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Poly(methyl vinyl ketone) as a Potential Carbon Fiber Precursor Joseph W. Krumpfer, Elisabeth Giebel, Erik Frank, Alexander Mueller, Lisa-Maria Ackermann, Catarina Nardi Tironi, Georgios Mourgas, Joerg Unold, Markus Klapper, Michael R Buchmeiser, and Klaus Müllen Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b04774 • Publication Date (Web): 17 Dec 2016 Downloaded from http://pubs.acs.org on December 18, 2016

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Chemistry of Materials

Poly(methyl vinyl ketone) as a Potential Carbon Fiber Precursor Joseph W. Krumpfer1,2, Elisabeth Giebel,3 Erik Frank,3 Alexander Müller,3 Lisa-Maria Ackermann,1 Catarina Nardi Tironi,1 Georgios Mourgas,3 Jörg Unold,3 Markus Klapper1, Michael R. Buchmeiser*3,4, and Klaus Müllen*1 1. Max 2

Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany

Department of Chemistry and Physical Sciences, Pace University, 861 Bedford Road, Pleasantville, NY 10570, USA

3.Institut

für Textilchemie and Chemiefasern (ITCF), Körschtalstr. 26, 73770 Denkendorf, Germany

4

Lehrstuhl für Makromolecular Stoffe und Faserchemie, Institut für Polymerchemie, Universität Stuttgart, Pfaffenwaldring 55, 70550 Stuttgart, Germany

[email protected]; [email protected]

Abstract Given their increasing importance in a variety of applications, the preparation of carbon fibers with well-defined chemical structures and innocuous byproducts has garnered a growing interest over the last decade. We report the preparation of medium molecular weight poly(methyl vinyl ketone) (PMVK) as a potential carbon fiber precursor material which can easily undergo carbonization via the well-known, acid-catalyzed Aldol condensation with water as a sole byproduct. Rheological studies further show that PMVK (MW~50 kg/mol) exhibit excellent physical and thermal properties for the spinning of single and multi-filament fibers and easily produce carbon yields of 25% at temperatures

1 ACS Paragon Plus Environment


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as low as 250 °C. Analysis of the carbonized product also suggests a more defect-free structure than commercially available carbon fibers.

Introduction Owing to their excellent mechanical properties, electrical conductivity, wide service temperature range, and chemical stability, carbon fibers (CFs) have attracted a great deal of academic and industrial interest over the last two decades.1-3 CFs are generally comprised of 92% carbon and exhibit tensile strengths up to 7 GPa, good creep resistance, low densities (1.75-2.00 g/mL) and high moduli (E ~ 900 GPa). For these reasons, CFs are widely used as reinforcing agents in light-weight polymer composites for aerospace, construction, medical and sporting goods. More recently, they have begun to play an increasingly important role within energy saving applications, from automobile bodies to wind turbines. 4-6 The quality of carbon fibers is generally considered to be directly dependent on the polymer precursor, as well as spinning and processing conditions. Currently, the most common source for high quality CFs is poly(acrylonitrile) (PAN).1-3, 7-15 However, despite its well-established role as a CF precursor, PAN still has several significant disadvantages, which prevent it from being an ideal material. Acrylonitrile is a toxic monomer, and the resulting polymer exhibits poor solubility in most solvents, necessitating the implementation of comonomers to facilitate fiber processing. Additionally, a major byproduct of this carbonization process is hydrogen cyanide (HCN), which adds additional environmental and safety concerns to the preparation of PAN-derived CFs. Furthermore, the chemistry of the carbonization process is still not fully understood with many potential


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and concomitant mechanisms having been suggested,16-17 which hinders a well-developed design strategy for future carbon fiber precursors. For these reasons, a tremendous effort has focused on finding alternative carbon fiber sources. Many polymers have been promoted as potential replacements for PAN fibers. Lignin6, 18-25 and cellulose26-28 have been suggested as inexpensive, bio-renewable sources, while carbon pitch represents a very cheap alternative, being the byproduct of petroleum refinement. Synthetic polymers,29-31 such as polyethylene, polystyrene and 1,2poly(butadiene), have also been explored. While in most cases these polymers are suitable precursors, they often suffer from processing issues, requiring the aid of plasticizers or other additives which ultimately induce defects in the resulting carbon fiber structure and decrease carbon yields below the benchmark values established by PAN. One class of polymers which has been almost exclusively ignored in the carbon fiber literature is poly(alkyl vinyl ketone)s, particularly poly(methyl vinyl ketone) (PMVK). With the exception of a single patent,32 PMVK has not been explored as a carbon fiber source in the scientific literature, despite previous reports of the crosslinking/cyclization of this polymer via simple, well-known Aldol condensations.33 In fact, the cyclization chemistry resembles, to some extent, the one proposed for PAN. In view of that, turbostratic CF structures as found in PAN-derived CFs can be expected for PMVK, as well. The reluctance with which it has been used as CF-precursor is likely due to the difficulties in obtaining high molecular weight polymers without crosslinking and the lack of readily apparent commercial applications. In fact, the literature on this polymer, which has been known since 1938,34 has been criticized as “sketchy or incomplete in nature.”35 Although there



