The influence of the temperature–time regime of dissolution of polyacrylonitrile copolymers in N-methylmorpholine-N-oxide on the rheological characteristics of spinning solutions and thermal properties of the processed polymer

Abstract Aprotic, highly polar solvents are often used to produce polyacrylonitrile (PAN) spinning solutions. Processing these solvents into fibers as highly concentrated solutions is difficult due to their extremely high viscosity. It has recently been shown that N-methylmorpholine-N-oxide (NMMO) allows for the production of concentrated PAN solutions (up to 55%). However, the high temperature at which the solutions are formed initiates chemical processes in the polymer-solvent system. Changes in polymer properties are reflected in the morphology and rheological behavior of the spinning solutions. The characteristics of the resulting product depend not only on the temperature but also on the time of solution preparation. At temperatures above 120°C, chemical transformations of PAN occur at a faster rate. Therefore, milder conditions are preferable for solution preparation, allowing for more predictable control of the polymer properties. Using various modes for producing PAN solutions in NMMO allowed us to track changes in the polymer's molecular weight (molecular weight distribution) and propose solutions suitable for fiber spinning. It was shown that at 120°C over 35–40 minutes, the PAN molecular weight decreases by a factor of 3.3, while the Mw/Mn values increase by almost 15%. The resulting PAN with different thermal histories exhibits thermal behavior typical of partially stabilized PAN. This means that cyclization processes are initiated in the PAN already at the stage of producing spinning solutions.