Enhancing β-Phase in Polyvinylidene fluoride

Polyvinylidene fluoride (PVDF) is known to exist mainly in α-spherulites phase in its pellet forms. However it is the β-crystallites form that is desirable for piezoelectric applications as it exhibits the highest dipolar moment. Electrospinning, due to the combined effect of stretching and electric field has already shown to enhance the presence of β-phase in the resultant fibers while fibers from force spinning (without static electric field) do not exhibit piezoelectricity [Lei et al 2015]. However, the piezoelectric effect of the membrane may be further enhanced by the arrangement of the fibers or by increasing the ratio of β-phase in the fibers.

There are conflicting results on the influence of voltage on PVDF polymorphism. Cozza et al (2013) reported that varying electrospinning parameters have no effect on the α/β crystal ratio. However, Sengupta et al (2017) showed with a single strand electrospun PVDF fiber that increasing voltage does lead to a general increase in its piezoelectric d₃₃ coefficient up to a value of -58.77 pm/V. He attributed this to the electric field encouraging dipole alignment of its molecules. Research have shown that altering the electrospinning parameters will have an influence on the β-phase in PVDF. Sengupta et al (2018) tested the influence of increasing electrospinning voltage on the fraction of β-phase in PVDF fibers. At 10kV, a β-phase fraction of 0.749 was recorded. At 12kV, the β-phase fraction was reduced to 0.715 before increasing voltage led to a gradual increase to 0.789 at 18kV. While it can be explained that increasing electrospinning voltage encourages stretching and poling thus favoring the transformation of α-phase to β-phase, there is an unexplained dropped in β-phase from 10kV to 12kV. It may be that the increased in acceleration of the jet due to higher voltage reduces the time for phase transformation before deposition on the collector while that voltage is insufficiently high to induce rapid transformation. He et al (2022) also showed that the β-phase of PVDF first increases with higher voltage but up to 20 kV before the β-phase ratio drops with further voltage increment. Costa et al (2010) found that by solvent selection, slower evaporation rate favors the formation of β phases in electrospun fibers while faster evaporation rate favors α phase. Huang et al (2008) showed that both α and β phases were present on nanofibers electrospun in ambient temperature from 15 °C to 45 °C. β phase was maximum when the ambient temperature was 25 °C and decrease slightly at higher temperature. This may be due to molecular chain movement at higher temperature.

A slower solvent vaporization rate may contribute to the formation of β-phase in electrospun PVDF fibers. Zaarour et al (2018) showed that electrospinning PVDF dissolved in a mixture of acetone (ACE) and N,N-dimethylformamide (DMF) yield greater crystallinity and β-phase at higher humidity. Electrospun PVDF fibers have a crystallinity and β-phase ratio of 44% and 55% respectively when the humidity is 2%, and 57% and 73% respectively when the humidity is 62%. The reduction in solvent vaporization rate at higher humidity may be attributed to skin formation on the surface of the fibers due to water condensation which traps solvents within its core. Although the solvents in the core will eventually escape, the delayed evaporation allows greater molecular alignment. In a study by Zaarour et al (2019) comparing the level of β-phase in smooth, wrinkled and porous electrospun PVDF fibers, they found that wrinkled surface PVDF fibers have the highest β-phase crystals, followed by smooth fibers and the least was in porous PVDF fibers. While the difference in β-phase crystals level were attributed to the fiber morphology, it may be influenced by the humidity of the electrospinning environment to obtain the various morphologies. Humidity level was kept low to produce the smooth fibers but a higher humidity of 60% was used to produce wrinkled fibers and humidity of 70% for porous fibers. Delay in evaporation due to skin formation may be the cause behind the higher β-phase crystals in wrinkled fibers. However, when interconnected pores start to form in the fibers, this may have disrupted β-phase crystals which led to their reduced amount compared to the others. In same study, comparison between aligned and random PVDF fibers showed no significant difference between the β-phase crystals level. Higher humidity in the electrospinning environment has been shown to increase the β-phase in PVDF and this has been attributed to skin formation on the surface of the electrospinning jet leading to delayed evaporation of the solvent within. Prasad et al (2020) carried this concept further by depositing the electrospinning PVDF jet directly into water. Electrospinning was carried out over an aluminium collector and a water bath collector. Comparing the crystal phase between the two collected PVDF fibers, they found that PVDF fibers exhibited greater β-phase and less α-phase when collected on water than those collected on aluminium substrate. In addition, electrospun PVDF fibers collected on water showed an additional small β-phase at 21.2° which was absent in the fibers collected on aluminium substrate. Less volatile solvent in the electrospinning of PVDF may not always be favorable to the formation of β-phase crystals. He et al (2022) suggested that a more volatile solvent is preferred as the use of less volatile solvent may cause bead formation on the fiber and the presence of beads disrupts the ordering of the polymer chain into β-phase crystals. Alternatively, a tip to collector distance may be increased to allow more time and distance for the electrospinning jet to stretch and solidify.

Representative pictures of samples fabricated by electrospinning of PVDF solutions with different morphologies. (a-c) Randomly oriented fibers, (a) wrinkled, (b) smooth, and (c) porous [Zaarour et al 2019].

A rotating collector may facilitate β-phase crystals ordering through mechanical stretching. With drum rotating speed up to 200 rpm, the electro-active phase increases from 87% to 91%. However, at high rotating speed up to 2000 rpm, the electro-active phase decreases, probably due to fiber breakages [He et al 2022].

