Microdomain morphology of poly(urethane urea) multiblock copolymers

As part of our continuing study of the solid-state morphology of poly(urethane urea) segmented block copolymers, we focus in the current paper on the use of atomic force microscopy to visualize the structure of phase-separated microdomains. Free surface and bulk images were obtained from two series of PUU films, for copolymers varying in hard segment content from 14 to 47 wt %. Using a progression of AFM tapping forces, the morphology of the hard domains at the free surfaces was found to be in the form of randomly oriented cylinders with additional spherical domains, both having lateral dimensions on the order of 5-10 nm. At higher tapping forces, phase images of microtomed surfaces of relatively high hard segment content PUU copolymers appear to exhibit hard segment-rich domains at two size scales. However, images of surfaces of specimens freeze-fractured at liquid N₂ temperatures (and acquired at relatively low tapping forces) reveal the larger structures to be aggregates of smaller hard microdomains.