Molecular Mobility, Phase Transitions, Transformation Plasticity and Memory Effect in Poly (tetrafluoroethylene) (PTFE)

The dynamic mechanical behaviour of polytetrafluoroethyne is investigated by means of high resolution low frequency mechanical spectroscopy. Two relaxation processes are present. The α process is located at 0.1Hz near 177K, and the α' relaxation is close to 410K. Both processes are sensitive to the amorphous/crystalline microstructure. We show for the fust time that the low temperature α process is associated with a glass transition, whereas the α' relaxation is to be related with defects motions in the disordered crystalline phase. Mechanical spectroscopy also show narrow peaks at the phase transitions (near 293 and 308 K). The study of deformation under a constant stress, during thermal cycles between 283 and 303K enabied us to show evidence of a transformation plasticity and a stress assisted memory effect related to the low temperature phase transition. Careful estimations are necessary to distinguish between the memory effect contribution and other superimposed deformations, due to elastic modulus changes, phase transformation dilatation. and thermal dilatation between 283 and 303K