# Development of polar phases in ferroelectric poly(vinylidene fluoride) (PVDF) nanoparticles

Edgar Gutiérrez-Fernández, Jose Sena-Fernández, Esther Rebollar, Tiberio A. Ezquerra, Francisco J. Hermoso-Pinilla, Mikel Sanz, Oscar Gálvez, Aurora Nogales

► To cite this version:

Edgar Gutiérrez-Fernández, Jose Sena-Fernández, Esther Rebollar, Tiberio A. Ezquerra, Francisco J. Hermoso-Pinilla, et al.. Development of polar phases in ferroelectric poly(vinylidene fluoride) (PVDF) nanoparticles. Polymer, 2023, 264, pp.125540. 10.1016/j.polymer.2022.125540. hal-05080028

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# Development of polar phases in ferroelectric poly(vinylidene fluoride) (PVDF) nanoparticles

Edgar Gutiérrez-Fernández<sup>a,b,c</sup>, Jose Sena-Fernández<sup>a</sup>, Esther Rebollar<sup>d</sup>, Tiberio A. Ezquerra<sup>a</sup>, Francisco J. Hermoso-Pinilla<sup>e</sup>, Mikel Sanz<sup>e</sup>, Oscar Gálvez<sup>e</sup>, Aurora Nogales<sup>a,*</sup>

## ARTICLE INFO

Keywords:

Ferroelectric polymer nanoparticles

Polar phase

PVDF

## ABSTRACT

Ferroelectric polymer nanoparticles of poly(vinylidene fluoride) (PVDF) were prepared by dialysis nano-precipitation. Particles with mean diameter of 230 nm show piezoelectric response as observed by Piezoresponse Force Microscopy (PFM). The origin of this ferroelectric behavior has been attributed to the presence of ferroelectrically active crystalline phases of PVDF in the nanoparticles (mainly β and γ phases) as demonstrated by differential scanning calorimetry (DSC) and temperature dependent Fourier transform infrared spectroscopy (FTIR) and X ray diffraction. The combination of the preparation method and temperature treatment can provide a method for tuning the ferroelectricity in PVDF homopolymer nanoparticles.

## 1. Introduction

Electroactive polymers like poly(vinylidene fluoride) (PVDF) and its copolymers are recently attracting interest in energy harvesting devices, due to their ability to convert mechanical strain into electrical energy [1, 2]. Piezo and ferroelectric properties of PVDF based materials are central to that conversion. The large difference in electronegativity between fluorine atoms and hydrogen and carbon atoms produces a strong electric dipole in PVDF monomer, perpendicular to the main chain. These monomer dipoles, upon crystallization, can produce conformational arrangements that give rise to an overall dipolar contribution per crystal unit cell, leading to ferroelectricity [3]. However, whereas PVDF copolymers with trifluoroethylene (PVDF-TrFE) crystallize at room temperature into an intrinsically ferroelectric phase [4], PVDF homopolymer can crystallize into five different crystalline phases, α, β, γ, δ and ε [5,6] being not all ferroelectrically active. The chain conformation is different in each of these phases, and subsequently, the arrangement of the chain dipoles is also different. The most common crystalline phase obtained from melt processing, the non-polar α phase, presents a *trans*-gauche-*trans*-gauche' (TGTG') conformation, with dipole cancellation. The β phase with a TTT arrangement, is electrically active since all

