from Merck, Mumbai, India with a density of 1.055, which was used as received without any pre-reaction treatment. Potassium hydroxide was obtained from Merck, Mumbai, India. Toluene was procured from Merck, Mumbai, India.

SYNTHESIS OF HYDROXYL TERMINATED PDMS

Polymerization was performed under dry nitrogen atmosphere in a 250 mL three-necked flask with a system regulating simultaneously temperature and stirring rate of the reaction mixture. 15 g (0.05 mol) of octamethylcyclotetrasiloxane (D₄) was distilled to remove the Si–H and Si–OH containing species and was introduced in the flask. 0.08 g (0.5%) of KOH, used as the catalyst, was finely grinded in nitrogen atmosphere and was added to the same flask. The reaction was continued for 2 h at 140 °C with a stirring rate of 200 rpm. Polymerization was evidenced by rise in viscosity of the resultant mixture. The reaction mixture was gradually cooled to room temperature and was left undisturbed overnight prior to work up. Unfilled hydroxyl PDMS was synthesized in order to make a comparative study of its various properties with those of the silica-filled PDMS vulcanizates.

OPTIMIZATION OF THE AMOUNT OF POTASSIUM HYDROXIDE REQUIRED FOR THE FILLER GENERATION

It is difficult to determine the exact amount of base required for nanosilica generation from TEOS during the nanocomposite synthesis through the FTIR studies. This is due to merging of the spectrum of PDMS with that of nanosilica. In order to avoid this, synthesis of nanosilica was pursued in toluene as the solvent with identical conditions as those of nanocomposite preparation. Calculated amount of TEOS was dissolved in 20 mL toluene in a three-necked flask purged with dry nitrogen gas and calculated amount of water was added using a microliter syringe to the resultant mixture. (This amount of water added was estimated from the amount of water generated as by-product in the anionic ring opening polymerization of D₄ with KOH. Since moisture has a dominant role in the silica generation by the solgel process, this amount of water was added to rule out the effect of moisture in the process of silica generation). Temperature of the reaction bath was raised to 138–140 °C with gradual addition of KOH in minute quantities (this was done in order to maintain identical conditions with those of the nanocomposite preparation). Aliquots were extracted from the reaction mixture using a microliter syringe at regular intervals and were analyzed by the FTIR spectroscopy. This process was continued for a prolonged duration until FTIR studies revealed termination of the desired reaction.

PREPARATION OF IN SITU PDMS–SILICA NANOCOMPOSITE

Fifteen grams of D₄ (distilled in the similar way as that with pristine PDMS) was taken in a three-necked round-bottomed flask purged with dry nitrogen and 0.08 g of KOH was added to it. The reaction was initiated at a temperature of 140 °C and allowed to continue for 2 h. This was followed by addition of excess amount of KOH (as optimized from the FTIR analysis of the reaction of TEOS with KOH) and simultaneous addition of TEOS which led to immediate generation of silica in the reaction mixture. Amount of TEOS and accordingly amount of KOH was varied in order to increase the concentration of the filler in the generating matrix and, thus, preparing samples with higher filler loading. The temperature was gradually reduced to 75 °C and the reaction was allowed to continue for another 1 h. This was followed by removal of the resulting by-product (ethanol) from the reaction mixture by distillation and was continued until stoichiometric amount of ethanol got collected. The resultant viscous mixture was left overnight. The polymeric part of the synthesized nanocomposites was found completely soluble in toluene. This suggested that the nanocomposites obtained by this method were uncrosslinked materials. Table 1 is a compilation of the sample designation along with their compositions.

TABLE I COMPOSITION OF THE SAMPLES WITH DESIGNATIONS.

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CURING OF SILICA/PDMS NANOCOMPOSITES

Thirteen grams of the nanocomposite obtained was stirred in 20 mL of toluene for 15 min to make a homogeneous solution. To this, 0.5 g of TEOS was added and the resulting mixture was stirred using a magnetic stirrer for a few minutes. This was followed by addition of 0.025 g of DBTDL and constant stirring for 1 min. The final mixture was degassed for 10 min to remove unwanted gaseous volatiles, cast in a Teflon petri dish, and left undisturbed overnight. This resulted in a transparent sheet of ∼0.5 mm thickness which was ultimately vacuum dried at 80 °C.

