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ELSEVIER European Polymer Journal 182 (2023) 111724 Contents lists available at ScienceDirect European Polymer Journal journal homepage: www.elsevier.com/locate/europolj EUROPEAN POLYMER JOURNAL eriosamiteA Efficient fluoride recovery from poly(vinylidene fluoride), poly(vinylidene fluoride-co-hexafluoropropylene) copolymer and poly (ethylene-co-tetrafluoroethylene) copolymer using superheated water with alkaline reagent Jin Hamaura a, Ryo Honma a, Hisao Hori Abdelatif Manseri b, Bruno Ameduri b a Department of Chemistry, Faculty of Science, Kanagawa University, 2946 Tsuchiya, Hiratsuka 259-1293, Japan b Institut Charles Gerhardt Univ Montpellier, CNRS, ENSCM, 34293, Montpellier, France ARTICLE INFO Keywords: Decomposition Fluoropolymer Mineralization Recovery Superheated water ABSTRACT Mineralization of poly(vinylidene fluoride) (PVDF), poly(vinylidene fluoride-co-hexafluoropropylene) [poly (VDF-co-HFP)] copolymer, and poly(ethylene-co-tetrafluoroethylene) copolymer (ETFE) in superheated water in the presence of an alkaline reagent was investigated with the aim of developing a technique to recycle the fluorine element. These polymers underwent almost complete defluorination to form fluoride ions (F-) in the reaction solution at a relatively low temperature (250 C) under Ar atmosphere. When PVDF was reacted with aqueous KOH (1.0 M) for 6 h, the amount of which corresponds to 10 times the molar amount of fluorine content (as atoms) in PVDF, the yield of F- released into the reaction solution reached 95 %. This transformation was accompanied by the formation of carbon rich reside, which consists of amorphous carbon. In contrast, for a treatment performed with O2 instead of Ar, significant differences were observed in the products: while r- ions efficiently produced in the reaction solution, carbon rich residue did not form and the major species that composed the total organic carbon content in the reaction solution was oxalate. Four consecutive runs (that is, after one reaction at 250 C under Ar was complete, new PVDF was charged to the reaction mixture and then reacted again) caused no decrease of the F- yield. When treated with aqueous KOH (1.0 M) under Ar at 250 C for 6 h, poly(VDF-co-HFP) and ETFE copolymers also completely mineralized to form F- ions with 100 and 98 % yields, respectively. Introducing Ca(OH)2 to the resulting reaction solutions of these (co)polymers after superheated water treatment produced pure CaF2, i.e., artificial fluorspar. 1. Introduction Fluoropolymers have been used in many industrial equipment because of their high stability against temperature, chemicals, ignition, UV-light irradiation, and other unique characteristics [1-6]. Among fluoropolymers, partially fluorinated polymers such as poly(vinylidene fluoride) (PVDF, --(CF2CH2)n--) and vinylidene fluoride (VDF) copolymers have combined characteristics of high stability of fluorochemicals with the melt processability of thermoplastic polyolefins. In contrast, poly(tetrafluoroethylene) (PTFE, --(CF2CF2)n--), the typical perfluorinated polymer, cannot be processed by melt molding [1,6]. Therefore, PVDF has been used for chemical- and electrical devices and energy-related applications such as lithium ion battery electrode binders and separators [3,5,7]. According to this trend, the production of PVDF reached the largest volume of fluoropolymer after PTFE [7]. Poly(ethylene-co-tetrafluoroethylene) copolymer (ETFE, --(CH2CH2CF2CF2)n--) is also a commercially available, melt-processable fluoropolymer used in harsh conditions as piping, wire insulation, protective films, fuel tubing, and other purposes [1,3]. While usages of the fluoropolymers have widely spread, waste treatment techniques do not catch up with their increasing demand. Incineration is an option to treat these polymer wastes [8]. However, hydrogen fluoride (HF) gas generated during the treatment seriously damages refractory bricks of incinerators. Therefore, most of the fluoropolymer wastes are disposed of in landfill, although some unfilled PTFE wastes from manufactures are reused in ram extrusion applications after cleaning and milling or they are degraded into low molar massPTFE by heating, and reused as micro powder, which was mixed with other materials [4]. * Corresponding author. E-mail address: M@kanagawa-u.ac.jp (H. Hori). https://doi.org/10.1016/j.eurpolymj.2022.111724 Received 11 August 2022; Received in revised form 11 November 2022; Accepted 18 November 2022 Available online 24 November 2022 0014-3057/ 2022 The Author(s). Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/bync-nd/4.0/). J. Hamaura et al. If fluorine atoms in fluoropolymer wastes are transformed into fluoride ions (F-) by means of environmentally benign methodologies, the F- ions can react with calcium hydroxide [Ca(OH)2] to form calcium fluoride (CaF2), which mineral is fluorspar. Nowadays, the mine production of high-purity fluorspar, suitable for hydrofluoric acid (i.e., aqueous HF) production, is limited in a few countries [9]. Because hydrofluoric acid is a raw material for all fluorochemicals (C-F bond is built by halogen exchange using HF) [10], the development of an efficient decomposition technique of fluoropolymers to F- ions would contribute toward closing the loop of fluorine element [11]. Superheated water (it is also mentioned as `subcritical water' or `pressurized hot water') is liquid water under high pressure at temperatures ranging from 100 C to 374 C, i.e., critical temperature. This water is recognized as environmentally benign in waste management because it enables to transform hazardous chemicals into harmless or to generate value-added compounds [12-17]. As a matter of fact, in the recycling of non-metallic component from electronic waste (E-waste), the superheated water reaction is analyzed to have a smaller environmental impact than that of pyrolysis [18]. Of course, lowering the reaction temperature is desirable in view of saving energy. We previously reported that PVDF [19] and VDF-based copolymers [19,20] efficiently mineralized in superheated water at a relatively low temperature (250 C) by use of potassium