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have been a number of studies utilizing this polymer over the last 50 years, application of this polymer in any meaningful manner has yet to be achieved. This absence of commercial utility arises from the preparation and physical properties of the homopolymer. The monomer forms an 85 % azeotrope with water,36 photopolymerizes at room temperature in the presence of light,37 but has a rather low free radical polymerization rate.38-39 Additionally, the monomer readily and preferentially swells the homopolymer making it difficult to recover. PMVK is also known to have a headto-tail structure34, 39 and degrade under UV light via polymer chain scission.40 The solid, dry polymer has a reported density of ~1.12 g/mL with a low softening point (30-50 °C) and poor mechanical properties.41 The onset of the thermal degradation of this polymer has been reported as ~200 °C.42 Despite these seemingly undesirable properties, attempts have been made utilizing PMVK in antibacterial coatings,43 electrically conductive films,44-45 polymer blends,46-47 photobiodegradable copolymers,48 encapsulation agents,49 and ATRP macroinitiators.50 Herein, we report the first study on PMVK as a potential carbon fiber precursor material. The preparation of medium molecular weight poly(methyl vinyl ketone) via free radical (FRP) polymerization and the subsequent melt extrusion of single- and multifilament polymer fibers thereof are presented. Thermal and rheological properties of PMVK are shown to be ideal for fiber formation, and the acid-catalyzed Aldol condensation of the side chains provides a chemical process for the formation of ladder structures within the fibers. This is prefaced by early work utilizing polymerized Nazarov reagent, poly(ethyl-3oxo-4-pentenoate) (PE3O4P), which provides the basis for our carbon fiber precursor design philosophy. The chemically and thermally treated fibers are shown to have mass


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residues well within the range of typical carbon fibers. Finally, initial attempts at optimizing a pyrolysis process for these fibers are presented.

Experimental Materials. 3-Buten-2-one (methyl vinyl ketone, MVK) (99%) was purchased from Sigma Aldrich, purified by vacuum distillation at 80 °C and 11 mbar, and stored at -20 °C prior to use. Azobis(isobutyronitrile) (AIBN), purchased from Sigma Aldrich, was purified by recrystallization from methanol prior to use and stored at - 20 °C.

Free Radical Polymerization. A typical free radical polymerization (FRP) was as follows. In a 2 L single-neck round-bottom flask with magnetic stir bar, 1 L anhydrous toluene was degassed with argon for 15 minutes. Cool MVK (150 g, 1.7M) was then added and the solution was bubbled with argon for an additional 20 minutes. The cold solution appeared slightly cloudy but became transparent upon warming to room temperature. Some care should be taken when handling the MVK, as the monomer has a very high vapor pressure and a pungent smell. Following this, 160 mg AIBN (0.16 mol%) was added, the flask was fitted with an argon inlet and connected to a bubbler under argon. The solution was then placed in an oil bath at 60 °C for 48 hours. Afterwards, 10 mL methanol was added to quench the reaction, and unreacted monomer was removed by rotary distillation at 60 °C at 500 mbar. The polymer was dried by slow reduction of the pressure to avoid expansion (ballooning) of the polymer. Typical yields of solid polymer ranged between 3555 % with an Mn = ~50 kg/mol, PDI = 1.7. 1H NMR (250 MHz, THF-d8): δ 2.11 (s, 3H, CH3-), 2.43 (s, 1H, -CH-), 1.66 (s, 1H, syn-CH2-), 1.90, 1.19 (2s, 1H, iso-CH2-).


13C-solid

state NMR


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(500 MHz): δ 213 (CH3-), 47 (-CH-), 24 (-CH2-). Elemental analysis (EA) found: 68.94 %C, 9.32 %H (calcd. 68.55 %C, 8.63 %H).

Polymer Analysis. Molecular weight analysis was performed via size exclusion chromatography (SEC) (Agilent Technologies Inc.) in THF at 40 °C against poly(methyl methacrylate) (PMMA) standards using a Cirrus Multi-Detector Software.

