The effect of the internal architecture of polymer micronetwork colloids on the dynamics in highly concentrated dispersions

Abstract

Motivated by the finding that colloidal dispersions of polymer micronetwork spheres with a cross-link density of 1:50 (inverse number of monomer units between crosslinks) show significant deviations from the dynamics of hard spheres in the colloid glass as seen by dynamic light scattering (DLS) (E. Bartsch, V. Frenz, H. Sillescu J. Non — Cryst. Solids 172–174 (1994), 88–97), we have undertaken a systematic study of the effect of the crosslink density on the dynamics at high concentrations. Long-time self-diffusion coefficients D ^L_S and collective diffusion coefficients D _c were measured for colloids with crosslink densities of 1:10, 1:20 and 1:50 by forced Rayleigh scattering (FRS) and the newly developed thermal diffusion FRS (TDFRS) technique, respectively. Whereas no dependence of D ^L_S on the crosslink density is found at low concentrations and the data coincide with theoretical results for hard spheres, strong effects of the internal architecture on self-diffusion are observed in the highly concentrated regime. Here, hard sphere behaviour is recovered only in case of the 1:10 particles, a glass transition being indicated at φ _g ∼ 0.59. Lowering the crosslink density leads to significantly higher (∼ three decades) values of D ^L_s at φ > φ _g. This may be due to an increased deformability of the spheres which could partially account for our DLS results. In contrast, D _c is more sensitive to variations of the degree of cross-linking. Here, a much faster increase of D _c with volume fraction as compared to hard spheres is observed already at low φ for 1:20 and 1:50 crosslinked particles, the effect being strongest for the lowest crosslink density. The results are tentatively interpreted in terms of a soft repulsive interaction potential, whose range increases on lowering the crosslink density.