Statistical mechanics of macromolecular networks without replicas

Abstract

We report on a novel approach to the Deam-Edwards model 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 {\it 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 $R_{\mbox{\tiny g}}$ of a RCGN is of the universal form $R_{\mbox{\tiny g}}^2=(0.26 \pm 0.01)a^2 N/M$, 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 modified version of Leibler's mean field theory for block copolymers.

24 pages, Revtex, 3 figures not included

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