Characterization of the Chain-Ends and Branching Structures in

Jun 29, 2012 - Commercially, the homopolymerization of vinylidene fluoride is accomplished by free radical polymerization in emulsion or suspension...
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Characterization of the Chain-Ends and Branching Structures in Polyvinylidene Fluoride with Multidimensional NMR Eric B. Twum,† Chun Gao,† Xiaohong Li,† Elizabeth F. McCord,‡ Peter A. Fox,‡ Donald F. Lyons,‡ and Peter L. Rinaldi†,* †

Department of Chemistry, University of Akron, Akron, Ohio 44325-3601, United States Experimental Station, E. I. du Pont de Nemours and Company, Wilmington, Delaware 19880-0402, United States

‡

S Supporting Information *

ABSTRACT: Multidimensional solution NMR (19F, 1H, and 13 C) has been used to determine chain-ends and backbone branching points and to obtain unambiguous 19F and 1H resonances assignments from these chain-ends and branching structures in poly(vinylidene fluoride) (PVDF). The multidimensional NMR methods employed in this study not only enabled the resonance assignments of the last monomer of the chain but also provided assignments for the last three monomer units of chain-end structures. The chain-end signals from PVDF were determined using spin−lattice relaxation measurements and 2D diffusion ordered spectroscopy (DOSY) analysis. 2D-NMR analyses were also used to assign resonances of chain branching points along the backbone of the polymer.

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INTRODUCTION Poly(vinylidene fluoride) (PVDF) is a resinous, nonreactive, semicrystalline fluoropolymer.1 Like poly(tetrafluoroethylene) (PTFE), it shows remarkable properties, such as stability to degradation by thermal, chemical, electrical and UV radiation treatment.2−4 Unlike PTFE, PVDF is a melt-processable fluoropolymer; it can be extruded and molded into parts directly. Because of its excellent chemical and thermal stability, PVDF is used in applications such as materials for chemical storage, cable insulation, pipes, materials for heat exchangers, gaskets, valves and coatings; it also has unusual piezo and pyroelectric properties.5,6 PVDF is also used as separators and binders for lithium ion batteries7,8 and backsheets for photovoltaics.9 VDF and VDF-based co- and terpolymers in general have many commercially important applications.10 Because of its unique molecular architecture, PVDF is one of the most studied fluoropolymers. Commercially, the homopolymerization of vinylidene fluoride is accomplished by free radical polymerization in emulsion or suspension.5,11−13 The different polymerization methods and different initiators used for homopolymerizing VDF can produce a variety of end group functionalities. These end groups can affect some of the physical properties of the polymer.14 The different end groups can lead to different degradation pathways, which influence the thermal stability, flame resistance, and rheology of PVDF.14,15 The characterization of the end groups in PVDF is therefore important in optimizing polymer performance. The low concentration of end groups in high molecular weight polymers makes characterization of polymer end groups challenging. The presence of chain branching can affect the properties of a polymer. The viscoelastic properties of a polymer, which © 2012 American Chemical Society

determine its performance during processing, are known to depend on factors such as chain branching.16 Branching in PVDF or VDF-based fluoropolymers/fluoroelastomers can be short chain branches (SCB’s) or long chain branches (LCB’s). SCB’s are usually produced from intramolecular hydrogen abstraction in a polymer chain; this is the so-called backbiting reaction.17 The production of LCB’s, on the other hand, can occur through hydrogen transfer from the polymer backbone to an existing radical, and propagation from the newly formed radical site in the backbone of the polymer. Detecting chain branches in commercial VDF and VDF-based fluoropolymer/ fluoroelastomer samples is very difficult due to the low concentration of these structures in commercial fluoropolymers/fluoroelastomers. This is even more challenging when both SCB’s and LCB’s are present in the same polymer. 19 F NMR spectroscopy potentially provides the most accurate way to characterize the chain-end and branching structures of fluorinated polymers. Knowledge of the coupling constants for 19F−19F, 19F−1H, and 19F−13C couplings are needed for accurate interpretation of the 19F NMR spectra. IR spectroscopy is another important technique for studying chain-ends of polymers. The IR technique is more sensitive and is especially useful for fluoropolymers that are insoluble in many solvents.15 However, with IR, one is only able to tell the presence of suspected end group functionalities. There is the possibility that the region of absorption of the end group signal might overlap with other signals. NMR spectroscopy not only reveals the kinds of end groups present in a fluoropolymer, but Received: May 2, 2012 Revised: June 19, 2012 Published: June 29, 2012 5501

