Differences in enthalpy recovery of gradient and random copolymers of similar overall composition: styrene/4-methylstyrene copolymers made by nitroxide-mediated controlled radical polymerization

Styrene/4-methylstyrene (S/MS) random and gradient copolymers were synthesized by nitroxide-mediated controlled radical polymerization (NM-CRP) and compared to random copolymers made by conventional free radical polymerization (ConvFRP). The gradient copolymers have molecular weight (MW) values approaching 85,000 g/mol, making these some of the higher MW gradient copolymers reported to date. Due to the proximity of the glass transition temperatures (Tg) of polystyrene (PS) and poly(4-methylstyrene) (PMS), there is no significant difference in Tg between the gradient and random copolymers, with both copolymer types yielding single Tgs that typically increase slightly with increasing MS content. While enthalpy relaxation studies demonstrate similarity in random copolymers made by NM-CRP and ConvFRP, they reveal significant differences between random and gradient copolymers. Gradient copolymers exhibit broad enthalpy recovery peaks, whereas random copolymers exhibit narrower enthalpy recovery peaks. The maxima in the enthalpy recovery peaks are at substantially lower temperature, as much as 17 °C, in the gradient copolymers as compared to random copolymers of equal overall composition. While random and gradient copolymers of a given overall composition exhibit similar enthalpy recovery values at a common physical aging time and quench depth relative to Tg, the major differences in the enthalpy recovery peaks indicate that differences in sequence distribution along the chain length can lead to unusual behavior in gradient copolymers relative to random copolymers.