the chain dipoles of the unit cell point to the same direction. A variety of processing techniques have been used to obtain the different phases of PVDF. Some of them are devoted to obtaining polar phases either from the melt [7] or from the solid state [8] by using mechanical (stretching, pressure ...) or electrical stimuli (poling). Another strategy to obtain polar phases in PVDF is the use of convenient additives like barium titanate (BaTiO₃) [9,10], nanoclays [11], titanium nanosheets [12,13] or perovskites [14]. This has been attributed to the enhancement of TT conformation due to the interaction of PVDF CF₂ groups with the charged surfaces of the additives. Also, the exposition of thin films to ionic liquids and temperature produces the γ polar phase in PVDF [15]. Polar phases in PVDF have been reported on nanoconfined systems [16, 17]. Nowadays, solution processing routes are being explored to generate ferroelectric phases of PVDF. The solution processing route is of particular importance for the fabrication of devices based on supported PVDF, for example in silicon wafers, in which mechanical processing methods are rather problematic. Polar phases of PVDF have been reported for films prepared from relatively polar solvents, like dimethyl formamide (DMF) [18], dimethyl sulfoxide (DMSO) [19] and from solvent mixtures [20] and nanoconfinement [17]. It seems that a higher fraction of polar phase is correlated with a higher polarity solvent [21].

<footnote><sup>a</sup> Instituto de Estructura de la Materia (IEM), CSIC, C/ Serrano 121, Madrid, 28006, Spain</footnote>

<footnote><sup>b</sup> XMaS/BM28-ESRF, 71 Avenue Des Martyrs, F-38043 Grenoble Cedex, France</footnote>

<footnote><sup>c</sup> Department of Physics, University of Warwick, Gibbet Hill Road, Coventry, CV4 7AL, UK</footnote>

<footnote><sup>d</sup> Instituto de Química Física Rocasolano (IQFR), CSIC, C/ Serrano 119, Madrid, 28006, Spain</footnote>

<footnote><sup>e</sup> Universidad Nacional de Educación a Distancia (UNED), Dept. Física Interdisciplinar, Fac. Ciencias Av. de Esparta s/n, 28232, Las Rozas, Madrid, Spain</footnote>

<footnote>* Corresponding author.
E-mail address: aurora.nogales@csic.es (A. Nogales).</footnote>
Also, including hydrated salts in the solution favors the formation of PVDF polar crystalline phases [20]. Solution processing is used in the fabrication of polymer membranes, where the casting polymer solution is immersed into an antisolvent coagulation bath. It is convenient to point out the similarities between the nanoprecipitation method we use and the fabrication of polymer membranes. In the latter process, the antisolvent gradually mixes with the solvent producing a phase separation between polymer rich and polymer poor regions, that finally coagulate leading to porous polymer membranes [22]. Recently, it has been reported that, phase inversion produced during cooling PVDF solutions in water enhanced the polar phases [23]. In the case of PVDF, final crystalline structure is highly dependent on the choice of the solvent. In our work, as with membrane processing, the starting point is a polymer solution, that is mixed afterwards with an antisolvent to produce a phase separation [24]. The miscibility of both, solvent and antisolvent, leads to different methods of nanoparticles preparation. Polymer nanoparticles prepared by miniemulsion are obtained from polymer solutions in immiscible solvents antisolvent pairs [25–27]. In this case, the use of surfactant maybe required. When the solvent/antisolvent pair is miscible, flash [24,28] and dialysis nanoprecipitation [24,29] can be used without any surfactant.

Nanocomposites with polar PVDF fillers provide a wide range of potential, interesting applications as organic, ferroelectric miniaturized devices. These may include, nonvolatile random access memories [30, 31], piezoelectric transductors [32] or energy harvesters [33]. In addition, the compatibility of these polymers with biological systems makes them of interest to tissue engineering applications [33]. Among the different types of polymer nano-objects, polymer nanoparticles have found applications in many scientific fields: photonic systems [34], adhesives [35], biomedical applications like targeting [36], molecular imaging [37], and drug delivery activated by external stimuli [38,39] and organic electronics [40]. Polymer nanoparticles offer high effective surface area, tunability through the preparation conditions and the possibility for dispersion in water, avoiding, during the deposition process, the handling of toxic organic solvents [24,41,42].

In this work we present a method to produce ferroelectric nanoparticles from originally non-polar PVDF polymer solution by the appropriate selection of: i) the precipitation method and ii) the solvent/non-solvent conditions. The obtained nanoparticles exhibit ferroelectric response as revealed by Piezoresponse Force Microscopy (PFM). The origin of this ferroelectric response is studied by differential scanning calorimetry and temperature resolved X ray scattering and infrared spectroscopy experiments.

