Studying the Physical and Chemical Properties of Polydimethylsiloxane Matrix Reinforced by Nanostructured TiO2 Supported on Mesoporous Silica

In this study, a reactive adsorbent filler was integrated into a polymeric matrix as a novel reactive protective barrier without undermining its mechanical, thermal, and chemical properties. For this purpose, newly synthesized TiO2/MCM/polydimethylsiloxane (PDMS) composites were prepared, and their various properties were thoroughly studied. The filler, TiO2/MCM, is based on a (45 wt%) TiO2 nanoparticle catalyst inside the pores of ordered mesoporous silica, MCM-41, which combines a high adsorption capacity and catalytic capability. This study shows that the incorporation of TiO2/MCM significantly enhances the composite’s Young’s modulus in terms of tensile strength, as an optimal measurement of 1.6 MPa was obtained, compared with that of 0.8 MPa of pristine PDMS. The composites also showed a higher thermal stability, a reduction in the coefficient of thermal expansion (from 290 to 110 ppm/°C), a 25% reduction in the change in the normalized specific heat capacity, and an increase in the thermal degradation temperatures. The chemical stability in organic environments was improved, as toluene swelling decreased by 40% and the contact angle increased by ~15°. The enhanced properties of the novel synthesized TiO2/MCM/PDMS composite can be used in various applications where a high adsorption capacity and catalytic/photocatalytic activity are required, such as in protective equipment, microfluidic applications, and chemical sensor devices.


Introduction
Polydimethylsiloxane (PDMS) is an organosilicon elastomer with a silicon-oxygen backbone of a [-Si (CH 3 ) 2 O-] repeated unit, also known as silicone rubber [1,2]. PDMS's valuable properties include hydrophobicity due to the methyl side groups, transparency, gas permeability, flexibility, biocompatibility, high strain-to-failure, and the ability to restore its original shape when stress is removed, in addition to being easily fabricated and having a low cost [2]. Furthermore, it can be sealed to itself and other materials without any adhesive when it is plasma-oxidized [3]. PDMS has many applications in various fields. It can be used for protective coatings against corrosion, ice-retarding, anti-biofouling and fire retardant, [4] in micropump and microvalve systems [5][6][7], as membranes [8], as wearable stretchable fabric for sensing and physical protection [9][10][11], and for piezoelectric usage [12]. Nevertheless, PDMS has some drawbacks that limit its application as a structural material. It has a low Young's modulus and it is susceptible to organic solvents, which cause swelling and the deformation of its structural configuration. Moreover, PDMS has a high coefficient of thermal expansion (CTE) (i.e., it has a dimensional instability to temperature changes and is a highly hydrophobic material with low surface energy). To overcome PDMS's disadvantages and to adjust it to its various application fields, much research has been carried

TiO 2 /MCM/PDMS Preparation
Thick films (~0.5 mm thickness) of a pristine PDMS matrix, as well as composite PDMS films embedded with TiO 2 /MCM at the different weight percentages of 2.5, 5, 10, and 15 wt%, were prepared.
First, MCM-41 mesoporous silica was modified by inducing a high loading (45 wt%) of nanostructured titanium dioxide (TiO 2 ) particles via the controlled hydrolysis of titanium butoxide and condensation within the pores of MCM-41. The preparation procedure of the TiO 2 /MCM catalysts is described in detail in our previous work [11].
Pristine PDMS and composite films were prepared using a SYLGARD TM 184 Silicone Elastomer Kit. For example, in order to prepare a composite film with a 10 wt% of TiO 2 /MCM, 0.22 g of the additive powder was sonicated in 2.5 mL of hexane for 2 h. The suspension was mixed with the PDMS precursors (2 g of the monomer and 0.2 g of the curing agent) and was poured into a polystyrene 6-well plate (9.61 cm 2 area) that had been degassed in a desiccator for 1 h to eliminate trapped air bubbles. Curing and drying were performed along the following temperature ramp: 30 • C for 1 h, 50 • C for 1 h, 70 • C for 1 h, and 85 • C for 3 h. At least three replicates were tested for each sheet. The obtained sheet thickness was 0.5 mm ± 0.01 mm. For comparison, pristine PDMS films with the same dimensions were also prepared.
Hereinafter, the pristine PDMS film is denoted as PDMS and the PDMS films embedded with different loadings of TiO 2 /MCM-41 are denoted as 2.5, 5, 10, and 15 wt% of TiO 2 /MCM/PDMS.

