Effects of the stabilization temperature on the structure and properties of polyacrylonitrile-based stabilized electrospun nanofiber microyarns

Polyacrylonitrile (PAN) precursor nanofiber microyarns were successfully synthesized using a modified electrospinning method, and then were stabilized at different stabilization temperatures. Effects of stabilization temperature on the structure, thermal, and mechanical properties of stabilized microyarns were studied by FTIR, XRD, DSC, TG, and mechanical behavior test. It was found that when the stabilization temperature was above 250 °C, cyclization and dehydrogenation reactions were basically completed, and PAN crystal microstructure was almost completely destroyed, as well as the thermally stable ladder structure was formed. The results also showed that the strength of the stabilized microyarns increased to a threshold value and started to reduce thereafter as the stabilization temperature increased.     

Thermal behavior of polyacrylonitrile polymers synthesized under different conditions and comonomer compositions

Polyacrylonitrile (PAN) polymers are used as precursors for carbon fiber production. This process requires an oxidative stabilization step, which can be studied by differential scanning calorimetry (DSC). In this sense, thermal behavior of PAN based terpolymers by different polymerization processes, compositions and itaconic acid concentrations in the reaction media were investigated. The obtained results showed that the addition of itaconic acid and methyl acrylate as comonomers resulted a lower heat flow during the process comparing to the PAN homopolymer. It suggested that these comonomers aid the oxidative stabilization stage for all studied process. The redox system polymerization at 40°C resulted in a lower heat flow. Itaconic acid decreases slightly initial and peak temperatures of the terpolymer and heat flow until concentration of 3y. The cyclization temperature decreases when MAis incorporated into the terpolymer compared to the MMA terpolymer and increases when MAA is the acidic monomer. Among terpolymers the AN/MA/AA polymer showed the best thermal behavior for carbon fiber producing.     

Mechanism and kinetics of the stabilization reactions of itaconic acid-modified polyacrylonitrile

The structural evolution and thermal behavior of polyacrylonitrile (PAN) homopolymer and copolymer [P(AN-IA)] containing about 1.5 mol% itaconic acid (IA) during stabilization in air were studied by Fourier transform infrared (FTIR) spectroscopy, differential scanning calorimetry (DSC) and thermogravimetry (TG). A new parameter Es=A1595cm⁻¹/A2243cm⁻¹ was defined to evaluate the extent of stabilization. The kinetic parameters, viz. activation energy (E_a) and pre-exponential factor (A) of the stabilization reactions were calculated by Kissinger method and Ozawa method. FTIR analysis indicated that the cyclization of nitrile groups was initiated at a lower temperature by the IA comonomer and the stabilization proceeded at a more moderate pace in P(AN-IA) than in PAN, while an IA additive was found to be decomposed and failed to initiate the cyclization at a lower temperature. The improvement effect of IA comonomer on the stabilization reactions was further confirmed by the dynamic thermal analysis and kinetic study.     

Graft copolymerization of itaconic acid-methacrylamide comonomers onto poly(ethylene terephthalate) fibers

The graft copolymerization of itaconic acid-methacrylamide (IA-MAAm) comonomers was carried out using benzoyl peroxide as initiator onto poly(ethylene terephthalate) (PET) fibers in an aqueous medium. The grafted fibers were characterized by FTIR, TGA, DSC and SEM analysis. Effect of various parameters on graft yield such as feed composition, feed and initiator concentration, reaction time and temperature were investigated. The graft yield in the presence of MAAm increased because of the synergistic effect of MAAm comonomer. While, the graft yield alone with the IA onto PET fiber was 2.2%, the use of MAAm as a comonomer increased the amount of IA introduced to the PET fiber up to 13.7%. The reactivity ratios for both monomers were determined by using a Fineman-Ross plot. The grafting rate and saturation graft yield was increased upon increasing the temperature between 65 °C and 85 °C. When the temperature increased further than 85 °C, the saturation graft yield decreased. The graft yield has shown an increase up to an initiator concentration of 1.0 × 10⁻² M and slightly decreased. The grafting increased the dyeability with acidic and basic dyes, and moisture absorption capacity but decreased the thermal stability of the fibers.     

