Morphology of bulk SBS block copolymers prepared by wet cryo-ultramicrotomy

Abstract

When a polystyrene-polybutadiene-polystyrene (SBS) block copolymer undergoes microphase separation, the resulting morphology is dependent on the chemical nature, component ratio, and molecular weights of the constituent molecules. Transmission electron microscopy studies have revealed that the morphology of block copolymers cast in various solvents is determined also by the selectivity of the preparatory solvent. This work compares the morphologies produced by thin-film casting of a toluene-SBS solution on water, with subsequent solvent evaporation, and those produced in bulk, without the use of any solvents. To obtain ultrathin sections of the latter, a new technique using cryo-ultramicrotomy is introduced. The technique utilizes sub-zero temperatures to harden the samples and allow reproducible sectioning; the optimal temperature is accurately predicted as the glass transition temperature for a hypothetical homogeneous blend of the same composition. Difficulties with the trough liquid necessary in wet sectioning are overcome by coating the surface of the glass knives with Teflon. Morphological differences arising from the two preparative methods are identified, and image-enhancing techniques are used to emphasize the interphase region described by various theories.