Characterization Methods
The chemical structure and composition of the samples were measured using attenuated total reflectance Fourier transform infrared (ATR-FTIR) and Raman spectroscopy. The infrared analysis was carried out with an ATR diamond crystal accessory using an INVENIO S FTIR spectrometer (Bruker, Bremen, Germany) equipped with a DGTS detector. FTIR spectra were recorded within the wavenumber range from 4000 cm −1 to 400 cm −1 at a 4 cm −1 resolution. The Raman spectra were recorded on a Malvern Morphologi 4-ID microscope (Malvern, UK) using a 785 nm wavelength excitation laser. The spectra were measured with a 1800 lines/mm grating monochromator with a spectral resolution of 4 cm −1 . The objective magnification was ×5 with an exposure time of 20 s under low-power conditions. Raman spectral analysis was carried out by creating a baseline. The baseline correction used asymmetric least squares smoothing to remove the effects of fluorescence, photoemission, photoluminescence, and other additive features in the spectra. The spectra were then normalized by dividing the baseline spectra by the 1414 cm −1 PDMS band, which removed uncertainties in regard to the laser-intensity fluctuations. Scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) were performed by using the Phenom ProX desktop SEM. Samples' images were obtained at 15 kV under low-vacuum conditions. The mapping of elements (Si, O, C, Ti) was carried out via cross-sectional imaging of the thick-film samples. The samples' mechanical properties were measured using the TA Instruments DMA Q800 in tension mode. For each material, a set of 5 film samples with dimensions of~0.5 mm × 2.5 mm × 20 mm were placed under a load control of 5 N/min at room temperature, and the displacement response was measured. Young's modulus was calculated from the slope of the stress-strain curve up to a 40% elongation to ensure analysis consistency for the samples' elastic region. The toughness was calculated based on the area under the stress-strain curve. The thermal properties of the samples were measured via differential scanning calorimetry (DSC) on a DSC823e from Mettler Toledo (Greifensee, Switzerland). The samples (~20 mg) went through three thermal cycles. During the first cycle, the samples were heated from −150°C to 100°C at a rate of 10°C/min, followed by 5 min of isothermal holding. During the second cycle, the samples were cooled from 100°C to −150°C at the same rate. Throughout the last cycle, the samples were heated in a similar manner as the first cycle. The glass transition temperatures (T g ) were measured for all samples during the last heating cycle. The normalized specific heat capacity (∆Cp n ) is calculated as follows [27]: where ∆Cp is the change in the specific heat capacity during the glass transition and x is the mass fraction of the adsorbent. In order to measure the CTE, film samples with the dimensions of~0.5 mm × 2.5 mm × 20 mm were placed in the TA Instruments DMA Q800 under a tensile loading of 0.1 N (about a 0.08 MPa stress level), and heated from room temperature to 150°C at a rate of 5°C/min. The samples' CTE was calculated based on the displacement-temperature slope in the temperature region of 45-70°C. Thermogravimetric analysis (TGA) was carried out on an STA6000 from PerkinElmer (Waltham, MA, USA). The samples (40 mg) were first equilibrated for 5 min at 30 • C, and then heated to 800 • C at a heating rate of 10 • C/min under a nitrogen atmosphere (60 mL/min). Peak degradation temperatures were found using the first-derivative curves.
Solvent swelling measurements were carried out by immersing pristine and composite PDMS samples (~0.5 mm × 5 mm × 20 mm) in a sealed vial that contained 5 mL of aqueous solution or toluene at room temperature for 3 h, until equilibrium was reached. The samples were wiped and weighed. The mass swelling ratio (R m ) was calculated based on the mass changes before and after swelling.
The sessile droplet method was used to measure the contact angle for each sample via the DSA 25drop shape analyzer from KRUSS. For each measurement, a 5 µL volume of either toluene or distilled water was placed on the surface. The procedure was repeated at least 5 times and the averaged values were reported. The optical properties of the film samples were observed through a UV-2401 PC spectrophotometer (Shimadzu, Kyoto, Japan) equipped with the ISR-2200 integrating sphere model. Spectra were recorded at room temperature between 200 and 800 nm by using a halogen lamp with a range of 400-800 nm and a deuterium lamp with a range of 160-400nm. The sample thickness was 0.5 mm.

Results and Discussion
Nanosized TiO 2 particles (~4 nm) were produced inside the pores of MCM-41 at a high loading of 45 wt% while preserving the texture and structure of the support. The surface area (520 m 2 /g) and pore volume (0.26 cm 3 /g) of the reactive adsorbent remained sufficiently high for adsorption and catalysis capabilities. The structural and textural properties of the TiO 2 /MCM adsorbent have been described in detail in our previous work [11].