CHARACTERIZATION

Fourier Transform Infrared Spectroscopy. — FTIR studies were carried out with aliquots from the reaction mixture using PerkinElmer FTIR spectrophotometer (model spectrum RX I), within a range of 400–4400 cm⁻¹ using a resolution of 4 cm⁻¹. An average of 16 scans was acquired for each sample.

Number Average Molecular Weight (M_n). — Number average molecular weight was determined by using ²⁹Si nuclear magnetic resonance (NMR) spectra of neat vinyl end-capped PDMS. ²⁹Si NMR spectra were taken using Bruker AM-360, 400 MHz NMR spectrometer with samples prepared having concentration of 50% w/v in CDCl₃ solution. The internal standard used was tetramethylsilane (TMS) and paramagnetic relaxation agent such as chromium acetylacetonate was added to ensure exact integration. A heteronuclear gated decoupled pulse sequence (NONOE) was used to acquire the ²⁹Si NMR spectra.

Transmission Electron Microscopy. — The samples for TEM analysis were prepared by ultracryomicrotomy with a Leica Ultracut UCT (Leica Microsystems GmdH, Vienna, Austria). Freshly sharpened glass knives with cutting edges of 45° were used to obtain cryosections of about 100–150 nm thickness at −150 °C. These cross sections were collected in sucrose solution and directly supported on a copper grid of 300 mesh. Microscopy was performed with JEOL 2100, Japan in which the electron microscope was operated at an accelerating voltage of 200 kV.

Mechanical Properties. — Tensile specimens were punched out from the solution cast sheets using ASTM Die- C. The tests were carried out on a Zwick UTM, Model – Z010 (Zwick GmbH and Co., Ulm, Germany) using a strain rate of 50 mm/min at 25 °C (ASTM D 412) at a cross-head speed of 500 mm/min at 25 °C. The average of three tests is reported here.

Dynamic Mechanical Analysis. — The dynamic mechanical spectra of the samples were obtained by using the dynamic mechanical analysis (DMA) of TA instruments (model Q800). The sample specimens were examined in tensile mode at a constant frequency of 1 Hz, a strain of 0.05%, and a temperature range from −130 to 50 °C at a heating rate of 2 °C/min. The data were analyzed by the TA Universal analysis software on a TA computer attached to the machine. Storage modulus (E′) and loss tangent (tan δ) were measured as a function of temperature for all the samples under identical conditions. The temperature corresponding to the peak in tan δ versus temperature plot was taken as the glass–rubber transition temperature (T_g).

Thermogravimetric Analysis. — The thermogravimetric analysis (TGA) analyses were carried out in TA instruments (model Q50), at the heating rate of 20 °C/min under air atmosphere up to 800 °C. The data were studied by the TA Universal analysis software on a TA computer attached to the machine. A small amount of material (around 5 mg) was used for the TGA study.

Wide Angle X-Ray Diffraction Studies. — The wide-angle X-ray diffraction (WAXD) patterns of the samples were documented in a Philips X-ray diffractometer (model PW-1710) using crystal monochromated Cu K_α radiation in the angular range 10–70° (2θ) and at 40 kV operating voltage and 20 mA current.

The d value was obtained from the Bragg's equation

where, n = 1, λ = wavelength of the X-ray used, and θ = half the Bragg's angle.

The crystallite size was calculated according to the Scherrer equation[23]

where L_hkl refers to the size of the crystallites at reflection of hkl. K is the Scherrer factor and β is the full width at half-maximum of the peak.

SYNTHESIS OF HYDROXYL TERMINATED PDMS

The synthesis of hydroxyl PDMS (ref [24]) has been carried out according to the reaction scheme shown in Figure 1. The curing of the hydroxyl functional PDMS has been done with TEOS and DBTDL following the principle of hydrolysis and condensation.

