Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Civil and Environmental Engineering, 2012.Cataloged from PDF version of thesis.Includes bibliographical references (p. 255-282).Nanotechnology has advanced to the point where almost any molecular functional group can be introduced into a composite material system. However, emergent properties attained via the combination of arbitrary components - e.g., the complexation of two weak polyelectrolytes - is not yet predictive, and thus cannot be rationally engineered. Predictive and reliable quantification of material properties across scales is necessary to enable the design and development of advanced functional (and complex) materials. There is a vast amount of experimental study which characterize the strength of electrostatic interactions, topology, and viscoelastic properties of polyelectrolyte multilayers (PEMs), but very little is known about the fundamental molecular interactions that drive system behavior. Here, we focus on two specific weak polyelectrolytes - poly(acrylic acid) (PAA) and poly(allylamine hydrochloride) (PAH) - that undergo electrostatic complexation, and can be manipulated as function of pH. While the driving mechanism investigated here is ionic interactions, the findings and atomistic approaches are applicable to a variety of systems such as hydrogen bonded polypeptides (e.g., protein structures), as well as similar polyelectrolyte systems (e.g., PSS, PDMA, etc.). Specifically, in this dissertation, the coupling of electrostatic cross-links and weak interactions, polyelectrolyte persistence length and molecular rigidity of PAA and PAH is investigated with full atomistic precision. Large-scale molecular dynamics (MD) simulations indicate the stiffening of PEMs cannot be explained by increased electrostatic cross-linking alone, but rather the effect is amplified by the increase in molecular rigidity due to self-repulsion. Based on MD simulations, a general theoretical model for effective electrostatic persistence length is proposed for highly flexible polyelectrolytes and charged macromolecules through the introduction of an electrostatic contour length which can applied to other chemical species. A focus on adhesion reveals the effective cross-linking strength exceeds the strength of ionic interaction alone, due to secondary effects (e.g., H-bonding, steric effects, etc.) Moreover, a derived elastic model for complexation reveals a critical bound for cross-link density and stiffness, indicating the required conditions to induce cooperative mechanical behavior. The key insight is that these critical conditions can be further extended for the coupling of flexible molecules in general, such as proteins or flexible nanoribbons. The results demonstrate how nanoscale control can lead to uniquely tunable mechanomutable materials from designed functional building blocks. While PEM systems are currently being developed for biosensor, membrane, and tissue engineering technologies, the results presented herein provide a basis to tune the properties of such systems at the nanoscale, thereby engineering system behavior and performance across scales. Understanding the bounds of mechanical performance of two specific polyelectrolyte species, and their joint interaction through complexation, provides a basis for coupling molecules with various functionalities. Similar to complete understanding the limitations of a steel beam in construction of a bridge, the systematic delineation of polyelectrolyte complexation allows quantitative prediction of larger-scale systems.by Steven W. Cranford.Ph.D