Additional

analysis was performed via gel permeation chromatography (Waters 150-C) in THF against polystyrene and PMMA standards using refractive index and UV detectors and found to be in good agreement with the SEC measurements. Structural analysis was performed via 1H nuclear magnetic resonance (NMR) spectroscopy using either a Bruker DPX 250 or Bruker DRX 500 spectrometer. Thermal gravimetric analysis (TGA) was performed using a TGA Q500 (TA Instruments) under nitrogen applying a heating rate of 10 K/min. Differential scanning calorimetry (DSC) was performed on a DSC Q 2000 (TA Instruments) applying a heating rate of 10 K/min. For determination of the glass transition temperature (Tg), the second heating cycle was used. Thermogravimetric analysis coupled to mass spectrometry (TGAMS) was accomplished under a helium atmosphere at 10 K/min on a NETSCH STA 449 F3 coupled with a QMS 403C Aelos system. Rheology measurements of the polymer melt were performed using an MCR 501 rheometer (Anton Paar) with parallel-plate geometry. The plate diameter was 25 mm with a gap of 1 mm. Storage and loss moduli (G’, G”) and the complex viscosity (η*) were determined from dynamic oscillatory experiments. Temperature sweeps were done with a heating rate of 10 K/min, a deformation of 3 % and an angular frequency of 10 s-1.


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Frequency sweeps were done at a given temperature in a range of 0.1-500 s-1 at a deformation of 1 %.

Fiber Melt Spinning. Lab-scale monofilament melt-spinning experiments were conducted using a HAAK MiniLab Micro-compounder (Thermo Fisher Scientific). Multifilaments were spun with the help of a single screw extruder (Haake Polylab OS, Thermo Fisher Scientific) with a 13 hole spinneret.

Fiber Stabilization and Carbonization of the Multifilament Fibers:. PMVK fibers were stabilized by placing them in a 3 L desiccator containing a beaker of 20 mL concentrated hydrochloric acid (38 %) and heating in an oven at 40 °C. The colorless PMVK fibers readily changed color with reaction time to a brown-red over time (Figure S1). In a second stabilization step, the pre-treated fibers were heated to 250 °C with a heating rate of 0.25 K/min. Fibers retained their shape, but turned black. Pyrolysis of the fibers was attempted under nitrogen using a Gero HTK 8GR/22-1G high temperature batch-type furnace with a heating rate of 10 K/min from room temperature. The final carbonization temperatures were varied from 100 to 1800 °C. Carbon yields of the fibers for the different stabilization and carbonization techniques were simulated using a NETSCH STA 449 F3. WAXS measurements of the pyrolysis products were performed using a Rigaku D/Max Rapid II at 40 kV and 30 mA with Cu Kα radiation (λ = 154.059 nm).



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Results and Discussion Design Strategy for CF Precursor Polymers. Our initial attempts at preparing a carbon fiber precursor system utilized a polymerized Nazarov reagent, poly(ethyl-3-oxo-4pentenoate) (PE3O4P), to eliminate any dangerous byproducts while allowing for intramolecular ring-closure and intermolecular cross-linking to form ladder-like polymers. It has been previously suggested that ladder polymers, or nanographene ribbons, should improve the performance of the resulting carbon fibers.51 While PE3O4P showed great potential for the preparation of these ladder-polymers, in hindsight, it is obvious that they are ultimately unsuitable as carbon fiber for reasons discussed below. Despite this, PE304P led to the development of a more rigorous design philosophy, and is therefore worth mentioning. Ethyl-3-oxo-4-pentenoate was prepared in 29% yield following a previously reported procedure52 and polymerized using AIBN at 40 °C for 24 hours to achieve molecular weights of Mn ~ 20 kg/mol (PDI = 1.6). The resulting polymer could then be treated with a catalytic amount of a strong base, here KOH, to produce the insoluble cyclized structures shown in Scheme 1 via intramolecular condensation reactions. n O KOH

O

O

n

n O

O O

O

O

+ O

+ HO O O

O

O

O

O O

+ H 2O Aldol

Claisen

Scheme 1. Aldol and Claisen products of the ring-closing reactions of poly(ethyl-3-oxo-4pentenoate).


