L. Catherine Brinson

Electrically Conductive “Alkylated” Graphene Paper via Chemical Reduction of Amine‐Functionalized Graphene Oxide Paper

2010 WILEY-VCH Verlag Gm Two-dimensional graphene nanosheets and graphene-based materials have garnered significant attention in recent years due to their excellent materials properties. Many graphenebased materials can be conveniently synthesized from graphite oxide (GO), which can be prepared in bulk quantities from graphite under strong oxidizing conditions. GO is a layered material featuring a variety of oxygen-containing functionalities with epoxide and hydroxyl groups on the basal plane and carbonyl and carboxyl groups along the edges, which provide a platform for rich chemistry to occur both within the intersheet gallery and along sheet edges. In addition, GO can be easily exfoliated into individual graphene oxide sheets, which can be reassembled into thin films or paper-like materials. For the latter case, flow-directed filtration of an aqueous graphene oxide dispersion produces very large sheets of a free-standing, foil-like material known as graphene oxide paper. This paper retains all the functional groups found in GO, preserving all of its native chemistry. While graphene oxide paper can be chemically modified in a facile fashion and has goodmechanical properties, it was found to be electrically conductive only after thermal annealing, which presumably converts it into graphene paper. Unfortunately, this thermal treatment also degrades its structural integrity. Graphene paper, fabricated via flow-directed filtration of an electrostatically stabilized aqueous graphene dispersion that was pre-prepared via hydrazine reduction of graphene oxide sheets, has excellent electrical conductivity and similar mechanical properties as graphene oxide paper maintained at temperatures below 100 8C. However, the hydrazine reduction of graphene oxide sheets can remove a significant amount of oxygen-containing functionalities and lead to graphene papers with low functional-group content. To produce functionalized graphene paper from graphene oxide sheets, we envisioned two strategies: 1) preparing functionalized graphene sheets before assembling them into ‘‘paper’’ or 2) reducing a pre-assembled, functionalized graphene oxide paper. Here, we present the successful preparation of a conductive, ‘‘alkylated’’ graphene paper via the post-synthetic modification of ‘‘alkylated’’ graphene oxide paper. By treating pre-assembled graphene oxide paper with hexylamine prior to hydrazine reduction, we can convert this insulating paper into conductive ‘‘alkylated’’ graphene paper while maintaining its well-ordered structure and good mechanical properties. Since reduction in the absence of hexylamine affords a less-ordered material with inconsistent conductivity, we attribute the uniform conductivity we observe for the ‘‘alkylated’’ paper to the structure-stabilizing presence of the hexylamine. GO prepared using the Hummers method was sonicated to yield aqueous dispersions of graphene oxide sheets, which were vacuum-filtered through an Anodisc membrane to yield graphene oxide paper (see Supporting Information (SI) for further details). Hexylamine-modified (HA-) graphene oxide paper was prepared by flowing a methanol solution of the amine (100mM) through the as-prepared wet paper, which already has a ‘‘well-stacked’’ structure. In contrast, if graphene oxide sheets aremodified first with hexylamine, they become hydrophobic and quickly precipitate in water, precluding the formation of well-ordered paper (Fig. S1 in SI). HA-graphene paper was then obtained by flowing an aqueous hydrazine monohydrate solution (2 M), a commonly used reducing agent for graphene oxide, through the as-prepared, wet HA-graphene oxide paper at 90 8C under vacuum assistance. Unmodified graphene paper was prepared by a similar reduction of unmodified wet graphene oxide paper. As the structures of the papers were already established during the assembly, our method conveniently omits the use of ammonia andmineral oil stabilizing agents found in an alternative method for preparing graphene paper from aqueous dispersions of graphene sheets. Functionalization prior to reduction is key to the proper preparation of HA-graphene paper (Fig. S2 in SI); performing reduction first removes themajority of reactive oxygen-containing functionalities from graphene oxide and prevents any substantial hexylamine functionalization. Successful hexylamine functionalization and reduction of the graphene oxide paper were confirmed by elemental analysis (EA) and Karl–Fischer titration (Table S2 in SI). As fabricated, graphene oxide paper has a Cgraphene/O ratio of 2.9 with a water content of 17wt%. In contrast, the water content for the HA-graphene oxide paper is significantly decreased to 1.49wt%

