Effect of Finite Chain Length on the Helix/Coil Coexistence Behavior of Polymers: Poly(oxymethylene)

Monte Carlo calculations were performed on poly(oxymethylene) chains using the rotational isomeric state model. The end-to-end distribution function was found to exhibit bimodal behavior, characteristic of a helix/coil coexistence, over a range of temperature and chain length. The first-order gauche distribution (P_G) and the second-order gauche pair distribution (P_GG) did not show any bimodality. The sequence length distribution, however, did show pronounced bimodality. The transition-like behavior was found to become sharper and shifts to lower temperature with a logarithmic dependence on the chain length. Thus the helix/coil coexistence behavior is a finite chain effect, with the transition temperature approaching 0 K for the infinite system. As expected, an external force on the ends of the chain was found to shift the coexistence temperature to higher temperatures. These results can be understood by analogy with the one-dimensional Ising model.