Effect of Molecular Weight of Methylphenylsiloxy‐Containing Vinyl‐Functionalized Terpolysiloxanes on Their UV‐Activated Crosslinking by Hydrosilylation and Mechanical Properties of Crosslinked Elastomers. Issue 5 (27th January 2021) - Ault, Charles A.
Abstract: Effects of molecular weight of methylphenyl‐containing vinylsiloxy‐functionalized terpolysiloxanes on their UV‐activated crosslinking by hydrosilylation at room temperature in air, shelf life stability of " all‐in‐one " pastes prepared from them for additive manufacturing, and mechanical properties of the resulting crosslinked elastomers, are investigated. It is found that while rheology of pastes containing base polymers, trimethylsilylated silica fillers, and thixotropic additives is not significantly affected by the base polymer molecular weight but is dominated by the filler concentration, the pastes based on higher molecular weight polymers exhibit faster crosslinking (corresponding to higher catalyst turnover numbers) and their crosslinked elastomers show transient strain‐induced crystallization. The latter appears in networks from terpolymers with degrees of polymerization (DP) of 240 and above (corresponding to about one half of the critical polydimethylsiloxane chain length for entanglement formation of DP = 460), within the temperature range of −80 to −30 °C, characteristic for polydimethylsiloxane melting transition. It is believed that this is the first time an observation of this chain length effect is reported for polysiloxane elastomers and that the properties reported herein can be expected to have major implications on the application potential of these polymers in both additive manufacturing and performance of their elastomers at sub‐ambient temperatures. Abstract : Methylphenyl‐containing vinylsiloxy‐functionalized terpolysiloxanes represent prime candidates for application of silicones in additive manufacturing. The rate of their UV‐activated hydrosilylation crosslinking increases at constant catalyst concentration with the decrease of polymer molecular weight and the resulting elastomers exhibit transient strain‐induced crystallization above the degree of polymerization of 240. This is the first observation of such dependence in a polysiloxane elastomer family.