Formation and saturation of β-phase of PVDF in DMF solvent: a comprehensive study

doi:10.21203/rs.3.rs-4022745/v1

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Formation and saturation of β-phase of PVDF in DMF solvent: a comprehensive study

https://doi.org/10.21203/rs.3.rs-4022745/v1

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In the present work, we report the effect of solvent content on the β-phase fraction in PVDF (poly(vinylidene fluoride)) films synthesized by the facile wet chemical route using DMF (N,N-Dimethylformamide) as the solvent. The amount of PVDF powder was kept fixed while the amount of DMF was varied. X-ray diffraction (XRD) revealed the transformation of the α-phase in the PVDF powder to β-phase in the synthesized PVDF films which was further confirmed by Fourier Transform Infrared Spectroscopy (FTIR) and Differential Scanning Calorimetry (DSC) measurements. Deconvolution of the XRD profiles of the films showed the increase of the β-phase content (viz. β/α ratio) with initial increase of the DMF volume. However, a surprising and previously unreported result emerged, i.e. the saturation of the β-phase beyond a certain volume of the solvent. The plausible mechanism for the formation and saturation of the β-phase in the PVDF films is discussed.

PVDF

DMF

β-phase saturation

H-bonding

dipole-dipole interactions

Poly(vinylidene fluoride) [(CH₂ = CF₂)_n], commonly abbreviated and referred to as PVDF, is a semi-crystalline polymer that has attracted immense attention due to its applicability in a wide spectrum of applications^1–4. Because of its favourable properties like ferroelectricity and piezoelectricity, in addition to being flexible, lightweight, non-toxic, etc., PVDF is used or is envisaged to use in/for energy harvesting, health monitoring, actuators, flexible electronics etc^1–4. PVDF can exist in five different phases depending on the chain conformations viz. α, β, γ, δ and ε phases⁵. Among these, the α-phase with trans-gauche (TGTGʹ) configuration is non-polar and the most common⁶. The β-phase, with all trans-planar zigzag (TTTT) configuration, is however more desirable from the perspective of applications due to its polar nature resulting in large ferroelectric polarization and piezoelectric response^5,7. The γ phase is also polar, but it is semi-electroactive in nature due to its lesser dipole moment in comparison to the β phase⁸. The δ phase is the polar version of the α phase having the same lattice constants and chain conformation⁹. Due to the 180° rotation of one of the chain axis of the δ phase, it has a non-zero value of the dipole moment apart from having similar lattice parameters as the α phase^8,10. The δ phase possess comparable piezoelectric properties to the β phase, however the δ phase usually requires ultra-high electric field based processing conditions⁸.

Different methods have been reported for the synthesis of the β-phase and/or the transformation from the α to the β-phase in PVDF. These include synthesis routes like Spin-coating^11,12, electrospinning^13,14 etc., or by adding fillers like Reduced-Graphene Oxide¹⁵, Carbon-Nanotubes^16,17, Nanoclay¹⁸, biowastes^19–21, 1,4-butadiene sulfone (BDS)⁷ etc. Post processing techniques like stretching^22,23, poling²⁴, quenching¹¹, annealing^25,26, hot pressing²⁷ are also reported to induce α to β-phase transformation in PVDF films.

