The validity and range of applicability of the Kirkwood-Riseman theory is examined by utilizing diffusivity data taken on infinitely dilute polystyrene solutions. The predictions of the theory are found to be in good agreement with diffusivity data obtained on the polystyrene–cyclohexane system at the theta temperature. It is concluded that diffusivity data which are in substantial disagreement with predictions based on the Kirkwood-Riseman approach are unreliable because of mixing effects which take place in a wedge interferometer when organic solvent–polymer systems are used. An extension of the Kirkwood-Riseman theory to polymer diffusion under nontheta conditions is presented and this modification satisfactorily represents diffusivity data when excluded volume effects are important.
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