FT-IR study of the stabilization reaction of polyacrylonitrile in the production of carbon fibers

The stabilized fibers prepared by heating PAN and a PAN copolymer in air and under reduced pressure have been examined by FT-IR spectroscopy in order to determine their chemical structures. Three kinds of reactions, cyclization, dehydrogenation, and oxygen uptake are found to occur almost simultaneously in the stabilization process in air by digital difference spectrum method. The effect of the comonomer is confirmed to accelate the dehydrogenation reaction and also the formation of acridone ring in the thermal stabilization in air by kinetic study.     

Structure and property of electrospinning PAN nanofibers by different preoxidation temperature

In this work, PAN fibers web was fabricated by Electrospinning, and then was pre-oxidated. Effect of the temperature on the structure and property of pre-oxidation web was discussed. The results showed that better level of pre-oxidation nanofibers web can be obtained when the pre-oxidation temperature is 250 °C. At this temperature, Infrared Spectroscopy showed that cyclization and dehydrogenation reaction have occurred and DSC curves showed that cyclization was basically complete, as well as moisture content can be appropriately controlled. Moreover, the preoxidated web with better breaking strength, elongation at break, and the initial modulus could be obtained.     

Synthesis and characterization of poly (acrylonitrile‐co‐acrylic acid) as precursor of carbon nanofibers

Synthesis of a co‐polymer of polyacrylonitrile (PAN) producing a carbon nanofiber out of PAN and co‐polymer of PAN and comparison between these products were examined. Free‐radical solution copolymerization of acrylonitrile (AN) with acrylic acid (AA) was studied. In this perspective, AA, and AN were used as a precursor for polymerization reactions; then copolymers were synthesized by using ammonium persulfate (APS) as an oxidant and carried in water/dimethylformamide (DMF) mixture. These polymers were used to obtain corresponding electrospun nanofibers. Synthesized P(AN‐co‐AA) was investigated by Fourier transform infrared spectroscopy‐attenuated total reflection (FTIR‐ATR) spectroscopy, and characteristic peaks for AN unit, AA were achieved. Thermal behavior was examined by using differential scanning calorimeter (DSC) and thermal gravimetric analyzer (TGA), and results indicated that addition of monomers to AN unit reduced the Tg value of homopolymer PAN compared to P(AN‐co‐AA), which provides improvement to the cyclization and the formation of a thermally stable aromatic ladder polymer chain formation. In order to prevent the shrinkage and maintain the molecular orientation on nanofiber webs during stabilization, tension was applied to the samples, and thermal oxidation varies at 200–300°C for different duration of times. Surface morphology of the fibers was observed with scanning electron microscope (SEM), and average nanofiber diameter was found 550 nm, and after carbonization it was reduced to 320 nm for homopolymer PAN, and for poly(AN‐co‐AA) average nanofiber diameter was found as 220 nm and reduced to 130 nm, respectively. Copyright © 2016 John Wiley & Sons, Ltd.     

Structure evolution mechanism of poly(acrylonitrile/itaconic acid/acrylamide) during thermal oxidative stabilization process

Polyacrylonitrile (PAN) polymers with different compositions were prepared by an efficient aqueous free-radical polymerization technique.Thermal properties of polyacrylonitrile homopolymer (PAN),poly(acrylonitrile/itaconic acid) [P(AN/IA)] and poly(acrylonitrile/itaconic acid/acrylamide) [P(AN/IA/AM)] were studied by Fourier transform infrared spectroscopy,X-ray diffraction,differential scanning calorimetry and thermogravimetry in detail.It was found that AM had the ability to initiate and accelerate thermal oxidative stabilization process,which was confirmed by the lower initiation temperature and broader exothermic peak in P(AN/IA/AM) as compared with that in P(AN/IA) and PAN.The intensity of heat releasing during the thermal treatment was relaxed due to the presence of two separated exothermic peaks.Accompanied by DSC analysis and calculation of the apparent activation energy of cyclization reaction,two peaks were assigned to the ionic and free radical induction mechanisms,respectively.The higher rate constant in P(AN/IA/AM) indicated that the ionic mechanism actually had a kinetic advantage at promoting thermal stability over the free radical mechanism.This study clearly show that the synthesized P(AN/IA/AM) terpolymers possess larger room to adjust manufacture parameters to fabricate high performance of PAN-based carbon fibers.     

