Rubaiya Fariha, Shofner Meisha L, Garten Lauren M
School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, United States.
Renewable Bioproducts Institute, Georgia Institute of Technology, Atlanta, Georgia 30332, United States.
ACS Omega. 2025 Mar 26;10(13):13339-13349. doi: 10.1021/acsomega.4c09915. eCollection 2025 Apr 8.
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/cm 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/cm 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.
虽然在基于纤维素的纸张和纸板中已经证实了负泊松比现象，并且计算研究表明在纤维素结构中应该会出现负泊松比行为，但纯纤维素纳米纤丝（CNF）薄膜的负泊松比响应尚未通过实验得到证实。在此，我们表明CNF薄膜确实会出现面外负泊松比响应，并且该响应的大小取决于薄膜密度、微观结构和样品几何形状。CNF薄膜通过真空过滤制备，然后在23°C和50%相对湿度下进行调湿处理。CNF水悬浮液的量从90 mL变化到450 mL，这会逐渐增加薄膜密度，直到对于悬浮液体积等于或大于270 mL的薄膜达到1.05±0.02 g/cm³的平稳状态。除了改变薄膜密度外，薄膜的纵横比从1.5变化到5（矩形CNF薄膜长度与宽度的比值），以确定样品尺寸如何影响负泊松比响应，特别是与样品夹持相关的应力场是否会限制纤维网络。通过这些研究，密度为1.05 g/cm³且薄膜纵横比为5的CNF薄膜在弹性区域表现出最高的负泊松比响应，通过线性拟合确定其负泊松比为 -5.3，在0.4%应变时的最大瞬时泊松比值为 -7.99。总体而言，这项工作深入了解了定义CNF薄膜负泊松比现象的加工 - 结构 - 性能关系，这使得CNF能够作为负泊松比超材料应用于广泛的领域，如传感、防护装备、复合材料和结构材料。
