Hygromechanics and Shape Memory of Wood Cell Wall Investigated with Multiscale Modeling

Abstract

Wood is an important material for mankind, not only because of its profound impact on the industry and everyday life, but also for its enormous potential for providing sustainable solutions to futuristic, advanced materials. One crucial characteristic of wood is the strong hygroscopicity. When subjected to moisture, a series of mechanical and physical phenomena are induced, such as swelling, weakening and shape memory effect, the fundamental mechanisms of which are still not clear. The difficulty originates from the limited resolution of current imaging methods and the intrinsic perplexing hierarchical nanoscale ultrastructure of wood. The ideal way to tackle these issues is to resort to numerical modeling that enables fast prototyping of material configurations with the knowledge of atomistic interactions. Therefore this study proposes a framework combining molecular dynamics and finite element method, where the microscopic mechanisms of various hygromechanical phenomena are elucidated at atomistic scale, and the crucial mechanical properties as functions of moisture extracted from molecular simulations are incorporated into finite element model that reducing the modeling complexity while retaining essential mechanisms. Softwood cell wall S2 layer is a representative system of wood mechanics, containing various hydrophilic biopolymers as well as their interfaces that are intensely affected by hydration. There is considerable interest in characterizing the mechanical performance of the components of the S2 layer. Arabinoglucuronoxylan is modeled using molecular simulation presenting a general picture of the thermodynamic and mechanical response of hydrophilic biopolymer upon moisture adsorption. The heat of adsorption, heat capacity, thermal expansion and elastic moduli, all as functions of moisture, are found to approach the value of pure water and moreover show a crossover occurring around m ~ 0.3. The saturation of the first layer of adsorption and the subsequent fast growth of the second layer of adsorption is asserted to be the main mechanisms underneath the crossover. Another biopolymer, i.e. uncondensed lignin, is used as a prototype to probe the mechanisms of moisture-induced swelling and weakening effect. The microscopic motion of polymer chain segments, or more precisely the local stiffness which is the ratio of temperature to caging size, nearly perfectly relates to macroscopic stiffness of the material. As an extension, the similarity and difference of the impacts of heat and moisture on biopolymer mechanics are also discussed. The time-temperature-moisture superposition is only valid phenomenologically, whereas mechanistically heat and moisture are different as revealed by dynamics and energetics analyses. Succeed in the investigation of bulk material, another aspect as important is the interfaces. Cellulose fibers feature superior tensile strength reinforcing the compliant matrix of the S2 layer. However, the overall mechanical performance of the composite is predominantly determined by the mechanics of the interfaces either between fibers or between fiber and matrix. Cellulose crystal interfaces display interfacial stick-slip behavior, where regardless of the various loading conditions, interfacial stress, shear velocity and interaction energy correlate with the density of interfacial hydrogen bonds. Moisture excessively concentrates at the interface and reduces the strength by several times, which can be seen as a mechanism for the molecular switch. The moisture dependent properties are then transferred to finite element models. A series of composite models of different material arrangements and boundary conditions are examined, and a possible mechanism of moisture-induced shape memory emerges. Contrary to the conventional glass transition or (re)crystallization hypotheses, a mechanism dominated by the stick-slip behavior of the interface is proposed: first, the breakage and reformation of hydrogen bonding at the fiber-matrix interface serve as the molecular switch responsible for shape fixation and recovery; second, the elastic energy stored in the fibril serves as the driving force of shape recovery. Following the exhibition of the hygromechanics of the basic components of the S2 layer, i.e. individual materials, matrices and interfaces, a state-of-the-art atomistic model softwood cell wall S2 layer complying with the most advanced experimental indications is proposed. The swelling coefficient, elastic moduli, and Poisson’s ratio show good agreement with experiments. Comprehensive rule of mixture analysis is carried out on the rich compilation of wood polymer hygromechanical data, elucidating the mechanical roles of interphase, different components of cell wall material, etc. In summary, this thesis presents a computational framework investigating the mechanics of a nanoscale fiber-reinforced composite, softwood cell wall S2 layer. The moisture dependent mechanical properties and mechanisms of moisture-induced swelling, weakening and shape memory effects are elucidated, filling the gap of experimental study and extends the understanding of the moisture effect on the fascinating natural material, wood.