## 2. Materials and methods

PVDF was obtained from Sigma Aldrich (Batch 182702, Weight average molecular weight, Mw, of 534 kg/mol, according to the manufacturer) and it was dissolved in Dimethyl Acetamide (DMAc, Sigma Aldrich, 99.8% purity) without further purification.

### 2.1. Preparation of the PVDF nanoparticles

PVDF nanoparticles were prepared using a dialysis nanoprecipitation method. For that, a 2 g/l PVDF solution in DMAc was prepared, by stirring at room temperature for 1 h. The solution was dialyzed against distilled water (Millipore grade) at room temperature using a dialysis membrane, previously cleaned and rinsed in DMAc. Dialysis tubing membranes (Visking DTV) by Medicell Int Ltd with a cutoff range of 12000–14000 g/mol and tube diameter of 25.5 mm were used for this purpose. Before utilization, the dialysis membrane, stored in a refrigerated saline solution, was rinsed several times in deionized water. The dialysis took around 24 h, during which water was changed periodically. After this process, a homogeneous PVDF nanoparticles colloid in water is obtained.

### 2.2. Characterization methods

Atomic force microscopy (AFM) images were collected in tapping mode with a Multimode 8 AFM (Bruker) with a Nanoscope V controller (Bruker) using gold-coated silicon probes (Tap300GB-G probes by BudgetSensors). To analyze the morphology of the nanoparticles, a drop of the prepared water colloid was deposited onto a silicon wafer. To estimate the size distribution of the prepared nanoparticles, the diameters of 50 particles were measured using the Nanoscope Analysis software. The ferroelectric response of the nanoparticles was observed by Piezoresponse Force Microscopy (PFM). PFM measurements were carried out by the same NanoScope V AFM (Bruker), in the piezoresponse mode. In this mode, a conducting AFM tip is used as a top electrode while a bias is being applied. Since ferroelectric materials are also piezoelectric, the electric field generated in the sample causes the domains with the polarization parallel to the field to extend, and the domains with opposite polarization to contract [43]. These variations of the topography produce deflections at the AFM probe, which are recorded at the photodetector. In PFM the microscope was used in the contact mode, with a low deflection set point (0.3 V), in order to avoid damaging the samples. Polymer nanoparticles were deposited by drop casting onto conductive silicon. The silicon wafer with the PVDF nanoparticles was attached to a metallic sample holder by using double sided carbon conductive tape (Ted Pella). For PFM, conductive PtIr-covered probes (model SCM-PIT, Bruker, k = 3 N/m) were used. Hysteresis cycles were recorded applying a tip bias ramp from –10 V to 10 V at a frequency of 0.1 Hz.

Temperature dependent grazing incidence X-ray scattering measurements at Wide Angle (GIWAXS) were performed at ALBA synchrotron (Cerdanyola del Vallès, Barcelona, Spain) at beamline NCD-SWEET. The X-ray beam wavelength was set at $\lambda = 0.1$ nm (E = 12.4 keV) using a Si(1 1 1) channel cut monochromator. The beamsize at the sample position was $60 \times 100 \ \mu\text{m}^2$ (vertical $\times$ horizontal). GIWAXS patterns were collected by a WAXS LX255-HS 2D (Rayonix) area detector placed at 109 mm. Sample to detector distance, detector tilts and reciprocal space were calibrated using $Cr_2O_3$. Data were normalized by the incident photon flux and the acquisition time. Flat field, polarization, solid angle and efficiency corrections were additionally applied to the GIWAXS images. The scattering vector $q$, ($q = (4\pi/\lambda)\sin(\theta)$, where $2\theta$ is the scattering angle) was defined with respect to the center of the incident beam. Azimuthal integration on the 2D images led to 1D scattering patterns, that are presented in this work after background subtraction. The temperature was controlled by using a LINKAM unit connected to a cooling system of liquid nitrogen.

