Synthesis of Polyacrylonitrile as Precursor for High-Performance Ultrafine Fibrids

Abstract

Polyacrylonitrile (PAN) copolymers with different methyl acrylate (MA) contents were synthesized via solution polymerization and used as precursors for high-performance PAN ultrafine fibrids. The chemical structures of the copolymers were characterized using Fourier-transform infrared spectroscopy and $^{13}C$ nuclear magnetic resonance spectroscopy. Their particle sizes and aspect ratios increased with increasing viscosity, and the degree of crystallinity increased with decreasing concentration of copolymer solution. In contrast, their peak temperature and heat of exotherm increased with decreasing concentration of the copolymer solution. The aromatization indices (AIs) of the fibrids increased with increasing heat-treatment time; however, the AIs decreased when the heat-treatment temperature was higher than the onset temperature of the copolymers. On the other hand, the crystal sizes of the fibrids decreased with increasing concentration of the copolymer solution when the MA content was held constant.

Introduction

Polyacrylonitrile (PAN) fibers were commercialized by DuPont in 1950 using a dry-spinning method in dimethylformamide (DMF) solvent. PAN fibers have excellent chemical and weather resistance and are used as industrial materials for filters. They also have a high melting point because of bonding and cross-linking structures between the dipoles induced by the high polarity of the nitrile groups, which are widely used in precursors for high-performance carbon fibers. In particular, the nitrile groups in PAN have continuous carbon chains and are in suitable locations to enable cyclization reactions. Thus, the PAN fibers can easily be converted to carbon fibers in a high yield.1-6

Generally, the preparation of carbon fibers from PAN fibers involves four stages: (i) preparation of the PAN precursor, (ii) stabilization, (iii) carbonization, and (iv) graphitization. The principal component of carbon fibers is carbon. The structure of carbon fibers is similar to that of graphite, and the diameter of the carbon fibers is approximately 10 mm. Carbon fibers are mainly used in carbon-fiber-reinforced composites. Carbon-fiber-reinforced composites have low weights, high rigidity, low heat expansibility, high dimensional stability, high electrical and thermal conductivity, and excellent chemical resistance.7-10

In this study, PAN copolymers with different contents of methyl acrylate (MA) were synthesized via solution polymerization. The chemical structures of the copolymers were characterized by Fourier-transform infrared (FT-IR) spectroscopy and 13C nuclear magnetic resonance (NMR) spectroscopy. Ultrafine PAN fibrids were prepared by wet spinning. The PAN fibrids were characterized via image analysis, optical microscopy, differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), FT-IR spectroscopy, and X-ray diffraction (XRD).

 

Experimental

Materials. Acrylonitrile (AN, 95％) and 2,2'-azobis-isobutyronitrile (AIBN, 99％) were purchased from Daejung Chemicals and Metals Co., Ltd (Korea). MA (98％) and itaconic acid (IA, 99％) were supplied by Junsei Chemical Co., Ltd (Japan). Dimethyl sulfoxide (DMSO, 99％) and dimethylacetamide (DMAc, 99.5％) were purchased from Samchun Chemical Co., Ltd (Korea).

Synthesis of PAN Copolymers. The desired amounts of AN, IA, and MA were added to a reactor followed by the addition of AIBN. The mixtures were gradually heated to 65 °C and reacted for 6 h under a nitrogen atmosphere. The resulting product was washed several times with distilled water and dried in vacuo. Finally, the product was obtained as a powder. The samples are abbreviated as M4-12. The integers in the sample name present the weight ratio of MA in the copolymers.

Preparation of PAN Fibrids. A copolymer solution was prepared by dissolving the copolymers in DMSO or DMAc. The flow rate of the copolymer solution and pressure were 1.2 mL/min and 0.2 MPa, respectively. Water (400 times the amount of the copolymer solution) was used as the coagulating agent. A slurry of fibrids was prepared using a screen filter (100 mesh). The particle sizes of the fibrids were adjusted using a colloid mill. Finally, 5 wt ％ slurries of the fibrids were prepared through several washing and solvent-removal cycles. The fibrids were heat-treated at 200-290 °C for 40-120 min. The fibrids were carbonized at 700-1400 °C under a nitrogen atmosphere. The fibrid samples are abbreviated as FM4-12; the integers in the sample name present the concentration of the copolymer solution.

