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, TC(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 Tc(soln), the phase separation temperature in solution, thereby suggesting that the different factors contribute toward the determination of the Tc(surf) and Tc(soln) values. For both series of thermoresponsive polymers, the increase in crowding of the polymer chains at the surface causes the value of Tc(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 Tc(surf) on M(n) as compared to that of Tc(soln).
Novel redox-active thermosensitive polymers (phenothiazine-labeled poly(ethoxyethyl glycidyl ether), PT-PEEGE), composed of a polyoxyalkylene backbone, ethoxyethoxymethyl side chains, and an electroactive phenothiazine end group, were prepared by base-catalyzed anionic ring-opening polymerization of ethoxyethyl glycidyl ether monomer in the presence of 10-(2-hydroxyethyl)phenothiazine. Phase separation of a 1.0 mmol dm(-3) (0.33 wt %) PT-PEEGE aqueous solution occurs at 28 degrees C. While the phase separation temperature (Tc) is almost constant in the concentration range above 1.0 mmol dm(-3), it increases at below 1.0 mmol dm(-3). A 10-fold decrease in the oxidation current of PT-PEEGE is observed above Tc and reflects the decrease in the apparent concentration of electroactive PT-PEEGE due to the phase separation. The redox response mainly comes from PT-PEEGE molecules in the dilute phase, resulting from the phase separation, and the half-wave potential and peak separation are independent of the phase separation. This thermally induced change in the redox response is reversible and is applied for the thermal control of the electrocatalytic reaction of glucose oxidase (GOx). The catalytic current in the presence of PT-PEEGE as an electron mediator decreases at temperatures higher than Tc. This originates from the phase separation of PT-PEEGE, and PT-PEEGE molecules which remained to be soluble participate in the electrocatalytic reactions of GOx as mediators.
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