Abstract
Abstract The dynamics of concentration fluctuations of polystyrene (M W = 20,000) in cyclohexane is studied as a function of temperature and concentration using the photon correlation spectroscopic technique. Over a 60°C temperature range the translational diffusion coefficient D C changes by more than two orders of magnitude. At a given temperature, D C first decreases with increasing concentration until a critical concentration C* at 22% is reached, beyond which D C increases with increasing concentration, reaching a maximum at higher concentration. The critical concentration C* is insensitive to temperature variations and can be estimated according to the equation for overlap concentration. The scattering intensity measurement as a function of concentration also shows a maximum at C*, suggesting that the slow down of concentration fluctuations due to the decrease in(∂u/∂c)P.T. is responsible for the intensity maximum. It is concluded that the emergence of the cooperative motion of the entangled network at C* is closely related to the effects of critical phenomena observed at the cloud point and the slow-down of translational diffusion.
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