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.