Nonlinear rheology of telechelic polymer networks

We report on the nonlinear rheology of aqueous solutions of telechelic associating polymers in the network regime. Different telechelic polymers with a poly(ethylene oxide) middle chain and semiperfluorinated end caps C8F17(CH2)(11) were synthesized and characterized with respect to their functionalization. At a telechelic concentration c = 4%, the aqueous solutions studied are highly viscoelastic and close to Maxwellian fluids. Due to the strong hydrophobicity of the end caps, the network relaxation time can be very long compared to that of polymers with fully hydrogenated hydrophobes. We have exploited these long relaxation times to explore the time-resolved stress responses obtained in start-up experiments. The steady shear stress versus shear rate curves exhibit two main and stable branches separated by a discontinuity. The first branch of the flow curve is Newtonian and shear thickening, whereas the second branch is shear thinning. At the onset of shear thickening, we show that the viscosity increase is related to the hardening of the network, i.e., to an increase of the effective elastic modulus. In the shear-thinning branch, extremely slow transients are observed and the steady state is reached for deformations as large as several hundreds of strain units. It is suggested that these slow relaxations, as well as the discontinuity in the shear stress, are the signatures of the onset of an inhomogeneous flow resulting from the breakdown or rupture of the network. (C) 2001 The Society of Rheology.