Polymer engineering science and viscoelasticity : an introduction

Introduction.- Stress and Strain Analysis and Measurement.- Characteristics, Applications and Properties of Polymers.- Polymerization and Classification.- Differential Constitutive Equations.- Hereditary Integral Representations of Stress and Strain.- Time and Temperature Behavior of Polymers.- Elementary Viscoelastic Stress Analysis for Bars and Beams.- Viscoelastic Stress Analysis in Two and Three Dimensions.- Nonlinear Viscoelasticity.- Rate and Time-Dependent Failure: Mechanics and Predictive Models.

Tuning the Mechanical Properties of Graphene Oxide Paper and Its Associated Polymer Nanocomposites by Controlling Cooperative Intersheet Hydrogen Bonding

The mechanical properties of pristine graphene oxide paper and paper-like films of polyvinyl alcohol (PVA)-graphene oxide nanocomposite are investigated in a joint experimental-theoretical and computational study. In combination, these studies reveal a delicate relationship between the stiffness of these papers and the water content in their lamellar structures. ReaxFF-based molecular dynamics (MD) simulations elucidate the role of water molecules in modifying the mechanical properties of both pristine and nanocomposite graphene oxide papers, as bridge-forming water molecules between adjacent layers in the paper structure enhance stress transfer by means of a cooperative hydrogen-bonding network. For graphene oxide paper at an optimal concentration of ~5 wt % water, the degree of cooperative hydrogen bonding within the network comprising adjacent nanosheets and water molecules was found to optimally enhance the modulus of the paper without saturating the gallery space. Introducing PVA chains into the gallery space further enhances the cooperativity of this hydrogen-bonding network, in a manner similar to that found in natural biomaterials, resulting in increased stiffness of the composite. No optimal water concentration could be found for the PVA-graphene oxide nanocomposite papers, as dehydration of these structures continually enhances stiffness until a final water content of ~7 wt % (additional water cannot be removed from the system even after 12 h of annealing).

Bio‐Inspired Borate Cross‐Linking in Ultra‐Stiff Graphene Oxide Thin Films

Adjacent graphene oxide nanosheets in a thin-film structure have been covalently cross-linked in a fashion similar to the cell walls of higher-order plants. The resulting ultra-stiff structure exhibits a maximum storage modulus of 127 GPa that can be tuned by varying borate concentration.

Effects of physical aging on long term creep of polymers and polymer matrix composites

Abstract For many polymeric materials in use below the glass transition temperature, the long term viscoelastic behavior is greatly affected by physical aging. To use polymer matrix composites as critical structural components in existing and novel technological applications, this long term behavior of the material system must be understood. Towards that end, this study applied the concepts governing the mechanics of physical aging in a consistent manner to the study of laminated composite systems. Even in fiber dominated lay-ups, the effects of physical aging are found to be important in the long term behavior of the composite. This paper first lays out, in a self-consistent manner, the basic concepts describing physical aging of polymers. Several aspects of physical aging which have not been previously documented are also explored in this study, namely the effects of aging into effective equilibrium and a relationship to the time-temperature shift factor. The physical aging theory is then extended to develop the long term compliance/modulus of a single lamina with varying fiber orientation. The latter is then built into classical lamination theory to predict long time response of general laminated composites. Comparison to experimental data is excellent. In the investigation of fiber oriented lamina and laminates, it is illustrated that the long term response can be counter-intuitive, stressing the need for consistent modeling efforts to make long term predictions of laminates to be used in structural situations.

A multivariant micromechanical model for SMAs Part 1. Crystallographic issues for single crystal model

