Macromolecules, Vol.36, No.2, 449-457, 2003
Perfluoroalkyl end-capped poly(ethylene oxide). Synthesis, characterization, and rheological behavior in aqueous solution
A series of perfluoroalkyl end-capped poly(ethylene oxide) (PEO) of molecular weight M-n = 10 000 and 20 000, noted F-HEUR-10K and F-HEUR-20K, with a well-defined structure were synthesized in two reaction steps. The reaction of PEO with a large excess of isophorone diisocyanate (IPDI) leads to an isocyanate-terminated prepolymer which is then reacted with the semifluorinated alcohol [C8F17(CH2)(11)-OH]. Experimental conditions were improved to prevent polycondensation in the first step of the synthesis. The degree of functionalization is controlled by F-19 NMR. The influence of the terminal perfluorinated group on the linear viscoelasticity of aqueous solutions has been investigated as a function of the polymer concentration C-pol, PEO chain length, degree of functionalization, surfactant (SDS), and temperature. A steep increase of the static viscosity eta(0), attributed to the sol-viscoelastic transition with the formation of a multiconnected network, is observed at C-pol approximate to 1 wt %. The stress relaxation function G(t) is best fitted by a stretched exponential G(t) = G(0) exp[-(t/tau)(alpha)], where G(0) is the elastic plateau modulus. The relaxation time T and static viscosity eta(0) were found to be temperature-dependent according to the Arrhenius law. The hydrophobic character of the end caps reinforced by the perfluorinated segments leads to larger relaxation times (several hundred seconds) and higher activation energies (approximately equal to 120 M mol(-1)) compared to those of hydrocarbon HEUR-type associative polymers.