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Thermal Behavior, Local-Scale Morphology, and Phase Composition of Spherulites in Melt-Crystallized Poly(vinylidene Fluoride) Films

A peer-reviewed version of this preprint was published in:
Crystals 2025, 15(1), 94. https://doi.org/10.3390/cryst15010094

Submitted:

20 December 2024

Posted:

23 December 2024

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Abstract
Synchrotron microbeam X-ray diffraction was employed to investigate the local-scale structure and solid-state phase transformation within individual spherulites of poly(vinylidene fluoride) (PVDF). In thin, non-oriented films, PVDF crystallizes into α and γ phases, forming distinct spherulitic morphologies: large, banded α-spherulites and smaller, irregular “mixed” spherulites dominated by the γ-phase. For samples crystallized at high undercooling (160 °C), the mixed spherulites primarily consisted of the γ-phase, with only a minor fraction of α-lamellae localized at the spherulite boundaries. At higher crystallization temperatures (165 °C), the α-phase was entirely absent from the mixed spherulites. High-temperature annealing induced a phase transformation from the α-phase to the γ-phase, initiating at the interface between α- and γ-spherulites. The transformation propagated radially along the b-axis of the α-spherulite, while its characteristic banded morphology remained intact. Radial scanning with an X-ray microbeam provided spatially resolved mapping of the structural transition within the α-spherulite at the micrometer scale, offering detailed insights into the transformation mechanism and its impact on the spherulitic structure. The fast crystal growth direction remained unaltered during the transition, suggesting minimal material transport and maintaining structural coherence.
Keywords: 
poly(vinylidene fluoride)
;  
banded spherulites
;  
crystal–crystal transition
;  
polarized optical microscopy
;  
microfocus X–ray diffraction
;  
atomic force microscopy
Subject: 
Chemistry and Materials Science  -   Polymers and Plastics

1. Introduction

Synchrotron microbeam X-ray diffraction has recently emerged as a powerful tool for morphological studies of polymer materials. In the context of polymer spherulites, this technique enables the exploration of local-scale morphological features such as crystal growth direction, orientation, and degree of order [1,2,3,4,5,6,7]. For banded spherulites, it provides insights into chirality of the helicoidal or spiral lamellar crystals, chain tilt with respect to the lamellar normal, polarity of the crystal growth direction and even local-scale thermal behavor by analyzing the periodic sequence of crystalline and small-angle peaks obtained from a series of 2D diffraction patterns measured along the spherulite radius [8,9,10,11,12,13,14,15,16]. These structural parameters are challenging to determine using other methods.
