Preparation And Rheological Properties Of Acrylonitrial / N-vinylformamide Copolymer

As one of the cutting-edge materials in the world,carbon fiber plays an important role in the development of the national economy and the modernization of national defense.With the rapid development of cutting-edge fields such as defense,military,aerospace,etc.,the demand for carbon fiber performance is also increasing,so the development of high-performance carbon fiber is of great significance.The quality of precursor is the basis and key for the preparation of high-performance carbon fibers.Due to the shortcomings of polyacrylonitrile(PAN)homopolymers,such as exothermic concentration during cyclization,poor flexibility,easy wire breakage,and poor spinnability,high performance carbon fiber generally cannot be obtained.Therefore,in the preparation of acrylonitrile-based carbon fiber precursors,a small amount of comonomers are usually introduced into the polymerization system to improve the properties of polyacrylonitrile.Comonomers commonly used mainly are divided into acidic comonomers,neutral comonomers and amide comonomers.Acidic comonomers is mainly used to improve the pre-oxidation properties of the polymer,but the increase of their content will lead to a decrease in the molecular weight of the copolymers.Neutral comonomers are usually used as the third monomer to improve the solubility and spinnability of the polymer.However,the sequence structure,sequence distribution and structural regularity are difficult to control in the terpolymerization.The cyclization reaction can be initiated through ionic mechanism by the introduction of amide comonomers.Meantime,amide comonomers can endow the precursor fiber with hydrophilicity and promote the miniaturization of the coagulated filament structure.Therefore,based on the above questions,the amide comonomer N-vinylformamide(NVF)was selected in this paper,and the poly(acrylonitrile-N-vinylformamide)copolymer(P(AN-co-NVF))was prepared by solution polymerization.A detailed study of the effect of comonomer content on the polymerization reaction,pre-oxidation properties and rheological properties was investigated by Fourier transform infrared spectroscopy(FTIR),X-ray diffraction(XRD),differential scanning calorimetry(DSC),thermogravimetry(TG)and Anton Paar high temperature rotational rheometer etc.It was found that the introduction of comonomer NVF can effectively improve the pre-oxidation properties of polyacrylonitrile,which is beneficial to the preparation of high-performance carbon fiber precursors.The main research contents of this paper are as follows:(1)The bicopolymer P(AN-co-NVF)was prepared by solution polymerization using amide monomer NVF as comonomer.The effect of copolymer content on polymerization conversion and molecular weight was studied.The effects of reaction temperature,initiator concentration and reaction time on the copolymerization of AN/NVF were studied in detail,and the optimal reaction temperature was 60?,the initiator concentration was 1.0 wt%,and the reaction time was 24 h.By controlling the reaction conversion rate,the reactivity ratio of the two monomers was measured.(2)The structural evolution and chemical reactions during the pre-oxidation of P(AN-co-NVF)were investigated by FTIR,XRD,DSC and TG,etc,and the effect of comonomer content on the pre-oxidation properties of polymers was studied in detail.Two parameters=₁₅₈₆/_2244(88)-1and=(-)/was defined to evaluate the extent of stabilization of P(AN-co-NVF)copolymers.The results show that the extent of stabilization of the polymers increases with the introduction of comonomer.The cyclization reaction is initiated by ionic mechanism at a lower temperature,which make the cyclization temperature decrease by 50°C.In the meantime,the exothermic peak is effectively broadened,which avoids concentrated exotherm and improves the pre-oxidation reaction of the polymer.Moreover,the introduction of NVF can improve the thermal stability and carbon yield of the copolymer to a certain extent.The conversion rate of the polymerization reaction,the molecular weight of the polymer and the thermal properties were considered,the optimal comonomer ratio was determined to be 98:2.The cyclization reaction activation energies of PAN and P(AN-co-NVF)were calculated by Kissinger and Ozawa methods,and the results showed that the cyclization reaction activation energy of P(AN-co-NVF)copolymer was lower than that of PAN homopolymer from 170 k J/mol to 75 k J/mol,which has a certain promotion effect on the cyclization reaction of polymers.(3)The rheological behavior of AN/NVF copolymer solution was tested by MCR-702 high temperature rotational rheometer produced by Anton Paar Company.The effects of concentration and temperature on apparent viscosity,non-Newtonian index and structural viscosity index were studied in detail.The results showed that the introduction of NVF can improve the spinnability of spinning precursors compared with PAN homopolymer.