Calorimetric measurements were carried out by means of a Perkin-Elmer DSC8500 instrument equipped with an Intracooler 2 sub-ambient device. The external block temperature was set at –100 °C. Indium standards were used for calibration. A lyophilized solid powder of nanoparticles was used as sample with weight of approximately 5 mg. The powder was enclosed in aluminum pans and heated from 0 °C to 190 °C at a rate of 10 °C/min. Bulk sample was measured as received, enclosed in aluminum pans.

IR spectra were acquired in transmission with a PerkinElmer, Frontier spectrometer in a 4500-500 cm⁻¹ range with a resolution of 2 and 4 cm⁻¹. The nanoparticles were deposited by spin-coating on special IR transparent silicon substrates (Si FZ 25.4 mm Ø x 1 mm polished window). The temperature range was varied between 30 and 175 °C by using a variable temperature cell (SPECAC). A spectrum for the background was taken at every temperature.

## 3. Results and discussion

### 3.1. Size and shape of the PVDF nanoparticles

The described dialysis nanoprecipitation for PVDF produces spherical nanoparticles with size below 500 nm, a mean diameter around 230
nm and standard deviation of 125 nm, as determined by measuring the size of a large set of nanoparticles by AFM (Fig. 1).

## 3.2. Evidence of ferroelectric functionality of the PVDF nanoparticles

To evaluate the ferroelectricity of the individual particles, voltage was applied through the AFM tip in contact with the nanoparticles. Fig. 2 shows an example of a PFM phase (Fig. 2a) and PFM amplitude (Fig. 2b) hysteresis cycle on the obtained PVDF nanoparticles. The piezoelectric signal is associated with local polarization switching and ferroelectric properties of the PVDF nanoparticles. The observed 180° phase as the voltage is increased and the hysteresis in lowering down the voltage probes that the nanoparticles are ferroelectric. This is a direct evidence of the presence of polar polymer crystals.

The hysteresis observed in the PVDF nanoparticles is narrower than that observed in the case of PVDF-TrFE copolymer nanoparticles [44].

## 3.3. Structure of the crystalline phase of the PVDF nanoparticles

DSC experiments on PVDF nanoparticles show an endothermic peak around 160 °C, which is associated with the melting point of crystalline phases [45] (Fig. 3a). Further analysis will elucidate the nature of this crystalline phase.

For the sake of comparison, a DSC heating experiment of a melt processed PVDF film is also shown in Fig. 3 a. The melt processed film was prepared by pressing the powder under mild pressure at 190 °C for 2 min, and subsequently cooling at room temperature. Although the subtle differences found in these two DSC temperature ramps are not enough to elucidate different crystalline phases, some observations can be highlighted. The nanoparticles DSC trace shows a strong and broad melting peak with onset at around 140 °C and peak maximum located at 156 °C followed by a small shoulder at 170 °C (marked with an asterisk in Fig. 3a). The DSC heating ramp of the melt processed film shows a narrower melting peak with onset close to 150 °C, and peak maximum

also at 156 °C.

<figure><img src="image_2.png" /><figcaption>Fig. 2. PFM phase (a) and amplitude (b) hysteresis cycle for PVDF nanoparticles.</figcaption></figure>

The melting point around 156 °C in both samples is coherent with the reported melting point of α and β phase [46], that are not distinguishable by DSC. However, the small feature that appears in the nanoparticles at around 170 °C suggests the presence of a small fraction of a higher melting temperature phase, most likely the γ phase since it is the one with higher melting point [47].

