Thin film thickness gradients and spatial patterning via salt etching of polyelectrolyte multilayers

We demonstrate the time- and concentration-dependent dissolution (etching) of polyelectrolyte multilayer (PEM) films comprised of poly(allylamine hydrochloride) (PAH) and poly(acrylic acid) (PAA) immersed in NaCl solutions. PEM thickness decreases to a reproducible unchanging value that depends upon the initial film thickness and the concentration of NaCl. As opposed to a top-down removal of PEM chains from the film, the dissolution mechanism involves the diffusion and association of chains throughout the PEM that leave the film in the form of polyelectrolyte complexes (PECs). The PEC phase diagram is invoked to explain the observations presented in this paper and work by previous researchers. We demonstrate the straightforward application of PEM salt etching to pattern films where the spatial position and amount of dissolved material are controlled, resulting in a multicolor reflector and a gradient-thickness film. In addition to providing a more comprehensive picture of PEM stability, these results may help to advance the technological impact of PEMs in biology and other fields where the ability to create gradients in film thickness could be advantageous.