# Capability of thermodesorption/pyrolysis DART FT-ICR MS to distinguish fluoropolymers and identify blend composition

Pierre Pacholski, Sébastien Schramm, Théo Voellinger, Umut Ugur Ozkose, Pierre Magri, Frank David-Quillot, Bruno Améduri, Frédéric Progent, Frédéric Aubriet

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Pierre Pacholski, Sébastien Schramm, Théo Voellinger, Umut Ugur Ozkose, Pierre Magri, et al. Capability of thermodesorption/pyrolysis DART FT-ICR MS to distinguish fluoropolymers and identify blend composition. Journal of Analytical and Applied Pyrolysis, 2026, 193 (1), pp.107361. 10.1016/j.jaap.2025.107361. hal-05248859

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# Capability of thermodesorption/pyrolysis DART FT-ICR MS to distinguish fluoropolymers and identify blend composition

Pierre Pacholski<sup>a,b,1</sup> <sup>1</sup>D, Sébastien Schramm<sup>b,*</sup>,<sup>2</sup> <sup>1</sup>D, Théo Voellinger<sup>b,3</sup> <sup>1</sup>D, Umut Ugur Ozkose<sup>c,d</sup>,<sup>4</sup> Pierre Magri<sup>b,5</sup> <sup>1</sup>D, Frank David-Quillot<sup>e,6</sup> <sup>1</sup>D, Bruno Améduri<sup>c,7</sup>, Frédéric Progent<sup>a,*</sup>,<sup>8</sup> Frédéric Aubriet<sup>b,*</sup>,<sup>9</sup> <sup>1</sup>D

## ARTICLE INFO

Keywords:

PVDF

Pyrolysis

DART

Fourier transform ion cyclotron resonance mass spectrometry

NMR

## ABSTRACT

Direct analysis in real time (DART) was coupled with a thermal desorption/pyrolysis (TDPy) device for the analysis of fluoropolymers. The product ions were analyzed using a Fourier transform cyclotron resonance mass spectrometer (FT-ICR MS). Two different polyvinylidene fluoride (PVDF) samples were studied individually and in a 50/50 mixture. This study demonstrates the capability of TDPy DART FT-ICR MS to provide information on PVDF polymers, including the determination of end-groups and the detection of comonomers.

The temperature program used enabled the desorption of the smallest oligomers (Mn ≈ 600 Da) below 400 °C, allowing for the identification of the end-groups, which ensured the differentiation of the PVDF. At temperatures above 400 °C, C<sub>w</sub>H<sub>x</sub>F<sub>y</sub> ions were predominantly formed as a result of the thermal cleavage of the PVDF backbone. Specific pyrolysis products observed for one PVDF sample suggested the presence of 4.6 mol% hexa-fluoropropylene (HFP), as determined by NMR measurements. The molar percentage of HFP was also determined by a new approach using TDPy DART MS. The analysis of a 50/50 PVDF blend revealed species from both polymers during the thermo-desorption and pyrolysis events, confirming the ability of the proposed methodology to determine the mol% of the HFP comonomer.

This represents the first TDPy DART FT-ICR MS study of fluoropolymers. Applicable to non-soluble or poorly soluble polymers, the proposed methodology enables the identification of end-groups, suggests the possibility of distinguishing fluoropolymers, and identifies blend composition. Additionally, the molar percentage of comonomers can be defined for poly(VDF-co-HFP) copolymers.

## 1. Introduction

Poly(vinylidene fluoride) (PVDF) is the second most produced

fluoropolymer after poly(tetrafluoroethylene) (PTFE) [1,2]. Its radical synthesis is operated by emulsion or suspension in water, using organic peroxide [3,4] or inorganic persulfate [5] initiators. PVDF has

<footnote><sup>a</sup> CEA, DAM, DIF, Arpajon F-91297, France</footnote>

<footnote><sup>b</sup> Université de Lorraine, LCP-A2MC (Laboratoire de Chimie et Physique-Aproche Multi-échelles des Milieux Complexes), Metz F-57000, France</footnote>

<footnote><sup>c</sup> Institut Charles Gerhardt, ICGM, Université de Montpellier, CNRS, ENSCM, Montpellier F-34293, France</footnote>

<footnote><sup>d</sup> Department of Chemistry, Faculty of Science and Letters, Piri Reis University, Tuzla, Istanbul TR-34940, Turkey</footnote>

<footnote><sup>e</sup> CEA, DAM, Le Ripault, Monts F-37260, France</footnote>

<footnote>* Corresponding authors.</footnote>

<footnote>E-mail addresses: sebastien.schramm@univ-lorraine.fr (S. Schramm), frederic.progent@cea.fr (F. Progent), frederic.aubriet@univ-lorraine.fr (F. Aubriet).</footnote>

<footnote n="1">¹ 0009-0007-7018-609X</footnote>

<footnote n="2">² 0000-0002-8500-8903</footnote>

<footnote n="3">³ 0009-0002-8998-3295</footnote>

<footnote n="4">⁴ 0000-0003-4807-6322</footnote>

<footnote n="5">⁵ 0000-0002-8645-5115</footnote>

<footnote n="6">⁶ 0009-0002-8749-8878</footnote>

<footnote n="7">⁷ 0000-0003-4217-6664</footnote>

<footnote n="8">⁸ 0009-0003-5994-4954</footnote>

<footnote n="9">⁹ 0000-0003-2457-7974</footnote>
piezoelectric properties, a high thermostability, and is resistant to oxidation, acids, solvents and ageing [6,7]. These properties allow the use of PVDF in sensors, transducers, actuators [8,9], engineering and chemical processing equipment [6,7,10,11], water treatment [12], coatings [13], binders for cathodes of Li-ion batteries [14–16]. Different analytical methods are used to characterize PVDF. Nuclear magnetic resonance (NMR) allows to obtain information on the chain structure (normal and reversed VDF-VDF addition), the end-groups, and the average molar mass [17–21]. Fourier transform infra-red (FT-IR) is useful to highlight the presence of functional groups [22,23]. Pyrolysis-gas chromatography (Py-GC), commonly hyphenated to mass spectrometry [24], enables the elucidation of the microstructure of polymers (end-groups, branching, cross linking) and for copolymers, the quantification of the co monomers [25,26].