Property changes of powdery polyacrylonitrile synthesized by aqueous suspension polymerization during heat-treatment process under air atmosphere

High molecular weight powdery polyacrylonitrile (PAN) polymers were prepared by aqueous suspension polymerization employing itaconic acid (IA) as comonomer and alpha,alpha(')-azobisisobutyronitrile (AIBN) as initiator at 60 degrees C. PAN polymers obtained with different monomer ratios were characterized by EA, DSC, FTIR and XRD. It is investigated that the oxygen element content in PAN polymers increased with the increase of required IA amounts in the feed and heat-treatment temperatures. DSC curves of PAN copolymers exhibited the triplet character, owing to the exothermic cyclization and oxidative reactions during heat-treatment process. Introduction of IA in the feed relaxed exothermic reactions of PAN polymers under air atmosphere. Structure and crystallinity changes were affected by required IA amounts in the feed and enhancement of heat-treatment temperatures. The characteristic functional groups (including C[triple bond]N, C=O, CH(2)) presented in FTIR spectra of PAN polymers indicated copolymerization reaction of AN and IA. Existence of some organic groups (C-O, C=C and/or C=N) indicated formation of ladderlike structure during heat-treatment process. PAN homopolymer had the better crystallinity (mainly peak intensity and peak area around 2theta = 17 degrees) than most RT-PAN copolymers. When heat-treatment temperature is around 210 degrees C, peak intensity, peak area, L(c) and CI of HT-PAN polymers corresponding to samples 1# and 2# got maxima, while crystallinity became weak at higher heat-treatment temperatures.     

Thermo-chemical reactions and structural evolution of acrylamide-modified polyacrylonitrile

<正>Thermal properties of acrylonitrile(AN)-acrylamide(AM) copolymers for carbon fibers were studied by DSC and in situ FTIR techniques in nitrogen(N_2) and air flows.The cyclization mechanism and stabilization behavior of polyacrylonitrile(PAN) were discussed.In N_2 flow,it was found that AM had the ability to initiate and accelerate cyclization process,which was confirmed by the fact that the initiation of nitriles shifted to a lower temperature.Compared to AN homopolymer,the initiation temperature of cyclization was ahead 32 K by introducing 3.59 mol%AM into the copolymer.The exothermic reaction was relaxed due to the presence of two separated exothermic peaks.Accompanied by DSC,in situ FTIR and calculation of activation energy,the two peaks were proved to be caused by ionic cyclization and free radical cyclization,respectively,and the corresponding cyclization mechanism was proposed.With increasing in AM content,the ionic cyclization tends to be dominant and the total heat liberated first increases and then decreases.For AN homopolymer,the activation energy of cyclization is 179 kJ/mol.For AN-AM copolymer(containing 3.59 mol%AM),the activation energy of ionic cyclization is 96 kJ/mol and that of free radical cyclization is 338 kJ/mol.In air flow,similar cyclization routes occur and the difference is the contribution of oxidation.The oxygen in environment has no remarkable effect on cyclization of AN homopolymer but retards the cyclization of AN-AM copolymers.For AN-AM copolymer with 3.59 mol%AM,the cyclization temperature is postponed 10℃in air.     

Einfluß von MA-Copolymergehalt der PAN-Fasern auf ihr Pyrolyseverhalten während der Oxidation

Zusammenfassung Der Verlauf der Dehydrierungs- und Zyklisierungsreaktionen, die während der Oxidation der PAN-Fasern stattfinden, wurde in einem breiten Temperaturbereich (120–320 °C) und bei verschiedenen Oxidationszeiten mit DTA- und IR-Spektroskopie-Messungen untersucht. Außerdem wurde der Einfluß des Methylacrylat (MA)-Comonomeren während der Oxidation der PAN-Fasern untersucht. Die Dehydrierungsreaktionen erfolgen in Luft vor den Zyklisierungsreaktionen. MA wird bei der oxidativen Stabilisierung nicht abgespalten, sondern teilweise in die Zyklisierung mit einbezogen. Gemäß den Oxidationsbedingungen (Temperatur, Zeit) wird das Methylacrylat mehr oder weniger abgebaut. Eine kontinuierliche Pyrolyse der Fasern (z. B. mit einer Aufheizrate von 10 °C/min) führt zu einem heftigen Abbau des MA oberhalb von 250 °C, so daß durch die Erzeugung von und die Zyklisierung beschleunigt wird. Bei isothermar Oxidation der Fasern unterhalb der Temperatur, wo die exothermen Reaktionen in den Fasern stattfinden (d. h. unterhalb von ungefähr 250 °C), ist die Erhitzung der Fasern mild, so daß nur ein kleiner Teil des MA abgebaut wird, d. h. die aktiven Zentren nehmen nicht zu und die Zyklisierung läuft langsam.

