Tuning Smart Microgel Swelling and Responsive Behavior through Strong and Weak Polyelectrolyte Pair Assembly

摘要

The layer-by-layer (LbL) assembly of polyelectrolytenpairs on temperature and pH-sensitive cross-linkednpoly(N-isopropylacrylamide)-co-(methacrylic acid), poly-n(NIPAAm-co-MAA), microgels enabled a fine-tuning of thengel swelling and responsive behavior according to the mobilitynof the assembled polyelectrolyte (PE) pair and the compositionnof the outermost layer. Microbeads with well-definednmorphology were initially prepared by synthesis in supercriticalncarbon dioxide. Upon LbL assembly of polyelectrolytes,ninteractions between the multilayers and the soft porousnmicrogel led to differences in swelling and thermoresponsivenbehavior. For the weak PE pairs, namely poly(L-lysine)/poly(Lglutamicnacid) and poly(allylamine hydrochloride)/poly(acrylic acid), polycation-terminated microgels were less swollen andnmore thermoresponsive than native microgel, whereas polyanion-terminated microgels were more swollen and not significantlynresponsive to temperature, in a quasi-reversible process with consecutive PE assembly. For the strong PE pair,npoly(diallyldimethylammonium chloride)/poly(sodium styrene sulfonate), the differences among polycation and polyanionterminatednmicrogels are not sustained after the first PE bilayer due to extensive ionic cross-linking between the polyelectrolytes.nThe tendencies across the explored systems became less noteworthy in solutions with larger ionic strength due to overall chargenshielding of the polyelectrolytes and microgel. ATR FT-IR studies correlated the swelling and responsive behavior after LbLnassembly on the microgels with the extent of H-bonding and alternating charge distribution within the gel. Thus, the proposednLbL strategy may be a simple and flexible way to engineer smart microgels in terms of size, surface chemistry, overall charge andnpermeability.