Monte Carlo simulation of the crystal structure of the low‐temperature phase of polytetrafluoroethylene

Abstract

AbstractAn energy analysis based on molecular field theory and Monte Carlo simulation is carried out for the crystal structure of the low‐temperature phase of polytetrafluoroethylene (PTFE). PTFE molecules in right‐handed and left‐handed 13/6 helical conformation are assumed to be packed in a hexagonal array, with each molecule having 13 possible orientations around the chain axis. Interactions between nearest‐neighbor chains are calculated from van der Waals interactions between nonbonded C‐C, C‐F, and F‐F atom pairs. First, an approximate molecular field calculation is made of the critical fluctuation of the chain orientation at the order–disorder transition point. This suggests that the low‐temperature phase has two chain stems in the unit cell. The Monte Carlo calculation, similar to that used for spin systems, is then applied to the equilibrium packing mode in the low‐temperature phase. A crystal structure containing two chain stems in the unit cell is again obtained. A large amount of disorder in the chain orientation is indicated and is believed to be essential in the PTFE crystal.