permanganate as an oxidizing agent, and this methodology was extended to treat ETFE [21]. In these systems, the fluorine content of the polymer was decomposed into F- ions in the reaction solution, and the carbon content was decomposed to CO2 in the gas phase and HCO3- in the reaction solution. For industrial processes, a technique that can achieve under simpler conditions is preferable. Regarding carbon content, conversion to carbon rich solid (e. g., amorphous carbon) instead of CO2 would be also an option. Herein, we report an effective method for complete defluorination of PVDF, poly(vinylidene fluoride-co-hexafluoropropylene) copolymer, poly(VDF-co-HFP), -[(CH2CF2)m(CF2CFCF3)]p-, and ETFE to F- ions, by means of superheated water in the presence of an alkali reagent (KOH or NaOH) at relatively low concentration (up to 1.0 M). Effect of coexisting gas on the reactivity of the polymers and the formation of `artificial fluorspar' upon addition of Ca(OH)2 to the resulting reaction solution are also described. So far, several studies on the alkaline-reagent induced degradation of PVDF [22-31] and a VDF-based copolymer [32] were reported. However, these previous studies aimed at the surface modification of the polymer, for example, to increase the adhesion of polymer surface [23]. The level of degradation that changes the bonding nature of PVDF surface is substantially lower than that required for waste treatment, because the latter requires whole degradation of the PVDF bulk. The present study is the first report on the decomposition of PVDF and related copolymers in superheated water combined with an alkaline reagent. 2. Experimental section 2.1. Materials and reagents Ar (99.99 %), O2 (99.999 %), and a standard gas mixture CO2 (1.00 %)/N2 were purchased from Nissan Tanaka (Saitama, Japan). PVDF powder was from SynQuest Laboratories (Alachua, FL). Size exclusion chromatography (SEC) showed that the weight-average molecular weight (Mw) was 6.5 105 and the polydispersity (Mw/Mn) was 2.5, relative to polystyrene standard. Poly(VDF-co-HFP) copolymer was the same as that used in our previous report [33]: the VDF/HFP molar ratio was 95.3/4.7 and the Mw determined by SEC in dimethylformamide was 4.5 105, relative to poly(methyl methacrylate), with the Mw/Mn of 3.2. Combustion ion chromatography demonstrated that the fluorine European Polymer Journal 182 (2023) 111724 percentages in PVDF and poly(VDF-co-HFP) copolymer were 60.7 and 59.4 wt%, respectively. The F- yields of the reactions were calculated according to these analytical values of the fluorine contents in the polymers. Granulated ETFE was obtained from Millipore Sigma (St. Louis, MO, USA). ETFE is essentially an alternating copolymer [1]. Consistently, the fluorine percentage in the ETFE was determined to be 58.6 wt% by combustion ion chromatography, which value was only slightly lower than that for complete alternating copolymer (59.3 wt%). As well as PVDF and poly(VDF-co-HFP) copolymer, the F- yields of the reactions for ETFE were calculated based on this analytical value. 1,3,5Trifluorobenzene (C6H3F3, >98 %) and other reagents were supplied from Fujifilm Wako Pure Chemical Industries (Osaka, Japan). 2.2. Superheated water treatment An autoclave, in which the internal volume was 31 mL, was mainly used. Some reactions for ETFE were also performed in a bigger autoclave (96 mL) attached with an impeller that can stir the reaction mixture. Each autoclave was fitted with a gold vessel to prevent contamination from the autoclave material (stainless steel, Japanese Industrial Standards SUS 316). In a typical run using 31 mL-autoclave, to the gold vessel in the autoclave was added the polymer powder (30 mg), followed by an aqueous solution of KOH or NaOH (10 mL, 0.25-1.00 M). Then, the autoclave was pressurized up to 0.60 MPa with Ar and sealed. Next, the autoclave was placed in an electric furnace and heated to the desired reaction temperature with a rate of ca. 10 C min-1. After holding a specified time, the autoclave was quickly cooled to 25 C by an electric fan. The gas in the autoclave headspace was recovered into a sampling bag and subjected to gas chromatography/mass spectrometry (GC/MS). After sampling the gas, the cap of the autoclave was opened in air. The liquid-solid mixture in the vessel was transferred into a polypropylene tube, which was subjected to centrifugation. The collected liquid phase was characterized by ion chromatography, total organic carbon (TOC) measurement, and attenuated total reflection infrared (ATR-IR) spectrometry. The solid residue was dried under vacuum and subjected to Raman spectroscopy, combustion-ion chromatography, and carbon analysis. Control reactions using O2 instead of Ar were also performed. Consecutive runs were also carried out. That is, after a charge of PVDF had been reacted in superheated water, an additional PVDF amount was charged to the resulting reaction mixture, and then the mixture was reacted again in superheated water. Such procedures were repeated. 2.3. Synthesis of artificial fluorspar The formation of CaF2 was investigated from the reaction solution generated from the superheated water treatment by adding Ca(OH)2. After PVDF (30 mg) was treated with 1.0 M of [KOH] under Ar at 250 C for 6 h, the resulting reaction mixture was centrifugated. The collected liquid was diluted to 25 mL with pure (Milli-Q) water. To the solution was added Ca(OH)2, which molar amount was the same as that of the F- molar amount contained in the solution. Shaking this mixture generated a white powder. The powder was collected and washed with 1.0 M of [HCl], followed by pure water. The purified powder was dried under vacuum for overnight, and then subjected to X-ray diffractometry (XRD). 