Study of the formation of a segmented polyester-polyurethane

Segmented polyurethanes are block copolymers of the (AB)n type characterized by a microphase-separated morphology. They are an important subclass of the family of thermoplastic elastomers presenting a broad range of applications, including adhesives and coatings. Typically, phase separation can go along with polymerization, being of great importance the simultaneous study of these two phenomena. In this work the development of a methodology for the study of the formation of a segmented polyester-polyurethane is presented. In a first stage, the determination through FTIR in ATR mode of the kinetics and phase separation has been undertaken. In a second step, the experimental size distributions of hard segments, by an original selective hydrolysis procedure was carried out. The polyurethanes have been prepared starting with 4,4’-methylene-diphenylene diisocyanate (MDI), policaprolactone diol (PCL) and 1,4-butanediol (BD) as chain extender, at various initial mole ratios. Along the polymerization, the system has typically presented a transition from a homogeneous state, during which the chemical kinetics was well described by an overall second order law, into a system presenting phase separation. The critical point, where phase separation starts, was characterized in terms of isocyanate conversion and number and weight average hard segments sizes. It became clear that phase separation requires not only a certain lower bound of the average hard segment size but also at least some minimum value of the concentration of hard segments. The occurrence of phase separation along the formation reaction modifies the size distribution of hard segments with respect to the geometrical distribution prevailing at homogeneous conditions. Moreover, average size tends to decrease as a consequence of phase segregation