Preparation and Mechanical Properties of Poly (vinyl acetate) / Silica Hybrids Obtained by a Sol-Gel Process: Effect of Methyl Groups in Silicon Alkoxides

A copolymer of vinyl acetate (VAc) and vinyl triethoxysilane (VTES) having 90 mole% of VAc component was synthesized and incorporated with silica by the sol-gel process involving silicon alkoxides. For the silicon alkoxides, we used tetraethoxysilane (TEOS), methyltriethoxysilane (MTES), and dimethyldiethoxysilane (DMDES). The mechanical properties of these copolymer hybrids were measured and then compared with each other. The tensile strength of the copoly(VAc/VTES)/TEOS hybrids was higher than that of the PVAc/TEOS hybrids due to bonding between the copolymer and the silica network. The addition of TEOS and MTES increased the strength to a maximum of 50 MPa and 43 MPa, respectively, whereas DMDES decreased it. The addition of 20 wt% of either MTES or DMDES increased the elongation. The dynamic moduli, G', of the hybrids prepared from TEOS and MTES were increased, whereas that of the hybrid from DMDES was decreased because this alkoxide has two methyl groups in the alkoxide chain and plasticized the silicon network. As the amount of added alkoxide was increased, the position of the tan δ peak shifted to a higher temperature range for TEOS and MTES , but to a lower range for DMDES. The hybrids prepared from TEOS were hard and brittle, but when we added both DMDES and TEOS with the copolymer we could control the mechanical properties of the hybrids from hard and brittle to tough and soft materials by varying the amount of added DMDES.