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OPTIMIZATION OF THE AMOUNT OF REAGENTS AND REACTION CONDITIONS FOR IN SITU PDMS–SILICA NANOCOMPOSITE SYNTHESIS

Study of the Progress of Nanosilica Generation Using FTIR Spectroscopy. — The amount of base is critical in nanocomposite synthesis, since the catalyst switches off its role with concentration and has a different role to play at a higher concentration. The base, which is the catalyst in the filler generation, acts in an adverse way toward the filler as its inherent character as a base dominates over its catalytic activity after a certain critical concentration. This concentration, undoubtedly, also depends upon the concentration of the precursor. Thus, concentration of the base required is essential for complete filler generation. In this study, the FTIR spectroscopy has been used strategically to estimate this critical concentration of the base. From the nature of the spectra obtained, the change in role of the base with concentration is evidently explicable. The peak at 3500 cm⁻¹ in Figure 2 shows an increase in the intensity which is the result of a collective increase in the amount of silanol functionality and alcohol formed and, thus, cannot be used in estimating, from quantitative aspect, the extent of silanol groups. However, the peaks centered at 968 cm⁻¹ and 793 cm⁻¹ reveal the extent of the silanol groups generated. With the increase in concentration of the catalyst, the peaks exhibit an increase in intensity initially, reach a maximum, and show a gradual decrease with further catalyst addition. Even there is a gradual shift in position of the peaks with increase in the catalyst concentration, which, in fact, acts as a base after a critical concentration thereby consuming the nascent filler. The peak at 968 cm⁻¹ in the inset of Figure 2 does not shift its position up to F7 (7.57 × 10⁻³ mol KOH), but for F8 (8.01 × 10⁻³ mol KOH) it shifts to 943 cm⁻¹. This shift continues and eventually for F12 (14.2 × 10⁻³ mol KOH) in Figure 2, the peak shows its existence at 939 cm⁻¹. Similar shift is observed for the absorption at 793 cm⁻¹. This peak shows a prominent shift to 772 cm⁻¹ for F12. With further increase in the base concentration, these peaks gradually disappear. Shift in the position for the above peak is due to Si–O–K linkages owing to the formation of potassium silicate which is similar to the observation made by Bal et al.[25] The peak at 793 cm⁻¹ ultimately disappears due to complete conversion of the silica into potassium silicate. There is also a marked change in the position and nature of the peak at 1088 cm⁻¹ which corresponds to the asymmetric Si–O–Si stretching as shown in Figure 2. For TEOS (F1), the peak appears at 1088 cm⁻¹ but gradually shifts toward lower frequency. For F7, its intensity is highest and is positioned at 1076 cm⁻¹. Surprisingly, it shows a marked shift to 1067 cm⁻¹ for F8 which is probably due to formation of the Si–O–K linkages in potassium silicate. This extent of shift in the peak position continues in an abrupt way with peak broadening and the peak ultimately appears at 997 cm⁻¹ due to complete conversion to potassium silicate. A comparative study on intensity of the peak at 1080 cm⁻¹ in Figure 3 shows that there is no significant change in its intensity. This is because the number of Si–O–Si bonds remains unchanged, though the nature of the product may vary: it may be the Si–O–Si bond of the precursor TEOS, of silica, or even that of potassium silicate.

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Another important aspect that needs to be discussed here is the nature of the silica formed in terms of polymorphism to its crystalline forms. The FTIR spectroscopy has been subtly used to detect the phenomenon of polymorphism in the case of nanosilica synthesis. Analysis of the standard amorphous nanosilica shows a broad peak at 1100 cm⁻¹ with a shoulder at 900 cm⁻¹. However, the crystalline polymorph of silica (quartz)[26] exhibits a prominent peak at 1100 cm⁻¹ along with a prominent shoulder at 1150 cm⁻¹. Surprisingly, the shoulder at 900 cm⁻¹ is found to have minimized and ultimately disappeared for the latter. Initially, the intensity of the shoulder is found to increase with an increase in the concentration of the base. The intensity of this shoulder reaches its maximum for the critical base concentration. This is followed by a gradual decrease owing to decrease in the amount of amorphous filler. The shoulder at 900 cm⁻¹, which is the characteristic of amorphous silica, increases in intensity initially due to an increase in the amount of silica generated. Once silica formation is complete, polymorphism to crystalline form (quartz) or some other metastable state is initiated. Amorphous silica has been reported to undergo polymorphism under several conditions, such as temperature, pressure, run duration, presence of any mineralizer (such as bases), and the primary form of silica.[27,28] In this process of silica generation, concentration of base is varied and that too at high temperature. The obvious consequence of this is polymorphism of silica. This has been justified by the WAXD analysis of the nanocomposites in the subsequent section.

