What is the excluded volume for an excluded volume chain?

Solvated polymers are generally expanded due to the effective interaction between the monomers. In Flory's lattice theory for polymers this 'excluded volume' is the same for a polymer solvated by its own monomers and a polymer in the absence of solvent. The magnitude of the effect is determined by a parameter , that is directly related to the lattice spacing. Computer simulations using a simple off-lattice model (dissipative particle dynamics) show that this is not true. Only an off lattice analogue of Flory's theory that takes into account the compressibility of the solvent quantitatively predicts the degree of expansion as a function of solvent density. We find that for the solvated model chains the distribution of mass around the centre of mass is to a good approximation Gaussian. This means that the diffusion coefficient can be calculated by simply treating the chain as an ideal chain with a Kuhn length that is modified to take into account the expansion.