Extrusion of poly(vinylidene fluoride) filaments: effect of the processing conditions and conductive inner core on the electroactive phase content and mechanical properties

The phase transformation from the alpha- to the electroactive beta-phase of poly(vinylidene fluoride) (PVDF) extruded filaments submitted to different stretching conditions was investigated. Sample filaments of alpha-PVDF thermoplastic were extruded and stretched uniaxially at different temperatures (80 A degrees C to 120 A degrees C) and stretch ratios (1 to 6). The stretched samples were studied and characterised by x-ray diffraction and quasi-static mechanical experiments. High beta-phase contents (similar to 80%) are achieved using a stretch ratio of 5 independently of the stretching temperature, between 80 A degrees C and 120 A degrees C. Subsequently, in order to obtain filament geometries and material configurations suitable for application, a two layer filament with coaxial layers was produced by coextrusion. The inner layer consisted of a commercially available grade of a conductive thermoplastic with a polypropylene (PP) matrix. For the outer layer the same grade of PVDF was employed. The double-layer filament was also stretched under the same conditions of the PVDF filaments and the results obtained shows that the inner layer material, acting as an electrode, does not have any influence in the PVDF crystallization process: PVDF crystallizes in the alpha-phase for stretch ratios of 1 and the alpha-to beta-phase transformation occurs for higher stretch ratios.