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acquisition time and a 3.4 μs (30°) pulse width were used. The data were zero-filled to 256k and the first three data points were backward linear predicted before Fourier transformation. The linear prediction was done to eliminate roll in the baseline. The 1H 1D-NMR spectra were collected with a 5.0 kHz spectral window, a 30.0 s relaxation delay, a 4.68 s acquisition time, a 9.5 μs (90°) pulse width, and gated 19F decoupling using CHIRP modulation (γBF/2π = 17.9 kHz).26,27 The data were zero-filled to 128k and the first three data points were backward linear predicted before Fourier transformation. Line broadening was set to 0.5 Hz for both the 1H and 19 F 1D-NMR spectra. The 13C 1D-NMR spectrum was collected with a 19 kHz spectral window, 1.0 s relaxation delay, 2.0 s acquisition time, and a 10 μs (90°) pulse width, with simultaneous 1H (WALTZ-16, γBH/2π = 2.7 kHz) and 19F (CHIRP, γBF/2π = 17.9 kHz) decoupling. The data were zerofilled to 256k; the first three points in the spectrum were backward linear predicted, and exponential weighting with line broadening of 3 Hz were applied before Fourier transformation. The T1 experiments were performed with the inversion recovery (relaxation-180°-τ-90° -acquire) technique. The 19F T1 experiments were performed with a spectral window of 18.9 kHz, 10 s relaxation delay, 1.0 s acquisition time, and a 10.2 μs (90°) pulse width, with gated 1H (WALTZ-16, γBH/2π = 2.7 kHz) decoupling; 10 τ delays ranging from 0.0625 to 32.0 s were used. The 1H T1 experiments were performed with 5.5 kHz spectral window, 20 s relaxation delay, 3.0 s acquisition time and a 12.5 μs (90°) pulse width, under gated 19F (CHIRP, γBF/2π = 17.9 kHz) decoupling; 10 τ delays ranging from 0.0625 to 32.0 s were used. Data were fitted to an exponential function using a three parameter fit. Acquisition of 2D-NMR Spectra. The one-bond (1JCF) and twobond (2JCF) 19F{13C} gHSQC experiments were performed with a previously described pulse sequence.28 The 19F dimension had 16.7 kHz spectral windows in both one-bond and two-bond experiments; the 13C dimension had spectral windows of 9 kHz and 19 kHz in the one-bond and two-bond experiments, respectively. The following parameters were used for the one-bond experiment: 0.06 s acquisition time, 1.0 s relaxation delay, and 90° pulse widths of 9.8 and 12.9 μs for 19 F and 13C, respectively. A total of 16 transients were averaged for each 2 × 320 increments using the States method of phase sensitive detection.29 The following parameters were used for the two-bond experiment: 0.1 s acquisition time, 1.0 s relaxation delay, and 90° pulse widths of 9.8 and 12.9 μs for 19F and 13C, respectively. A total of 32 transients were averaged for each 2 × 384 increments using the States method of phase sensitive detection. WURST-4030 decoupling (γBC/ 2π = 12.8 kHz) was used to decouple 13C during the acquisition time. Delays were optimized depending on the couplings of interest. The delay, Δ = 1/4JCF, was set to 0.96 ms (1JCF = 260) for a one-bond 19 13 F{ C} correlation experiment, and to 8.3 ms (2JCF = 30) for a twobond 19F{13C} correlation experiment. The data were zero-filled to a 4096 × 4096 data matrix and weighted with sinebell and shifted sinebell functions prior to Fourier transformation. The 19F−19F gDQCOSY (gradient double quantum filtered COSY) experiment was performed with the standard Varian pulse sequence, using continuous 1H WALTZ-16 decoupling (γBH/2π = 2.7 kHz). The data were collected with a 16.7 kHz spectral window, 1.0 s relaxation delay and 9.8 μs (90°) pulse width. Eight transients were averaged for each 2 × 512 increments using the States method of phase sensitive detection in the f1 dimension. Processing was done with sinebell and shifted sinebell weighting functions and zero-filling to a 4096 × 4096 data matrix prior to Fourier transformation. The 19F{1H} gHETCOR (heteronuclear correlation) experiment was performed using the standard Varian pulse sequence with an 19F spectral window of 16.7 kHz, an 1H spectral window of 4 kHz, a 1.0 s relaxation delay, a 0.15 s acquisition time with gated 1H WALTZ-16 decoupling (γBH/2π = 2.7 kHz), and 90° pulse widths of 9.8 and 12 μs for 19F and 1H, respectively. Eight transients were averaged for each 2 × 256 increments in the f1 dimension. The data were zero-filled to a matrix of 4096 × 2048 points and processed with sinebell and shifted sinebell weighting functions prior to Fourier transformation. The gHETCOR spectrum had many weak peaks hidden by the tails of

also provides the linkage of the end groups to the main chain of the polymer. NMR can also give an excellent quantitative measure of the number of such end groups. While their work did not investigate chain branching, Wormald et al.18 used 19F and 1H 2D-NMR to investigate chain-end resonances of a vinylidene fluoride telomer (VDFT) with MW of about 1200 Da. Pianca et al.,15 have reviewed the characterization of end groups in some fluoropolymers including PVDF. Russo et al.19 characterized low MW PVDF with 1D-NMR, their chain-end assignments were solely based on their 1D-NMR, from which they correlated the polymer microstructure to the different reaction pathways that might occur during the polymerization. No evidence for branching was seen in their spectra of PVDF. Elucidation of the reaction process and mechanism of VDF−methanol telomerization using different initiators was carried out by Duc et al.20 Another study on PVDF that is notable is that of Guiot et al.21 who used 19 F and 1H NMR spectroscopy to investigate the microstructure and reaction mechanism for the preparation of PVDF. This study uses multidimensional NMR techniques to assign the resonances of the last three monomer units of various chain-end structures, from both SCB’s and LCB’s, in a high MW PVDF. In most previous reports, only 1D-NMR has been used to assign the last monomer or last two monomers of the chain, and the resonances from SCB’s are not explicitly identified. This work describes the use of T1 relaxation, 2D DOSY and other 2D-NMR techniques to study chain-end structures, SCB’s, and LCB’s in PVDF.

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EXPERIMENTAL SECTION

Materials were used as received. Acetone-d6 (99.9%), was purchased from Cambridge Isotope Laboratories Inc. Trichlorofluoromethane (CFCl3, 99.5%), used as internal standard for 19F NMR and TMS (99.9%), used as internal standard for 1H and 13C NMR, were purchased from Sigma-Aldrich. Most commercial PVDF is prepared with an organic peroxide initiator22 for color stability,14 however, the PVDF characterized in this work was prepared with a persulfate-initiator to simplify end group analysis. The details concerning the polymerization procedure used are found in ref 23. Samples for NMR analyses were prepared by dissolving 50 mg of the PVDF in about 700 μL of the acetone-d6 solvent and heating in a sonicator to about 45−50 °C to ensure complete dissolution. The solution was then transferred into a 5 mm NMR tube for NMR studies. The PVDF sometimes crystallizes from solution with time; therefore the temperature is often raised to about 45−50 °C in a sonicator to redissolve all the crystals prior to NMR analyses at 30 °C. Instrumentation. All the NMR spectra were acquired on a Varian Direct-Drive 500 MHz spectrometer equipped with VnmrJ 2.2D software, five broad-band rf channels and a 5 mm 1H/19F/13C triple resonance pulse field gradient (PFG) probe. The specially made probe from Varian is suitable for fluoropolymers as it has a minimum of fluorine-containing materials near the detection coil to minimize interference from background signals that are common in standard probes. It has a single channel, doubly tuned to 1H and 19F with the capability of producing very short 90° pulse widths. A duplexer was used to combine the signals from the 1H and 19F rf channels and direct them through the dual-tuned 1H/19F high frequency channel of the probe. The detected 1H or 19F signals returning from the probe are separated by the duplexer and the signal of interest (1H or 19F) is directed to the receiver. Acquisition of 1D-NMR Spectra. The 19F 1D-NMR spectra were collected with a 16.7 kHz spectral window with inverse gated 1H decoupling (to suppress NOE’s in order to retain quantitative accuracy) using WALTZ-16 modulation (γBH/2π = 2.7 kHz).24,25 For the purpose of quantitation, a 20.0 s relaxation delay, a 1.0 s 5502

