Microstructural and mechanical properties of silica–PEPEG polymer composite xerogels

We prepared and characterized silica–polyethylene-block-polyethylene glycol (PEPEG) nanocomposite xerogels and investigated the role of addition of the elastic PEPEG oligomer in a brittle porous silica matrix by relating microstructure to mechanical properties. For PEPEG < 7 wt.%, the oligomer fills smaller pores in the silica matrix; but, as evidenced by atomic force microscopy and Fourier transform infrared spectroscopy, PEPEG partially disrupts matrix formation at higher concentrations. Consequently the pore size distribution shifts towards larger sizes and the specific surface area of the composite decreases. The toughness increases continuously with PEPEG addition. The elastic modulus of the composite increases twofold for 7 wt.% PEPEG, but decreases on further addition of PEPEG. Microhardness and structural loss factor also show a similar optimum for PEPEG at around 5–7 wt.%. The analytical and empirical models agree with the experimental modulus values for lower PEPEG addition, but fail to explain the maximum because of their inability to capture the observed microstructural changes in the composite structure.