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Summary The process of the dehydration and cyclization reactions that occur during the oxidation of the PAN-fibers, were studied with measurements by DTA and IR-spectroscopy in a wide area of temperatures (120–320 °C) and for different time of oxidation.It was additionally examined the influence of the methylacrylate (MA)-comonomer during the oxidation of the PAN-fibers. The dehydration reactions take place before the cyclization reactions in the presence of the air. MA is not separated during the oxidative stabilization, but it is partially included in the cyclization. According to the conditions of the oxidation (temperature, time) is the methylacrylate more or less decomposed. One continuous pyrolysis of the fibers (e. g. with one rate of heating about 10 °C/min) lead to a rapid decomposition of the MA above 250 °C, so the activated centers are also increased because of the production of >C=O bonds and the cyclization is accelerated. In an isothermal oxidation of the fibers below the temperature, where the exothermal reactions occur in the fibers (i. e. under about 250 °C), is the heating of the fibers mild, thus only a small part of the MA is decomposed, i. e. the activated centers don't increase and the cyclization goes slowly.

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Mit 5 Abbildungen und 3 Tabellen     

Influence of homogenization and drying on the thermal stability of microfibrillated cellulose

Homogenization has been used to release microfibrils from cellulose fibres to produce microfibrillated cellulose (MFC). Oven drying, atomization or freeze-drying were used to dry MFC. Morphological differences were observed linked to the compaction of the system and the formation of microfibril agglomerates. Thermal stability of the dried MFC, checked by TGA, decreased after homogenization and drying. Char level at the end of the pyrolysis was higher than for cellulose fibres. Derivative TGA (dTGA) showed a shoulder around 250 °C for the dried MFC. Volatile degradation product detection by FTIR spectroscopy (FTIR) coupled to TGA and DSC showed that the shoulder corresponds to expected dehydration reactions of the cellulose. Increasing the contacts between microfibril(s) (bundles) and agglomerates of the freeze-dried MFC by compression promoted dehydration reactions. Homogenization and drying modified the thermal properties of the MFC. No significant influence of freeze-drying kinetics on the thermal behaviour of the MFC was observed.     

Effects of synthetic and natural zeolites on morphology and thermal degradation of poly(lactic acid) composites

Poly(lactic acid) (PLA) composites containing 5 wt% synthetic (type 4A) and natural (chabazite) zeolites were prepared using extrusion/injection molding. Morphological, structural, and thermal properties of composites were investigated by scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), and differential scanning calorimetry (DSC). DSC results revealed that the glass transition and melting temperatures were not significantly changed; however, the incorporation of both type 4A and chabazite zeolites enhanced the nucleation of PLA crystallites as well as increased the percent crystallinity. Thermal degradation properties of PLA and PLA/zeolite composites were studied by non-isothermal thermogravimetric analysis (TGA) in nitrogen atmosphere. TGA results showed that at temperatures above 300 °C, PLA/type 4A synthetic zeolite composites were thermally decomposed more easily than the PLA and PLA/chabazite natural zeolite composites. The apparent activation energies of thermal degradation of PLA and PLA/zeolites composites estimated using both the Flynn-Wall-Ozawa and Kissinger methods followed the same order: PLA/type 4A < PLA/chabazite < PLA.     

Effects of deformation-induced orientation on cyclization and oxidation of polyacrylonitrile fibers during stabilization process

Orientation of copolymer polyacrylonitrile(PAN) chains during their deformation prior to stabilization and the further effect on the stabilization were investigated in detail.Results reveal that the orientation of PAN chains presents a saturation point of 69.51%when the deformation ratio reaches approximately 1.07,meanwhile the cyclization rather than the oxidation has a stronger dependence on the orientation of PAN chains during stabilization.The cyclization is facilitated that the cyclization degree is increasing while the activation energy is decreasing obviously as a consequence of the developing orientation of PAN fibers before the saturation point;however,it is restrained during the further deformation of PAN fibers after the point.The resulting carbon fibers obtained from the PAN fibers prepared at the saturation point possess the highest mechanical properties of 4.07 GPa in tensile strength and 249.0 GPa in tensile modulus.     

