Comprehensive and quantitative study on the thermal oxidative stabilization reactions in poly(acrylonitrile‐co‐itaconic acid) copolymer

Thermal oxidative stabilization (TOS) reactions, mainly cyclization, oxygen uptake, dehydrogenation, crosslinking, and disproportionation termination of cyclization in poly(acrylonitrile‐co‐itaconic acid) (PAI), are tracked quantitatively by the combination of second‐derivative and cure‐fitting operations in the Fourier transform infrared spectroscopy spectra (FTIR) range of 1800–1000 cm^?1 and 2300–2100 cm^?1. Except the cyclization and disproportionation of cyclization are chemical‐reaction‐controlled process, the other three reactions are both diffusion‐controlled and chemical‐reaction‐controlled processes. The formation of free and conjugated carbonyl group need the optimal cyclization rate, adequate unreacted linear PAI chains, and appropriate oxidation ability coordinated. The higher temperature, the faster rate and the higher extent of the cyclization and the crosslinking reactions as well as the faster disproportionation termination of cyclization. The length of the formed cyclic structures became shorter as the evolution of TOS process, especially for 265 °C. The crosslinking, oxygen uptake, dehydrogenation, and termination of cyclization reactions become dominant with the proceeding of TOS process, especially as the cyclization extent above 0.8 and at high temperature such as 265 °C. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 45934.     

Structure evolution mechanism of polyacrylonitrile films incorporated with graphene oxide during oxidative stabilization

Oxidative stabilization is a key process for polyacrylonitrile (PAN)-based carbon materials. During this process, oxygen has a significant influence on the formation of cyclization structure and crosslinking structure of PAN matrix. Here, graphene oxide (GO) was used as filler in PAN matrix, the structure evolution of GO/PAN composite was studied during oxidative stabilization. Solubility measurement revealed that the crosslinking degree increased, while the cyclization degree of stabilized films decreased after GO incorporation. The effect of GO on the structure of PAN film was characterized by XRD, DSC, DMA, FTIR, and XPS. These results verified that GO could initiate the cyclization reaction of PAN at a lower temperature. The carboxylic groups on GO sheets might take part in the cyclization reactions during the heat treatment. Moreover, the oxygen-containing molecules released from GO decomposition during the heat treatment were beneficial to the formation of crosslinking structure. The possible mechanism of the structure evaluation was proposed in this article. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47701.     

Effect of boric acid on the stabilization of poly(acrylonitrile-co-itaconic acid)

The effect of boric acid on the stabilization of poly(acrylonitrile-co-itaconic acid) (P(AN-co-IA)) was studied in detail by differential scanning calorimetry (DSC) incorporating with thermogravimetry (TG) in air and nitrogen. In nitrogen atmosphere the ionic cyclization of the doped P(AN-co-IA) copolymer was badly retarded by boric acid, however, the free radical cyclization was little influenced. In air atmosphere boric acid was found to weaken the first exothermic peak corresponding to dehydrogenation and cyclization reactions, and enhance the second exothermic peak corresponding to the oxidative reactions resulting in a great expansion of the heat evolved. The activation energy of stabilization was calculated with Kissinger method, the results indicated that boric acid seemed to be effective to reduce the activation energy. TG studies showed that boric acid could reduce the weight loss during stabilization, which could be attributed to the crosslinking of polymer chains caused by the reaction between boric acid and the hydroxyl groups.     

DSC and DMA studies on silane‐grafted and water‐crosslinked LDPE/LLDPE blends

Effects of silane grafting and water crosslinking reactions on crystallizations, melting behaviors, and dynamic mechanical properties of the LDPE/LLDPE blends are investigated using DSC and DMA. From DSC data, cocrystallization of LDPE and LLDPE does not occur, but cocrosslinking of these two polymers is evidenced at the experimental temperature of 100°C, a temperature lower than melting temperatures of both polymers. The water crosslinking reactions of the LLDPE‐rich blends enable development of a new phase having a melting endotherm in between that of LDPE and LLDPE. From the thermal fractionation data, interaction between LDPE and LLDPE is observed, and compatibilization of the blends can be achieved by the crosslinking reactions. From DMA data, the storage moduli of the blends are not found to be consistent with their degrees of crosslinking. The storage moduli of the blends are not simply determined by the degree of crosslinking but determined by very complicated but unclear factors. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 1808–1816, 2001     

Effect of the nature and mole fraction of acidic comonomer on the stabilization of polyacrylonitrile

