Nearest-neighbor interaction model pair approximation

The A-units and B-units of the two pol3mier species both have the same volume Vc and occupy the cells of a regular lattice with coordination number z. It is assumed that the interaction energies are purely enthalpic and effective between nearest neighbors only. Excess contributions kTx, which add to the interaction energies in the separated state, arise for all pairs of unlike monomers. The parameter x = ( 2)x was devised to deal with this model and therefore depends on the size of the cell. Flory evaluated this model with the tools of statistical thermod3mamics. Using approximations, he arrived at Eqs. (4.5) and (4.6). 

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