Bond vector probability distribution function of bulk molecules

In our previous work we studied dynamics of the interfacial layer between flowing polymer melt and a solid wall. We showed that the ensemble-averaged behavior of the polymer molecules grafted on the wall could be successfully described in terms of the so-called bond vector probability distribution function (BVPDF). As was shown, the BVPDF satisfies a certain nonlinear integro-differential equation with all the major relaxation mechanisms taken into account. The goal of the present work is to extend the developed formalism to the bulk flow region. For that the corresponding equation of motion for the BVDPF is derived which allows for reptation and the isotropic boundary conditions inherent to the bulk chains