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Scheme 1. Reaction Pathways for the Homopolymerization of VDF using an Initiator I2

increments in the f1 dimension. The FID was weighted with sinebell and shifted sinebell weighting functions in both dimensions and zerofilled to a matrix of 8192 × 512 before Fourier transformation. The 2D DOSY experiments were performed with the bipolar gradient pulse pair longitudinal eddy-current delay (Dbppled) pulse sequence.31 The temperature of the probe was calibrated with ethylene glycol to obtain exact temperatures for the measurement. The gradient strengths were calibrated, and a correction factor was calculated with a doped D2O standard. The correction factor obtained gave diffusion coefficients of 1.901 × 10−9 m2/s and 2.995 × 10−9 m2/s at 25 and 40 °C, respectively for the Varian doped D2O standard sample. The 19F 2D DOSY was performed with a spectral window of 17 kHz, 5.0 s relaxation delay, 1.0 s acquisition time, 64 transients and a 10 μs (90°) pulse width. The diffusion delay (del) was set to 0.1 s and the eddy-current storage delay (delst) was set to 0.005 s. Continuous 1 H WALTZ-16 decoupling (γBH/2π = 2.7 kHz) was applied during the entire length of the experiment; 14 spectra with gradient amplitudes nonlinearly spaced from 0.0191 to 0.5445 T/m were used. The 1H DOSY was performed with 3.6 kHz spectral window, 5.0

strong peaks. The weighting functions were adjusted differently for each region depending on the peaks of interest, in order to suppress the signals from strong peaks and to optimally resolve the weak cross peaks of interest. The 19F−19F selective COSY experiment was performed using the pulse sequence previously described.28 The following parameters were used: 1.0 s relaxation delay, 0.06 s acquisition time; four transients were averaged for each 2 × 256 increments in the f1 dimension. The pulse widths used for the f1 and f 2 dimensions were selective for the desired spectral windows. The FID was weighted with sinebell and shifted sinebell weighting functions in both dimensions, and zero-filled to a matrix of 2048 × 2048 points before Fourier transformation. Continuous 1H WALTZ-16 decoupling (γBH/2π = 2.7 kHz) was applied to decouple protons. The 19F homonuclear 2D-J experiment had a spectral window of 8 kHz in the 19F dimension and 450 Hz in the f1 (J) dimension. It was performed with 9.8 μs (90°) pulse width, 1.0 s relaxation delay, 0.2 s acquisition time, with continuous 1H WALTZ-16 decoupling (γBH/2π = 2.7 kHz). A total of 32 transients were averaged for each 2 × 64 5503

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s relaxation delay, 2.0 s acquisition time, 64 transients, and an 8.6 μs (90°) pulse width using continuous 19F (CHIRP, γBF /2π= 17.9 kHz) decoupling. The diffusion delay and the eddy-current storage delay were set to 0.05 and 0.005 s, respectively; 13 spectra with gradient amplitudes nonlinearly spaced from 0.0191 to 0.5445 T/m were used. Data were processed using the standard DOSY processing software in Varian’s VnmrJ 2.2D.

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RESULTS AND DISCUSSION Structure and Nomenclature. The homopolymerization of VDF through free radical polymerization involves several reactions. Scheme 1 shows some reactions that occur during this process. The initiator can attack the VDF monomer from either the CH2 or CF2 end. However, the radical centered on the CF2 carbon is more stable than one centered on the CH2 carbon, therefore VDF usually polymerize in such a way that the free radical is located on the CF2 group. A growing PVDF chain favors the addition of a VDF monomer in a normal head−tail fashion; however, there is a possibility for a monomer to add to a growing chain by an inverse addition. Chain-ends and branches will contain a mixture of structures from various permutations of normal and inverse addition. At the initiation end of the polymer, inverse addition transiently produces a HOCF2CH2− chain-end which eliminates HF and hydrolyzes to HOOCCH2− chain-ends, which were not detected. Chain branching can also occur in different ways, such as through backbiting reactions. Three possibilities are shown in Scheme 1: (1) inversion and 6-membered ring transition state, (2) 5-membered ring transition state, and (3) 7-membered ring transition state. Branching can also occur by chain transfer to polymer, and by incorporation of an unsaturated polymer chain-end into the polymer. Termination may occur through coupling of two radicals or by a disproportionation reaction. Termination by chain transfer to solvent is absent in the polymer studied here, since the polymer was prepared under solvent-less conditions The designation 0 and 2 are used in this paper, where 0 and 2 represents CH2 and CF2, respectively, as is used in some literature on PVDF.32,33 The hypothetical structure of PVDF given in structure 1 below, shows both a SCB and a chain-end from chain transfer, and also illustrates the 0/2 nomenclature used. The 19F resonances are designated with upper case letters, the label K (which was arbitrarily chosen) designates the fluorines of CF2 groups in long sequences derived from normal head−tail addition. Labels A−F designate 19F resonances/ atoms of CF2 groups in sequences with a single inversion in the chain; labels with a single prime (e.g., A′, B′, C′, ...) designate 19 F atoms of CF2 groups in sequences from a double inversion. Fluorines labeled with double primes (e.g., A″, B″, C″, ...) designate signals from chain-end structures, and those with triple primes (e.g., A‴, B‴, C‴, ...) are 19F resonances suspected to arise from either long chain branching or short chain branching species. 13C resonances are labeled with lower case letters, where labels a−f designate carbons bound to fluorines, and t−z are carbons without attached fluorines (e.g., CH2 carbons). 1H resonances/atoms are labeled with numbers (see Supporting Information). 2D-NMR Methods Used for Resonance Assignments. Several different 2D-NMR techniques were used to obtain the resonance assignments of the chain-ends and branching structures from VDF polymerization. The different techniques complemented one another to give unambiguous assignment of the chain-end and branching resonances.

