Rediscovering silicones: molecularly smooth, low surface energy, unfilled, UV/vis-transparent, extremely cross-linked, thermally stable, hard, elastic PDMS.

We demonstrate the preparation of extremely cross-linked poly(dimethylsiloxane) (PDMS)-based materials and report optical, mechanical, and surface properties. Transparent monolithic molded objects are prepared catalytically with no byproducts; parts per million levels of platinum (catalyst) remain in the articles. Essentially the same material was prepared in 1993 and described as a "hard transparent glass." We confirm the thermal stability and chemical structure described in this report. We show that the catalytic reaction used, which was reported in 1999 always to exhibit a "violent exotherm", can be controlled conveniently using a low (parts per million) catalyst concentration. The combination of low surface energy, transparency, hardness, elasticity, and thermal stability makes this an unusual and interesting material. That it can be prepared from commercially available low-viscosity monomers adds to its interest. We comment that the class of materials known as siloxanes or silicones and PDMS in particular is not currently generally well understood (or taught) and review aspects of the structure, properties, and cross-linking chemistry of PDMS.