Characterization and Measurements. The weight-average molecular weight and molecular weight distribution (Md) were estimated via gel permeation chromatography on an HLC-8320 GPC instrument (Tosoh Co.). FT-IR spectra of the samples (in KBr pellets) were recorded using a Thermo Nicolet Avatar 370 spectrometer. The optical density ratio (ODR) was calculated using the following equation:

where Tb2240 and Tb1730 are the baseline transmittances at 2240 cm−1 (C≡N) and 1730 cm−1 (C=O), respectively, and Tp2240 and Tp1730 are the peak transmittances at 2240 cm−1 and 1730 cm−1, respectively. The extent of reaction (EOR) was calculated using the following equation:

where I1600 and ICN are the peak absorption intensities at 1600 cm−1 and 2240 cm−1, respectively.

The 13C NMR spectra were recorded on an FT-NMR spectrometer (JNM-ECA 600 MHz, JEOL Ltd.) using DMSO-d6 as the solvent.

The viscosity of the copolymer solution was measured with DMAc as the solvent using a digital viscometer (DV II+pro viscometer, Brookfield) at 25 °C and a torque of 70%.

Images of the PAN fibrids in the slurry state were investigated using an optical microscope (ICS-305B, Sometech Inc.). The particle sizes and morphological properties of the fibrids were measured using an image analyzer (Lixell, Sympatec) with a Heywood shape factor of 1.

The thermal properties of the fibrids were investigated via differential scanning calorimetry (DSC; TA Q-series, TA Instruments) at a heating rate of 5 °C/min under a nitrogen atmosphere. The thermal decomposition of the fibrids was studied using thermogravimetric analysis (TGA, Q-series, TA Instruments) at 35-950 °C with a heating rate of 20 °C/ min under a nitrogen atmosphere.

The change in the crystal structure was investigated using an X-ray diffraction analyzer (X’PERT MRD, Phillips) at 40 kV and 40 mA. The crystal size was calculated using the Scherrer equation:

where λ is the wavelength of CuKα (1.5418 Å) irradiation, B is the full width at half maximum intensity, 2θ = 17 °C, and K is a constant (0.89). The aromatization index (AI) was calculated using following equation:

Table 1.Characteristics of Tertiary PAN Copolymers

where Ia and Ip are the peak intensities at 2θ = 25.5° and 2θ = 17°, respectively.

 

Results and Discussion

Characterization of PAN Copolymers. Table 1 shows the weight-average molecular weight and molecular weight distribution (Md) of tertiary PAN copolymers as a function of monomer composition. When the MA content was varied, the and Md values ranged from 197000-229000 g/mol and 1.57-1.63, respectively. The values of the copolymers vary slightly with varying MA contents; whereas, the Md of the copolymers was fairly constant at 1.60 ± 0.03.

Figure 1 shows the FT-IR spectra of the PAN copolymers with various contents of MA. At higher MA contents, the intensity of the C≡N peak at 2240 cm−1 decreased, but that of the C=O peak at 1730 did not change significantly. The relative peak transmittances, i.e. the ratios of the C=O/C≡N peaks (ODR), were calculated and are shown in Table 1; the ODR increased with increasing MA content.11,12

Figure 1.FT-IR spectra of tertiary PAN copolymers with various contents of MA.

Figure 2 shows the 13C NMR spectra of the copolymers. The chemical shift at 30 ppm corresponds to the methylene (CH) and methane (CH2) carbon atoms and the chemical shift at 120 ppm corresponds to nitrile carbon atom. The carbonyl carbon in IA and MA appears at 175 ppm, and the methoxy (OCH3) carbon in MA appears at 52 ppm. As shown in Table 1, the area of the peak at 52 ppm increased with increasing MA content, but is not proportionate to the ODR in the FT-IR spectra.13,14 Through FT-IR and 13C NMR spectroscopy, the chemical structures of the copolymers were confirmed.

