The random phase approximation for polymer melts with quenched degrees of freedom

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Abstract
There are many concentrated polymer systems of interest (e.g. crosslinked rubber networks) where the chains retain their configurational freedom but the translational degree of freedom is absent. To investigate the effect of the loss of this degree of freedom on the density fluctuations we have considered a simple case of a melt of polymer chains where one end of each chain is anchored to a fixed point in space. The random phase approximation (RPA) used for melts cannot be immediately applied to this problem, since the loss of translational freedom couples together density fluctuations belonging to different wavevectors. The quenched chain end variables must first be accounted for using a “replica” calculation, then the RPA can be used for the annealed configurational degrees of freedom. An explicit analytic result has been obtained, which shows that despite the loss of the translational degree of freedom the density fluctuations in the quenched system remain similar to those in the true melt out to spatial scales of the order of N1/4R, where R is the size of a single chain coil (∼N1/2) and N is the degree of polymerisation. We have also considered the concentration fluctuations a binary blend of chains with anchored chain ends. The situation is shown to be very similar to a diblock polymer melt in so far as when the Flory interaction parameter exceeds a given value microphase separation occurs. Again the loss of the translational freedom of the chains makes little difference to the stabilily criterium

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