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Solid-state NMR and IR confirmed the formation of carbon-carbon double bonds via Aldol condensation. Solution NMR kinetic studies additionally revealed the evolution of ethanol, indicating the Claisen condensation as a concurrent reaction. Elemental analysis showed the insoluble product to be 61.30% C and 7.94% H (theo.: 67.73% C, 6.50% H for the Aldol product). The above-mentioned NMR results are provided in the Supporting Information (Figure S1, Figure S2). In terms of chemical objectives, this polymer system seems quite successful, despite the additional Claisen condensation, which can be viewed as a “minor defect.” However, for the purposes of a CF precursor, PE3O4P is unsuitable for a number of reasons. First, the monomer is notoriously difficult to produce, with nearly 30 different preparations with sub-40 % yields having been reported over the last 60 years.53 For an industry that utilizes literally tons of monomer for CF precursor polymers, this represents a serious shortcoming. Additionally, the monomer is not stable at room-temperature and easily polymerizes via the activated carbon-carbon double bond.54 Furthermore, due to the high reactivity of the diketo-methylene (pKa ~ 12), the condensation reaction is thermally induced above 60 °C,55 which makes the polymer unsuitable (i.e. cross-linking) for many spinning processes, and is the reason for the low temperature polymerization conditions. It quickly became clear that a reexamination of our design philosophy had to incorporate not only the chemical aspects of carbon fiber precursors but also take into consideration the importance of the thermal and physical properties. While none of these issues are altogether surprising in retrospect, they allow for a more practical and composite design to be established. First, the polymer should be indefinitely stable (storable) under reasonable conditions. Second, the polymer should not undergo chemical



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processes without a catalyst during fiber spinning. This necessarily means the polymer must also have a high thermal stability for processing. Third, the polymer should show a relatively low glass transition or melting temperature with no chain entanglement in order to have suitable rheological properties. Fourth, the monomer unit should be “small”, or utilize easily removable solubilizing groups to increase the carbon yield of the resulting fiber. Fifth, the polymer should ideally undergo a single chemical reaction for stabilization/carbonization for better understanding of the resulting structures. Ideally, byproducts of this reaction should be innocuous and/or non-existent. Finally, the monomer/polymer should be easily scalable. With these guidelines in mind, we realized that the polymer of the simplest vinyl alkyl ketone, poly(methyl vinyl ketone) (PMVK), could address many of these points. MVK is commercially available, and the methyl protons are still capable of undergoing Aldol condensations (pKa ~ 22), but are far more stable and less reactive than the methylene units of PE3O4P. Furthermore, MVK would allow only a single reaction type between the repeat units (Scheme 2), thus simplifying the chemistry and increasing the theoretical carbon yield. In fact, the theoretical carbon yield for a perfect ladder structure derived from the condensation of neighboring chains would be 92.26% without pyrolysis. We note that Scheme 2 shows an idealized structure, and in reality it will form crosslinked networks via reactions with neighboring chains, similar to PAN, but which do not possess heteroatoms. For these reasons, this polymer seemed to be an ideal candidate for further investigation.

H+, ∆T O

n O

+ n H 2O O

n


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Scheme 2. Acid-catalyzed aldol condensations of adjacent polymer repeat units.

Polymerization and Properties of Methyl Vinyl Ketone. Free radical polymerizations of MVK initiated by AIBN resulted in polymers with number-average molecular weights (Mn) ranging from 31 - 221 kg/mol with polydispersity indices (PDI) in the range of 1.5-2.8 (Supporting Information, Table S1). Polymerization kinetics were studied via 1H-NMR spectroscopy in toluene-d8 and showed a strong dependence on the concentration of the monomer (Figure 1). Despite the relatively slow kinetics at 1.8 M, it was found that these concentrations were important to avoid autoacceleration of the polymerization as seen at monomer concentrations of 2.9 M in Figure 1. This effect is presumably due to the Tromsdorff-Norrish effect, as a gel-like polymer product is produced under these conditions. Two separate trials were performed using initial monomer concentrations of 1.8 M at both 60 °C and 80 °C and are provided to show the reproducibility of this polymerization in Figure 1.



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Figure 1. Polymerization kinetics of methyl vinyl ketone performed at 60 °C (open) and 80 °C (closed) with initial monomer concentrations of 2.9M and 1.8M. Two separate trials for

each 1.8 M reaction are presented.