Chemical Structure and Composition
ATR-FTIR spectroscopy was used to study the formation of bonds in the pristine PDMS and in the composite materials ( Figure 1). The FTIR of all the samples showed excellent agreement with the previous FTIR results for PDMS [18,[28][29][30][31][32][33]. No additional peaks were observed for the composite materials, suggesting that no chemical bonds were formed between the adsorbent and the PDMS matrix [34]. A peak at~2140 cm −1 , corresponding to Si-H stretching, was not observed for any sample, as shown in Figure 1, verifying that a hydrosilylation crosslinking reaction occurred and that a similar high crosslink density was achieved for all samples [31,32]. The peaks at 2965 cm −1 correspond to C-H stretching, the peaks at 1410 cm −1 correspond to -C-H bending, and the peaks at 1260 cm −1 correspond to the -CH 3 deformation in Si-CH 3 . The observed peaks at 1023 and 1082 cm −1 are attributed to Si-O-Si flexing and stretching vibrations, respectively. Si-C bands and rocking peaks for Si-CH 3 were observed in the 825-865 and 785-815 cm −1 regions, respectively [33]. There is a higher absorption below 700 cm −1 , compared to in the pristine PDMS, due to the broad peaks at~660 cm −1 that correspond to Ti-O-Ti stretching in TiO 2 anatase crystals [35].
Raman spectroscopy was used as a complementary method for the study of the chemical structure ( Figure 2). The Raman vibration modes of the pristine PDMS were found to be 488, 614, 687, and 707cm −1 , as seen in Figure 2 for numbers 1, 2, 3, and 4, respectively. These vibrations were also found in previously published data [19,36,37], and they correspond to Si-O-Si symmetric stretching, Si-CH 3 symmetric rocking, Si-CH 3 symmetric rocking, and Si-C symmetric stretching. The vibration band at 787 cm −1 (number 5 in Figure 2) is attributed to both CH 3 asymmetric rocking and Si-C asymmetric stretching. bands are found for oxides [38], and the appearance of new vibration bands at 336 and 642 cm −1 can be attributed to the Ti-O anatase [39,40]. The 534 cm −1 band is assigned to the delocalized vibrational modes arising from nonbridging oxygen in the silica network [41].
Polymers 2023, 15, x FOR PEER REVIEW PDMS, due to the broad peaks at ~660 cm −1 that correspond to Ti-O-Ti stretching anatase crystals [35]. Raman spectroscopy was used as a complementary method for the stud chemical structure (Figure 2). The Raman vibration modes of the pristine PDM found to be 488, 614, 687, and 707cm −1 , as seen in Figure 2 for numbers 1, 2, 3 respectively. These vibrations were also found in previously published data [1 and they correspond to Si-O-Si symmetric stretching, Si-CH3 symmetric rocking symmetric rocking, and Si-C symmetric stretching. The vibration band at 787 cm ber 5 in Figure 2) is attributed to both CH3 asymmetric rocking and Si-C asy stretching. With the addition of TiO2/MCM, higher intensity levels of the 405, 418, cm −1 bands are found for oxides [38], and the appearance of new vibration band and 642 cm −1 can be attributed to the Ti-O anatase [39,40]. The 534 cm −1 band is a to the delocalized vibrational modes arising from nonbridging oxygen in the si work [41].  Raman spectroscopy was used as a complementary method for the study of the chemical structure ( Figure 2). The Raman vibration modes of the pristine PDMS were found to be 488, 614, 687, and 707cm −1 , as seen in Figure 2 for numbers 1, 2, 3, and 4 respectively. These vibrations were also found in previously published data [19,36,37] and they correspond to Si-O-Si symmetric stretching, Si-CH3 symmetric rocking, Si-CH symmetric rocking, and Si-C symmetric stretching. The vibration band at 787 cm −1 (num ber 5 in Figure 2) is attributed to both CH3 asymmetric rocking and Si-C asymmetric stretching. With the addition of TiO2/MCM, higher intensity levels of the 405, 418, and 687 cm −1 bands are found for oxides [38], and the appearance of new vibration bands at 336 and 642 cm −1 can be attributed to the Ti-O anatase [39,40]. The 534 cm −1 band is assigned to the delocalized vibrational modes arising from nonbridging oxygen in the silica net work [41].

Mechanical Properties of the TiO 2 /MCM/PDMS Composite Materials
The stress-strain curves for all TiO 2 /MCM/PDMS composites were measured using the DMA under a controlled load. A representative example for each material is presented in Figure 3.

