Abstract: In this paper, molecular orientation behavior of polymethylene chains was investigated on the basis of considering long-distance interactions by using the enumeration calculation method and the rotational isomeric state(RIS) model. The chain lengths are from N=12 to N=24, and the Lennord-Jennes potential was adopted in the nonlocal interaction calculations. The molecular orientation function(Legen-dre polynomial) may be well expressed as <P₂(ζ)>/(λ²-λ^-1)=a(λ²-λ^-1)+b here a and b only depend on the chain length, and decrease with the increase of chain length, especially for b. In the meantime, we also calculate the Helmholtz free energy change in the process of tensile deformation. Comparisons with Gaussian chains are made, and our results are close to those obtained from the Roe and Krigbaum theory.

Key words: Polymethylene chain, Molecular orientation, Elastic deformation, Rotation isomer state model