The influence of inter-domain interactions on the physical properties of polyurethanes

The phase separated structure of polyurethane foams is kinetically trapped due to the vitrification of hard segment domains, which results in an interconnecting hard domain morphology. This new morphological picture depicts the importance of the inter-connectivity provided by both the small portions of the long hard segments and the disordered regions that serve as an indirect bridge between hard domains. Deuterium substitution has been used to specify different regions of the interconnecting phase-separated structure in polyurethanes. Thin film samples were prepared isothermally using foam formulations with different water contents to obtain very different degrees of domain inter-connectivity. By analyzing the relative rate of substitution at different relative humidity and temperature of the environment, it is possible to differentiate the degree of phase separation for various samples with different degrees of phase separation. The infrared spectroscopic features, mainly amide vibrations, of various parts of hard segments have been identified. The deuterium substitution rates measured revealed that the size and dispersion of hard-segment domains can vary significantly as a function of reaction temperature and composition of reactants. It is also possible to differentiate the interfacial region from the phase-separated domains. Based upon the interconnecting hard domain morphology, infrared dichroic studies further demonstrated the contribution of hard domains to reinforcement of polyurethane materials. Plastic deformation within the hard domain was observed even at small strain, giving a molecular insight to the stress-softening effect and energy hysteresis associated with polyurethanes. The viscoelastic properties of polyurethane studied by stress relaxation experiments have again shown the importance of inter-connectivity to the overall mechanical properties of this material. A correlation between the exact water placement and overall deterioration of physical properties has been established. Lignin has been successfully incorporated as a reinforcing component to enhance polyurethanes. It was not until the dispersion state of the lignin in polyol was improved and both the processing conditions and catalyst package were optimized that the modulus and strength of the polyurethane material showed significant enhancement. It is found that lignin most likely functions as the hard segments to achieve better inter-connectivity of the physical network of the hard domain morphology