Mechanical Properties of the TiO2/MCM/PDMS Composite Materials
The stress-strain curves for all TiO2/MCM/PDMS composites were measur the DMA under a controlled load. A representative example for each material is p in Figure 3. The tensile stress-strain curves show a higher elongation at break with the of a filler, and a maximum value of 150 MPa for the toughness of 10 wt% TiO2/M active adsorbent was obtained, as compared to a value of 62 MPa for the pristin The toughness further decreases as the TiO2/MCM content increases from 10 w wt%. The characteristic nonlinear strain-hardening effect disappears in the plast of the composite after the addition of a > 5 wt% filler. This effect is due to the fille interactions limiting the chains' movements.
Based on the results obtained from the tensile testing of the composite mate mechanical properties were evaluated, including the Young's modulus ( Figure 4 imum strain (Figure 4b), and tensile strength ( Figure 4c). For the pristine PD Young's modulus, tensile strength, and maximum strain are 0.81 MPa, 1.54 M 115.39%, respectively. Pure PDMS usually has a Young's modulus between 1.32 MPa, and a tensile strength from 3.51 to 5.13 MPa. These differences can be expl the curing conditions (humidity, curing agent ratio, temperature, etc.), which ha shown to have a significant effect on the final mechanical, thermal, and physical ties of PDMS [42]. With the incorporation of TiO2/MCM, the mechanical propert enhanced significantly, as all samples had a higher elastic modulus, tensile streng imum strain, and toughness compared to the pristine PDMS. Incorporation of the TiO2/MCM into the PDMS polymer produced a maximum strain with the hig provement of 134% without undermining the maximum Young's modulus (1 which was doubled compared to that of the pristine PDMS, and significantly in the tensile strength of the material (2.4 MPa). Above a 10 wt% of TiO2/MCM, modulus and the tensile strength were reduced. Increasing the adsorbent conce resulted in more contact sites and higher interfacial interactions between the chains and the TiO2/MCM particles. Consequently, the stiffness and the streng composites were increased due to the improved stress transfer from the PDMS m the adsorbent [43]. The decrease in tensile properties above a 10 wt% is due to mation of bigger aggregates that reduce the dispersion of the filler in the PDMS leading to a reduction in stress transferring efficiency. This finding is supporte The tensile stress-strain curves show a higher elongation at break with the addition of a filler, and a maximum value of 150 MPa for the toughness of 10 wt% TiO 2 /MCM reactive adsorbent was obtained, as compared to a value of 62 MPa for the pristine PDMS. The toughness further decreases as the TiO 2 /MCM content increases from 10 wt% to 15 wt%. The characteristic nonlinear strain-hardening effect disappears in the plastic region of the composite after the addition of a >5 wt% filler. This effect is due to the filler-matrix interactions limiting the chains' movements.
Based on the results obtained from the tensile testing of the composite materials, the mechanical properties were evaluated, including the Young's modulus (Figure 4a), maximum strain (Figure 4b), and tensile strength (Figure 4c). For the pristine PDMS, the Young's modulus, tensile strength, and maximum strain are 0.81 MPa, 1.54 MPa, and 115.39%, respectively. Pure PDMS usually has a Young's modulus between 1.32 and 2.97 MPa, and a tensile strength from 3.51 to 5.13 MPa. These differences can be explained by the curing conditions (humidity, curing agent ratio, temperature, etc.), which have been shown to have a significant effect on the final mechanical, thermal, and physical properties of PDMS [42]. With the incorporation of TiO 2 /MCM, the mechanical properties were enhanced significantly, as all samples had a higher elastic modulus, tensile strength, maximum strain, and toughness compared to the pristine PDMS. Incorporation of the 10 wt% TiO 2 /MCM into the PDMS polymer produced a maximum strain with the highest improvement of 134% without undermining the maximum Young's modulus (1.6 MPa), which was doubled compared to that of the pristine PDMS, and significantly increased the tensile strength of the material (2.4 MPa). Above a 10 wt% of TiO 2 /MCM, Young's modulus and the tensile strength were reduced. Increasing the adsorbent concentration resulted in more contact sites and higher interfacial interactions between the polymer chains and the TiO 2 /MCM particles. Consequently, the stiffness and the strength of the composites were increased due to the improved stress transfer from the PDMS matrix to the adsorbent [43]. The decrease in tensile properties above a 10 wt% is due to the formation of bigger aggregates that reduce the dispersion of the filler in the PDMS matrix, leading to a reduction in stress transferring efficiency. This finding is supported by the SEM/EDS images of the TiO 2 /MCM/PDMS composites at various TiO 2 /MCM wt% and by Ti element mapping (see Supplementary Materials, Figure S1). Figure S1 shows that the TiO 2 /MCM particles were dispersed uniformly in the PDMS matrix. As the loading increased above 10 wt%, larger aggregates at a higher concentration were formed, which reduced the dispersion, and can explain the higher standard deviation.  Figure S1). Figure S1 shows that th TiO2/MCM particles were dispersed uniformly in the PDMS matrix. As the loading i creased above 10 wt%, larger aggregates at a higher concentration were formed, whic reduced the dispersion, and can explain the higher standard deviation.

