The interplay between gelation and phase separation in PAN/DMSO/H2O blends and the resulted critical gels

Abstract

Isothermal aging experiments are carried out in Polyacrylonitrile/Dimethylsulfoxide/H2O (PAN/DMSO/H2O) blends, and rheological and optical methods are used to investigate the interplay between phase separation and gelation. Two types of gel structures are found. In most samples, gelation is induced by phase separation in PAN concentrated regions, yet it will subsequently slow the kinetics of phase separation down by altering the concentration balance and fluctuation. As a result, the drop of light transmittance becomes slower and a phenomenon called “RT reversal” is observed. At last, the phase separated morphology is replaced by gel structure, which inherits the inhomogeneity in PAN content distribution. Another type of gel grows from homogeneous solution and its structure disperses uniformly in space. Moreover, the temperature dependence of critical gel properties, like its size limits and thermodynamics, is discussed. It is found that the self-similar structure in homogeneous critical gel covers wider length scales and possesses stronger gel stiffness than those of inhomogeneous ones. The activation energy for gelation is also calculated, which reveals no obvious dependence on original PAN concentration or temperature.