Kinetic studies up to 38 hours for 1.8 M MVK at 60 °C displayed a nearly linear increase in monomer consumption with an ultimate conversion of approximately 60 % (Figure S3), suggesting that long polymerization times are necessary for appreciable yields. This is a comparable conversion to polyacrylonitrile.56-57 Reproducible molecular weights of ~ 30 kg/mol and ~ 50 kg/mol could be achieved under given conditions of 1.8 M, 80 °C, 18 h and 1.8 M, 60 °C, 48 h, respectively. While additional investigations into the cause of this slow polymerization rate were not performed, we speculate that the monomer is capable of stabilizing the propagating radical, thus slowing the polymerization rate. A representative NMR spectrum of poly(methyl vinyl ketone) is provided in the Supporting Information (Figure S4), and shows the polymer to be atactic with an exclusively head-to-tail conformation, which is in agreement with previously reported results.39 Attempts at anionic polymerizations did not yield polymer, and reversible addition-fragmentation chain-transfer (RAFT) polymerizations did not produce the desired molecular weights even at long polymerization times (72 h) under the above conditions. A previous study suggests that high monomer concentrations might be necessary to produce the desired molecular weights via RAFT,55 which likely derives from the slow polymerization. Differential scanning calorimetry (DSC) of PMVK revealed a glass-transition temperature (Tg) of ~ 35 °C. Melting and crystallization transitions were not observed in


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the range of -20 to 150 °C. Thermogravimetric analysis (TGA) under nitrogen gave a degradation onset temperature of 290 °C. Mass spectroscopy of the degradation products showed the presence of water and monomer, suggesting that Aldol condensation between adjacent repeat units and depolymerization are the major, concurrent degradation processes. All thermal data is provided in the Supporting Information.

PMVK Monofilament Fiber Formation. Initial fiber spinning trials were conducted using three different polymer samples, P1 - P3. Molecular weight data is provided in Table 1.

Table 1. Molecular weight data for PMVK samples tested for melt spinning. Sample

Mn (kg/mol)

Mw (kg/mol)

PDI

P1

50

113

2.3

P2

111

354

3.2

P3

240

524

2.2

Rheological measurements of P2 and P3 were only possible above 180 °C, below which the polymer did not flow and the rheometer was not able to compact to the necessary height of 1 mm. Above a temperature range of 180 - 240 °C, the storage modulus of P2 was always below the loss modulus, while for P3 a modulus crossover occurred at 235 °C (Figure S7). The high temperature crossover in P3 is likely due to entanglement of the high molecular weight polymer chains, which does not occur for the lower molecular 13 ACS Paragon Plus Environment


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weight samples and suggests that a critical entanglement molecular weight exists above Mn = 111 kg/mol. Unlike P2 and P3, rheological measurements for the lower molecular weight P1 could be made over the range of 120 - 180 °C, well below the degradation temperature of PMVK. Over this temperature range, the storage modulus, G’, was below the loss modulus, G” (Figure 2). Temperature-dependent viscosities were also measured for the above samples. Typically, workable viscosities for homogeneous melt-spinning fall within the range of 200 - 800 Pa·s.58 From Figure 2, we can see that P1 exhibits these viscosities at temperatures between 150 and 165 °C. This is particularly ideal since this range is well above the glass transition temperature, while still sufficiently below the degradation temperature. P2 displayed suitable viscosities only at temperatures above 200 °C, while for P3, only above 240 °C (Figure S7). Additional frequency-dependent flow sweeps were performed at temperatures from 150 to 165 °C for P1 (Figure 3) During the measurements, a yellow discoloration of the sample was noted, suggesting that a small amount of the polymer undergoes a cyclization reaction at these temperatures. Minor differences in viscosity determined via temperature and frequency sweeps, respectively, at a given temperature are attributed to temperature-induced changes in the thermo-sensitive samples. Table 2 summarizes the zero-shear viscosities of P1. The determination of zerosheer viscosities of P2 and P3 were not possible, as the temperature-induced crosslinking and degradation occurred too rapidly for measurement.


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Figure 2. Temperature-dependent parallel plate rheological measurements of P1.

Figure 3. Frequency-dependent viscosity measurements of P1 at various temperatures.

Table 2. Zero-sheer viscosities of P1 at various temperatures. T (°C)

150

155

160

165

η0 (Pa·s)

1300

880

670

550

Lab-scale melt spinning was performed using a Haake MiniLab twin-screw extruder with a monofilament spinneret with a diameter of 500 µm and approximately 10 g of polymer. As anticipated, samples P2 and P3 could not be spun at the necessary elevated 15 ACS Paragon Plus Environment


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temperatures, as the onset of degradation occurred. On the other hand, spinning of P1 was readily performed at 165 °C at a rate of 60 m/min to produce fibers with diameters between 55-70 µm. Despite the low Tg of the polymer, the fibers rapidly cooled immediately following spinning to form continuous monofilaments, which could be lifted from the spool directly after spinning. The as-spun fibers are white in appearance and do not display any discoloration. However, if not stored in a cool, dry environment, the filaments formed a film after ~18 hours (overnight) as the polymer easily absorbs moisture and causes a depression in its glass transition temperature. Scanning electron microscopy of the individual fibers shows homogeneous, non-porous structures with smooth surfaces (Figure 4). Slight roughness on the edge surface can be seen in the cut fibers in Figure 4c and d. This is attributed to the cutting of the fiber. Additionally, by applying a winding speed of 75 m/min, fibers with diameters of approximately 15 µm could be prepared without breakage (Figure 4d).


