Mechanical relaxations in a series of polyurethanes with varying hard to soft segment ratio

Abstract

A series of nine polyurethanes with a systematically varying hard to soft segment ratio was prepared; their dynamic mechanical spectra were studied over a wide temperature range. These polyurethanes were prepared using Adiprene L-100 which contains polyether units (forming the soft segment material) and diphenylmethane-4,4′di-isocyanate which formed the bulk of the hard segments. The chain extender was 1,3-diaminopropane. The samples were studied at 2 per cent static strain and at a frequency of 110 Hz. They showed tan δ maxima in three temperature regions, but each sample did not necessarily show all three transitions. In general, these transitions occurred at about -130°, -20° and over a range from about 11° to over 225°. These transitions were referred to as the a, β and γ transitions in order of decreasing temperature. The transitions were interpreted in terms of molecular motion. The γ transition was thought to arise through a Schatzki type mechanism while the β transition for most samples represented the glass transition and was caused by micro-Brownian motion of the more flexible parts of the chain. The a transition, which only occurred when MDI was also present, exists in the 15-20° range for the samples with the higher soft segment concentrations; it was thought to be the result of phenyl group rotation. As the hard segment concentration was increased, spherulites were detected and the a transition shifted to considerably higher temperatures, but a shoulder remained at about 20° until hard segment concentration had reached a high level. The high temperature transition was the result of crystallite fusion. © 1972.