PVDF films can be synthesized by dissolving PVDF powder in a polar organic solvent²⁸. The dissolution of the polymer in the solvent can happen if three conditions are satisfied- (i) higher dispersion of the solute on the solvent (ii) intermolecular force between the solute molecules and solvent molecules (iii) hydrogen bonding between the solute molecules and the solvent molecules²⁹. A variety of solvents have been studied and 8 solvents viz. tetramethyl phosphate, tetramethyl urea, triethyl phosphate, Dimethylacetamide (DMA), Dimethylformamide (DMF), Dimethyl sulfoxide (DMSO), N-methyl-2-pyrrolidone (NMP), and Hexamethylphosphoramide (HMPA) were reported to dissolve PVDF³⁰. In a study reported by Li et al.³¹, the dipole moments of the solvents DMSO, DMF, Tetrahydrofuran (THF) and Methyl ethyl ketone (MEK) were considered for the variation of the β phase content in PVDF films. The PVDF was mixed with the solvents at 15 wt% concentration. These solvents had the dipole moments of 4.1, 3.8, 2.7 and 1.8 respectively. It was found that the solvent having the highest dipole moment, i.e., 4.1 for DMSO, gave the highest β phase content in the PVDF films synthesized at crystallization temperature of 60°C. Similar studies were carried out where the formation of the β phase in PVDF has been done with DMF and other solvents^32–36. In a molecular dynamics based study carried out by Park et al.³² the crystallization behaviour of PVDF at different concentrations of 9 to 67 weight percent (%) of PVDF were studied in the solvents NMP and DMF (the number of NMP and DMF molecules were kept fixed at 1030 and 1395). It was seen that, in both the solvents, as the PVDF content increased, the β phase increased; however, PVDF showed better crystallization behaviour with DMF than that with NMP, which resulted in higher β phase formation with DMF. Ma et al.³⁶ studied the formation of the β phase of PVDF films synthesized with mixture THF/DMF at three different ratios viz. 9:1, 8:2 and 5:5 (keeping the PVDF amount fixed at 10 wt %). It was seen that as DMF was increased, the solubility of the PVDF resin in the mixture along with the formation of the β phase was improved, and the highest amount of the β phase was observed in the 5:5 mixture. Thus, it was inferred that increase in the amount of DMF enhanced the dipolar interactions and hydrogen bonding between PVDF and DMF molecules facilitating the greater formation of the β phase.

In the present work, a comprehensive study on the effect of solvent content on the β phase formation in PVDF has been done by varying the amount of solvent (viz. DMF) while keeping the amount of PVDF constant. As expected from previous literature, an initial increase in the β-phase fraction is observed upon increasing the DMF content. However, interestingly, saturation of the β-phase beyond a particular volume of DMF is witnessed. To our best knowledge, such a saturation of the β phase depending on the amount of solvent and/or PVDF has not been reported till date. This observed saturation forms the prime focus of this paper and a plausible reason for the same is discussed.

2.1. Materials:

Polyvinylidene fluoride (PVDF) (-[CH₂ = CF₂]_n-)(Avg Mw = 530,000 by GPC), N,N-Dimethylformamide(DMF) were purchased from Sigma Aldrich, France and was used as received.

2.2. Synthesis of PVDF Films:

The PVDF films were synthesized via wet chemical method for homogeneous mixing of the polymer in the solvent. As shown in Fig. 1, 1g of Poly-(vinylidene fluoride) (PVDF) powder and 4ml of N,N-Dimethylformamide (DMF) solvent were mixed together and stirred on a magnetic stirrer at an RPM of 600 for 3 hours at a temperature of 60°C. It has been revealed from earlier studies that the rate of crystallization of PVDF depends on the evaporation rate of the solvent, and evaporation temperatures below 70°C aids in the formation of the β-phase of PVDF³⁷. Similar reactions were carried out using different amounts of DMF, viz. 5 ml, 7.5 ml, 9 ml, 10 ml, 12 ml, 15 ml, 18 ml and 20 ml, but each having only 1 g PVDF. After 3 hours, the solution was poured into a petri dish dried inside vacuum oven at approximately 55°C for 4–6 hours to completely remove the DMF. In the case of the solutions having DMF of more than 10 ml, the drying process took about 12 hours. When the drying was complete, a thin round-shaped polymer film that is transparent and flexible was obtained. Henceforth, the films are named P-D4, P-D5, P-D7.5, P-D9, P-D10, P-D12, P-D15, P-D18 and P-D20, where the numbers after D indicates the amount of the DMF in ‘ml’.

2.3. Characterization:

X-Ray Diffraction (XRD)

The phase of the synthesized films and PVDF powder was investigated via the X-ray diffraction method using X-Ray power diffractometer ‘Bruker D8 Advance Diffractometer’ (Germany) with Cu-Kα radiation (λ = 1.54Å).

