Layered, Erasable Polymer Multilayers Formed by Hydrogen-Bonded Sequential Self-Assembly

Abstract

Robust multilayers can be formed on solid surfaces, and subsequently destroyed by changing the environmental conditions, by the layer-by-layer sequential assembly of monomolecular films of a polyacid and polybase from aqueous solution. Interlayer hydrogen bonding produces stable multilayers up to the point where altered pH or other environmental stimulus introduces an unacceptably large electrical charge within them. This is demonstrated for the polyacids poly(acrylic acid), PAA, and poly(methacrylic acid), PMAA, and for the polybases poly(vinylpyrrolidone), PVPON, and poly(ethylene oxide), PEO, in D 2 O. The adsorption was quantified by Fourier transform infrared spectroscopy in attenuated total reflection (FTIR-ATR). The ratio between suppressed ionization of the carboxylic groups within the film and their ionization in solution, as directly measured by FTIR-ATR, was used to estimate the fraction of hydrogen-bonded carboxylic groups; this was ∼0.5 in PVPON/PMAA but only ∼0.1 in the PEO/PMAA system, though the dielectric environments appeared to be similar. The critical pH, at which these hydrogen-bonded layers disintegrated, was controlled by a balance of internal ionization and a fraction of carboxylic groups that formed hydrogen bonds with either PVPON or PEO. The critical point was at pH = 6.9 for the PVPON/PMAA films (relatively strong hydrogen bonding), but pH = 4.6 for the PEO/PMAA films (in which a smaller fraction of segments participated in hydrogen bonding). It was even less, pH = 3.6, in the PEO/PAA system, which contained a larger proportion of ionized groups at a given pH owing to the higher acidity of PAA. As a second avenue to control the stability of these multilayer films, ionic strength was varied systematically. In the PEO/PMAA system, the multilayers were stable up to pH = 4.6 in the environment of 10 mM ions (this ionic strength resulted from the buffer solution to control pH), but the multilayers were stable up to higher pH, pH = 5.15, when 0.4 M NaCl was added. The reason is that a higher ionic strength reduced the intensity of electrostatic repulsion between a given number of ionized groups within the multilayer assembly. A slight weakening of stability with decrease of molecular weight was observed (these experiments concerned the PVPON system) as expected from fewer hydrogen bonds per molecule. Finally, experiments with added rhodamine 6G dye showed that dye or drug molecules can be incorporated into such multilayers and then released as needed at preselected conditions a feature that may be used in drug release devices.