Differential scanning calorimetry (DSC) and thermogravimetric analysis measurements were used to study the influence of the acidic comonomers acrylic acid, methacrylic acid, and itaconic acid on the exothermic reactions occurring during the heat treatment of acrylonitrile copolymers under air atmosphere. The presence of these reactions was noticed in the DSC exotherms of the copolymers, which appear as doublets. These doublets were resolved into their constituent peaks, arising due to the occurrence of oxidative and cyclization reactions, and the area under the resolved peaks was considered as the extent of the reaction. The comonomers under study were found to enhance the secondary oxidation reactions and retard the extent of cyclization reactions. The degree of cyclization decreases abruptly with the increase in the comonomer content beyond ⋍ 2 mol %. © 1996 John Wiley & Sons, Inc.     

Thermoanalytical Investigations on the Effect of Atmospheric Oxygen on HTPB resin

TG-DSC studies, carried out on hydroxy-terminated polybutadiene (HTPB) in air and nitrogen atmospheres show three transitions which give rise to (1) an exothermic DSC peak and mass gain in TG at 170°C–240°C, seen only in air (2) exothermic peak and low mass loss, both in air and nitrogen, due to depolymerization, cyclization and cross linking, and (3) final mass loss corresponding to pyrolysis in nitrogen and combustion in air, appearing respectively as an endothermic peak and as a sharp exothermic peak in the two atmospheres. The FTIR spectrum of the product isolated from TG after the mass gain step shows addition of oxygen to the butadiene back bone. Arrhenius activation parameters (E and A) were computed for the exothermic oxygen addition reaction. The Ozawa method refined by MKN two-term approximation for p(x) function gave results quite comparable to those from Kissinger method.     

New polymers from epoxidized soybean oil with pyridine derivatives

In this study 4‐methylpyridine (4MP), 4‐vinylpyridine (4VP) and poly(4‐vinylpyridine) (P4VP) were separately reacted with epoxidized soybean oil triglycerides (ESO) to give plant oil based thermoset polymers. The addition reaction of pyridine with epoxide followed by a rearrangement results in formation of pyridone units and these were polymerized via a Diels–Alder reaction. DMA, DSC, TGA and IR spectroscopy were used for the characterization of the products. 4MP‐ESO, P4VP‐ESO and P4VP‐ESO‐in situ polymers were crosslinked yielding rigid infusible polymers. Glass transition temperatures (T_g) of 4MP‐ESO and P4VP‐ESO‐in situ were found as ?10.5 and 70.5 (32.3 as shoulder) °C respectively, by DMA analysis. Storage moduli of 4MP‐ESO and P4VP‐ESO‐in situ at 25°C were 13.7 and 187.2 MPa, respectively. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Oxidative stabilization of PAN/VGCF composite

An oxidative stabilization process to prepare carbon films was carried out for a new kind of precursor using the composite of polyacrylonitrile (PAN) and vapor‐grown carbon fiber (VGCF) by a process of gelation/crystallization from dilute solutions. It was found that the new precursor has special features in the stabilization process different from those of the homopolymer in regard to thermal and morphological aspects. In the stabilization process under heat treatment at 180–350°C in an oxidative atmosphere, it was inferred that, although the introduction of VGCF hinders the initiation and propagation step of the cyclization and dehydrogenation reactions, the precursor helps the oxidation and the additional aromatization and intermolecular crosslinking reactions in the stabilization process, thus promoting the formation of the later carbon product in film's shape with good performance. From characteristic works by wide‐angle X‐ray diffraction (WAXD), FTIR, Raman, and DSC, the changes of the precursors in structure, morphology, and mechanical property in terms of different heat‐treat temperatures and tensions were studied. Through a series of experimental results, the effect of the VGCF's introduction on those changes was discussed. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 87: 2063–2073, 2003     

Synthesis and properties of new polymerized monomer reactants matrix resins of pyrrolone–benzimidazole copolymers containing pyridine unit