Mass spectrometry is also a versatile tool that can be used to study polymers [27,28]. Some PVDF have been investigated by MALDI [27], electrospray ionization (ESI) [29,30], atmospheric solid analysis probe (ASAP) [31] and direct analysis in real time (DART) [32,33]. The solubilization of PVDF in a polar and volatile solvent or its homogenous mixing with a matrix are among the preparation steps required to conduct ESI and MALDI experiments, respectively. As PVDF is known to be resistant to solubilization in the classically-used organic solvents [34], its analysis by ESI or MALDI is often difficult, except for low molar mass PVDF [27,35]. ASAP and DART, both introduced in 2005 [36,37], are ambient ion sources which do not require solubilization or other extensive sample preparation steps. ASAP operates by thermal desorption of a solid or liquid sample introduced in a glass probe. Ionization is operated by a corona discharge [36]. DART produces a stream of excited atoms or molecules (typically He\* or N\_2\*) heated up to 500 °C [37], which interacts with the atmosphere components and the sample to mainly produce in the gas phase [M+H]\+ and [M+NH\_4]\+ ions in (+) DART and [M-H]\-, [M+O\_2]\*- [M+NO\_2]\-, [M+NO\_3]\-, [M+HCO\_3]\- in (-) DART [33,37–40]. Because DART is a fully open-air ion source, it can be used to analyze large samples as polymer pellets, which is not possible by ASAP.

The DART ion source may also be coupled to a thermo-desorption/pyrolysis (TDPy) device, which consists in a hot stage placed under the ionization region, between the outlet of the DART and the inlet of the mass spectrometer [32]. The species generated in gas phase are ionized by DART. With the gradual increase in temperature, the nature and amounts of generated species vary [32,41]. Time- and temperature-resolved TDPy DART MS analyses have been successfully performed for different polymers, including some fluorinated ones [32, 41–45].

A TDPy DART polymer mass spectrum presents hundreds to thousands of signals, the interpretation of which can be tedious [32,41,44, 45]. Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) has been extensively used to study complex mixtures such as plant extracts [46], environmental organic matter [47], and biomass pyrolysis products [48,49]. The very high resolution achieved allows for the distinction of isobaric species, while the high *m/z* measurement accuracy, typically better than 0.3 ppm, enables the assignment of a molecular formula to each ion. Molecular formula attribution is a key point for the data interpretation. Different graphical representation, such as class diagram, Van Krevelen, and double bound equivalence (DBE) vs carbon number plots, provide a deep understanding of the sample. Another useful data representation is Kendrick diagram [50], which was first used for polymers by Sato et al. [51] and Fouquet et al. [52,53].

In a previous study, DART ion source was used to distinguish four PVDF samples [33]. The present work focuses on additional information accessible by using a TDPy device. The ability of TDPy DART hyphenated to an FT-ICR MS instrument to distinguish two PVDF samples was assessed. Each PVDF was initially analyzed to obtain a deep understanding of their composition, thermal desorption and pyrolysis behaviors. NMR measurement were also conducted to support some TDPy

DART results. Additionally, a 50/50 w/w mixture of both PVDF was investigated to evaluate the capability of TDPy DART MS to identify the composition of a fluoropolymer blend.

## 2. Materials and methods

### 2.1. Samples

Two PVDF samples were considered. The first one (named Kynar) was provided by Arkema (Colombes, France) while the second one (named uPVDF) was an unknown PVDF sample provided by the CEA (commissariat à l'énergie atomique et aux énergies alternatives). For both samples, neither the average molar mass nor the dispersity was known. Both PVDFs were analyzed individually and in a 50/50 (weight/ weight) mixture. The detailed procedure used to prepare the uPVDF-Kynar blend is given in supplementary information (SI 1). Briefly, three solutions were prepared to obtain a uPVDF/Kynar blend and solubilized/reprecipitated samples of each polymer, aiming to assess the effect of solubilization on thermodesorption/pyrolysis behavior. uPVDF and Kynar were dissolved in MEK or MEK/DMSO and combined to form an equal-mass blend. Water was added to induce precipitation, followed by washing, filtration, and vacuum drying. Since no significant differences were observed between raw and reprecipitated samples, only raw sample results are presented in this study.

### 2.2. Mass spectrometry instrumentation

For analysis, 1 mg of sample was cut from a polymer pellet, using a scalpel and pliers. Samples were positioned in a copper cup, then placed on the heating plate of the IonRocket TDPy device (Biochromatino Inc., Fujisawa, Kanagawa-ken, Japan), located 0.5 cm below the gap between the DART ion source and the Vapur interface (1.7 cm). The temperature program was controlled and set as follow: 35 °C for 1 min, heating to 300 °C at 100 °C/min, hold for 2 min, heating to 600 °C at 100 °C/min, hold until the end of the 15-minutes run.

DART-SVP ion source (IonSense, Saugus, MA, USA) experimental parameters were detailed in a previous study [33]. Shortly, high purity helium (Air Liquide, Paris, France) was heated in the DART source at 300 °C. The voltage on the exit grid was set to 350 V.

