Die swell of thermoplastic polyurethanes: A peculiar behavior

The instantaneous and equilibrium isothermal die swell ratios of thermoplas tic polyurethanes have been measured by two methods. The incidence of some structural, geometrical, and processing parameters has been analyzed. The e xperimentation reveals the atypical behavior of these polymers related to a structural evolution during processing that greatly depends on the residen ce time in temperature under shearing and that probably results from the cu mulated effects of multiple recombinations and chain cuts. This thermo-mech anical degradation affects the rheological properties of these polymers. Th e maximal behavior variations are observed for short times and stabilize ab ove a specific critical time. Taking into account the effect of these rheol ogical alterations on the generating mechanisms of die swell allows us to i nterpret all the experimental results (atypical or not) in terms of magnitu de and direction of variation of the phenomenon. (C) 2001 John Wiley & Sons , Inc. J Appl Polym Sci 80: 1710-1724, 2001.