Crystalline Beta Phase as a Function of Temperature in Electrospun and Melt Extruded PVDF

Polyvinylidene fluoride (PVDF) is a semicrystalline polymer with multiple phases, of which the most electroactive is the beta phase. As a flexible polymer with high piezoelectric output (when there is a large crystalline beta phase fraction (F_CB)), it is an attractive option for physiological and structural health monitoring. Traditionally piezoelectric PVDF is produced through melt extrusion followed by a poling step by which the film is stretched under high temperature and a strong electrical field to increase the F_CB. Electrospinning is an alternative production technique in which PVDF in solution is extruded through a needle under high voltage, producing a mat of nanofibers with high F_CB without the need for another poling step.
However, the F_CB decreases when exposed to high temperatures, causing hesitancy towards using piezoelectric PVDF in high temperature environments or using PVDF in electronics where high temperature processing steps are required. Literature conflicts on the degree of degradation and the temperature at which this occurs. There is limited evidence to suggest that the ability of the PVDF to bow and flex while being exposed to high temperatures causes degradation in the crystalline beta phase. Furthermore, current academic literature has focused on the commercial melt extruded/poled PVDF. It is reasonable to believe that electrospun PVDF’s fibrous nature may lead to a different temperature response than a connected film of PVDF produced by melt extrusion. This lack of knowledge means it is not possible to make informed decisions on whether to use electrospun or melt extruded PVDF in certain applications. It also further elucidates constraints on manufacturing with respect to temperature exposure.
The study utilises a range of temperature exposures from ambient to 200°C over a range of times; 1 minute, 1 hour, and 24 hours. These correspond to different use cases, such as short-term soldering, an hour of curing an ink onto the PVDF, and 24 hours of use in a high-temperature environment. Both electrospun and commercial PVDF are exposed to these temperatures and FTIR is performed to obtain the beta phase content, DSC is performed to obtain the crystallinity, and the product of the two represents the crystalline beta phase.
The results show markedly different temperature responses between electrospun and melt extruded PVDF. Both the crystallinity and overall beta phase content as a function of temperature varied between the two forms of PVDF with the electrospun form showing a higher F_CB than the melt extruded form.
In conclusion, electrospun PVDF can be produced with a higher F_CB than commercially obtained melt extruded PVDF. Further, electrospun PVDF shows less crystalline beta phase degradation when exposed to high temperatures for both short and extended periods of time. Paired with a more streamlined production process, electrospinning provides a viable alternative for PVDF production when temperature resistance is a key requirement in either manufacturing or when in use.