2.4. Instrumental analysis The fluorine weight percentages in the initial polymers and the solid residues formed from the reactions were determined by combustion ion chromatography at Nissan Arc (Yokosuka, Japan). The instrument consisted of a combustion unit (AQF-100, Nittoseiko Analytech, Yamato, 2 J. Hamaura et al. Japan; matrix combustion temperature, 1100 C) and an ion chromatograph (Dionex ICS-3000, Thermo Fisher Scientific, Waltham, MA). The F- concentrations in the reaction solutions were determined by an ion-chromatograph (IC-2001, Tosoh, Tokyo, Japan) attached with an analytical column (TSKgel Super IC-Anion, Tosoh). The mobile phase was an aqueous solution consisting of 6 mM sodium tetraborate, 15 mM boric acid, and 0.2 mM sodium hydrogen carbonate, and the flow rate was 0.8 mL min-1. Another ion-chromatograph with an analytical column (TSKgel Super IC-AP) was also used to quantify organic acid anions in the reaction solutions. The mobile phase was an aqueous solution consisting of 1.7 mM sodium hydrogen carbonate, 1.8 mM sodium carbonate, and acetonitrile (23 vol%). The total organic carbon (TOC) concentrations in the reaction solutions were determined by a TOC instrument (N/C 3100 BU, Analytik Jena, Jena, Germany). In this instrument, a halogen removal column was employed to avoid corrosion of the non-dispersive IR detector. Raman spectra of solid residues and PVDF before the treatment were measured at 532 nm excitation using a Raman imaging microscope (WITec 300, Oxford Instruments, Harpenden, UK). A FTIR spectrometer (Spectrum 100, Perkin Elmer, Waltham, MA, USA) attached with a diamond ATR cell was used for observing ATR-IR spectra of the reaction solutions. The sample droplets were placed into the ATR cell, evaporated to dryness with a stream of N2 gas, and the IR spectra were recorded. Carbon contents in the residues were determined by a carbon/sulfur analyzer (EMIA-Expert, Horiba, Kyoto, Japan) or a total carbon analyzer (N/C 3100 BU with HT 1300, Analytik Jena, Jena, Germany). The collected gas after the superheated water reactions was analyzed by means of a GC/MS instrument (QP2010 SE, Shimazu, Kyoto, Japan) attached with a fused-silica capillary column (Rt-Q-BOND, Restek, Bellefonte, PA). The carrier gas was helium, and the sample injection temperature was maintained at 120 C. The sample gas was injected into the GC/MS system in split mode (20/1 ratio) while the analyses were conducted in full-scan mode (m/z 2.0-200). The temperature program of the column oven was as follows: keeping at 30 C for 5 min, raising to 200 C at 20 C min-1 rate, and maintaining at that temperature for 20 min. XRD patterns of the artificial fluorspar were recorded by using an X-ray diffractometer with copper K radiation (MultiFlex; Rigaku, Tokyo, Japan). In the present study, the F- yield, the remaining TOC ratio, and the CO2 yield were determined by the following Eq. (1)-(3), respectively. F- yield = [(F- moles in the reaction solution) / (fluorine atom moles in the initial polymer)] (1) TOC ratio = [(TOC moles in the reaction solution) / (carbon atom moles in the initial polymer)] (2) CO2 yield = [(CO2 moles in the gas phase) / (carbon atom moles in the initial polymer)] (3) 3. Results and discussion 3.1. Decomposition of PVDF 3.1.1. Effect of OH- First, the effect of KOH concentration on the PVDF reactivity was examined. Fig. 1A, 1B, and 1C display KOH concentration dependences of F- amount in the reaction solution, TOC amount in the reaction solution, and CO2 amount in the gas phase, respectively, obtained from reactions of PVDF at 250 C for 6 h. When the treatment was performed without KOH, almost no reaction occurred. Traces of F- (1.4 mol; yield ~ 0 %) were noted in the resulting reaction solution (Table 1, entry 1). Likewise, the TOC amount (3.7 mol) in the reaction solution and the CO2 amount (2.1 mol) in the gas phase were negligible (both yields were ~ 0 %). These data indicate that PVDF is stable in pure superheated European Polymer Journal 182 (2023) 111724 Fig. 1. Initial KOH concentration dependences of (A) the F- amount in the reaction solution, (B) the TOC amount in the reaction solution, and (C) the CO2 amount in the gas phase. PVDF (30 mg; fluorine amount as atoms, 959 mol; carbon content as atoms, 937 mol) was reacted in the presence of KOH under Ar in superheated water at 250 C for 6 h. Error bars at 1.0 M of [KOH] were obtained from two replicate experiments under the same reaction conditions. water (i.e., under Ar) at 250 C. In contrast, an addition of KOH dramatically increased the PVDF reactivity in superheated water at this temperature. When 0.125 M of [KOH] was employed, the formed F- amount jumped to 794 mol, revealing 83 % yield (Fig. 1A). In addition, when the [KOH] was further increased to 1.0 M (charged amount, 10 mmol in 10 mL), the molar amount of which corresponds to a 10-fold molar excess (=10 mmol/959 mol) relative to the amount of fluorine atoms (959 mol) in the initial PVDF, the F- amount increased to 910 mol (Table 1, entry 2, the average value of two reactions), which corresponds to 95 % yield. Therefore, the fluorine content in the initial PVDF led to an almost complete mineralization. The KOH-induced F- formation was accompanied by increasing TOC amount that gradually increased with higher [KOH] up to 1.0 M (Fig. 1B). This observation indicates that nonfluorinated organic compounds generated with increasing [KOH], because the F- yield was considerably high even at low [KOH] such as 0.125 M (Fig. 1A, the F- yield was 83 %). When [KOH] was 1.0 M, the TOC amount reached 144 mol (Table 1, entry 2, the average value of two reactions), which corresponds to 15 % of the carbon molar amount in the initial PVDF. In the gas phase, very few CO2 amount was detected, and the amount reduced when the reactions were performed with KOH (Fig. 1C). At [KOH] of 1.0 M, the CO2 amount was only 0.5 mol (the yield was ~ 0 %, the average value of two reactions, Table 1, entry 2). When [KOH] was increased from 0 to 1.0 M, the pH of the resulting 3 J. Hamaura et al. Table 1 Products from superheated water treatment of PVDF and copolymers.a Entry Polymerb Alkaline reagent Coexisting gas T P (MPa) [Conc.