Fourier Transform Infrared Spectroscopy (FTIR)

The Fourier Transform Infrared (FTIR) spectroscopy was performed in the transmittance mode using Thermonicolet (Anexus) 1600 series at room temperature from 450-1600cm^− 1.

Differential Scanning Calorimetry (DSC)

The melting temperature of PVDF powder and P-D films was determined using Differential Scanning Calorimeter Model (DSC), Hitachi High-Tech Science Corporation, Japan, Model DSC 7020. The PVDF powder and the films were heated up to 240°C at a scan rate of 10°C/min.

The structural transformation of the polymer films was studied using X-Ray Diffraction method. The as purchased PVDF powder shows peaks at 17.8°, 18.4°, 19.9°, 26.6° (Fig. 2) assigned to the planes (100), (020) and (110), all corresponding to the electrically inactive α-phase^17,38–40.

Figure 3(a) shows the XRD pattern of the P-D4 film. It is apparent that the XRD pattern of the film is distinctly different from the purchased powder. A small α-phase peak at 18.7° corresponding to the (100) plane and a β-phase peak at 20.4° corresponding to the (200) plane is seen in the P-D4 film. The peak at 26.6° corresponding to the α-phase also disappears in the film. Similar observations are made in all the other synthesized films (Fig. 3(b)) indicating the transformation from α-phase in the powder to predominantly β-phase in the films. All the films (Fig. 3(b)) show peaks in the range of 2θ = 20.4° to 20.6, which are the characteristic peaks of β-phase^{18,20,23,41,42} along with small α-phase peaks ~ 18.7°. The transformation from α to β phase was induced without the addition of any fillers or any kind of post-synthesis processing. It can be inferred that the synthesis route might have a significant contribution to the induction of the β-phase in the polymer films. This will be discussed in detail later.

The FTIR investigation of PVDF powder (Fig. 4.) shows distinct dips/vibrational bands at 490 cm^− 1, 615 cm^− 1, 762 cm^− 1, 796 cm^− 1, 881 cm^− 1, 975 cm^− 1, 1071 cm^− 1 and 1401 cm^− 1, assigned to the α-phase^40,43–47. Some bands of small intensity can be seen at 510 cm^− 1, 840 cm^− 148 and 1280cm^{− 149–51} which are assigned to the β-phase. The band at 490 cm^− 1 shows the presence of α-phase which arises due to in-phase combined bending and wagging of the CF₂ group⁴³, that at 615 cm^− 1 arises due to out of phase CF₂ bending and CCC skeletal vibration^44,45,52, dip at 762 cm^− 1 arises due to CF₂ bending and skeletal bending mode, those at 796 cm^− 1 and 975 cm^− 1 arises due to CH₂ twisting of the polymer skeleton⁴⁶. A wide band is seen at 1188cm^− 1 which occurs due to the stretching frequencies of CF₂⁵³. The intensity of the β-phase dips/bands is lesser than those of the α-phase deeps, which shows that PVDF powder lies mostly in the electrically inactive α-phase.

