Gelation process of polyacrylonitrile solutions as studied using small-angle neutron scattering techniques.

We studied the structural formation process of polyacrylonitrile (PAN) gelation by a wide-spatial scale. In micron-scale structure, light transmittance and gelation time were evaluated. The presence of water in the mixed solvent accelerated the phase separation and gelation because of the poor solubility of PAN in water, while in the case of low water concentration, transparent gelation process could be observed. Further, we conducted in situ small-angle neutron scattering measurements to clarify the gelation processes of 10 wt% PAN solutions with various water contents with new-type spectrometer in MLF, J-PARC, Japan. The nanometer-scaled inhomogeneities in the polymer chains develop immediately after quenching. In the opaque-gel case, we observed the nanometer-scaled correlation of crystalline cross-linking points and smooth boundary of two micron-scaled phases due to liquid-liquid phase separation. Cross-linking points begin to emerge and grow just after quenching. The correlation length is independent of the annealing time, then the cross-linking point exists locally and micro-gelation occurs during phase separation. By contrast, in the transparent-gel case, we did not observe scattering from the surfaces of the cross-linking points; we were able to measure only the correlation length. The correlation length was found to increase throughout the gelation process, and the entanglements of the polymer chains play a role similar to that of cross-linking points. [ABSTRACT FROM AUTHOR]