Hybrid thiol-acrylate-epoxy polymer networks: Comparison of one-pot synthesis with sequential reactions and shape memory properties

Abstract

Hybrid polymer networks are synthesized using thiol-, acrylate- and epoxide-functionalized reactants. The thiols react with both acrylate and epoxide groups, but acrylate and epoxide groups are unreactive with each other. Past research efforts on this system have employed a combination of sequential photo-initiated and thermally initiated reactions of thiols with (meth)acrylate and epoxide functional groups and have led to single-phase materials. Analysis of reaction kinetics shows that thiol-acrylate reactions are much faster than thiol-epoxy reactions; consequently, we have also developed a one-pot synthesis of thiol-acrylate-epoxy hybrid networks using room temperature reactions and 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) as catalyst. For the DBU-catalyzed reaction conditions tested, such one-pot reactions yield materials with properties identical within error to those of materials made by sequential reactions involving thiol-acrylate first and thiol-epoxide second. With low molecular weight reactants, homogeneous, single-phase materials are produced which exhibit the same thermal and mechanical properties regardless of whether reactions are sequential or simultaneous. With some higher molecular weight reactants, e.g., 2000 g/mol diacrylate, novel phase-separated thiol-acrylate-epoxide polymer networks are produced with properties that are highly tunable by sequential reaction order. Beyond good mechanical properties, some of the resulting phase-separated networks are very good shape memory polymers, with shape-fixity values above 95% and shape-recovery values above 99% after multiple cycles.