pH-Responsive Random Copolymer Films with Amine Side Chains
By Bai, Dongshun; Ibrahim, Zulkifli & Jennings, G. Kane
Published in The Journal of Physical Chemistry C
2007
Abstract
We have designed pH-responsive copolymer films that consist of polymethylene (PM) with dilute randomly distributed 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 reaction of 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 polymer composition on pH response was investigated by electrochemical impedance spectroscopy (EIS). PM-CONHR films are more hydrophilic in their uncharged state than PM-CO2H films and thereby exhibit lower resistance against ion transfer. When the pH is decreased, the amine groups become protonated or charged, dramatically increasing their hydrophilicity and reducing the resistance of the film against ion transfer by around 2 orders of magnitude over the pH range of 7 to 4. In contrast, PM-CO2H films become protonated or neutralized as the pH is decreased and exhibit a 5 order of magnitude increase in resistance over a similar range of pH. The hydrophobicity of the PM-CONHR films can be enhanced by capping the acid chloride groups with a mixture of DMEDA and n-butylamine to yield a film with some pH inert groups that boost film resistance in the uncharged state and increase the magnitude of the change in film resistance. The resistance of the film against ion transfer correlates well with the hydrophobicity of its dilute side chain component, as quantified by the octanol-water partition coefficient (Kow).