Abstract
The glass transition temperatures and the viscosities (over a very large temperature range) have been measured as a function of molecular weight, for two different series of perfluoropolyethers. The results have been interpreted according to well-known free-volume models. The Williams-Landel and Ferry equation describes the viscosity data with values of f g (the fractional free volume at T g ) close to 0.04 and with values of α f (the free-volume expansion coefficient) close to 7 × 10 −4° C −1. The critical molecular weight, M c , is approximately 5000–6000 for perfluoropolyethers from hexafluoropropene and 7800 for perfluoropolyethers from tetrafluoroethene.
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