Results of Optimization of Amount of Reagents and Reaction Conditions. — From the FTIR study of TEOS hydrolysis and silica generation, it is found that for complete hydrolysis of 4.46 × 10⁻³ mol TEOS, 7.57 × 10⁻³ mol KOH is required. In the similar way, concentration of the base is determined for varying amounts of TEOS which is summarized in Table 1. Nanocomposites were synthesized using the reagents following the method mentioned in the experimental section.

CHARACTERIZATION OF THE SYNTHESIZED PDMS

NMR Analysis and Determination of Number Average Molecular Weight (M_n). — Figure 4 shows the ²⁹Si NMR spectra of synthesized hydroxyl PDMS. The signals at 0, −10, and −21 correspond to TMS, silanol, and Si–O–Si (backbone), respectively. Based upon the analysis in our previous work,[29] it is found that the synthesized PDMS has a degree of polymerization of 802 and that its M_n is ∼60 000. M_n value remains same for the nanocomposites.

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CHARACTERIZATION OF THE NANOCOMPOSITES

TEM Analysis. — Figure 5 shows the transmission electron micrograph of 2 phr TEOS-loaded PDMS vulcanizate. The individual nanosilica particles are found to be well dispersed in the matrix domain. However, image analysis has been done using the Image J software for precision. Image analysis has been initiated by setting the scale for measurement. The distance in pixels has been converted into known distance (say 100 nm). This is followed by background subtraction where a “sliding paraboloid” or a legacy “rolling ball” algorithm has been used for correction of uneven illuminated background. The image has been subjected to smoothening to remove undesired noise and then converted to a threshold image or 8 bit binary image. This shows clearly the distribution of the dispersed phase in the matrix domain as shown in Figure 5. This has been followed by selection of a particular region within the scanned area using the rectangular tool (displayed by the black hollow rectangular box in the figure) and analyzing the same by studying the plot profile. Plot profile is a display of the variation in gray value as a function of distance in terms of the scale set for measurement. For the selected area in Figure 5, the corresponding plot profile is shown in Figure 5. The peaks in Figure 5 correspond to the particles dispersed in the matrix where the length of the base of each peak corresponds to the diameter of the individual particles since the matrix possesses a zero gray value. The nature of the peaks gives an indirect evidence for the spherical shape of the filler. Measurement of these values results in a mean diameter of 15 nm.

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Mechanical Properties. — The mechanical properties show tremendous improvement with in situ filler generation. Maximum tensile strength is observed for PD 2T with minimum filler concentration (Table 2). This is followed by gradual decrease in tensile stress with increase in the amount of TEOS. Elongation at break shows a twofold increase for PD 2T but goes on decreasing with higher amount of TEOS, though it is higher for samples with low-to-moderate loading of the precursor. It is known that greater extent of interaction between the generated filler and the matrix enables stress transfer to the dispersed phase. The seminal factor for tensile strength is the filler particle size since it is the same that determines the interfacial area per unit volume. This shows that with increasing amount of the precursor and hence the filler, agglomeration is a prominent phenomenon.[30] Agglomerates may prevent macromolecular chain movements thereby leading to this decrease. Figure 6 shows how tensile strength changes with increase in mean particle size of the nanoparticle which is obtained from the TEM analysis. Though tensile strength decreases with an increase in the filler concentration beyond 2 phr in the matrix, the modulus shows a gradual increase. This is evident from the results compiled in Table 2. If there is a good matrix–filler adhesion, the fracture, instead of going through a direct path, goes from particle to particle, and consequently, the composite shows a greater elongation at break (EAB).[31] This phenomenon happens in the case of high extent of interaction between the two phases. In the case of high filler concentration, the filler preferentially agglomerates. The latter being stress concentrators reduces the elongation at break.[32]

TABLE II COMPARISON OF THE MECHANICAL PROPERTIES OF THE NANOCOMPOSITES.

