Nature and mechanism of the effect of thermal prehistory on the structure and molecular mobility in a polyurethane capable of crystallization

Abstract

Various physical methods have been used to investigate the structure and molecular mobility of a crystallizable polyurethane based on diphenylmethanediisocyanate and 1,4-butanediol as a function of thermal prehistory. Various forms of molecular movement have been observed and identified: these are caused by the appearance of local and segmental forms of movement in the polyurethane macromolecules, and also by molecular mobility within crystalline regions. It has been established that the latter includes a number of processes of molecular motion caused by the melting of the initial assembly of crystallites that had been formed as a result of the previous thermal treatment and subsequent reorganization of the crystallites during heating of the specimen. These processes of molecular motion substantially affect the mechanical behaviour of the polyurethane at temperatures above the glass region. It has been shown that the system of hydrogen bonds in the crystallizable polyurethane investigated is directly connected with the characteristics of its structure and undergoes considerable changes when the latter is reorganized during annealing and heating.