Kinetics of cyclization reaction in poly(acrylonitrile/methyl acrylate/dimethyl itaconate) copolymer determined by a thermal analysis

The PAN/MA/DMI (poly(acrylonitrile/methyl acrylate/dimethyl itaconate)) copolymer was synthesized for being used as a carbon fiber precursor. Its high melt-spinnability at 175–210 °C has been demonstrated elsewhere but the cyclization kinetics was thoroughly investigated by DSC analysis herein. The isothermal analysis based on the Kamal equation demonstrates that at a given temperature, the cyclization rate constant k of PAN/MA/DMI system is five to seven times that of a regular PAN/MA system. The activation energy E_a of the PAN/MA/DMI system, owing to the acid-catalysis by the DMI compound, is approximately 30 kJ/mol lower than that of a regular PAN/MA system. An isothermal Avrami analysis confirmed the above finding. Furthermore, based on the obtained nucleation index n, DMI may induce intermolecular cyclization which can rapidly stabilize the fiber shape of PAN/MA/DMI precursor. A nonisothermal analysis, obtained from the Ozawa's method, revealed the same tendency as did the isothermal results, but the difference between the average activation energies E_a obtained with and without DMI was reduced from 30 to 25 kJ/mol. Finally, a cyclization mechanism that involves the formation of a cyclic anhydride was proposed.     

Thermal oxidation products and kinetics of polyethylene composites

The thermal oxidation behavior of high-density polyethylene (HDPE) composites was investigated at 60 °C, 90 °C and 110 °C, using Fourier transform infrared (FTIR) spectroscopy and thermogravimetric analysis (TGA). The inorganic fillers do not modify the thermal oxidation mechanisms of HDPE. However, they have great effects on the thermal oxidation kinetics—both the activation energy and the pre-exponential factor increase. That means that although the addition of inorganic fillers retards the onset of thermal oxidation of HDPE, once the oxidation begins, it proceeds much faster than that of HDPE. Stability evaluation of HDPE composites by TGA was not consistent with the result by FTIR. The inorganic fillers influence the oxidation products and their distribution greatly. HDPE/STC and HDPE/mica oxidations were delineated by large amount of carbonyl formation, especially esters and ketones, while HDPE/wollastonite and HDPE/diatomite showed minimum carbonyl formation. In HDPE composites, there is a good relation between the carboxylic formation and the carbonyl index.     

Structural evolution of poly(acrylonitrile‐co‐dimethyl itaconate) copolymer during thermal oxidative stabilization

The structural evolution of poly(acrylonitrile‐co‐dimethyl itaconate) [P(AN‐DMI)] copolymer was investigated by Fourier transform infrared spectroscopy (FTIR) in detail and compared with the polyacrylonitrile (PAN) homopolymer. The extent of cyclization reactions was calculated from the FTIR data. It was found that DMI comonomer had the ability to promote the cyclization reactions significantly at the temperature of 240°C, compared to the PAN homopolymer. The results of quantitative FTIR analysis in the range of 1000–1800 cm^?1 showed that the DMI comonomer not only promoted the cyclization reactions, but also facilitated the oxygen uptake reactions, especially the conjugated carbonyl group in an acridone ring in the ladder polymer chains, which proved that DMI comonomer had the potential ability to make successful thermal oxidative stabilization (TOS) process. The positive effects of DMI comonomer on TOS reactions and carbon yield were further confirmed by the dynamic thermogravimetry (TG) analysis. Copyright © 2015 John Wiley & Sons, Ltd.     

衣康酸对聚丙烯腈原丝结构和性能的影响

控制单体配比 ,采用丙烯腈 (AN)与衣康酸 (IA)自由基溶液共聚 ,以偶氮二异丁腈为引发剂在溶剂二甲基亚砜中合成了聚丙烯腈原丝纺丝溶液 ,并纺制了碳纤维前驱体聚丙烯腈原丝 .通过元素分析、IR、DSC、13 C NMR等手段 ,讨论了共聚单体IA对共聚反应及聚丙烯腈原丝结构和性能的影响 .共聚反应时 ,共聚单体IA的加入量控制在AN/IA =98/2 (W/W )较合适 .利用IR谱 ,可定量分析IA在共聚中的摩尔分数 .经13 C NMR分析 ,随着共聚单体IR在共聚物中的摩尔分数的增加 ,共聚物的全同规整度增加 ,达到一定值后又呈下降趋势 .共聚单体IA能在较低温度时引发聚丙烯腈原丝的氧化、环化放热反应 ,且能减缓放热效应 .