H-shaped double graft copolymers: Effect of molecular architecture on morphology

Abstract

The morphologies formed by block copolymers with a double-graft, H or S2IS2 architecture were investigated using transmission electron microscopy (TEM) and small angle neutron scattering (SANS). Here S and I represent blocks of polystyrene and polyisoprene, respectively. These materials were synthesized using anionic polymerization and chlorosilane linking, and they were characterized using size exclusion chromatography, membrane osmometry, and low-angle laser light scattering. This characterization work confirmed the desired molecular architectures and narrow molecular weight distributions. The results of morphological characterization indicate that one can understand complex grafting architectures by decomposing them into fundamental building blocks, which are taken as the component single graft structures out of which the larger structure is constructed. We propose rules for dividing structures into these components, which we call constituting block copolymers. The morphological behavior of the more complex architecture is approximately equivalent to that of the constituting block copolymer structure. Through the use of the constituting block copolymers we map the experimentally determined morphological behavior of the H architecture onto the morphology diagram calculated by Milner for miktoarm stars [Macromolecules 27, 2333 (1994)]. Mapping the H architecture onto the morphology diagram in this way produces general agreement between experimental results and the model. However, it is found that in the case of the H architecture, as well as in previously published results for I2S and I3S miktoarm star materials, that the morphology diagram slightly overestimates the amount of shift in the order-order transition lines produced by asymmetry in molecular architecture. This overestimation in the theory is attribute to a junction point localization effect which was neglected in Milner’s calculation.