We now need a model that we can use to calculate dynamical properties of polymeric systems. In order to be able to concentrate on concepts, and not on mathematical problems, we need a model which is as simple as possible. We therefore adjust the Gaussian chain such that it serves our purposes.
When a polymer chain moves through a liquid every bead, whether it represents a monomer or a larger part of the chain, will feel a certain friction together with random forces . Of course the motion of a bead through the liquid will, like in the previous chapter, induce a velocity field in the liquid which will be felt by all the other beads. To first order we might however neglect this effect and consider the solvent as being some kind of indifferent ether, only producing the friction. The model defined in this way is the Rouse model . When applied to dilute polymeric solutions it gives rather bad results, indicating the importance of the hydrodynamic interactions . When applied to polymeric melts we expect the model to be much more appropriate, because in polymeric melts the friction may be thought of as being caused by the motion of the chain relative to the rest of the material, which to a first approximation may be taken to be at rest; propagation of a velocity field like in a normal liquid is highly improbable, meaning that there is no hydrodynamic interaction. It will turn out in Chapter 8 that the Rouse model is a very useful model to describe polymeric melts.