A facile spinning approach towards the continuous production of aligned nanocellulose films

In this work, we present an alternative approach to cellulose nanofibril film (CNF) production, taking inspiration from the wet spinning of fibers to wet spin films. During the spinning process, a CNF suspension is injected into a coagulation bath, where the partially aligned CNF network is locked. The CNF alignment of the dry films is then detected by wide angle X-ray scattering (WAXS). The comparison between the ultimate strengths and strengths at breaks of the films produced with different process parameters, including the suspension injection rate, bath pH, and bath flow rate, indicated no significant change in mechanical properties, suggesting a reliable and constant outcome for large-scale film fabrication. Furthermore, the produced films demonstrated high total light transmittance of 93 % at the wavelength of 550 nm, making them suitable for optoelectronic applications. Polarized optical microscopy revealed that even a low degree of CNF alignment can lead to anisotropic optical properties. Moreover, an anisotropic response to humidity was observed, in which the films preferentially bend in the perpendicular direction of the CNF orientation, thus opening a way for humidity-driven actuators.
This work was financially supported and part of the Academy of Finland's Flagship Program under Projects No. 318890 and 318891 (Competence Center for Materials Bioeconomy, FinnCERES). BioEconomy and RaMi infrastructures were employed in this work. Karl Håkansson kindly acknowledge the financial support from the Nils and Dorthi Troedsson Research fund. T.A. acknowledges the Chair in Sustainable Materials at EPFL, co-funded by BASF, Logitech, Nestlé, and SIG. Dr. Hoang M. Nguyen is acknowledged for his help in acquiring the AFM images.