Out-of-Plane Auxetic Behavior in Cellulose Nanofibril Films

While auxeticity has been established in cellulose-based paper and paperboards and computational studies suggest auxetic behavior should occur within cellulose structures, the auxetic response of neat cellulose nanofibril (CNF) films has not yet been experimentally established. Here, we show that an out-of-plane auxetic response does occur in CNF films and that the magnitude of the response is dependent on the film density, microstructure, and sample geometry. CNF films are fabricated using vacuum filtration, followed by conditioning at 23 °C and 50% relative humidity. The CNF aqueous suspension amount is varied from 90 to 450 mL, which progressively increases the film density until reaching a plateau of 1.05 ± 0.02 g/cm³ for films with suspension volumes equal to or greater than 270 mL. Aside from varying the film densities, the aspect ratio of the film is varied from 1.5 to 5 (ratio of rectangular CNF film length to width) to determine how the sample dimensions contribute to the auxetic response, specifically if stress fields associated with gripping of the sample constrain the fiber network. From these studies, CNF films with a density of 1.05 g/cm³ and a film aspect ratio of 5 exhibit the highest auxetic response in the elastic region with a negative Poisson's ratio of -5.3 as determined by linear fitting and the largest instantaneous Poisson' ratio value of -7.99 at 0.4% strain. Overall, this work provides insight into the processing-structure-property relationships that define auxeticity in CNF films, which can enable the use of CNFs as auxetic metamaterials in a broad range of applications such as sensing, protective gears, composites, and structural materials.