(M)] ( C) 1 PVDF none Ar 250 4.4 2 PVDF KOH Ar [1.0] 3 PVDF NaOH Ar [1.0] 4 PVDF KOH Ar [1.0] 5 PVDF KOH O2 [1.0] 6 Poly(VDF-co-HFP) KOH Ar [1.0] 7 Poly(VDF-co-HFP) KOH O2 [1.0] 8 ETFE KOH Ar [1.0] 9 ETFE KOH Ar [1.0] 250 4.4 250 4.4 200 2.3 250 4.1 250 4.4 250 4.2 250 5.2 200 2.3 a Internal volume of the autoclave, 31 mL; initial solution volume, 10 mL; reaction time, 6 h. b Charged weight, 30 mg. c Obtained from two replicate experiments under the same reaction conditions. European Polymer Journal 182 (2023) 111724 F- (mol) [yield (%)] 1.4 [0] 910 4 c [95 1] 858 [89] 846 [88] 866 [90] 934 [100] 951 [101] 903 [98] 1.1 [0] TOC (mol) [ratio (%)] 3.7 [0] 144 25c [15 3] 225 [24] 32 [3] 149 [16] 188 [21] 200 [22] 243 [26] 29 [3] CO2 (mol) [yield (%)] 2.1 [0] 0.5 0.0 c [0] 0.4 [0] 0.2 [0] 0.4 [0] 0.2 [0] 0.3 [0] 0.1 [0] 0.1 [0] Fig. 2. Raman spectra of (A) the solid residue formed from a reaction of PVDF in superheated water with 1.0 M of [KOH] under Ar and (B) PVDF before the treatment. For (A), the treatment was performed at 250 C for 6 h. reaction solution increased from 3.9 to 13.6. In such a highly basic solution, even if CO2 was formed, most of the molecules were transformed into CO32- in the reaction solution. Instead, black solid residues were produced when the reactions were performed with KOH. The carbon the residue showed that the carbon and fluorine amounts were 744 and 5.4 mol, respectively, indicating that the molar amount of carbon was 138 times higher than that of the fluorine. This carbon amount corresponds to 79 % of the carbon content (937 mol) in the initial PVDF. On the other hand, the fluorine amount corresponds to only ~ 1 % of the fluorine content (959 mol) in the initial PVDF, which is consistent with the fact that F- ions were produced in the reaction solution with a high yield (95 %). The carbon recovery--[(total moles of carbon atoms in residue, TOC, and CO2)/(moles of the carbon atoms in initial PVDF)] was 95 % [= (744 + 144 + 0.5) / 937 100]. This result indicates that the carbon amount was well-accounted for by the residue, TOC in the reaction solution, and CO2 in the gas phase. To identify this carbon rich residue, Raman spectroscopy was employed. Fig. 2A and 2B display the spectra for the carbon-rich residue and PVDF before the treatment, respectively. In the spectrum of PVDF, intense peaks were noted at 2984, 1432, and 794 cm-1 (Fig. 2B). These signals are attributed to CH2 groups [27,28,34-36]. In addition, several peaks appeared around 1300-870 and 606 cm-1, which are assigned to CF2 groups [27,34-36]. The peaks ascribed to CH2 and CF2 groups were not observed in the spectrum of the carbon-rich residue (Fig. 2A). Instead, two intense bands were observed at 1596 and 1367 cm-1. This feature was very similar to that noted for amorphous carbon [37-39], showing two bands characteristic of stretching of C--C bonds. Furthermore, when PVDF was reacted in 1.0 M of [KOH], trace amount (0.02 mol) of C6H3F3 was detected in the gas phase (total-ion current chromatogram in the GC/MS measurement is displayed in Fig. S-1 in Sup- plementary material). According to these results, the PVDF degradation mechanism caused by OH- can be explained as follows (Scheme 1). First, OH- favors the deprotonation of methylene moiety flanked between two electron-withdrawing CF2 groups [Eq. (4)]: -CH2CF2-CH2CF2-CH2CF2- + OH- -CH2CF2-CH(-)CF2-CH2CF2- + H2O (4) percentage of the residue obtained from reactions with 1.0 M [KOH] was 44 wt% (the average value of two reactions). In contrast, the fluorine percentage of the residue was 0.51 wt%. These values and the weight of 4 J. Hamaura et al. European Polymer Journal 182 (2023) 111724 Scheme 1. Proposed mechanism for PVDF decomposition induced by OH-. The deprotonated anion is unstable to form a C--C bond in the polymer chain, accompanied by releasing F- into the reaction solution [Eq. (5)]: -CH2CF2-CH(-)CF2-CH2CF2- -CH2CF2-CH=CF-CH2CF2- + F- (5) The initial step is also described by Ross et al. [26], who monitored PVDF surface degradation by IR spectroscopy, using 12 M NaOH solution at 80 C, although the IR spectra in this report revealed that most of CH2 and CF2 units in bulk PVDF remained after such a treatment for 254 h. A further deprotonation in the polymer chain results in an additional C--C bond and F- release [Eq. (6, 7)]. 150 C led to very little PVDF decomposition, producing 22.8 mol of F- (2 % yield); 14.2 mol of TOC amount (2 % of the carbon content in the initial PVDF); and 0.25 mol of CO2 (~0% yield). Elevating temperature to 200 C dramatically enhanced the F- amount (846 mol), which corresponds to 88 % yield (Table 1, entry 4). Furthermore, at 300 C, the F- amount reached 958 mol (100 % yield) while the TOC amount also increased to 159 mol (17 % of the carbon content of initial PVDF). The CO2 amount in the gas phase also slightly increased to 1.2 mol, although the yield was negligible (~0%). As described above, the F- amount jumped at 200 C, and kept steady at higher temperature (Fig. 3A). In contrast, the TOC amount remained low up to 200 C, then -CH2CF2-CH=CF-CH2CF2- + OH- -CH2CF2-CH=CF-CH(-)CF2- + H2O (6) -CH2CF2-CH=CF-CH(-)CF2- -CH2CF2-CH=CF-CH=CF- + F- (7) The sequence of these steps results in the formation of a carbon rich residue. The generation of C6H3F3 trace in the gas phase can be explained by chain scission during the defluorination processes. When the superheated water reactions were performed with KOH, the TOC amount in the reaction solution increased with increasing [KOH] (Fig. 1B). This phenomenon is also consistent with the occurrence of chain scission during the defluorination processes. As expected, not only KOH, but also NaOH induced the defluorination of PVDF to F-. When PVDF was reacted with 1.0 M of [NaOH] at 250 C for 6 h, the F- amount was 858 mol, or 89 % yield (Table 1, entry 3). Next, the effect of the reaction temperature on the PVDF decomposition was examined by using 1.0 M of [KOH] at a constant reaction time of 6 h. Fig. 3A, 3B, and 3C display the temperature dependences of the amounts of F-, TOC, and CO2, respectively. The reaction performed at increased at 250 C and kept the same at higher temperature (Fig. 3B). This difference suggests that the defluorination proceeded at first [Eq. (4-7)], followed by chain scission in the remaining polymer. Effect of the reaction time was also examined for reactions in 1.0 M of [KOH] at 250 C (Fig. 4). Almost complete mineralization of the fluorine content was observed even at a short reaction time of 2 h (Fig. 4A): the F- amount reached 903 mol or 94 % yield. The F- amount remained almost constant after 2 h (e.g., at 18 h, it was 933 mol, which corresponds to 97 % yield) whereas the TOC amount was 146 mol (or 16 % of the carbon content in the initial PVDF). The CO2 amount in the gas phase was almost constant during the reaction time (Fig. 4C). After 18 h, it was 0.4 mol (yield ~ 0 %). 