Reinforcement of poly(dimethylsiloxane) networks by blended andin-situgenerated silica fillers having various sizes, size distributions, and modified surfaces

Abstract

Elastomeric networks of poly(dimethylsiloxane) (PDMS) were reinforced with silica particles introduced in two rather different ways. The first was by the simple blending of separately-prepared particles into hydroxyl-terminated PDMS chains that were subsequently end-linked, and the second was by in-situ precipitation through the catalyzed "sol-gel" hydrolysis and condensation of a tetraalkoxysilane. Both approaches permitted control of particle sizes and particle-size distributions. Strongly bimodal size distributions in which there were equal weights of small and large particles were of particular interest, and were achieved in the in-situ approach by using two consecutive steps for generating the two sizes of particles. In addition, some in-situ generated particles were treated to make their surfaces more hydrophobic using co-hydrolyses of dialkoxysilanes with the tetraalkoxysilane. X-ray scattering measurements and electron microscopy were used to characterize the particles and their dispersion within the PDMS matrix. In the in-situ approach, an increased amount of catalyst gave larger particles, but in smaller amounts. It also gave better dispersions, both with regard to the particles within the matrix in general, and in the mixing of the two sizes of particles amongst themselves. The resulting composites were also characterized with regard to their mechanical properties in elongation, both in continuous extension and in near-equilibrium measurements. In the latter case, the extents of stress relaxation were also measured and found to be smaller for the in-situ reinforced systems. The bimodal size distributions did not give mechanical properties superior to those of the unimodal ones, with most of the improvements in mechanical properties coming from the smaller particles. The surface-modified silica particles, however, did seem to give improved mechanical properties, presumably through stronger polymer-filler interfacial interactions.