Fluorinated polymers are known to produce negatively charged ions under DART ionization conditions [32,33,43,54]. Accordingly, the ions generated by TDPy DART were analyzed by a Solarix 7 T 2XR FT-ICR mass spectrometer (Bruker Daltonics, GmbH, Bremen, Germany). Their *m/z* value ranged from 229 to 2000. The transient size was set to 4 megapoints, 2.2370 s length, and measured using quadripolar detection (2ω). The accumulation time was set to 5 ms. One mass spectrum was acquired every 2.4 s during the 15-minutes run. External calibration was performed by directly introducing an uPVDF pellet in the DART gas stream using the HO-(*C*₂H₂F₂)ₙ·H polymer distribution with n = 6–14, detected as [M-H]⁻, [M+O₂]⁺⁻ and [M+NO₂]⁻ which was identified in a previous work [33]. The background ion of the deprotonated palmitic acid (*m/z* 255.23295) was used for on-line lock mass calibration to avoid peak splitting. Alternatively, the lock mass calibration was set on identified ions from the sample: (*C*₂H₂F₂)₈F⁻ (*m/z* 531.09860) for Kynar, (*C*₂H₂F₂)₄C₃HF₈ (*m/z* 445.04542) and (*C*₂H₂F₂)₉F⁻ (*m/z* 595.11106) for the uPVDF.

MS/MS was achieved by collision induced dissociation (CID) in the quadrupole cell of the FT-ICR at a collision energy varying from 0 to 50 eV. The collision gas was high purity argon (Air Liquide, Paris, France).

### 2.3. Post-acquisition data processing

Data were processed using DataAnalysis 5.0 (Bruker Daltonics, GmbH, Bremen, Germany) to sum mass spectra in a selected zone of the thermogram. They were internally calibrated using a list of known ions
over the whole *m/z* range. The residual standard deviation (RSD<sub>cal</sub>) was systematically lower than 0.15 ppm. All peaks with a signal-to-noise ratio (S/N) higher than three were exported as a peak list to be processed using Punc'Data [55] an in-house software, that permits the attribution of molecular formula and visualization/plotting of data.

The constraints for the determination of the ion elemental composition were C<sub>4-60</sub>H<sub>0-60</sub>O<sub>0-10</sub> in the first pass and C<sub>4-60</sub>H<sub>0-60</sub>F<sub>0-60</sub>O<sub>2-13</sub>N<sub>1</sub> in the second one. Indeed, the end-groups may involve oxygen and, oxidation could occur during pyrolysis under atmospheric conditions. Moreover, nitrite and nitrate adducts have been observed in (-) DART [32,56]. Both anions and radical anions were considered. The *m/z* error allowed for attribution was typically 3 times the RSD<sub>cal</sub> (≈ 0.45 ppm). Attribution was considered satisfying when more than 95 % of peaks were attributed and the error followed a normal distribution, centered on zero. The <sup>13</sup>C isotopic distribution of the most intense ions was verified. The inclusion of additional hetero-atoms, such as sulfur or chlorine, in the formula search criteria did not lead to any new attributions.

The data were plotted as Kendrick diagrams, class diagrams, and double bound equivalent (DBE) vs carbon number diagrams. Kendrick diagrams display the Kendrick fractional mass (KFM) on the y-axis and the *m/z* ratio on the x-axis. In this study, KFM were calculated using the C<sub>2</sub>H<sub>2</sub>F<sub>2</sub> repeat unit of the PVDF. In the generated diagrams, species differing solely by the number of repeating units (C<sub>2</sub>H<sub>2</sub>F<sub>2</sub>) exhibit identical KFM values. Consequently, they appear horizontally aligned, facilitating their identification. This approach has been thoroughly described in previous work [33]. Polymer ions differing by their end-groups and/or adducts have different KFM values. The class diagrams are histograms displaying the percentage of ion current attributed

to C<sub>w</sub>H<sub>x</sub>O<sub>y</sub>, F<sub>z</sub>, F<sub>z</sub>O<sub>1</sub>, F<sub>z</sub>O<sub>2</sub>, F<sub>z</sub>O<sub>3</sub>, F<sub>z</sub>O<sub>4</sub>, F<sub>z</sub>O<sub>5-10</sub> and F<sub>z</sub>N<sub>1</sub>O<sub>y</sub> chemical classes. The F<sub>z</sub> chemical class corresponds to ions containing only carbon, hydrogen, and fluorine atoms. The DBE vs carbon number diagrams enabled the identification of anions (half-integer DBE value) and radical anions (integer DBE value), as well as the number of unsaturations.

## 2.4. NMR analysis

The <sup>1</sup>H and <sup>19</sup>F NMR spectra were obtained using a Bruker AC 400 Spectrometer, operating at 400 MHz for <sup>1</sup>H and 376 MHz for <sup>19</sup>F. Acetone-d<sub>6</sub> and DMSO-d<sub>6</sub> (Eurisotop – Grenoble, France) were used as solvents. The temperature was maintained at 298 K. Chemical shifts were expressed in parts per million (ppm). The experimental parameters for acquiring the <sup>1</sup>H [<sup>19</sup>F] NMR spectra were as follows: number of scans 100 [150], acquisition time 4.5 s [0.7 s], flip angle 90° [30°], pulse delay 2 s [5 s], and a pulse width of 5 µs for <sup>19</sup>F NMR.

## 2.5. Thermogravimetric analysis

The thermogravimetric analysis (TGA) was conducted using a TGA Q500 V20.13 instrument from TA instruments, under a nitrogen gas flow of 40 mL/min. Approximately 10 mg of the sample was placed in a platinum crucible and analyzed following a procedure similar to that used for TDPy DART analysis. The temperature program was controlled and set as follows: first, a five-minute isotherm at 35 °C, followed by a ramp from 35 °C to 300 °C at 100 °C/min, with 2-minute isotherm at 300 °C. Finally, a ramp from 300 °C to 600 °C at 100 °C/min was applied, with a 2-minute isotherm at the final temperature of 600 °C.

