Physical properties of perfluoropolyethers: Dependence on composition and molecular weight

Abstract Three copolymeric perfluoroethers with the structure CF 3 [(OCF 2 CF 2 ) p (OCF 2 ) q ] OCF 3 , having different p/q ratios, have been fractionated. The fractions obtained have been characterized by Gel Permeation Chromatography and 19 F‐NMR. The viscosity η the specific volume v and the glass transition temperature, T g have been measured by standard techniques for all the above samples as well as for some other perfluorinated polyethers. The temperature dependence of viscosity of the unfractionated samples is described by the W.L.F. equation. The values of f g (fractional free‐volume at T g ) and of a f (free‐volume expansion coefficient) are independent of composition, for p/q ratios from 0.53 to 1.15. The critical molecular weight, M c , is of the order of 8–9,000. From the molecular weight dependence of specific volume, the contribution to the molar volume of the in‐chain CF 2 group and the excess molar free volume of the chain ends have been determined. The limiting value of T g for an infinite molecular weight polymer was found to depend linearly on the compositional ratio O/C and the extrapolated values for polytetrafluoroethylene and for the homopolymer (CF 2 O) n were found to be respectively 200 K and 120 K.