For example, from a hypothetical PVDF chain-end structure 2, the gDQCOSY experiment will show five-, four-, and threebond correlations between the resonances of FA−FC, FA−FB, and FB−FC, respectively. The 19F{1H} gHETCOR will give 3JFH correlations between resonances FA−HA, FA−HB, FB−HB, and FC−HC. The fact that both FA and FB show 3JFH correlations to resonance HB indicate that 19F resonances FA and FB are correlated through four-bonds. The 19F{13C} gHSQC experiment will show 1- and 2-bond correlations. For instance, in structure 2, resonances FA and FB will both show 2-bond gHSQC correlations to the same carbon. This also confirms that the COSY correlation between resonances FA−FB is through four-bonds.

Assignment of Chain-End Resonances in PVDF with 1D- and 2D-NMR. Figure 1 shows the full 19F 1D-NMR spectrum of PVDF in acetone-d6. The labeling described above has been used. The detailed assignments of the backbone resonances (A−F and A′−F′) can be found in ref 23. The splitting patterns observed from the 1H decoupled and 1 H coupled 1D spectra can be used to make tentative assignments prior to using 2D-NMR. Expansions from the 470 MHz 19F 1D-NMR spectrum of PVDF are given in Figure 2. The 19F resonance F″ appears as a triplet in the broadband 1 H decoupled spectrum (Figure 2b), with a coupling constant of ca. 10 Hz (indicative of 4JFF). In the 1H coupled spectrum (Figure 2a), this resonance appears as a broad peak with unresolved couplings. Information from the 19F 1D-NMR spectrum shows that 19F resonance F″ has a single 4JFF correlation; thus its other coupled neighbors must be protons. This 4JFF correlation is seen in the gDQCOSY spectrum (Figure 3) and the selective COSY spectrum (Supporting Information, Figure S5). 19 F resonance F″ shows two 19F−1H correlations in the 19 1 F{ H} gHETCOR spectrum (Figure 4); one to a 1H 5504

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Figure 1. 470 MHz 19F 1D-NMR spectrum of PVDF with 1H gated decoupling. Spectral inserts are selected regions from 1H decoupled spectrum with the K peak presaturated and expansion around the 220 region. The saturation reveals peaks hidden near the base of the K peak.

Figure 4. Expansion around peaks from the 470 MHz gHETCOR spectrum of PVDF.

19

F{1H}

groups, while the CH2 resonance less shielded at 3.78 ppm is consistent with that of a CH2 group bound to a more electronegative group X as in structure 3. The most likely electronegative atom would be an oxygen atom from the initiator decomposition (X = OH). The possibility of X being a sulfate ester is ruled out since the ester bond undergoes hydrolysis to give an alcohol end group.15 The persulfate initiated polymerization can produce −CF2CH2OSO3− and −CH2CF2OSO3− chain-ends during the initiation process.15 However, according Pianca et al.15 the ester bonds in both end groups undergo hydrolysis as they are quite labile. The ester bond in the former gives a −CF2CH2−OH end group, while hydrolysis of the latter gives a carboxylic acid end group −CH 2 CO 2 H. The −CF2CH2−OH end groups are detected and assigned as seen above. Resonances of carboxylic acid end groups are not observed in our spectra.

Figure 2. Expansions from the 470 MHz 19F 1D-NMR spectrum of PVDF: (a) 1H coupled; (b) 1H decoupled.

resonance near 2.81 ppm (labeled 12), and the other to a 1H resonance near 3.78 ppm (labeled 19). The 1H resonance near 2.81 ppm is consistent with a CH2 group between two CF2 Figure 5 shows an expansion around the region containing F resonance F″ from the two-bond 19F{13C} gHSQC spectrum. The figure shows that resonance F″ has two-bond correlations to two different CH2 13C resonances. The less shielded correlation near 64 ppm is consistent with a CH2 group attached to an oxygen atom of an alcohol. The 2D experiments establish that the correlation between resonances F″ and A″ is a 4JFF correlation as speculated from the 19F 1DNMR. The carbon labeling is found in the supporting documentation. The labeling was done taking into consideration the 13C resonances identified from the 19F{13C} gHSQC 2D-NMR spectrum. The 19F resonance labeled H″ shows a very weak correlation to 19F resonance G″ in the gDQCOSY spectrum (Figure 3). This correlation is assigned as a three-bond correlation for the following reasons. First, the positions of the two signals suggest they are both more likely to be found in 022 sequences, in which case they would be correlated through a weak 3JFF correlation. Second, the signal G″ has a correlation with the proton resonance labeled 4 in the 19F{1H} gHETCOR 19

Figure 3. 470 MHz 19F−19F gDQCOSY spectrum of PVDF with continuous 1H decoupling. 5505

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Figure 5. Expansion around correlations to 19F resonance F″ from the 470 MHz 19F{13C} 2-bond gHSQC spectrum of PVDF.

spectrum, Figure 4. The 1H resonance labeled 4 is a singlet with 19 F decoupling, and a triplet in the 19F coupled spectrum. It is assigned as a methyl group based on its position in the 1H 1DNMR spectrum. 19F resonance G″ does not show other 19F−1H correlations, suggesting that its other nearest neighbor is not a CH2 group; hence, its correlation with 19F resonance H″ is a three-bond 19F−19F correlation. From the 19F 1D-NMR spectrum in Figure 2, 19F resonance G″ shows a very weak residual coupling in the spectrum obtained with 1H decoupling, ruling out 4JFF coupling with 19F resonance H″, which would be relatively large. From the foregoing discussion, it is evident that 19 F resonance G″ is an end-group signal having a correlation with a terminal methyl group, as in structure 4.

