Thermal ageing of pyroelectricity in PVDF and P(VDF-TrFE) copolymers

A study of the thermal ageing of the pyroelectric properties of commercially available bioriented PVDF and P(VDF-TrFE) 75-25 mol% films is reported. The analysis of pyrocurrent spectra has been carried out in a large temperature range of -100 degrees C up to +150 degrees C. The pyroelectric coefficient p(3) shows a distinct temperature dependence below and above thermal transitions, and especially the glass transition. We consider how this dependence changes upon ageing. The influence of both ageing temperature and ageing time was investigated. ThermoStimulated Currents spectroscopy (TSC) has been used as a complementary technique of investigation. We have paid a particular attention to the evolution of the relaxation modes associated with the glass transition and with the a, transition as a function of ageing conditions. Experimental results on beta-PVDF show that the decay of the pyroelectric activity is linearly dependent on the annealing temperature. Identical ageing conditions induce a lower decay of p(3) in P(VDF-TrFE) 75-25 than in PVDF, attesting the higher stability of the macroscopic polarization in fluorinated copolymers. The evolution of the decay of p(3) as a function of the ageing time is more complex. Indeed, p(3) decreases quickly below a short characteristic ageing time (a few minutes), and much slower at longer times. This characteristic time is ageing temperature dependent. The evolution of the relaxation mode associated with the glass transition shows that irreversible crystallinity changes occur at ageing temperatures higher than +110 degrees C. So, crystallization phenomena do not play a major role in the decay of p(3). Besides, the alpha(c) transition seems to play an important role in the ageing of pyroelectricity. Indeed, any loss of pyroelectric activity is detected after ageing at temperatures lower than the alpha(c) transition.