Characterization of Shape-Memory Trifunctionally Cross-Linked Polyurethanes,with Varying Hard and Soft Segments

A family of potential shape-memory trifunctionally cross-linked polyurethanes (SMPs) was achieved with varying the hard segments. The molecular weight of the soft segment poly(oxytetramethylene) (PTMO) was held constant. The polymers were employed with and without chain extender. They were characterized by means of structural studies and creep tests over a range of temperatures, and hence their shape-memory performance was determined. More crystallization was observed in the materials with hard domains based on the flexible diisocyanate 4,4′-dibenzyl diisocyanate (DBDI) extended with 1,4-butanediol (BDO). A series of key parameters characterizing shape-memory performance was determined: the temperature position of the peak (T_peak), the half-width of the peak (δT), and the minimum value of the 30 s isochronal creep modulus E(30 s) E_min. While the peak temperature and modulus increased with increasing cross-link density, the width of the peak was observed to decrease with increasing cross-link density. The materials were found to vary systematically with details of the network structure.