Thermal Properties
Glass transition temperatures of the PDMS composites with different loadings of a sorbents were evaluated via DSC. Figure 5 shows the DSC curves during the secon

Thermal Properties
Glass transition temperatures of the PDMS composites with different loadings of adsorbents were evaluated via DSC. Figure 5 shows the DSC curves during the second heating cycle. Pristine PDMS showed a T g of −123 • C (Table 1); similar results were also found in ref. [30]. The addition of the catalytic adsorbent (TiO 2 /MCM) slightly increased the T g (≤2°C), as can be viewed in Figure 5 and Table 1, indicating that the T g is not significantly affected by the filler as reported previously [43,44]. This phenomena is due to the fact that the glass transition is only affected by the fraction of the unmodified polymer outside the interfacial layer [45].  (Table 1); similar results were also found in ref. [30]. The addition of the catalytic adsorbent (TiO2/MCM) slightly increased the Tg (≤ 2℃), as can be viewed in Figure 5 and Table 1, indicating that the Tg is not signif icantly affected by the filler as reported previously [43,44]. This phenomena is due to the fact that the glass transition is only affected by the fraction of the unmodified polymer outside the interfacial layer [45].  The reduction in the mobility of the PDMS composites is reflected by the decrease in the normalized specific heat capacity, which was pronounced with the addition o TiO2/MCM (Table 1). ∆Cpn (Equation (1)) relies on a proportion of the polymer participat ing in the glass transition and is relative to the degree of freedom of segmental motion [46,47]. As presented in Table 1, the values of ∆Cpn for TiO2/MCM/PDMS composites are all lower than those for pristine PDMS. This reduction seems to be related to the polymeradsorbent interfacial interactions that create an immobilized polymer layer on the surface of the adsorbent [27,46].
The samples' CTE was determined from the measured displacement-temperature curves under tensile loading. All samples showed a relatively constant CTE value between 45℃ and 70℃; hence, this region was used to calculate the CTE. The CTE of the PDMS matrix was found to be 290 ppm/℃, which was slightly lower than the value (310 ppm/℃ reported by Dow for the SYLGARD™ 184 [48]. This difference can be explained by the change in curing conditions (time, temperature, etc.), which have a significant effect on the properties of PDMS, as reported by other studies [34,42]. The CTE values that were obtained for the TiO2/MCM/PDMS composites with a 2.5, 5, and 10 wt% adsorbents were 110, 110, and 150 ppm/℃, respectively, which were 48-62% lower than that of the pristine PDMS. Thus, the addition of TiO2/MCM induces a higher thermal stability of the material  The reduction in the mobility of the PDMS composites is reflected by the decrease in the normalized specific heat capacity, which was pronounced with the addition of TiO 2 /MCM ( Table 1). ∆Cp n (Equation (1)) relies on a proportion of the polymer participating in the glass transition and is relative to the degree of freedom of segmental motion [46,47]. As presented in Table 1, the values of ∆Cp n for TiO 2 /MCM/PDMS composites are all lower than those for pristine PDMS. This reduction seems to be related to the polymer-adsorbent interfacial interactions that create an immobilized polymer layer on the surface of the adsorbent [27,46].
The samples' CTE was determined from the measured displacement-temperature curves under tensile loading. All samples showed a relatively constant CTE value between 45°C and 70°C; hence, this region was used to calculate the CTE. The CTE of the PDMS matrix was found to be 290 ppm/°C, which was slightly lower than the value (310 ppm/°C) reported by Dow for the SYLGARD™ 184 [48]. This difference can be explained by the change in curing conditions (time, temperature, etc.), which have a significant effect on the properties of PDMS, as reported by other studies [34,42]. The CTE values that were obtained for the TiO 2 /MCM/PDMS composites with a 2.5, 5, and 10 wt% adsorbents were 110, 110, and 150 ppm/°C, respectively, which were 48-62% lower than that of the pristine PDMS. Thus, the addition of TiO 2 /MCM induces a higher thermal stability of the material. The new reactive mesoporous adsorbent, which has a low CTE of silica-ceramic materials, introduced its higher dimensional stability when above room temperature (45-70 • C) to the final modified PDMS [25]. According to the literature [23,24], some PDMS chains may penetrate the pores of MCM-41 due to capillary forces. As a result, their thermal expansion will be limited by the framework of MCM-41.