3.1.2. Combination of OH- With O2 The reactions described above were performed under Ar. Changing the coexisting gas from Ar to O2 induced significant differences on the PVDF reactivity, especially on the fate of the PVDF carbon content. Fig. 5 5 J. Hamaura et al. European Polymer Journal 182 (2023) 111724 Fig. 3. Effect of temperature on (A) the F- amount in the reaction solution, (B) the TOC amount in the reaction solution, and (C) the CO2 amount in the gas phase. PVDF (30 mg) was reacted in the presence of 1.0 M of [KOH] in superheated water under Ar for 6 h. Error bars at 250 C were obtained from two replicate experiments under the same reaction conditions. displays the molar amounts of main products from the 6 h-reactions in superheated water under O2 using 1.0 M of [KOH], and under these conditions, the reaction temperatures varied from 150 to 300 C. The elevation of temperature from 150 to 200 C enabled to dramatic increase of the F- amount. For a reaction performed at 250 C, this amount was 866 mol, or 90 % yield (Table 1, entry 5), which was similar to that using Ar (95 %, Table 1, entry 2). The TOC ratio at this temperature (16 %, Table 1, entry 5) was also the same as that obtained from the reaction using Ar (15 %, Table 1, entry 2). However, the TOC amount showed a unique temperature dependence (Fig. 5B), different from that observed for the reactions under Ar (Fig. 3B). In the presence of O2, the TOC amount increased with increasing temperature to 200 C, then turned to decrease at higher temperature. Furthermore, the solid residue, observed for the reactions under Ar, almost disappeared when PVDF reacted under O2 above 200 C, at which the TOC amount was 186 mol (i.e., 20 % of the carbon content in the initial PVDF). Finally, at 300 C, the amount decreased to 132 mol, or 14 % of the carbon content in the initial PVDF. This result suggests that the carbon rich moiety generated after releasing F- [Eq. (5-7)] can decompose in the presence of O2. Fig. 5C shows the temperature dependence of the CO2 amount by this treatment that resulted in trace (0.4-0.6 mol) while the temperature varied from 150 to 300 C. To identify the organic component that generates TOC amount, the reaction solution was monitored by ATR-IR spectroscopy. When the reaction was performed at 250 C with O2 for 6 h, which gave 90 % F- Fig. 4. Effect of reaction time on (A) the F- amount in the reaction solution, (B) the TOC amount in the reaction solution, and (C) the CO2 amount in the gas phase. PVDF (30 mg) was reacted in the presence of 1.0 M of [KOH] under Ar in superheated water at 250 C. Error bars at 6 h were obtained from were obtained from two replicate experiments under the same reaction conditions. yield and 16 % of the remaining TOC ratio (Table 1, entry 5), the spectrum of the resulting reaction solution after drying displayed two intense absorptions at 1646 and 1374 cm-1, accompanied by a broad adsorption around 3200-3300 cm-1 (Fig. S-2 in Supplementary material). Indeed, this pattern was similar to that of potassium oxalate [40]. Therefore, this reaction solution was subjected to ion chromatography, which conditions were adjusted for the detection of organic acid anions. Consistently, the measurement detected 41.0 mol of oxalate anion [(COO-)2], indicating that the carbon atom amount was 82.0 mol (=41.0 2), which accounts for 55 % of the TOC amount (149 mol) in the reaction solution. Therefore, the majority of the TOC amount was accounted for by oxalate anion. 3.1.3. Consecutive runs Because PVDF (30 mg) was almost completely defluorinated to form F- by superheated water treatment with 1.0 M of [KOH] at 250 C for 6 h, we examined consecutive runs. That is, after one reaction using PVDF with 1.0 M of [KOH] under Ar at 250 C for 6 h was complete, new PVDF (30 mg) was charged to the resulting reaction mixture, and then the second reaction was performed under Ar at 250 C for 6 h. Four consecutive runs were performed, where the yield of F- ions accumulated from the first to nth run (n 4) was defined by Eq. (8). F- yield until nth run = [(F- moles detected after nth run) /(moles of F atoms in PVDF charged from 1st to nth run)] (8) 6 J. Hamaura et al. European Polymer Journal 182 (2023) 111724 Fig. 5. Temperature dependence of PVDF decomposition in the presence of O2 ([KOH] = 1.0 M): (A) the F- amount in the reaction solution, (B) the TOC amount in the reaction solution, and (C) the CO2 amount in the gas phase. The reaction conditions were the same as those described in the caption of Fig. 3, except that O2 was used instead of Ar. Error bars at 200 C were obtained from two replicate experiments under the same reaction conditions. Fig. 6. F- amount and F- yield after consecutive runs for PVDF decomposition. PVDF (30 mg) was reacted with 1.0 M of [KOH] in superheated water at 250 C under Ar for 6 h. After the reaction, the autoclave was cooled to room temperature, and new PVDF (30 mg) was charged to the resulting reaction mixture, purged with Ar, then the mixture was further heated at 250 C for 6 h. Fig. 7. Effect of temperature on (A) the F- amount in the reaction solution, (B) the TOC amount in the reaction solution, and (C) the CO2 amount in the gas phase. Poly(VDF-co-HFP) copolymer (30 mg, fluorine amount as atoms; 938 mol; carbon content as atoms, 902 mol) was reacted in the presence of 1.0 M of [KOH] in superheated water under Ar for 6 h. The F- amounts and the yields are displayed in Fig. 6. The former increased linearly with the number of consecutive runs, whereas the latter ones were similar (93-98 %): no decrease was observed, indicating that at least four consecutive runs were possible under the present reaction conditions. 