A series of new polymerized monomer reactants (PMR) matrix resins of poly(pyrrolone‐benzimidazole)s containing a pyridine unit (PPBP) were synthesized by polycondensation of monoethyl ester of cis‐5‐norbornene‐endo‐2,3‐dicarboxylic acid, 2,6‐diphenyl ester pyridinedicarboxylic acid or 3,5‐diphenyl ester pyridinedicarboxylic acid, and diethyl ester of 4,4′‐oxydiphthalic acid with 3,3′‐diaminobenzidine in a mixing solution of anhydrous ethyl alcohol and N‐methylpyrrolidone under given temperature and pressure conditions. The resulting resin solutions showed good solubility in polar organic solvents and stability at room temperature. The corresponding PPBP matrix resin, molded powder, and molded plate were prepared by undergoing amidation, imidization, cyclization, and crosslinking reactions when the reaction temperature was increased from 80 to 350°C, successively; the crosslinking structure was formed by the reverse Diels–Alder reaction at 270–290°C under 50 MPa pressure (2.5–3.5 MPa displayed by the pressure meter). The chemical reactions and properties of the resulting PPBP were studied by means of FTIR, TGA, and DMA methods, and the results indicated that the kinds of PPBP materials retain excellent thermal stability and processability; when the initial decomposition temperature was above 620°C the T_g was at 413.5°C for 3,5‐PPBP‐20 molded plate. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 91: 3981–3990, 2004     

Crosslinking characterization of a polyimide: AFR700B

Crosslinking in AFR700B polyimide was studied using several techniques. Crosslinking could not be detected using Fourier transform infrared spectroscopy (FTIR) and differential scanning calorimetry (DSC). Swelling in n-methyl pyrrolidone (NMP) showed network formation occurs at cure temperatures ≥325°C (617°F), but the reaction kinetics could not be determined from the data. Dynamic mechanical analysis (DMA) studies have shown that the storage modulus (G′) increased with increasing cure temperature up to 350°C (662°F), and was constant above 350°C (662°F), indicating that crosslinking via the reverse Diels-Alder reaction was occurring. A broad secondary transition was seen in the loss modulus (G″) curves, centered between 150 and 180°C (302 and 356°F). This secondary transition appeared at a cure temperature of 300°C (572°F) and became more dominant with increasing temperature and time. This secondary transition was not seen for cures <300°C (572°F) when the chains have not crosslinked. Therefore, this is likely due to a crosslink bond rather than one in the backbone. At 1 atm, crosslinking followed second-order kinetics based on the increase in glass transition temperature (T_g). The Arrhenius plot of the rate constant showed a break in the slope, possibly indicating a change in reaction mechanism. At 1.38 MPa (200 psi), the T_g data was too scattered to determine the kinetics.     

Correlation of chemical shrinkage of polyacrylonitrile fibres with kinetics of cyclization

It has been established that the most important step in the production of carbon fibres from polyacrylonitrile (PAN) precursor fibres is the oxidative thermal treatment applied. During this treatment, physical phenomena and chemical reactions take place accompanied by the shrinkage of the fibres, which has a physical or chemical origin, depending on the nitrile cyclization reactions. The aim of the present study is to establish a correlation between the chemical shrinkage of PAN fibres and the kinetics of cyclization reactions. Based on the isothermal treatment of PAN fibres, we developed a method in order to distinguish between physical and chemical shrinkages. The onset time for the chemical shrinkage follows a relationship with temperature. Chemical shrinkage versus cyclization time data were fitted with the exponential rise to the maximum of the curves. Furthermore, the cyclization kinetics was studied using differential scanning calorimetry and the kinetic parameters determined were identical to those calculated from the chemical shrinkage, demonstrating that the latter is directly related to the kinetics of the cyclization reactions. It was therefore concluded that according to the method developed to distinguish the physical from the chemical shrinkages: (1) there exists a certain onset time for a given treatment temperature to trigger the chemical shrinkage; (2) cyclization reactions do not start below a limiting temperature of 168 °C; (3) at 340 °C, the temperature where the cyclization reactions are completed, the maximum shrinkage is 24%; and (4) the oxidized PAN fibres contain mainly ladder polymer structures with approximately symmetrical sequences connected in angled positions. Copyright © 2007 Society of Chemical Industry     

氧对PAN分子侧氰基反应进程的影响

采用固体13C-CP／MAS,FT-IR和DSC方法分析自制二元共聚PAN纤维在不同气氛下的热稳定化 行为,研究氧对PAN分子侧氰基反应的影响。结果表明:在无氧气氛下处理时,PAN分子先发生环化反应; 而在有氧气氛下,先发生脱氢反应,再发生环化、氧化反应,氧化反应中氧与PAN分子链结合生成C=O, HC-OH等官能团,氧可以促进环化、脱氢反应的发生,PAN链状分子结构逐渐演变为耐热的含氧梯型聚合 物结构。     

