The Equation of State of a Gaussian Phantom Network with Defined Cross   Link Functionality
  A phantom network of Gaussian chains far from the point of gelation can be described as a gas of interacting particles represented by the cross links. The type of particles varies with the network functionality, whereas the type of interaction depends on the properties of the connecting chains. In a mean field approximation the Equation of State can be calculated using Mayer's cluster expansion. The resulting isothermal compressibility is compared for different cross link functionalities. 
Benjamin Paul, Michael Schulz, and Harry L. Frisch