The crystalline structure of the prepared nanoparticles was studied by X-ray scattering experiments. Fig. 3 b shows the X-ray diffracted intensity as a function of the modulus of the scattering vector q from the nanoparticles. For comparison, the diffractogram of a pure PVDF a crystalline phase, obtained from a film prepared from the melt, is presented. Both patterns show enough differences to indicate that a large fraction of the crystalline phase in the nanoparticles is not the non-polar α phase. The X ray diffractogram obtained for the nanoparticles exhibits

<figure><img src="image_3.png" /><figcaption>Fig. 1. (a) Topographical images, with low (left) and high (right) magnifications, taken by AFM of PVDF nanoparticles prepared by dialysis. (b) Histogram for the measured diameters of PVDF nanoparticles prepared by dialysis. Histogram has been adjusted with a log-normal distribution. Total counts = 70.</figcaption></figure>
<figure><img src="image_4.png" /><figcaption>Fig. 3. (a) DSC heating traces (b) X ray diffraction and (c) FTIR spectra at room temperature of the PVDF nanoparticles (grey curves) and of the melt processed film (pink curves). Discontinuous lines in (b) indicate the position of the main reflections of the α phase (red) β phase (green color) and γ phase (blue color). With the same color code, the discontinuous lines in (c) indicate the position of the main IR bands of the α, β and γ phases of crystalline PVDF. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)</figcaption></figure>

two main Bragg peaks, at 13.0 nm⁻¹ and at 14.2 nm⁻¹ with a small shoulder at 14.6 nm⁻¹. The peak at 13.0 nm⁻¹ can be associated either to the 020 reflection of the α phase or to the 020 reflection of the γ phase [48], while the peak at 14.2 nm⁻¹ can be attributed to the 110 reflection of the α phase, or to the 110 reflection of the γ phase. The presence of the shoulder located around 14.6 nm⁻¹ can be assigned to the 101/021 reflection of the γ phase or to the 200/110 reflection of the β phase. Anyway, this shoulder cannot be associated to any of the reflections of the α phase [48]. Therefore, it can be considered as a signature of the presence of electrically active (EA) phases in the nanoparticles. On the other hand, the diffractogram of the melt processed film (Fig. 3 b, pink line) is fully associated to the α phase. In the supporting information, together with the X ray scattering from PVDF NPs and a melt processed films, the results from films prepared by spin coating PVDF solutions on two different solvents are presented (Fig. S1).

IR spectroscopy also reflects the presence of the EA phases in the nanoparticles and just a residual presence of the α phase. Following the flow diagram proposed by Cai et al. [49] the band at 1071 cm⁻¹ appears in the spectra of α, β and γ crystalline phases, and therefore, can provide an indication of the presence of crystallinity in the sample, while the bands located at 1234 cm⁻¹ and 763 cm⁻¹ revealed the presence of γ phase, and those located at 1274 cm⁻¹ and 1430 cm⁻¹ are associated to the β phase. The bands at 840 cm⁻¹ and 510 cm⁻¹ are the result of several overlapping bands associated to the EA phases. At 763 cm⁻¹ and 614 cm⁻¹ the IR spectrum of the PVDF nanoparticles shows two weak bands, associated to the presence of the α phase. As can be observed in the IR of the melt pressed film (Fig. 3b, pink line), these two bands are very intense in a pure α phase sample.

From the IR spectra of the PVDF NPs, an estimation of the EA phase fraction (Φ<sub>EA</sub><sup>IR</sup>) can be obtained [49]. The 840-833 cm⁻¹ band can be assigned to EA phases while the band at 763 cm⁻¹ is associated to the non-polar α phase [49]. The fraction of EA phases over the total crystalline phase can be calculated by considering the Lambert Beer law, and therefore:

$$ \Phi_{EA}^{IR} = \frac{I_{840}}{\left(\frac{K_{840}}{K_{763}}\right)I_{763} + I_{840}} \times 100\% \quad (1) $$

where, I<sub>840</sub> and I<sub>763</sub> are the absorbances at 840 cm⁻¹ and 763 cm⁻¹, and K<sub>840</sub> and K<sub>763</sub> are the absorption coefficients at the respective wave numbers, whose values are 7.7·10⁴ and 6.1·10⁴ cm² mol⁻¹, respectively [50].