The thermal stability of the TiO 2 /MCM/PDMS composites at high temperatures was evaluated via the thermogravimetric analysis, which measured the sample's weight loss as a function of the temperature. Several characteristic parameters were used to analyze the thermal stability and degradation process, such as: the temperature for a 5% weight loss (T 5 ), the temperature for a 10% weight loss (T 10 ), and the temperature at which the maximum rate of mass loss is observed (T max ), in addition to the char residues [43,49]. The TGA thermograms of the pristine PDMS and PDMS composites are shown in Figure 6. The major weight loss of the pristine PDMS and PDMS composites occur in the temperature range of 400-690 • C, with two main degradation steps. The first step is around 450 • C, which is due to the decomposition of hydrocarbon species, such as methane and ethylene, from the vinyl terminal sites of PDMS, as well as the decomposition of Si-C 2 H 3 . The second step is around 650 • C, which is due to the decomposition of silicon-based oligomers and depolymerization [30,49]. It appears that the incorporation of TiO 2 /MCM, as a function of adsorbent loading, increased the thermal stability parameters of T 5 , T 10 , and T max of the PDMS composites ( Table 2). The initial decomposition temperature, T 5 , increased with the addition of the adsorbent, rising from 399 • C for the pristine PDMS to 422 • C for the 15 wt% TiO 2 /MCM/PDMS composite. The T 10 change was more pronounced, and the temperature increased from 453 • C for the pristine PDMS to 502 • C for the 15 wt% TiO 2 /MCM/PDMS sample. The T max for the pristine PDMS was found to be 628 • C, whereas the T max of the 2.5-15 wt% TiO 2 /MCM/PDMS was 676-682 • C. Thus, introducing the TiO 2 /MCM ceramic particles to the PDMS improves the thermal stability of the polymer. Furthermore, the char residues of the PDMS composites (69.9-71.2%) were higher than the char residue of the pristine PDMS (59.5%). This can be considered as an additional indication that adding TiO 2 /MCM improves the PDMS composites' resistance to thermal degradation, but not in a linear manner. In summary, the thermal degradation results of the TiO 2 /MCM/PDMS composites are strengthened by the lower ∆Cp n and CTE results which were presented earlier. penetrate the pores of MCM-41 due to capillary forces. As a result, their thermal expansion will be limited by the framework of MCM-41. The thermal stability of the TiO2/MCM/PDMS composites at high temperatures was evaluated via the thermogravimetric analysis, which measured the sample's weight loss as a function of the temperature. Several characteristic parameters were used to analyze the thermal stability and degradation process, such as: the temperature for a 5% weight loss (T5), the temperature for a 10% weight loss (T10), and the temperature at which the maximum rate of mass loss is observed (Tmax), in addition to the char residues [43,49]. The TGA thermograms of the pristine PDMS and PDMS composites are shown in Figure 6 The major weight loss of the pristine PDMS and PDMS composites occur in the temperature range of 400-690 °C, with two main degradation steps. The first step is around 450 °C, which is due to the decomposition of hydrocarbon species, such as methane and ethylene, from the vinyl terminal sites of PDMS, as well as the decomposition of Si-C2H3. The second step is around 650 °C, which is due to the decomposition of silicon-based oligomers and depolymerization [30,49]. It appears that the incorporation of TiO2/MCM, as a function of adsorbent loading, increased the thermal stability parameters of T5, T10, and Tmax of the PDMS composites ( Table 2). The initial decomposition temperature, T5, increased with the addition of the adsorbent, rising from 399 °C for the pristine PDMS to 422 °C for the 15 wt% TiO2/MCM/PDMS composite. The T10 change was more pronounced, and the temperature increased from 453 °C for the pristine PDMS to 502 °C for the 15 wt% TiO2/MCM/PDMS sample. The Tmax for the pristine PDMS was found to be 628°C, whereas the Tmax of the 2.5-15 wt% TiO2/MCM/PDMS was 676-682 °C. Thus, introducing the TiO2/MCM ceramic particles to the PDMS improves the thermal stability of the polymer. Furthermore, the char residues of the PDMS composites (69.9%-71.2%) were higher than the char residue of the pristine PDMS (59.5%). This can be considered as an additional indication that adding TiO2/MCM improves the PDMS composites' resistance to thermal degradation, but not in a linear manner. In summary, the thermal degradation results of the TiO2/MCM/PDMS composites are strengthened by the lower ΔCpn and CTE results which were presented earlier.