In this paper, we present an application of synchrotron microfocus X-ray diffraction to investigate a solid-state phase transformation within a single spherulite of poly(vinylidene fluoride) (PVDF). The high spatial resolution afforded by this technique enables the precise localization of transformed regions and provides insights into the mechanisms driving the phase transition.
Poly(vinylidene fluoride) (PVDF) has been extensively studied over the past 40 years due to its exceptional properties, which are exploited in various technological applications [17,18,19,20]. PVDF exhibits excellent mechanical performance, high chemical resistance, and good thermal stability, along with outstanding pyro- and piezoelectric coefficients. These properties are heavily influenced by processing conditions, which determine the crystal phase composition [21,22]. PVDF primarily crystallizes in three polymorphs: the nonpolar α-phase (lattice parameters: a=4.96Å, b=9.64Å, and c=4.62Å (fiber repeat), chain conformation TG+TG–) [23,24], the strongly polar β–phase (all–trans conformation) and the moderately polar γ–phase (a=4.97Å, b=9.66Å, c=9.18Å (fiber repeat); chain conformation T3G+T3G–) [25,26]. These phases can be identified using techniques such as NMR, [23] Fourier-transform IR spectroscopy (FTIR) [28,29,30], X-ray diffraction [31,32], and differential scanning calorimetry (DSC) [33].
Polar phases, particularly the β-phase, are of significant interest due to their pyro- and piezoelectric properties. Pure β-phase PVDF films can be obtained through stretching [34,35], nucleation agents [36], or application of strong electric fields [37]. However, the β-phase easily transforms into the nonpolar α-phase at elevated temperatures [38]. In contrast, the γ-phase demonstrates thermal stability up to 190 °C and can be generated through additives or high-temperature treatment [39,40]. Notably, annealing above 130 °C results in a solid-state α→γ transformation, as observed in DSC [41,42,43] and FTIR studies [44,45,46,47].
Temperature-resolved polarized optical microscopy (POM) reveals the kinetics of the α→γ transformation. In thin, melt-crystallized PVDF films, two spherulite types are observed: large, highly birefringent α-spherulites with concentric bands and smaller, less birefringent γ-spherulites with no apparent banding due to morphological irregularity [48,49,50,51]. Banded α-spherulites grown or annealed above 165 °C partially melt at 174–178 °C, with residual banded regions persisting up to 190 °C, identified as the γ-phase [46].
Despite prior studies using AFM [52,53], scanning electron microscopy [54], and other methods, the crystalline lamellar orientation and phase stability in PVDF films remain poorly understood, particularly in relation to film thickness and thermal treatment. To address this, we undertook a detailed morphological study of quasi-two-dimensional spherulites in melt-crystallized PVDF films, employing microfocus X-ray scattering combined with AFM and optical microscopy.

