Phase behavior and some calorimetric and physical properties of diurethanes from perfluoropolyether macromonomers

Density and calorimetric measurements have been carried out on copolymeric perfluoropolyethers of molecular weight ranging between 500 and 4700, terminated at both chain ends either with -CH2OH or ethylurethane groups. Samples of the second series are models for perfluoropolyethers based polyurethanes. Density measurements point out that the terminal alcoholic group brings about a negative excess volume, which increases with decreasing the molecular weight; a lower excess volume is found for urethane-terminated molecules. Calorimetric traces, T-g, and Delta c(p) at T-g show that alcoholic-terminated molecules are amorphous and one phase systems, while urethane-terminated compounds are crystalline; phase separation is observed for this last series when the molecular weight of the fluorinated segment is larger than 1000. The trend of T-g with molecular weight is discussed for one- and two-phase systems in the light of current theories for the glass transition. The decrease of the melting point with increasing the molecular weight for the urethane series can be described by the Flory equation for random copolymers. (C) 1999 John Wiley & Sons, Inc.