Figure 2.13C NMR spectra of tertiary PAN copolymers: (a) methylene carbon atom, (b) methine carbon atom, (c) nitrile carbon atom, (d) carbonyl carbon atom, (e) methoxy carbon atom.

Morphology. It is well known that the comonomer in PAN polymers acts as a plasticizer to improve the segmental mobility of the polymer chains and their solubility, resulting in improved radioactivity. In particular, the polarity, resonance, and steric hindrance of MA are similar to those of AN; therefore, MA is the best comonomer for AN.15

Figure 3 shows microscopic images of PAN fibrids prepared from aqueous suspensions in DMSO and DMAc. When DMSO was used as the solidification solvent, the prepared fibrids were ultrafine cylindrical filaments; in contrast, when DMAc was used as the solvent, the prepared fibrid was a thin film. These results can be explained by the diffusion rates of the solvents in solvent/non-solvent systems; the diffusion rate of DMSO in a DMSO/H2O system (0.5-1.8× 10−10 m2/s) is greater than that of DMAc in DMAc/H2O system (0.4-0.6 × 10−10 m2/s) resulting in rapid solidification of the polymer solution to the fiber phase by the external force of the fluids before deformation.16,17

Figure 3.Microscopic images of PAN fibrids prepared from aqueous suspensions in (a) DMSO and (b) DMAc.

Table 2 shows the morphologies of the PAN fibrids as a function of the MA content and solution concentration. The viscosity of 5 wt ％ FM4 (370 cPs) was higher than that of 7.5 wt ％ FM6 (240 cPs); however, the particle size of 5 wt ％ FM4 was smaller than that of 7.5 wt ％ FM6. These results indicate that the density and mobility of the polymers are main factors in the sizes of the prepared fibrids. The particle size and morphology of FM6 did not vary significantly with increasing MA content. However, the particle sizes and morphologies of FM12 vary significantly with increasing MA content because of the relatively high mobility of the polymer.18

Table 2.Morphologis of PAN Fibrids as a Function of MA Contents and Solution Concentration

Table 3 shows the particle size and morphology of PAN fibrids prepared using the colloid milling process. The deformation rate (ΔE) is defined as the difference in the particle size before and after milling. The ΔE values of the FM6 fibrids increased significantly with increasing concentration of polymer solution; however, ΔE of the FM12 fibrids increased slightly because of the rigidity of the fibrids induced by the mobility of the polymer chains. Fibrids with a low MA content and high concentration are relatively rigid, easily cut and compressed by external physical force, and undergo deformation of their particle size and morphology. The morphological properties such as the aspect ratio, convexity, and sphericity increased after milling because the fibrids were pressed along their length and deformed in a planar fashion during milling.19

From the above results, it is apparent that moist PAN fibrids are thin films with a two-dimensional planar structure, and disperse in water in a very soft form. In particular, the flexibility of the fibrids increased with increasing segmental mobility of the copolymers. Initially, the PAN fibrids had higher dispersion stability of the carbon fibers and increased contact surface between the carbon fibers and PAN fibrids caused by their thin and soft structures, which results in a fine and strong paper-like structure.

Table 3.Particle Size and Morphology of PAN Fibrids Prepared Using Colloid Milling Process

Figure 4.DSC curves of PAN fibrids as a function of (a) concentration of copolymer solution and (b) duration of heattreatment under nitrogen atmosphere.

Thermal Behaviors. Generally, when the content of acidic monomers, such as IA or acrylic acid (AA), increases, the initial temperature and peak temperature of cyclization of the PAN fibrids decreases, and the heat of exotherm decreases and becomes broad. However, neutral monomers such as MA hinder or inhibit the cyclization of the nitrile groups, and thus increase the initial temperature and peak temperature of cyclization.20

Figure 4(a) shows the DSC curves of PAN fibrids as a function of the concentration of the copolymer solution under a nitrogen atmosphere. The peak temperature and heat of exotherm increased with decreasing copolymer concentration; this can be attributed to the arrangement and increased crystallinity of the polymer chains with decreasing copolymer concentration and viscosity of the polymer solution when the same external force was applied to solidify the liquids.