2. Materials and Methods

2.1. Sample Preparation

PVDF (Solef 6010, Mw = 166kg/mol, Mw/Mn=1.7) films were prepared by solution–casting from dimethyl formamide onto glass slides. The resulting films (ca. 20µm thick) were dried under vacuum, melted at 210 °C, and subsequently crystallized at varying temperatures under nitrogen to prevent oxidative degradation: at 160 °C for 3 hours (sample 1), at 162.5 °C for 48 hours (sample 2) and at 165 °C for 88 hours (sample 3). To simplify measurement descriptions, general notations were assigned to specific regions of the films (Figure 1a). For the non–banded spherulites the notations are as follows: I – center of spherulite, II – region between center and periphery, III – boundary between non–banded spherulites and region with low birefringence VI (will be explained below); for the banded α–spherulites: IV – region close to boundary between non–banded and α–spherulites, V – region far from the boundary.

2.2. Polarized Optical Microscopy

A polarized optical microscope (Olympus Provis AX70, Olympus, Tokyo, Japan) equipped with an LNP420 heating stage (Linkam Scientific Instruments Ltd., Salfords, UK) was used for optical texture observations. Heating experiments were performed immediately after crystallization at a heating rate of 3 °C/min, without cooling to room temperature. Image analysis, including optical density quantification, was conducted using custom routines designed in Igor Pro 8 (Wavemetrics, Portland, USA).

2.3. Differential Scanning Calorimetry

DSC measurements were performed using a Mettler Toledo 822e heat flux DSC (Greifensee, Switzerland) equipped with an LN2 cooling system. The instrument was calibrated with the melting points and enthalpies of indium and zinc standards.

2.4. Synchrotron X–Ray diffraction

Wide-angle X-ray scattering (WAXS) measurements were conducted at beamline BM26 of the European Synchrotron Radiation Facility (ESRF, Grenoble, France) using a photon energy of 10 keV. The X-ray beam (~30 × 50 µm2) was focused on the sample using banded Kirkpatrick-Baez (KB) mirrors. PVDF films were removed from glass substrates by flotation on water for X-ray measurements. Diffraction patterns were recorded with a CCD camera, and temperature control was achieved with a DSC600 heating stage (Linkam Scientific Instruments Ltd., Salfords, UK) under LN2 flow. The scattering vector magnitude, s = 2sinθ/λ, where θ is the Bragg angle and λ – wavelength, was calibrated using several diffraction orders of silver behenate.
Two-dimensional (2D) microfocus X-ray scattering experiments were performed on the ID13 beamline at ESRF using a 1.0 Å wavelength. The crossed-Fresnel optics generated a 500 × 500 nm2 beam spot. Diffraction patterns were recorded with a MARCCD 165 detector (pixel size: 80 µm) in transmission geometry, with the sample surface oriented perpendicular to the beam. The region of interest was identified using an on-axis optical microscope based on morphological differences between α- and γ-spherulites. Sample scanning was performed with a 1.0 µm step size using an x-y gantry, and online exposure normalization was achieved with an upstream beam monitor.

