Benzene-polystyrene systems, diffusion

Caroline and co-workers have recently reported measurements of translational diffusion coefficients in solutions of PS in two mixed-solvent systems at or near theta conditions. In the solvent CCb-methanol (85), they observed the diffusion theta state, defined when the coefficient y of Equation 41 equals 0.5, to occur at 25°C and a volume fraction of CCI4, (fyCCU = 0.8025. In this system there is strong preferential adsorption of the polymer for CCI4, and it is not possible to define a true theta state such that y = a = V2 and A2 = 0 simultaneously. Under diffusion theta conditions, the concentration dependence of Dt apparently is closely described by the Pyun-Fixman hard-sphere model. In the mixed solvent benzene—2 propanol, polystyrene exhibits a true theta condition at T = 25.5°C and (benzene) = 0.04. Frost and Caroline confirmed that y = 0.5 within experimental error in this system (86) and report that values of the parameter fcf are scattered between the extreme values corresponding to the predictions of Yamakawa (and Imai) and the soft-sphere model of Pyun-Fixman (or the Freed theory). 

As polymer molecular weight increases, the number of chain ends decreases. The chain ends represent a discontinuity and may form sites for permeant molecules to be sorbed into glassy polymers. For example, for a series of polystyrene samples, the diffusivity of a range of organic vapours (acetone, benzene, etc) decreased by a factor of almost ten as molecular weight increased from 10000 to 300000. However, in other systems, molecular weight has been found to have no influence on the transport of liquid permeants. ... 

Adam et al. carried out systematic measurements of the diffusion coefficient of semi-dilute solutions of polystyrene in benzene using QELS and CGD methods. They found (cf. Fig. 12) that the diffusion coefficients obtained by both methods are equal within experimental error. The diffusion coefficient scales with the concentration as. 67 o.o2 depends only on concentration. These results are identical with those obtained by Munch et al. from QELS experiments performed on the same system PS-benzene (cf. Section D-II) and the results of J. Roots et al. who found for PS-toluene by means of CGD experiments that D c ... 

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