Prediction of Thermoplasticity in Lignocellulose-Based Materials using Molecular Simulations

With increasing interest for sustainably sourced and renewable materials, lignocellulose-based biopolymers are a natural candidate for such applications. However, most biopolymers, including lignocellulose, are intrinsically rigid and are therefore difficult to shape with e.g. extrusion and other tools for processing thermoplastic polymers. The lignocellulose must thus be modified somehow, with e.g. chemical modifications and/or plasticizers, to become thermoplastic. In this thesis, the molecular and atomistic-level mechanisms that govern the interactions important for thermoplasticity within lignocellulosic materials are investigated through the use of molecular dynamics (MD) simulations combined with experimental validation. The chemical structures of the lignocellulosic components, in particular cellulose and lignin, are explored to improve processability, mechanical performance, and thermodynamical behavior. The first part of the work focuses on cellulose and dialcohol cellulose, a ring-opened derivative which have demonstrated promising properties for processing, both via simulations and experiments. An increased degree of ring opening enhances the molecular mobility and lowers the glass transition temperature, both in dry and moist conditions, facilitating thermoplastic behavior while maintaining mechanical performance. Subsequent studies, extend the investigation into a broader set of cellulose modifications and ring openings, including aldehyde- hydroxylamine and carboxyl functionalization, identifying how the different types of modifications affect the thermodynamic and mechanical properties. The role and effect of moisture content, and the presence of functional groups are thoroughly investigated. Plasticizers in cellulose and dialcohol cellulose systems were also evaluated, revealing that the plasticizer size and mobility influence the stability and thermal softening of the systems, with sorbitol and glycerol in particular showing especially promising results. The thermo-mechanical behavior of lignin is also examined under the influence of temperature and moisture content, linking lignin softening with effects on hot-pressed unbleached paper through a combined simulation and experimental study. The study showed that wet hotpressing is an efficient way for improving the mechanical properties of paper. This thesis demonstrates how molecular dynamics simulations can provide a better understanding of the internal structure of materials. It shows how MD simulations can guide the development of new thermoplastic materials, especially by examining properties that are difficult or even impossible to observe experimentally.