Multiresponsive Clay-Containing Layer-by-Layer Films

We report on polymer/clay layer-by-layer films responsive to multiple stimuli. Temperature- and salt-responsive films were constructed using assembly of poly(N-isopropylacrylamide) (PNIPAM) and montmorillonite clay nanosheets. An additional pH response was achieved by depositing and cross-linking hybrid, dual-network PNIPAMday/PNIPAMpoly-(methactylic acid) (PMAA) multilayers. Both types of films remained stable in a wide pH range and were highly swollen. For example, PNIPAMclay films swelled up to similar to 14.5 times their dry film thickness in low-salt solutions at 25 degrees C, as shown by laser scanning confocal microscopy. At temperatures higher than PNIPAM's lower critical solution temperature (LCST) of 32 degrees C, or In 0.3 M Na(2)5O(4) solutions at room temperature, both PNIPAM/day and PNIPAMday/PNIPAMPMAA films reversibly deswelled as a result of collapse of PNIPAM chains. Films of both types showed a decrease in permeability to fluorescein-tagged dextrans of various molecular weights. Importantly, film permeability to dextrans was decreased at temperatures above PNIPAM's 1051, and the effect could be reversed by lowering the temperature. Inclusion of PMAA within multilayers provided an additional pH response to film swelling and permeability. Hybrid PNIPAMday/PNIPAMPMAA films showed drastic deswelling at low pH values due to the onset of hydrogen bonding between PNIPAM and PMAA, and the diffusion of 70 kDa dextran through multilayers at acidic pH was completely blocked. These multiresponse features of day-containing films make them promising candidates for applications in sensing, actuation, and controlled delivery.