This calculation leads to a fraction of ferroelectric phase of around 75% at room temperature from the total crystalline phase of the nanoparticles. From the combined analysis of the room temperature experiments of DSC, X ray diffraction and IR spectroscopy we conclude that the ferroelectric response observed by PFM (Fig. 2) arises from a mixture of β and γ crystalline phases present in the nanoparticles.

### 3.4. Variation of the fraction of polar phases with temperature

The thermal stability of the different crystalline phases with temperature was studied by temperature resolved X-ray scattering and FTIR spectroscopy. Fig. 4 a shows the X-ray diffracted intensity as a function of the scattering vector from room temperature to 180 °C.

The diffraction pattern changes as temperature increases, with variations in the relative intensity of the different Bragg peaks. To quantify these changes, the peak intensity was separated into different contributions: a broad amorphous halo and Lorentzian peaks associated to different Bragg reflections at 13.0 nm⁻¹ and at 14.2 nm⁻¹ and 14.6 nm⁻¹. Only three peaks associated to crystalline contributions were considered since, some of the Bragg reflections that could be associated to different crystalline phases are effectively overlapped, within the experimental error. Fig. 4 b shows examples of the fittings obtained at different temperatures. As temperature increases, the contribution of the peak at 14.6 nm⁻¹ univocally associated to the EA phases becomes more important. Above 145 °C, the intensity of this peak decreases and finally vanishes at 165 °C, while the other two peaks at 13.0 and 14.2 nm⁻¹ remain visible. This effect can be clearly observed by considering the area below the 14.6 nm⁻¹ peak related to the total area associated to crystalline peaks (Fig. 5). The monotonous increase of this peak in the range from room temperature to 145 °C indicates an increase of the content of EA phases in the nanoparticles due to annealing effects. The change of tendency observed at 145 °C can be related to melting of the β phase crystals, that is reported to occur at lower temperatures than that of the γ phase [51].

The two remaining peaks in the diffractograms observed at temperatures above 165 °C can be associated either to the ferroelectric γ phase or to the non-polar α phase. As mentioned above, the content of α phase at room temperature, obtained from FTIR, is around 25% of the total crystalline phase. Temperature dependence FTIR experiments (Fig. 6) were performed to further investigate the nature of the remaining high temperature phases in the nanoparticles.

In the measured temperature range, the 1071 cm⁻¹ band, common to α, β and γ phases, can be observed, informing about the presence of crystalline phases [49]. This band at 1071 cm⁻¹ can be observed in the FTIR spectra even at 140 °C although its intensity decreases significantly. The bands associated exclusively to either the α phase (975 cm⁻¹ and 760 cm⁻¹) or the β phase (1275 cm⁻¹) are absent in the FTIR spectra recorded at temperatures above 140 °C, suggesting that these phases are molten. However, those bands associated to the EA phase (833-840 cm⁻¹ and 1429-1431 cm⁻¹) remain visible even at 175 °C. In fact, the estimation of Φ<sub>EA</sub><sup>IR</sup>, obtained through eq. (1) (Fig. 5, right axis) also exhibits a sudden increase above 140 °C, and above this temperature, the remaining PVDF crystals are those corresponding to the ferroelectric phases. Accordingly, we can associate this phase to the γ one.

These results are in agreement with the previously discussed calorimetric studies that point to a higher melting temperature of the γ phase of PVDF than those of α and β phases [47]. In the same work it is also suggested that, when samples are annealed at high temperatures, a transition from the α phase to the γ one takes place. Our DSC results evidence a small melting peak occurring at high temperature that points also to the presence of a small fraction of crystals with higher melting
<figure><img src="image_5.png" /><figcaption>Fig. 4. (a) X ray scattered intensity from the PVDF nanoparticles as a function of increasing temperature. (b) Examples of the separation of the crystalline and amorphous contributions at different temperatures. In the bottom graph, lines indicate the position of the Bragg reflection associated to the α (red), β (green) and γ (blue) phases [48]. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>Fig. 5. (○, Left axis) Temperature dependence of the area associated to the Bragg peak at 14.6 nm⁻¹, associated EA phase, relative to the total area of all the Bragg peaks in the diffractograms of Fig. 4. (□, Right axis) Φ<sup>IR</sup><sub>EA</sub> obtained through eq. (1).</figcaption></figure>