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Figure 4. Photographs (a, b) and scanning electron micrographs (c, d) of as-spun poly(methyl vinyl ketone) (P1).

Stabilization of Monofilament PMVK Fibers. Early attempts at inducing the Aldol condensation of adjacent repeat units in PMVK (Scheme 2) followed previous efforts for PE3O4P. Figure 5 shows solid-state NMR spectra for HCl- and KOH-catalyzed reactions in toluene (0.1 M). By comparing the spectra in Figures 5a and 5b, one can see that the consumption of the ketone group at 212 ppm occurs readily in 0.1 M KOH at 60 °C to form the predicted carbon-carbon double bond seen at 125 ppm. However, the appearance of the peaks at δ = 68 and 74 ppm suggests that while the expected ring-closure occurs, there is an incomplete elimination of water resulting in the formation of tertiary alcohols. This is not seen when using 0.1 M HCl at 110 °C, suggesting the near-complete elimination of water from the polymer chain. Quantification of the propagation length of the ladder structure could not be directly achieved, but the broadness of the peak at δ =130 ppm suggests a variety of species corresponding to different lengths. Additionally, the intermolecular and intramolecular reactions could not be separated, but both are likely to occur during this treatment.



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Figure 5.

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state NMR spectra of a) untreated PMVK fibers, b) PMVK fibers after

KOH-treatment at 60 °C in toluene and c) PMVK fibers after HCl-treatment at 110 °C in toluene.

While the above data suggests that the Aldol condensation reactions proceed as expected under the above conditions, SEM images of the post-treatment fibers revealed a high degree of fusion between the filaments, even at temperatures as low as 40 °C (Figure S8). This fusion is attributed to capillary forces, which draw the fibers together in the solvent and elevated temperature for processing, which occurs above the polymer’s glass transition temperature. For these reasons, a solvent-free, vapor-phase approach was developed. As KOH does not have a sufficient vapor pressure, HCl vapor at 40 °C was used instead. Initial studies were performed by placing the fibers in a 20 mL scintillation vial with a 2 mL vial containing 150 μL of concentrated HCl. It should be noted that never during this procedure does liquid HCl come into contact with the fibers. Attempts utilizing ethylamine and piperidine vapor swelled the polymer resulting in the loss of structure and subsequent


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trials on base-catalyzed ring-closure were not pursued. Fiber samples were exposed to HCl vapor for various times. A rapid color change to yellow became readily apparent even after 15 minutes of exposure. Upon longer exposure times, fibers transition from yellow to red to a near black while becoming increasingly brittle (Figure S1). This color change is attributed to an increase in the conjugation length of the polycyclic polymer structure. Throughout this process, the fiber structure is maintained and no fiber merging was observed in SEM (Figure 6a,b). Despite visible changes in the polymer’s appearance, elemental analysis of the HCl-treated fibers did not show a significant change in %C from the original polymer samples even after 24 hours (Table 3). An additional thermal treatment of the fibers at 80 °C for 2 hours was performed on the samples, and an increase in the %C was observed. It should be noted that while the fibers were removed from the HCl vapor source for the thermal treatment, no attempt was made to evacuate the fibers to remove any residual acid.



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Table 3. Elemental and thermal gravimetric analysis of the stabilized fibers. Stabilization Conditions and Times

Elemental Analysis

Vapor Treatment Thermal Treatment

TGA Residue

Sample

40 °C (h)

80 °C (h)

%C

%H

(% mass)

a

0

0

68.94

9.32

~0

b

2

0

65.69

6.44

11.2

c

2

2

72.35

7.98

29.0

d

24

0

62.99

7.18

15.6

e

24

2

73.45

7.87

40.0

SEM images of the same fibers (Figures 6c, d) demonstrated that this additional thermal treatment does not cause any loss of the fiber structure but does increase their brittleness. Thermal gravimetric analysis of the treated samples in nitrogen up to 1000 °C at a heating rate of 10 K/min (Figure S10) also showed a dramatic increase in residual mass percent for both vapor treatment times after additional thermal treatment. Residual mass percents are reported in Table 3. However, following heating to 1000 °C, the resulting residue did not retain their fiber shape, but rather formed a single mass.


