Conformational and orientational order and disorder in solid polytetrafluoroethylene

The low pressure phase diagram of solid polytetrafluoroethylene (PTFE/Teflo n) has been investigated using constant temperature-constant pressure molec ular dynamics techniques and a new all-atom potential model for fluorocarbo ns. The simulation was started in an ordered low temperature phase in which the molecules are parallel and have a helical conformation with a pitch of uniform magnitude and sign (chirality). Tn accordance with experiment, a t ransition to an orientationally disordered state is observed 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 temperature. The orientat ional and conformational disorder is accompanied by translational diffusion along the chain direction. At a still higher temperature melting sets in. On cooling. the disordered solid phase is recovered and its structure is sh own to be identical to that generated on heating. On further cooling, a spo ntaneous ordering transition is observed 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 indicat es a deficiency in the potential model.