Kinetic studies of colloidal crystallization of thermo-sensitive gel spheres of poly(N-isopropylacrylamide)

Abstract

Crystal growth rate coefficients, k of the colloidal crystallization of thermo-sensitive gel spheres of poly(N-isopropylacrylamide) were measured from the time-resolved reflection spectroscopy mainly by the inverted mixing method in the deionized state. Crystallization of colloidal silica spheres were also measured for comparison. The k values of gel and silica systems increased sharply as the sphere concentration and suspension temperature increased. The k values of gel system were insensitive to the degree of cross-linking in the range from 10 to 2 mol% of cross-linker against amount of the monomer in mole and decreased sharply when the degree of cross-linking decreased further to 0.5 %. The k values increased as gel size increased. The k values of gel systems at 20 °C were small and observed only at the very high sphere concentration in volume fraction, whereas those at 45 °C were high but smaller than those of silica systems. Induction time (ti) after which crystallization starts, increased as the degree of cross-linking increased and/or the gel size decreased at any temperatures, when comparison was made at the same gel concentration. The ti values at 45 °C were high and decreased sharply with increasing sphere concentration, whereas those at 20 °C were high only at the very high sphere concentrations. Significant difference in the k and ti values between the soft gels and hard silica spheres was clarified. These kinetic results support that the electrical double layers play an important role for the gel crystallization in addition to the excluded volume of gel spheres. It is deduced further that the electrical double layers of the gel system form from the vague interfaces (between soft gel and water phases) compared with those of typical colloidal hard sphere system.