How does sequence length heterogeneity affect the dilute solution conformation of copolymers?

Sequence length heterogeneity (SLH) 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 molar mass of a dimensionless size parameter, the ratio of the viscometric radius and the radius of gyration, irrespective of chemical heterogeneity or molar mass 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.