The four main chain structures of copolymers comprised of co-monomers A and B include: alternating, block, grafted, and random. Polymer blends will also vary in composition depending on the process employed.
Due to the nature of polymerization reactions, the distribution of co-monomer A and B will change with respect to the polymer’s molecular weight. Consequently, changes in chemical composition are reflected in the refractive index increment (dn/dc) across the molecular weight distribution whereas the dn/dc of a homopolymer remains constant. In order to measure absolute molecular weight for copolymers it is imperative to measure the actual dn/dc at each fraction of the copolymer chain distribution. In the case of purely alternating and alternating block copolymers, dn/dc values would also remain constant due to the equal distribution of component A and B, and therefore these copolymer can be treated as homopolymers. Similarly, a constant dn/dc can be used for molecular weight calculation of copolymers comprised of components A and B that have near-identical dn/dc values.
Using the advanced capabilities of tetra-detection GPC incorporating RI, UV, LS, and VIS, Polyanalytik offers copolymer and polymer blend analysis to elucidate absolute molecular weight, molecular weight distribution, intrinsic viscosity, size, chain conformation, and co-monomer and polymer blend composition. General requirements for this analysis include UV absorption of one co-monomer or polymer contained in the blend and the technique is limited to two co-monomers or two types of polymers in a blend.
The detector responses from the RI and UV detectors are dependent on the respective concentrations, dn/dc values, and dA/dc values of components A and B at each molecular fraction of the copolymer distribution. The dn/dc and dA/dc values of each component are easily measured via tetra-detection GPC of the homopolymers of A and B.
The actual dn/dc at each molecular fraction of the copolymer or polymer blend distribution is calculated based on the concentrations of component A and B which is obtained from the ratio of RI/UV detector signals. Absolute molecular weight averages (Mn, Mw, Mz) and molecular weight distribution (Mw/Mn) are then calculated based on the light scattering detector response using the actual dn/dc of the copolymer.