Layer‐by‐layer assembly of weak‐strong copolymer polyelectrolytes: A route to morphological control of thin films

Multilayer films were assembled from a strong polyelectrolyte (poly(diallyldimethylammonium chloride), PDADMAC) and a copolymer containing both strongly charged styrene sulfonate moieties and weakly charged maleic acid moieties (poly(4-styrenesulfonic acid-co-maleic acid), PSSMA). Growth of PSSMA/PDADMAC multilayers was linear, as characterized by UV-vis spectroscopy and quartz crystal microgravimetry. The influence of both the pH of the PSSMA adsorption solutions and the ratio of SS:MA in the PSSMA on multilayer properties was investigated. Fourier transform infrared spectroscopy results showed that the ionization of carboxylic acid groups in PSSMA/PDADMAC multilayers did not vary significantly with changes in the PSSMA assembly pH. However, the multilayers showed different thicknesses, surface morphologies, and stability to post-assembly pH treatment. We also demonstrate that PSSMA/PDADMAC multilayers are significantly more stable than PSSMA/PAH multilayers after post-assembly pH treatment (i.e. the films remain intact when exposed to pH extremes). In addition, the surface morphology of two films (PSSMA 1:1 assembled at pH 5.8, post-treated at pH 2 and PSSMA 3:1 assembled at pH 5.8, post-treated at pH 11) changed significantly when the films were exposed to solutions of different pH and, in the former case, this change in film morphology was reversible. The porous morphology after treatment at pH 2 could be reversed to give a significantly smoother film after subsequent exposure to water for 24 h. Our results demonstrate that by the rational choice of the assembly pH of PSSMA, stable and pH-responsive films can be obtained via the sequential assembly of PSSMA and PDADMAC. These films have potential in controlled release applications where film stability and pH-responsive behavior are essential.