Quasielastic light scattering studies of dynamics of dilute and semi-dilute polymer solutions

Abstract

The dynamics of concentration fluctuations of polystyrene ([Mbar] _w = 20 000) in cyclohexane are studied as a function of temperature and concentration using the photon correlation spectroscopic technique. Over a 60°C temperature range, the translational diffusion coefficient [Dbar] _c changes by more than two orders of magnitude. At a given temperature, [Dbar] _c first decreases with increasing concentration until a critical concentration C* at 22 per cent is reached, beyond which [Dbar] _c increases with increasing concentration, reaching a maximum at higher concentration. The critical concentration C* is insensitive to temperature variations and cannot be identified with the overlap concentration of the entangled polymer. The scattering intensity as a function of concentration also shows a maximum at C*, suggesting that the slowing down of concentration fluctuations due to the decrease in (∂μ/∂C) _P,T is responsible for the intensity maximum. It is concluded that the emergence of the cooperative motion at C* is closely related to the effects of critical phenomena observed at the cloud point.