<figure><img src="image_3.png" /><figcaption>**Fig. 1.** Thermograms of the uPVDF sample (top) and Kynar (bottom). Three zones (named u-A, u-B, u-C and k-A, k-B, k-C for uPVDF and Kynar, respectively) of sum mass spectrum were defined. The temperature program from 35 °C to 600 °C is represented by a red line.</figcaption></figure>
# 3. Results and discussion

## 3.1. uPVDF and Kynar samples

### 3.1.1. Thermograms

The total ion current (TIC) and the temperature for both samples are shown in respect with the time (Fig. 1). The TIC of the uPVDF stayed almost at baseline level until the first peak was detected at 7.3 min (454 °C). The TIC decreased to reach baseline level around 8.4 min, and a second peak was noted at 590 °C). Kynar behaved similarly (Fig. 1), as the signal remained almost at baseline level until the observation of a first peak at 7.7 min (498 °C). A second peak appeared at 8.4 min (568 °C). These results are in good agreement with a previous PVDF TDPy DART study [32], and the thermogravimetric analysis [57–59], showing that PVDF thermal degradation occurs at T > 400 °C. The previously reported thermogravimetric analyses were conducted with a continuous increase in temperature over the time. To better compare the TDPy DART results with the TGA results, a specific temperature program including isotherms, was used. The TGA results are reported in SI 2. For both PVDF samples, only one significant mass loss was observed. This occurred at 565 °C (93.7 % mass loss) for uPVDF and at 536 °C (75.3 % mass loss) for Kynar. For the Kynar sample, a small weight loss was also noted during the 300 °C isotherm, corresponding to less than 0.1 % of the initial mass. The TGA behavior of both PVDF samples was in agreement with the second peak observed by TDPy DART, despite a shift of about 30 °C. This shift could be explained by the location where the temperature measurement for the TDPy DART, which was not at the copper crucible but on the heater. Moreover, the TGA mass loss regions were relatively broad. For uPVDF, it began at 520 °C and ended at 590 °C, while for Kynar, it spanned from 480 °C to 570 °C. An alternative explanation for Kynar might be the combination of the two TDPy DART peaks into the single one observed in TGA. It should also be noted that the signal recorded during TDPy DART analysis is an ion current, which is significantly influenced by the ionization efficiency of the species released into the gas phase upon the increase of the temperature. Consequently, the TIC cannot be considered in terms of quantitative information. The comparison of the thermograms obtained by TGA and TDPy DART assessed that very low amounts of species were released into the gas phase at temperatures below 500 °C, but they were efficiently ionized by DART. As a result, they differed from the species corresponding to the thermal degradation of uPVDF and Kynar at 590 °C and 568 °C, respectively.

Three different regions of the thermograms were specifically investigated (Fig. 1). The mass spectra of each of these regions were summed to yield the A, B and C mass spectra (Fig. 2 and SI 3). The A mass spectrum corresponds to the sum of the individual mass spectra obtained between 1 and 6 min. This ensured to investigate the species relative to the low-signal observed below 400 °C. The region summed to obtain the B and C mass spectra were sample dependent (Fig. 1). They are assigned to the first and second peaks, respectively. The u-A, u-B, u-C and k-A, k-B, k-C notations were used to identify the mass spectra obtained in the A, B and C temperature ranges for uPVDF and Kynar, respectively (Fig. 1).

### 3.1.2. A Region of the uPVDF and Kynar thermograms

In A region, non-fluorinated species in the m/z < 400 range were highlighted, corresponding to background ions (palmitic and stearic acid at m/z 255.23295 and 283.26426, respectively) as well as potential additives or pollution coming from the sample. Their molecular formula was C<sub>w</sub>H<sub>x</sub>O<sub>2-w</sub> with DBE value ranging from 1 to 5 (C<sub>w</sub>H<sub>x</sub>O<sub>y</sub> class, Fig. 3). Above m/z 400, oligomer distributions, with peaks separated by the number of VDF (C<sub>2</sub>H<sub>2</sub>F<sub>2</sub>) were obtained. Kendrick plots were drawn using the C<sub>2</sub>H<sub>2</sub>F<sub>2</sub> base unit (Fig. 4). Observed oligomers were written (C<sub>2</sub>H<sub>2</sub>F<sub>2</sub>)<sub>n</sub>α+ω, α and ω being the end-groups and any other atoms not part of the VDF repeat unit. The ions of the oligomer distributions were relative to [(C<sub>2</sub>H<sub>2</sub>F<sub>2</sub>)<sub>n</sub>H<sub>2</sub>O - H]<sup>-</sup> for the uPVDF while [(C<sub>2</sub>H<sub>2</sub>F<sub>2</sub>)<sub>n</sub>C<sub>4</sub>H<sub>10</sub>O<sub>3</sub> - H]<sup>-</sup> and [(C<sub>2</sub>H<sub>2</sub>F<sub>2</sub>)<sub>n</sub>C<sub>4</sub>H<sub>10</sub>O<sub>4</sub> - H]<sup>-</sup> were evidenced for Kynar

<figure><img src="image_4.png" /><figcaption>Fig. 2. Mass spectra of the u-A, u-B and u-C regions of the uPVDF sample. The formulae of the annotated *m/z* are given on the figure inlet.</figcaption></figure>

($n = 5–15$). The difference of the KFM for each end-groups allowed an easy distinction of both samples.

In Fig. 3, the F<sub>z</sub>O<sub>1</sub> group of uPVDF corresponds to oligomers bearing α+ω = H<sub>2</sub>O end-groups detected as [M-H]<sup>-</sup>. The F<sub>z</sub>O<sub>3</sub> ions are [M+O<sub>2</sub>]<sup>+</sup> adduct anions. F<sub>z</sub>O<sub>3</sub> and F<sub>z</sub>O<sub>4</sub> classes of Kynar indicate the presence of α+ω = C<sub>4</sub>H<sub>10</sub>O<sub>3</sub> and C<sub>4</sub>H<sub>10</sub>O<sub>4</sub> end-groups, respectively. These ion series have been previously observed by DART [33], suggesting that species detected below 400 °C by TDPy DART correspond to the thermo-desorption of small oligomers, as discussed by Cody *et al.* [32].