3.2. Decomposition of poly(VDF-co-HFP) copolymer Because PVDF was efficiently mineralized in superheated water in the presence of an alkaline reagent, this methodology was applied to decompose poly(VDF-co-HFP) copolymer. The microstructure of this copolymer contains VDF-HFP-VDF units. In such a case, F- is expected to be generated not only from the VDF units, but also from the HFP ones [Eq. (9), (10)] [41]. -CH2CF2-CF2C(CF3)F-CH2CF2- + OH- -CH2CF2-CF2C(CF3) F-CH(-)CF2- + H2O (9) -CH2CF2-CF2C(CF3)F-CH(-)CF2- -CH2CF2-CF2C(CF3)=CHCF2- + F- 7 (10) J. Hamaura et al. European Polymer Journal 182 (2023) 111724 Fig. 8. Temperature dependence of poly(VDF-co-HFP) copolymer decomposition in the presence of O2: (A) the F- amount in the reaction solution, (B) the TOC amount in the reaction solution, and (C) the CO2 amount in the gas phase. The reaction conditions were the same as those described in the caption of Fig. 7, except that O2 was used instead of Ar. Furthermore, if the HFP-VDF-HPF units are present, they are likely to release more F- [41]. Fig. 7 displays the effect of temperature on the amounts of F-, TOC, and CO2, generated from reactions with 1.0 M of [KOH] under Ar for 6 h. When the reaction was performed at 150 C, little reaction proceeded, giving 43.1 mol of F- (i.e., 5 % yield); 26.6 mol of TOC amount (3 % of the carbon content in the initial copolymer), and 0.11 mol of CO2 (~0% yield). Elevating reaction temperature enhanced the decomposition of this copolymer. The F- formation greatly increased above 200 C (Fig. 7A). When the reaction was performed at 250 C, the F- amount reached 934 mol, which corresponds to 100 % yield (Table 1, entry 6). That is, the fluorine content in initial poly(VDF-co-HFP) copolymer was completely mineralized. The TOC amount also increased with increasing temperature (Fig. 7B). At 250 C, it reached 188 mol, which corresponds to 21 % of the carbon content in initial poly(VDF-co-HFP) (Table 1, entry 6). Small amount of CO2 (0.1-0.2 mol) was also detected at each temperature (Fig. 7C), although the yield was ~ 0 %. Fig. 8 displays results of (A) F-, (B) TOC, and (C) CO2 formations from this copolymer, where the reactions were performed in the presence of O2 for 6 h. The temperature dependence of the amount of each product was almost identical with that performed under Ar. The reaction at 250 C generated 951 mol of F- (101 % yield), 200 mol of TOC (22 % of the remaining ratio), and 0.3 mol of CO2 (0 % yield) (Table 1, entry 7). Fig. 9. ETFE reactions using KOH or NaOH in a bigger autoclave (96 mL): Effect of temperature on (A) the F- amount in the reaction solution, (B) the TOC amount in the reaction solution, and (C) the CO2 amount in the gas phase. ETFE (30 mg) was reacted in the presence of 1.0 M of [KOH] or [NaOH] in superheated water under Ar for 6 h. Error bars for the reaction using NaOH at 260 C were obtained from two replicate experiments under the same reaction conditions. 3.3. Decomposition of ETFE First, decomposition of ETFE was examined by use of a 31 mLautoclave, which was used for reactions of PVDF and poly(VDF-co-HFP) copolymer as described above. For a reaction performed with 1.0 M of [KOH] at 250 C for 6 h, the F- amount was 903 mol, or 98 % yield while the TOC amount was 243 mol, corresponding to 26 % of the carbon content in the initial ETFE (Table 1, entry 8). This means that the fluorine content in initial ETFE was completely mineralized, similar to PVDF and poly(VDF-co-HFP) copolymer. Significant differences in the reactivity between ETFE and PVDF were observed at lower temperature. For a reaction performed at 200 C, very small F- amount (1.1 mol, 0 % yield) was detected from ETFE (Table 1, entry 9). In contrast, PVDF produced F- with a much higher yield (88 %) under the same conditions (Table 1, entry 4). This difference suggests that OH- can abstract H+ from the methylene moiety in PVDF easier than that from ETFE. In PVDF, each -CH2- unit from normal head to tail addition is bounded to two -CF2- units, which are strongly electron withdrawing. The alternation of -CH2- and -CF2- units may facilitate the abstraction of H+ and subsequent C--C bond formation and F- release [Eq. (4-7)]. In contrast, ETFE consists of alternation of -CH2CH2- and -CF2CF2- units; each -CH2- unit is adjacent to a -CF2- unit and a -CH2- unit. This microstructure may suppress the H+ abstraction. That is, the difference in the 8 J. Hamaura et al. European Polymer Journal 182 (2023) 111724 Fig. 10. Temperature dependence of ETFE decomposition using NaOH in the presence of O2: (A) the F- amount in the reaction solution, (B) the TOC amount in the reaction solution, and (C) the CO2 amount in the gas phase. The reaction conditions were the same as those described in the caption of Fig. 9 (NaOH), except that O2 was used instead of Ar. Error bars at 260 C were obtained from two replicate experiments under the same reaction conditions. environment around the -CH2- group results in different behaviors of both polymers at such a temperature (200 C). Next, temperature dependence of the reactivity of ETFE was investigated by use of a bigger autoclave (96 mL) with 1.0 M of [KOH] or [NaOH]. Both these alkaline reagents caused no significant difference in the amounts of F-, TOC, and CO2 (Fig. 9A, 9B, and 9C, respectively). While the F- and TOC amounts were low below 230 C, they increased at higher temperatures. For a reaction performed with 1.0 M of [KOH] at 260 C for 6 h, the F- amount reached 874 mol or 94 % yield (entry 1, Table S-1 in Supplementary material). Likewise, when the reaction was performed with NaOH under the same conditions, the F- amount reached 925 mol, or 100 % yield (as average value of two reactions, entry 2, Table S-1 in Supplementary material). Reactions under O2 atmosphere were also performed (Fig. 10). At 260 C, the F- amount reached 893 mol or 97 % yield (as average value from two reactions, entry 3, Table S-1 in Supporting Information). It should be noted that when the reaction was performed at 320 C in the presence of O2, the TOC amount decreased to 64 mol (Fig. 10B), that is, 7 % of the carbon content in the initial ETFE. In contrast, a reaction carried out under Ar at 320 C did not decrease the TOC value (321 mol or 34 % of the carbon content in the initial ETFE) from the value at 260 C (see Fig. 9, NaOH plots). This result suggests that non-fluorinated organic component in the water was oxidatively mineralized by O2. Fig. 11. XRD patterns of the artificial fluorspar obtained from (A) PVDF, (B) poly(VDF-co-HFP) copolymer, and (C) ETFE. Ca(OH)2 was added to the reaction solution from superheated water treatment, and the formed precipitate was washed with aqueous HCl and pure water, and dried. 