CARBON-BASED COMPLEX STRUCTURE AND MODEL DEVELOPMENT

Growing concerns about the environment and energy crisis prompt a search for effective carbon-based materials due to their low cost, renewability, sustainability, easy accessibility and excellent properties. We study the model development, structure and properties of graphene oxide, cellulose and their nanocomposites in order to obtain a better fundamental understanding of carbon complex materials and construct a structure-property relationship via reactive molecular dynamics simulations. In chapter 3, the model development of GO is studied. Theoretical GO models developed so far present a good description of its chemical structure. However, when it comes to the structural properties, such as the size and distribution of vacancy defects, the curvature (or roughness), there exist significant gaps between computational models and experimentally synthesized GO materials. We carry out reactive molecular dynamics simulations and use experimental characteristics to fine tune theoretical GO models. Attentions have been paid to the vacancy defects, the distribution and hybridization of carbon atoms, and the overall C/O ratio of GO. The GO models proposed in this work have been significantly improved to represent quantitative structural details of GO materials synthesized via the modified Hummers method. The temperature-programmed protocol and the computational post analyses of Fourier-transform infrared spectroscopy, X-ray photoelectron spectroscopy, vacancy size and curvature distribution, are of general interest to a broad audience working on GO structures from other synthesis methods and other two-dimensional materials and their composites. In Chapter 4, we outline the state-of-the-art understanding of cellulose structures, and discuss in details cellulose interactions, dissolutions and decompositions via computational methods of molecular dynamics (MD) and reactive molecular dynamics (RxMD) simulations. In addition, cellulose characterizations, beneficial to validate and support computational results, are also briefly summarized. Such a state-of-the-art account of atomistic computational studies could inspire interdisciplinary collaborations, optimize process design, promote cellulose-based materials for emerging important applications and shed a light on fundamental understandings of similar systems of biomolecules, polymers and surfactants. In Chapter 5, we investigate the fundamental mechanism of how cellulose structure transforms under pyrolysis conditions and the practical guideline of how cellulose properties are fined tuned accordingly. A series of reactive molecular dynamics calculations has been designed to reveal the structural evolution of crystalline cellulose under pyrolysis treatments. Through the detailed analysis of cellulose configuration change, hydrogen bonding network variation, reaction and redistribution of carbon, oxygen and hydrogen elements, and Young’s modulus, a molecule level insight of crystalline cellulose and its structural evolution under pyrolysis conditions has been constructed via reactive molecular dynamics simulations. We anticipate those theoretical results could effectively promote the design, the manufacture, and the optimization of cellulose based materials for relevant emerging applications. In Chapter 6, we combined the results from previous chapters and explore a new composite material that incorporating amorphous cellulose chains on GO surface, which is barely reported by recent publications. A series of RxMD simulations have been carried out to reveal the mechanical properties of pure GO and cellulose-GO nanocomposites. Two different cellulose-GO composites are constructed, namely, cellulose (monolayer)-GO model and cellulose (multilayer)-GO model. The tensile deformation, Young’s modulus and mechanical strength of GO and cellulose-GO composites have been recorded and calculated under the temperature of 300, 500 800 K, with two strain rates of 10-4/fs and 10-5/fs. We hope the GO model with the simultaneously description to both structural and chemical properties can provide a new fundamental understanding of the mechanical performance of GO and cellulose-GO composites, and could add some advancement to existing knowledge of carbon-based materials

Systematic Hydrogen‐Bond Manipulations To Establish Polysaccharide Structure–Property Correlations

A dense hydrogen‐bond network is responsible for the mechanical and structural properties of polysaccharides. Random derivatization alters the properties of the bulk material by disrupting the hydrogen bonds, but obstructs detailed structure–function correlations. We have prepared well‐defined unnatural oligosaccharides including methylated, deoxygenated, deoxyfluorinated, as well as carboxymethylated cellulose and chitin analogues with full control over the degree and pattern of substitution. Molecular dynamics simulations and crystallographic analysis show how distinct hydrogen‐bond modifications drastically affect the solubility, aggregation behavior, and crystallinity of carbohydrate materials. This systematic approach to establishing detailed structure–property correlations will guide the synthesis of novel, tailor‐made carbohydrate materials