### 3.1.3. B and C regions of uPVDF and Kynar thermograms

The u-B, u-C, k-B and k-C mass spectra were more complex than the u-A and k-A ones. Nevertheless, the presence of PVDF distributions was clearly evidenced by the horizontal alignments on the Kendrick diagrams presented in Fig. 4. The attribution of these ion series is given in Table 1.

The ions obtained for uPVDF mainly presented a KFM in the [0.01;0.05] range. For Kynar, the ion KFM was generally lower than 0.01. More than 80 % of the attributed ion current for uPVDF and more than 90 % for Kynar, were relative to anion. The ions with a -0.5 DBE were attributed to [M+F]<sup>-</sup> and those with a 0.5 DBE to [M-H]<sup>-</sup>. The relative distribution of both ion series was very close. The formation of [M+F]<sup>-</sup> adduct ions could be explained by the thermal dehydrofluorination of PVDF [59–61]. The produced HF molecules reacted in
<figure><img src="image_5.png" /><figcaption>Fig. 3. Class diagrams of the A, B and C regions of uPVDF and Kynar.</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>Fig. 4. Kendrick diagrams in the [-0.10;0.10] KFM range of u-A, u-B, u-C of uPVDF (top) and k-A, k-B, k-C of Kynar (bottom). The dot size and color scale are proportional to the relative intensity of the peak. PVDF series are annotated with letters from a to p (see Table 1 for assignment). Specific series from the sample are underlined and in red color.</figcaption></figure>

gas phase with atmospheric H₂O to release H₃O⁺ and F⁻ ions and the latter adducted to neutral species formed by TDPy of PVDF. This type of adduct was not observed in the u-A and k-A mass spectra and can be considered indicative of the thermal degradation of the polymer. Interestingly, [M+F]⁻ ions with longer VDF chains than those obtained for [M-H]⁻ anions were formed (Table 1), meaning that fluoride attachment allowed the ionization of the heaviest oligomers.

Ions with an unexpected -1.5 DBE value were detected for Kynar and, to a lesser extent, for uPVDF (Fig. 5). The corresponding formulae were (C₂H₂F₂)₆CH₃F₄ (m/z 475.09242) and (C₂H₂F₂)₆H₃F₂ (m/z 425.09557) ions were deeply investigated. First, the measured and theoretical ¹³C isotopic pattern were compared and

allowed to confirm the number of carbon atoms. MS/MS experiments were also conducted to obtain structural information. HF losses were evidenced, as reported previously by MS/MS of PVDF ions [29,33]. CID performed with a 50 eV kinetic energy yielded the consecutive loss of 15 HF for m/z 475.09242 and 14 HF for m/z 425.09557 meaning that both species encountered a total dehydrofluorination to produce the C₁₃F⁻ and C₁₂H⁻ fragments ions, respectively (SI 4 and 5). The MS/MS results, the observed ¹³C isotopic pattern and, the m/z measurement accuracy allowed us to confirm the (C₂H₂F₂)₆CH₃F₄ and (C₂H₂F₂)₆H₃F₂ ion attribution. According to the -1.5 DBE value, the ions did not correspond to a single neutral ionized by anion attachment, but rather to heterodimers sharing the same fluorine anion and had to be written [H(C₂H₂F₂)ₙ-mCF₃ + H(C₂H₂F₂)ₘH + F]⁻ and, [H(C₂H₂F₂)ₙ-mF + H(C₂H₂F₂)ₘH + F]⁻, respectively. The successive HF losses were thought
<table><caption>**Table 1**

List of the most abundant ion distributions detected for uPVDF and Kynar. A letter is assigned to each alignment observed on Fig. 5.</caption><thead><tr><th>Alignment</th><th>Series formula</th><th>Range of n</th></tr></thead><tbody><tr><td>a</td><td>[HO(C<sub>2</sub>H<sub>2</sub>F<sub>2</sub>)<sub>n</sub>H - H]<sup>-</sup></td><td>6-15</td></tr><tr><td>b</td><td>[H(C<sub>2</sub>H<sub>2</sub>F<sub>2</sub>)<sub>n</sub>C<sub>3</sub>F<sub>7</sub> - H]<sup>-</sup></td><td>3-14</td></tr><tr><td>c</td><td>[H(C<sub>2</sub>H<sub>2</sub>F<sub>2</sub>)<sub>n</sub>C<sub>3</sub>F<sub>7</sub> + F]<sup>-</sup></td><td>2-26</td></tr><tr><td>d</td><td>[H(C<sub>2</sub>H<sub>2</sub>F<sub>2</sub>)<sub>n</sub>C<sub>2</sub>F<sub>5</sub> - H]<sup>-</sup></td><td>4-15</td></tr><tr><td>e</td><td>[H(C<sub>2</sub>H<sub>2</sub>F<sub>2</sub>)<sub>n</sub>C<sub>2</sub>F<sub>5</sub> + F]<sup>-</sup></td><td>3-22</td></tr><tr><td>f</td><td>[H(C<sub>2</sub>H<sub>2</sub>F<sub>2</sub>)<sub>n</sub>C<sub>3</sub>F<sub>5</sub> - H]<sup>-</sup></td><td>5-16</td></tr><tr><td>g</td><td>[H(C<sub>2</sub>H<sub>2</sub>F<sub>2</sub>)<sub>n</sub>CF<sub>3</sub> + F]<sup>-</sup></td><td>4-21</td></tr><tr><td>h</td><td>[H(C<sub>2</sub>H<sub>2</sub>F<sub>2</sub>)<sub>n</sub>F - H]<sup>-</sup></td><td>5-14</td></tr><tr><td>i</td><td>[H(C<sub>2</sub>H<sub>2</sub>F<sub>2</sub>)<sub>n</sub>F + F]<sup>-</sup></td><td>5-19</td></tr><tr><td>j</td><td>[H(C<sub>2</sub>H<sub>2</sub>F<sub>2</sub>)<sub>n</sub>CH<sub>2</sub>F - H]<sup>-</sup></td><td>6-13</td></tr><tr><td>k</td><td>[H(C<sub>2</sub>H<sub>2</sub>F<sub>2</sub>)<sub>n</sub>CH<sub>2</sub>F + F]<sup>-</sup></td><td>5-16</td></tr><tr><td>l</td><td>[H(C<sub>2</sub>H<sub>2</sub>F<sub>2</sub>)<sub>n</sub>mCF<sub>2</sub>H + H(C<sub>2</sub>H<sub>2</sub>F<sub>2</sub>)<sub>n</sub>F + F]<sup>-</sup></td><td>4-19</td></tr><tr><td>m</td><td>[H(C<sub>2</sub>H<sub>2</sub>F<sub>2</sub>)<sub>n</sub>mH + H(C<sub>2</sub>H<sub>2</sub>F<sub>2</sub>)<sub>m</sub>F + F]<sup>-</sup></td><td>5-20</td></tr><tr><td>n</td><td>[H(C<sub>2</sub>H<sub>2</sub>F<sub>2</sub>)<sub>n</sub>C<sub>4</sub>H<sub>9</sub>O<sub>4</sub> - H]</td><td>6-15</td></tr><tr><td>o</td><td>[H(C<sub>2</sub>H<sub>2</sub>F<sub>2</sub>)<sub>n</sub>C<sub>4</sub>H<sub>9</sub>O<sub>3</sub> - H]</td><td>5-15</td></tr><tr><td>p</td><td>[(C<sub>2</sub>H<sub>2</sub>F<sub>2</sub>)<sub>n</sub> - H]<sup>-</sup></td><td>4-17</td></tr></tbody></table>