Figure 6. Expansions from the 500 MHz 1H and 470 MHz 19F 1DNMR spectra of PVDF: (a) 1H NMR spectrum without 19F decoupling; (b) 1H NMR spectrum with 19F decoupling; (c) 19F NMR spectrum without 1H decoupling; (d) 19F NMR spectrum with 1 H decoupling.

resonance J″, respectively, (see structure 5). 1H resonance 23 shows a triplet splitting with a coupling constant of ca. 4.5 Hz in the 1H 1D-NMR spectrum obtained with 19F decoupling (Figure 6b). The splitting pattern and the coupling constant of ca. 4.5 Hz are consistent with a 3JHH coupling from a CH2 group (1H resonance 10). The 19F 1D-NMR spectrum in Figure 6d shows that resonance J″ is a triplet when obtained with 1H decoupling (coupling from 19F resonance E″); however, resonance J″ shows a large doublet splitting (coupling from 1H resonance 23) with unresolved couplings in the 19F 1D-NMR spectrum obtained without 1H decoupling (Figure 6c). From the foregoing discussion, 1H resonance 23 is consistent with that of a methine proton in the chain-end structure − CH2CF2H.

Results from the selective COSY experiment (Supporting Information, Figure S5b) indicate that 19F resonance H″ has a correlation with 19F resonance B″. The correlation is consistent with a four-bond 19F−19F coupling based on its positions in the 19 F 1D-NMR spectral regions (19F resonance B″ is in the 020 region, while 19F resonance H″ is in the 022 region). Furthermore, a three-bond correlation between resonances B″ and H″ is not possible as that would require there be three consecutive CF2 groups along the backbone; this is not possible in PVDF. Detection of a five-bond correlation is highly unlikely from such a low occurrence structure having a very weak signal intensity in the 19F 1D-NMR spectrum. Referring to structure 5, 19F resonance J″ is correlated with 19 F resonance E″ in the gDQCOSY spectrum (Figure 3). This cross-peak is assigned to a four-bond correlation as both resonances E″ and J″ show correlations to the 1H resonance labeled 10 (see Supporting Information, Figure S3a and S3e). 19 F resonance J″ shows a correlation with the 1H resonance 23 in the 19F{1H} gHETCOR spectrum (Supporting Information, Figure S6). The 1H NMR signal near 6.28 ppm (labeled 23, Supporting Information, Figure S1) in Figure 6a shows a triplet of triplets splitting in the 1H 1D-NMR spectrum obtained without 19F decoupling (with coupling constants of ca. 4.5 and 55 Hz). The coupling constants of 4.5 and 55 Hz are consistent with 3JHH and 2JFH couplings from 1H resonance 10 and 19F

19 F resonance I″, like J″, shows a large doublet splitting from a methine chain-end (Figure 6c), with other unresolved coupling in the 1H coupled spectrum. Resonance I″, like J″, is from a CF2 group in a chain-end structure −CH2CF2H (6), as it also shows a correlation to 1H resonance 23 (Supporting Information, Figure S6). Both I″ and J″ have similar 13C chemical shifts in the one-bond gHSQC spectrum (Figure 7). They also show correlations to similar 1H resonances in the 19F−1H gHETCOR spectrum (Figure 4 and Supporting Information, Figures S3e and S6).

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Figure 7. Expansion around correlations in the 470 MHz 19F{13C} 1bond gHSQC spectrum of PVDF obtained with 1H decoupling.

Figure 9. Expansion around the region containing resonances of structures from normal addition in the 470 MHz 19F−19F gDQCOSY spectrum of PVDF.

From Figure 8, showing an expansion around peak containing region in the 19F−19F selective COSY spectrum of (T1) measurements were used to identify the chain-end resonances from PVDF. Chain-end groups of polymers have shorter correlation times as a result of their faster motion when compared to the motion of the rest of the backbone atoms in a polymer. As a result, the average T1’s for resonances from chain-ends are longer when compared with the average T1’s of units in the polymer backbone. A summary of some relevant T1 data is presented in Table 1. T1 measurements at a variety of temperatures between 22 and 50 °C (see Table 1 and Supporting Information) showed that at ambient temperature (22 °C), motion of the backbone atoms is near the T1 minimum and that motion of the chain-end atoms are on the short correlation time side (“extreme narrowing region”) of the T1 minimum. Thus, from results obtained from the T1 studies, resonances from chain-ends have longer T1’s and can clearly be distinguished from those of the backbone. For instance, at 22 °C resonances F″, G″ and J″ attributed to resonances from chain-ends, have T1’s of 1.06, 1.18, and 1.96 s, respectively, whereas resonances like C, D and E have T1’s of 0.38, 0.32, and 0.36 s, respectively (thus the latter are from backbone structures). The backbone resonance assignments have been proven in a complementary paper using various 1D- and 2DNMR experiments (Table 1).23 With the exception of resonance G‴, the T1’s of the 19F resonances labeled with triple primes, e.g. F‴ are relatively short when compared with other backbone resonances. From these results, it can be inferred that these resonances are somewhat different from the other backbone resonances. These resonances were assigned to branching structures based on the 2D-NMR experiments (vide inf ra). Because of the lower intensities of such signals, the errors from their T1 measurements were higher. The T1 values from these branching structures change only slightly with increasing temperature; their values actually decreased slightly in going from 40 to 50 °C for resonance A‴ (see Supporting Information). This is consistent with T1 on the long correlation time side of the T1 minimum. The motion at the branch point in a polymer chain is slower than that of a straight chain. This is expected, as a branch will restrict C−C bond rotation along the backbone for those groups near branch site.

Figure 8. Expansion around peak containing region from 470 MHz 19 F−19F selective COSY spectrum of PVDF. The H−T peak at −91.23 ppm in the 19F 1D-NMR plotted along the F1 axis is presaturated.

