Visible-light photopolymerization of epoxy-terminated poly(dimethylsiloxane) blends: Influence of the cycloaliphatic monomer content on the curing behavior and network properties

Abstract

Siloxane-based networks were obtained by photoinitiated cationic ring-opening polymerization of mixtures composed of two different siloxane monomers: poly(dimethylsiloxane) diglycidyl ether (PDMS-DGE) and bis [2(3,4-epoxycyclohexyl) ethyl]-tetramethylsiloxane (ECE-TMA). The photoinitiated polymerization of PDMS-DGE is very sluggish and thus finds little use in high-speed photocuring applications. However, this monomer undergoes fast visible-light photopolymerization when combined with ECE-TMA. This behavior was ascribed to copolymerization effects induced for the higher reactivity of the cycloaliphatic epoxy monomer (ECE-TMA). It was demonstrated that the polymerization rate, as well as the thermo-mechanical properties of the resulting networks, can be adjusted by varying the ratio between both monomers. Plots of PDMS-DGE conversion versus ECE-TMA conversion evidenced a non-ideal copolymerization behavior, with a preferential addition of ECE-TMA units during the first stages of network formation. The composition of the mixture was varied over the entire range of concentrations without evidence of phase separation before or during photopolymerization at room temperature. The resulting networks exhibited a behavior that changed from a rubbery to a glassy state at room temperature depending on the mixture composition, as revealed by dynamic mechanical analysis. The storage modulus and damping factor were successfully correlated with the crosslinking density and network topology. The high transparency of the mixtures allowed to reach a high epoxy conversion by photopolymerization, even for thick samples (1?2 mm in thickness), which constitutes a significant advance for applications requiring highspeed curing protocols at ambient conditions.