Thomas S. Gates

THE STRESS-STRAIN BEHAVIOR OF POLYMER-NANOTUBE COMPOSITES FROM MOLECULAR DYNAMICS SIMULATION

Stress–strain curves of polymer–carbon nanotube composites generated from molecular dynamics simulations of a single-walled carbon nanotube embedded in polyethylene are presented. A comparison is made between the response to mechanical loading of a composite with a long, continuous nanotube (replicated via periodic boundary conditions) and the response of a composite with a short, discontinuous nanotube. Both composites are mechanically loaded in the direction of, and transverse to, the nanotube axis. The long-nanotube composite shows an increase in the stiffness relative to the polymer and behaves anisotropically under the different loading conditions considered. The short-nanotube composite shows no enhancement relative to the polymer, most probably because of its low aspect ratio. The stress–strain curves from molecular dynamics simulations are compared with corresponding rule-of-mixtures predictions. Published by Elsevier Ltd.

Characterization of viscoelastic properties of polymeric materials through nanoindentation

Nanoindentation testing was used to determine the dynamic viscoelastic properties of eight polymer materials, which include three high-performance polymers and five densities of high-density polyethylene. It was determined that varying the harmonic frequency of nanoindentation does not have a significant effect on the measured storage and loss moduli of the polymers. Agreement was found between these nanoindentation results and data from bulk dynamic mechanical testing of the same materials. Varying the harmonic amplitude of the nanoindentation had a limited effect on the measured viscoelastic properties of the resins. However, storage and loss moduli from nanoindentation were shown to be sensitive to changes in the density of the polyethylene.

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.

Effect of Nanotube Functionalization on the Elastic Properties of Polyethylene Nanotube Composites

The effects of the chemical functionalization of a single-wall carbon nanotube in nanotube/polyethylene composites on the bulk elastic properties are presented. Constitutive equations are established for composites containing both functionalized and nonfunctionalized nanotubes using an equivalent-continuum modeling technique. The elastic properties of both composite systems are predicted for amorphous and crystalline polyethylene matrices with various nanotube lengths, volume fractions, and orientations. The results indicate that for the specific composite materials considered in this study most of the elastic stiffness constants of the composite with functionalized nanotubes are either less than or equal to those of the composite without functionalized nanotubes.

Experimental characterization of nonlinear, rate-dependent behavior in advanced polymer matrix composites

In order to support materials selection for the next-generation supersonic civilian-passenger transport aircraft, a study has been undertaken to evaluate the material stress/strain relationships needed to describe advanced polymer matrix composites under conditions of high load and elevated temperature. As part of this effort, this paper describes the materials testing which was performed to investigate the viscoplastic behavior of graphite/thermoplastic and graphite/bismaleimide composites. Test procedures, results and data-reduction schemes which were developed for generating material constants for tension and compression loading, over a range of useful temperatures, are explained.

Investigation of the effect of single wall carbon nanotubes on interlaminar fracture toughness of woven carbon fiber—epoxy composites:

Single wall carbon nanotubes (SWCNTs) were introduced in the interlaminar region of woven carbon fiber—epoxy composites and the mode-I delamination behavior was investigated. Pristine (P-SWCNT) and functionalized (F-SWCNT) nanotubes were sprayed in the mid-plane of these laminates and delamination was initiated using a teflon pre-crack insert. The composite laminates were produced using vacuum-assisted resin transfer molding process. The interlaminar fracture toughness (ILFT) represented by mode-I critical strain energy release rate (GIc) for the initiation of delamination was measured using double cantilever beam tests. The specimens with pristine nanotubes and functionalized nanotubes showed a small effect on the ILFT. The specimens with P-SWCNTs showed stable crack growth and the potential for enhanced crack bridging along with slightly higher GIc than F-SWCNT specimens. Scanning electron microscopy images showed enhanced fiber—matrix interfacial bonding in the specimens with F-SWCNTs. However, large unstable crack propagation was observed in these F-SWCNT specimens from load—displacement curves and crack propagation videos. This research helps in understanding the differences in mechanisms by addition of functionalized and unfunctionalized (pristine) nanotubes to the woven carbon fiber—epoxy matrix composite laminates.

