pH-Responsive Copolymer Films with Amine Side Chains
Dongshun Bai, Zulkifli Ibrahim, and Kane G. Jennings. Chemical Engineering Department, Vanderbilt University, VU Station B 351604, 24th & Garland Avenues, 107 Olin Hall, Nashville, TN 37235
We have designed pH-responsive copolymer films that consist predominately of polymethylene (PM) with randomly distributed amine-terminated side chains. The films are grown on gold surfaces by (1) exposure to an ether solution of diazomethane and ethyl diazoacetate to produce Au–[(CH2)A-(CO2Et)B]- (denoted as PM-CO2Et), (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) 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 resistances against ion transfer. When pH is decreased, the amine groups become protonated or charged, dramatically increasing their hydrophilicity and greatly reducing the resistance of the film against ion transfer by up to 3 orders of magnitude over the pH range of 7 to 4. In contrast, PM-CO2H films become deprotonated or charged as pH is increased 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-butyl amine 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.