The P-D4 film (Fig. 5(a)) shows dips/vibrational bands at 510 cm^− 1, 600 cm^− 1, and 745 cm^− 1, which are the characteristic peaks of the β-phase⁴⁵. Band at 510 cm^− 1 arises due to bending of the CF₂ group^48,54, that at 745 cm^− 1 arises due to in-plane bending or rocking⁴⁸. A dip at 490 cm^− 1 shows the presence of α-phase which arises due to in-phase combined bending and wagging of the CF₂ group⁴³. The combined FTIR profiles of PVDF powder (Fig. 5(b)) and P-D films show noticeable changes in the band/dip positions. The α-phase dips at 490 cm^− 1, 615 cm^− 1, 762 cm^− 1, 796 cm^− 1, 881 cm^− 1, 975 cm^− 1, 1071 cm^− 1 and 1401 cm^− 1 are very prominent in PVDF powder with dips of small intensities at 510 cm^− 1 and 840 cm^− 1 of the respective β-phase. In all the P-D films, the α-phase dips are seen to be disappearing and the dips of the β-phase at 510 cm^− 1, 600 cm^− 1, 745 cm^− 1 and 840 cm^− 1 are increasing in intensity. The dip at 840 cm^− 1 which arises due to out-of-phase combined rocking motion of the CH₂ group and the asymmetric stretching of the CF₂ group^44,45,48 is observed to be appearing like a very small well in the P-D7.5, P-D9, P-D10 and P-D15 films showing the presence of the β-phase (Fig. 5(b)). The α-dip at 615 cm^− 1 in PVDF powder disappears and the dip at 600 cm^− 1 appears depicting the β-phase. Another β-phase dip at 745 cm^− 1 is seen in all the films, which is absent in PVDF powder. The α-phase dips at 975 cm^− 1, 1071 cm^− 1 and 1401 cm^− 1 disappears in all the films. This change in the P-D films in comparison to PVDF powder, suggests the ongoing transformation of the α-phase into the β-phase, which is in good agreement with the XRD results.

The melting temperatures of PVDF powder and P-D films were determined using DSC analysis (Fig. 6). Melting endotherm of PVDF powder shows onset melting temperature at 135°C and maximum at around 157°C which is around the reported melting temperature of the α-phase⁵⁵. In P-D films, the melting temperature onsets around 133°C with maximum at 163°C for the P-D7.5 film, at 159°C for the P-D15 film and at 161°C and 162°C for the P-D4 and P-D12 and P-D18 films⁵⁵,⁵⁶. The melting point of the PVDF powder at 157°C and the P-D films at ~ 162°C, corresponding to the melting point of the α-phase and the β-phase, is in fair agreement with that of literature⁵⁵. The endotherm of PVDF powder lies in the range 135°C to 166°C with ΔT_m = 31°C; whereas the endotherms P-D films lie in the range 133°C to 170°C with ΔT_m = 37°C which shows the different crystallization behaviour of the P-D films⁵⁷. The transformation from the α-phase to the β-phase when the as purchased PVDF powder is used to synthesize the films, is thus confirmed from XRD, FTIR and DSC.

As described, the synthesized films are in the β-phase. In order to quantify the amount of the β-phase in all the films, deconvolution of the XRD peak was carried out. Peak positions corresponding to the amorphous region (a), the α-phase (α) and the β-phase (β) were assigned to the XRD profiles of the P-D films and deconvolution was done. Figure 7 shows the deconvolution of the XRD profile of the P-D4 film. Similarly, the deconvolution was done for the P-D5, P-D7.5, P-D9, P-D10, P-D12, P-D15, P-D18 and P-D20 films. The β and α phase content present in the films were calculated by first determining the area under the β and α curves and then dividing both by their respective |F_hkl|² values [See Supplementary information for the heuristic derivation of the |F_hkl|² values].

Figure 8 shows the plot for %β/α of the P-D films with the increasing DMF content. It is seen in Fig. 8 that the β/α ratio increases linearly from 84–95% as the amount of DMF is increased from 4 ml to 12 ml. Further increase in DMF amount does not show any increase in the β/α ratio and saturates at 95% for 12 ml, 15 ml, 18 ml and 20 ml DMF. It has been reported that reduction in the solvent evaporation rate favours the formation of the β-phase³⁷. In the present scenario, it can be well understood that at higher DMF content, there is a reduction in the solvent evaporation rate, since all the films were synthesized at same temperature of 60°C and dried at 55°C and for the same time. Hence the β-phase should increase with the increasing DMF content. This can explain the initial increase in the β-phase content in the films when the DMF content is increased from 4 ml to 12 ml, but cannot explain the saturation of the β-phase content when the DMF content is increased beyond 12 ml.

