Tunable, Catalyst-Free Preparation of Silicone Gels

Silicone gels are typically comprised of silicone elastomers swollen with silicone oil; catalysts used for cure and/or their byproducts remain in the final material. Complications arising from these catalyst residues, including effects on thermal behavior, slow changes in mechanical properties due to continued catalyst reactivity, toxicity due to leaching, etc., can all work to compromise gel applications. We previously reported a catalyst-free, high-temperature, autoxidation cure method for elastomers whereby crosslinking is initiated through radical-mediated hydrogen abstraction by oxygen from a hydrosilane (SiH) starting material; propagation proceeds through a radical chain. We now report that this method is particularly amenable to silicone gel synthesis and describe the preparation of a library of silicone gels with various mechanical properties and gel fractions. Young’s modulus of silicone gels prepared using the autoxidation cure method is readily tuned by the SiH concentration in the starting materials and the time and temperature of cure, as shown from 220 to 300 °C. Starting material ratios of linear siliconeshydride-functional polydimethylsiloxane (PDMS) plus, optionally, nonfunctional PDMS oilwere varied to control the SiH concentration in the starting materials, which ranged from 0.3 to 7.2 mol %. All materials were fully cured in a week at 220 °C, contained <30% extractable materials, and exhibited Young’s moduli from 0.78 to 2.75 MPa. Softer gels were produced (>0.02 MPa) with higher levels of the extractable material (up to 92% mass loss following extraction) by removing the formulations from heat prior to full cure. Higher cure temperatures allowed analogous gel structures to be realized in shorter reaction times, in as little as 2.8 h at 300 °C, but resulted in brittle materials. However, gels can be reliably synthesized for as little as 1.5 h at 250 °C when the oxidation is performed in an oxygen atmosphere.