Modeling of helix reversal defects in polytetrafluoroethylene II. Molecular dynamics simulations

Molecular mechanics and dynamics simulations have been utili zed to probe t he nature of helix reversal activity in polytetrafluoroethylene (PTFE). The results of the simulations indicated that helix reversals do form and migr ate in PTFE crystals. At low and intermediate temperatures, the most import ant defect structure was a helix reversal band: two helix reversals which b racket a small chain segment having the opposite helical sense from the par ent molecule. The size of this reversal band defect was equal to approximat ely half of the helical repeat unit in the low and intermediate temperature phases. In the high temperature phase where intermolecular effects are dim inished, a wider distribution of reversal band sizes was observed during th e simulations. The impact of helix reversal activity on rotational disorder was also examined. In addition to the rotational disorder created by the p resence and motion of helix reversals, a mechanism was identified by which significant reorientation of a chain segment (having the same helical sense as the host chain) about the molecular axis can occur when it is bracketed by two helix reversal bands. (C) 1999 Elsevier Science Ltd. All rights res erved.