PVDF, it is seen that 19F resonance I″ is correlated with 19F resonance D″. This correlation is assigned as a 4JFF correlation based on the chemical shifts of the two resonances. It must be noted that the shift of 19F resonance D″ is identical to that of resonance C″ in the 19F 1D-NMR spectrum (Figure 1). 19 F resonance E″ is correlated to 19F resonance C″ (Figure 9). Both 19F resonances are in the 020 region; therefore, the most plausible way they could be correlated is through a 4JFF correlation as in structure 5. No 19F−1H correlations are seen for resonance C″ in the gHETCOR spectrum, this might be due to its position in the 19F spectrum. The huge signals near resonance C″ are likely to mask its 19F−1H correlations, especially if the 1H chemical shifts of the adjacent methylenes are similar to the 1H chemical shift arising from head−tail addition. Identification of Chain-End Resonances by 19F Spin− Lattice Relaxation (T1) Analysis. Spin−lattice relaxation 5507

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Table 1. 19F and 1H NMR Assignments for PVDF in Acetone-d6 19

F resonances

peak label

δF (ppm)

K A B C D E F A′ B′ C′ D′ E′ F′ A″ B″ C″ D″ E″ F″ G″ H″ I″ J″ A‴

−91.23 −91.14 −91.14 −91.67 −95.05 −113.55 −115.83 −91.28 −95.36 −113.80 −115.50 −115.67 −115.95 −90.93 −90.97 −91.40 −91.42 −92.33 −104.55 −107.60 −114.00 −114.55 −114.69 −90.49

C‴ C‴ D‴ F‴ F‴ G‴

−93.93 −94.47 −94.58 −97.96 −98.51 −107.35

1

recovery spin−lattice relaxation (T1) technique is used to identify chain-end signals. However, the DOSY experiment can also be used; signals from chain-ends have higher apparent diffusion coefficients compared to signals from the backbone when the data is fit to a single exponential. The 19F 2D DOSY NMR spectrum of PVDF is shown in Figure 10. From this spectrum and the calculated diffusion

H resonances

peak label

δH (ppm)

14 16 14 13 6 17 6

2.94 2.99 2.94 2.93 2.35 3.02 2.35

17 17 6 8 8 12

3.02 3.02 2.35 2.41 2.41 2.81

11 10 19 4

2.76 2.75 3.76 1.79

23 23

6.28 6.28

assignmentsa 202020202 0202022 202020202 002020202 2002020 202022002 020220020 NA 2002022 002022002 200220020 200220022 020220022 In-02020−02022CH3 −020202-H 0202-H −020202-H In-02020−02022CH3 −02022CH3 −0202-H −020202-H −0202− CH(02-)2see text NA see text

3

1.78

−022CH3

19

F T1b,c

D × 10−10 m2/sc,d

0.38 0.36 0.36 0.38 0.32 0.36 0.34 0.37 0.30 0.36 0.33 0.34 0.34 0.45 NM NM NM 0.87 1.06 1.18 0.60 1.12 1.96 0.27

0.36 NM NM 0.34 0.35 0.33 0.35 NM 0.32 0.31 0.33 0.33 0.31 NM NM NM NM 0.49 0.89 0.37 0.33 0.46 0.74 0.29

NM NM 0.25 0.23 0.23 0.54

NM NM 0.33 NM NM 0.33

NA denotes resonances not assigned. bT1 measurements at 22 °C. c NM denotes value not measured. dMeasured by 19F NMR. a

The T1 value obtained for 19F resonance G‴ is longer when compared to those obtained for the backbone resonances, but shorter when compared to the T1’s from resonances known to come from chain-end structures. The T1 value, the chemical shift, and the correlations seen for resonance G‴ showed that it is from a chain-end group. However, the relatively short T1 value of resonance G‴ (compared to other chain-ends, e.g., G″) suggest that it is a different kind of chain-end. Both resonances G″ and G‴ are from −022CH3 chain-ends, the relatively short T1 value for resonance G‴ suggests that it might be from a SCB (vide inf ra). The motion of chain-ends from SCB’s is somewhat slower than the motion of other chain-ends as rotation along the branch is restricted. Identification of Chain-Ends with Diffusion Ordered Spectroscopy (DOSY) NMR. It is common knowledge that the rate of diffusion of a molecule depends on its size and shape. Small molecules are known to diffuse faster compared to large molecules. The dependence of diffusion on molecular size provides a way to distinguish the chain-ends of polymers from the resonances of the polymer backbone. Diffusion ordered spectroscopy (DOSY) NMR has been used to determine the MW distributions of polydispered polymers,34 and the polydispersity index of polymers.35 Traditionally, the inversion

Figure 10. 470 MHz 19F{1H} 2D DOSY NMR spectrum of PVDF obtained under continuous 1H decoupling.

coefficients presented in Table 1, resonances from chain-ends are shown to have higher apparent diffusion coefficients compared to those of the backbone signals. Most of the chain-ends are from low MW molecules; their average diffusion coefficient is weighted heavily by the fast diffusion of the low MW polymer chains in the mixture (assuming PDI is not low). Backbone resonances and chain-ends from SCB’s on the other hand are mostly from high MW molecules. Their average diffusion coefficients are therefore weighted heavily by the slow diffusion of high MW polymer chains in the mixture. 5508

