Conformational Source of Comonomer Sequence-Dependent Copolymer Glass-Transition Temperatures

This brief review addresses the source of the dependence of copolymer glass transition temperatures (Tgps) on their comonomer sequences. Here we show that a comparison of the conformational entropies obtained from the Rotational Isomeric State (RIS) conformational models of the poly-A and poly-B homopolymers and their resultant poly-A/B co-polymers, i.e., ΔSconf = (XASA + XBSB) - SA/B (X = comonomer fraction), can be used to predict/understand the Tgps of copolymers. For copolymers with ΔSconf ~ 0, we expect their Tgps to follow Fox behavior and to depend only on copolymer composition, because of the similar conformational flexibilities of the A and B homo- and A/B-copolymers. When the conformational entropy ΔSconf is negative the A/B copolymer is assumed more flexible than the weighted sum of polymer-A and polymer-B conformational entropies, resulting in Tgps that are lower than expected from the Fox equation. Conversely, a positive ΔSconf suggests the copolymer’s lower flexibility, resulting in higher Tgps than expected from the Fox relation. We use the successful comparison of the observed dependence of numerous copolymer Tgps to demonstrate the validity of using their calculated RIS conformational entropies to predict their comonomer sequence dependencies.