<→DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> Poly(acrylonitrile-<i>co</i>-3-aminocarbonyl-3-butenoic acid methyl ester):A better precursor material for carbon fi ber than acrylonitrile terpolymer <→---------------------start--------------------->
  Chinese Chemical Letters  2014, Vol.25 Issue (09):1275-1278   PDF    
Poly(acrylonitrile-co-3-aminocarbonyl-3-butenoic acid methyl ester):A better precursor material for carbon fi ber than acrylonitrile terpolymer
An-Qi Jua,b , Meng-Juan Lia, Miao Luoc, Ming-Qiao Gea    
a Key Laboratory of Eco-textiles, Ministry of Education, Jiangnan University, Wuxi 214122, China;
b College of Textile & Clothing, Jiangnan University, Wuxi 214122, China;
c School of Pharmacy, East China University of Science and Technology, Shanghai 201620, China
Abstract: In order to improve the stabilization and spinnability of polyacrylonitrile, a bifunctional comonomer containing both ester and amide groups was synthesized to prepare poly(acrylonitrile-co-3-aminocarbonyl-3-butenoic acid methyl ester) [P(AN-co-ABM)] copolymers used as the carbon fiber precursor instead of poly(acrylonitrile-acrylamide-methyl acrylate) [P(AN-AM-MA)] terpolymer. The differential scanning calorimetry and thermogravimetry results show that the stabilization of P(AN-co-ABM) have been remarkably improved by ABM compared with P(AN-AM-MA) terpolymer, such as lower initiation temperature, broadened exothermic peak and smaller activation energy. Moreover, the spinnability of P(AN-co-ABM) is also improved by ABM due to the lubrication of ester groups in ABM. This study clearly shows that P(AN-co-ABM) copolymer is a better material for use as a carbon fiber precursor than P(AN-AM-MA) terpolymer.
Key words: Polymerization     Stabilization     DSC     Spinnability     Carbon fiber    
1. Introductionl

Carbon fiber has been extensively applied in high-tech aerospace, defense areas,and civil engineering. More than 90% of carbon fibers used today are made from polyacrylonitrile (PAN)-based precursor, but PAN homopolymer has rarely been used as a carbon fiber precursor due to its poor spinnability and stabilization [1, 2]. Thus, many terpolymers of acrylonitrile are used as carbon fiber precursors [3, 4],such as poly (acrylonitrile-methyl acrylate-itaconic acid),poly (acrylonitrile-methyl methacrylate-itaconic acid) and poly (acrylonitril-methyl acrylate-acrylic acid),in which the acidic comonomers,acrylic acid,methacrylic acid and itaconic acid,are incorporated to reduce the cyclization temperature and relax heat release,and the neutral comonomers,methyl acrylate and methyl methacrylate,are used to improve the solubility and spinnability of PAN [5]. However,it is difficult to improve the stabilization and spinnability in acrylonitrile terpolymers concurrently due to the different reactivities of the acidic comonomer and neutral comonomer,which often results in poor performance carbon fibers [6]. Additionally,the sequence structure of acrylonitrile terpolymers is hard to control. In order to replace ternary polymers with binary polymers,the bifunctional comonomer 3-aminocarbonyl-3-butenoic acid methyl ester (ABM) containing both ester and amide groups was synthesized to prepare poly(acrylonitrile-co-3-aminocarbonyl-3-butenoic acid methyl ester) [P(AN-co-ABM)] copolymer used as carbon fiber precursor in this work. For comparison,the poly(acrylonitrile-acrylamide-methyl acrylate) [P(AN-AM-MA)] and PAN were also synthesized. The stabilization mechanism and spinnability of P(AN-co-ABM),P(AN-AM-MA) and PAN were studied in detail. 2. Experimental 2.1. Synthesis ofβ-methylhydrogen itaconate

To a 100 mL round-bottomed flask,13.00 g of itaconic acid, 14.20 mL of methanol and 0.50 mL of benzoyl chloride were added and the mixture was refluxed at 65°C for 0.5 h and then cooled to room temperature. The reaction mixture was distilled under reduced pressure to remove excess methanol and followed by standing to get precipitation. The precipitation was recrystallized from benzene-petroleum ether (v/v,1:1).

