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The phase transition from the non-polar alpha-phase to the polar beta-phase of poly(vinylidene fluoride) (PVDF) has been investigated using micro-Raman spectroscopy, which is advantageous because it is a nondestructive technique. Films of alpha-PVDF were subjected to stretching under controlled rates at 80 degrees C, while the transition to beta-PVDF was monitored by the decrease in the Raman band at 794 cm(-1) characteristic of the alpha-phase, along with the concomitant increase in the 839 cm(-1) band characteristic of the beta-phase. The alpha-->beta transition in our PVDF samples could be achieved even for the sample stretched to twice (2x-stretched) the initial length and it did not depend on the stretching rate in the range between 2.0 and 7.0 mm/min. These conclusions were corroborated by differential scanning calorimetry (DSC) and X-ray diffraction experiments for PVDF samples processed under the same conditions as in the Raman scattering measurements. Poling with negative corona discharge was found to affect the alpha-PVDF morphology, improving the Raman bands related to this crystalline phase. This effect is minimized for films stretched to higher ratios. Significantly, corona-induced effects could not be observed with the other experimental techniques, i.e., X-ray diffraction and infrared spectroscopy.

This paper contains a brief overview on the recent developments of corona charging of polymers, with emphasis on the constant current corona triode. This latter method, which has been successfully applied to several types of polymer, is a legacy from Prof. Bernhard Gross' work in São Carlos, Brazil. Following a short introduction to corona charging, the experimental setups are described, especially with regard to the advantages in the constant current method. A few examples are given of the use of the constant current corona triode in the investigation of electrical properties of nonpolar and ferroelectric polymers. The application of corona charging to pole nonlinear optic NLO polymers is discussed, including the perspectives for the constant current c harging method for the NLO eld. I IntroductionInterest in corona charging of dielectrics arose in the 1940s following Carlson's invention of the rst electrophotographic system, since sensitizing the photoconductive plate by corona discharge was proven more effective than other methods. A number of experimental arrangements have been used since then. For instance, in the corotron setup corona wires are placed above the photoconductive plate, whereas in the scorotron system a metallic grid is inserted between the corona wires and the plate, the grid being biased to a voltage that approximates the potential required for charging the plate. Both systems permit very rapid charging. From the 1960s corona discharge became extensively employed for charging polymer foils for electret production and also for investigating charge stability in these electrets. Corona systems for such purposes may b e simply a two-electrode arrangement comprising a point or wire and a plate or cylinder. Prof. Bernhard Gross realized, however, that invaluable information was lost during the charging process. In the 1970s he then instituted a project for developing corona triodes in which in addition to good charging control, the charging current and sample surface potential could be measured. Several versions of the corona triodes were developed, where the greatest advance was achieved when the constant current method was introduced. With this new technique, not only charging uniformity could be substantially improved but also interpretation of experimental results on charging transport was facilitated because one important parameter, the charging current, was xed. The constant current method was itself improved over the years, especially with regard to making it as versatile as possible in terms of the range of charging currents and nal charging potentials available and the accuracy in the surface potential measurement. In this paper we shall describe the most recent advances in the constant current corona triode method, and its use for charging and investigating the electrical properties of a variety of polymers. In Section 4.1, experimental results and theoretical models will be discussed