In a search for designed materials that mimic those found in biological systems, combining polymers with inorganic materials in sol-gel chemistry processes has gained momentum in the research world since the 1990's. This work methodically investigated the properties of several related polymer-SiO₂ hybrids prepared by a simple methodology that could be applied to a range of other polymers and organically modified inorganic starting materials. The preparation and drying conditions were varied systematically to provide the best method for combining poly (methyl methacrylate) (PMMA), polystyrene (PS), polystyrene-co-poly (methyl methacrylate) (PS-co-PMMA) and polystyrene-polyisoprene-polystyrene (PS-PI-PS) with pre-hydrolyzed tetraethoxysilane (TEOS).
Thermogravimetric analysis (TGA) provided the weight percent of silica contained in each hybrid and the percentage of retained solvent. The derivative of the TGA curve is useful for illuminating the effect of silica on the degradation pathway of the polymer. Fourier transform infrared spectra (FTIR) probed the chemical interaction between the hybrid constituents. Dynamic mechanical analysis (DMA) provided mechanical data over a range of working frequencies and temperatures. Tapping mode atomic force microscopy (TM-AFM) was used in preference to standard electron microscopies to provide the phase distribution of the constituents as well as a 3D topographical image.
Overall, PMMA interacted with pre-hydrolyzed TEOS to produce more thermally stable hybrids that were stronger at high temperatures. The PS showed that pre-hydrolyzed TEOS could be added as filler as it did not change the thermal or mechanical properties appreciably. The PS-co-PMMA mixed with the pre-hydrolyzed TEOS showed higher glass transition temperatures with increasing silica content. The PS-PI-PS did not combine well with pre-hydrolyzed TEOS. One of the major findings was that small amounts (a few %) of residual solvent and hydrolysis byproducts (ethanol) act as a plasticizer that significantly reduces mechanical stiffness of the hybrids below that expected.