While the study by Zaarour et al (2019) showed that electrospun porous PVDF fibers have less β-phase compared to smooth fibers, Abolhasani et al (2022) showed a very different result. Electrospun non-porous PVDF fibers showed a β-phase of about 77% while porous PVDF fibers showed a β-phase of up to 92% for the most porous fiber. It is important to note that the porous fibers were fibers with inner pores but smooth outer surfaces. The higher β-phase was attributed to trapped molecular chains in the β-phase due to confined space between the pores. To obtain porous fibers, water which is a non-solvent was added to the PVDF solution to induce phase separation during electrospinning. The higher β-phase translates into higher voltage response when the membrane experiences a mechanical vibration. For the non-porous PVDF fibers, a voltage response of 4.4 V but with highly porous fibers, a voltage response of 22.6 v was recorded. To increase the β-phase in non-porous PVDF fibers, 0.1wt% of graphene was able to increase the β-phase to 84%. However, further increase in graphene leads to reduction in the β-phase. The increased β-phase with the graphene addition was able to significantly increase the voltage response of non-porous fibers from 4.4V to 14.8 V although this is still lower than the voltage obtained from porous PVDF fibers.

Solvents generally exhibit different properties and they may have varying effects on the characteristics of electrospun fibers. Yin et al (2022) found that higher dipole moments in the solvent consistently yield higher β-phase fraction in electrospun PVDF. Using different combinations of solvents, properties such as boiling point and dipole moment can be varied and compared to the crystallinity of the electrospun fiber. Using the following solvents, dimethyl sulfoxide (DMSO), N, N-dimethylformamide (DMF), acetone (ACE), and tetrahydrofuran (THF), all the electrospun fibers showed crystallinity of around 50% and β-phase above 80%. Electrospun fibers with the highest β-phase came from solvents with the highest dipole moment. This has been attributed to the enhancement of end-to-end length and dipole alignment of PVDF molecules by solvents with a high dipole moment. There is no correlation found between the boiling point of the solvent mixtures and the β-phase composition.

One method of increasing β-phase in electrospun fibers is to use additives. Yoon and Kelarakis (2014) showed that by adding Lucentite nanoclay into PVDF solution, the resultant electrospun fibers exhibited the presence of β-phase while neat fibers predominantly crystallize in the nonpolar α-phase. It is not clear why the presence of nanoclay enhances the formation of β-phase. Addition of nanoclay does improve the conductivity of the solution and this may have play a role in β-phase formation. Ahn et al (2013) showed that the addition of multiwalled carbon nanotube (MWCNT) to PVDF solution increases β-phase formation. This has been attributed to the nucleation effect of MWCNT which encourages crystallization. The same effect may be responsible for higher β-phase in the PVDF fibers when nanoclay was added. Li et al (2014) have also added silver nanowires into PVDF solution for electrospinning and similarly found an increase in β-phase fraction. The role of silver nanowires functioning as nucleation site was believed to be the reason for it. The use of additives in electrospinning PVDF may not alway result in an increase in the crystallinity and β-phase of the fibers. In a study by Song et al (2022), the addition of 1D carbon nanotubes (CNTs) and 2D graphene oxide (GO) into PVDF solution led to a reduction in both the crystallinity, relative and volume fraction of β-phase compared to neat electrospun PVDF fibers. In neat electrospun PVDF, the crystallinity was 46.7%, but the introduction of either CNTs at concentration of 0.01 wt% and GO at concentration of 0.5% resulted in crystallinity of 30.8% and 26.4% respectively. The reduction in crystallinity has been attributed to physical obstruction of PVDF molecules due to the presence of CNT and GO. While greater β-phase in neat PVDF increases the piezoelectric coefficient d₃₃, it is not the only factor that increases it. The addition of CNT and GO to PVDF may reduce the crystallinity of neat PVDF, but it increases the piezoelectric coefficient d₃₃. 0.01 wt% loading of CNT into PVDF increases the d₃₃ by 60%. The increase in d₃₃ may be the result of higher electrical conductivity due to the presence of CNT and better orientation of β-phase crystals. The addition of GO had a less positive effect on d₃₃ with loading of GO greater than 0.5 wt% resulted in a d₃₃ that was lower than neat PVDF. GO is lower conductivity and possible flocculation of GO may reduce electron transfer and disrupted the orientation of β-phase crystals.

XRD patterns of electrospun nanofibers: (a) PVDF and (b) PVDF/1.5 wt% Lucentite [Yoon et al 2014]

The piezoelectric properties of PVDF fibers can be enhanced by the modifying the electrospinning setup. Kang et al (2017) found that the content of β phase of electrospun PVDF fibers were higher when they are collected on step collectors where there is lateral field concentration at the steps. Comparing force spun fibers and electrospun fibers, only the latter fibers showed β phase form which suggest that the presence of electric field is crucial for β phase transformation [Lei et al 2015]. However, a study by Lin et al (2009) showed that assisted stretching by gas jet electrospinning yield PVDF fibers with greater β phase compared to conventional electrospinning.

There are mixed opinions regarding the use of heat annealing on PVDF films and membranes to influence its crystallinity. A key appeal of PVDF and its β-phase is its piezoelectric properties. Silva et al (2011) reported that piezoelectric PVDF thin film will lose its piezoelectric properties as the temperature increases above 80 °. However, they attributed this to depoling of the material rather than the degree of crystallinity or β-phase content. Nevertheless, any change in crystal phase will affect its piezoelectric performance. Parker et al (2018) tested the stability of electrospun polyvinylidene fluoride (PVDF) membrane in particular, its crystal phase. Their study showed that thermal annealing of the electrospun PVDF membrane at temperatures up to 100 ° resulted in more β phase conversion either at the expense of α phase or amorphous phase. The increase in β phase may give rise to greater piezoelectricity. Nevertheless, it will be interesting to investigate the effect of elevated temperature on piezoelectric property in electrospun PVDF membranes.

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