Jeffrey A. Hinkley

Molecular dynamics simulation of thermal and mechanical properties of polyimide–carbon-nanotube composites

An aromatic polyimide and its mixture with randomly distributed carbon nanotubes (NTs) are simulated by using molecular dynamics, repeated energy minimization and cooling processes. The glass transition temperatures are identified through volume-temperature curves. Stress-strain curves, Youngs moduli, densities and Poisson ratios are computed at different temperatures. It is demonstrated that the carbon NT reduces the softening effects of temperature on mechanical properties and increases the ability to resist deformation.

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.

Mechanical and swelling behaviour of well characterized polybutadiene networks

Abstract Endlinking of hydroxyl-terminated polybutadiene with the appropriate isocyanate has been used to prepare well characterized networks. Two networks have been studied with molecular weights of the prepolymers being 6100 and 2400 g/mole by g.p.c. Cylindrical specimens were prepared and the derivatives of the stored energy function with respect to the stretch invariants were determined by torque and normal force measurements in torsion. From these data the Valanis-Landel stored energy function derivatives w′(λ) were determined for both networks. The stored energy function for the junction constraint model of Flory, which is a special form of the Valanis-Landel function, has been fitted to that determined from the experiments. The contributions, ΔAph and ΔAc, to the stored energy function from the phantom network and from the junction constraints respectively do not agree with predictions from the topologies of the networks. In spite of this the form of w′(λ) for the junction constraint model gives an excellent ‘curve fit’ to the data. Comparison is also made with equilibrium swelling.

Structure-property study of piezoelectricity in polyimides

High performance piezoelectric polymers are of interest to NASA as they may be useful for a variety of sensor applications. Over the past few years research on piezoelectric polymers has led to the development of promising high temperature piezoelectric responses in some novel polyimides. In this study, a series of polyimides have been studied with systematic variations in the diamine monomers which comprise the polyimide while holding the dianhydride constant. The effect of structural changes, including variations in the nature and concentration of dipolar groups, on the remanent polarization and piezoelectric coefficient is examined. Fundamental structure-piezoelectric property insight will enable the molecular design of polymers possessing distinct improvements over state-of-the-art piezoelectric polymers including enhanced polarization, polarization stability at elevated temperatures, and improved processability.

Fracture toughness of polyimide films

Abstract Two aromatic polyimides and an aromatic polyamide-imide were tested in single-edge-notched tension. Fracture toughnesses, Kc, normalized to 25 μm film thickness ranged from 1.65 to 5.4 MPa m 1 2 . LARC-TPI, a thermoplastic polyimide, showed evidence of crazing ahead of a growing crack whereas the other materials formed a shear yielded zone.

Computational Materials: Modeling and Simulation of Nanostructured Materials and Systems

Abstract The paper provides details on the structure and implementation of the Computational Materials Program at the NASA Langley Research Center. Examples are given that illustrate the suggested approaches to predicting the behavior and influencing the design of nanostructured materials such as high-performance polymers, composites, and nanotube-reinforced polymers. Primary simulation and measurement methods applicable to multi-scale modeling are outlined. Key challenges including verification and validation of models are highlighted and discussed within the context of NASA ’s broad mission objectives. Introduction “I have not failed. I’ve just found 10,000 ways that don’t work.” -Thomas Alva Edison (1 847-1 93 1) Each distinct age in the development of humankind has been associated with advances in materials technology. Historians have linked key technological and societal events with the materials technology that was prevalent during the “stone age,” “bronze age,” and

Spiral Tunneling Cracks Induced by Environmental Stress Cracking in LaRC™-TPI Adhesives

Abstract Some currently-available formulations of LaRC™-TPI, a thermoplastic polyimide originally developed at NASA-Langley, were found to be highly susceptible to environmental stress cracking when exposed to solvents such as acetone, toluene, diglyme and methyl ethyl ketone. The combination of stress and solvent led to rapid cracking in films and adhesive layers of this material system. Residual cool-down stresses induced when the LaRC-TPI is used as an adhesive or coating led, in the presence of a solvent, to dense “mud crack” patterns which relieve a portion of the stored energy. Because these through-the-thickness cracks are not able to relieve the stored energy in the vicinity of the adherends, additional fractures in the form of curious spiral tunnel cracks initiated and grew inward within each adhesive fragment. Micrographs of the spiral fractures are given, along with a qualitative explanation for the failure process as observed in adhesives and coatings.

Resin Systems and Chemistry: Degradation Mechanisms and Durability

This chapter focuses on the chemistry of polymer resins and the changes that occur when resins are exposed to environmental factors that can cause degradation. The exposures that are considered are elevated temperatures (with or without oxygen); contact with water and other fluids; radiation; and mechanical loads. Some general observations about the types of effects to be expected for each exposure condition are outlined first. Then, the chemistries of the various classes of resins that are in current use as matrices for high-performance composites are described. Separate sections treat epoxies, bismaleimides, PMR-type thermosets, phenyethynyl-terminated imides, and high-temperature thermoplastics. For each resin type, the formulation and curing are briefly discussed. This discussion is followed by a review of the available literature on mechanisms of long-term degradation as determined by spectroscopy, chromatography, and other analytical techniques. Consequences to constituent properties are also described: increases or decreases in glass transition temperatures; shrinkage and cracking; and changes in mechanical stiffnesses, strengths, and toughnesses.

Structure and properties of melt-extruded laRC-IA (3,4′-ODA 4,4′-ODPA) polyimide fibers

LaRC-IA polyimide fibers were extruded from the melt and drawn. The fibers were characterized by sonic pulse propagation, polarized optical microscopy, calorimetric analysis, X-ray diffraction, and tensile testing. When the amorphous, essentially isotropic, as-extruded filaments were heated slowly in a calorimeter, no “cold” crystallization could be detected. However, when these filaments were drawn at temperatures above the Tg of the polymer, they crystallized rapidly to produce oriented semicrystalline structures. The consequent increases in modulus and strength were significant, with the properties reaching the levels of commercial fibers of intermediate tenacity.

Dye Penetrant Induced Micro cracking in High Performance Thermoplastic Polyimide Composites

The possibility of spurious microcracking in three high performance thermoplastic polyimide composite materials due to zinc iodine dye penetrant was studied. The material systems were IM7/LaRCTM-IAX, IM7/LaRCTM-IAX2, and IM7/LaRCTM-8515. Specimens from each material system were subjected to one of three immersion tests. The first immersion test involved soaking composite specimens previously prepared with different polishing techniques in dye penetrant. In the second test, specimens were soaked in the individual components of the dye penetrant. The final test involved soaking the specimens in one of six immersion liquids followed by soaking in dye penetrant. Results showed that the composite materials have sufficiently high thermal residual stresses to drive microcracking in the presence of dye penetrant without external mechanical loading. There was no evidence that the different polishing techniques had an effect on dye penetrant induced stress cracking. The individual components of the dye penetrant did not produce microcracks in the composites. Some combination of the components must be present to induce microcracking. Observations also revealed that polishing had an effect on the microcracking process of the composites that were initially soaked in immersion liquids then dye penetrant.