White crystals,84.52% yield. IR (KBr,cm-1 ):vmax 3004,2955 (C-H),1726,1691 (C55O),1636 (C55C),1237,1170 (C-O). 1H NMR (400 Hz,DMSO-d6,r.t.,TMS): δ12.616 (s,1H,COOH),6.149 (d,1H, J= 1.20 Hz,CH2=),5.763 (d,1H,J= 1.20 Hz,CH2=),3.580 (s,3H, OCH3),3.336 (s,2H,CH2) (see Fig. S1 in Supporting information). 2.2. Synthesis of ABM

To a 150 mL three-necked flask,1.44 g of MHI,40.00 mL of chloroform,1.10 mL of thionyl chloride and 0.10 mL of N,Ndimethylformamide were added and the mixture was refluxed at 70°C for 0.5 h and then cooled to room temperature. The reaction mixture was distilled under reduced pressure to remove excess thionyl chloride. Chloroform (40 mL) was added to the remaining residue. Then anhydrous ammonia was passed into the solution at 0°C until no more precipitate formed. The precipitate was filtered off and washed with chloroform (3×40 mL). The obtained chloroform solution was distilled under reduced pressure to leave crystalline residue.

Yellow crystals,82.49% yield. IR (KBr,cm-1 ):vmax 3409,3174 (N-H),3000,2953 (C-H),1736,1670 (C55O),1645 (N-H),1605 (C55C),1174 (C-O). 1H NMR (400 Hz,DMSO-d6,r.t.,TMS): δ 7.594 (s,1H,CONH2),7.031 (s,1H,CONH2),5.893 (s,1H,CH255),5.531 (d, 1H,CH255),3.587 (s,3H,OCH3),3.3329 (s,2H,CH2) (see Fig. S2 in Supporting information). 2.3. Preparation of acrylonitrile polymers

PAN homopolymer,P(AN-AM-MA) and P(AN-co-ABM) were prepared by solution polymerization using DMSO as reaction media under nitrogen atmosphere. A typical polymerization is described as: 30,0000gof AN,1.6516 g of ABM,0.2532 g of AIBN and 94.9547 g of DMSO were added into a 250 mL three-necked flask equipped with condenser tube and stirrer. The polymerization was carried out at 60°C and terminated by methanol after 24 h. Then,the reaction mixture was added to excessive methanol with vigorous agitation to precipitate the polymer. The isolated polymer was washed with methanol for several times and then dried at 60°C under vacuum to a constant mass. The details of PAN polymers used in this study are given in Table 1.

Table 1
Experimental and calculated parameters of polymer samples.
2.4. Characterization

Proton nuclear magnetic resonance 1H NMR (400 MHz) spectra and 13C NMR spectra were recorded on a Bruker DMX-400 NMR spectrometer in dimethyl sulfoxide (DMSO-d6) as solvent at room temperature. The differential scanning calorimetry (DSC) and thermogravimetry (TG) curves of powder samples were carried out on TA instrument Modulated DSC 2910 and Netzsch TG 209 F1 thermal analyzer,respectively. The sample (3-4 mg) was scanned under air atmosphere (40 mL min-1 ) for TG analysis and under N2 (40 mL min-1 ) for DSC analysis. The oxygen (O) contents of P(ANco-ABM) copolymer and P(AN-AM-MA) terpolymer were determined on an Elementar Vario EL III elemental analyzer. Rheological measurements were recorded on a Haake RS150L Rotational rheometer at 70°C from 0 s-1 to 1000 s-1 ,and the concentration of polymer solutions was 18 wt%. 3. Results and discussion

3.1. DSC studies of PAN,P(AN-AM-MA) and P(AN-co-ABM)

Fig. 1 shows the DSC curves of PAN homopolymer,P(AN-AMMA) and P(AN-co-ABM) heated at 10°C min-1 from ambient temperature to 400°C under N2 (40 mL min-1 ). The parameters obtained from the exotherms,including the initiation temperature (Ti ),the termination temperature (Tf ) and their difference (ΔT=Tf×Ti ),the first peak temperature (Tp1 ,the peak at low temperature),the second peak temperature (Tp2 ,the peak at high temperature),the released heat (ΔH),and the velocity of releasing heat (ΔH/ΔT),are listed in Table 2.

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Fig. 1. DSC curves of PAN,P(AN-AM-MA) and P(AN-co-ABM) heated at 10°C min-1 .

Table 2
Parameters for DSC curves of PAN polymers.

