Polyelectrolyte exchange and diffusion in microgel multilayer thin films

The exchange of the polycation, poly-l-lysine (PLL), within polymeric multi-layer films constructed with anionic poly(N-isopropylacrylamide)–co-acrylic acid (pNIPAm-co-AAc) microgels was explored. Via incorporation of a fluorescent tag on PLL, the incorporation and distribution of PLL was visualized by fluorescence microscopy and optical extinction techniques. Using UV-vis spectrophotometry, the absorbance of PLL during multi-layer film formation was monitored. Distinctive “in” and “out” diffusive properties of the polycation with limited exchange was observed. Additionally, mechanical deformation was used to probe the influence of self-healing on PLL redistribution across an entire multi-layer film depth; CLSM was utilized to compare the fluorescence profile of these films before and after healing, and a lack of polycation exchange throughout the entire microgel-polyelectrolyte film depth was revealed. Importantly, these results stand in contrast to those observed previously in purely linear polyelectrolyte films, which suggests polycation-microgel interactions that are somewhat unique as compared to the interactions observed for purely linear polyelectrolyte complexation.