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DMA Studies. — Figure 7 shows the storage modulus plot as a function of temperature. A unique nature of the plot is observed for the nanocomposites that follow altogether a different trend when compared with that of the unfilled elastomer. Storage modulus (E′) values when compared at different temperatures provide an exquisite but exceptional trend. At −130 °C, the nanocomposites exhibit a greater magnitude of E′ compared to the virgin elastomer. However at T_g, E′ for the nanocomposites shows a drastic fall. This follows a decent rise in E′ followed by a gradual and smooth fall resulting in a hump. Quite interestingly, the size of the hump gradually decreases with increase in the filler loading. The storage modulus values for the unfilled and filled samples at different temperatures are compiled in Table 3. From the plot (Figure 7) it is clearly understood that in the glassy region nanocomposites exhibit higher modulus, which increases with increase in filler loading. But once the T_g is reached, there is a catastrophic fall in the storage modulus. This is probably due to the decrease in number as well as size of the crystalline domains in the nanocomposites. There is a decrease in the size of the crystalline domains which act as physical crosslinks imparting strength to the matrix.[33] From the values of E′ for the virgin elastomer, it is beyond doubt that the crystalline domains contribute appreciably to the modulus of the matrix.[34] Comparison of the storage modulus of the nanocomposites shows that magnitude of the storage modulus (at −70 °C) decreases gradually with filler loading. This is due to gradual decrease in crystalline domain size or percentage crystallinity of the polymer matrix. However, with increase in temperature, the storage moduli of the nanocomposites are found to be much higher compared to the neat rubber which is due to the reinforcing effect of the filler on the elastomeric matrix.[35]

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TABLE III COMPARISON OF THE E′ AT DIFFERENT TEMPERATURES FOR UNFILLED AND FILLED PDMS VULCANIZATES.

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Similarly, the plot of tan δ versus temperature in the inset of Figure 7 shows a unique prototype. With increase in the filler content, the height of the peak at T_g shows a regular increase. This observation is exceptional and has not been reported till date. However, there is no shift in T_g suggesting that amorphous regions in the polymer are unaffected. In this case, height of peak at T_g increases probably due to decrease in size of crystalline domains and decrease of crystallinity of the matrix. This is by virtue of the method of synthesis of the in situ nanocomposites.

TGA Analysis. — Figure 8 shows the TGA and differential thermogravimetric (DTG) thermograms of the unfilled and in situ silica-filled nanocomposites. Both the temperature of onset of degradation (T_i) and the temperature of maximum degradation (T_max) increase with an increase in the silica content. Thus, oxidative thermal stability increases for PDMS vulcanizates on filler incorporation and the extent of improvement increases with increasing silica concentration. T_max increases by 12 °C, 28 °C, 79 °C, 97 °C, and 20 °C, while T_i increases by 5 °C, 14 °C, 36 °C, 57 °C, and 27 °C for 2, 4, 6, 8, and 10 phr TEOS-filled nanocomposites, respectively. In addition to this, rate of degradation also shows a drastic decrease from 39.8%/min for PD 0T to 9.5%/min for PD 10T.

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It is found that the mode of degradation changes with the changing filler amount. For lower filler content, there is a single step sharp degradation. But from 6 phr TEOS-loaded sample, it changes to a two-step degradation process as observed from the DTG plots (inset of Figure 8). The literature shows that the degradation pattern of PDMS depends upon the heating rate.[36] For lower heating rates, the first stage degradation takes place at 340 °C, whereas the second begins at a comparable temperature (400 °C) with major products consisting of a mixture of oligomers, CO₂, and water. The lower temperature of first maximum degradation is assumed to be due to the catalytic role played by oxygen in depolymerizing PDMS into volatile cycle oligomers. Above 400 °C, breakdown of crosslinked structures takes place by molecular splitting of the cyclic oligomers at a higher rate. At a higher heating rate, only one hump is observed in the DTG curve.[37] This explanation can be subtly used to explain the results obtained here. If the rate of degradation is considered, it gradually decreases with the increasing filler content. When rate of degradation is high, the products of first step degradation have minimal stability to be noticed in the DTG plot. In other words, the double hump corresponding to the two-stage degradation merges to give a single hump. However, with increase in filler content, the thermo-oxidative stability of the nanocomposites increases. This factor reduces rate of degradation for the materials, which imparts thermal stability to the volatile products of the first degradation step. Hence, in these cases, prominent double-hump features are visible in the DTG plots. Thus, the thermal stability of the nanocomposites is explained.