2.5. Atomic Force Microscopy

PVDF film surfaces were analyzed using a Nanoscope IIIa MultiMode AFM (Veeco Metrology Group, Santa Barbara, USA) in tapping mode, suitable for imaging soft materials. Tapping mode silicon probes (length: 220 µm, resonant frequency: 150–200 kHz, stiffness: ~40 N/m) were used. The free oscillation amplitude (A0) ranged from 50 to 100 nm. Imaging was performed in both light and hard tapping mode, whereby the set–point amplitude, Asp, was (0.8–0.9)A0 and (0.4–0.5)A0, respectively. Light tapping was used for high–resolution imaging of top surface structures, whereas discriminative visualization of amorphous, mesophase and crystalline regions was performed in hard tapping. These complementary modes allowed detailed surface characterization of the PVDF films.

3. Results

3.1. Analysis of Polarized Microscopy Images

Variable-temperature optical microscopy was used for the preliminary analysis of the crystalline phase composition of the samples. Images of both Sample 1 and Sample 3, taken at 165 °C, reveal the presence of banded α-spherulites. Additionally, smaller, non-banded spherulites with high birefringence, indicative of radial orientation, are observed (Figure 1a, c). However, some non-banded spherulites exhibit peripheral regions with low birefringence (Figure 1a, Region VI). These spherulites display less regular shapes and fringed boundaries. The relative fraction of such peripheral regions decreases with increasing crystallization temperature, and their detailed morphology is discussed later.
In situ POM measurements during heating of Sample 1 show that α-spherulites disappear completely at 185 °C, leaving only non-banded spherulites visible (Figure 1b). In contrast, in Sample 3, at temperatures above the melting point of the α-phase, a higher-melting crystalline phase with jagged boundaries becomes apparent alongside non-banded spherulites (Figure 1d). At 190 °C, the non-banded spherulites in both samples begin to melt, while the birefringence of the higher-melting phase within the α-spherulites remains unchanged until 192 °C (Figure 1e).
These melting events, visualized through POM, are consistent with the differential scanning calorimetry (DSC) results (Figure 1f), which show three distinct melting peaks with similar onset temperatures. Considering the quasi-2D nature of the spherulites, detailed analysis of the POM images allows for the identification of the melting behavior of each microscopic texture.
The optical density of spherulites observed in polarized light can be attributed to the birefringence of the crystals, which arises from the specific lamellar orientation within different regions [55]. Assuming that the lamellar orientation remains unchanged during heating, birefringence can be used as an indicator of the local presence of crystalline phases.
As evidenced by DSC data, recrystallization processes do not play a significant role during heating after crystallization at temperatures above 160 °C. Therefore, any decrease in optical density in a selected region is attributed to the melting of specific crystal modifications. Figure 1g,h display the derivative of optical density as a function of temperature for Samples 1, 2, and 3, measured in the α-spherulites and mixed spherulites, respectively.
For Sample 1, the derivative curve for α-spherulites shows a peak at 174 °C (Tm1), corresponding to the melting of the α-phase, consistent with the DSC data (Figure 1g). For Samples 2 and 3, this peak shifts to higher temperatures, and its intensity decreases, while a new weak peak emerges at 193 °C (Tm3), attributed to the melting of a high-temperature phase. In mixed spherulites of Sample 1, two distinct peaks are observed at 176 °C (Tm1) and 179 °C (Tm2), corresponding to the melting of the α- and γ-phases, respectively. The relative content of the α-phase decreases with increasing crystallization temperature, and for samples crystallized at 162.5 °C and 165 °C, the α-phase is no longer detected in mixed spherulites. In these cases, the γ-phase peak shifts to higher temperatures, indicating thicker and more perfect lamellae.
The presence of the α-phase in predominantly γ-phase non-banded spherulites was previously suggested by Lovinger using IR spectroscopy [41]. These spherulites, referred to as "mixed" spherulites, show a clear decrease in α-phase content as the crystallization temperature increases. However, in this study, the fraction of the α-phase in mixed spherulites is directly estimated using POM.
To further investigate the phase composition in both α- and mixed spherulites, X-ray diffraction with a focused beam (~30 µm in diameter) was employed. This approach enables the structural characterization of individual spherulites at different temperatures, providing precise insights into their crystalline phases.

