Synthesis of Gradient Copolymers and Their Properties in Dilute Solution and the Melt

Abstract

Gradient copolymers are copolymers where the instantaneous copolymer composition varies along the length of the chain. In this dissertation, we developed a mathematical model to assist us in synthesizing gradient copolymers with pre-specified composition profiles using atom transfer radical polymerization (ATRP). Gradient copolymers of 2-hydroxyethyl methacrylate, HEMA, and 2-(dimethylamino)ethyl methacrylate, DMAEMA, were synthesized with linear, parabolic, and hyperbolic (Tanh) composition profiles, having targeted molecular weights and controlled molecular weight distributions, and targeted overall compositions. While dilute aqueous buffer solutions of random copolymers of HEMA/DMAEMA have a single cloud point temperature, dilute aqueous buffer solutions of gradient copolymers of HEMA/DMAEMA undergo clouding over a range of temperatures. HEMA/DMAEMA gradient copolymers begin aggregating when the least soluble chain end becomes insoluble, but macroscopic clouding does not set in until a significant portion of the chains becomes insoluble. Furthermore, the macroscopic cloud point temperature of gradient copolymer solutions is suppressed relative to that of solutions of random copolymers with the same overall composition, with the magnitude of suppression increasing with increasing gradient strength of the gradient copolymer. Finally, gradient copolymers aggregate more slowly than random copolymers, with aggregation rate decreasing as gradient strength increases.

Gradient copolymers of styrene, S, and 4-acetoxystyrene, AS, are capable of microphase separating in the melt. When compared with microphase separation in block copolymers, microphase-separated gradient copolymers have an increased affinity to form an alternating lamellar morphology, an observation that is qualitatively in agreement with recent theoretical predictions. However, quantitative comparison of the morphology of actual microphase-separated gradient copolymers with theoretical phase diagrams is complicated because the predicted morphology depends sensitively upon the composition profile of the gradient copolymer. Furthermore, gradient copolymers synthesized by controlled free-radical polymerizations have sub-unity gradient strengths, which will reduce the segregation strength of these copolymers. With aid from our model, we synthesized S/AS gradient copolymers with pre-specified composition profiles to match those for which theoretical phase diagrams exist. We introduced effective segregation strengths and effective volume fractions to account for sub-unity gradient strengths. Using these corrections, we demonstrate agreement between predicted morphologies and actual morphologies of gradient copolymers.