The effects of orientation-dependent interactions on the segmental orientation in weakly deformed polymer systems were reexamined on the basis of lattice theories and gas-of-rods theories. Both the freely jointed chain and the wormlike chain were considered as polymer models. It was shown in a somewhat general fashion that the orientation-dependent excluded-volume effect or packing effect could give rise to a considerable amount of excess orientation over that predicted in the absence of such an effect. For a dry system of freely jointed chains comprising rodlike segments of axial ratio 2, for example, the relative excess orientation was estimated to range roughly from 30 to 100%, depending on the theories. This value of the axial ratio seems to approximate the flexibilities of common vinyl polymers, and in this context, the volume effect would never be a minor one even for the “flexible” class of polymers. The nature of and the differences among the various statistical models were discussed in some detail.
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