3.2. WAXS Studies at Different Temperatures

2D diffraction patterns measured on both types of spherulites in sample 1 reveal well-oriented structures (Figure 2). At room temperature, the pattern obtained from the mixed spherulite (region V, Figure 2a) shows reflections at 020αγ (4.83 Å) and 110αγ (4.42 Å), characteristic of both crystalline modifications, as well as peaks at 100α (4.98 Å) and 120α (3.48 Å), specific to the γ-phase. The azimuthal intensity maximum of the 020α peak aligns with the radial direction of the banded spherulite, indicating that the growth direction of the α-spherulite is parallel to the b-axis. A weak SAXS signal is observed perpendicular to the radial direction.
In the mixed spherulite (region II, Figure 2b), the diffraction pattern also includes peaks at 020αγ and 110αγ, along with the γ-specific 021γ (3.31 Å) reflection, but lacks the 100α and 120α peaks. Similar to the α-spherulite, the 020αγ reflection has its maximum along the radial direction. The SAXS signal is more pronounced in the mixed spherulite, suggesting that lamellar stacks are at least partially oriented edge-on relative to the film surface.
At 180 °C, the diffraction pattern from region II indicates the melting of the mixed spherulite, with no crystalline reflections or SAXS signal observed (Figure 2c). In contrast, the crystalline peaks of the γ-modification are still present at 180 °C in the α-spherulite (Figure 2d), with the orientation of these reflections remaining unchanged from room temperature.
These X-ray measurements confirm that the non-banded spherulites in sample 1 primarily consist of the γ-phase, while the banded spherulites contain only α-crystals. However, it is important to note that the X-ray beam integrates the signal over an area spanning tens of microns. To further investigate the phase composition and lamellar orientation in points I–VI on a micrometer scale, atomic force microscopy (AFM) and microfocus X-ray diffraction were employed.

3.3. Microstructure of the Mixed Spherulite

The microstructure of the samples was analyzed using AFM. Scanning the mixed spherulite in sample 1, which exhibits low birefringence at its periphery, revealed a complex morphology (Figure 3a). The topography image shows that the center of the spherulite (region I) is composed of curled, cylinder-like entities with diameters ranging from 300 to 600 nm (Figure 3b). Similar lamellar scrolls have been reported in PVDF films crystallized above 168 °C [54]. Moving radially outward from the center (region I) to the intermediate region (region II), the orientation of the scroll axes transitions from perpendicular to the film surface to inclined along the radial direction (Figure 3c). In contrast, at the low-birefringence region (region VI), flat-on lamellae with slightly curled edges along the tangential axis are observed, alongside bright, out-of-plane banded lamellae (Figure 3d).
These variations in lamellar morphology were further corroborated by microfocus X-ray diffraction. In region I, the diffraction pattern reveals only a non-oriented 110αγ reflection (Figure 3e). In region II, where the scrolls are inclined, an additional 020αγ reflection aligned along the radial direction is observed (Figure 3f).
The crystalline b-axis of the γ-phase aligns along the scrolls and gradually orients radially as one moves away from the center of the mixed spherulite. This change in orientation likely results from the "quasi–2D" nature of the spherulites. During the initial growth phase, when the lateral size of the spherulite is smaller than the film thickness, the spherulite behaves as a three-dimensional (3D) object (region I). Here, lamellar scrolls diverge from the nuclei formed deep within the film and orient radially. As the spherulite grows and transitions into a two-dimensional (2D) morphology, only scrolls parallel to the surface continue to grow (region II).
In region VI, where birefringence is low, X-ray patterns show a strong, non-oriented 110αγ peak, while the 020αγ reflection vanishes. The absence of the 020αγ peak suggests the presence of tangentially curled lamellae in addition to planar lamellae observed by AFM. Curling these flat-on lamellae around the tangential axis transitions the spherulite back into a 3D structure.
At the boundary between mixed and α-spherulites, these 3D lamellae may grow atop the α-spherulites, forming a double-layered mixed zone (region III) (Figure 4a). Microfocus WAXS measurements confirm this structure (Figure 4b). In region III, both α and γ crystals are present, while regions I, II, and VI lack the 120α peak, indicating the pure γ-phase. Conversely, the absence of the 111γ peak in the α-spherulite (region V) confirms the pure α-phase. The width of the mixed region (region III) along the spherulite boundary is at least 10 µm in the radial direction.
The mechanism of interpenetration between the two spherulites requires further study. Besides the double-layer mechanism, interpenetration could occur via the advancing growth front of α-spherulites, which permits γ-phase crystallites to grow between α-lamellae. Regardless, microfocus X-ray diffraction reveals that the α-modification in mixed spherulites is limited to the interface with α-spherulites.