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Figure 6. Scanning electron micrographs of monofilament fibers after vapor stabilization with HCl at 40 °C for 2 h (a) and 24 h (b), and the same fibers after an additional 2 h thermal treatment at 80 °C, (c) and (d) respectively.

The above results suggested that while enough ring-closure of the polymer chains occurs at low temperatures to stabilize the fiber structure, the elimination of water is thermally activated. Solid state NMR (Figure 7) and IR (Figures S11 and S12) analysis confirm this assumption and prove the consumption of the carbonyl group after treatment with HCl vapor and subsequent thermal treatment. Figure 7 suggests that long HCl vapor exposure followed by a short thermal treatment is sufficient to consume the carbonyl group (δ =212 ppm) to near completion and form the expected carbon-carbon double bond (δ =125 ppm). From these results, a two-stage, low temperature stabilization process appears to greatly increase the carbon yield of these fibers.



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Figure 7.

13C-solid

Page 22 of 35

state NMR spectra of a) untreated PMVK fibers, b) after 2 hour HCl-

vapor treatment, c) after 2 hour HCl-vapor and 2 hour thermal treatment, d) after 24 hour HCl-vapor treatment, and e) after 24 hour HCl-vapor and 2 hour thermal treatment.

Upscaling and Multifilament Fiber Formation. So far, poly(methyl vinyl ketone) has addressed many of the design criteria discussed earlier in the text. We have observed that the polymer can be easily stored in cool, dry environments without degradation or deformation, and the polymer is smoothly spun under reasonable conditions above its Tg, but well below its degradation point. Additionally, the stabilization of the fibers is easily achieved using well-known chemistry with an innocuous byproduct. However, until now, only small batches (10 g) of polymer have been discussed. With this in mind, the preparation of kilogram-scale, multi-filament fibers were investigated.


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Due to glassware limitations, several batches of polymer with yields ranging from 21 to 347 g were prepared and blended together to achieve kilogram-scale quantities for multifilament fiber formation. Molecular weight data for each individual batch size can be found in the Supporting Information (Table S2). Little difficulty was found in scaling this polymerization to several hundred-gram batches, which suggests this polymerization could be scaled to larger quantities with the appropriate facilities. The combined polymer showed nearly identical thermal properties with a Tg of 36 °C and a thermal degradation onset at approximately 280 °C. SEC measurements revealed an Mn of 59 kg/mol (PDI = 1.88) Temperature-dependent viscosity measurements indicated a temperature range suitable for melt-spinning between 135 and 155 °C. Multi-filament spinning was performed with approximately 1 kg of blended polymer using a single screw extruder with a spinneret of thirteen 300 μm holes at 142 °C. Continuous spinning occurred over several hours without breakage. In order to prevent fusion of the individual filaments, the fibers were immediately cooled by passing through liquid nitrogen prior to winding. Fibers were stored under a nitrogen atmosphere to prevent absorption of water. Initial stabilization was performed as described above using a 24 h HCl vapor treatment at 40 °C followed. Optical microscopy shows as-spun multifilament fibers with diameters of approximately 70 μm (Figure S13). After the stabilization treatment, though, there was an approximately 20% shrinkage in the polymer fiber, resulting in fibers with diameters of nearly 55 μm.

Preliminary Pyrolysis Investigations. Similar to the monofilament fibers, a direct carbonization of the stabilized, multifilament fibers at 1000 °C led to a loss of the fiber



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Page 24 of 35

structure. Additionally, low carbon yields were observed (13 wt. %) by TGA. Therefore, a pre-pyrolysis step was investigated by heating to 250 °C at a rate of 0.25 K/min in either a nitrogen or air environment. This temperature represents a value just above the onset of the degradation process of the untreated homopolymer. As such, we expected degradation of the fibers to occur at this point should there be significant residual, unreacted ketogroups. Yet, in all cases, the fibers retained their individual shape after pre-pyrolysis. An overview of the processing steps is given in Figure 8. Interestingly, fibers heated in the nitrogen environment showed no appreciable change in % yield (12 wt.%) when compared to the untreated samples (13 wt.%). However, under atmospheric conditions, an increase in residue yield to 25 wt.% was observed. While this does not quite yet compare to the values derived from PAN, it does represent a value that is comparable to the carbon yield of cellulose based carbon fibers and typically considered suitable for manufacturing.1-2, 7 This is further supported by the elemental analysis of the pre-pyrolysis products (71.11 %C, 5.75 %H, C:H = 12.36). From these values, it was determined that a pre-pyrolysis approach significantly improved the carbon yields of the multifilament fibers and places them in the same range as the monofilament fibers.

