Microstructure and dynamics of nanocellulose films: Insights into the deformational behavior

Abstract

Cellulose nanocrystals (CNCs) thin films draw considerable interest in engineering and technological applications due to their excellent mechanical and physical properties associated with dynamic and microstructural features. Here, we employ coarse-grained molecular dynamics (CG-MD) simulations to investigate how the dynamics and microstructure change in the CNC films under tensile deformation. Our results show that the Young’s modulus can be quantitatively predicted by the power-law scaling relationship with initial packing density, where higher density leads to an increase in both modulus and strength. By evaluating the molecular local stiffness during the tensile process, our findings show that CNC film with a higher density exhibits a higher degree of dynamic heterogeneity, which is greatly reduced under deformation. Our results further demonstrate that randomly oriented CNCs tend to be more aligned with the tensile direction associated with higher free volume and porosity during the deformation; however, the dynamics of CNC is more associated with the degree of local packing and density rather than the CNC orientation. Our study provides fundamental insights into deformational mechanisms associated with the microstructure and dynamics of CNC films at a molecular level, aiding in the tailored design of cellulose-based materials for their mechanical performance.

Introduction

With the increasing demands in multifunctionality, recyclability, and environmental friendliness [1], [2], cellulose-based nanomaterials have drawn considerable attentions as sustainable and renewable materials that offer potential alternatives to conventional petroleum-derived polymers in various applications, such as nanocomposites [3], [4], advanced manufacturing [5], [6], electronics [7], tissue engineering [8], [9], and food packaging [10], [11]. As the most abundant biopolymer on earth, a cellulose chain typically has a high molecular weight consisting of glucose repeat units; multiple cellulose chains can stack together to form a hierarchical paracrystalline structure, which can be extracted from a wide range of biological sources (e.g., wood, plant, bacterial, algae, and tunicates) [12], [13], [14], [15], [16]. Cellulose can be further chemically or mechanically processed in the form of cellulose nanocrystals (CNCs) or nanofibers (CNFs) [17], [18], [19], [20], which exhibit remarkable mechanical properties due to rich interchain/intrachain hydrogen-bonding. In particular, CNCs exhibit a rod-like shape with a high aspect ratio (i.e., 3−5 nm in cross section dimensions, 50−500 nm in length) and possess stiff and predominantly crystalline structures after removal of amorphous domains, achieving remarkably high strength and stiffness (i.e., comparable to structural steel) [15]. These exceptional features have rendered CNCs as ideal candidates of nanoscale building blocks for the development of structural materials [21], [22], [23].

Recently, there is a particular upsurge of interests in nanocellulose-based thin films [24], [25], [26] comprised of readily self-assembled CNCs/CNFs, forming the so-called cellulose nanopapers. These cellulose thin films or nanopapers have been produced by several approaches, such as spin coating [27], Langmuir–Schaeffer [28], and Langmuir–Blodgett technique [29]. Cellulose nanofibers are often randomly distributed in ordinary nanopapers, exhibiting strong network-forming characteristics [30], yielding interesting physical properties, including optical transparency [31], low thermal expansion [24], high porosity and permeability, in addition to excellent structural performance [32]. For instance, light-weight porous nanocellulose derivative foams, exhibiting high specific surface area and high ductility and toughness, have received a great deal of attention and are of interest for applications involving mechanical energy absorption and sound insulation [32]. The full potential of nanopapers, however, is still limited due to their complex microstructural features arising from the random orientation of CNFs/CNCs. Considerable efforts have been made towards this direction. For instance, CNCs within suspensions can be oriented in a given direction by applying external force with multiple experimental techniques, including electric [33], [34], magnetic fields [35], [36], and shear casting [35], [36]. The internal structure of processed CNC nanopapers can also adopt various other architectures, such as highly aligned, brick-and-mortar type, Bouligand (or twisted plywood) microstructures [37], [38], as well as isotropic assemblies, chiral nematic ordering fashion [39], [40]. Moreover, it has been shown that the mechanical, barrier properties, and impact tolerance are highly dependent upon the microstructural arrangements (e.g., nanofiber orientation) of CNCs [30], [37], [41], [42]. In particular, homogeneous dispersion of CNCs forming random network yields overall isotropy of physical and mechanical properties, and the porosity and density can be readily varied to achieve tunable permeability usage and lightweight performance. Despite considerable efforts, it still remains largely elusive that how the microstructural features (e.g., density, nanocrystal orientation) impact the dynamics and deformational response for describing structure–property relationship of cellulose nanopapers having a network topology.

To uncover the underlying molecular mechanisms associated with mechanical properties of nanocellulose films, several microstructure-based modeling approaches have been developed to interpret the mechanical behavior of materials and guide the design. Fibrous network model based on finite element method (FEM) has been useful in predicting the elastic modulus of cellulose nanopaper and elucidating the effect of inter-fiber bonds density and bonds stiffness on the modulus [43], [44]. Moreover, Meng et al. developed a theoretical crack-bridging model to investigate the alignment effect on the fracture toughness of cellulose nanopaper [45]. Despite the success of continuum and theoretical modeling, they may lack of preserving nanoscopic details necessary to accurately capture the mechanical performance of nanocellulose films. Molecular dynamics (MD) simulations have been proved to be highly useful in this regard. All-atomistic (AA) MD simulations have been extensively employed to accurately present the nanoscale deformation and failure behavior of CNCs [46], [47], [48], [49]. Zhu et al. proposed a molecular chain pull-out model to reveal the hydrogen bond breaking and re-forming mechanism, which in turn dictate the enhanced energy dissipation during sliding [50]. More recently, coarse-grained (CG) MD simulations have gained tremendous popularity due to its access to larger spatiotemporal scale as well as providing avenues for parametric studies. These methods have been successfully applied to CNC materials, both during quasi-static [40], [51], [52], [53] and dynamic deformation procedures [37]. For instance, a CG model proposed by Shishehbor and Zavattieri offers a promising scheme to capture mechanical and interfacial features of CNC-based materials [54]. Ray et al. recently establish a bottom-up CG modeling which is capable of modeling cellulose fibers ranging from nanometers to microns and studying the deformation process of a cellulose nanopaper [55].

In this work, we aim to better understand the dynamics and mechanical performance of nanocellulose thin films consisting of randomly oriented CNCs. By employing an atomistically-informed CG model developed for CNCs [40], we systematically explore the microstructural features of CNCs (i.e., density, porosity, and nanocrystal orientation) and their coupling with the dynamics and mechanical properties of a random network of CNC nanopaper under tensile deformation. Specifically, first, the mechanical properties of CNC film with various packing density are explored to understand their scaling relationship. Next, by evaluating molecular local stiffness, we gain valuable insights into the dynamical heterogeneities of CNC film in quasi-static tension. Finally, porosity analysis and orientation distribution are discussed to provide insights into the underlying mechanism of deformational behaviors of the CNCs. Our simulation results highlight the critical role of density and microstructure in the mechanics and dynamics of CNCs thin film at a fundamental level, paving the way for tailored design of lightweight performance of cellulose-based materials.