The DSC curves of PAN polymers were obtained under N2 atmosphere,so there were no oxidative reactions occurred during this process and the exothermic peaks were attributed to the cyclization reactions. As shown in Fig. 1,there is one sharp exothermic peak in PAN homopolymer and the cyclization reactions can only be initiated through a free radical mechanism, resulting in a large amount of heat to be released at the same time, which breaks molecular chains and further results in defects in the final carbon fiber. Although the initiation temperature of P(ANAM-MA) decreases from 244.16°C to 205.25°C compared with PAN homopolymer,the heat release of P(AN-AM-MA) is still concentrative and expeditious because there is only one exothermic peak in DSC curve of P(AN-AM-MA). Whereas in P(AN-coABM) copolymer,there are two exothermic peaks and the cyclization reactions can be initiated through both radical and ionic mechanisms,which broadens the exothermic peak and effectively avoids centralized heat release. The lower exothermic peak (peak 1) is assigned to cyclization reactions initiated by the comonomer ABM through an ionic mechanism. As shown in Scheme 1,the nitrogen of the amide group (-CONH2) in ABM can initiate a nucleophilic attack on the carbon atom of adjacent nitrile groups and then induce molecules to cyclize. The DSC curve of P(AN-co-ABM) was deconvolved by muti-peak fittingviaorigin 7.5 as shown in Fig. S3 in the Supporting information. The result shows that about 30.37% (based on the peak areas) of cyclization reactions were initiated by ABM through the ionic mechanism. As shown in Table 2,theTi of P(AN-co-ABM) is much lower than that of PAN homopolymer and P(AN-AM-MA) terpolymer,suggesting that the cyclization reactions are more easily initiated in P(AN-co-ABM) copolymers than in PAN homopolymer and P(AN-AM-MA) terpolymer. P(AN-co-ABM) has the largest DHamong the three PAN polymers,implying more cyclization reactions occurred in P(AN-co-ABM) copolymer,and it is helpful in improving the extent of cyclization. Moreover,P(AN-co-ABM) has the smallestDH/DT, which can avoid centralized heat release effectively. As show in Table 1 (Section 2),the content of AM and MA in the feed stock is the same,however the content of AM in the resulting P(AN-AMMA) terpolymer is much lower than that of MA due to the lower reactivity of AM,and thus leads to a single exothermic peak in the DSC curve of P(AN-AM-MA). Furthermore,the MA in P(AN-AMMA) chains hinders the ionic cyclization reactions from proceeding along the molecular chains as shown in Scheme 2. Thus,P(AN-coABM) copolymer is much better than P(AN-AM-MA) terpolymer on improving the stabilization of PAN. The DSC curves of PAN,P(ANAM-MA) and P(AN-co-ABM) heated at different rates are shown in Fig. S4 (Supporting information). Based on Fig. S4,Kissinger’s method is used to calculate the activation energy (Ea ) of cyclization reactions [7]. The calculatedEa of the cyclization reactions for PAN homopolymer and P(AN-AM-MA) terpolymer is about 168.37 and 131.58 kJ mol-1 ,respectively,while for the P(AN-co-ABM) copolymer,theEa is splitted into two parts. The first part,calculated from the first exothermic peak,is assigned to ionic cyclization reactions, and the calculated Ea is about 90.12 kJ mol-1 which is much smaller than that of PAN homopolymer and P(AN-AM-MA) terpolymer,confirming that the cyclization of nitrile groups in the P(AN-co-ABM) copolymer has been promoted by ABM. The second part,assigned to radical cyclization reactions,is calculated from the second exothermic peak,and the Ea value (188.33 kJ mol-1 ) approaches that of PAN homopolymer. The Ea result further demonstrates that the P(AN-co-ABM) copolymer is much better than P(AN-co-ABM) terploymer on improving the stabilization of PAN.

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Scheme 1. Cyclization in P(AN-co-ABM) initiated through ionic mechanism.