3.4. Synthesis of artificial fluorspar Finally, we examined the transformation of F- ions obtained from superheated water treatment into CaF2 to close the loop on the fluorine element. After PVDF was treated with 1.0 M of [KOH] under Ar at 250 C for 6 h, the F- ions in the reaction solution were reacted with Ca(OH)2. The powder formed was washed with aqueous HCl, followed by pure water. The XRD pattern of the dried powder showed peaks only assigned to CaF2 (Fig. 11A). The weight of the collected pure CaF2 indicated that 77 % of the fluorine atoms in the initial PVDF were recovered into CaF2. The same treatment was applied to the solutions from the reaction of poly(VDF-co-HFP) copolymer with 1.0 M of [KOH] at 250 C for 6 h and from that involving ETFE with 1.0 M of [NaOH] at 260 C for 6 h under Ar. Both reaction solutions generated pure CaF2 (Fig. 11B and 11C) with 84 % and 73 % yields (based on the fluorine content in the initial polymer) for poly(VDF-co-HFP) copolymer and ETFE, respectively. 4. Conclusions Mineralizations of PVDF, poly(VDF-co-HFP) and ETFE copolymers in superheated water in the presence of an alkaline reagent were investigated. These polymers underwent a quasi-complete defluorination, releasing F- ions into the reaction solution at a relatively low temperature (250 C) under Ar atmosphere. When PVDF was reacted in the presence of 1.0 M of [KOH] at 250 C for 6 h, which amount corresponds 9 J. Hamaura et al. to 10 times the molar amount of fluorine content (as atoms) in PVDF, the F- yield released into the reaction solution reached 95 %. This transformation was accompanied by formation of carbon rich residue consisting of amorphous carbon. When the PVDF reaction was performed with O2 instead of Ar, significant differences were observed in the products: while F- ions were efficiently produced, carbon rich residue did not form. In such conditions, oxalate was the major species that composed the total organic carbon content in the reaction solution. Four consecutive runs (that is, after one reaction of PVDF with 1.0 M [KOH] at 250 C under Ar for 6 h was complete, new PVDF was charged to the reaction mixture and reacted again) were performed, and no decrease of the F- yield was observed. The fluorine atoms in poly(VDF-co-HFP) and ETFE copolymers also completely mineralized to form F- ions in the reaction solutions with 100 and 98 % yields, respectively, after treatment with 1.0 M of [KOH] for 6 h under Ar. Addition of Ca(OH)2 into the reaction solutions from superheated water treatment for these polymers and subsequent washing procedures with 1.0 M of [HCl] and pure water gave pure CaF2, i.e., artificial fluorspar, with 77, 84 and 73 % yields for PVDF, poly(VDFco-HFP) and ETFE copolymers, respectively. Further efforts using similar approaches for recycling other fluoropolymers are under progress in our laboratories. Data Availability The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study. CRediT authorship contribution statement Jin Hamaura: Investigation, Validation. Ryo Honma: Investigation. Hisao Hori: Conceptualization, Investigation, Project administration, Supervision, Validation, Writing - original draft. Abdelatif Manseri: Writing - review & editing. Bruno Ameduri: Conceptualization, Writing - review & editing. 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 The data that has been used is confidential. Acknowledgements This work was supported by Japan Science and Technology Agency (JST) CREST Grant Number JPMJCR21L1. BA thanks the French Fluorine Network (GIS). Appendix A. Supplementary material Supplementary data to this article can be found online at https://doi. org/10.1016/j.eurpolymj.2022.111724. References [1] J.G. Drobny, Technology of Fluoropolymers, 2nd ed., CRC Press, Boca Raton, FL, 2009. [2] B. Ameduri, B. Boutevin, Well-Architectured Fluoropolymers: Synthesis, Properties and Applications, Elsevier, Oxford, UK, 2004. [3] J. Gardiner, Fluoropolymers: origin, production, and industrial and commercial applications, Aust. J. Chem. 68 (2015) 13-22, https://doi.org/10.1071/CH14165. [4] R. Dams, K. Hintzer, Industrial aspect of fluorinated oligomers and polymers, in: Fluorinated Polymers, vol. 2, Applications, B. Ameduri, H. Sawada (Eds.), Royal Society of Chemistry, Cambridge, 2017, pp. 3-31. [5] B. Ameduri, S. Fomin (Eds.), Fascinating Fluoropolymers and their Applications, Elsevier, Amsterdam, 2020. European Polymer Journal 182 (2023) 111724 [6] G.J. Puts, P. Course, B.A. Ameduri, Polytetrafluoroethylene: synthesis and characterization of the original extreme polymer, Chem. Rev. 119 (2019) 1763-1805, https://doi.org/10.1021/acs.chemrev.8b00458. [7] Fluoropolymer Market Overview; Future Market Insights: Newark, 2022. https:// www.futuremarketinsights.com/reports/fluoropolymers-market. [8] K. Aleksandrov, H.J. Gehrmann, M. Hauser, H. Matzing, D. Pigeon, D. Stapf, M. Wexler, Waste incineration of polytetrafluoroethylene (PTFE) to evaluate potential formation of per- and poly-fluorinated alkyl substances (PFAS) in flue gas, Chemosphere 226 (2019) 898-906, https://doi.org/10.1016/j. chemosphere.2019.03.191. [9] Mineral Commodity Summaries 2021, U.S. Geological Survey, Reston, 2021, pp. 60-61. https://pubs.usgs.gov/periodicals/mcs2021/mcs2021.pdf. [10] R.D. Chambers, Fluorine in Organic Chemistry, Blackwell, Oxford, 2004. [11] M. Schlipf, T. Schwalm, Closing the recycling loop. Up-Cycling of end-of-life fluoroplastics, Kunststoffe Int. 6 (2014) 58-60. [12] P.G. Jessop, W. Leitner (Eds.), Chemical Synthesis Using Supercritical Fluids, Weinheim: Wiley-VCH, 1999. [13] A.A. Peterson, F. Vogel, R.P. Lachance, M. Froling, M.J. Antal, J.W. Tester, Thermochemical biofuel production in hydrothermal media: a review of sub- and supercritical water technologies, Energ. Environ. Sci. 1 (2008) 32-65, https://doi. org/10.1039/B810100K. [14] G. Brunner, Near critical and supercritical water. Part I. Hydrolytic and hydrothermal processes, J. Supercrit. Fluids 47 (2009) 373-381, https://doi.org/ 10.1016/j.supflu.2008.09.002. [15] E. Furimsky, Hydroprocessing in aqueous phase, Ind. Eng. Chem. Res. 52 (2013) 17695-17713, https://doi.org/10.1021/ie4034768. [16] Z. Knez, M.K. Hrncic, M. Colnik, M. Skerget, Chemicals and value added compounds from biomass using sub- and supercritical water, J. Supercrit. Fluids 133 (2018) 591-602, https://doi.org/10.1016/j.supflu.2017.08.011. [17] H. Hori, R. Honma, Decomposition of fluoropolymers by their mineralization in subcritical water, in: B. Ameduri, S. Fomin (Eds.), Opportunities of Fluoropolymers, Elsevier, Amsterdam, 2020, pp. 303-331. [18] R. Wang, Z. Xu, Recycling of non-metallic fractions from waste electrical and electronic equipment (WEEE): a review, Waste Manag. 