3.4. Microstructure of the α–Spherulite

As mentioned earlier, under polarized light, α-spherulites exhibit periodic dark and bright bands, with the banding period varying based on the crystallization temperature (Figure 5a). The local morphological study of the α-spherulites in sample 3 reveals periodically twisting crystals, identifiable via AFM as alternating edge-on and flat-on lamellar stacks (Figure 5b,c). The one-dimensional power spectral density analysis derived from AFM images taken near (region IV) and far (region V) from the interface between α- and mixed spherulites indicates a consistent banding period of 4.2 µm, aligning well with POM data (4.3 µm) (not shown here). High-magnification AFM images further resolve individual lamellae within the region with scrolls and regular edge-on stacks (inset, Figure 5b,c). Differences in the phase composition between regions IV and V were further identified through microfocus X-ray diffraction analysis.
The microfocus X-ray diffraction patterns of sample 3, measured along the radial directions of mixed and α–spherulites (red arrow in Figure 5a), reveal three distinct crystalline structures (Figure 6a–c). The X-ray pattern of region II (Figure 6a) displays the 110γ (4.98 Å) and 021γ (4.27 Å) reflections, characteristic of the γ–phase. During the radial scan, the azimuthal position of the 110γ peak fluctuates, indicating local orientational disorder of the inclined scrolled lamellae of the γ–modification. This behavior confirms that, unlike sample 1, the mixed spherulites of sample 3 crystallized at higher temperatures consist solely of the γ–phase, as corroborated by the POM analysis (Figure 1h).
Region V (Figure 6c) exhibits the 100α (4.98 Å) and 120α (3.48 Å) reflections, which are characteristic of the α–phase, and the absence of the 021γ peak. The azimuthal position of the 020α reflection aligns with the radial direction of the banded spherulite (dashed arrow in Figure 6b,c), indicating that the growth direction of the α–spherulite is parallel to the b–axis of the α unit cell.
In contrast, the intermediate region IV (Figure 6b) shows only peaks typical of the γ–crystalline phase. These findings confirm that the high-melting phase (region IV) corresponds to the γα–modification, formed from the α–phase. The morphological difference between the γ and γα phases lies in the γα–phase's oriented 020αγ peak, similar to the α–phase. This suggests that the lamellar orientation remains unchanged during the α-γ phase transition, as corroborated by POM observations.
The radially integrated diffraction intensity, plotted as a function of scan distance (axis y) and reciprocal space vector q (axis x), is shown in Figure 6d. Oscillations in the intensity of the 110αγ reflection within the γα– and α–regions exhibit a periodicity of approximately 5 µm, corresponding to the banding periodicity (Figure 6e,f). This intensity variation can be attributed to regular lamellar twisting, which causes the lamellae to move in and out of reflection conditions during the scan. The discrepancy between the banding period obtained from microfocus WAXS and POM may arise from slight deviations in the scan direction from the exact radial orientation.
The solid-state α-γ transformation observed during prolonged annealing at high temperature begins at the interface between mixed and α–spherulites and propagates radially along the b–axis of the α–modification. This mechanism explains the jagged boundary observed in optical micrographs. Once the transformation front reaches the α–spherulite center, γα–phase growth continues radially outward in all directions. Such a solid-state transformation in semi-crystalline polymers appears to be unique to PVDF.
The supramolecular morphology of the spherulite remains largely unaltered during the α-γ transformation. The lamellar thickness of the γα–phase closely matches that of the α–modification. High-resolution AFM images of edge-on lamellar stacks in the γ– and α–phases (Figure 5b,c insets) reveal a long period of 13.8 nm in the α–spherulite. This value was determined from the 1D SAXS-type correlation function and its second derivative, the interface distribution function [56,57] (Figure 5d). The long period corresponds to the position of the first subsidiary maximum in the correlation function and the first minimum in the interface distribution function. In the center of the mixed spherulite (region I), the long period of the curled lamellar stack is 10.4 nm (not shown here), which is significantly smaller than that in the α–spherulite. The interface distribution function indicates a linear crystallinity of approximately 50% and a γ–crystal thickness of 5.2 nm. This value aligns well with theoretical predictions based on scroll formation models, which estimate crystal thicknesses of 3–6 nm for scroll diameters of 300–600 nm [58]. In region IV, the long period increases to approximately 20 nm (Figure 5d), likely due to the inclination of the scrolls relative to the film normal, further modifying the local morphology. The higher melting temperature of the γα–phase compared to the γ–phase in mixed spherulites can be attributed to the increased lamellar thickness and reduced surface energy associated with specific stresses in the curled crystals. In α–spherulites, helicoidal structures formed by regular lamellar stacks transform into the γ–phase during annealing. These transformations yield thicker lamellae in the γα–phase compared to the less regular scrolls in mixed spherulites, explaining the higher thermal stability of the α-derived γα–phase.

