| Summary: | The evolution of microstructure in polyurethane foams is a complex process involving competing kinetics of polymerization, crosslinking, and phase separation. Hard segments form in-situ and the increase in molecular weight induces phase separation. Coincident with these processes is a significant rise in viscosity through crosslinking of the trifunctional polyether soft segment. The inherent temperature rise that accompanies foaming complicates matters further and makes morphology control difficult. The resulting morphologies are complex and cannot be explained by traditional views of polyurethane morphology. Based on morphological and mechanical data, a new model of the phase-separated structure is proposed. This model incorporates the notion of interconnectivity of ordered hard domains through bridges of either extremely long hard segments or constrained soft material. To identify factors responsible for the final morphology in these polyurethanes and to correlate mechanical properties to the polymer microstructure without the complication of a cellular structure, films were synthesized isothermally. By exploiting the influence of temperature on the relative rates of phase separation and crosslinking, it was possible to create very different morphologies. Infrared spectroscopy, atomic force microscopy, and transmission electron microscopy show that films prepared at low reaction temperatures have organized, continuous hard domains. These materials have very high moduli and mechanically behave as continuous structures. In contrast, films prepared at high temperatures exhibit isolated, poorly ordered hard domains. These films are much weaker mechanically, although the volume fraction of hard segment is the same. The differences in morphology are accredited to varying rates of crosslinking relative to phase separation. At low reaction temperatures, phase separation proceeds faster than chemical crosslinking, but at higher temperatures, crosslinking dominates. Factors potentially responsible for the change in relative rates are addressed. Analysis of hard segment mass distribution by Matrix-Assisted Laser Desorption Ionization mass spectrometry shows that hard segment length is not a critical factor governing the phase separation behavior. Instead, an increase in viscosity associated with an increase in crosslinking rate with temperature is responsible for the observed homogeneous morphology in films prepared at high reaction temperatures. However, hard segment length does contribute to the mechanical properties by providing connectivity between domains.
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