Variational principle for the Kirkwood theory for the dynamics of polymer solutions and suspensions

The general theory of Kirkwood for the dynamics of polymer solutions and suspensions is reformulated in the form of a variational principle. This formulation is analogous to Lagrangian mechanics and the time evolution equation is derived from a certain scalar functional. The advantage of this formulation is that: (i) it states the essential physics of the Kirkwood theory without using the Riemannian geometry; and (ii) it suggests various new schemes of approximation. Two examples are given to illustrate the usefulness of the formulation, i.e., the dynamics of rodlike polymers and the deformation of an elastic particle in elongational flow.