A simple statistical thermodynamic treatment of the dependence of lateral order in molecular arrangement on temperature for the crystallite of polyether type polymer -{-A-(B) _b-} _n-is put forward in this paper: this treatment is analogous to the elementary theory of orderdisorder transformation for a binary alloy. The degree of lateral order (f_p) is given by following equation, _??_
where b is the number of atomic group B in a repeating unit, l is the dimension of a segment moving along the direction of chain axis in the crystallite, Δεr is the difference in bond energy between ordered and disordered arrangements for a pair of repeating units, k is Boltzmann constant and T is absolute temperature.
The heat of fusion of polyether type polymer (ΔH_f) is given by following equation, where f_p is the degree of lateral order in molecular arrangement calculated from above equation at the melting point (T_m) and ΔH_AA, ΔH_BB and ΔH_AB are the heats of fusion for A-A, B-B and A-B bonds repectively, _??_
The heats of fusion of polyethers of polymethyleneoxide series calculated by this equation are in good agreement with measured values.