to ensure crosslinking between the two neutrals attached to the fluorine anion [62]. Alternatively, -1.5 DBE value ions might also result from association of a HF molecule produced by the thermal degradation of PVDF with a neutral oligomer to yield [H(C<sub>2</sub>H<sub>2</sub>F<sub>2</sub>)<sub>n</sub>CHF<sub>2</sub> + HF + F]<sup>-</sup> or [H(C<sub>2</sub>H<sub>2</sub>F<sub>2</sub>)<sub>n</sub>H + HF + F]<sup>-</sup> ions. This latter hypothesis would be supported by the mass spectrum obtained after the selection of both parent ions (SI 4 and 5). Without activation, the loss of one HF was systematically observed. The relative abundance of the mono-dehydrofluorination ion ranged from 50% to 90% of those of the parent ion.

The other species observed were [M+O<sub>2</sub>]<sup>-</sup>, commonly formed by DART [38,39]. Overall, the main part of the ions was relative to C<sub>w</sub>H<sub>x</sub>F<sub>y</sub> species (F<sub>z</sub> class, Fig. 3), which were not detected during the thermo-desorption step. Consequently, these C<sub>w</sub>H<sub>x</sub>F<sub>y</sub> species resulted

from the thermal degradation of the PVDF. Despite the pyrolysis of the polymers occurring in open-atmosphere (presence of atmospheric oxygen), the relative abundances of F<sub>z</sub>O<sub>1</sub> and F<sub>z</sub>O<sub>3,4,5-10</sub> classes (Fig. 3) were lower than 10% of the TIC for uPVDF. Only F<sub>z</sub>O<sub>1</sub> ions were detected for Kynar. Thus, the contribution of α+ω = C<sub>4</sub>H<sub>10</sub>O<sub>3</sub> and C<sub>4</sub>H<sub>10</sub>O<sub>4</sub> end-group ions was marginal for the k-B and k-C mass spectra. For both PVDF, the compound class distribution was very close in the B and C regions.

Assuming that the α+ω end-groups of low molar mass PVDF observed by thermodesorption in A region were identical to long PVDF chains, the large amounts of F<sub>z</sub> compounds likely resulted from the cleavage of the both sides of the PVDF chains at temperature of 454 and 498 °C for uPVDF and Kynar, respectively [61]. This behavior is one of the well-known PVDF degradation by pyrolysis [24,59,61]. In pyrolysis processes involving long PVDF chains, the proportion of pyrolysis products that retain end groups is significantly lower than that of the degradation products derived from the polymer backbone, which constitute the majority of the species observed in the mass spectra. This process explains why end groups are no longer observed in the pyrolysis products (B and C regions). For the products formed after double-chain scission, exhibiting a generic formula of C<sub>w</sub>H<sub>x</sub>F<sub>y</sub>, the DBE versus carbon number diagrams (Fig. 5, panels u-B and k-B) reveal the presence of low-abundance ions with DBE values between 2 and 6, whose formation arises from HF losses. Following the double scission stage, the second pyrolysis step (Fig. 5, panels u-C and k-C) results in the formation of lower-mass fragments and a noticeable increase in the abundance of high-DBE ions (2 < DBE < 8). This increase in DBE is associated with cross-linking processes and additional HF elimination. This trend is particularly evident in the uPVDF sample. While ions with 2 < DBE < 8 were almost absent in the u-B stage, they appear clearly in the u-C stage. These compounds can therefore be classified as olefins, polyolefins, or even aromatic compounds.