Effects of composition of hardener on the curing and aging for an epoxy resin system

Different mixture ratios of Shell Epon 828 (based on diglycidyl ether of bisphenol A, DGEBA) and Shell EPI‐CURE 3046 (based on triethylenetetramine, TETA) were evaluated under different environments of isothermal curing at 80°C in DSC, room temperature curing in air, and aging in water at 45°C. The curing reactions were monitored using differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), and infrared spectroscopy (IR). It was shown that the initial curing rate increased with the amount of hardener. However, the epoxy groups in samples with excess hardener were prone to reaction with primary amines located at the ends of TETA molecules, resulting in a less dense epoxy network. During aging in water at 45°C, significant effects of water on the postcure and the increased water absorption with an increase of hardener amount were observed. The DMA results show that the samples with hardener around stoichiometric composition have the greatest storage modulus while curing in air environment. However, the samples with hardener much less than stoichiometric composition have greater storage modulus under aging in water at 45°C. in water at 45°C. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 99: 580–588, 2006     

Development of new poly(ϵ‐caprolactone)/chitosan films

This study reports for the first time the production of poly(?‐caprolactone)/chitosan (PCL/CHT) films by solvent casting using a mixture of formic acid/acetone (70:30 vol%). Both uncrosslinked and crosslinked films with the natural crosslinker genipin were developed. The mechanical properties of the samples were analyzed by dynamical mechanical analysis (DMA) in the hydrated state. DMA was also successfully used for the first time to monitor in situ the crosslinking process as a function of time and crosslinker concentration. The compatibility between the polymers in the blended films was analyzed by Fourier transform infrared (FTIR) spectroscopy and differential scanning calorimetry (DSC). It was found that phase distribution is highly dependent on the blend composition. The developed films could potentially be used in different applications, such as tissue engineering scaffolds. © 2013 Society of Chemical Industry     

Cross‐linking of ethylene‐octene copolymer (EOC) by dicumyl peroxide (DCP)

Ethylene‐octene copolymer (EOC) was crosslinked by dicumyl peroxide (DCP) at various temperatures (150–200°C). Six concentrations of DCP in range 0.2–0.7 wt % were investigated. cross‐linking was studied by rubber process analyzer (RPA) and by differential scanning calorimetry (DSC). From RPA data analysis real part modulus s', tan δ, and reaction rate were investigated as a function of peroxide content and temperature. The highest s'_max and the lowest tan δ were found for 0.7% of DCP at 150°C. Chain scission was analyzed by slope analysis of conversion ratio, X in times after reaching the maximum. Less susceptible to chain scission are temperatures in range 150–170°C and peroxide levels 0.2–0.5%. Heat of reaction was analyzed by DSC at various heating rates (5–40°C min⁻¹). It was found to be exothermic. By projection to zero heating rate, the reaction was found to start at 128°C with the maximum at 168°C. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Preparation and characterization of shape memory polymer networks based on carboxylated telechelic poly(ɛ-caprolactone)/epoxidized natural rubber blends

Shape memory polymer networks were prepared from blends of end-carboxylated telechelic poly(?-caprolactone) (XPCL) and epoxidized natural rubber (ENR). The XPCL/ENR blends can form cross linked structure via interchain reaction between the reactive groups of each polymer during molding at high temperature. Degree of crosslinking of the blend network and their thermomechanical properties were characterized by gel content measurement, DSC and DMA. We found that the degree of crosslinking and crystalline melting transition temperatures was dependent upon the blend compositions as well as the molecular weight of the XPCL segment in the blends. The blends showed a good shape memory behavior such as good shape fixity as well as a high final recovery rate when they exhibit crystalline melting transition with a sufficiently high degree of crosslinking. And the response temperature of the recovery was well matched with T_m of the samples.     

