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Effect of molecular weight on shape memory behavior in polyurethane films



Understanding the relationship between the number-average molecular weight (Mn) and the shape memory behavior of polymers is crucial for a complete picture of their thermomechanical properties, and hence for the development of smart materials, and, in particular, in textile technology. We report here on the study of shape memory properties as a function of Mn of polymers. Shape memory polyurethanes (SMPUs) of different Mn were synthesized, with various catalyst contents or molar ratio(r = NCO/OH) in the composition. In particular, two types of SMPU, namely Tm and Tg types according to their switch temperature type, were synthesized to compare the influence of Mn on their shape memory behavior. X-ray diffraction, differential scanning calorimetry, dynamic mechanical analysis, and shape memory behavior results for the SMPUs are presented. The results indicate that the melting temperature (Tm), the glass transition temperature (Tg), the crystallinity, and the crystallizability of the soft segment in SMPUs are influenced significantly by Mn, before reaching a critical limit around 200 000 g mol−1. Characterization of the shape memory effect in PU films suggests that the Tm-type films generally show higher shape fixities than the Tg-type films. In addition, this shape fixity decreases with increasing Mn in the Tg-type SMPU, but the shape recovery increases with Mn in both types of SMPU. The shape recovery temperature, in contrast, decreases with Mn as suggested by the result of their thermal strain recovery. It is concluded that a higher molecular weight (Mn > 200 000 g mol−1) is a prerequisite for SMPUs to exhibit higher shape recovery at a particular temperature. Copyright © 2007 Society of Chemical Industry