Viscoelastic Behavior of Semidilute Solutions of Multisticker Polymer Chains

Multisticker polymer chains consisting of water-soluble polyacrylamides hydrophobically modified with low amounts of N,N-dihexylacrylamide have been prepared by a free radical micellar polymerization technique. This process gives multiblock copolymers in which the number and length of the hydrophobic blocks can be tuned by varying the surfactant over hydrophobe molar ratio. The viscoelastic behavior of semidilute solutions of various series of copolymers with variable molecular weights (M w ≈ 4.2 × 104−2.7 × 106), 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 in the zero-shear viscosity η 0 = f(C) curves: a first unentangled regime where the viscosity increase rate strongly depends on N H and [H]; a second entangled regime where the viscosity follows a scaling behavior of the polymer concentration with an exponent close to 4, whatever [H] or N H. The linear viscoelasticity can be described by (i) a slow relaxation process with a plateau modulus that only depends on polymer concentration and (ii) other faster complex relaxation processes. In the latter regime, the results can be quite well accounted for by a hindered reptation model.