Preparation Of P(AN-AMPS) And P(AN-MA-AMPS) By Solution Polymerization And Their Performance Study

Polyacrylonitrile（PAN）based large tow carbon fibers are more suitable for large scale applications in modern transportation and energy equipment and other industrial fields due to their excellent characteristics such as high single line yield,high post-processing efficiency and low cost of use,which are of great significance for the development of strategic emerging industries.High quality PAN filament is the basis for the production of high performance carbon fibers,and the quality and nature of the filament determines the final performance of the finished carbon fiber.Due to the large bundle of PAN raw filaments,the pre-oxidation process is prone to heat storage and concentrated heat release,resulting in easy fusion of fibers or the formation of skin core structure,affecting production safety and carbon fiber performance.Therefore,other comonomers need to be introduced into the PAN molecular chain to optimise and improve the thermal response properties of PAN.In this paper,azo diisobutyronitrile（AIBN）was used as initiator,dimethyl sulfoxide（DMSO）as solvent,acrylonitrile（AN）,2-acrylamido-2-methylpropanesulfonic acid（AMPS）and methyl acrylate（MA）as comonomers.Firstly,P（AN-AMPS）copolymers with different AMPS contents were prepared by solution polymerization,and the effects of different AMPS contents on the structure,molecular weight and distribution,thermal properties,crystalline properties and hydrophilicity of the P（AN-AMPS）copolymers were systematically investigated.Further,different ratios of P（AN-MA-AMPS）copolymers were prepared,and the effects of different AN/MA/AMPS ratios on the structure and properties of P（AN-MA-AMPS）copolymers were investigated.The ratios with the best pre-oxidation properties were selected and the evolution of the structure and properties of P（AN-MA-AMPS）copolymers during the pre-oxidation process were analysed in detail.The influence of different polymerization factors on the conversion rate,molecular weight and distribution of the products,crystalline structure and chain structure of the AN/MA/AMPS copolymerization reaction was investigated,and P（AN-MA-AMPS）solutions with different solid contents were configured to study their rheological properties.The results showed as follows.（1）Compared to the PAN homopolymer,the introduction of AMPS improved the pre-oxidative cyclization properties,thermal stability,crystallinity and hydrophilicity of P（AN-AMPS）copolymer,especially when the AN/AMPS mass ratio was 97/3,the pre-oxidative cyclization properties and thermal stability showed the best performance,with the lowest cyclization onset temperature（202.6℃）The highest exothermic and carbon yields were achieved（910.0 J/g and 58.69%）.However,as the AMPS content increased,the molecular weight of P（AN-AMPS）gradually decreased and the distribution became wider.At the same time,the exothermic peak temperature of the cyclization of P（AN-AMPS）gradually increased,and the overall cyclization reaction shifted to a higher temperature.（2）Compared to PAN homopolymer,P（AN-AMPS）and P（AN-MA）copolymers,P（AN-MA-AMPS）copolymers exhibited better pre-oxidative cyclization performance,more significant heat releasing effect,larger exothermic range,and more ring structures were formed at lower temperatures to obtain higher E_s values.The best pre-oxidation cyclization and thermal stability performance was achieved when AN/MA/AMPS（wt%）=97.5/1.5/1,with the lowest cyclization onset temperature（219.1℃）,the largest exothermic range（104.5℃）and the highest exothermic and carbon yields（812.9 J/g and 58.07%）.（3）With the increase of the pre-oxidation temperature,the colour of P（AN-MA-AMPS）changed from white to light yellow,then to brown and finally to black;the content of C and N elements in P（AN-MA-AMPS）gradually decreased,while the content of O elements gradually increased;In the FT-IR spectra,the stretching vibrational peak representing-C≡N at 2244 cm^-1weakened,the bending and stretching vibrational peaks representing C-H bonds at 1454 cm^-1and 1365 cm^-1 also weakened,while the stretching vibrational peak representing C=N bonds at1618 cm^-1 and the peaks representing C=C and C=N in the ring trapezoid structure at 1590 cm^-1appeared and gradually strengthened;the grain size and crystallinity of P（AN-MA-AMPS）tended to increase and then decrease,the crystalline regions in the polymer gradually decreased and finally disappeared completely,while the amorphous regions gradually increased.（4）In the AN/MA/AMPS（wt%）=97.5/1.5/1 polymerization system,As the reaction time,reaction temperature,initiator dosage and monomer concentration increase,the conversion rate of the polymerization reaction all increased accordingly;the molecular weight of the polymer tended to increase and then decrease with increasing monomer concentration,while the molecular weight decreased with increasing reaction temperature and initiator dosage.The optimum polymerization process parameters for higher conversion and higher molecular weight were selected:total monomer concentration[C_t]=22wt%,initiator concentration[AIBN]=0.3～0.6wt%,reaction temperature T=60℃,reaction time t≥12h.（5）The crystallinity of P（AN-MA-AMPS）gradually increased with the increase of reaction temperature and initiator concentration,and tended to decrease and then increase with the increase of monomer concentration,while the change of crystallinity of the samples was inversely proportional to the change of molecular weight.Changing the copolymerization reaction factors had little effect on the stereoregularity of the PAN copolymer,and the stereoregularity of P（AN-MA-AMPS）was still dominated by atactic structures,followed by isotactic structures.The apparent viscosity,energy storage modulus and loss modulus of the P（AN-MA-AMPS）/DMSO solution decreased with increasing temperature and increased with increasing solid content.