Brush-sheathed particles diffusing at brush-coated surfaces in the thermally responsive PNIPAAm system

Phase-contrast microscopy and particle tracking algorithms are used to study the near-surface diffusion of poly(N-isopropylacrylamide) (PNIPAAm) brush functionalized micron-sized silica microspheres after sedimentation from aqueous suspension onto planar substrates coated with a similar polymer brush above and below the lower critical solution temperature (LCST) of PNIPAAm, 32 degrees C. A small negative charge on the wall and the particles (zeta potential = -6 mV) prevents adhesion above and below the LCST. The near-surface translational diffusion coefficient (D(surface)) is compared to the bulk-phase translational diffusion coefficient (D(bulk)), which was measured by dynamic light scattering. We find that D(surface)/D(bulk) is approximately equal to 0.6 at temperatures T < 32 degrees C but rises abruptly to approximately 0.8-0.9 at T > 32 degrees C. Near-surface diffusion is expected to be slower than bulk diffusion owing to hydrodynamic coupling to the wall, implying reduced hydrodynamic coupling at the higher temperatures, perhaps mediated by enhanced electrostatic repulsion above the LCST transition.