Dual pH-sensitive layer-by-layer films containing amphoteric poly(diallylamine-co-maleic acid)

Layer-by-layer (LbL) thin films were prepared using an amphoteric copolymer consisting of alternating diallylamine and maleic acid monomer units (PDAMA), and pH-induced decomposition of the films was studied. LbL deposition of poly(styrene sulfonate) (PSS) and PDAMA on a solid surface afforded thin films at pH 3.0, while thin films did not form at pH 8.0. In contrast, PDAMA-poly(N-ethyl-4-vinylpyridine) (PEVP) films were constructed by LbL deposition of PDAMA and PEVP at pH 8.0, though films could not be obtained at pH 3.0. These results were explained based on the amphoteric nature of PDAMA, which is net positively and negatively charged at pH 3.0 and 8.0, respectively. PSS-PDAMA films were stable at pH 3.0-4.0 in a 150 mM NaCl solution and decomposed upon immersion in solutions with pH 4.3 or higher as a result of the reversal of the net PDAMA charge from positive to negative. In contrast, PDAMA-PEVP films disintegrated at pH 4.0 or lower. Thus, PDAMA-containing LbL films were found to be dual pH-sensitive: the films can be decomposed at acidic or neutral pH depending on the type of counter polymer in the film. PSS-PDAMA and PDAMA-PEVP films were also sensitive to salt concentration, where a higher concentration of NaCl in the solution in which the films were immersed facilitated decomposition of the films. The effect of salt concentration was attributed to partial breakage of the ion pairing between PDAMA and the counter polymers as a result of competitive binding with Na(+) and Cl(-) ions.