Viscoelastic behavior of semidilute solutions of multisticker polymer chains

Multisticker polymer chains consisting of water-soluble polyacrylamides hyd rophobically modified with low amounts of N,N-dihexylacrylamide have been p repared by a free radical micellar polymerization technique. This process g ives multiblock copolymers in which the number and length of the hydrophobi c blocks can be tuned by varying the surfactant over hydrophobe molar ratio . The viscoelastic behavior of semidilute solutions of various series of co polymers with variable molecular weights (M-w approximate to 4.2 x 10(4)-2. 7 x 10(6)), hydrophobe contents ([H] = 0.5-2 mol %), and hydrophobic block lengths (N-H = 1-7 units per block) has been investigated as a function of polymer concentration, C, using steady-flow and oscillatory experiments. In the semidilute range, two different regimes can be clearly distinguished i n the zero-sheer viscosity eta(o) = f(C) curves: a first unentangled regime where the viscosity increase rate strongly depends on N-H and [H]; a secon d entangled regime where the viscosity follows a scaling behavior of the po lymer concentration with an exponent close to 4, whatever [H] or N-H. The l inear viscoelasticity can be described by (i) a slow relaxation process wit h a plateau modulus that only depends on polymer concentration and (iii oth er faster complex relaxation processes. In the latter regime, the results c an be quite well accounted for by a hindered reptation model.