Static and dynamic light scattering experiments on semidilute solutions of polystyrene in cyclohexane between the Θ-temperature and the binodal curve

Static and dynamic light scattering experiments are carried out with solutions of a high molar mass polystyrene (Mw=0.96×106 g mol-1) in cyclohexane at different concentrations of the polymer. At the Θ-temperature TΘ the critical concentration of the system is located in the semidilute regime. The temperature is varied between TΘ and the temperature of phase separation TP of the solutions. At TΘ the distribution of the amplitudes of the relaxation rates of the autocorrelation function of the electric field is bimodal (slow and fast relaxation rates) in the semidilute regime. It becomes monomodal approaching TP. The contribution of the fast relaxation rates disappears and the weak q-dependence of the slow mode changes over to a q-dependence similar to that of the critical relaxation rate in binary liquid mixtures of low molar mass components close to the critical temperature Tc. On approaching the temperature of phase separation by decreasing the temperature, concentration fluctuations with long range correlations develop. The change of the q-dependence of the slow relaxation rate is rationalised using the transient gel model of Brochard and de Gennes. Here it is assumed that approaching TP transient gels formed by interpenetrating polymer coils become part of the concentration fluctuations with long range correlations. The dynamics of the solutions in the vicinity of the binodal curve monitored by dynamic light scattering experiments is dominated by the dynamics of the concentration fluctuations with long range correlation.