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Several attempts have been made to account for polymer chain stiffness or excluded volume effects in non-Newtonian viscosity theory with the use of a modified Gaussian probability for intrachain distances. This method is here tested for the freely rotating chain by an application to the equilibrium expectation of the end-to-end vector R in the presence of an external potential −R· f. The Gaussian parameters are varied as f increases to keep Gaussian estimates of the bond length and bond angle constant. Numerical tests are made for the limits of a freely jointed chain and the worm chain, and the modified Gaussian approach is found to work very well.
Fixman et al. (Thu,) studied this question.