Colloidal Rod-Like Particles with Temperature-Driven Tunable Interactions

Temperature-sensitive rod-like colloidal particles were synthesized by grafting a temperature-responsive polymer, poly(2-(dimethylamino)ethyl methacrylate) (PDMA), on the surface of high aspect ratio silica rods by surface-initiated atom transfer radical polymerization. The stability of the grafted polymer on the surface of the particles in aqueous solutions was found to deteriorate with time, leading to a gradual decrease of the polymer content of the hybrid colloids, which was attributed to the mechanically activated hydrolysis of the labile bonds at the polymer-silica interface. The polymer degrafting was significantly suppressed by first growing a hydrophobic poly(methyl methacrylate) block onto the particle surface to act as a barrier layer for the penetration of water molecules at the polymer-particle interface, followed by chain-extension with the hydrophilic PDMA chains. Dynamic light scattering, microscopy, and rheological measurements revealed that the PDMA block conferred a temperature-responsive behavior to the rod-like particles, which formed aggregates at temperatures above the lower critical solution temperature (LCST) of the polymer. However, in contrast to their spherical counterparts, the polymer-grafted rod-like particles did not exhibit complete thermo-reversibility upon lowering the solution temperature below the LCST of PDMA, which was reflected by different values of the diffusion coefficient for the heating and cooling cycles, indicating an irreversible rod particle aggregation upon increasing the temperature.