Diffusion of single polyelectrolytes on the surface of poly (N-isopropylacrylamide) brushes

Lateral diffusion of a single polyelectrolyte probe, poly(2-vinylpyridine) (P2VP), on the surface of thermoresponsive poly(N-isopropylacrylamide) (PNIPAM) brushes was studied by fluorescence correlation spectroscopy. Measurements were conducted at a condition of a low pH value and a low salt concentration., under which the P2VP chain is fully charged and at its extended coil conformation. The temperature dependence Of the Surface diffusion coefficient (D-s) of P2VP was studied at varying PNIPAM thickness, in which D, increased with the elevation of temperature and reached a maximum value around 32-35 degrees C before it dropped at further increased temperature. The experimental results indicate a linear dependence of the friction force experienced by the probe on the brush thickness. The thermovariation of D, demonstrates the decrease of the surface friction force at temperature lower than 32-35 degrees C due to the decrease of the solvent viscosity and a sharp increase after the LCST transition because of the change of the PNIAPM chain rigidity.