Chemical Stability
Solvent swelling tests were used to study the effect of the adsorbent filler on the PDMS's chemical stability. The swelling of the TiO 2 /MCM/PDMS composites was measured in the organic solvent toluene, as a hydrophobic representative material, and in water as a hydrophilic, physiologic representative material. The mass swelling ratio (R m ) is calculated based on the mass change as follows: where m 0 and m symbolize the weights of the PDMS before and after absorbing toluene and water, respectively. With regard to water absorption, the swelling ratio results were negligible, which indicates that there is no effect due to the incorporation of TiO 2 /MCM adsorbents in the PDMS matrix. The hydrophobicity of the PDMS was not impaired. On the other hand, for toluene absorption, the composite samples showed a significant decrease (about 40%) in the swelling ratio, as compared to the pristine PDMS (Figure 7).

Chemical Stability
Solvent swelling tests were used to study the effect of the adsorbent filler on PDMS's chemical stability. The swelling of the TiO2/MCM/PDMS composites was m ured in the organic solvent toluene, as a hydrophobic representative material, and in ter as a hydrophilic, physiologic representative material. The mass swelling ratio (R calculated based on the mass change as follows: where m0 and m symbolize the weights of the PDMS before and after absorbing tolu and water, respectively. With regard to water absorption, the swelling ratio results were negligible, w indicates that there is no effect due to the incorporation of TiO2/MCM adsorbents in PDMS matrix. The hydrophobicity of the PDMS was not impaired. On the other hand toluene absorption, the composite samples showed a significant decrease (about 40% the swelling ratio, as compared to the pristine PDMS (Figure 7). In general, a decrease in the swelling ratio is associated with an increase in the cr link density. However, as shown previously through the FTIR analysis, all samples gardless of the TiO2/MCM concentration, reached a similar degree of crosslinking, im ing that the adsorbent has a weak interaction with the PDMS polymer matrix. As gested by Gupta et al. [30], the adsorbent reduces the free volume between the poly molecules, and thus, partially inhibits the solvent from entering the matrix during sw ing.

Surface Hydrophilicity/Hydrophobicity
The surface hydrophilic/hydrophobic properties were evaluated via contact a In general, a decrease in the swelling ratio is associated with an increase in the crosslink density. However, as shown previously through the FTIR analysis, all samples, regardless of the TiO 2 /MCM concentration, reached a similar degree of crosslinking, implying that the adsorbent has a weak interaction with the PDMS polymer matrix. As suggested by Gupta et al. [30], the adsorbent reduces the free volume between the polymer molecules, and thus, partially inhibits the solvent from entering the matrix during swelling.

Surface Hydrophilicity/Hydrophobicity
The surface hydrophilic/hydrophobic properties were evaluated via contact angle measurements using either toluene or water. As expected, the pristine PDMS matrix demonstrated hydrophobic surface properties, as the water contact angle was 100 • , which is higher than 90 • . Similar results were also achieved by Lamberti [19]. The addition of 2.5-15wt% TiO 2 /MCM slightly decreased the contact angle to the range of 85 • -95 • due to the hydrophilic nature of the TiO 2 /MCM additive as compared to the PDMS matrix. Since the incorporation of the TiO 2 /MCM adsorbent just slightly reduced the hydrophobic nature of the PDMS, it can still be used in, for example, microfluidic systems, where a uniform laminar flow of an aqueous solution without friction and deformation is required [50]. Furthermore, the hydrophobic nature of PDMS was depicted by the low contact angle of 29 • when introduced to toluene. The addition of 2.5-15 wt% TiO 2 /MCM increased the contact angle to the range of 37 • -49 • , yielding a surface that is less hydrophobic and more stable against organic solvent swelling as compared to pristine PDMS. Figure 8 demonstrates the images of the contact angle as a function of time for the pristine PDMS, 5 wt% TiO 2 /MCM/PDMS composite, and 15 wt% TiO 2 /MCM/PDMS composite. It can be noticed that the addition of a reactive adsorbent reduced the deformation curvature, as well as the relaxation time (from 110 s to 60 s), i.e., the PDMS composites are less deformable and can quickly recover their original shape. In cases where exposure to an organic substance is required, absorption and deformation of the structure may occur, and therefore, TiO 2 /MCM/PDMS composites are beneficial. demonstrated hydrophobic surface properties, as the water contact angle was 100°, which is higher than 90°. Similar results were also achieved by Lamberti [19]. The addition of 2.5-15wt% TiO2/MCM slightly decreased the contact angle to the range of 85°-95° due to the hydrophilic nature of the TiO2/MCM additive as compared to the PDMS matrix. Since the incorporation of the TiO2/MCM adsorbent just slightly reduced the hydrophobic nature of the PDMS, it can still be used in, for example, microfluidic systems, where a uniform laminar flow of an aqueous solution without friction and deformation is required [50]. Furthermore, the hydrophobic nature of PDMS was depicted by the low contact angle of 29° when introduced to toluene. The addition of 2.5-15 wt% TiO2/MCM increased the contact angle to the range of 37°-49°, yielding a surface that is less hydrophobic and more stable against organic solvent swelling as compared to pristine PDMS. Figure 8 demonstrates the images of the contact angle as a function of time for the pristine PDMS, 5 wt% TiO2/MCM/PDMS composite, and 15 wt% TiO2/MCM/PDMS composite. It can be noticed that the addition of a reactive adsorbent reduced the deformation curvature, as well as the relaxation time (from 110 s to 60 s), i.e., the PDMS composites are less deformable and can quickly recover their original shape. In cases where exposure to an organic substance is required, absorption and deformation of the structure may occur, and therefore, TiO2/MCM/PDMS composites are beneficial.