4. Conclusions

In this study, we conducted a comprehensive analysis of the morphology, thermal behavior, and phase transformations of relatively thick (~20 µm) melt-crystallized PVDF films. The selected film thickness provided an optimal balance for analysis using techniques suitable for both thin films and bulk materials. By combining direct-space methods (polarized optical microscopy and AFM) with reciprocal-space techniques (microfocus WAXS), we demonstrated that the local structure of the films varies significantly depending on the crystallization temperature.
For samples crystallized at high undercooling (160 °C), the mixed spherulites predominantly contained the γ–phase, with a minor fraction of α–lamellae confined to the spherulite boundaries. This is the first direct evidence of this phase composition. The high-birefringence regions of the mixed spherulites exhibited a unique morphology with curled lamellae diverging radially from the center to the periphery, where the orientation of the scrolls gradually aligns along the radial direction. The scrolling axis, parallel to the b–axis, determines the optical and structural properties of the spherulites. In regions of the mixed spherulites with low birefringence, lamellae curled around the tangential a–axis, affecting optical properties by altering the direction of the b–axis and the incident light. This lamellar curling leads to the disappearance of the 020αγ reflection and disorientation of the 110αγ reflection on X-ray diffraction patterns. With increasing crystallization temperature, the fraction of mixed spherulites increased, while the fraction of low-birefringence regions in these spherulites decreased. For films crystallized at 165 °C, no α–phase was detected in the mixed spherulites.
The morphology of the α–spherulites was distinct. Twisted lamellae organized into continuous helicoids with the b–axis aligned along the growth direction. Microfocus X-ray diffraction revealed that during high-temperature annealing, regions within the α–spherulites underwent a transformation to the γ–phase. This transformation began at the interface between α– and γ–spherulites and propagated radially along the b–axis without altering the banded morphology of the α–spherulites. Scanning along the radial direction of a single PVDF spherulite enabled the spatial identification of structural evolution on the micrometer scale.
The differences in crystal phase and morphology between mixed and α–spherulites had a pronounced effect on their thermal behavior. Quantitative analysis of optical density in POM images during heating revealed three melting events in the range of 174–193 °C. The first melting, corresponding to the α–phase, occurred in the α–spherulites. At higher temperatures, the melting of the γ–phase in mixed spherulites was observed, followed by the high-temperature melting of the γα–phase within α–spherulites. These observations were corroborated by X-ray diffraction, which tracked the thermal evolution of the crystal structures in single α– and mixed spherulites during heating. The increase in melting temperature was attributed to the greater crystal thickness of the γα–phase compared to that in mixed spherulites, as well as to surface stresses induced by lamellar curling.
The PVDF films provided direct evidence of the critical role of the crystallization mechanism in determining the final spherulite morphology. The γ–phase formed directly from the melt exhibited a completely different supramolecular structure and thermal behavior compared to the γ–phase produced through the solid-state α→γ transformation. Notably, microfocus X-ray scattering proved invaluable for investigating local structural features within relatively thick polymer films, surpassing the capabilities of standard techniques like selected-area electron diffraction. Thicknesses above 10 µm were found necessary to support complex three-dimensional morphological structures such as scrolls and helicoids, which have characteristic dimensions of 3–5 µm.
This study of melt-crystallized PVDF films serves as a model for investigating the structure and properties of specific morphological entities across scales from nanometers to hundreds of microns. The integrated experimental approach provides new insights into the correlations between local structure and macroscopic properties, enabling the architectural analysis of materials across hierarchical levels.

Author Contributions

For research articles with several authors, a short paragraph specifying their individual contributions must be provided. The following statements should be used “Conceptualization, D.A.I.; methodology, M.R.; formal analysis, A.V.C.; data curation, D.A.I., D.V.A., M.R., D.S.P.; writing—original draft preparation, D.V.A.; writing—review and editing, D.A.I.; supervision, D.A.I.; project administration, A.V.C., D.V.A. All authors have read and agreed to the published version of the manuscript.”

Funding

This research was funded by the Ministry of Science and Higher Education of the Russian Federation within State Contract 075–15–2022–1117 from 30 June 2022.

Data Availability Statement

The data available on request.

Acknowledgments

The authors gratefully acknowledge the excellent technical support provided by the personnel of the BM26 and ID13 beamlines at the ESRF in Grenoble, France. Special thanks are extended to M. Burghammer of the ID13 beamline for his invaluable assistance with the experiments.

Conflicts of Interest

The authors declare no conflicts of interest.

