Structure, properties, and phase transitions of melt‐spun poly(vinylidene fluoride) fibers

Abstract

Three different experimental techniques were used to study structural phase transitions in meltspun poly(vinylidene fluoride) fibers, which were produced with different process parameters and processed in the draw-winding process at different temperatures and draw ratios. The fibers are examined with the help of wide-angle X-ray diffraction at elevated temperatures, differential scanning calorimetry with stochastic temperature modulation, and dynamic mechanical analysis. An oriented mesophase and deformed crystal structures can be observed in all fibers and assigned to the mechanical stress occurring in the processes. Furthermore, several phase transitions during melting and two mechanical relaxation processes could be detected. The observed transitions affect the crystal geometry, the orientation distribution, anisotropic thermal expansion, and the mechanic response of the fiber samples. The relaxation processes can be related with an increasing amount of crystalline b-phase in fibers drawn at different temperatures. The detailed information about phase transitions and the related temperatures are used to produce fibers with an extended amount of b-phase crystallites, which are responsible for piezoelectric properties of the material.

Simulation of α- and β-PVDF melting mechanisms

“…The increase in melting point with increased chain length is expected [24] and is partially due to the increased density. It is also consistent with the fact that the α-and β-PVDF crystals with infinite chains gave the highest melting points.…”

“…Both of these melting points are relevant to the present study, since we simulate crystals made from finite and infinite chains. In addition, recent studies on mechanically drawn PVDF fibres by Steinmann et al [24] showed that and increase in temperature converts the β-PVDF into α-PVDF, which then melts or changes into γ-PVDF. This appears to contradict data that shows that β-PVDF has a higher melting point than α-PVDF [25,26], or that α-and β-PVDF are melting at the same temperature [27].…”

Thermal Analysis of Phase Transitions and Crystallization in Polymeric Fibers

“…Possible applications include direction sensitive and spatially resolved strain measurement, which is useful for health monitoring in medical / smart textiles or structural health monitoring in fiber reinforced composites. However, suitable process conditions for fiber spinning, drawing and further processing steps have to be found, whereas phase crystallites have to be formed and not be destroyed along the process chain [39][40][41][42]. Therefore, methods of thermal analysis were developed to identify the presence of the phase, which are validated by additional X-ray diffraction measurements (WAXD) and dynamic mechanical analysis (DMA) [40,42].…”

“…However, suitable process conditions for fiber spinning, drawing and further processing steps have to be found, whereas phase crystallites have to be formed and not be destroyed along the process chain [39][40][41][42]. Therefore, methods of thermal analysis were developed to identify the presence of the phase, which are validated by additional X-ray diffraction measurements (WAXD) and dynamic mechanical analysis (DMA) [40,42]. In the following section of this chapter, these methods and their validation will be demonstrated, and they will be applied to gather information about phase transitions during melt spinning, drawing and heat treatment.…”

Textile piezoelectric sensors – melt spun bi-component poly(vinylidene fluoride) fibres with conductive cores and poly(3,4-ethylene dioxythiophene)-poly(styrene sulfonate) coating as the outer electrode

“…It was recently found that the same effect can be achieved with sufficient cold drawing, i.e. drawing in the temperature interval between T g and T m , during the melt spinning process of PVDF textile fibres (Lund and Hagström 2010;Steinmann et al 2011).…”