Siloxane modified hybrid materials by the sol-gel process
Joardar, Saikat Sutej
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The first part of this research constituted the study of oligomeric incorporation into an inorganic network leading to the fonnation of a hybrid material by the sol-gel process. The oligomeric species chosen was silanol terminated poly(dimethylsiloxane) (PDMS) and the inorganic oxide precursor chosen was titanium tetra-isopropoxide (TIP). The rationale for choosing these materials stemmed from the desire to make materials for optical filters since a gradient of optical properties can be derived based on the composition and the initial reaction conditions. 29Si NMR studies were conducted to investigate the incorporation process which led to the conclusion that the PDMS chains were being degraded by the TIP. This problem led to the synthesis of TIP endcapped PDMS which was then used as the oligomeric component in the hybrid materials. The endcapping changed the functionality of the oligomer to six, and more importantly, its reactivity which was now closer to that of the TIP. Also, having the same functionality at the ends as the precursor affects the compatibility of the two components. Hybrid materials in the form of free standing films were fabricated from varying TIP/pDMS compositions and different initial reaction conditions. The hybrid materials synthesized were then subjected to a systematic structure-property relationship study. The degree of phase mixing or the encapsulation of the siloxane phase by the inorganic oxide increases with higher TIP content. This behaviour was seen by both the dielectric and dynamic mechanical studies. The modulus and the stress to break increased with higher TIP content while the strain to break decreased. At the 50/50 TIP/PDMS composition there was an abrupt change in the mechanical properties which signified a morphological change in these materials. The refractive indices of the materials also showed a trend similar to that of the mechanical properties. Three different cure procedures, viz., room temperature, thermal cure at 70 °C and microwave cure at 70 °C were utilized to fabricate the films. More phase separation occurred in the microwave cured over the thermally cured materials as evident from dielectric and dynamic mechanical studies. However, the effect diminished with higher TIP content in the materials. Changing the initial reaction conditions by varying the initial water and acid content changed the properties observed in the final products. Dielectric and dynamic mechanical studies showed that the phase separation was higher with increased water content. Also, the modulus of the materials made with increased water content was higher. Decreased acid content led to better dispersion of the two components as shown by the dielectric and dynamic mechanical experiments. The effect of using a less reactive inorganic component tetra-ethyl orthosilicate (TEOS), instead of TIP was also studied. The TEOS/pDMS materials showed more phase mixing than the TIPIPDMS materials, although, the dielectric tan Î´ peak maxima appeared at a lower temperature. The TEOS/PDMS materials were also less stiff than the TIP/PDMS materials. Structural studies were conducted by means of solid-state NMR, electron microscopy and small angle x-ray scattering (SAXS). With few exceptions, these studies correlated very well with the properties observed in these materials. The spin-lattice relaxation times obtained from the solid-state NMR correlated with the better phase mixing observed in the TEOSIPDMS materials relative to the TIPIPDMS materials. Electron microscopy in the backscattered mode showed the presence of microphase separation in these materials. At the 50/50 TIPIPDMS composition connectivity of the inorganic oxide phase was complete, perhaps encapsulating the siloxane phase totally. Microstructural differences were observed due to the variation in the cure modes. The domain sizes of the oxide phase were finer in the thermally cured as compared to the room temperature cured materials. Higher water content led to a very different microstructure when compared to materials made with lower water content. Smaller spherical particulate type structures were observed in the material made with higher water content. The domain sizes of the oxide phase in the TEOS/PDMS material were bigger than those in the TIP/PDMS material of equivalent composition. These differences may be intimately related to the reactivity difference between TIP and TEOS. The SAXS data supported the microstructure seen by electron microscopy and the properties observed by dynamic mechanical, dielectric and mechanical studies.
- Doctoral Dissertations