Preparation Of High-Quality Polyacrylonitrile For Carbon Fiber By Mixed Solvent Method

Polyacrylonitrile(PAN)is the important precursor for the preparation of carbon fibers.High-quality PAN polymer made from acrylonitrile(AN)and a small amount of comonomers is one of the important conditions for improving the mechanical properties of carbon fibers.Excellent PAN spinning dope is the premise of preparing high-quality PAN spinning dope.The proper method of polymerization is the key to preparing high-performance PAN spinning dope.The mixed solvent method using water and organic solvents as the reaction medium to prepare PAN copolymers has the advantages of both homogeneous solution polymerization and aqueous phase polymerization.It can prepare PAN copolymers with high molecular weight and suitable molecular weight distribution,which is conducive to obtaining high-quality PAN yarn.There have been studies on the initiation of aqueous precipitation copolymerization of AN monomer by a single water-soluble initiator without alkali metal ions,namely ammonium persulfate(APS)and azodiamidine hydrochloride(V50),but there are few studies on AN copolymerization in mixed media initiated by APS or V50.Therefore,this paper uses the initiator V50 and APS,organic solvents and water as the mixed reaction medium to study the effects of various polymerization factors on the polymerization of the mixed solvent method.Through the analysis of the physical and chemical structure of the PAN polymer,the relationship between the various polymerization process parameters and the structure and properties of the final PAN polymer was explored,which provide theoretical guidance for the industrialization of PAN-based carbon fibers.(1)In the improved mixed solvent precipitation polymerization system,the transition metal halide FeCl3 is introduced as a catalyst and the organic acid succinic acid as a ligand to form a ternary initiation system.At the same time,V50 is used as the initiator,and methyl acrylate(MA)was used for copolymerization monomer.By studying the effects of feeding methods,mixed solvent ratio,comonomer content,and ligand content on the polymerization reaction.It is compared with PAN polymers prepared by homogeneous solution polymerization,aqueous precipitation polymerization,aqueous suspension polymerization and general mixed solution precipitation polymerization.The results show that compared with a mixed reaction medium composed of dimethylformamide(DMF)and water,the molecular weight of the PAN copolymer obtained is higher when DMSO is used;When the catalyst system is first complexed,then the initiator is added,and the monomer is added dropwise to perform polymerization,the obtained PAN copolymer has a high viscosity average molecular weight and a suitable distribution;with the increase of water content,both the conversion rate and molecular weight of the polymer increased continuously;as the comonomer content MA increased,the conversion rate increased first and then decreased.The conversion rate is the largest when the MA content is 2wt%;as the content of the initiator increases,the viscosity average molecular weight gradually decreases;as the content of the ligand succinic acid increases,both the conversion rate and the viscosity average molecular weight increase and then decrease;As the catalyst content increases,the conversion and viscosity average molecular weight continue to decrease.When the ratio of FeCl3 and succinic acid is 1:2,the reaction catalysis effect is good,and a PAN polymer having a viscosity average molecular weight of 216,000 and a molecular weight distribution of 2.9 is prepared.The structural properties of the AN/MA copolymerization reaction are affected by the comonomer MA.The content of oxygen(O)in the polymer increases with the increase of the comonomer MA,indicating that the content of MA in the molecular chain increases.The peak intensity of the C=O group at 1735 cm-1,which represents the MA chain link in the FTIR chart,is constantly increasing,and it is the same as the result of elemental analysis.After the comonomer is added,the initial exothermic temperature moves in the lower temperature direction,the width of the exothermic peak increases,so the exothermic reaction rate is moderated.(2)V50 and APS are used as initiators,and polymerization is performed in a low water mixed system(water content does not exceed 10%by weight).The results show that:?For the water-initiated homogeneous mixed system using V50,when the organic solvent is DMF,the molecular weight is significantly lower,so DMSO is used as the solvent;with the increase of water content,the conversion rate increases first and then decreases,and the viscosity average molecular weight is between 180,000 and 200,000.When the mixed solvent ratio is 94:6,a high-quality PAN polymer with a conversion rate of 58.6%,a viscosity average molecular weight of 186,000,and a molecular weight distribution of 1.6 is obtained;As the comonomer IA content increases,both the conversion rate and the viscosity average molecular weight increase first and then decrease;As the reaction temperature increases,the conversion rate increases and the viscosity average molecular weight decreases;With the increase of the initiator concentration,the conversion rate increases and the viscosity average molecular weight decreases;With the extension of the reaction time,the conversion increased,but the viscosity average molecular weight almost no change.?In a water-less homogeneous mixed system initiated by APS,when only the solvent DMSO is used,the viscosity of the reaction system is relatively large.As the water content of the mixed solvent system increases,the polymerization reaction time becomes longer,the initiator concentration becomes larger,the polymerization system tends to generate gels,and the membrane forming property of the polymer stock solution becomes worse.After adding DMF,the viscosity of the reaction system becomes smaller,and the membrane formation becomes better.As the DMF content decreases,the conversion rate does not change significantly,and the molecular weight gradually increases;As the amount of IA in the comonomer increases,the molecular weight and conversion of the polymerization product increase first and then decrease;as the initiator concentration increases,the molecular weight of the polymer decreases and the conversion rate increases;as the polymerization temperature increases,the conversion rate increases and the molecular weight decreases;with the increase of the polymerization reaction time,the molecular weight of the polymer is basically unchanged,and the monomer conversion rate increases.The study on the structure and properties of PAN polymer prepared by water-less homogeneous polymerization shows that as the amount of IA increases,the content of O element in the PAN polymer increases,indicating that the content of IA in the molecular chain increases.At the same time,the peak intensity of the C=O group at 1735 cm-1 representing the IA chain link in the FTIR diagram was increased.Compared with the PAN homopolymer,the exothermic peak width of the PAN copolymer becomes larger,and the addition of IA eases the cyclization reaction.