SynopsisParameters characterizing the interaction of nonidentical polymer molecules in dilute solutions of mixtures of two polymers have been defined on the basis of Stockmayer's theory of light scattering in multicomponent systems. Various procedures for evaluation of the interaction parameters from experimental data have been compared. Specific features of the investigation of such interactions are demonstrated with the scattering behavior of the system polystyrene-poly-(methyl methacrylate)-dioxane.
The composition heterogeneity has been calculated for some block and graft copolymers. General relationships have been derived for a model of the poly(A‐g‐B) or poly‐(A‐b‐B) type fulfilling the assumption of random grafting. Numerical results have been calculated for special cases of particular interest. These include a copolymer having independent molecular weight contributions from units of species A and B (a special case of which is a block copolymer) and a graft copolymer where the grafting is not accompanied by crosslinking or degradation side reactions and no homopolymer B is formed. Two other model systems represent the effects of the presence or absence of both homopolymers. For the above models, the dependence of the composition heterogeneity on the structure‐determining parameters (average number of grafts in the molecule, polydispersity index of the molecular weight of the backbone and graft) is discussed. The heterogenities thus calculated are in many cases comparatively low. The composition heterogeneity of the system is in most cases increased by the presence of homopolymers.
The interaction of unlike polymer molecules (components 2 and 4) in a ternary solution can be regarded as selective exclusion or desorption of one polymer by another. A relation is derived between the coefficient of selective sorption and the interaction parameters A24 and A244 which are analogs of the second and third virial coefficients. The ratio between the apparent light‐scattering molecular weight and the true value for a polymer solute in a ternary system with one component of a binary solvent polymeric is more involved than in a ternary system in which both solvent components are of low molecular weight. Under certain conditions, the introduction of polymer component 2 into a dilute solution of polymer component 4 may lead to a decrease in the total intensity of scattered light.
The sensitivity of the light‐scattering method for the determination of the composition heterogeneity of copolymers is discussed in terms of the accuracy of measurements of the intensity of scattered light. When the composition is independent of molecular weight, a nomographic solution has advantages. The nomograph is used to investigate the feasibility (a) of the determination of heterogeneity by a single measurement at zero refractive index increment of the copolymer and (b) of the determination of the heterogeneity from two measurements. The results show that in the case of low heterogeneities the light‐scattering method can be used only at higher molecular weights. By using the heterogeneities calculated elsewhere, the possibility of determining them for the individual types of copolymers is discussed. It appears that reported discrepancies between measured and calculated heterogeneities are mostly found in cases where the sensitivity of the light‐scattering method for the determination of the composition heterogeneity is low.
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