When 1:00 PM - 3:00 PM Mar 20, 2015
Where GM Room, Lurie Enginering Center
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Molecular dynamics simulation of structures and interfaces in amorphous/ordered composites


Katherine Sebeck
Thesis Defense

John Kieffer, advisor

 

This thesis describes studies of the structure-property relationships of amorphous and amorphous/ordered composite materials at the nanoscale, using molecular dynamics simulations. Both inorganic and organic bulk amorphous systems, as well as the interfaces between ordered substrates and two different amorphous systems have been investigated.

 

A series of soda lime silicate glasses were simulated, with varying concentrations of silica, sodium and calcium. The clustering of cations and the second-neighbor connectivities affect the vibrational modes and mechanical properties. Mean-field theory is unable to account for mixed modifier effects in soda lime silicates.

 

The structure and mechanical properties of a dynamically reacted bulk epoxy network were studied. , An improved polymerization method for continuously monitoring properties as a function of network growth, including volumetric shrinkage and internal stresses, is demonstrated. The elastic properties change minimally during the growth of the network within the achieved degree of conversion. However, the tensile strength increases. Systems with a surplus of amine hardener reach higher degrees of epoxide conversion, but formation of an infinite network is delayed.

 

As a simple model system for understanding interfaces between amorphous and ordered materials, a thin alkane film was placed onto a metallic substrate. The ordered substrate creates a layered polymer configuration within the first 10 Å of the surface, as shown by density profiles, pair correlation functions, and monomer orientation statistics. This structural change also affects the mechanical properties of the system. 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, a series of epoxy/carbon laminate systems were investigated, comparing different epoxy layer thicknesses and amine functionality. The overall cure and shrinkage behavior mimicks the bulk epoxy, though the percolation of an infinite cluster is delayed. Annealing of structures in the process of determining the glass transition temperature shows 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.