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For instance resonances I″ and J″ are both from the −0202− H chain-end sequences. The diffusion coefficient of J″ is higher than that of I″. Resonance I″ is proposed to be a chain-end from a SCB, while resonance J″ is a “true” chain-end. Results from the T1 analysis (Table 1) and from the 2D DOSY experiment complement one another in terms of identifying the chain-end resonances from PVDF. An anomaly from the 19F DOSY spectrum is that resonances G″ and G‴ are both from the chain-end structure −022−CH3, but with resonance G‴ belonging to a SCB showing similar diffusion coefficients (Figure 10 and Table 1). The diffusion coefficient of resonance G‴ from a SCB is expected to be heavily weighted by the slow diffusion of high MW polymer chains in the mixture (this is what is seen for resonance G‴ in the DOSY spectrum). However, resonance G″ from a “true” chain-end also shows a similar slow diffusion coefficient to that of the backbone resonances instead of showing higher apparent diffusion coefficient. Furthermore, resonance E″ from the same structure as J″ has a little lower diffusion coefficient in the DOSY spectrum. Work is currently underway with copolymers made from VDF and other comonomers that may shed more light on these results, which may be due to effects of overlapping resonances and/or low signal-to-noise of these resonances. The 1H 2D DOSY spectrum given in the Supporting Information show that some of the diffusion coefficients are in agreement with those obtained from the 19F DOSY spectrum, while other do not agree. For most of the 1H signals from chain-ends structures, apart from being very weak, they are close to strong signals from backbone resonances; their diffusion coefficients might therefore be inaccurate due to overlap effects. Assignment of Branching Chains. Branching structures produced during ethylene polymerization have been studied extensively as early as the 1950s.36 The different types of branching structures from both SCBs and LCBs, the possible mechanisms for their formation, and the amount of these branches have all been subjects of discussion.37−40 In the case of PVDF and PVDF-based co- and terpolymers, the production of branches during polymerization has been mentioned,15,16,41,42 although in most cases no concrete evidence has been offered for the presence of these structures.15,41 Wormald et al.,41 suggested that the peak near −107 ppm in their spectrum was a result of chain branching. Pianca et al.,15 speculated that the presence of −CF2H and −CH3 end groups in PVDF are a result of short chain branches produced by an intramolecular 1−5 hydrogen shift. However, in both cases, no explicit NMR data were used to substantiate these conclusions. Hedhli et al.,16 studied randomly branched PVDF and suggested that in their case, the mechanism for branching is through chain transfer; the abstraction of hydrogen from the polymer backbone by a radical. They suggested that the radical in their case could be a persulfate radical or a radical generated by Luperox 230. Their mechanism was supported with 13C 1DNMR, 13C DEPT and 1H{13C} HSQC 2D-NMR data. While they did not rule out the possibility of chain branching from backbiting reactions, no evidence for a backbiting reaction was found in their NMR data. Figure 11 shows a region from the 19F−19F gDQCOSY spectrum of PVDF, which gives NMR evidence supporting the presence of chain branching. The presence of a methine proton (a branching point) creates a stereogenic center in the polymer chain; CF2 fluorines and CH2 protons close to this center will be diastereotopic and will each produce two different

Figure 11. Expansion from the 470 MHz 19F−19F gDQCOSY NMR spectrum of PVDF.

resonances. 19F resonances C‴ and F‴ have characteristics consistent with such fluorines; the expected 2JFF correlation between the two resonances is observed (Figure 11), with a coupling constant of ca. 265 Hz. The other correlation between resonances C‴ and F‴ is assigned as a 4JFF coupling over the methine group (structure 7). 19F resonance F‴ also show correlation to 19F resonance A‴ (Figure 11 and structure 7), which is attributed to 4JFF coupling. On the basis of these correlations, structure 7 is proposed as one type of structure from LCB in the PVDF studied. One mechanism for the production of this structure is hydrogen abstraction from the polymer backbone by a growing chain, followed by propagation of a new chain from the branch site. Resonances B‴ and E‴ also have characteristics of diastereotopic fluorines, that results from the creation of a stereogenic center in the polymer chain. The structure of resonances B‴ and E‴ is speculated to be similar to that of resonances C‴ and F‴ but with a monomer inversion. The signal intensities of C‴/F‴ relative to B‴/E‴ are consistent with these assignments. The patterns these resonances produce in the 19F homonuclear 2D-J spectrum (Supporting Information, Figure S7) are also consistent with these assignments.

The 19F resonance labeled G‴ shows similar correlations to those exhibited by resonance G″, including one-bond and twobond 19F−13C gHSQC correlations (see Supporting Information, Figure S4), and a 19F−1H gHETCOR correlation to a CH3 proton (see Figure 4). 19F resonance G‴ is therefore an end group signal of a −2CH3 group. Although not detected, resonance G‴ is suspected to have a three-bond 19F−19F COSY 5509

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correlation to another 19F in the 022 region, resulting in possible structure −022−CH3, just as resonance G″ shows a three-bond COSY correlation with resonance H″. Resonance G‴ is speculated to be from the end group of a SCB for the following reasons. Its T1 is intermediate between that of an end group and that of a polymer backbone unit. At 30 °C, the T1 of resonances G″ and G‴ are 1.25 and 0.60 s, respectively. While both are shown to be chain-ends from the correlations seen in the 2D-NMR experiments, the difference between their T1 values leads to the proposed SCB structure for resonance G‴. The close proximity of the groups assigned to resonance G‴ with the backbone gives these groups mobility similar to that of a backbone structure, hence their shorter T1 value. This postulate is further supported by the 1H T1 data shown in the Supporting Information. 1H resonance 3 has a T1 of 1.29 s at 30 °C, while 1H resonance 4 has a T1 of 1.85 s at 30 °C. 1H resonance 3 and 19F resonance G‴ are correlated, while 1H resonance 4 and 19F resonance G″ are correlated (Figure 4). The shorter T1 value for 1H resonance 3 compared to 1H resonance 4, gives credence to the speculation that 19F resonance G‴ is a chain-end from a SCB. The 2D DOSY discussed above also supports the proposed assignment of resonance G‴. Resonance G‴ from a SCB is primarily from high MW molecules, its average diffusion coefficient is therefore weighted heavily by the slow diffusion of high MW polymer chains in the mixture. Hence the diffusion coefficient of G‴ is smaller than that of G″. However, as discussed above, the diffusion coefficient of G″ is lower than expected for a chainend signal; work is underway with copolymers that may shed more light on this result. As discussed above, 19F resonance I″, like resonance J″, is a −0202−H end group. However, from their T1 values, it is seen that the T1 of resonance J″ is almost twice that of resonance I″ at every temperature (e.g., at 30 °C, resonances I″ and J″ have T1’s of 1.28 and 2.19 s, respectively), therefore, as far as relaxation is concerned, I” behaves like a SCB and J” behaves like a polymer chain-end (LCB). Again from the 19F 2D DOSY spectrum, resonance I″ behaves like a backbone resonance (SCB). Comparing the correlations of resonances J″ and I″, resonance J″ is correlated with E″, and resonance E″ is in turn correlated with resonance C″. On the other hand, resonance I″ shows COSY correlation with resonance D″; D″ has no additional COSY correlations. On the basis of these data, structure 8 or 9 is proposed as the source of resonances D” and I”. 19F resonance E″ has a chemical shift of −92.33 ppm, while resonance D″ has a chemical shift of −91.42 ppm (the absolute difference between the two shifts is 0.91 ppm). Referring to structure 8 (a proposed SCB structure), it involves an inversion of a VDF monomer (at the branch site) and a backbiting reaction through a 6-membered ring transition state. This proposed structure is consistent with the predicted chemical shifts using the empirical methods developed by Murasheva et al.43 The calculated chemical shifts using their method agree with experimental results within ±1 ppm. If the structure of the branch point were that predicted by a 5-membered ring transition state, the chemical shift of D″ would be expected to occur at about −9 ppm relative to the chemical shift of E″ in structure 5. The relatively small shift difference between the chemical shifts of resonances E″ and D″ is not consistent with a structure formed by backbiting through a 5-membered ring transition state. Again 5-membered ring transition states are strained and less probable. The difference between the shifts of resonances D″ and E″ (0.91 ppm) is not large enough to rule

