Local variation of the dielectric properties of poly(vinylidene fluoride) during the α- to β-phase transformation
The phase transformation from the non-polar α-phase to the polar electroactive β-phase of polyvinylidene fluoride (PVDF) has been investigated using the fluorescence from Nile red. Films of α-PVDF doped with Nile red were stretched at controlled rates at a temperature of 80 °C to produce the α- to β-phase transition. The thermo/mechanical dependent changes in the crystalline structure are related to the physical rotation of the polar (CH2-CF2) group, which can be monitored by steady state fluorescence techniques. The degree of phase transformation is related to variation in the fluorescence, which in turn is linked to local dielectric constant of the polymer. The variation of the refractive index is more associated to the alignment of the polymeric chains than to the phase transformation. Thus, fluorescence is a suitable technique to monitor phase transitions coupled to a variation in the polarity of the dielectric medium.