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gauche (TGTG’) with a unit cell that is centrosymmetric due to the anti-parallel packing of the two chains contained in the cell. Dipole moments are randomly aligned in the crystalline phase of the polymer, therefore resulting in a non-polar form [95, 97, 98]. Form III (γ -phase) can be formed via solution crystallization by using N,N-dimethylformamide (DMF), dimethylamine (DMA) or dimethyl sulfoxide (DMSO) [99] and by melt crystallization with high temperature and high pressure. It can also be transformed to β-phase by drawing. The configuration is an intramolecular mix of both α-phase and β-phase that is (T3GT3G’) [95]. Therefore, the piezoelectric effect of form III is lower than form I but higher than form II. Form IV (γ -phase) is produced by the transformation of non-polar α-phase by subjecting to a high electric field and so producing an inversion of dipole moments, so they become non-centrosymmetric. It can also be transformed to β-phase by subjecting to high electric field [95]. As mentioned earlier, the most important polymorph of PVDF is β-phase for providing enhanced PE properties. Due to the thermoplastic nature of the polymer, it can easily be melt-processed and molten PVDF will exhibit non-polar α-phase unless it is processed under the right combination of mechanical, thermal and electrical conditions [100]. If the polymer is subjected to the optimized conditions, randomly oriented dipole moments will align to form polar β-phase. However, it is important to consider that the piezoelectric effect of PVDF is only stable up to its Curie temperature which is about 80–90 ◦ C. The most frequently used piezoelectric fibre production methods are discussed further in the next sections.
10.2.2.1
Electrospinning of Piezoelectric Textile Structures
Electrospinning is an electrostatic fibre formation technique which enables fibre production with a diameter ranging from few nanometres to several micrometres via the application of an electrical field [101, 102]. There are two established electrospinning techniques: solution electrospinning where the polymer is dissolved in an appropriate solvent or a mixture of solvents and melt electrospinning where a molten-stage thermoplastic polymer is used to form fibres. The main components of an electrospinning system are a syringe pump (feeding unit), a spinneret (with a metallic tip), a high-voltage power supply and a grounded collector (Fig. 10.4). There are a number of parameters that affect the fibre formation including solution viscosity, feeding speed, diameter of the hole(s) on the spinneret, applied high voltage, collector (size, shape, material, etc.) and the distance between the spinneret and the collector. A certain polarity is injected into the polymer solution or the molten polymer via the high-voltage power supply, and the produced fibres are collected by a conductive collector having the opposite polarity. In the literature, works on electrospinning are mostly concentrated on solution electrospinning since almost all thermoplastic polymers require elevated temperatures to melt. For solution electrospinning, the polymer is dissolved in an appropriate solvent at a suitable temperature. The solution is then introduced into the capillary tube, which has a spinneret with a metallic tip. Due to the high electric field applied via the voltage power supply, an electric charge