Investigation Of Gelation And Phase Separation Behavior In PAN Solutions

PAN/DMSO/H₂O system is widely used in wet-spinning,dry-wet spinning,gel spinning for PAN fiber formation processes.Gelation and phase separation in PAN/DMSO/H₂O system are complicated processes,and study for the processes may provide theoretical referencees for PAN fiber formation,which are beneficial for technology optimization and structure control,thus improving property of the final PAN-based carbon fiber.In this work,gelation and phase separation behavior in the PAN/DMSO/H₂O system was investigated by rheological and optical methods.The PAN polymer with suitable molecular weight and molecular weight distribution was synthetized by solution radical polymerization.The rheological behavior of the gelation and phase separation during cooling process for the PAN/DMSO/H₂O blends with various compositions was studied.The incipient precipitation temperature,where a distinct decrease in relative light transmittance occurred,was considered as the cloud point?CP?.The validity of Chambon-Winter model was evaluated in these blends and used to measure the critical gel point?GP?.It was found that CPs were always higher than GPs,and they both increased with increasing PAN or H₂O contents.Based on the CPs and GPs,dynamic phase diagrams were constructed and two different gelation mechanisms with and without phase separation were proposed.For the gelation accompanied with phase separation,gelation is first induced by phase separation,for the gelation without phase separation,gelation is caused by the the polarity interactions between nitrile groups.Moreover,phase separation mechanism of nucleation and growth was deduced by the cooling rate dependence of storage modulus and relative transmittance.During the isothermal gelation and phase separation process of the PAN/DMSO/H₂O system,Winter-Chambon criteria could be applied whereas only limited size in self-similar structure of the critical gel was detected.During the isothermal gelation process whether phase separation occurred or not,the self-similar structure size increased with decreasing temperature,which expanded toward smaller length scale.When temperature decreased,the phase separation and gelation rates both increased,the relaxation exponent n decreased and the fractal dimension df increased,indicating more elastic and tighter structure formed in the related critical gels.Also,the corresponding gel stiffness S increased.It is also found that for the blends in which phase separation accompanied gelation occurred,gelation was initially induced by phase separation.On the other hand,gelation could slow the phase separation kinetics,and this effect was more pronounced in relatively higher PAN concentration blends.The apparent gelation activation energy was minus,which indicated that the isothermal gelation process with or without phase separation was exothermic and spontaneous.