Table 4 shows the thermal properties of the PAN fibrids as a function of the MA content with different heat-treatment durations. The initial and peak temperatures before heat treatment increased with increasing MA content. However, the initial and peak temperatures after heat treatment show different tendency. The heat of exotherm decreased with increasing MA content, and the peak temperature and heat of exotherm decreased with increasing heat-treatment time.21

Figure 4(b) shows the DSC curves of the PAN fibrids as a function of the duration of the heat-treatment at 200 °C. The initial and peak temperatures present the initial cyclization temperature and cyclization peak temperature, respectively. The initial temperature and peak temperature decreased with increasing heat-treatment time. The lower initial temperature was due to the increase in the number of initial active sites in the polymer chains with increasing heat-treatment time, resulting in easier cyclization reactions at low temperature. The range of the peak temperature decreased and the peak temperature increased with increasing heat-treatment time, which is because of the increased thermal stability of the copolymers after cyclization.

Figure 5 shows the TGA curves of the FM6 and FM12 fibrids as a function of heat-treatment temperature. The char yields of both fibrids prepared by heat treatment at 250 °C for of 80 min were greater than 46％; however, the char yields of both fibrids were less than 42％ when the heattreatment temperature was below 180 °C. These results indicate that the cyclization reaction and yield of carbon fibers increased with increasing heat-treatment temperature. The char yield of the FM6 fibrid treated at 230 °C was greater than that treated at 250 °C, which is because of the increased oxidative stabilization reaction. The char yield varied with MA content, but the difference in the char yields increased with increasing heat-treatment conditions. Thus, the final yield of carbon fibers can be increased by optimizing the stabilization conditions.22,23

Table 4.Thermal Properties of PAN Fibrids as a Function of MA Content with Different Heat-treatment Durations

Figure 5.TGA curves of PAN fibrids as a functions of heattreatment temperature under nitrogen atmosphere.

FT-IR Analysis of the PAN Fibrids. To investigate the chemical changes of the PAN fibrids, FT-IR spectra were measured during stabilization. Figure 6(a) shows the FT-IR spectra of the PAN fibrids as a function of the duration of heat treatment at 200 °C. Figure 6(b) shows the FT-IR spectra of the PAN fibrids as a function of the heat-treatment temperature for 80 min. As the heat-treatment time or temperature increased, the intensity of the CN stretching peak at 2240 cm−1 gradually decreased and that of a broaden peak at 1600 cm−1, which corresponds to conjugated C=C structure, (C=N)n structure, or combination of the two structures, increased. As shown in Figure 6(b), above temperature of 230 °C, a broader peak appeared at 2200 cm−1, which corresponds to conjugated nitriles induced by dehydrogenation or tautomerization and isomerization of ladder polymers. The carbonyl peak in MA and IA at 1730 cm−1 gradually disappeared with increasing heat-treatment temperature or duration.22,24

Figure 6.FT-IR spectra of PAN fibrids as a function of (a) duration of heat treatment at 200 °C and (b) heat-treatment temperature for 80 min.

Figure 7 shows the EORs of PAN fibrids as a function of heat-treatment duration or temperature. With longer heat treatment time, the EORs of all the fibrids increased, but the range of the increase decreased with increasing MA content. This result is similar to that of the AI values, which were obtained from DSC analysis, and supports that MA hinders or inhibits cyclization. The EOR curves show a maximum value at the heat-treatment temperature of 230 °C; the increasing range of the reacting weight decreased above this temperature. This phenomenon can be observed via the char yield of the TGA curves; when the surface of the high-temperature press was hotter than 230-240 °C during the compressed isothermal stabilization process, chemical shrinkage occurred, i.e., the surfaces of the fibrics rapidly contracted, thereby hindering the oxidative and dehydrogenation reactions of stabilization.

Figure 7.Extent of reaction of PAN fibrds as a function of (a) heat-treatment duration at 200 °C and (b) temperature for 80 min.

