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 a process is located at 0.1 Hz near 177 K, and the alpha' relaxation is close to 410 K. Both processes are sensitive to the amorphous/crystalline microstructure. We show for the first time that the low temperature a process is associated with a glass transition, whereas the alpha' 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 303 K enabled 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 303 K.