Thermal response of poly(ethoxyethyl glycidyl ether) grafted on gold surfaces probed on the basis of temperature-dependent water wettability

Two series of thiol-modified poly(ethoxyethyl glycidyl ether) with different chain-end groups and molecular weights (PT-PEEGE-SH and Bu-PEEGE-SH), which undergo lower critical solution temperature (LCST)-type phase separation in an aqueous milieu, are grafted onto gold substrates through Au - S bonding. The water wettability of the resultant polymer-tethered surface discontinuously varies with temperature, and this alteration of wettability is reversible according to the variation in temperature of the environment. For all the polymers examined, the transition temperature on surface, T_c(surf), the temperature at which half the discontinuous change in surface wettability occurs, increases with the number-average molecular weight (M_n). This tendency does not necessarily agree with the relationship between M_n and T_c(surf), the phase separation temperature in solution, thereby suggesting that the different factors contribute toward the determination of the T_c(surf) and T_c(soln) values. For both series of thermoresponsive polymers, the increase in crowding of the polymer chains at the surface causes the value of T_c(surf), to increase due to an increase in the interchain interaction in the outermost region of the tethered polymer chains and reduction in the chain mobility. The greater interactions between neighboring chains at the surface explain the larger dependency of T _c(surf) on M_n as compared to that of T_c(surf).