Optical Properties
The effects of the incorporation of adsorbent particles on the optical properties of PDMS were studied. The ultraviolet-visible (UV-Vis) spectra of PDMS and the 2.5, 5, 10, and 15 wt% TiO2/MCM/PDMS composites are shown in Figure 9. The pristine PDMS was relatively transparent with a transmittance above 92% in the range from 350 to 800 nm. As the loading increased from 2.5 to 15 wt%, the transmittance of the PDMS composites decreased to be in the range of 27%-54%, as can be viewed due to the higher absorbance in the visible range. The TiO2/MCM particles can cause light scattering, which decreases the transmittance of PDMS gradually as the particle concentration increases [25]. In the UV range, pristine PDMS has a negligible absorbance between 250-400 nm. With the addition of TiO2/MCM, the absorbance increased significantly and had a clear cutoff at 400 nm, which was due to the optical properties of the TiO2 anatase crystalline structure that absorb in the UV range, but not in the visible range [51]. The addition of TiO2/MCM to PDMS enables both adsorption and photocatalytic activity. These features can be applied in applications for environmental pollutants' treatment [52].

Optical Properties
The effects of the incorporation of adsorbent particles on the optical properties of PDMS were studied. The ultraviolet-visible (UV-Vis) spectra of PDMS and the 2.5, 5, 10, and 15 wt% TiO 2 /MCM/PDMS composites are shown in Figure 9. The pristine PDMS was relatively transparent with a transmittance above 92% in the range from 350 to 800 nm. As the loading increased from 2.5 to 15 wt%, the transmittance of the PDMS composites decreased to be in the range of 27%-54%, as can be viewed due to the higher absorbance in the visible range. The TiO 2 /MCM particles can cause light scattering, which decreases the transmittance of PDMS gradually as the particle concentration increases [25]. In the UV range, pristine PDMS has a negligible absorbance between 250-400 nm. With the addition of TiO 2 /MCM, the absorbance increased significantly and had a clear cutoff at 400 nm, which was due to the optical properties of the TiO 2 anatase crystalline structure that absorb in the UV range, but not in the visible range [51]. The addition of TiO 2 /MCM to PDMS enables both adsorption and photocatalytic activity. These features can be applied in applications for environmental pollutants' treatment [52].

Conclusions
A novel reactive mesoporous adsorbent material, i.e., TiO2/MCM, was synthesized and embedded inside a PDMS matrix as a potential protective barrier due to its capabili ties of adsorption and the catalytic degradation of hazardous materials. The mechanical thermal, optical, and chemical stability properties of TiO2/MCM/PDMS composites at dif ferent weight percentages (2.5-15 wt%) were studied. The composite materials demon strated superior mechanical (Young's modulus, tensile strength, and maximum strain and thermal (CTE, ΔCp, and Tmax) properties compared to the pristine PDMS matrix. Fur thermore, the TiO2/MCM addition increased the dimensional stability in a hydrophobi environment, as the swelling was reduced by 40% and the relaxation time was decayed (from 110 s to 60 s). Regarding the optical properties, the adsorbent addition reduced transparency as a function of composite loading. However, incorporating TiO2/MCM en ables UV absorption, thus inducing the photocatalytic ability of the PDMS composite. Th results indicate that the 10 wt% TiO2/MCM addition to the PDMS matrix is optimal, du to the combined effects of the adsorption/catalytic reactivity and mechanical/thermal/op tical/chemical stability properties. Such reinforcements can also be applied in other appli cations where the combined effects of TiO2 nanoparticles and MCM as a reactive adsor bent are favorable, such as organic/biologic pollutants' degradation of wastewater, chem ical sensor devices, coatings against biofouling in microfluidic devices, and coating against corrosion.
Supplementary Materials: The following supporting information can be downloaded a www.mdpi.com/xxx/s1: Figure S1

Conclusions
A novel reactive mesoporous adsorbent material, i.e., TiO 2 /MCM, was synthesized and embedded inside a PDMS matrix as a potential protective barrier due to its capabilities of adsorption and the catalytic degradation of hazardous materials. The mechanical, thermal, optical, and chemical stability properties of TiO 2 /MCM/PDMS composites at different weight percentages (2.5-15 wt%) were studied. The composite materials demonstrated superior mechanical (Young's modulus, tensile strength, and maximum strain) and thermal (CTE, ∆Cp, and T max ) properties compared to the pristine PDMS matrix. Furthermore, the TiO 2 /MCM addition increased the dimensional stability in a hydrophobic environment, as the swelling was reduced by 40% and the relaxation time was decayed (from 110 s to 60 s). Regarding the optical properties, the adsorbent addition reduced transparency as a function of composite loading. However, incorporating TiO 2 /MCM enables UV absorption, thus inducing the photocatalytic ability of the PDMS composite. The results indicate that the 10 wt% TiO 2 /MCM addition to the PDMS matrix is optimal, due to the combined effects of the adsorption/catalytic reactivity and mechanical/thermal/optical/chemical stability properties. Such reinforcements can also be applied in other applications where the combined effects of TiO 2 nanoparticles and MCM as a reactive adsorbent are favorable, such as organic/biologic pollutants' degradation of wastewater, chemical sensor devices, coatings against biofouling in microfluidic devices, and coatings against corrosion.