Effect of Interactions, Molecular and Phase Structure on the Properties of Polyurethane Elastomers

Linear polyurethanes with various stoichiometry were prepared from 4,4'-methylenebis(phenyl isocyanate), 1,4-butanediol and poly (tetrahydrofuran) in melt reaction. The molecular weight of the polymers was estimated from the equilibrium torque measured during the reaction, mechanical properties were characterized by tensile testing and DMA measurements, while phase separation by the determination of transparency. Changes in the NCO/OH ratio led to the expected changes in molecular weight. Both strength and elongation-at-break increased with increasing isocyanate content up to the equimolar ratio of the two functional groups, but decreased only slightly at larger NCO/OH ratios. On the other hand, modulus assumed minimum value in the same composition range. The extent of phase separation is the smallest and the number of mobile soft segments the largest at around the equimolar ratio of the two functional groups. While viscosity depends only on molecular weight, strength and deformability are strongly influenced also by interactions, which on the other hand drive phase separation and completely determine stiffness.