A method of lattice model approximation of the solution of rigid polymer is presented. In the present method, a rigid polymer molecule is regarded as a multiple size molecule occuping definit number of lattice points of a simple cubic lattice. The combinatory factor, i. e., the explicit formula of the number of distinguishable configurations of the molecules is constructed by the method of pseudo-assembly approximation (Watari, Kurata, Busseiron-Kenkyu, 37, 81 (1951)), in order to calculate the second virial coefficient of the osmotic pressure of the solution. Making the lattice constant to be infinitesimal, the configuration in space of the molecules becomes continuous with respect to their positions, their directions being restricted only to three directions of the principal axes of cubic lattice. In the case of spherical molecule, since the positional continuity also assures the directional one, the second virial coefficient obtained by the present method is in prefect accordance with that of Zimm (J. Chem. Phys., 14, 164 (1946)). This proves the present method to be rigorous in this case. However, those of non-spherical molecules can be determined only approximately due to the restriction of the directions of molecules. The approximate second virial coefficients of rectangular and circular rod molecules are calculated, and the results are compared with the rigorow ones obtained by Ishihara (J. Chem. Phys., 18, 1446 (1950)).
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