Novel Characterization of Critical Micelle Concentrations of Block and Gradient Copolymers In Homopolymer: Effects of Sequence Distribution, Composition and Molecular Weight

Over the past thirty years, many theoretical studies have examined the effects of block copolymer composition and molecular weight as well as homopolymer molecular weight on the critical micelle concentration (CMC) of block copolymer in homopolymer. However, because of a lack of appropriate experimental methods, very few experimental studies have been performed to test the theoretical results. Here we describe a simple, novel, fluorescence-based method for characterizing the CMC values of styrene-containing block copolymers in homopolymer and extend this approach to a novel class of materials known as gradient copolymers, in which the sequence distribution along the length of the copolymer chain gradually varies from one comonomer to another.

At concentrations below the CMC, the fluorescence spectrum of the styrene units in styrene/methyl methacrylate block or gradient copolymers in blends with poly(methyl methacrylate) exhibit a strong peak centered at 280-285 nm, which is associated with fluorescence from a single excited-state phenyl ring and is called monomer fluorescence. At concentrations above the CMC, a second peak centered near 330 nm becomes evident. This lower energy fluorescence arises from excited-state dimers which form from two phenyl rings arranged in a sandwich-like conformational state and is called excimer fluorescence. This is easily explained because when micelles are formed, there is a high local concentration of styrene units in the micelle core, which accommodates excimer formation and fluorescence. A plot of the ratio of excimer fluorescence intensity to monomer fluorescence intensity as a function of weight percent copolymer increases abruptly at the CMC, allowing for simple, quantitative determination of CMC values. Our studies reveal that the CMCs of block copolymer in homopolymer are approximately an order of magnitude lower than those of gradient copolymers of similar molecular weight and composition. Studies to test particular theoretical pictures of micelle formation of block copolymers in homopolymer are underway, and our investigation is being extended to examine a broad range of styrene-containing copolymer and homopolymer systems.