Kinetics of the cyclization and isomerization reactions in polyacrylonitrile based carbon fiber precursors during thermal-oxidative stabilization

The kinetics of reactions in polyacrylonitrile (PAN) based carbon fiber (CF) production should be of significance to the guidance of process control, fiber structure formation. PAN precursor fibers were isothermally stabilized at 210, 225, 240, 255, and 270 °C, respectively, for 10 to 100 min in an air oven to study the kinetics of the cyclization and isomerization reactions. The structural evolution of PAN precursor fibers during thermal-oxidative stabilization was characterized by Fourier transform infrared (FTIR) spectroscopy and solid state ¹³C nuclear magnetic resonance (¹³C NMR). The results indicate that the FTIR absorbance of  CN (the resultant of the cyclization) in PAN shows a trend of first increasing and then decreasing. And then the NMR peak assigned to the carbon atoms linking imino groups ( NH ) proves the isomerization of  CN into  NH in pyridone structure. Based upon the FTIR absorbance method, the entire process of the cyclization and isomerization reactions is considered as a consecutive first-order reaction. A kinetic model for the consecutive reaction has been established via the evaluation of the reaction rate constants of two single reactions. According to the model, the simulated kinetic curves of the characteristic groups ( CN,  CN , and  NH ) conform to the FTIR absorbance trends of these groups based on experimental data. This study is expected to furnish in-depth information on the crucial reaction kinetics during stabilization of PAN precursors, which is of advantage to the process optimization of the CF production. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48819.     

Mechanism and kinetics of oxidation during the thermal stabilization of polyacrylonitrile fibers

The thermal behavior and structural evolution during the thermal stabilization of polyacrylonitrile (PAN) fibers in N₂ and air were investigated using differential scanning calorimetry and solid‐state ¹³C nuclear magnetic resonance. It was found that an oxidation reaction, that generated carbonyl (C?O) groups could occur at 160°C which has not been reported in the literature. It is proposed that the cyclized structures in the PAN macromolecule chains are a prerequisite for the oxidation. Further investigations indicate that with more cyclized structures in the PAN macromolecule chains, the oxidation proceeds more readily, which is consistent with the proposed mechanism. The kinetic parameters for the oxidation and cyclization reactions were estimated using the Kissinger method. The activation energies for the reactions of oxidation and cyclization for PAN fibers are about 96.4 kJ/mol and 190.0 kJ/mol, respectively, which implies that the cyclization is the rate determining step during the thermal stabilization of PAN fibers. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Mass DSC/TG and IR ascertained structure and color change of polyacrylonitrile fibers in air/nitrogen during thermal stabilization

The structure evolution was studied by mass spectrum (MS), differential scanning calorimetry (DSC) and thermogravimetry (TG), Fourier transform infrared (FTIR) spectroscopy. The results indicated that the CN and CC groups appeared gradually with the increase of the temperature in air and nitrogen. The CO groups appeared because of oxidative reaction in air. The CN, CC and CO groups were all chromophores. The effect of conjugated CN and CC on the absorption of the visible light was shifted to longer wavelengths and indicated π‐π* transition. There was a strong bathochromic effect as the number of CC bonds were increased. The effect of CO and –NH₂ on the absorption of the visible light was shifted to longer wavelengths and indicated n‐π* transition. Oxygen could facilitate chemical reactions in air. Hence, the color of PAN in air was deeper than in nitrogen at the same temperature. The structural change of PAN in air was faster and more complex than in nitrogen. PAN fibers treated in air turned black after 230°C. However, PAN fibers turned black at 350°C in nitrogen. The MS and FTIR indicated that cyclization occurred before dehydrogenation during stabilization in air and nitrogen. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Reaction rate and MWD changes in crosslinking of PVC,with dithioltriazine

The reaction rate of crosslinking of PVC with dithioltriazine has been studied by following gel formation and changes in the molecular weight distribution (MWD). Compounding was performed on a roll mill at 145°C and crosslinking by heat treatment at 180 or 90°C. In this system crosslinking is executed by the thiolate anion, formed in situ by reaction with MgO. We have studied the catalyzing effect of several polyols in order to achieve a more efficient reaction. Most likely, these catalysts work by chelating the Mg²⁺ ions, thus increasing the nucleophilic character of the thiolate. With the most efficient ones, ditrimethylolpropane and HO(CH₂CH₂)_6-7H, complete crosslinking can be obtained in 3 min at 180°C, i.e., at processing temperatures. We also followed the changes in the MWD before gelation at a considerably lower temperature, 145°C, and found an extensive molecular enlargement even after 5-10 min. Most surprisingly, μM_n increased up to 100% without formation of insoluble material. By ¹H-NMR measurements on low molecular weight extracts, we have shown this to be due to a fast and selective reaction with allylic chlorine in the unsaturated end groups, ～ CH₂? CH?CH? CH₂Cl, formed in the mechanism of chain transfer to monomer. Due to this reaction, formulations with too high reactivity may crosslink during processing, which calls for a careful balancing of the reactivity for each processing case.