Compositional Heterogeneity of Copolymers by Combined GPC and Lalls

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Abstract
A method for determining the compositional heterogeneity of copolymers is described. A low-angle laser light-scattering (LALLS) photometer is added on-line to a dual-concentration-detector gel-permeation chromatograph (GPC) in order to measure heterogeneity at each elution volume interval. Experimental results for polystyrene-poly(di-methylsiloxane) diblock copolymers and blends are discussed, and these give overall heterogeneity parameters which are in good agreement with theory. The technique shows, however, that some of the samples originally thought to be copolymers were actually blends or contained homopolymer. Both random errors and errors inherent in the analysis are examined. The largest inherent error arises from the assumption that there is no distribution of molecular weights within a single GPC fraction. An alternative to this assumption is described, and its use considerably reduces the inherent errors. All types of error are reduced by selecting a solvent such that the refractive index increment of the copolymer under investigation is small, and the refractive index of the solvent should ideally fall between those of the two copolymer components. The accuracy of the technique increases linearly with copolymer molecular weight and the square of the difference in component refractive index increments.