Glass Transition Temperatures of Copolymers: Molecular Origins of Deviation from the Linear Relation

Abstract

The glass transition temperature (Tg) is an important material parameter in determining polymer properties. The main molecular structure factors affecting Tg are chain stiffness and cohesive energy, causing polymers to exhibit the complex micro-structure dependence of Tg. The composition dependence of the Tgs of binary random copolymers mainly shows as three cases: the linear relation, the negative deviation (or concave curve), or the positive deviation (convex curve). The study of the Tg’s composition dependence can guide the design of copolymer materials with desirable properties. On the other hand, it can provide insights into understanding the precise nature of the glass transition. In this study, the root cause of the Tg deviation from the linear relation in random copolymers was revealed, and new molecular origins were proposed. First, the theoretical basis of why there exists a linear Tg relation for copolymers was clarified, and the influence of the specific polar interactions between components was discussed briefly. More importantly, it has been identified that the negative Tg deviation is related to copolymers composed of the mono-substituted (M-type) monomer and asymmetrical di-substituted (AD-type) monomer, and the positive Tg deviation is related to copolymers composed of the M-type monomer and symmetrical di-substituted (SD-type) monomer. The molecular origin of the Tg deviations from the linear relation is the destruction of the special conformational structures (e.g., the locally rigid conformational structure in the AD-type homopolymer or the dynamic flexible structure in the SD-type homopolymer) caused by copolymerization. The behaviors for more complex binary copolymers (AD–AD, SD–SD, and AD–SD types) were also discussed. Finally, the variation in Tg of a statistical copolymer as a function of composition was summarized.