The morphological behavior of model graft copolymers

The effect of graft molecular architecture on the formation of self-assembling morphologies of strongly phase-separated, amorphous block copolymers have been systematically investigated. Three series of samples across a range of component volume fractions were characterized using transmission electron microscope and small angle x-ray and neutron for different model architectures of polystyrene-polyisoprene single graft and double graft copolymers. The single graft architecture was an asymmetric single graft , ASG, formed by grafting a polystyrene block on a polyisoprene backbone. This architecture is considered the fundamental unit, or constituting block copolymer, from which all more complex graft architecture with multiple trifunctional junction points are constructed. The ASG architecture is found to shift the volume fraction windows in which specific strongly microphase separated morphologies are observed to higher volume fractions of the PS graft material than in the corresponding linear diblock copolymers with a same molecular weight and chemical composition. As the polystyrene is grafted from the center of polyisoprene backbone, the ASG architecture becomes an I$\sb2$S star architecture. The morphological behavior of the I$\sb2$S block copolymers was predicted by the morphological diagram of S. T. Milner for miktoarm stars (Macromolecules, 27, 2333 (1994)). However, it is found that the morphology diagram overestimates the amount of shift in the order-order transition lines produces by asymmetry in molecular architecture. This overestimation in the theory is attributed to an effect of chain crowding close the junction points. The effect of asymmetric grafting of the single PS chain on the PI backbone was investigated in a series of materials where the single PS graft is asymmetrically located along the PI backbone. Additionally the effect of multiple graft architecture was explored with S$\sb2$IS$\sb2$ (H-shaped) and (SI)I$\sp\prime$(SI) $\pi$-shaped) materials, each of which has two trifunctional branch points per molecule. It was found that to a good approximation the behavior of the double graft materials can be mapped onto the behavior of the single graft materials by considering the double graft molecules to be divided into component single graft parts