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Effect of Increasing the Hard Segment Percentage on the Mechanical Response of Selected Polyurethane Films

This paper describes a study which investigated aspects of the mechanical responses of selected polyurethane films (PUs) when the hard segment percentage was varied. The range of structures achieved included the chain extender ethylene glycol (EG), the macrodiol polytetrahydrofuran (PTHF) (molar mass 2000), and two di-isocyanates: 4,4′-methylene bis(phenyl isocyanate) (MDI) and 4,4′-dibenzyl diisocyanate (DBDI) generating dibenzyl-based hard segments of conformational mobility. The influence of increasing the hard segment percentage from 20 to 80%, on the mechanical properties was followed. The study of the PUs stress—strain response included tensile tests, and load—unload cycles at constant rate of extension, with measurement of hysteresis and strain recovery. As shown, the mechanical properties of such materials were greatly enhanced by hard-phase crystallinity, through its effect on the flow stress. Increasing the hard segment percentage led systematically to increases in the input strain energy elongation. With increasing hard segment percentage the polymers based on DBDI hard segments, displayed higher stiffness and strength than the conventional MDI-based PUs. Both features of the response were attributed to differences in hard phase plastic flow stress, resulting from crystallinity in the DBDI phase. With increasing hard segment percentage, lower strain recovery and strain energy recovery on cycling were observed in the case of PUs derived from dibenzyl structures.