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Scheme 2. Cyclization in P(AN-AM-MA) initiated through ionic mechanism.
3.2. TG studies of PAN,P(AN-AM-MA) and P(AN-co-ABM)

Fig. 2 shows the TG curves of PAN homopolymer,P(AN-AM-MA) and P(AN-co-ABM) heated at 5°C min-1 from ambient temperature to 700°C under air atmosphere (40 mL min-1 ). The TG curves of all PAN polymers can be roughly divided into three stages according to the rate of mass loss. The first stage is up toca.265°C exhibits a slow rate of mass loss. It is known that both cyclization and dehydrogenation reactions occur in this stage [8]. The cyclization reactions,theoretically,do not cause any mass loss [9],therefore the mass loss in this stage is mainly attributed to the dehydrogenation reactions. The magnified image in Fig. 2 shows that the mass loss of P(AN-co-ABM) copolymer is larger than that of PAN homopolymer and P(AN-AM-MA) terpolymer at the same temperature in this stage,indicating that the dehydrogenation reactions of P(AN-co-ABM) have been promoted by ABM,which is similar to the incorporation of β-methylhydrogen itaconate into PAN chains [9]. The probable reason is that the cyclization reactions of P(AN-co-ABM) can be improved by ABM through an ionic mechanism,which is beneficial to dehydrogenation reactions. The second step is up to 470°C and the rate of mass loss becomes rapid in this stage,which is mainly ascribed to the release of volatile particles formed by random degradation of polymer chains. The last stage is above 470°C with the fastest rate of mass loss among the three stages. In this stage,the mass loss of P(AN-coABM) is smaller than that of PAN and P(AN-AM-MA) at the same temperature,suggesting that the structural changes from linear to ladder induced by cyclization and dehydrogenation reactions have been promoted by ABM,and provides P(AN-co-ABM) better stability against high temperature treatment than PAN homopolymer and P(AN-AM-MA).

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Fig. 2. TG curves of PAN,P(AN-AM-MA) and P(AN-co-ABM) heated at 5°C min-1 .
3.3. Spinnability studies of PAN,P(AN-AM-MA) and P(AN-co-ABM)

The flow curves of PAN,P(AN-AM-MA) and P(AN-co-ABM) in DMSO solutions at 70°C are shown in Fig. 3. Obviously,all PAN polymer solutions show shear thinning at high shear rates,which are caused by the breakage of entangled networks and shear alignment of molecules [10]. As shown in Fig. 3,thehaof P(AN-coABM) and P(AN-AM-MA) is smaller than that of PAN homopolymer at low shear rate. The intermolecular forces of P(AN-co-ABM) and P(AN-AM-MA) are weakened by the introduction of comonomers ABM,AM and MA into PAN chains owing to the large molecular volume of ABM and lubrication of the ester group in ABM and MA. The structural viscosity index (Δη),which represents the degree of structuralization of polymer solution,can be calculated by following equation (Eq. (1)) [11]:

where ηa is the apparent viscosity of polymer solutions and ˙ gis the shear rate. It has been reported that the Δη can be used as an indication of spinnability [12]. The smaller the Δη value,the better the spinnability. The calculated Δη of PAN/DMSO solution is 7.89, while the Δη of P(AN-co-ABM)/DMSO and P(AN-AM-MA)/DMSO solutions is 6.14 and 5.92,respectively,hinting that the spinnability of P(AN-co-ABM) and P(AN-AM-MA) has been improved compared with PAN homopolymer due to the introduction of ester groups into the polymer chains. The spinnability of P(AN-co-ABM) is almost as good as that of P(AN-AM-MA).
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Fig. 3. Plots of lgha versuslg ˙ gfor PAN polymers in DMSO solutions at 70°C.
4. Conclusion

A novel P(AN-co-ABM) copolymer used as carbon fiber precursors was designed and prepared by solution polymerization. The DSC results show that the cyclization of P(AN-co-ABM) has been significantly improved by ABM compared with P(AN-AMMA) terpolymer with lower initiation temperature,smaller activation energy and broadened exothermic peak,which is mainly attributed to the ionic cyclization reactions induced by ABM. The TG results confirm that the dehydrogenation of P(AN-coABM) has also been promoted by ABM. Additionally,P(AN-coABM) has better spinnability than PAN as a result of the lubrication of ester groups in ABM,which is conducive to the preparation of high performance carbon fibers. Acknowledgment

s Financial support of this work from Important National Research Program ‘‘863’’ (No. 2012AA030313-1),Undergraduate Innovation Project (No. 1065210232130740) and The Fundamental Research Funds for the Central Universities (No. JUSRP11450) were gratefully acknowledged. Appendix A. Supplementary data

Supplementary data associated with this article can be found,in the online version,at http://dx.doi.org/10.1016/j.cclet.2014.03.009.

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