Abstract A general 3-D multivariant model for shape memory alloy constitutive behavior is further developed in this paper. The model is based on the habit planes and transformation directions for variants of martensite and uses a thermodynamic and micromechanics approach to develop the governing equations for thermomechanical response. The model accounts for the self-accomodating group structure exhibited during martensitic plate formation and utilizes this concept in its calculation of the interaction energy between variants. In this paper, we expand the multivariant model to consider the impact of inclusion shape on model predictions. A direction selection scheme is proposed for penny shaped inclusions and is based on the fact that several habit plane variants tend to cluster about one of the {011} or {001} poles. We also explore in detail the crystallographic basis of material response and the impact of specific crystallographic changes on the macroscopic single crystal constitutive response. Differences between type I and type II twinning are examined and it is shown that choice of the proper twinning type is essential to capture experimental data. The grouping structure is examined and several different options published for a NiTi alloy are implemented and results compared. Several concepts, i.e. artificial merging, exclusive and non-exclusive grouping, are raised to assist exploration of NiTi grouping possibilities. The anisotropy of the single crystal material response is illustrated and implications on higher level modeling are discussed. It is noted that properly representing the details of the crystallographic microstructure is crucial to obtaining accurate macroscopic stress–strain predictions.

Evolution of order during vacuum-assisted self-assembly of graphene oxide paper and associated polymer nanocomposites

Three mechanisms are proposed for the assembly of ordered, layered structures of graphene oxide, formed via the vacuum-assisted self-assembly of a dispersion of the two-dimensional nanosheets. These possible mechanisms for ordering at the filter-solution interface range from regular brick-and-mortar-like growth to complete disordered aggregation and compression. Through a series of experiments (thermal gravimetric analysis, UV-vis spectroscopy, and X-ray diffraction) a semi-ordered accumulation mechanism is identified as being dominant during paper fabrication. Additionally, a higher length-scale ordered structure (lamellae) is identified through the examination of water-swelled samples, indicating that further refinements are required to capture the complete formation mechanism. Identification of this mechanism and the resulting higher-order structure it produces provide insight into possibilities for creation of ordered graphene oxide-polymer nanocomposites, as well as the postfabrication modification of single-component graphene oxide papers.

A multivariant micromechanical model for SMAs Part 2. Polycrystal model

Abstract An averaging scheme is developed to simulate the behavior of a polycrystalline shape memory alloy (SMA) specimen using the Multivariant Micromechanics approach. An untextured polycrystalline specimen is assumed to be formed by a number of randomly oriented single crystal grains. The previously developed Multivariant technique is used to model the response of each single crystal grain subjected to its stress field seen in the polycrystalline sample. Using spherical grains, the Eshelby–Kroner approach is used to formulate the interaction between grains and to determine the stress state in each individual grain. This model successfully captures the basic features of SMA polycrystalline response to loading and temperature. In addition, comparison is made to recent experimental data with fully triaxial load states. Reasonable qualitative agreement is obtained and some issues related to crystallography of the material model are addressed.

A Hybrid Numerical-Analytical Method for Modeling the Viscoelastic Properties of Polymer Nanocomposites

In this paper, we present a novel hybrid numerical-analytical modeling method that is capable of predicting viscoelastic behavior of multiphase polymer nanocomposites, in which the nanoscopic fillers can assume complex configurations. By combining the finite element technique and a micromechanical approach (particularly, the Mori-Tanaka method) with local phase properties, this method operates at low computational cost and effectively accounts for the influence of the interphase as well as in situ nanoparticle morphology. A few examples using this approach to model the viscoelastic response of nanotube and nanoplatelet polymer nanocomposite are presented. This method can also be adapted for modeling other behaviors of polymer nanocomposites, including thermal and electrical properties. It is potentially useful in the prediction of behaviors of other types of nanocomposites, such as metal and ceramic matrix nanocomposites.

A new model to simulate the elastic properties of mineralized collagen fibril

Bone, because of its hierarchical composite structure, exhibits an excellent combination of stiffness and toughness, which is due substantially to the structural order and deformation at the smaller length scales. Here, we focus on the mineralized collagen fibril, consisting of hydroxyapatite plates with nanometric dimensions aligned within a protein matrix, and emphasize the relationship between the structure and elastic properties of a mineralized collagen fibril. We create two- and three-dimensional representative volume elements to represent the structure of the fibril and evaluate the importance of the parameters defining its structure and properties of the constituent mineral and collagen phase. Elastic stiffnesses are calculated by the finite element method and compared with experimental data obtained by synchrotron X-ray diffraction. The computational results match the experimental data well, and provide insight into the role of the phases and morphology on the elastic deformation characteristics. Also, the effects of water, imperfections in the mineral phase and mineral content outside the mineralized collagen fibril upon its elastic properties are discussed.