To explain the saturation of the β-phase content beyond the certain concentration of DMF, we consider the various interactions occurring between PVDF and DMF. Due to the weak hydrogen bonding C = O^….H-C between DMF and PVDF, the interchain forces in the solid PVDF are disrupted and the PVDF gets dissolved into the DMF^58,59. Molecular interactions occurring between PVDF and DMF (schematically shown in Fig. 9) involve (i) the dipole-dipole interactions between the C = O group and CH₂-CF₂, (ii) the formation of hydrogen bonds at the interface between the PVDF nucleus and DMF molecules, and (iii) a weaker H-bonding between fluorine atoms of the PVDF skeleton and the methyl hydrogen of the DMF solvent⁶⁰,⁶¹. This facilitates the better orientation and packing of the CH₂-CF₂ dipoles leading to the formation of, or the transformation from the α phase to the electrically active all-trans (TTTT) conformation β-phase of PVDF⁵⁸. In case of the P-D4 to P-D10 films, the H-bonds between PVDF and DMF increases with increasing DMF content consequently leading to the increase in the β-phase content. In case of the P-D12 film, the interaction between PVDF and DMF in terms of H-bonds and dipole-dipole interactions is maximum thereby resulting in the maximum β-phase content. Further H-bonding does not occur with the additional DMF (i.e., when DMF is increased beyond 12 ml) as there is no site left on the PVDF chain to form H-bonding with the additional DMF. This leads to the saturation of the β-phase content in the P-D15, P-D18 and P-D20 films.

Therefore, it is evident that by varying the amount of the solvent DMF and keeping the amount of PVDF fixed in the films, there has been a significant transformation of the α-phase into the β-phase without adding any fillers or performing any post-synthesis processing.

In this work, the effect of altering DMF content is examined in relation to the transition from the electrically inert α-phase of PVDF powder to the electrically active β-phase in PVDF films synthesized via the wet chemical route. The study emphasises the important role that variations in solvent concentration and synthesis process have in promoting the transition from one phase to another. Our results, which are in line with previous research, confirm via XRD, FTIR, and DSC investigations, the existence of the β-phase in the films.

The deconvolution of the XRD patterns of the synthesized films reveal the significant presence of the β-phase. The linear relationship between the percentage of β/α content and DMF concentration is mostly explained by the persistence of H-bonding and dipole-dipole interactions between PVDF chains and DMF molecules. In the P-D12 film, this tendency reaches its apex. Additional increase in DMF content do not result in further increase in the β/α ratio, which leads to the saturation seen in the P-D15, P-D18, and P-D20 films. This saturation is observed due to the unavailability of H-bonding sites on the PVDF chain for further interactions with the extra DMF molecules. This saturation in the β phase content at higher DMF volume is an interesting outcome and has never been reported before (neither for DMF nor for any other polar solvent). Consequently, these films, rich in piezoelectric β phase content, hold promise for the development of energy harvesting devices suitable for real-world applications.

CONFLICT OF INTEREST

The authors declare no conflict of interest.

ACKNOWLEDGEMENTS

The authors express their sincere gratitude to the Central Instrumentation Centre, (CIC), University of Petroleum and Energy Studies (UPES), Dehradun for synthesis and characterization support. The authors thank UPES SEED Grant (UPES/R&D-SEED-INFRA/11032022/03) for the DSC instrument.

Ethical Approval

Not Applicable

Funding

P.K. acknowledges the UGC-DAE CSR Project (Ref. CRS/2021-22/02/481) for financial support. P.K and S.K acknowledges the SEED project grant from UPES for financial support. S.D acknowledges UPES for PhD fellowship.

Availability of data and materials

Not Applicable

Author Contribution

All authors contributed equally

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Formation and saturation of β-phase of PVDF in DMF solvent: a comprehensive study

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Abstract

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1. INTRODUCTION

2. EXPERIMENTAL

2.1. Materials:

2.2. Synthesis of PVDF Films:

2.3. Characterization:

3. RESULTS

4. DISCUSSION

5. CONCLUSION

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References

Additional Declarations

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