Studies of the concentration dependence of the conformational dynamics of solutions containing linear, star or comb homopolymers

The concentration dependence of the conformational dynamics of polymer solutions as revealed by measurements of oscillatory flow birefringence (o.f.b.) has been obtained for narrow-distribution linear, comb and regular-star molecules for concentrations in the range c[η]≲ 11. The data obtained show that the relaxation-time spectrum is affected markedly by concentration. The longest relaxation time is affected most; for the linear, comb or 3-armed star polystyrenes studied to date, τ1 exhibits an exponential dependence on concentration for c[η] < 3. The Muthukumar theory correctly predicts the dependence of τ1 on concentration for solutions containing linear molecules for which c[η]⩽ 5. There is some indication that τ1 as a function of concentration exhibits two different concentration regimes, one above and one below the onset of significant entanglement effects. The shortest relaxation times are almost unaffected by concentration. The measured frequency dependences of o.f.b. properties for finite concentrations have also been compared with the predictions of the Muthukumar and Freed and the Muthukumar theories for the concentration dependence of the relaxation times for the bead–spring model.