I5S miktoarm star block copolymers:: Packing constraints on morphology and discontinuous chevron tilt grain boundaries

A morphological study of three I5S six-arm miktoarm star block copolymers is presented. These miktoarm stars are comprised of five arms of polyisoprene (PI) and one arm of polystyrene (PS) joined together at a single junction point. The strong segregation limit theory for the morphological behavior of miktoarm stars predicts that these materials should form spherical morphologies, but only lamellar and cylindrical morphologies were observed by TEM and SAXS. These results are similar to previously reported discrepancies between experimentally observed morphological behaviors of miktoarm stars and the predictions of the theory. Previous work has attributed the discrepancies to the neglect of the effect of the multifunctional junction points on calculated free energies. The current results suggest that, in addition to this, geometrical packing constraints prevent the formation of morphologies such as spheres and cylinders in highly asymmetric miktoarm stars in which the minor volume fraction component would need to occupy the matrix phase. Finally, unusual broken chevron tilt grain boundary morphologies were observed in a lamellar I5S material. We attribute these new structures to the asymmetric energy penalties for interfacial bending which result from the molecular asymmetry of the miktoarm stars.