Journal of Statistical Physics. Vol. 77. Nos. 1/2. 1994, p. 491-500.
No modern theory of polymer excluded volume adequately describes the crossover from poor solvent to good solvent conditions; a fundamental difficulty is a singularity in the binary cluster integral. Mayer expansion techniques are applied to a model with a pair interaction between monomers to clarify the distinction between "geometric" and "solvent" contributions to excluded volume. Detailed calculations are undertaken for a hard-core potential and a mimic Lennard-Jones potential The significance of the singularity in the binary cluster integral for calculations in the crossover regime is discussed.