out a branching structure 9 formed through a 7-membered ring transition state. Therefore, either 6-membered ring transition state with monomer inversion (structure 8) or 7-membered ring transition state (structure 9) backbiting reactions is responsible for SCB in the polymer. The presence of both SCB and LCB structures in the PVDF studied show that several different mechanisms, including backbiting reactions are responsible for chain branching in PVDF and VDF-based fluoropolymers/fluoroelastomers.

Estimating the Numbers of Chain-Ends and Branching Units. Several assumptions were made in performing the calculations of the percentage of the chain-ends and branching structures in the polymer studied. One assumption is that there is one fluorine atom for each carbon atom in the polymer, since a monomer unit has two carbons and two fluorines. The same assumption is made for the number of hydrogens. This assumption is not entirely true as the formation of certain end groups like −CF2CH2CH2COOH (resonances from this structure were not detected in this work) involves the conversion of a CF2 group into a −COOH group and hence loss of two fluorine atoms. Other assumptions are that for every CF2 group there is one monomer, and for every CH2 group there is one monomer. Given the small number of end groups in the polymers studied, the calculations will have a small systematic relative error on the order of 0.1%. On the basis of these assumptions, the numbers of the various chain-ends and branching units per 1000 carbons were calculated (Table 2). Table 2. Concentration of Chain-Ends and Branching Units per 1000 Carbon via 19F NMR peak label

assignment

number/1000 C’s

F‴ F″ G″ G‴ I″ J″ E″

−CH2CF2−CH(CH2CF2−)CF2− HO−CH2CF2CH2− −CF2CF2CH3 −CF2CF2CH3 −CH2CF2CH2CF2H −CH2CF2CH2CF2H −CH2CF2CH2CF2H

1.60 0.82 0.48 0.64 1.24 1.15 1.17

The calculation indicates that there are about 1.6 LCB units for every 1000 carbon atoms. Again the calculation indicates that the −CH2CF2H end group is the most prevalent in the PVDF under study (2.4/1000 C’s relative to all other resonances), followed by the −CF2CH3 end group (1.1/1000 C’s) and the least prevalent end group is the −CH2CF2CH2− OH end group (0.8/1000 C’s). The assumptions made in these calculations are not valid for polymers with low molecular weight, or with a large number of branching units.

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CONCLUSIONS The ability to acquire 1 H, 19F, and 13 C NMR from hydrofluoropolymers provide complementary information for the assignment of resonances, including chain-ends and chain branching. Information from the 19F NMR experiments was extremely helpful as a result of the high sensitivity for NMR detection of 19F nuclei and the large chemical shift dispersion in 19 F NMR spectra. 5510

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on copolymers of vinylidene fluoride (VDF) and hexafluoropropylene (HFP). He will be greatly missed.

The use of the selective COSY experiments enabled the detection of weak signals that could not be detected in the traditional COSY experiments, and also gave a relatively simple spectrum. The selective COSY experiment is particularly useful when dealing with the large 19F spectral window when groups of 19F resonances are well separated from one another. The selective COSY experiment is faster, as fewer t1 increments must be collected to provide 2D-NMR spectra with high digital resolution in the indirectly detected dimension. Two important consequences of this are that (1) high resolution spectra prevent undesirable disappearance of cross-peaks when antiphase multiplet components are not resolved and (2) signal components in congested regions of the spectrum are better resolved. Assignments made from 2D-NMR experiments are supported by results obtained from T1 measurements, DOSY experiments and comparison of the relative integrals of resonances from atoms within each structure, for those signals that could be measured reliably. This work confirms assignments made earlier in the literature using 1D-NMR and empirical chemical shift calculations; it also provides assignment of some resonances not previously assigned. Assignment of the resonances from chain-ends and chain branching structures help in understanding the polymerization mechanism. This knowledge helps guide process changes that can lead to improved products.

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ASSOCIATED CONTENT

S Supporting Information *

Figures showing 1D and 2D-NMR spectra, a calculation of percentage inversion of monomer in PVDF, and tables summarizing 1H and 19F T1 at various temperatures. This material is available free of charge via the Internet at http:// pubs.acs.org/.

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AUTHOR INFORMATION

Notes

The authors declare the following competing financial interest(s):Dupont has provided partial support for this work. The polymers studied are related to some of Dupont's commercial products.

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ACKNOWLEDGMENTS We wish to acknowledge the support of The Ohio Board of Regents and The National Science Foundation (CHE-0341701 and DMR-0414599) for funds used to purchase the NMR instrument used for this work. We thank NSF (DMR-0905120) and E. I. du Pont de Nemours and Company for its support of this work. We wish to thank A. Zens and the probe manufacturing group at Varian Instruments for their effort in providing some of the instrument hardware used in this work. We also wish to thank the staff of the Magnetic Resonance Center at the University of Akron, especially S. Stakleff and V. Dudipala for their help in maintaining the instruments used for this work. The authors would also like to acknowledge A. Moore who prepared the original PVDF samples.

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REFERENCES

DEDICATION

The authors would like to dedicate this paper to Dr. Walter W. Schmiegel, who passed away on April 11, 2012. Walter was a pioneer chemist in studying the fluoroelastomer cross-linking chemistry, and published some of the earliest 19F NMR studies 5511

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