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

by Katherine M. Sebeck

Institution: University of Michigan
Department: Materials Science and Engineering
Degree: PhD
Year: 2015
Keywords: molecular dynamics, epoxy, glass, simulation, computational materials; Materials Science and Engineering; Engineering; Science
Record ID: 2061228
Full text PDF: http://hdl.handle.net/2027.42/111417


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 ??, 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.