Synthesis and glass transition behavior of high molecular weight styrene/4-acetoxystyene and styrene/4-hydroxystyrene gradient copolymers made via nitroxide-mediated controlled radical polymerization

Styrene (S)/4-acetoxystyrene (AS) gradient copolymers were synthesized by moderate-temperature, nitroxide-mediated controlled radical polymerization. Hydrolysis of styrene/4-acetoxystyrene gradient copolymers produced styrene/4-hydroxystyrene (HS) gradient copolymers. Molecular weight (MW) characterization via gel permeation chromatography demonstrated that these materials were made in a "controlled" manner, while intrinsic viscosity measurements revealed that the MWs exceeded 100 000 g/mol, with apparent viscosity-average MW values ranging between 100 000 and 385 000 g/mol, making these materials the first high MW gradient copolymers ever synthesized. Characterization of the glass transition temperature, T-g, revealed different behavior depending on the type of gradient copolymer produced. Using a normal thermal history for measuring T-g by differential scanning calorimetry (DSC), linear gradient copolymers exhibited one T-g, with a value intermediate to the T(g)s of polystyrene (PS) and poly(4-acetoxystyrene) (PAS) or poly(4-hydroxystyrene) (PHS). In contrast, "blocky" gradient copolymers with overall S content greater than or equal to55 mol % yielded two T(g)s, one near the T-g of PS and the other intermediate to the T(g)s of PS and PAS or PHS, indicating microphase separation. When the cumulative composition of the "blocky" gradient copolymer was majority AS or HS, only one Tg was resolved, with a value near the T-g of PAS or PHS. For S/AS and S/HS gradient copolymers of identical chain length, overall fractional S content, and strength of gradient, physical aging at 90 degreesC provided resolution of a second T-g via DSC, allowing comment on how manifestations of microphase separation depend on gradient structure and the strength of the comonomer repulsive interactions.