The specific [M-H]<sup>-</sup> and [M+F]<sup>-</sup> ions of H(C<sub>2</sub>H<sub>2</sub>F<sub>2</sub>)<sub>n</sub>C<sub>2</sub>F<sub>5</sub> and H(C<sub>2</sub>H<sub>2</sub>F<sub>2</sub>)<sub>n</sub>C<sub>3</sub>F<sub>7</sub> in u-B and u-C (Fig. 4, Table 1) appeared to reveal some

<figure><img src="image_7.png" /><figcaption>**Fig. 5.** DBE vs. Carbon number plots of the A, B and C regions of the thermograms for uPVDF and Kynar. The DBE is calculated according to the ion formula: a semi-integer value corresponds to an anion and an integer value corresponds to a radical anion. The dot size and color scale are proportional to the relative abundance.</figcaption></figure>
polymer structure differences leading to specific thermal degradation mechanism for uPVDF. One explanation might be relative to different amounts of head-to-head or tail-to-tail chain defects in the two PVDFs. The polymerization of VDF goes by a head-to-tail route [6,63], meaning the monomer chaining is [(CH₂-CF₂)-(CH₂-CF₂)]n. Head-to-head [(CH₂-CF₂)-(CF₂-CH₂)]n and tail-to-tail [(CF₂-CH₂)-(CH₂-CF₂)]n inversion rates were evidenced by ¹⁹F NMR measurement to be ranging from 4% to 5% for uPVDF and Kynar (SI 6), respectively. This result is in accordance with the inversion rate generally reported in the literature [19,64–66]. Consequently, [M-H]⁻ and [M+F]⁻ ions of H(C₂H₂F₂)nC₂F₅ and H(C₂H₂F₂)nC₃F₇ on the u-B and u-C mass spectra of uPVDF could not be explained by a larger amount of CF₂-CF₂ chain defects. In addition to signals linked to VDF, the ¹⁹F NMR spectrum of uPVDF displayed between –71 to –77 ppm a signal assigned to CF₃ of a CF(CF₃)-CF₂ hexafluoropropylene (HFP) unit (SI 6) [21,28]. Thus, uPVDF was not a VDF homopolymer but a poly(VDF-co-HFP) copolymer. The molar percentage of HFP was evaluated to 4.6 [6,21,67,68]. This explains why it was not evidenced by direct (–) DART analysis [33] and by thermal desorption. Indeed, the detected oligomers contained less than 20 VDF units. In contrast, the pyrolysis of long chains ensured the observation of the b, c, d and e ions involving HFP units (u-B mass spectrum, see Fig. 4 and Table 1). According to a procedure inspired by those used by ¹⁹F NMR (Table 2), the molar HFP percentage has been evaluated according to the equation given in SI 7. Briefly, for the B region, ion abundances were weighed by the number of HFP units they contained, divided by the total number of VDF and HFP units, multiplied again by the number of HFP units, and normalized to the total ion current (TIC) before being summed (see calculations in the SI 7). A 5.1 mol% of HFP was obtained (Table 2), in relatively close good agreement with ¹⁹F NMR (4.6 mol%).

The *m/z* of ions on the mass spectrum of the C region was in a smaller range (400–1000), than in the B region (Fig. 4) meaning that shorter VDF chains were formed for both samples. The most abundant distribution of uPVDF was [H(C₂H₂F₂)₆–₁₄F - H]⁻. The H(C₂H₂F₂)nCF₃, H(C₂H₂F₂)nC₂F₅, H(C₂H₂F₂)nC₃F₇ compounds were also detected as [M-H]⁻ and [M+F]⁻ with the same n-value range. The difference of a CF₂ unit between these ion series was highlighted by an oblique alignment of points on the Kendrick diagram (Fig. 4), and suggested that cleavage could occur in the HFP unit itself. Ions differing by CF₂ and C₂F₄ units were previously observed by Py-GC/MS in the study of HFP-containing fluoropolymers [69,70]. For Kynar, the ions of the k-C mass spectrum were similar to those obtained in k-B region.

This behavior may be explained by the formation of free radical species during polymer pyrolysis that could be responsible of cross-linking [6,71,61,62,71,72]. The crosslinked species were formed during the first pyrolysis step (B region) but were initially not released in gas phase, resulting in a drop of the TIC on the thermogram. When the temperature was high enough (C region), the thermal energy was sufficient to break a high number of bonds, releasing pyrolysis products from the crosslinked residue in the gas phase. Those products correspond to shorter and more unsaturated compounds. For both PVDFs, the number of peaks (uPVDF 1789 vs 1075 and, Kynar 997 vs 816) as the variety of KFM values were greater in the C region (Fig. 4) than in the B one. In addition, the increase in DBE as well as in ion abundance for ions with a DBE > 2 (Fig. 5) confirmed the occurrence of advanced pyrolysis processes involving the well-known HF elimination [31,32,59,72,73].

For uPVDF, the distribution of the ions for a given ion series in the B

<caption>**Table 2**</caption>

Molar percentages of HFP comonomer obtained by ¹⁹F NMR and TDPy DART MS for uPVDF, Kynar and the 50/50 uPVDF/Kynar blend.

<table><thead><tr><th></th><th>uPVDF</th><th>Kynar</th><th>uPVDF/Kynar 50/50 blend</th></tr></thead><tbody><tr><td>HFP content (mol%) by ¹⁹F NMR</td><td>4.6</td><td>-</td><td>2.2</td></tr><tr><td>HFP content (mol%) by TDPy DART</td><td>5.1</td><td>-</td><td>2.7</td></tr><tr><td>MS</td><td></td><td></td><td></td></tr></tbody></table>

region led to an intriguing behavior. Indeed, an odd-even alternation was observed (Fig. 4 and Fig. 5). For example, the abundance of the [H(C₂H₂F₂)₆,₇,₈C₃F₇ - H]⁻ ions was 1.4 × 10⁶, 5.1 × 10⁵ and, 9.0 × 10⁵ arbitrary unit, respectively. The presence of at least one HFP unit was assumed to be relevant of a specific pyrolysis degradation pathway. The fine understanding of this behavior is still under study.

