| Abstract
| - We have designed pH-responsive copolymer films that consist of polymethylene (PM) with dilute randomlydistributed side chains terminated with dimethylamine groups. The films are grown on gold surfaces by (1)exposure to an ether solution of diazomethane and ethyl diazoacetate to produce a random copolymer Au-[(CH2)A-(CHCO2Et)B] (denoted as PM-CO2Et, A ≫ B), (2) hydrolysis of the ester groups to carboxylic acids(PM-CO2H), (3) complete conversion of the acids to acid chlorides (PM-COCl), and (4) quantitative reactionof the acid chlorides with N,N-dimethylethylenediamine (DMEDA) to yield a film (PM-CONHR, R represents−-CH2CH2N(CH3)2) with amide-linked, amine-terminated side groups that are pH active. The effect of polymercomposition on pH response was investigated by electrochemical impedance spectroscopy (EIS). PM-CONHRfilms are more hydrophilic in their uncharged state than PM-CO2H films and thereby exhibit lower resistanceagainst ion transfer. When the pH is decreased, the amine groups become protonated or charged, dramaticallyincreasing their hydrophilicity and reducing the resistance of the film against ion transfer by around 2 ordersof magnitude over the pH range of 7 to 4. In contrast, PM-CO2H films become protonated or neutralized asthe pH is decreased and exhibit a 5 order of magnitude increase in resistance over a similar range of pH. Thehydrophobicity of the PM-CONHR films can be enhanced by capping the acid chloride groups with a mixtureof DMEDA and n-butylamine to yield a film with some pH inert groups that boost film resistance in theuncharged state and increase the magnitude of the change in film resistance. The resistance of the film againstion transfer correlates well with the hydrophobicity of its dilute side chain component, as quantified by theoctanol−water partition coefficient (Kow).
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