XRD Analysis. Table 5 shows the XRD data for the PAN fibrids as a function of the MA content and concentration of copolymer solution. When the concentration of the polymer solution was held constant, the Lc decreased with increasing MA content; however, this decrease was not significantly. When the MA content was s held constant, the Lc decreased with increasing concentration of copolymer solution. These results indicate that the arrangement and degree of crystallinity of the copolymer chains increased with decreasing concentration and viscosity of the copolymer solution when the same fluid external force was applied. This is in agreement with the DSC results shown in Figure 4. Thus, the structure of the fibrids can be optimized by adjusting the solidified liquid composition and rotor rotational speed, which are related to the concentration and viscosity of the polymer solution.

Table 5.XRD Data of PAN Fibrids as a Function of MA Content and Concentration of Copolymer Solution

The PAN fibrids composed of linear polymers show complete crystal peaks at 2θ=17° and 29.5°. The crystal peak at 2θ=29.5° was induced by the effects of the heat-treatment temperature and elongation of the fibrids. As the PAN fibrids were heated above the initial temperature of cyclization, the linear structure became cyclized, and crystallization occurred in the amorphous region resulting in a new peak for the aromatic structure at 2θ=25.5°. The intensity of this new peak increased and those of the two initial peaks decreased following oxidative stabilization. Thus, the degree of stabilization can be evaluated via the AI values.25,26

Figure 8(a) shows the XRD patterns of FM6 fibrids treated at various temperatures for 80 min. The intensity of the peak at 2θ=17° decreased and that of the peak at 2θ=25.5° increased with increasing heat-treatment temperature. The crystal peak at 2θ=25.5° for the fibrids prepared at 180 °C and 200 °C for 80 min does not completely disappear because of incomplete cyclization.

Figure 8(b) shows the XRD patterns of FM6 fibrids as a function of heat-treatment time. As the stabilization time increased, the peak at 2θ=17° broadened, the crystal size of (100) plane decreased, and the intensity of the peak at 2θ=25.5° broadened and increased. The AI values of the fibrids prepared via heat treatment for 20 min and 80 min were 27θ and 37θ, respectively. After carbonization at 1340 °C, the rotational peak of (002) plane increased significantly with increasing AI value. Thus, PAN fibrids were converted to ultrafine carbon fibers through oxidative stabilization above 200 °C for 80 min and carbonization above 1300 °C under an inert gas. The ultrafine carbon-fiber- converted fibrids have high electrical conductivity and a large contact surface, which decrease resistance of the final carbon-fiber papers.27,28

Figure 8.(a) XRD patterns of PAN fibrids treated at various heat treatment temperatures for 80 min; (b) XRD patterns of PAN fibrids as a function of heat-treatment time: (1) heat-treated at 200 °C for 20 min; (2) heat-treated at 200 °C for 80 min; (c) carbonization of (a) sample at 1340 °C; (d) carbonization of (b) sample at 1340 °C.

Figure 9 shows the AI values of the PAN fibrids as a function of the heat-treatment temperature. The cyclization reaction of the linear polymer to a ladder polymer occurs rapidly above 230 °C. FM6 fibrids prepared at the heat-treatment temperature of 250 °C have low AI values because MA hinders cyclization, as evident from the DSC and FT-IR results. Generally, when a comonomer such as MA is introduced into PAN chains, the balance between the chains in the crystal region is destroyed by the bulky and flexibility of the MA chains. Thus, when the MA content is higher than the critical value, MA acts as a defect that hinders cyclization in the crystal region.29,30

Figure 9.Aromatization index values of PAN fibrids as a function of heat-treatment temperature for 80 min.

 

Conclusions

PAN tertiary copolymers with various contents of MA were synthesized and the chemical structures of the copolymers were characterized using FT-IR and 13C NMR spectroscopy. PAN ultrafine fibrids were prepared via wet spinning method. The use of DMSO and DMAc as solidification solvents resulted in ultrafine cylindrical filament and thin film fibrids. The peak temperatures and heat of exotherm of the PAN fibrids increased with decreasing concentration of copolymer solution. The EORs of the PAN fibrids increased with increasing heat-treatment time and the crystal size decreased with increasing concentration of copolymer solution when the MA content was held constant. After stabilization, the PAN fibrid was successfully converted into fine carbon fibers by heat-treatment at 1340 °C under inert gas. The data reported in this paper suggests that the ultrafine PAN fibrids have excellent properties and are promising candidates for carbon-fiber-paper applications.