Morphology of polyurethanes revisited by complementary AFM and TEM

Cast, segmented polyetherurethanes with 30 and 50% hard-segment content (HSC), respectively, were studied by transmission electron microscopy (TEM) and atomic force microscopy (AFM). Multi-phase segregation was observed in both samples on two levels (micro and nano) of structural organization. Spherulites with a prominent radial structure, built of branched fibrils and globules, were captured on the micrometer level. The use of AFM enabled us to investigate the nanostructure in the polyurethanes studied here. In the sample with low (30%) HSC, nano-scale phase separation was observed by AFM in areas outside the crystalline aggregates. The morphology in these domains exhibited short, rodlike hard domains embedded in the matrix of the soft segments. The other sample (50% HSC) contained four identifiable morphological features. These included spherulites, globules, bundles of lamellae, and nanophase-separated, rodlike hard domains, embedded in the soft-segment matrix. The globules did not have any internal structure visible by AFM down to the nanometer scale. We speculate that the globules form as a result of macro-phase segregation, due to incompatibility of the reactants, during synthesis and may thus be identified as pockets of free hard segments. The AFM phase imaging has been very useful to observe the bundles of lamellae and the nanoscale phase-separated structures, which were not captured by TEM, due to large differences in AFM phase signal contrast between the hard and the soft domains.