2-D nano-scale finite element analysis of a polymer field

Abstract Two types of 2-D nano-scale finite elements, the chemical bond element and the Lennard–Jones element, are formulated based upon inter-atomic and inter-molecular force fields. A nano-scale finite element method is employed to simulate polymer field deformation. This numerical procedure includes three steps. First, a polymer field is created by an off-lattice random walk, followed by a force relaxation process. Then, a finite element mesh is generated for the polymer field. Chemical bonds are modeled by chemical bond elements. If the distance between two non-bonded atoms or monomers is shorter than the action range of the Lennard–Jones attraction (or repulsion), a Lennard–Jones element is inserted between them. Finally, external load and boundary conditions are applied and polymer chain deformation is simulated step by step. During polymer deformation, failed Lennard–Jones bond elements are removed and newly formed Lennard–Jones elements are inserted into the polymer field during each loading step. The process continues until failure occurs. Two examples are presented to demonstrate the process. Stress–strain curves of polymer fields under unidirectional tensile load are derived. Continuum mechanical properties, such as modulus and polymer strength, are determined based upon the stress strain curve. Further, throughout the deformation process one observes polymer chain migration, nano-scale void generation, void coalescence and crack development.

Creep and Physical Aging in a Polymeric Composite: Comparison of Tension and Compression

To help address the lack of engineering data on the time-dependent behavior of advanced polymeric composites, a combined experimental and analytical research program was initiated to investigate the similarities and differences of the effects of physical aging on creep compliance of IM7/K3B composite loaded in tension and compression. Recently developed novel experimental apparatus and methods were employed to test two matrix dominated loading modes, shear and transverse, for two load cases, tension and compression. The tests, run over a range of sub-glass transition temperatures, provided material constants, material master curves and aging related parameters. Assessment of these new results indicated that although trends in the data with respect to aging time and aging temperature are similar, differences exist due to load direction and mode. An analytical model proposed previously by the authors for tension loading was used for predicting long-term tension and compression behavior using short-term data as input. These new studies indicated that the model worked equally as well for the tension or compression loaded cases. Comparison of the loading modes indicated that the predictive model provided more accurate long-term predictions for the shear mode as compared to the transverse mode. Parametric studies showed the usefulness of the predictive model as a tool for investigating long-term performance and compliance acceleration due to temperature.

Constitutive Modeling of Nanotube-Reinforced Polymer Composites

In this study, a technique is presented for developing constitutive models for polymer composite systems reinforced with single-walled carbon nanotubes (SWNT). Because the polymer molecules are on the same size scale as the nanotubes, the interaction at the polymer/nanotube interface is highly dependent on the local molecular structure and bonding. At these small length scales, the lattice structures of the nanotube and polymer chains cannot be considered continuous, and the bulk mechanical properties can no longer be determined through traditional micromechanical approaches that are formulated by using continuum mechanics. It is proposed herein that the nanotube, the local polymer near the nanotube, and the nanotube/polymer interface can be modeled as an effective continuum fiber using an equivalent-continuum modeling method. The effective fiber serves as a means for incorporating micromechanical analyses for the prediction of bulk mechanical properties of SWNT/polymer composites with various nanotube lengths, concentrations, and orientations. As an example, the proposed approach is used for the constitutive modeling of two SWNT/polyimide composite systems. Key words, carbon nanotubes, composites, mechanical properties, modeling, nanotechnology Subject classification. Structures and Materials

THE EFFECT OF CHEMICAL FUNCTIONALIZATION ON MECHANICAL PROPERTIES OF NANOTUBE/POLYMER COMPOSITES

The effects of the chemical functionalization of a carbon nanotube embedded in a nanotube/polyethylene composite on the bulk elastic properties are presented. Constitutive equations are established for both functionalized and nonfunctionalized nanotube composites systems by using an equivalent-continuum modeling technique. The elastic properties of both composites systems are predicted for various nanotube lengths, volume fractions, and orientations. The results indicate that for the specific composite material considered in this study, most of the elastic stiffness constants of the functionalized composite are either less than or equal to those of the non-functionalized composite.