Formation of worm-like micelles from a polystyrene–polybutadiene–polystyrene block copolymer in ethyl acetate

Abstract

An investigation has been made of micelle formation by a polystyrene–polybutadiene–polystyrene block copolymer in ethyl acetate, which is a selectively bad solvent for polybutadiene; the number-average molecular weights of the three blocks were 12900, 66000 and 13700, respectively. As the temperature was raised dilute solutions of the copolymer in ethyl acetate showed a distinct cloud-point, which moved to higher temperatures with increases in concentration. On raising the temperature further the solutions gradually became clear again.

Light scattering and electron microscopy studies showed that the onset of cloudiness was due to the formation of long worm-like micelles from spherical micelles. The clearing of solutions at higher temperatures occurred because the micelles dissociated to form copolymer molecules in the free-chain form. The cores of both types of micelle were composed of swollen polybutadiene blocks and the outer fringes of the micelles of polystyrene blocks. The spherical micelles had a fairly narrow size distribution. The worm-like micelles had a wide distribution of contour lengths but possessed a narrow size distribution in the radial direction.

A second polystyrene–polybutadiene–polystyrene copolymer having shorter blocks than the first copolymer showed a similar type of solution behaviour at lower temperaturs.