Investigation Of The Gelation Process And Gel Properties Of Polyacrylonitrile Solutions

Because the melting temperature is much higher than the thermal-decomposition temperature,PAN-based carbon fibers are usually prepared through solution spinning in both laboratory andindustrial. In the spinning process, when the PAN solution enters into the coagulation bath, bothgelation and phase separation could happen. These two processes basically determine the quality ofPAN precursor and the performance of the following carbon fiber. Therefore, the research of therheological and gelation behavior of PAN solution is very essential. In this thesis, starting with thesynthesis of PAN copolymer with targeted molecular weight，we studied the rheological andgelation behavior of PAN-DMSO-H2O system using rheological methods.Firstly, PAN copolymer was synthesized by radical polymerization, which was controlled byseveral factors, such as the amount of the initiator and polymerization inhibitor, polymerizationtemperature and polymerization time. AN/IA copolymers with targeted molecular weight andmoderate molecular weight distribution were successfully prepared and characterized by GPC,FTIR and1H NMR. With these raw materials, we studied the effect of PAN concentration,non-solvent H2O content on rheological properties and gelation behavior of PAN-DMSO-H2Osystems. Analysis of gelation mechanism was proposed by polar-polar interactions, and the gelstructure was established by the rheological images. At last, the effect of temperature, solvent, PANmolecular weight on gelation behavior of PAN solution were also studied, and an empirical lawabout the effects of the three factors on PAN solution gelation behavior was obtained.All the above results showed that increasing the concentration of PAN or H2O content causedthe shear thinning phenomenon and the non-Newtonian behavior of the PAN-DMSO-H2Osolutions more prominent. The increased concentrations could intrigue the gelation process athigher temperatures. However, their influences on the structure and strength of PAN gel wereopposite. Increasing PAN concentration could enhance the gel strength near the critical gel point,while the situation was reversed by increasing the concentration of H2O. When the temperaturewas below the critical gel point, the gelation process became quicker as the temperature decreasesand the stiffness of the gel was strengthened. Because PAN has better solubility in DMSO thanDMF, the gelation phenomenon in PAN-DMSO-H2O solution appeared later than PAN-DMF-H2Osolution with all the concentrations were kept the same. On the other hand, PAN with larger molecular weight inclined to tangle with each other and form a cross-linked structure, whichcaused the gelation more spontaneously. With the help of Arrhenius equation and thetime-temperature superposition principle, the activation energies of PAN gelation and coolingprocess were calculated. The results showed that, the PAN gel formation and growth is aspontaneous process, and after the gel transition the PAN system lost the quality of fluid.