## 3.2. Blend of uPVDF/Kynar

A 50/50 (w/w) mixture of uPVDF and Kynar was studied according to the same procedure. The results are reported in Fig. 6 and SI 8. Similarly, to the previous results, the signal remained close to the baseline before the appearance of two peaks (Fig. 6). The first one at 7.6 min (485 °C) was obtained at a closer temperature to that observed for Kynar (498 °C) than for uPVDF (454 °C). The second peak at 8.4 min (565 °C) appeared at a temperature close to what was obtained for Kynar (568 °C). The second peak of uPVDF occurred at a higher temperature (590 °C).

The mass spectrum obtained in the u/k-A range displayed several oligomer distributions (Fig. 6). These ions were [(C₂H₂F₂)nH₂O - H]⁻, [(C₂H₂F₂)nC₄H₁₀O₃ - H]⁻ and, [(C₂H₂F₂)nC₄H₁₀O₄ - H]⁻ with n = 6–13, corresponding to the thermo-desorbed products of uPVDF and Kynar. The presence of two distinct domains on the Kendrick diagram for the specific ions of each PVDF allowed for a rapid visual diagnosis (Fig. 6).

Numerous horizontal alignments relative to different PVDF series were highlighted on the Kendrick diagrams from the u/k-B and u/k-C. For the thermogram B region, ion series in the 400–1300 *m/z* range were evidenced. They included ions with KFM < 0.01 observed in the study of uPVDF and Kynar, as well as the KFM > 0.01 uPVDF specific species. For these latter, the odd-even alternation was still observed, meaning that the thermal degradation mechanism of uPVDF was not affected by Kynar. It is also worth to note that KFM < 0.01 ions were among the most abundant species, similarly to those of Kynar. Consequently, the identification of both uPVDF and Kynar in the blend was possible.

For the u/k-C region of the thermogram, the ions detected were in the 400–800 *m/z* range, corresponding to shorter polymer chains than in the u/k-B region. The main part of the ions presented a KFM < 0.01, similarly to what was obtained for Kynar. This suggests that the residue formed after the first pyrolysis step was mostly composed of degraded Kynar. The appearance of the second pyrolysis peak at a similar temperature for Kynar and the u/k blend supported this assertion.

Finally, no specific ions to the blend were evidenced, whatever the considered thermogram region. All ions of the blend were also observed for uPVDF and/or Kynar. Thus, the results presented allowed for the identification of both samples in the blend and are encouraging to use TDPy DART as a means of polymer materials description.

## 4. Conclusion

TDPy DART MS was coupled to FT-ICR MS for the analysis of VDF (co)polymers. Under 400 °C, the main mechanism corresponded to the thermo-desorption of small pristine oligomers. The ions obtained gave information on the polymer end-groups and enabled to distinguish uPVDF from Kynar. When the temperature increased, a two-step pyrolysis of the polymer occurred. In a first one, the formation of C<sub>w</sub>H<sub>x</sub>F<sub>y</sub> species indicated the loss of the end-groups by cleavage of both sides of the polymer. The same ions were obtained in the second pyrolysis step, but they were smaller and more unsaturated. This behavior was explained by the formation of crosslinks during the first pyrolysis step, which were pyrolyzed in a second step. The uPVDF and Kynar pyrolysis led to common species with KFM < 0.01. In contrast, KFM > 0.01 species were specific to uPVDF linked to small HFP amounts, quantified by NMR (4.6 mol%) and TDPy DART MS (5.1 mol%). Hence, the pyrolysis gave insights on the structure of the polymer backbone. These results demonstrated the capacity of TDPy DART FT-ICR MS to obtain (1)
<figure><img src="image_8.png" /><figcaption>**Fig. 6.** Thermogram of the uPVDF/Kynar 50/50 blend (top). Three zones (u/k-A, u/k-B, u/k-C) were defined. The corresponding Kendrick diagrams in the 0.10 / -0.10 KFM range (bottom). The dot size is proportional to the relative abundance. PVDF series are annotated with a to p letters (see Table 2). The sum mass spectra are shown in *supplementary information*.</figcaption></figure>

information on polymer end-groups (thermodesorption products) (2) insights on the structure of the polymer chain and detection of a low comonomer content (pyrolysis products), which was not evidenced by single-DART (3) the monomer composition of a copolymer and (4) the description of a polymer blend. The proposed methodology therefore offers several advantages over traditional NMR and Py-GC/MS techniques. Since polymer solubilization is not required, TDPy DART FT-ICR MS enables the analysis of insoluble polymers—an aspect that is particularly challenging when using liquid state ¹⁹F NMR for determining the molar percentage of comonomers. For polymers with long chains, NMR spectroscopy offers limited insight into end-group structures, as the corresponding signals are often of quite low intensity and fall below the detection threshold. For similar reasons, identifying end-group structures using Py-GC/MS is also often challenging.

### CRediT authorship contribution statement

**Frédéric Progent:** Writing – review & editing, Supervision, Project administration, Methodology, Funding acquisition, Conceptualization.
**Frédéric Aubriet:** Writing – review & editing, Supervision, Project administration, Methodology, Funding acquisition, Conceptualization.
**Umut Ugur Ozkose:** Investigation, Formal analysis, Data curation.
**Pierre Magri:** Investigation, Formal analysis, Data curation.
**Frank David-Quillot:** Investigation, Formal analysis, Data curation.
**Bruno Améduri:** Writing – review & editing, Supervision, Methodology.
**Pierre Pacholski:** Writing – original draft, Investigation, Formal analysis, Data curation.
**Sébastien Schramm:** Writing – review & editing, Supervision, Project administration, Methodology, Funding acquisition, Conceptualization.
**Théo Voellinger:** Visualization, Software.

### 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.

### Acknowledgments

We acknowledge the CEA for financial support. BA thanks the French Fluorine Network (GIS). This work was partially supported by the MassLor research infrastructure at the University of Lorraine, for FT-ICR MS analyses.

### Appendix A. Supporting information

Supplementary data associated with this article can be found in the online version at doi:10.1016/j.jaap.2025.107361.

### Data availability

Data will be made available on request.

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