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Unique properties of ultrathin nanocellulose films revealed

By Frieda Wiley August 27, 2019
nanocellulose films
Four topographic images obtained by atomic force microscopy of cellulose nanofiber (CNF) thin films with surface charge densities—that is, the amount of electric charge on the surface of a given area—of the CNF network at (a) 400 µmol/g; (b) 800 µmol/g; (c) 1000 µmol/g; and (d) 1360 µmol/g. The insets are optical microscopy images of the corresponding films.

Due to the current high interest in using environmentally friendly materials, researchers are exploring cellulose nanofibers (CNFs), due to their natural abundance and biocompatibility in addition to their remarkable strength, flexibility, and light weight. Their smooth texture, for example, makes CNFs desirable for optoelectronics applications. This material is also used in nanocomposites and transparent conductive paper. However, as with any application, CNFs have limitations. One major issue is that the performance and properties of CNFs are affected by structural changes due to humidity. Humidity can compromise the integrity of the surface and promote degradation by promoting swelling while reducing the material’s mechanical strength. Previous research using polyurethane acrylate has demonstrated this phenomenon. Now, Stephan V. Roth of the Deutsches Elektronen-Synchrotron DESY in Hamburg, Calvin Brett of KTH Royal Institute of Technology in Stockholm, and their colleagues have characterized these humidity-driven structural changes at the nanoscale in ultrathin nanocellulose films. They published their results in a recent issue of Macromolecules.

“This work is…the first of its kind to use in situ grazing incidence small-angle x-ray and neutron scattering techniques to study the change(s) in nanocellulose topography in ultrathin films under different humidity conditions,” says Benjamin Hsaio, director at the Center for Integrated Energy Systems at Stony Brook University in Stony Brook, who did not participate in the study.

The investigators studied nanoscale morphological rearrangements in low-surface roughness hydrophilic ultrathin CNF films deposited on a Si substrate by airbrush spraying, enabling layer-by-layer analysis in a synchrotron beamline at the PETRA III DESY facility. The researchers measured properties such as surface-sensitive x-ray scattering, surface-sensitive neutron scattering, and contact angle measurements.

Imaging of CNF surfaces with four different surface charge densities—see Figure—revealed the lack of variation in the overall homogeneity of the material. However, each sample also had larger aggregates of CNF as noted by the brighter spots observed using optical microscopy. Additionally, the contact angle of water (to monitor wettability of the four samples) decreased in an inverse linear fashion from approximately 27° to 14° relative to the surface concentration expanding from 400 µmol/g to 1000 µmol/g, respectively—a feature the researchers attribute to the increased smoothness of the surface.

“The results convincingly demonstrated the reversible structural changes of nanocellulose by humidity, which is somewhat expected,” Hsiao says. “However, the detailed analysis showed that the average distance between large nanocellulose aggregates are more susceptible to humidity, but that the distance between the smaller fibrils is not—which is quite surprising.”

Brett, a graduate student and first author of the article, says previous industry understanding has been that water covers individual fibers; however, their research findings suggest that water may be predominantly assembled around only the larger agglomerates, which would affect the way the coating can modify or regulate humidity.

The method and results of their study, according to the researchers’ article, hold implications not only for cellulose-based materials, but also for hierarchical materials constructed with “moisture-sensitive building blocks.”

“There will be variations of how these nanoscale components look and behave, given the natural variability of trees or plants as a function of species, season, [and] location of growth,” Brett says. “For that reason, it is tough to fabricate well-defined engineering materials for specific applications based on natural nanocomponents.”

Read the article in Macromolecules.