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Preprints 143589 g001
Figure 1. Optical microscopy images of Sample 1: (a) at 160 °C, (b) at 180 °C, and Sample 3: (c) at 165 °C, (d) at 182 °C, (e) at 187 °C. Differential scanning calorimetry (DSC) scans of Samples 1, 2, and 3 are included, with the sample numbers corresponding to their respective notations. Changes in optical density during heating are shown for (f) α-spherulites and (g) mixed spherulites.
Figure 1. Optical microscopy images of Sample 1: (a) at 160 °C, (b) at 180 °C, and Sample 3: (c) at 165 °C, (d) at 182 °C, (e) at 187 °C. Differential scanning calorimetry (DSC) scans of Samples 1, 2, and 3 are included, with the sample numbers corresponding to their respective notations. Changes in optical density during heating are shown for (f) α-spherulites and (g) mixed spherulites.
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Figure 2. Wide-angle X-ray scattering diffraction patterns of a single α-spherulite: (a) measured at room temperature and (b) at 180 °C; and a mixed spherulite: (c) measured at room temperature and (d) at 180 °C.
Figure 2. Wide-angle X-ray scattering diffraction patterns of a single α-spherulite: (a) measured at room temperature and (b) at 180 °C; and a mixed spherulite: (c) measured at room temperature and (d) at 180 °C.
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Figure 3. (a) Polarized optical microscopy (POM) image of sample 1, showing a mixed spherulite with a low-birefringence region (VI). Atomic force microscopy (AFM) scans (10 × 10 µm) and corresponding microfocus WAXS patterns: (b,e) region I (center of the spherulite), (c,f) region II (intermediate radial region), and (d,g) region VI (low-birefringence region at the periphery). Arrows on the WAXS diffractograms indicate the radial direction of the spherulite.
Figure 3. (a) Polarized optical microscopy (POM) image of sample 1, showing a mixed spherulite with a low-birefringence region (VI). Atomic force microscopy (AFM) scans (10 × 10 µm) and corresponding microfocus WAXS patterns: (b,e) region I (center of the spherulite), (c,f) region II (intermediate radial region), and (d,g) region VI (low-birefringence region at the periphery). Arrows on the WAXS diffractograms indicate the radial direction of the spherulite.
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Figure 4. (a) AFM scan (10 × 10 µm2) highlighting the boundary between α– and mixed spherulites (region III); (b) 1D WAXS profiles corresponding to regions II, III, and V, respectively; (c) schematic representation of the mixed spherulite morphology, with Roman numerals indicating the corresponding regions identified in the POM image.
Figure 4. (a) AFM scan (10 × 10 µm2) highlighting the boundary between α– and mixed spherulites (region III); (b) 1D WAXS profiles corresponding to regions II, III, and V, respectively; (c) schematic representation of the mixed spherulite morphology, with Roman numerals indicating the corresponding regions identified in the POM image.
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Figure 5. (a) Polarized optical microscopy image of sample 3, indicating the scan direction for the microfocus diffraction experiment. The red points represent the measurement positions of the microfocus WAXS data shown in Figure 6a–c. (b) AFM image (5 × 5 µm2) of region V inside the α-spherulite, showing lamellar scrolls. (c) AFM image (5 × 5 µm2) of region IV inside the α-spherulite, with the inset highlighting magnified edge-on lamellae. (d) Correlation and interface distribution functions calculated from high-resolution AFM images for region V (solid line) and region IV (dashed line).
Figure 5. (a) Polarized optical microscopy image of sample 3, indicating the scan direction for the microfocus diffraction experiment. The red points represent the measurement positions of the microfocus WAXS data shown in Figure 6a–c. (b) AFM image (5 × 5 µm2) of region V inside the α-spherulite, showing lamellar scrolls. (c) AFM image (5 × 5 µm2) of region IV inside the α-spherulite, with the inset highlighting magnified edge-on lamellae. (d) Correlation and interface distribution functions calculated from high-resolution AFM images for region V (solid line) and region IV (dashed line).
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Figure 6. (a–c) 2D microfocus WAXS patterns of PVDF recorded at various positions along the scan, illustrating two distinct crystalline modifications: (a) γ–phase within the mixed spherulite, (b) γα–phase, and (c) α–phase within the α–spherulite. (d) Radially integrated diffraction intensity plotted as a function of the scan position. (e) Intensity variation of the 110αγ reflection along the scan distance. (f) 1D power spectral density analysis of the γα and α regions in the banded spherulite, revealing a peak at s=0.2 µm-1 or d-spacing of 5.0 µm corresponding to the periodic band width observed along the scan.
Figure 6. (a–c) 2D microfocus WAXS patterns of PVDF recorded at various positions along the scan, illustrating two distinct crystalline modifications: (a) γ–phase within the mixed spherulite, (b) γα–phase, and (c) α–phase within the α–spherulite. (d) Radially integrated diffraction intensity plotted as a function of the scan position. (e) Intensity variation of the 110αγ reflection along the scan distance. (f) 1D power spectral density analysis of the γα and α regions in the banded spherulite, revealing a peak at s=0.2 µm-1 or d-spacing of 5.0 µm corresponding to the periodic band width observed along the scan.
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