α - to - β Transformation on PVDF Films Obtained by Uniaxial Stretch

The α to β phase transformation of PVDF through the stretching process at different temperatures was investigated. The optimum stretching conditions were studied and characterised by infrared spectroscopy and differential scanning calorimetry. The maximum β−phase content was achieved at 80oC and a stretch ratio of 5. Accompanying the phase transformation, a orientation of the polymer chains and a packing of the crystalline structure is observed. The stretch ratio does not significantly affect the degree of crystallinity. Introduction Poly(vinylidene fluoride) (PVDF) is being widely investigated due to its outstanding pyroand piezoelectric properties [1-7]. These properties generated various applications, especially in the field of sensor and actuator devices and technologies [1,2]. The existence and the optimization of these properties are intimately related with the crystallinity level, structure and orientation of the crystalline part of the polymer, which in turn heavily depend on the processing conditions [1]. PVDF is a semi-crystalline polymer which shows polymorphism and is commonly crystallized in non-polar crystalline α-phase. The piezoand pyroelectric properties mainly depend on the β-phase content and its characteristics. Thus, the increment of the β-phase content has always attracted great interest [1,2,6,7]. The β-phase can be induced by several techniques, the most common being the mechanical stretching of the α-phase films at a suitable temperature. The α-phase is developed directly from the melt. The conversion of α into β-phase takes place when stretching at temperatures below 100°C, using a stretch ratio between 3 and 5. In this work the formation of the β-phase through the stretching process at different temperatures was studied. The optimum stretching conditions were studied and characterised by infrared spectroscopy. Due to its effects on pyroand piezoelectric properties of films, the changes in crystallinity are also reported.