A Computational Materials-by-Design Approach for Cellulose-Based Nanocomposite

In recent times, there has been a transition away from traditional engineering materials to more advanced materials that are multi-functional and exhibit improved thermomechanical properties. Much of the inspiration for these advanced materials comes from natural biological systems that fulfill the needs of diverse organisms through clever material organization and mechanics. A better understanding of the underlying mechanics of natural systems that give rise to versatile mechanofunctionality provides insight into building new synthetic, bio-inspired materials. One specific approach to developing these new materials is to directly integrate biomolecules with impressive material properties into synthetic polymer materials to develop new polymer nanocomposites. Cellulose nanocrystals (CNCs) are one such biomolecule that exhibit impressive mechanical, thermal, and optical properties and are excellent candidates for inclusion in cellulose-polymer nanocomposites and all-cellulose thin films. CNCs serve as the key reinforcing component of wood, itself an all-natural nanocomposite, and offer advantages over comparable, synthetic materials such as aramid fibers due to their natural availability and reduced environmental impact.;The traditional approach to the development of new materials involves significant experimentation and iteration to determine design parameters that optimize material properties. For CNC-based nanocomposites, this approach has only been moderately successful and the properties of these new materials, specifically mechanical properties such as elastic modulus and failure strength, have not been optimized. In this work, we describe a new approach to the materials design process termed computational materials-by-design. In this approach, computational tools are used to accelerate and improve the design process. For this specific system, this involves exploring the fundamental mechanics of CNCs (materials discovery), understanding how processing techniques can be used to change surface and interfacial properties (materials development), and describing how nanoscale simulation results can be used to predict macroscale material performance (property optimization). This computational approach offers numerous advantages over traditional materials design as it is iterative, scalable, cost effective, and applicable across multiple time and length scales. Specifically, our approach aims at probing and tuning the molecular level behavior of CNC-based nanocomposites to address current shortcomings in the materials and further suggest strategies for improving their thermomechanical properties.;To address these current shortcomings and further understand interfacial mechanics of these nanocrystals that are imperative to hierarchical material performance, here we present a computational materials-by-design approach to studying CNC-based nanocomposites. We begin with materials discovery where we aim to elucidate the natural behaviors and underlying mechanics of CNCs. We characterize and explain clear size dependence of the fracture energy of these materials that illuminates natural design principles. Further, we utilize nanoscale simulations to probe the interfacial traction-separation behavior of CNC interfaces and reveal key contributions of interfacial chemistry and hydrophobicity. We then shift to materials development where we utilize molecular simulation to characterize the effect of chemical surface modification on interfacial mechanics and water adsorption, and further demonstrate the effects of polymer grafting on polymer relaxation dynamics. Finally, our approach aims to optimize the properties of these new materials. To this effect, we first present multiscale models that allow us to predict bulk, macroscale properties of polymer nanocomposites directly from nanoscale simulations. Second, we provide a molecular level explanation for a longstanding phenomenon for the drying creep behavior of nanoporous solids. All together, these studies shed light on the importance of nanoscale interfacial chemistry and mechanics, as well as natural design principles that provide guidance toward developing new high performance cellulosic materials using a computational materials-by-design approach.