Domain structure and viscoelastic properties of graft copolymer

Summary

The domain structures of film specimens cast from benzene solutions of a series of graft copolymer of poly(methyl acrylate) with styrene with different degrees of grafting, from 10 to 80 vol% of styrene component, were investigated with phase-contrast and dye-staining microscopy. The graft copolymers were synthesized by coupling polystyryllithium, obtained by anionic polymerization technique, onto a well-fractionated PMA, so that the coupling density of the grafted segment along the backbone segment was varied but the molecular weights of the grafted and backbone segments were kept constant to cover the above range of volume fraction of styrene component for the series of graft copolymer.

It was found that with an increase of the volume fraction of styrene component (grafted segment) the domain structure changes from spheres of styrene component dispersed in a matrix of MA component, to alternating lamellae of the two components, and to spheres of MA component dispersed in a matrix of styrene component. Two types of rodlike domains, usually found for block copolymers as the intermediates between the spherical and lamellar domains, were missed.

The domain formation mechanism was discussed in terms of a quasi-equilibrium phenomenon of micelle formation, microphase separation between the grafted and backbone segments, at a critical micelle concentration. TheGibbs free energies of five types of micelle formations; anA sphere in aB shell micelle, anA rod in aB sheath micelle, alternating lamellar micelle ofA andB, aB rod in anA sheath, and aB sphere in anA shell micelle, were discussed in terms of molecular and thermodynamic parameters. Comparing the free energy levels of the five types of micelle formations with each other, it was revealed that the free energy levels for forming the two types of rodlike micelles can not be the lowest at any volume fraction of grafted segment.