Selective water uptake within micelle-containing layer-by-layer films of various architectures: A neutron reflectometry study

Swelling of micelle-containing layer-by-layer (LbL) films of various architectures has been studied by neutron reflectometry (NR). Multilayers of the first type were constructed using poly(2-(dimethylamino)ethyl methacrylate)-block-poly(N-isopropylacrylamide) (PNIPAM-b-PDMA) block copolymer micelles (BCMs) alternately assembled with poly(4-styrene sulfonate) (PSS). NR data showed that the films maintained their layered structure, but deuterated PSS (dPSS), deposited within every 5^th layer as a marker, was highly interdiffused into neighboring layers. In situ NR measurements demonstrated that the films swelled homogeneously by ∼35% in an aqueous environment. The second type of multilayer contained three zones: bottom and top stacks consisting of PDMA/dPSS homopolymer assemblies, and BCMs deposited in the middle stack as (BCM/dPSS)_n, where n is the number of deposition cycles, equal to 1 or 2. The individual micellar layer deposited in a single deposition step (n = 1) only partially covered the surface, whereas a complete layer of micelles was achieved after two deposition cycles. In situ NR study of these stacked films revealed different degrees of water uptake by film internal strata. While layers of assembled micelles took up ∼38% water by volume when dry films were exposed to an aqueous environment at 25°C, the bottom homopolymer stack was able to take up only 11-20% water. In addition, the film architecture and the degree of surface coverage by BCMs were found to be important factors enabling the study of selective swelling of film strata. NR-enabled observations of selective swelling of assembled amphiphilic BCMs allow one to correlate film swelling on the nanoscale with internal structure, and present a powerful approach for future studies of BCM-containing systems, which are useful in actuation, sensing, and controlled delivery applications.