Molecular Dynamic Simulation of Structures and Interfaces in Amorphous/Ordered Composites.

This thesis describes molecular dynamics simulation studies of the structure-property relationships of molecular network systems, including inorganic and organic bulk amorphous systems, as well as two different amorphous polymers at the interface with ordered substrates. A series of soda lime silicate glasses were simulated, with up to 50% total modification and varying ratios of sodium and calcium. The clustering of cations and second-neighbor connectivity affect vibrational modes and the compressibility vs. pressure behavior. Mean-field theory is unable to account for mixed modifier effects in soda lime silicates. The structure and tensile behavior of a dynamically reacted bulk epoxy network were studied, demonstrating an improved polymerization method for continuously monitoring properties as a function of network growth, including volumetric shrinkage and internal stresses. A bifunctional epoxy resin is reacted with two aliphatic amines at room temperature, comparing simulation size, amine functionality, and stoichiometry. The elastic properties change by only 1-2 GPa during the growth of the network within the achieved degree of conversion. Tensile strength increases by ~100 MPa. Systems with surplus amine hardener reach higher degrees of epoxide conversion, but lag in formation of an infinite network. As a simple model system for amorphous/ordered interfaces, a thin alkane film was placed onto a metallic substrate. The ordered substrate creates a layered polymer configuration within the adjacent 10 Å, as shown by density profiles, pair correlation functions, and monomer orientation statistics. This structural change also affects the mechanical properties, as the elastic moduli of nanoconfined alkane systems are higher than would be expected for a simple laminate composite, based on extrapolating from the bulk properties of the two materials. Lastly, epoxy/carbon laminate systems were investigated, comparing different epoxy layer thicknesses and amine functionality. The cure and shrinkage behavior mimic the bulk epoxy, though the percolation of an infinite cluster is delayed. Post-annealed structures show a nearly uniform decrease in both the elastic modulus and tensile strength. Local heterogeneity is important in predicting nanoscale mechanics for all systems investigated. Larger system size provides better accuracy in determining mechanical properties of simulated highly cross-linked network polymers.PHDMaterials Science and EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/111417/1/kabeck_1.pd

Microscale elastic properties of interphases in polymer matrix composites: correlating spatial mapping with cure history

Polymer matrix composites with textile reinforcement are used in a wide range of aerospace and industrial applications. Continuum mechanical predictions of the composite behaviors have been inaccurate and resorted to empirical corrections, because of the lack of polymer materials property information. The length scales involved make experimental measurement of the elastic properties of the matrix within fiber tows and proximity to individual fibers difficult. However, micro-Brillouin and Raman light scattering provide sufficiently high spatial resolution to probe the mechanical properties and chemical composition of the matrix, without interfering with the thermo-mechanical equilibrium of the material. The elastic properties of epoxy resin have been measured between and within the fiber tows of a composite with this technique, and compared to a bulk epoxy resin. Using this approach, the elastic properties have also been monitored in situ, during epoxy cure under different thermal and chemical conditions. To interpret and enhance these results, experiments are complemented with molecular dynamics simulations of the interface extrapolating findings to nanometer length scales. We observe that matrix materials in close proximity to fibers have a diminished elastic modulus compared with both bulk epoxy and material between tows. To explain the underlying reason for this finding we identify the extent to which residual stresses, chemical inhomogeneities, or purely structural rearrangements near the interface contribute to this effect. Finally, we correlate the spatial distribution of mechanical properties with the cure history

