Effect of Molecular Weight of Methylphenylsiloxy‐Containing Vinyl‐Functionalized Terpolysiloxanes on Their UV‐Activated Crosslinking by Hydrosilylation and Mechanical Properties of Crosslinked Elastomers

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.

One-Component, Vinyl-Functional Fluorosilicone Pastes for Direct Ink Write Additive Manufacturing

“…It can be seen from these figures that in the 5 mol % MeTfpS-containing terpolymer series, the higher the terpolymer DP was, the faster the paste cross-linked. This was consistent with the higher concentration of reacting Si–Vi groups in the lower DP polymers requiring longer reaction times when the same concentration of the reaction catalyst was used and is similar to our previously reported observation for the corresponding MePhS-containing terpolymer pastes . Consistent with this, it can also be seen from Figure A that for a constant terpolymer DP of 600, an increase in the content of MeTfpS repeat units decreased the cross-linking time required, and from Figure B that pastes from MeTfpS-containing terpolymers cross-linked somewhat faster than those from the previously reported corresponding MePhS-containing terpolymers .…”

“…Like the closely related elastomers from MePhS-containing terpolymers reported earlier, 20 the MeTfpS-containing variants having DPs of 240 and higher exhibited strain-induced crystallization, SIC, within the −80 to −40 °C temperature range when subjected to DMA. This phenomenon was dependent on polymer composition and disappeared for elastomers from polymers having 10 and 15 mol % MeTfpS repeat units (i.e., those that were less like pure PDMS).…”

“…In Figure , the SEC monitoring data for polymer A of Figure A (DP = 600 and 5 mol % MeTfpS) are compared with the corresponding data previously reported for their all-DiMeS (i.e., unmodified PDMS), DiPhS-, DiEtS, and MePhS-containing terpolymer counterparts (I, II, and III, respectively). , It can be seen from this figure that polymerization of MeTfpS-containing terpolymer A behaved very similarly to the corresponding polymerizations of its all-DiMeS-, MePhS-, and DiEtS-containing counterparts I and II, with the only exception being the polymerization of DiPhS-containing terpolymer, III, which showed a characteristic inhibition period caused by low solubility of cyclic DiPhS tetrasiloxane in its DiMeS counterpart, D 4 …”

“…Figure 14A,B shows cure times as a function of the polymer molecular weight for the two series of MeTfpS-containing terpolymer pastes evaluated in this work, and a comparison of the behavior of the 5 mol % MeTfpS-containing terpolymer pastes with the corresponding MePhS-containing terpolymer pastes with different crystallization-suppressing repeat units reported earlier. 20 It can be seen from these figures that in the 5 mol % MeTfpS-containing terpolymer series, the higher the terpolymer DP was, the faster the paste cross-linked. This was consistent with the higher concentration of reacting Si−Vi groups in the lower DP polymers requiring longer reaction times when the same concentration of the reaction catalyst was used and is similar to our previously reported observation for the corresponding MePhS-containing terpolymer pastes.…”

“…where z was kept at less than 0.01 (1 mol %), y ranged from 0.03 to 0.07 (3–7 mol %), and x ranged from 0.92 to 0.96 (92–96 mol %). − The DiPhS (I), DiEtS (II), and MePhS (III) repeat units were introduced into these polymers to act as suppressors of crystallization, , which is a characteristic of pure PDMS, occurring between −50 and −40 °C. , Hence, when modified in this manner, the resulting terpolymers become precursors for completely amorphous elastomers that retain their characteristic elasticity to temperatures as low as −100 °C (i.e., just above their respective glass temperature, T g ) and eliminate any potential concerns on the cross-linking kinetics and desired mechanical properties of the resulting rubbers. , Vinylsiloxy-functional groups (both in –Si(CH 3 ) 2 –CHCH 2 chain ends and in –[Si(CH 3 )(CHCH 2 )–O]– repeat units) were introduced to serve as reactive sites for subsequent cross-linking by hydrosilylation. ,, …”