Structure-property behavior of hybrid materials incorporating oligomeric species with inorganic silicates by a sol-gel process

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Virginia Polytechnic Institute and State University

A sol-gel process has been utilized to develop novel hybrid materials incorporating organic and inorganic species. The objectives of this project were to study the feasibility of this new bridging route and, if successful, study the structure-property relationships of these new materials. In this thesis, tetraorthosilicate was used to combine with one of three types of oligomers, which included silanol terminated polydimethylsiloxane (PDMS), triethoxysilane endcapped polytetramethyleneoxide (PTMO), and PTMO with multiple triethoxysilane functional groups. The general reaction scheme was to first generate silanols from both components through the hydrolysis reaction, and then form the network structure by a co-condensation reaction with the silanol groups. The preparation of these hybrid materials was successful. Most of these hybrid materials were obtained without significant cracking problems with an initial TEOS loading up to 80 wt%, and the final products were always transparent.

For the PDMS containing systems, the tensile strength was always lower than 8 MPa. and the elongation at break was in the range of 5-25%. Dynamic mechanical results showed a bimodal tanδ behavior, which was attributed to two different physico-chemical environments of the oligomers: one was PDMS rich phase and one represented more dispersed PDMS oligomers. The dispersion of the oligomers increased with acid and silicate content of the system, and this postulation of better PDMS dispersion was strongly supported by the SAXS results which showed a systematic decrease in the mean square electron density fluctuation.

The mechanical properties of PTMO containing materials were considerably enhanced compared to the PDMS hybrid systems. Depending on the composition and oligomeric molecular weight, the tensile strength could reach 33 MPa. Also, the range of the elongation at break increased up to ca. 100%. The tanδ spectra showed a single, broad maximum at temperatures much higher than the Tg of pure PTMO oligomers, which indicated the absence of a pure oligomer phase. A broad maximum in the SAXS profile was observed in most cases, implying the existence of a correlation distance in these PTMO containing materials. To rationalize all the experimental observations, a schematic model was suggested which contains highly condensed TEOS clusters and mixed regions of partially condensed TEOS and PTMO. This model was further supported by swelling data and by agreement between the SAXS correlation length and the estimated PTMO end-to-end distance.

The systems prepared with PTMO possessing multiple triethoxysilane groups showed the most promising results in terms of mechanical properties. The tensile strength ranged from ca. 30 to 55 MPa., and the ambient modulus was nearly 10⁸ Pa. Also, a yield point was observed in some cases and was postulated to be an indication of partial continuity of the silicate phase.