Multiscale Modeling Of The Hierarchical Structure Of Cellulose Nanocrystals

Cellulose constitutes the most abundant renewable polymeric resource available today. It considered an almost inexhaustible source of raw material, and holds great promise in meeting increasing demands for environmentally friendly and biocompatible products. Key future applications are currently under development for the automotive, aerospace and textile industries. When cellulose fibers are subjected to acid hydrolysis, the fibers yield rod-like, highly crystalline residues called cellulose nanocrystals (CNCs). These particles show remarkable mechanical and chemical properties (e.g. Young Modulus ~200 GPa) within the range of other synthetically-developed reinforcement materials. Critical to the design of these materials are fundamental material properties, many of which are unavailable in the existing literature. A multiscale framework has been developed to predict and describe the thermo-mechanical characteristics of cellulose nanocrystals using state-of-the-art computational tools capable of connecting atomistic based simulations to experiments through continuum based modeling techniques. First-principle density functional theory and molecular dynamic simulations were utilized at the atomistic level. Longstanding issues regarding the elastic and thermal expansion anisotropies for crystalline cellulose have been studied in terms of the single-crystal elasticity tensor and the thermal expansion tensor components. First-principles phonon calculations via van der Waals density functionals as well as reverse non-equilibrium molecular dynamics simulations were used to gain a fundamental understanding of defect-free, crystalline cellulose thermo-mechanical properties. Entropy, enthalpy, constant pressure heat capacity, thermal expansion tensor, thermal conductivity, Young\u27s modulus, and Poisson\u27s ratio, were computed over a wide range of temperatures (0 to 500 K). A comprehensive study of the hydrogen bond structure that characterizes crystalline cellulose has been conducted in an attempt to ascertain the roles that inter- and intra- molecular hydrogen bonds play in determining the mechanical properties of CNCs. Five different force fields/parameter sets were compared with experimental results and first-principles simulations in terms of their ability to predict the following properties: lattice parameters and angles, linear elasticity tensor and linear thermal expansion tensor. Continuum based modeling techniques were used to answer fundamental questions regarding the role of hydrogen bonding in the mechanical response of CNCs. A variety of finite element-based continuum models were specifically developed for cellulose chains and non-bonding interactions (van der Waals, Coulomb and hydrogen bonds). As a result, a complete multiscale framework capable of reproducing the mechanical behavior of cellulose nanocrystals has been developed

Molecular Dynamics Simulations of Cellulose and Dialcohol Cellulose under Dry and Moist Conditions

The development of wood-based thermoplastic polymers that can replace synthetic plastics is of high environmental importance, and previous studies have indicated that cellulose-rich fiber containing dialcohol cellulose (ring-opened cellulose) is a very promising candidate material. In this study, molecular dynamics simulations, complemented with experiments, were used to investigate how and why the degree of ring opening influences the properties of dialcohol cellulose, and how temperature and presence of water affect the material properties. Mechanical tensile properties, diffusion/mobility-related properties, densities, glass-transition temperatures, potential energies, hydrogen bonds, and free volumes were simulated for amorphous cellulosic materials with 0-100% ring opening, at ambient and high (150 \ub0C) temperatures, with and without water. The simulations showed that the impact of ring openings, with respect to providing molecular mobility, was higher at high temperatures. This was also observed experimentally. Hence, the ring opening had the strongest beneficial effect on “processability” (reduced stiffness and strength) above the glass-transition temperature and in wet conditions. It also had the effect of lowering the glass-transition temperature. The results here showed that molecular dynamics is a valuable tool in the development of wood-based materials with optimal thermoplastic properties

Classical and reactive molecular dynamics: Principles and applications in combustion and energy systems

Molecular dynamics (MD) has evolved into a ubiquitous, versatile and powerful computational method for fundamental research in science branches such as biology, chemistry, biomedicine and physics over the past 60 years. Powered by rapidly advanced supercomputing technologies in recent decades, MD has entered the engineering domain as a first-principle predictive method for material properties, physicochemical processes, and even as a design tool. Such developments have far-reaching consequences, and are covered for the first time in the present paper, with a focus on MD for combustion and energy systems encompassing topics like gas/liquid/solid fuel oxidation, pyrolysis, catalytic combustion, heterogeneous combustion, electrochemistry, nanoparticle synthesis, heat transfer, phase change, and fluid mechanics. First, the theoretical framework of the MD methodology is described systemically, covering both classical and reactive MD. The emphasis is on the development of the reactive force field (ReaxFF) MD, which enables chemical reactions to be simulated within the MD framework, utilizing quantum chemistry calculations and/or experimental data for the force field training. Second, details of the numerical methods, boundary conditions, post-processing and computational costs of MD simulations are provided. This is followed by a critical review of selected applications of classical and reactive MD methods in combustion and energy systems. It is demonstrated that the ReaxFF MD has been successfully deployed to gain fundamental insights into pyrolysis and/or oxidation of gas/liquid/solid fuels, revealing detailed energy changes and chemical pathways. Moreover, the complex physico-chemical dynamic processes in catalytic reactions, soot formation, and flame synthesis of nanoparticles are made plainly visible from an atomistic perspective. Flow, heat transfer and phase change phenomena are also scrutinized by MD simulations. Unprecedented details of nanoscale processes such as droplet collision, fuel droplet evaporation, and CO2 capture and storage under subcritical and supercritical conditions are examined at the atomic level. Finally, the outlook for atomistic simulations of combustion and energy systems is discussed in the context of emerging computing platforms, machine learning and multiscale modelling