34 (2014) 1455-1469, https://doi.org/10.1016/j.wasman.2014.03.004. [19] R. Honma, H. Hori, F. Reis da Chunha, N. Horiike, L. Steinbach, B. Ameduri, Permanganate-induced efficient mineralization of poly(vinylidene fluoride) and vinylidene-fluoride based copolymers in low-temperature subcritical water, Ind. Eng. Chem. Res. 58 (2019) 13030-13040, https://doi.org/10.1021/acs. iecr.9b01921. [20] H. Hori, R. Honma, K. Igarashi, A. Manseri, B. Ameduri, Oxidative mineralization of poly[vinylidene fluoride-co-2-(trifluoromethyl)acrylic acid] copolymers in superheated water, Ind. Eng. Chem. Res. 61 (2022) 1386-1397, https://doi.org/ 10.1021/acs.iecr.1c04299. [21] H. Hori, J. Hamaura, Efficient mineralization of ethylene-tetrafluoroethylene copolymer in superheated water with permanganate, Polym. Degrad. Stabil. 190 (2021), 109621, https://doi.org/10.1016/j.polymdegradstab.2021.109621. [22] H. Kise, H. Ogata, Phase transfer catalysis in dehydrofluorination of poly (vinylidene fluoride) by aqueous sodium hydroxide solutions, J. Polym. Sci. Pol. Chem. 21 (1983) 3443-3451, https://doi.org/10.1002/pol.1983.170211208. [23] D.M. Brewis, I. Mathieson, I. Sutherland, R.A. Calyless, R.H. Dahm, Pretreatment of poly(vinyl fluoride) and poly(vinylidene fluoride) with potassium hydroxide, Int. J. Adhes. Adhes. 16 (1996) 87-95, https://doi.org/10.1016/0143-7496(95)00053-4. [24] R. Crowe, J.P.S. Badyal, Surface modification of poly(vinylidene difluoride) (PVDF) by LiOH, J. Chem. Soc. Chem. Commun. 14 (1991) 958-959, https://doi. org/10.1039/C39910000958. [25] P. Maccone, G. Brinati, V. Arcella, Environmental stress cracking of poly (vinylidene fluoride) in sodium hydroxide. Effect of chain regularity, Polym. Eng. Sci. 40 (2000) 761-767, https://doi.org/10.1002/pen.11205. [26] G.J. Ross, J.F. Watts, M.P. Hill, P. Morrissey, Surface modification of poly (vinylidene fluoride) by alkaline treatment 1. The degradation mechanism, Polymer 41 (2000) 1685-1696, https://doi.org/10.1016/S0032-3861(99)003432. [27] G.J. Ross, J.F. Watts, M.P. Hill, P. Morrissey, Surface modification of poly (vinylidene fluoride) by alkaline treatment part 2. Process modification by the use of phase transfer catalysis, Polymer 42 (2001) 403-413, https://doi.org/10.1016/ S0032-3861(00)00328-1. [28] S. Zhang, J. Shen, X. Qiu, D. Weng, W. Zhu, ESR and vibrational spectroscopy study on poly(vinylidene fluoride) membranes with alkaline treatment, J. Power Sources 153 (2006) 234-238, https://doi.org/10.1016/j.jpowsour.2005.05.020. [29] Q. Liu, C. Lee, H. Kim, Performance evaluation of alkaline treated poly(vinylidene fluoride) membranes, Sep. Sci. Technol. 45 (2010) 1209-1215, https://doi.org/ 10.1080/01496391003775808. [30] W. Lang, X. Zhang, J. Shen, H. Xu, Z. Xu, Y. Guo, The contrastive study of chemical treatment on the properties of PVDF/PFSA and PVDF/PVP ultrafiltration membranes, Desalination 341 (2014) 72-82, https://doi.org/10.1016/j. desal.2014.02.027. [31] M.M. Ahmed, J. Hruza, M. Stuchlik, V. Antos, J. Mllerova, M. Rezanka, Revisiting the polyvinylidene fluoride heterogeneous alkaline reaction mechanism in propan2-ol: An additional hydrogenation step, Eur. Polym. J. 156 (2021), 110605, https://doi.org/10.1016/j.eurpolymj.2021.110605. [32] S. Mitra, A. Ghanbari-Siahkali, P. Kingshott, K. Almdai, H.K. Rehmeier, A. G. Christensen, Chemical degradation of fluoroelastomer in an alkaline environment, Polym. Degrad. Stab. 83 (2004) 195-206, https://doi.org/10.1016/ S0141-3910(03)00235-0. 10 J. Hamaura et al. [33] H. Hori, H. Tanaka, K. Watanabe, T. Tsuge, T. Sakamoto, A. Manseri, B. Ameduri, Hydrogen peroxide induced efficient mineralization of poly(vinylidene fluoride) and related copolymers in subcritical water, Ind. Eng. Chem. Res. 54 (2015) 8650-8658, https://doi.org/10.1021/acs.iecr.5b01716. [34] K. Tashiro, Y. Itoh, M. Kobayashi, H. Tadokoro, Polarized Raman spectra and LOTO splitting of poly(vinylidene fluoride) crystal form I, Macromolecules 18 (1985) 2600-2606, https://doi.org/10.1021/ma00154a041. [35] L. Lauchlan, F. Rabolt, Polarized Raman measurements of structural anisotropy in uniaxially oriented poly(vinylidene fluoride) (form I), Macromolecules 19 (1986) 1049-1054, https://doi.org/10.1021/ma00158a019. [36] P. Nallasamy, S. Mohan, Vibrational spectroscopic characterization of form II poly (vinylidene fluoride), Indian J. Pure Appl. Phys. 43 (2005) 821-827. [37] A.C. Ferrari, J. Robertson, Resonance Raman spectroscopy of disordered, amorphous, and diamondlike carbon, Phys. Rev. B 64 (2001), 075414, https://doi. org/10.1103/PhysRevB.64.075414. European Polymer Journal 182 (2023) 111724 [38] E.F. Sheka, Y.A. Golubev, N.A. Popova, Graphene domain signature of Raman spectra of sp2 amorphous carbons, Nanomaterials 10 (2020) 2021, https://doi.org/ 10.3390/nano10102021. [39] T. Jawhari, A. Roid, J. Casado, Raman spectroscopic characterization of some commercially available carbon black materials, Carbon 33 (1995) 1561-1565, https://doi.org/10.1016/0008-6223(95)00117-V. [40] NIST Chemistry Web Book, SRB 69, Infrared spectrum, Potassium oxalate. https:// webbook.nist.gov/cgi/cbook.cgi?Name=potassium+oxalate&Units=SI&cIR=on, 2022 (accessed 9 August 2022). [41] W.W. Schmiegel, Crosslinking of elastomeric vinylidene fluoride copolymers with nucleophiles, Angew. Makromol. Chem. 76-77 (1979) 39-65, https://doi.org/ 10.1002/apmc.1979.050760103. 11