Poly(Vinylidene
Fluoride-Co-Hexafluoropropylene)/Amethyst
Composites: Preparation, Properties, and Potential for
Sustainable Energy Materials

Surapoj Khawkom, Nathapong Sukhawipat, Suchet Glabsawat, Tanapat

Rompoche, Jureeporn Yuennan, Nikruesong Tohluebaji, Pamela Pasetto,

Tharanin Panyachan

► To cite this version:

Surapoj Khawkom, Nathapong Sukhawipat, Suchet Glabsawat, Tanapat Rompoche, Jureeporn Yuennan, et al.. Poly(Vinylidene Fluoride-Co-Hexafluoropropylene)/Amethyst Composites: Preparation, Properties, and Potential for Sustainable Energy Materials. Solid State Phenomena, 2024, 366, pp.21-28. 10.4028/p-og6yqx. hal-05261904

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# Poly(Vinylidene Fluoride-co-Hexafluoropropylene)/Amethyst Composites: Preparation, Properties, and Potential for Sustainable Energy Materials

Surapoj Khawkom<fnref n="1" />,<fnref n="2" />,a, Nathapong Sukhawipat<fnref n="1" />,<fnref n="2" />,b*, Suchet Glabsawat <fnref n="1" />,<fnref n="2" />,c,
Tanapat Rompoche<fnref n="1" />,<fnref n="2" />,d, Jureeporn Yuennan<fnref n="3" />,e, Nikruesong Tohluebaji<fnref n="4" />,f,
Pamela Pasetto<fnref n="5" />,g, Tharanin Panyachan<fnref n="1" />,<fnref n="2" />,h

**Keywords:** Poly(Vinylidene Fluoride-co-Hexafluoropropylene), Amethyst, Composite film, Mechanical properties, Green materials

**Abstract.** This study investigates the fabrication and potential application of novel sustainable energy materials from poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP)/amethyst composite films. Solution processing and casting techniques were employed to prepare these composites with varying amethyst content (0 – 0.5 phr). The film characteristics, including appearance, dispersion, morphology, density, and mechanical properties, were comprehensively analyzed. Optical microscopy (OM) revealed good dispersion of amethyst at low loadings, while higher loadings exhibited a tendency towards agglomeration. Notably, the mechanical properties of the composites improved with increasing amethyst content. Tensile strength increased from 5.57 to 13.04 N/mm<fnref n="2" />, strain at break increased from 4.33 to 15.74%, and Young's modulus increased from 156.00 to 258.21 N/mm<fnref n="2" />. The combined properties of PVDF-HFP and amethyst suggest their potential for application in sustainable energy generation. Future work will be directed towards exploring this possibility through further characterization relevant to energy applications.

## Introduction

The growing demand for sustainable energy solutions has driven extensive research into novel materials with superior electrical, mechanical, and thermal properties. Among these materials, polymers and their composites have gained significant attention due to their versatility, ease of processing, and tunable properties. Poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) is a particularly promising polymer due to its excellent chemical resistance, mechanical strength, and high dielectric constant [1–3]. These attributes make PVDF-HFP an ideal candidate for applications in energy storage [4,5], sensors [6,7] and actuators [8]. However, to further enhance these vital properties, additional efficiency is required, which can be achieved by incorporating various additives such as zinc oxide (ZnO) [1], ferromagnetic cobalt ferrite (CoFe₂O₄) [2] and silver

<footnote n="1">¹Division of Polymer Engineering and Rubber Industrial Technology, Department of Mechanical Engineering Technology, College of Industrial Technology, King Mongkut's University of Technology North Bangkok, Bangkok 10800, Thailand</footnote>

<footnote n="2">²Center of Sustainable Energy and Engineering Materials, College of Industrial Technology, King Mongkut's University of Technology North Bangkok, Bangkok 10800, Thailand</footnote>

<footnote n="3">³Surface Technology Research Unit (STRU), Faculty of Science and Technology, Nakhon Si Thammarat Rajabhat University, Nakhon Si Thammarat, 80280, Thailand</footnote>

<footnote n="4">⁴Faculty of Science and Technology, Princess of Naradhiwas University, Narathiwat, 96000, Thailand</footnote>

<footnote n="5">⁵Institut des Molecules et Materiaux du Mans, UMR CNRS 6283, Le Mans Universite, 72085 Le Mans Cedex 9, France</footnote>

<footnote>a. surapojn@gmail.com, b. nathapong.s@cit.kmutnb.ac.th, c. kong.suchate@gmail.com, tanapatn30@gmail.com, d. jureeporn_yue@nstru.ac.th, e. Nikruesong.t@pnu.ac.th, f. Pamela.Pasetto@univ-lemans.fr, h. tharanin.panyachan@gmail.com</footnote>

<footnote>*Corresponding author: nathapong.s@cit.kmutnb.ac.th</footnote>
nanoparticles (AgNPs) [3,4]. However, these additives are synthetic materials. To turn waste into materials valuable, this work focuses on exploring the use of amethyst as a novel additive to enhance the properties of PVDF-HFP.

