Effects of Catalyst Concentration on the Morphology and Mechanical Properties of Polyisobutylene-based Thermoplastic Polyurethanes

We studied the morphology and mechanical properties of a series of polyisobutylene-based thermoplastic polyurethanes (TPUs) with the same composition, but synthesized with tin (II) 2-ethylhexanoate catalyst concentrations ranging from 0.04 to 1mol% relative to 4,4-methylenebis(phenyl isocyanate) (MDI). All of the TPUs were found to have incomplete microphase separation between the soft segment (SS) and hard segment (HS), and the degree of microphase separation was not significantly affected by catalyst concentration. However, changing the catalyst concentration resulted in a substantial change in the microphase separated structure. Such differences may arise from the difference in the average chain length distribution of HS, which was confirmed by oxidation of the TPUs. Interestingly, the catalyst concentration also had a dramatic effect on the mechanical properties of these TPUs. When the catalyst concentration was at or below 0.1mol%, the TPUs exhibited ultimate tensile strength of 20-21 MPa, compared to 10 MPa when the catalyst concentration was at or above 0.4mol%. These results may help better understand the structure-property relationships of TPUs, and more importantly, develop biostable TPUs with high mechanical strength.