Figure 8. Overview of the stabilization and carbonization processes for multifilament PMVK fibers.


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Despite this improvement in carbon yield, subsequent pyrolysis under nitrogen by heating from room temperature to temperatures between 800 and 1800 °C at a rate of 10 K/min once again resulted in the loss of the fiber structure, suggesting that traditional high-temperature carbonization approaches may not be suitable for PMVK. Despite this, analysis of the carbonaceous product via wide-angle X-ray diffractometry (WAXS) was performed and revealed interesting results, especially when compared to a commercially available T300 carbon fiber (Toray). From Figure 9, a sharpening of the reflections is observed

with

increasing

carbonization

temperature.

At

800°C,

only

broad,

uncharacteristic signals were observed. At 1000 °C, the reflection corresponding to the (002) plane appears at 22.5° with only a slight shift to 25.8° at higher temperatures. This corresponds to a shift in interlayer spacing from 4 Å at 1000 °C to 3.5 Å for above temperatures. Notably, this suggests that the interlayer spacing of the pyrolyzed PMVK fibers is closer to graphene (3.4 Å) than those observed in commercially available PANbased carbon fibers (3.6 Å). Figure 10 shows an increase in the crystalline height Lc and length La with increasing temperatures up to 1600 °C. At 1800 °C pyrolysis temperature, the crystallite dimensions were further reduced.



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Page 26 of 35

Figure 9. WAXS diffractograms of the PMVK fiber products after pyrolysis at different temperatures.

Notably, above a pyrolysis temperature of 1200 °C, the crystallites observed on PMVKderived carbon were larger than in a commercial T300 sample. The same trend was observed in the Raman spectra (Figure 11). A minimum in the D/G ratio as well as a maximum in the 2D/G ratio were achieved (Table S3) at a carbonization temperature of 1600 °C, maching the maximum of Lc and La from the WAXS analysis. The 2D-peak at 2902 cm-1 (see Figure S14 and Table S4, for peak analysis) also had a maximum value at this carbonization temperature.

Figure 10. Development of crystallite dimensions Lc (▲), La (●) and interlayer distances

d002 (■) with carbonization temperature of the PMVK fibers. Reference values of a commercial T300 PAN-based CF (Toray, open symbols) analyzed by the same method.


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Figure 11. Raman spectra of the of the PMVK fibers after pyrolysis at different temperatures. All together, these features point towards an optimized stacking of the carbon layers.59-61 A rather unusual additional peak was observed at 1556 cm-1, responsible for an asymmetric peak shape in the G-band. This peak probably results from rearrangements in the sp2-structure. These results suggest that with further optimization of the pyrolysis procedure, fibers with a defect-free formation of carbon crystallites than those found in PAN-based carbon fibers might be possible using PMVK.

Conclusions Poly(methyl vinyl ketone) provides an enticing material for a carbon fiber precursor and displays many promising qualities as a candidate for future investigation. Conditions for the free radical polymerization of methyl vinyl ketone have been optimized to reproducibly give polymers of the appropriate molecular weight and rheological behavior



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Page 28 of 35

for single- and multi-filament fiber spinning. The resulting fibers can be easily and successfully treated to produce ladder-like structures through chemical methods while retaining their fiber shape. Thermal stabilization of the multi-filament fibers at 250 °C give carbon yields up to 25 %, making it comparable to other commercially available carbon fibers through a single, well-understood chemistry with a single, innocuous byproduct. Initial pyrolysis studies suggest that with further optimization of processing conditions, PMVK can produce highly defect-free fibers, as seen in both the interlayer spacing of 3.5 Å and the values for Lc and La. In order to utilize these findings for the preparation of CFs with high mechanical strength, further efforts will have to focus on the elaboration of a continuous, technical scale multi-filament spinning, stabilization and carbonization process that allows for the preparation of small diameter (≤10 µm) PMVK-derived multifilament CFs.

Supporting Information Available: NMR data on PE3O4P, kinetic plots of polymerization, 1H

NMR, DSC, TGA, rheology of poly(methyl vinyl ketone), SEM micrographs of fibers, TGA,

FT-IR, Raman data of fibers. This material can be obtained free of charge from http://www….

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TOC Graphic

O

O

O

HCl vapor


Poly(Methyl Vinyl Ketone) as a Potential Carbon ... - ACS Publications

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