Composition and molecular weight distribution of block copolymers

Abstract

Abstract The composition distribution (CS) of diblock copolymers of styrene and methyl methacrylate, prepared anionically via a sequential addition procedure, has been investigated by two methods, one based on the theory of light scattering from copolymer solutions and the other on the principle of adsorption chromatography. Calculations were carried out to show that if the structure of the sample block copolymer is described by random coupling statistics, information of the molecular weight distribution (MWD) for either one of the block chains can be drawn from light scattering data obtained with one single solvent in which the solute is invisible. In practice, bromobenzene was used as a solvent appropriate for this purpose. Since the MWD for the precursor polystyrene is known, the MWD for the poly(methyl methacrylate) block was determined by this method. By using these data and assuming the Schulz distribution for MWD, the CD curve was calculated. Fractionation of the block copolymer sample by composition was made on a semipreparative scale (300 mg) by employing a glass cylinder packed with activated silica gel. The elution was carried out with a binary mixture of ethyl acetate and benzene. The fractionation was achieved without interference of molecular weight. Nine fractions having different styrene contents were recovered and analyzed to construct the CD curve. The CD curves obtained by the chromatographic method were in good agreement with those drawn from the light scattering data.