Wide-Angle X-Ray Diffraction Studies. — The WAXD patterns of the neat PDMS vulcanizate and that of the nanosilica-filled systems are shown in Figure 9. From the plots it is quite evident that analysis has to be made in two aspects: one regarding the change in crystallinity of matrix material in the nanocomposites compared to the unfilled one, and the other on the polymorphic modification of the filler.

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PDMS shows a sharp reflection at 12.5° corresponding to the centered tetragonal unit cell.[38] This study marks a gradual shift in the position of this peak toward lower 2θ value. While for the unfilled vulcanizate, the peak appears at 12.5°, it shifts to 12.1° for 8 phr TEOS-filled system. Moreover, the peak undergoes a decent broadening with an increasing amount of in situ generated filler. This change in the peak corresponds to a decrease in the size of the crystalline domains present in the polymer. However, no new reflection appears suggesting that packing structure or the unit cell pattern is not affected.[38] Factors such as dislocations and point defects are responsible for this change in the XRD pattern. The d values and crystallite size are calculated using eqs 1,2, respectively. The domain size is found to reduce from 2.53 nm for PD 0T to 1.85 nm for PD 8T as shown in Table 4. This explains the drastic fall in low-temperature storage modulus for the nanocomposites.

TABLE IV DETERMINATION OF d-SPACING FOR THE REFLECTION AT 110 PLANE AND CRYSTALLITE SIZE.

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Next comes the characterization of the filler in terms of crystallinity since some additional peaks are detected in the plots for nanocomposites which are absent in the unfilled sample and that too their numbers and positions vary with the increasing concentration of the filler. Fenner constructed the interrelationship between the six polymorphic forms of silica (both low- and high-temperature modifications), viz., quartz, tridymite, and crystobalite.[39] Besides these forms, there exist a number of metastable phases. The skeletal structures of quartz, tridymite, and crystobalite consist of tetrahedral SiO₄ units with different pattern of connectivity and orientation in space. The resembling structures of the latter two are from the six-membered rings of the tetrahedral SiO₄ units, which are connected in planar networks.[40] The variation of the modes of connecting layers is the root cause of the polytype modifications of tridymite. In contrast to the structure of crystobalite and tridymite, the main structural constituent of quartz is the spiral chain of SiO₄ tetrahedra along the threefold or sixfold axes leading to a three-dimensional skeleton formation on condensation.[40] The presence of mineralizers accelerates the process of transformation and even decreases the temperature for this process.[27] It has been reported that presence of OH⁻ ions in various concentrations has a parallel effect to that of pressure.[28] In fact, increase in rate of 100% conversion to quartz was observed with increasing hydroxyl concentration.[41,42]

In this study, it is found that with increasing concentration of the base, transformation of crystalline form of silica is predominant. For 2 phr loading of TEOS, the silica generated has a diameter of 15 nm and from the reflections in the XRD pattern of the nanocomposite, polymorphic form of silica is found to be similar to that of tridymite. This is in accordance with the observation of Wu et al. who reported that polymorphic modification of silica depends upon the particle size.[43] With an increase in the concentration of the precursor, the amount of catalyst increases and this results in a polymorphic form which is similar to that of quartz. This is due to the fact that with higher concentration of base the characteristic reflections which are signatures for the XRD pattern of quartz gradually intensify. It is found that the reflections at 2θ values 20.83°, 23.33°, and 25.37° for PD 2T, which are characteristics of the tridymite polymorph, are found to diminish for higher loadings of TEOS, while new reflections 50.67° and 65.15°, which are characteristics of quartz, gradually intensify. Thus, XRD clearly indicates that the base is first used in silica generation followed by polymorphic modification of silica, i.e., conversion from amorphous form to crystalline form.