Spectroscopic analysis of phase-separation kinetics in model polyurethanes

The phase-separation behavior of well-defined polyurethanes, consisting of the reaction product of methylenebis(p-phenyl isocyanate) (MDI) and butanediol (BD) as the hard segments and poly(propylene oxide) (PPO) as the soft segment, were characterized. The success in utilizing vibrational spectroscopy for the study of phase separation depends on the existence of bands sensitive to mixed and phase-separated states. In a previous study, spectroscopic features characteristic of urethane linkages dispersed in the soft segments as compared to interurethane hydrogen bonding confined to hard-segment domains were assigned. With a specially designed sample cell, a phase-mixed structure was trapped at a temperature 60 deg C below the glass transition temperature of the soft segments. When this quenched sample was brought up to a higher temperature, the increase of one spectroscopic component and the corresponding decrease of the other provided a direct measurement of phase-separation kinetics. The initial findings associated with a well-defined model polyurethane are reported. The phase separation behavior has been interpreted to proceed by a nucleation and growth mechanism with a two-dimensional growth initially, followed by a diffusion-dominated mechanism. Additional studies for the entire family of model polyurethanes will be published separately. 20 ref.--AA