Amethyst, a naturally occurring form of quartz, has been historically valued for its aesthetic appeal and piezoelectric properties [9,10]. The integration of amethyst into polymer matrices presents an intriguing opportunity to enhance the functional properties of the composite materials.

Recent advances in composite material preparation techniques have enabled the development of PVDF-HFP/nanoparticle composites with enhanced performance characteristics [3,11–14]. The incorporation of amethyst into PVDF-HFP is expected to not only improve the mechanical properties of the composite but also impart additional functionalities such as improved piezoelectric response and energy harvesting capabilities.

Therefore, motivated by the potential for performance enhancement, this study investigated the preparation methods, structural properties, and potential applications of PVDF-HFP/amethyst composites in sustainable materials. To clarify the relationship between material composition and functional properties, a systematic investigation has been undertaken. This involved exploring the preparation process and characterizing the resultant composites with varying amethyst content ranging from 0 to 0.5 wt%. Specifically, the morphology and mechanical properties of the composites were evaluated.

# Experimental

## Materials

Poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) with a purity more than 99.8% (grade Solef 11010/1001) was purchased from Postar Chemicals Co., Ltd., Bangkok, Thailand. *N,N*-Dimethylformamide (DMF), used as a solvent, with a purity of 99.8% (AR grade), was obtained from Quality Reagent Chemical Products. The coupling agent Silane-69 (industrial grade) was sourced from Lab Line Enterprises, Haryana, India. Amethyst, prepared from amethyst gemstones from Tanzania, Africa, was supported from a gem polishing shop in Chanthaburi province, Thailand. All chemicals were used without further purification. PVDF-HFP and amethyst were dried at 100°C overnight before use.

## Methods

### Preparation of Amethyst

Amethyst composition in the PVDF-HFP/amethyst composite film was prepared as a powder substance. Initially, various 1 to 4 mm diameter of amethyst ore was cleaned by using the anionic surfactant and had been soak int water- anionic surfactant mixture for 48 hours. Then, the amethyst ore was diluted with the DI water in the ultrasonic bath for 30 min. The ore had been crushed into less 1 mm-size by using the iron crusher, which result in a grinded amethyst. To make a rough texture of powder, a grinded amethyst was scuffed by the hard contact between iron mortar and pestle until a grinded texture was collapsed into a powder. Finally, the rough amethyst powder had been spined with the in-house customized ball milling blade with a 35,000 rpm for 1 hour, which resulting in the fine amethyst powder.

### Preparation of PVDF-HFP/Amethyst Composite Film

PVDF-HFP was dissolved in DMF at a concentration of 15 wt% in a 250-ml round bottom flask. The mixture was stirred at room temperature at 500 rpm for 30 minutes. Subsequently, 3 parts per hundred resin (phr) of Silane-69 was added dropwise to the solution and stirred for an additional 20 minutes at room temperature at the same stirring rate. Finally, amethyst was incorporated into the solution at concentrations of 0, 0.1, 0.3, and 0.5 phr, with stirring at 500 rpm for 20 minutes at room temperature. The resulting composite solution was then cast onto a Teflon mold with dimensions of 15 cm x 7 cm x 0.5 cm. The cast film was allowed to dry at room temperature for 24 hours before being further dried in an oven at 80°C for 2 hours. The PVDF-HFP/amethyst composite film was subsequently stored in a desiccator until testing.
<table><caption>**Table 1.** Formulation of PVDF-HFP/amethyst composite.</caption><thead><tr><th rowspan="2">Sample</th><th colspan="3">Composition</th></tr><tr><th>PVDF-HFP (phr)</th><th>Amethyst (phr)</th><th>Silane (phr)</th></tr></thead><tbody><tr><td>PVDF-HFP-0Am</td><td>100</td><td>0</td><td>3</td></tr><tr><td>PVDF-HFP-0.1Am</td><td>100</td><td>0.1</td><td>3</td></tr><tr><td>PVDF-HFP-0.3Am</td><td>100</td><td>0.3</td><td>3</td></tr><tr><td>PVDF-HFP-0.5Am</td><td>100</td><td>0.5</td><td>3</td></tr></tbody></table>

## Characterization

The structural characteristics were identified using X-ray diffraction (XRD). A Philips X'Pert MPD system, equipped with Cu-Kα radiation (1.54 Å) operating at a voltage of 30 kV and a current of 35 mA, was employed. The 2θ range was evaluated from 10° to 80°.

