Preparation Of High Molecular Weight Polyacrylonitrile Used To Manufacture Carbon Fibers

Polyacrylonitrile (PAN) precursors used to produce carbon fibers are polymerized by acrylonitrile and a small quantity of comonomers. Itaconic acid (IA) is the most frequently employed. The effective approach to manufacture high performance carbon fibers is adopting high molecular weight PAN copolymers as solute to prepare spinning dope. Compared to the conventional homogenous solution radical polymerization, it is propitious to utilize aqueous deposited polymerization to synthesize high molecular weight PAN polymers. And the polymerizations have higher conversions. In this polymerization system, water (H2O) is used as reaction medium, whose chain transfer constant is 0. And chain transfer reactions to AN radicals can be avoided. Chain branching also reduces. The main objective of this paper was specific to preparation of high molecular weight PAN used to manufacture carbon fibers by aqueous deposited polymerization technology. Effects of various polymerization factors on aqueous deposited copolymerization of AN and IA were investigated through Ubbelohde viscometer (UVM), differential scanning calorimeter (DSC), thermogravimetric analysis (TGA), element analysis (EA), wide angle X-ray diffraction (WAXRD), Fourier transform infrared spectroscopy (FTIR), nuclear magnetic resonance (NMR) and rotational viscometer. Structures and properties of the obtained PAN polymers were studied, whose relationships with the parameters of polymerization technology were also discussed. And valuable theoretical achievements applied to industrialized production stage of carbon fibers were acquired.As a single water-soluble ammonium salt, ammonium persulphate [(NH4)2S2O8: APS], was adopted as initiator to polymerize AN and IA. This was not identical with many researches, which employed complex initiation systems containing alkali metal ions. It is indicated that high molecular weight PAN copolymers with proper IA contents can be synthesized by this method. At the beginning, the polymerization system was separated into two phases, monomer phase and water solution phase. During the polymerization process, APS was decomposed into sulfate ion radicals (SO4(?)) under heating to initiate radical copolymerization of AN and IA. When the white PAN polymer precipitate emerged, polymer particle phase appeared. With the consumption of AN in the polymerization system, AN was transported from monomer phase to water solution phase. And then AN was absorbed to polymer particle phase. The polymerizations occurred in both water solution phase and polymer particle phase. With the increase of initiator concentration, total monomer concentration and polymerization temperature, molecular weights of PAN polymers all decreased, while polymerization conversions increased. With the extension of polymerization time, both molecular weights and polymerization conversions heightened.Structures and properties of PAN copolymers were affected by copolymerization of AN and IA. Because of different reactivity of monomers, the copolymer composition was not the same as the monomer ratio in the feed. With the increase of IA content in the feed, polymerization conversions and molecular weights firstly increased and then decreased. While the oxygen (O) content in the polymer increased. And the carbon (C), nitrogen (N) and hydrogen (H) contents in the polymer decreased. Stretching vibration of carboxyl group (C=O) around 1737cm-1 in FTIR assigned to IA chain segments increased. When the required IA amount in the feed was 2wt%, the polymerization conversion and molecular weight got the maximum values. Crystallinity and grain size decreased with the introduction of IA in the feed.Polymerization conversions and molecular weights of AN/IA copolymers could be controlled by using molecular weight regulators during aqueous deposited copolymerizations of AN and IA. With the increase of isopropanol (IPA) and n-dodecyl mercaptan (n-DDM) in the feed, polymerization conversions and molecular weights of PAN copolymers increased. But there were few changes of C, N, H and O contents in PAN copolymers. Crystallinity and grain size increased with the introduction of IPA in the feed. While chemical structures had few changes. Moreover, triad stereoregularity was not basically affected by IA and IPA.When the total monomer concentration C1 was 22wt%, the initiator concentration [APS] was 0.8wt%, the polymerization temperature T was 60℃, and the polymerization conversion was about 6.2%, values of monomer reactivity ratios were calculated by Kelen-Tudos (K-T) method and Fineman-Ross (F-R) method. Corresponding to the former method, the r1(AN) value was 0.64 and r2(IA) was 1.37. And the latter r1(AN) value was 0.61 and r2(IA) was 1.47. Evidently, the reactivity of IA is higher than AN. With the increase of polymerization conversion and temperature, monomer reactivity ratio of AN increased. Whereas monomer reactivity ratio of IA decreased. Kinetics equation of the copolymerization system was obtained, i.e. Rp[APS]0.538[M]1.696. And the activation energy was also figured out, i.e. E=180.8kJ/mol.Thermal properties of PAN polymers synthesized by aqueous deposited polymerization were characterized by the measuring methods above. It is shown that thermal properties of PAN polymers were improved with the introduction of IA comonomer. Cyclization reactions could be initiated through ionic mechanism under low temperature. DSC curves of PAN homopolymer and copolymers exhibited multiple peaks under inert atmosphere or air atmosphere. When there was oxygen in heating atmosphere, multiple peaks became more evident. With the increase of molecular weight of PAN copolymers, characteristic temperatures of DSC exothermic peaks had few changes. And the exothermic peaks represented doublets or triplets. The multiple peaks in DSC curves of PAN polymers were attributed to many exothermic reactions in macromolecular chains during heating process. Structure and property changes of PAN polymers heat-treated under air atmosphere were investigated. It is indicated that colours of heat-treated PAN polymers became from faint yellow to black gradually with the increase of the heat treatment temperature. And O contents in the polymers increased. Contents of C, N and H elements decreased. Crystallinity and grain sizes of heat-treated PAN polymers firstly increased and then decreased. At the later period of pre-oxidation reactions, diffraction peaks around 2θ≈25.5°corresponding to amorphous areas in the heat-treated PAN polymers appeared and then increased by degrees. In FTIR spectra, stretching vibrations and bending vibrations of methylene group (CH2) around 2940cm-1 and 1455cm-1, and stretching vibrations of nitrile group (C=N) around 2244cm-1 decreased by degrees and then disappeared. Stretching vibrations of double bond group (including C=C and C=N) around 1600cm-1 representing the ladder structure appeared and then became stronger by degrees. Whereas FTIR absorption peaks of the two oxygen-containing functional groups, including stretching vibrations of C=O around 1737cm-1 and C-O single bond around 1180cm-1, maintained in the PAN polymers and heat-treated PAN polymers all the time.Rheological properties of different PAN solutions were discussed. The solvents used in concentrated solutions of PAN copolymers were dimethylsulphoxide (DMSO), dimethyl formamide (DMF) and dimethylacetamide (DMAc). Only DMSO was utilized in dilute solutions of PAN polymers. In the dilute solution system of PAN homopolymer, its Huggins curve owned better linear relation under the testing concentration region. Because of different polyelectrolyte effects in dilute solutions of PAN copolymers, their Huggins curves only had linear relation under the high concentration region. Different degrees of deviations were shown under the low concentration region. Apparent viscosities of different PAN concentrated solutions were studied. It is shown that PAN spinning solution is a shear-thinning fluid. Their apparent viscosities became larger with the increase of molecular weight and solid content. As for PAN spinning solutions prepared by different solvents, the apparent viscosity values changed in sequence as follows, i.e. PAN/DMSO>PAN/DMAc> PAN/DMF. PAN spinning solutions were temperature-sensitive. The apparent viscosities decreased with the increase of testing temperature.Structures and properties of different PAN fibers were discussed. It is convenient to enhance tensile strength of PAN fibers by increasing average molecular weights and crystallinity of PAN fibers. It is found that AN/IA copolymers synthesized by aqueous deposited polymerization could be used to prepare high performance PAN fibers by dry-jet wet spinning technology. Furthermore, the PAN fiber had different DSC curve and WAXRD curve from the corresponding PAN copolymer. Combining several laboratory experiments, PAN copolymers with viscosity average molecular weight (Mv) ranging from 20×10 to 25×10 were preferable to manufacture high performance PAN fibers. And the final PAN fiber, whose fiber fineness was 1.07dtex and tensile strength was up to 7.54cN/dtex, had been produced by dry-jet wet spinning technology using the PAN copolymer with Mv equaling to 24×104.