Atomic force microscopy visualization of poly(urethane urea) microphase rearrangements under aqueous environment

Polyurethane biomaterials are a critically important class of polymers used in a variety of medical devices. It has been suggested that the good blood compatibility of polyurethanes arises from nanoscale chemical heterogeneities at the surface as a consequence of the microphase separated morphology. In this study, we used tapping mode atomic force microscopy with phase imaging under aqueous conditions to visualize the distribution of the surface microphases for a series of poly(urethane urea) block co-polymers with varying hard segment content. The surfaces were prehydrated for 24 h under a flow of 1 mM phosphate buffer. Topographic images showed the formation of nanometer-sized raised features on the surface, having lateral dimensions of 50-70 nm and heights of 10-15 nm. Phase images, reflecting the local distribution of the mechanical properties under aqueous conditions, were quite different from those obtained in ambient conditions, consistent with water-induced structural reorientation. Images suggest that there is little soft phase material at the polymer surface in the presence of water, while images acquired after dehydration of the samples show that the surface layer remains rich in hard domains, indicating that the films do not return to their original states over the time period studied.