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 1 H 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 g s 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.

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ABSTRACT:The critical micelle concentrations (CMCs) of styrene-methyl methacrylate (S-MMA) block and gradient copolymers present in a homopolymer poly(methyl methacrylate) (PMMA) matrix were determined using an intrinsic fluorescence technique based on the ratio of excimer to monomer fluorescence from styrene repeat units. The homopolymer molecular weight (MW) and copolymer MW, composition, and sequence distribution were varied to determine their effects on the CMC, and comparisons were made to theory. Although the effects of these parameters on micelle formation have been the focus of significant theoretical study, few experimental studies have addressed these issues. The MW of the S block (forming the micelle core) has a strong effect on the CMC. For example, an order of magnitude reduction in the CMC (from $ 1 to $ 0.1 wt %) is observed when the S block MW is increased from 51 to 147 kg/mol while maintaining the MMA block and PMMA MWs at 48-55 kg/mol. Increasing the PMMA matrix MW also has a strong an effect on the CMC, with the CMC for a nearly symmetric S-MMA block copolymer with each block MW equal to 48-51 kg/mol decreasing by a factor of 5 and by several orders of magnitude when the matrix MW is increased from 55 to 106 kg/mol and 255 kg/mol, respectively. In contrast, similar changes in the MMA block MW have little effect on the CMC. Finally, when present in a 55 kg/mol PMMA matrix, a 55 kg/mol S-MMA gradient copolymer with a styrene mole fraction of 0.51 exhibits a factor of 6 larger CMC than a block copolymer of similar MW and composition.

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