Drying-induced bending deformation of cellulose nanocrystals studied by molecular dynamics simulations
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ORIGINAL RESEARCH
Drying-induced bending deformation of cellulose nanocrystals studied by molecular dynamics simulations Yu Ogawa
. Yoshiharu Nishiyama
. Karim Mazeau
Received: 7 May 2020 / Accepted: 10 September 2020 Ó Springer Nature B.V. 2020
Abstract Drying cellulosic materials from their water-swollen state can collapse their ultrastructure and alter their macroscopic material properties such as mechanical strength and water-retention ability. However, at the single-crystal or molecular level, little is known about the deformation of cellulose upon drying. We thus investigate herein the drying-induced deformation of a cellulose crystal by using an atomistic molecular dynamics simulation that considers a hydrated system composed of two short cellulose crystals, a lower one fixed to a flat substrate and an upper one free to deform. To mimic vacuum drying, the water is gradually removed from the system. As the drying proceeds, the upper cellulose crystal bends and forms a tight contact with the lower cellulose crystal. This result underlines the importance of lateral deformation of cellulose crystals in the collapse of the cellulose ultrastructure and provides insights into the molecular mechanisms responsible for modifying the properties of cellulose materials. Keywords Cellulose Drying Lateral deformation Molecular dynamics
Y. Ogawa (&) Y. Nishiyama (&) K. Mazeau CERMAV, CNRS, Univ. Grenoble Alpes, 38000 Grenoble, France e-mail: [email protected] Y. Nishiyama e-mail: [email protected]
Introduction Cellulose is extracted and processed in the aqueous condition in many industrial processes, from chemical pulping to fiber and textile production to nanocellulose production. As a result, producing cellulosic materials often requires drying or water removal. Drying a cellulosic material modifies its physical properties; for instance, in the process of paper manufacturing, pulp fibers stiffen and shrink laterally upon drying (Minor 1994; Giacomozzi and Joutsimo 2017). In addition, once dried, the pulp does not fully recover to the original state upon rewetting, and repeated drying and wetting produces a more rigid and brittle material. This phenomenon, called ‘‘hornification,’’ poses a challenge to paper recycling because hornified fibers do not reswell to their initial hydrated state when resuspended in water, which weakens recycled paper. Such degradation of the material properties upon conventional drying (air, oven, contact, etc.) occurs not only with pulp fibers but also with different cellulosic materials such as hydrogels and nanocellulose materials (Peng et al. 2012; Ra¨ma¨nen et al. 2012). Retaining the fine structure of cellulosic materials and minimizing the impact of drying often requires more gentle and costly drying methods such as freezedrying or critical-point drying (Jin et al. 2004; Heath and Thielemans 2010; Hoepfner et al. 2008; Beaumont et al. 2017). Conversely, simple air drying also produces some exceptional ma
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