Cure kinetics and interfacial phenomena in polymer matrix composites

Polymer matrix composites with textile reinforcement are used in a wide range of aerospace and industrial applications. Continuum mechanical predictions of the composite behaviors have been inaccurate, possibly because of the lack of information with regard to polymer materials properties, especially near the interfaces with the reinforcing fibers. Concurrent micro-Brillouin and Raman light scattering provides sufficiently high spatial resolution to probe the mechanical properties and chemical composition of the interphase regions of the matrix, without interfering with the thermo-mechanical equilibrium of the material. Using this technique, we mapped the elastic properties of epoxy resin in between and within the fiber tows of a composite, revealing that the modulus exhibits a marked spatial inhomogeneity in proximity of fibers, with a decrease of up to 5% compared to that of bulk epoxy resin in the regions of highest fiber density (see Fig. 1).1 We estimate that it would take a deformation of four times the failure strain to cause such a change in modulus based on residual stresses. Hence, the origin must lie elsewhere. Using the same methodology, we then monitored the elastic properties in situ, during epoxy cure under different thermal and chemical conditions. We find that depending on the reaction rate, the elastic modulus evolves differently as a function of the degree of cure: the faster the rate, the more the modulus lags behind of what would be expected from the amount of cross-links that have formed according to the degree of cure. This is because the overall modulus is based on the stiffness resulting from bonded and non-boned network connections, the latter arising the optimization of network packing that ensues after a slow structural relaxation.2 Provided enough time, the same final modulus is reached, unless network formation is impeded by the under-supply of hardener. To interpret and enhance these results, experiments are complemented with molecular dynamics simulations of the interface. Accordingly, the one-sided confinement of polymer adjacent to a fiber surface results in clearly detectable structural features, e.g., layering and densification, as well as changes in the elastic properties within a spatial extent that reaches significantly beyond the region of distinguishable structural features. In conclusion, we attribute the inhomogeneity in mechanical properties to a combination of hardener depletion and an impediment of structural relaxation due to unilateral confinement that lowers the extent and effectiveness of non-bonded interaction

Alloy Partitioning Effect on Strength and Toughness of κ-Carbide Strengthened Steels

Alloy partitioning during heat treatment in a lightweight precipitation hardened steel was investigated using transmission electron microscopy and atom probe tomography. The mechanical properties are discussed as a function of the effect of solution treatment temperature and aging time, giving rise to variations in chemical modulation. A wrought lightweight steel alloy with a nominal composition of Fe-30Mn-9Al-1Si-1C-0.5Mo (wt. %) was solution-treated between 1173–1273 K and aged at 773 K. Lower solution treatment temperatures retained a finer grain size and accelerated age hardening response that also produced an improved work hardening behavior with a tensile strength of −1460 MPa at 0.4 true strain. Atom probe tomography indicated these conditions also had reduced modulation in the Si and Al content due to the reduced aging time preventing silicon from diffusing out of the κ-carbide into the austenite. This work provides the framework for heat-treating lightweight, age hardenable steels with high strength and improved energy absorption

Thermal Conductance in Cross-linked Polymers: Effects of Non-Bonding Interactions

Weak interchain interactions have been considered to be a bottleneck for heat transfer in polymers, while covalent bonds are believed to give a high thermal conductivity to polymer chains. For this reason, cross-linkers have been explored as a means to enhance polymer thermal conductivity; however, results have been inconsistent. Some studies show an enhancement in the thermal conductivity for polymers upon cross-linking, while others show the opposite trend. In this work we study the mechanisms of heat transfer in cross-linked polymers in order to understand the reasons for these discrepancies, in particular examining the relative contributions of covalent (referred to here as “bonding”) and nonbonding (e.g., van der Waals and electrostatic) interactions. Our results indicate cross-linkers enhance thermal conductivity primarily when they are short in length and thereby bring polymer chains closer to each other, leading to increased interchain heat transfer by enhanced nonbonding interactions between the chains (nonbonding interactions being highly dependent on interchain distance). This suggests that enhanced nonbonding interactions, rather than thermal pathways through cross-linker covalent bonds, are the primary transport mechanism by which thermal conductivity is increased. We further illustrate this by showing that energy from THz acoustic waves travels significantly faster in polymers when nonbonding interactions are included versus when only covalent interactions are present. These results help to explain prior studies that measure differing trends in thermal conductivity for polymers upon cross-linking with various species