A Molecular Dynamics Study of the Cellulose-Graphene Oxide Nanocomposites: The Interface Effects

Cellulose has gained increasing attention due to its abundance and renewability. Obtained through a strong acid hydrolysis treatment of cellulose microfibrils, cellulose nanocrystals (CNCs) stand out among all hierarchical cellulose structures with appealing mechanical and optical properties that have been utilized as a reinforcing nanomaterial for the advanced material design. The cellulose nanocrystal graphene oxide (CNC-GO) nanocomposite film has been developed and successfully applied in portable and bendable sensing optoelectronics, energy storage and electromagnetic pulse protection devices. New material phenomena have been observed through experimental characterizations, but they lack fundamental understanding due to the experimental limitations in nanoscale. Therefore, a systematic and theoretical study at atomic level is desired to address the key factors responsible for the associated material properties of the CNC-GO nanocomposite, especially at its interface. We adopt molecular dynamics (MD) simulation techniques to investigate the role of the hydrogen bonds in the CNC-GO interface interaction with respect to the CNC slab orientation, the CNC slab thickness, the GO oxidation type, and the water content at the interface. The objective is to understand the role of hydrogen bonds at the CNC-GO interface in CNC morphological variations and the mechanical property enhancement. We systematically investigate (1) the crystallography of the CNC-GO nanocomposites and their lattice parameter variations for the suppression of (200) facet in the X-ray diffraction (XRD) spectrum; (2) the hydrogen bond formation, types and distributions of the CNC due to the CNC-GO interface interaction; and (3) the mechanical property variations due to the interface hydrogen bonding of the CNC-GO nanocomposites. Through systematic molecular dynamics simulations of a set of simplified CNC-GO sandwich structures, the mechanism behind local (200) facet manipulation, as well as the global morphological variations, can be elucidated. It will shed light on the correlations between interface types and mechanical loading responses along with the interface water molecules for the mechanical performance enhancement. This research provides an understanding of intrinsically manipulating the CNC-GO interface and potentially engineering the cellulose based nanocomposite materials and mechanical properties for future advanced materials development

Systematic Hydrogen‐Bond Manipulations To Establish Polysaccharide Structure–Property Correlations

A dense hydrogen‐bond network is responsible for the mechanical and structural properties of polysaccharides. Random derivatization alters the properties of the bulk material by disrupting the hydrogen bonds, but obstructs detailed structure–function correlations. We have prepared well‐defined unnatural oligosaccharides including methylated, deoxygenated, deoxyfluorinated, as well as carboxymethylated cellulose and chitin analogues with full control over the degree and pattern of substitution. Molecular dynamics simulations and crystallographic analysis show how distinct hydrogen‐bond modifications drastically affect the solubility, aggregation behavior, and crystallinity of carbohydrate materials. This systematic approach to establishing detailed structure–property correlations will guide the synthesis of novel, tailor‐made carbohydrate materials

Modeling of negative Poisson’s ratio (auxetic) crystalline cellulose Iβ

Energy minimizations for unstretched and stretched cellulose models using an all-atom empirical force field (Molecular Mechanics) have been performed to investigate the mechanism for auxetic (negative Poisson’s ratio) response in crystalline cellulose Iβ from kraft cooked Norway spruce. An initial investigation to identify an appropriate force field led to a study of the structure and elastic constants from models employing the CVFF force field. Negative values of on-axis Poisson’s ratios nu31 and nu13 in the x1-x3 plane containing the chain direction (x3) were realized in energy minimizations employing a stress perpendicular to the hydrogen-bonded cellobiose sheets to simulate swelling in this direction due to the kraft cooking process. Energy minimizations of structural evolution due to stretching along the x3 chain direction of the ‘swollen’ (kraft cooked) model identified chain rotation about the chain axis combined with inextensible secondary bonds as the most likely mechanism for auxetic response