The work described in this thesis includes research done on two major projects, chemical heterogeneity and sequence length heterogeneities in copolymers. In the chemical heterogeneity project, we present a method by which to obtain the absolute, chemical-heterogeneity-corrected molar mass averages and distributions of copolymers and apply the method to two gradient random copolymers of styrene and methyl methacrylate and styrene and t-butyl methacrylate. In the first copolymer, the styrene percentage decreases from approximately 30% to approximately 19% as a function of increasing molar mass while in the second it varies between 60% and 70% as as a function of molar mass. The method consists of separation by SEC with detection using multi-angle static light scattering (MALS), differential viscometry (VISC), differential refractometry (DRI), and ultraviolet absorption spectroscopy (UV), and relies on the preferential absorption of styrene over methyl methacrylate and tert-butyl methacrylate at 260 nm. Using this quadruple-detector SEC/MALS/UV/VISC/DRI approach, the percentage of styrene (%St) in each elution slice is determined. This %St is then used to determine the specific refractive index increment, corrected for chemical composition, at each elution slice, which is then used to obtain the molar mass at each slice, corrected for chemical composition. From this corrected molar mass and from the chemical-composition-corrected refractometer response, the absolute, chemical-heterogeneity-corrected molar mass averages and distribution of the copolymer are calculated. The corrected molar mass and intrinsic viscosity at each SEC elution slice are used to construct a chemical-heterogeneity-corrected Mark-Houwink plot. The slice-wise corrected M data are used, in conjunction with the MALS-determined RG,z of each slice, to construct a conformation plot corrected for chemical heterogeneity. The corrected molar mass distribution (MMD) of the gradient copolymers extends over an approximately 30,000 g/mol wider range than the uncorrected MMD in the styrene-methyl methacrylate random copolymer and 60,000 g/mol in the case of styrene-tert-butyl methacrylate random copolymer. Additionally, correction of the Mark-Houwink and conformation plots for the effects of chemical heterogeneity shows that the copolymer adopts a more compact conformation in solution than originally concluded.
The sequence length heterogeneity (SLH) project shows a novel method for detecting SLH based on a change in the conformation of the copolymer in solution. Sequence length heterogeneity is defined as the change, as a function of copolymer molar mass (M), in the average number of continuous monomers of a given repeat unit. SLH can influence polymeric properties such as thermal stability, mechanical behavior, transparency, and the ability of copolymers to reduce interfacial surface tension. Here, we demonstrate the relation between SLH and the change as a function of M of a dimensionless size parameter, the ratio of the viscometric radius and the radius of gyration, irrespective of chemical heterogeneity or M polydispersity. Multi-detector size-exclusion chromatography (SEC) provides for a convenient method by which to experimentally establish this relation and, consequently, a method by which to determine whether SLH is present in a copolymer, whether the degree of randomness of a copolymer changes across the molar mass distribution (MMD), or whether two copolymers differ from each other in degree of randomness at a given M and/or across their MMDs. Results from our SEC and FT-IR measurements of block, random, alternating, and gradient copolymers of styrene (S) and methyl methacrylate (MMA) and their respective homopolymers agree with results from a probability theory based model of SLH in linear random copolymers. The multi-detector SEC method employs instrumentation available in most polymer separations laboratories and the relations developed should be applicable to copolymers other than the S-MMAs studied here.