Dynamics of critical fluctuations of a macromolecular solution

Abstract

The angular and the spectral distribution of scattered light intensity and the hydrodynamic shear viscosity from polystyrene (Mn = 1.96×105, Mw/Mn < 1.02) in cyclohexane were measured at the critical solution concentration above the critical consolute temperature in the one phase region. In comparing the linewidth, correlation length, and shear viscosity data with the modified mode−mode coupling theory of Kawasaki, we note that the magnitude of a ’’scaled linewidth’’ Γ+ (≡6πη*pΓ/kBTK3) as a function of Kξ is a factor of 1.5−3 higher than the single universal curve of Kawasaki and Lo, where η*p, Γ, kB, T, K, and ξ are, respectively, the shear viscosity, the linewidth, the Boltzmann constant, the absolute temperature, the magnitude of the momentum transfer vector, and the correlation length. Since the theory describes only the critical part of the linewidth Γc, we separate the decay rate due to critical fluctuations and the finite molecular size of the polymer. The resultant scaled linewidth Γ* (≡6πη*pΓc/ kBTK3) as a function of Kξ can then be brought down to the universal curve. Our comparison between existing polymer data and theory represents a first attempt to take into account the finite molecular dimension of the system in decay rate of order parameter fluctuations. The data exhibit considerable scatter after the background correction. Consequently, there seems to be a need to consider more carefully the manner in which the backgrounds are subtracted before more crucial comparisons between experiments and theory can be made.