Reversible gelation of concentrated polyacrylonitrile solutions

The reversible gelation of concentrated polyacrylonitrile solutions was studied by creep measurements under constant shearing stress at different stages of the reaction. The pure viscous 20% solution in dimethyl formamide were apparently stable at 25°C. When they were cooled down below 0°C. a retarded elasticity appeared and the solutions became completely elastic gels after a certain time. The rate of gelation increased strongly as the temperature decreased. Furthermore, when the gels were heated back to 25°C. the gelation process continued after a short melting period. A simple empirical equation describes the creep of the gels in the completely elastic state and the results are discussed in terms of a generalized Maxwell model. The gelation is to be ascribed to the formation of a tridimensional network. The crosslinks are assumed to be crystallization nuclei.