Uniquely Broad Glass Transition Temperatures of Gradient Copolymers Relative to Random and Block Copolymers Containing Repulsive Comonomers

Nitroxide-mediated controlled radical polymerization is used to synthesize gradient and block copolymers of styrene (S) and 4-acetoxystyrene (AS), and conventional free radical polymerization is used to synthesize S/AS random copolymers. The S/AS copolymers are hydrolyzed to yield S/4-hydroxystyrene (HS) copolymers. Gel permeation chromatography and 1H NMR of aliquots taken during polymerization yield proof of the controlled nature of the gradient copolymer structures. The glass transition temperature (T g) responses are compared using the derivative of differential scanning calorimetry heat curves, with the temperature range over which the derivative exceeds a base level being equated to the T g breadth. A single, narrow T g is obtained in each random copolymer, consistent with a single phase of limited compositional nanoheterogeneity. Two narrow T gs are evident in each block copolymer, consistent with well-developed nanophases containing nearly pure S or nearly pure AS or HS units with a very narrow interphase yielding no indication of an intermediate T g. In contrast, T g breadths of ∼65−80 °C are observed in many S/HS gradient copolymers, consistent with ordered nanostructures in which the unit cell composition varies sinusoidally. The possibility of capitalizing on the broad T g of gradient copolymers in damping applications is discussed.