The elemental composition of the samples was analyzed using an X-ray fluorescence (XRF) analysis, model XGT-5200. The analysis was conducted with a live time of 100 seconds and a processing time setting of P1. The XGT diameter was set to 1.2 mm, with the X-ray tube operating at a voltage of 50 kV and a current of 1.000 mA. A 5-element X-ray filter was employed during the analysis, and no cell was used.

The density of the PVDF-HFP/amethyst composite was determined using ASTM D4635-16. A vernier caliper was used to accurately measure the length and thickness of composite films. The specimens were precisely weighed to calculate the density using equation (1).

$$ \rho = m/V \qquad (1) $$

Where ρ is density (g./m³), m is mass (g) and V is volume (cm³).

The mechanical properties of the PVDF-HFP/amethyst composite films were measured using Testometric on a universal testing machine (Instron model 3365) in accordance with ASTM D638 (M500-25AT). The size of the specimen was 100 x 10 mm². The instrument included a 1 kN load cell and a 25 mm gauge length extensometer with a crosshead speed of 10 mm/min. Tensile strength, strain at peak and elongation at peak were reported.

The morphology and dispersion of amethyst in the PVDF-HFP/amethyst composite films were studied with an optical microscope at a magnification of 40X and 100X, using Xenon (DN-117M).

## Results and Discussion

Fig. 1(a) presents the XRD pattern of amethyst, with 2θ ranging from 10° to 80°. Several significant peaks are observed at 20.85°, 26.63°, 40.28°, 50.60°, 55.31°, and 59.94°, corresponding to the crystal planes 100, 011, 111, 112, 103, and 113, respectively [9]. These peaks confirm the crystalline structure of amethyst. Furthermore, XRF analysis, shown in Fig. 1(b), confirms that the primary composition of amethyst is silica (SiO₂), accounting for 96.52% of the content. The analysis also detected iron (Fe) contamination at 2.72%, along with trace amounts of chromium (Cr), manganese (Mn), and nickel (Ni) at 0.37%, 0.27%, and 0.12%, respectively. The combined results from XRD and XRF analyses confirm both the crystalline structure and the elemental composition of the amethyst that could be used as an additive for polymer matrix.
<figure><img src="image_2.png" /><figcaption>Fig. 1. (a) XRD pattern and (b) XRF spectrum of amethyst.</figcaption></figure>

<figure><img src="image_3.png" /><figcaption>Fig. 2. Appearance and OM images at magnification 10X and 40X of PVDF-HFP/amethyst composite with various content of amethyst – (a) 0 phr, (b) 0.1 phr, (c) 0.3 phr and (d) 0.5 phr.</figcaption></figure>

Figure 2 presents optical microscopy (OM) images of the PVDF-HFP/amethyst composite films, highlighting their appearance, morphology, and dispersion of amethyst particles. The neat PVDF-HFP film exhibited exceptional clarity due to the absence of embedded amethyst, minimizing light scattering and enabling unimpeded light transmission (Figure 2a). Conversely, incorporating amethyst into the PVDF-HFP matrix renders the film opaque. This increased opacity could be attributed to the light-scattering properties of the crystalline amethyst particles. For the morphology and dispersion of additive in the PVDF-HFP matrix, at 10x magnification (Figure 2a), the neat PVDF-HFP film revealed a clear morphology with only minimal surface cracks. For the composites containing 0.1 phr and 0.3 phr amethyst (Figure 2b and 2c, respectively), homogeneous dispersion of the additives throughout the PVDF-HFP matrix was observed. Interestingly, at a higher loading of 0.5 phr (Figure 2d), the amethyst particles appeared to clump together into larger aggregates. This suggests a less uniform distribution compared to lower loadings. Figure 2e illustrates a possible model for this observed amethyst aggregation.
<figure><img src="image_4.png" /><figcaption>Fig. 3. Density of PVDF-HFP/amethyst composite films (varying amethyst content).</figcaption></figure>

