A transparent cyclo-linear polyphenylsiloxane elastomer integrating high refractive index, thermal stability and flexibility

A new type of cyclo-linear structured high phenyl content siloxane elastomer (Ph-CLS-E) was prepared through a “one-pot” hydrosilylation reaction that takes advantage of the reactivity differences of cyclic and linear precursors and a vinyl-diphenylpolysiloxane crosslinker. The cyclo-linear structure is based on the hydrosilylation of 1,5-dimethyl-1,5-divinyl-3,3,7,7-tetraphenylcyclotetrasiloxane (ViPh-Cyclo) and an α,ω-bis(dimethylsiloxyl)polydiphenylsiloxane oligomer (H-Ph-Linear). The cyclo-linear precursor can further react with the α,ω-bis(methyldivinylsiloxyl)poly(methylvinylsiloxane-diphenylsiloxane) (Vi-ViPh-Crosslinker) to form the Ph-CLS-E elastomer, in which the molar ratio of Ph : Si > 1. Ph-CLS-Es are transparent elastomers with a high refractive index of ≥1.580 (@589 nm) and an excellent thermal stability of Td 5wt% ≥ 437 °C. Meanwhile, the flexibility of Ph-CLS-E is improved with the increasing content of the cyclo-linear unit. The maximum elongation at break of Ph-CLS-E reaches ∼189%. This research indicates that the high phenyl content cyclo-linear structure inhibits the thermal decomposition of the siloxane chain even at low crosslinking density. It provides an effective way to fabricate siloxane elastomers with the combination of high RI, excellent flexibility and thermal stability, which shows application prospects in optoelectronic encapsulation and packaging.