Nanoscale contacts between materials are found in advanced technologies from nanomanufacturing to nanodevices to scanning probe microscopy. These applications require quantitative understanding of the nanoscale contact properties, such as contact area and adhesion. Continuum mechanics models are extensively used to describe and predict the behavior of such contacts, but are based on assumptions that may not hold true at the nanoscale. Additionally, the buried nature of the contact interface makes direct measurement of nanocontact a challenge. Alternatively, molecular dynamics (MD) simulations provide atomic details of the contact interface and can be used to investigate contact at the nanoscale. Here, physically realistic models of nanoscale contact were created with identically matched materials, geometry, crystallographic orientation, degree of amorphization, adhesion and loading conditions. The contact area was then computed from the positions and interactions of atoms. The results demonstrated that significant variation in the value of contact area can be obtained, depending on the technique used to determine it. Next, contact area from MD simulations was compared with predictions from electron transport theories. Electron transport theories were shown to underpredict the contact area by more than an order of magnitude. This low conductance of the nanocontact could not be explained by electron scattering off of defects, and was instead attributed to approximately a monolayer of insulating oxide on the contact surfaces. The effect of native oxide on contact area measurements was further investigated using a newly developed method to approximate conductance based on the distance between atoms in channels across the contact. Then, we compared the contact area calculated with ballistic transport and tunneling theories to that obtained using the known positions of atoms in the contact. The difference was small for very thin (<0.1 nm) or thick (>1.0 nm) oxides, where ballistic transport and tunneling theories work well; however, the difference was significant for oxides between these limits, which is expected to be the case in many practical applications. The prediction of contact area from continuum mechanics models also relies on accurate knowledge of the work of adhesion between the surfaces. An important assumption in these models is that the work of adhesion is constant for a given pair of materials. Our results challenge this assumption, instead showing a significant increase in work of adhesion with increasing pressure, and analyses suggest the presence of stress-driven chemical reactions in the contact. Overall, this research defined and computed contact properties using MD simulations and the results demonstrated the limits of empirical models, including continuum theories and electron transport theories, to describe contact at the nanoscale.

Aromatic polyamides, the most famous of which is poly(p-phenylene terephthalamide) (PPTA), are well-known for their excellent mechanical properties , thermal stability , and chemical resistance. These properties make PPTA ideal for many different uses, including aerospace and military applications where performance-to-weight ratio is critical. However, the processing of PPTA is challenging and expensive due to its solubility in only highly aggressive polar solvents and its high melting temperature (~500 C°). Therefore, it is desirable to design modified systems that improve processability without compromising other properties. It was reported that by introducing aliphatic compound into PPTA, the new polyamides, aromatic-aliphatic polyamides, had improved processability. However, the mechanical properties of these novel polyamides remained unknown. This dissertation seeks to study the mechanical properties of PPTA and four related amatic-aliphatic polyamides, PAP5-PAP8. First, a ReaxFF force field developed by Liu et al. was identified to be the optimum for our systems after evaluating all of the nine candidate force fields based on crystal structures as well as intermolecular hydrogen-bonding and π-π interactions. Then the Liu ReaxFF force field was used to simulate stress-strain behavior in the chain and transverse-to-chain directions for PPTA and PAP5. In the chain direction, PAP5 had higher ultimate stress and failure strain than PPTA; however, the stiffness of PAP5 was lower than PPTA at low-strain (0-2%) while the reverse was observed at high-strain (last 5% before failure). This contrast, and differences in the transverse direction properties, were explained by the methylene segments of PAP5 that confer conformational freedom, enabling accommodation of low strain without stretching covalent bonds. Next, in order to study the aliphatic chain length effect on the stress-strain response, we extended the models pool by varying the numbers of carbon atoms in the aliphatic chain to obtain another three aromatic-aliphatic polyamides, i.e., PAP6, PAP7, and PAP8. Tensile strain was applied to each polymer crystal in the chain direction and the mechanical response was characterized. The modulus at high strain was similar for all polymers, but the modulus calculated at low strain decreased with increasing aliphatic chain length. The decrease in the low-strain modulus with increasing chain length was attributed to the observation that polymers with longer aliphatic chains were wavier in the quiescent state such that they could accommodate low strain without deforming covalent bonds. Extension of wavy chains occurred through an intra-chain process for all polymers, quantified by the bond dihedral angles. In addition, for polymers with an even number of non-aromatic carbons, the strain response involved slip between chains within the hydrogen bonded sheets. The ultimate stress of the polymers exhibited an odd-even effect which was explained by differences in hydrogen bonding and ring-ring coplanarity prior to failure; polymers with an even number of carbon atoms had less favorable H-bonding and poorer ring alignment. The results revealed direct correlations between aliphatic chain length, intra- and inter-chain interactions, and the mechanical properties of polyamide crystals. Overall, the results of this dissertation contribute to establishing the framework of fundamental knowledge needed to design and optimize high-performance polyamides for various applications.