Degradation of PVDF in photocatalytic membranes in gaseous environments.

Abstract

Highlights

PVDF was degraded when combined with TiO₂ upon high power UV exposure.
CF₂O was formed as reaction product, CO₂ and CF₄ as end products.
Degradation was accelerated using extreme UV conditions compared to sunlight.

Poly(vinylidene fluoride) (PVDF) and poly(vinylidene fluoride-co-hexa-fluoropropylene) (PVDF-HFP) are fluoropolymers used for applications including, but not limited to, immobilization of photocatalytic materials for water and air purification due to its high resistance to ultraviolet (UV) exposure and extreme environmental conditions. The stability of these fluoropolymers in combination with titanium dioxide (TiO) nanopowder was investigated. Membranes were prepared using the phase inversion method. Fourier transform infrared spectroscopy (FT-IR) and X-ray photoelectron spectroscopy (XPS) analysis after 21 h of intense UV exposure showed a degradation of both PVDF and PVDF-HFP. A decrease in PVDF(-HFP) absorbance relative to TiO $_{2}$ absorbance in the FT-IR spectra, a decrease of the F (1s) peak and a significant increase of the Ti (2p) peaks in the XPS spectra were observed. It was demonstrated that this degradation results from reaction with TiO $_{2}$ upon UV exposure. Gas analysis using a quadrupole mass spectrometer (QMS) strongly indicated the gradual formation and subsequent conversion of (highly toxic) carbonic difluoride CF $_{2}$ O, reaching a maximal concentration after 2.5 h and a full removal after 10 h of UV exposure. A strong indication of the production of carbon dioxide (CO $_{2}$ ) and tetrafluoromethane (CF $_{4}$ ) as end products was obtained.

Graphical abstract

Introduction

Over the last decades, PVDF polymers have gained interest for numerous applications. Because of its piezoelectric and pyroelectric properties, PVDF can be used for sensors, transducers and energy harvesting devices [1], [2], [3]. PVDF is also known for its high chemical [4], thermal [5] and UV resistance [6]. It is applied in battery [7], [8] and filtration technology [9]. Additionally, PVDF can be manufactured as a flexible material, making it useful for wire insulation [4]. Its low refractive index makes it even useful for fishing lines, reducing the visibility in water [10]. Often, PVDF is combined with HFP to create a copolymer PVDF-HFP. This enhances the mechanical strength of the material [9], [11].

PVDF(-HFP) belongs to the large group of per- and polyfluoroalkyl substances (PFAS). These are characterized by their strong C-F bonds, making these compounds very persistent, resulting in potential health risk for humans and the environment [12], [13]. When using this polymer in applications where its merits outweigh its disadvantages, it is important to make sure that the material remains intact during use and that it can be recovered or recycled after use.

Over the last years, there has been an increased interest in PVDF(-HFP) for photocatalytic applications. Porous membranes consisting of this polymer and a photocatalyst can be synthesized for water [14], [15] or air purification [16], [17]. Here, PVDF(-HFP) mainly acts as an immobilizer for the photocatalyst, and it is crucial that this polymer remains stable without any degradation to avoid release of photocatalytic material, this fluoropolymer or degradation products into the environment.

When combining this polymer with a photocatalyst, it is important to understand the photocatalytic mechanism and its impact on the polymer. Previously, some research regarding the stability of PVDF(-HFP) during photocatalytic activity was performed. A change in structure and tensile strength of TiO/PVDF dual layer hollow fibre membranes was observed after long-term exposure to UV light in aqueous conditions [18]. The formation of cracks [19] and even degradation of the structure to powder was observed for similar membranes upon UV exposure in gaseous conditions [20].

In the present research, the stability of PVDF(-HFP) combined with a photocatalyst (TiO $_{2}$ ) upon long exposure to high-intensity UV light in a gaseous atmosphere is investigated. Several techniques are used to analyse possible degradation of PVDF(-HFP) in different experimental conditions. Degradation products are identified after gas analysis using a QMS. The goal of this research is to study the degradation behaviour of this fluorine-containing polymer when combined with TiO $_{2}$ and to understand the formation of possible reaction products. To the best of our knowledge, this is the first systematic study of the instability of PVDF(-HFP) TiO membranes under UV light exposure in a gaseous environment, also including the identification of reaction products, and offering new insights into their long-term photocatalytic durability.

Section snippets

Materials

Both poly(vinylidene fluoride-co-hexa-fluoropropylene) (PVDF-HFP (94% PVDF, 6% HFP), average M = 400,000 g/mol, Sigma-Aldrich, St. Louis, MO, USA) and poly(vinylidene fluoride) (PVDF, average M $_{W}$ = 534,000 g/mol, Sigma-Aldrich) were used as polymer matrix. Titanium dioxide (TiO $_{2}$ P25, $\geq$ 99.50%, Evonik Industries, Essen, Germany) nanopowder was used as photocatalyst. It consists of 85% anatase and 15% rutile, and has a specific surface area in the range 35–65 m/g. The average crystal size was

Results and discussion

It should be noted that the first indication of degradation of PVDF(-HFP) in combination with TiO was found by membranes becoming brittle after long-term UV exposure and starting to fall apart to powder, when handling these samples.

Conclusion

In this research, the stability of photocatalyst-containing PVDF(-HFP) membranes upon UV exposure was investigated. Since PVDF(-HFP) is often used as an immobilizer for photocatalysts and this fluorine-containing polymer is a PFAS compound, possible degradation reactions need to be understood.

Several UV exposure experiments (365 nm, 280 mW/cm) were performed for different types of membranes (PVDF(-HFP) without or with photocatalyst) in different gaseous conditions (e.g., Ar/O $_{2}$ and N $_{2}$ /O). The

Abbreviations

The following abbreviations are used in this manuscript:

ATR	Attenuated total reflectance
DI	Deionized
FT-IR	Fourier transform infrared spectroscopy
LED	Light emitting diode
NMP	N-methyl-2-pyrrolidone
PFAS	Polyfluoroalkyl substances
PVDF	Poly(vinylidene fluoride)
PVDF-HFP	Poly(vinylidene fluoride-co-hexa-fluoropropylene)
QMS	Quadrupole mass spectrometer
RhS	Rhodiasolv® PolarClean
TEP	Triethyl phosphate
UV	Ultraviolet
XPS	X-ray photoelectron spectroscopy

CRediT authorship contribution statement

Ewoud Cosaert: Writing – review & editing, Writing – original draft, Visualization, Validation, Software, Methodology, Investigation, Formal analysis, Data curation. Hadis Mortazavi Milani: Writing – review & editing, Visualization, Validation, Methodology. Geraldine J. Heynderickx: Writing – review & editing, Supervision, Project administration, Funding acquisition, Conceptualization. Dirk Poelman: Writing – review & editing, Validation, Supervision, Resources, Project administration, Funding

Environmental implication

This research focuses on polyvinylidene fluoride (PVDF), a member of the per- and polyfluoroalkyl substances (PFAS) group. Due to their persistence, PFAS pose long-term environmental risks and were brought to the attention by health organizations for the last couple of years. The study examines the photocatalytic degradation of PVDF and identifies potential reaction and end-products. As PVDF is frequently used with photocatalysts in water and air purification systems, understanding its

Funding

This research was funded by Ghent University , grant number 01G00319.

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.

Acknowledgements

Henk Vrielinck is acknowledged for the contribution to the FT-IR experiments. Matthias Minjauw and Aditya Chalishazar are acknowledged for performing XPS measurements. Magalie Delbeke is thanked for the assistance with the graphical abstract.

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Degradation of PVDF in photocatalytic membranes in gaseous environments.

Abstract