Adsorption of PNIPAmx-PEO20-PPO70-PEO20-PNIPAmx pentablock terpolymer on gold surfaces: effects of concentration, temperature, block length, and surface properties

Abstract

The effects of concentration, relative block length and environmental temperature as well as the surface chemical and wetting properties of solid substrates on the adsorption behaviors and mechanisms of a series of pentablock terpolymer poly(N-isopropylacrylamide)_x-poly(ethylene oxide)₂₀-poly(propylene oxide)₇₀-poly(ethylene oxide)₂₀-poly(N-isopropylacrylamide)_x (PNIPAm_x-PEO₂₀-PPO₇₀-PEO₂₀-PNIPAm_x or PNIPAm_x-P123-PNIPAm_x) with x of 10, 63 and 97 on gold were studied by using a quartz crystal microbalance (QCM) technique. It was found that increasing the solution concentration did not alter the adsorption mechanism of thickness growth mode but increase the adsorption amount of PNIPAm₉₇-P123-PNIPAm₉₇ on a bare gold substrate at 20 °C. Increasing the length x of PNIPAm block decreased the adsorption rate constant and shifted the adsorption mechanism from the densification adsorption process for PNIPAm₁₀-P123-PNIPAm₁₀ to the thickness growth mode for PNIPAm₆₃-P123-PNIPAm₆₃ and PNIPAm₉₇-P123-PNIPAm₉₇ on bare (unmodified) gold substrate at 20 °C. The adsorption mechanisms of PNIPAm₉₇-P123-PNIPAm₉₇ at 20 °C on the hydrophobic and hydrophilic gold surfaces were the thickness growth mode and densification adsorption process, respectively. A complex adsorption behavior with large adsorption amounts was observed at the lower critical solution temperature (LCST) of PNIPAm block, i.e. 34.7 °C, for the adsorption of PNIPAm₉₇-P123-PNIPAm₉₇ not only on hydrophobic gold substrates but also on hydrophilic gold substrates. The adsorption mechanism of PNIPAm₉₇-P123-PNIPAm₉₇ micelles at 45 °C was the densification adsorption process regardless of the surface wetting and chemical properties of gold substrate. Overall, the adsorption behavior and mechanism of PNIPAm_x-P123-PNIPAm_x pentablock terpolymers were mainly determined by the interactions of the pentablock terpolymers with different chain conformations in dilute aqueous solutions at various temperatures and the gold substrates with surface wetting and chemical properties.