Structures of regenerated cellulose films revealed by grazing incidence small-angle x-ray scattering

The characteristic in-plane length scales of ultrathin films of regenerated cellulose are measured using noncontact atomic force microscopy (NC-AFM) and grazing incidence small-angle x-ray scattering (GISAXS) in ambient atmosphere and under various humidity conditions. The aim is to elucidate the structural basis for the excellent compatibility of cellulose supports to planar lipid membranes. Films are deposited on silicon wafers by Langmuir-Blodgett (LB) transfer and spin coating. NC-AFM height profiles and the resulting calculated power spectral density functions indicate that both kinds of cellulose films have almost identical root-mean-square roughness values (0.7-0.8 nm) and very similar characteristic length scales (32 nm), respectively. GISAXS measurements, both above and below the critical angle of total external reflection, show that the dominant length scales in the bulk and near the surface of the films are comparable (~50 nm). The origin of these length scales can be attributed to the bundle of rodlike molecules of cellulose that result during the regeneration process (i.e., as a consequence of the cleavage of the silyl side chains of trimethylsilylcellulose). Exposure of the cellulose samples to various humidities shows that above a relative humidity of 97% a significant swelling of the films occurs, which is consistent with our previous findings. The swelling of films with more than 30 LB monolayers of cellulose induces a remarkable out-of-plane rearrangement of the cellulose bundles, due to a reduced influence of the solid substrate compared to thinner films with only eight to ten LB monolayers.