STUDIES ON THERMALLY STIMULATED SHAPE-MEMORY EFFECT OF SEGMENTED POLYURETHANES

The shape memory behavior of a series of polycaprolactone/methane diis ocyanate/butanediol (PCL/MDI/BDO) segmented polyurethanes of different composition was studied. The molecular weight of PCL diols was in the range of 1600-7000, and the hard segment content varied from 7.8 to 2 7% by weight. Film specimens for shape memory measurements were prepar ed by drawing at temperatures above the melting temperature of the sof t segment crystals and subsequent quick cooling to room temperature un der constrained conditions. The shape memory process was observed and recorded in a heating process. Parameters describing the recovery temp erature, ability, and speed were used to study the influence of struct ure and processing conditions an the shape memory behavior of the samp le. It was found that the high crystallinity of the soft segment regio ns at room temperature and the formation of stable hard segment domain s acting as physical crosslinks in the temperature range above the mel ting temperature of the soft segment crystals are the two necessary co nditions for a segmented copolymer with shape memory behavior. The res ponse temperature of shape memory is dependent on the melting temperat ure of the soft segment crystals. The final recovery rate and the reco very speed are mainly related to the stability of the hard segment dom ains under stretching and are dependent on the hard segment content of the copolymers. (C) 1997 John Wiley & Sons, Inc.