Conformational and orientational order and disorder in solid polytetrafluoroethylene

The low-pressure phase diagram of solid poly(tetrafluoroethylene) (PTFE/Teflon) has been investigated using const.-temp./const.-pressure mol.-dynamics techniques and a new all-atom potential model for fluorocarbons. The simulation was started in an ordered low-temp. phase in which the mols. are parallel and have a helical conformation with a pitch of uniform magnitude and sign (chirality). In accordance with expt., a transition to an orientationally disordered state is obsd. upon heating. The coherent helical winding of CF2 groups also disappears abruptly at the transition but short helical segments remain and become equally distributed between left and right chirality with increasing temp. The orientational and conformational disorder is accompanied by translational diffusion along the chain direction. At a still higher temp., melting sets in. On cooling, the disordered solid phase is recovered and its structure is shown to be identical to that generated on heating. On further cooling, a spontaneous ordering transition is obsd. but the system fails to recover a uniform helical ground state. Instead, the high-pressure ordered monoclinic all-trans (alkane-like) structure is obtained: an observation that indicates a deficiency in the potential model. [on SciFinder (R)]

Published in:
Molecular Physics, 97, 3, 355-373

 Record created 2006-02-27, last modified 2018-03-17

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