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Nanomaterials at Liquid/Liquid Interfaces: Assembly and Rheology

Abstract
This dissertation concentrated on the behavior of nanomaterials at liquid/liquid interfaces. A strategy of segregating acid-treated SWCNTs at oil/water interfaces was developed by adding amine-terminated polystyrene (PS-NH2) in the oil phase. Electrostatic binding between carboxylic acid of SWCNTs and amine drives the assembly of SWCNTs, monitored by pendant drop tensiometry and confocal microscopy. A sharp transition of interfacial segregation against SWCNT solution pH was revealed, with the transition point at the pKa of carboxyl. The reduced SWCNT surface charge density at low pH was found to be beneficial for segregation due to the attenuated repulsions between adsorbed SWCNTs. Along with the charge effect, multivalency indispensably contributes to this sharp transition. This co-assembly technique is extensible to many materials with multiple carboxyl functionalization. The interfacial rheology of the SWCNTs/PS-NH2 co-assembled thin-films was quantified by oscillatory dilation and stress relaxation performed on the pendant drop rheometry. The stiffness is enhanced by almost a factor of five compared to pure PS-NH2 films. Two relaxations processes were identified, a fast one attributable to the confinement-mediated Rouse-like chain dynamics of end-attached PS-NH2 and the slow one was associated with adsorption/desorption (attachment/detachment). This reversibility over long timescale ultimately endows the films with fluid-like terminal behavior. Among the variables that affect positions and strengths of these relaxations are SWCNT, PS-NH2 bulk concentrations and water phase pH. In timescale ranges intermediate between the two relaxations, the co-assembled films display “soft-glass” behavior, with storage and loss moduli characterized by nearly equal power-law exponents. The rheology of gold nanoparticle-surfactants layer at oil/water interface was also studied by the similar technique with a strong size-dependence found. A characteristic size of ~5-10 nm, marking the transition from a solid-like to fluid-like behavior are identified, with solid-like for size less than 5 nm and fluid-like for size larger than 10 nm. In the transitional size range (5 nm-10 nm), a transient “plateau modulus” or “power-law” behavior was observed at the intermediate timescale, though the assemblies were still fluid-like in nature. These understandings provided valuable guidance to manipulating the behavior and properties of nanomaterials at liquid/liquid interfaces.
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