Statistical mechanics of macromolecular networks without replicas

We report on a novel approach to the Deam-Edwards model (1976) for interacting polymeric networks without using replicas. Our approach utilizes the fact that a network modelled from a single non-interacting Gaussian chain of macroscopic size can be solved exactly, even for randomly distributed crosslinking junctions. We derive an exact expression for the partition function of such a generalized Gaussian structure in the presence of random external fields and for its scattering function S 0 . We show that S 0 of a randomly crosslinked Gaussian network (RCGN) is a self-averaging quantity and depends only on crosslink concentration M/N, where M and N are the total numbers of crosslinks and monomers. From our derivation we find that the radius of gyration R g of a RCGN is of the universal form R g 2 =(0.26+or-0.01)a 2 N/M, with a being the Kuhn length. To treat the excluded volume effect in a systematic, perturbative manner, we expand the Deam-Edwards partition function in terms of density fluctuations analogous to the theory of linear polymers. For a highly crosslinked interacting network we derive an expression for the free energy of the system in terms of S 0 which has the same role in our model as the Debye function for linear polymers. Our ideas are easily generalized to crosslinked polymer blends which are treated within a modifed version of Leibler`s mean-field theory (1980) for block copolymers.