Fig. 3a presents the density of PVDF-HFP/amethyst composites with varying amethyst content (0, 0.1, 0.3, and 0.5 phr). The density increased from 1.53 g/cm³ to 1.61 g/cm³ as the amethyst content increased from 0 phr to 0.3 phr. However, a decrease in density to 1.51 g/cm³ was observed at 0.5 phr amethyst content. This trend can be attributed to the interplay between the densities of the matrix (PVDF-HFP, 1.53 g/cm³) and the additive (amethyst, 2.57 g/cm³). When the amethyst content is low (up to 0.3 phr), its higher density contributes to an overall increase in the density of composite. However, at higher loadings (0.5 phr), agglomeration of the amethyst particles might be occurring. This agglomeration creates free volume within the composite, resulting in a decrease in density. This observation is potentially supported by the findings from the OM images (Fig. 1). This observation aligns with the findings of Werken et al. [15], reported that exceeding the well-dispersed content of carbon reinforcement in a polymer matrix, leads to agglomeration. These agglomerates create free volume between the matrix and the poorly dispersed additive, ultimately decreasing the composite density.

Fig. 4 presents the results of mechanical property testing conducted using a Universal Testing Machine in accordance with ASTM D638 standards. Fig. 4(a) shows the stress-strain curves for PVDF-HFP/amethyst composites film with 0, 0.1, 0.3, and 0.5 phr amethyst content. The results clearly demonstrated the influence of amethyst on mechanical behavior. Neat PVDF-HFP exhibited a tensile strength of 5.57 N/mm², which increased to 7.53 N/mm² with the addition of 0.1 phr amethyst, followed by further increments to 12.73 N/mm² and 13.04 N/mm² for 0.3 phr and 0.5 phr amethyst, respectively. These findings highlighted the reinforcing effect of amethyst. Fig. 4(b) presents the strain-at-break results for the composites. The strain values decreased from 15.74% to 11.30%, 9.93%, and 4.33% for 0, 0.1, 0.3, and 0.5 phr amethyst, respectively. This decrease in strain could be attributed to the disruption of crystallinity of PVDF-HFP by amethyst, leading to a less ordered crystal structure and lower strain accommodation [16,17]. However, the unusually high strain value of 15.74% for 0.5 phr amethyst suggested the formation of free volume due to amethyst agglomeration during stretching, as observed in Fig. 2. Initially, the reduced crystallinity and disordered chain arrangement may contribute to the low strain. However, it is hypothesized that the high stress applied during tensile testing induces chain reorientation in the PVDF-HFP matrix, leading to a more ordered structure and continued increase in stress with reducing strain. Figure 4c shows the Young's modulus values for the composites, which increase from 156.00 N/mm² to 164.41 N/mm², 234.01 N/mm², and 258.21 N/mm² for 0, 0.1, 0.3, and 0.5 phr amethyst, respectively. This enhancement in modulus is attributed to the inherent crystallinity and stiffness of amethyst, which reinforces the PVDF-HFP matrix and enhances its ability to resist deformation under applied stress. Our findings are consistent with the observations reported by Sengupta et al. [16].
<figure><img src="image_5.png" /><figcaption>Fig. 4. (a) stress at peak, (b) strain at peak and (c) Young's modulus of PVDF-HFP/amethyst composite films with various amethyst content.</figcaption></figure>

**Conclusion**

This study successfully fabricated PVDF-HFP/amethyst composite films via solution processing and casting. The amethyst content was varied within a range of 0–0.5 phr. Optical microscopy (OM) revealed good dispersion of amethyst at low loadings, with a tendency towards agglomeration at higher contents. The mechanical properties, including tensile strength (increased from 5.57 to 13.04 N/mm²), strain at break (decreased from 15.74 to 4.33 %), and Young's modulus (increased from 156.00 to 258.21 N/mm²), all exhibited enhancement with increasing amethyst content. The combined properties of PVDF-HFP and amethyst suggest their potential application in energy generation. To explore this hypothesis further, future work will focus on the characterization of piezoelectric properties, energy storage capabilities, and other relevant aspects to establish their viability as novel green energy materials.

**Acknowledgement**

This research was funded by College of Industrial Technology, King Mongkut's University of Technology North Bangkok (Grant No. Res-CIT702/2024). Moreover, we would like to thank Teanchai Chantakit for supporting us with the amethyst ore and the powder fabrication development.
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