temperature (marked with an asterisk in Fig. 3a). From our results we
cannot conclude that such α→γ transition occurs. However, the com-
bined interpretation from FTIR spectroscopy, X-ray diffraction and DSC
experiments evidences that PVDF nanoparticles prepared by dialysis
nanoprecipitation from DMAc solution in water are ferroelectric and a
temperature treatment on these nanoparticles can provide a way of
tuning the ferroelectric phase. Furthermore, we should inquire why
ferroelectric phases appear when processing PVDF this way. Pigliacelli
et al. [52] have studied the formation of PVDF colloids in water through
sonication in the presence of a surfactant. In this case, they reported an
unambiguous crystallization into apolar α phase. In our case, the
mechanism of formation is different. First of all, since no surfactant is

<figure><img src="image_7.png" /><figcaption>Fig. 6. FTIR Spectra from the PVDF nanoparticles as a function of increasing temperature.</figcaption></figure>

used, there is indeed an effective interaction between the polymer and
the water during the aggregation of the chains. Second, the exchange
between DMAc and water is gradual, so we could expect a slow (guided)
crystallization while the solubility of the medium is being reduced.
Therefore, the replacement of polar DMAc for another polar medium
(water) during the dialysis procedure would be the reason why PVDF
chains are prone to crystallize into a polar configuration. However, we
cannot inquire further the mechanism of crystallization and is out of the
scope of this work. Surface layer polar phases appear as a result of strong
interactions between PVDF chains and polar water molecules. In our
case the morphology in the form of nanoparticles present a very high
surface to volume ratio, and therefore, maximizes the formation of polar
phases for PVDF in contact with water.

## 4. Conclusions

PVDF nanoparticles with ferroelectric properties have been prepared by dialysis nanoprecipitation of a DMAc solution against water. The ferroelectric properties of these nanoparticles were proven by piezoresponse force microscopy. The origin of these ferroelectric properties can be attributed to the presence of a large fraction of ferroelectrically active crystals in the nanoparticles. We have proved by IR spectroscopy and X ray diffraction, that these crystals correspond to polar β and γ phases. The temperature dependence of the IR spectra and X ray diffraction patterns reveals that each of the crystalline phases of PVDF in the nanoparticles is stable in different temperature windows, providing a route to tuning the ferroelectric properties in these systems.

### CRediT authorship contribution statement

All authors have contributed to the work on this manuscript. **Edgar Gutiérrez-Fernández:** prepared the polymer NPs and carried out the X ray experimental work and analysis. **Jose Sena-Fernández:** prepared the polymer NPs and carried out the X ray experimental work and analysis. **Esther Rebollar:** performed the AFM and PFM work. **Tiberio A. Ezquerra:** performed the AFM and PFM work. **Francisco J. Hermoso-Pinilla:** performed the IR experiments. **Mikel Sanz:** performed the IR experiments. **Oscar Gálvez:** performed the IR experiments. **Aurora Nogales:** performed the AFM and PFM work, wrote the manuscript, which was revised and discussed by all the rest of the authors.

### Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

### Data availability

Data will be made available on request.

### Acknowledgments

The authors gratefully acknowledge the financial support of the Spanish Ministry of Science and Innovation (MICINN) through projects PID2019-107514GB-100/AEI/10.13039/501100011033 and PID2019-106125GB-100/AEI/10.13039/501100011033. Also, 'Programa Operativo de Empleo Juvenil' (YEI) Comunidad de Madrid (CAM, PEJ-2019-AI/TIC-15699). The synchrotron experiments were performed at NCD-SWEET beamline at ALBA Synchrotron with the collaboration of ALBA staff. The authors acknowledge also J.P. Sánchez for technical support.

## Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.polymer.2022.125540.

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