Interfacial Assembly of Soft Polymeric Janus Nanoparticles Towards Structuring Liquids

The focus of this dissertation is to understand the basic principles that govern interfacial assembly towards structuring liquids via the jamming of interfacially active soft polymeric Janus nanoparticles(sJNPs) and develop such structured liquids to be adaptive. Liquid interfaces have desirable characteristics as a platform for materials, ion and charge transport, and material synthesis. There have been numerous studies on the use of surfactants and compatibilizers to create large interfacial areas by stabilizing one liquid in another but structuring the liquids into shapes other than spherical has been difficult to achieve. Colloidal particles are interfacially active (a 0.5mm particle can reduce the interfacial energy by 104 kBT) and form a monolayer at the interface between two liquids to reduce interfacial energy. The size dispersity of the particles plays a large role in determining what type of monolayer is obtained. If the size dispersity is unity, a 2D crystallization occurs at high areal densities, whereas for polydisperse particle size distributions, the 2D assemblies will remain disordered and vitrify. Such behavior is well-documented for colloidal systems but changing the shape of the liquid supporting the assemblies leads to a fracturing of the assemblies, due to the longer time scale of the segregation of the particles to the interface relative to the deformation being applied. Compare to colloidal particles, the time scale of nanoparticle assembly is reduced by factor of ~104, based on the diffusion coefficient of the particles in solution. Due to their size, uniformly functionalized nanoparticles have an interfacial binding energy that is 10k¬BT or less and, as such, the particles are weakly bound to the interface, and exchange with particles in the bulk dispersions. Consequently, any compressive forces applied to the nanoparticles, as for example when the shape of the liquid domains change and the system reduces the interfacial area to minimize energy, will cause the nanoparticles to be ejected from the interface, and the liquid will assume an equilibrium spherical shape. One strategy to increase the binding energy is to have a heterogenous surface functionality, where one functional group prefers one liquid, and the second functional group preferentially interacts with the second liquid. If the different functionalities are sequestered to different hemispheres of the particle surface, one part of the particle interacts with one liquid and the second part with the other liquid i.e. the particle is Janus. The binding energy of Janus-type nanoparticles, like their surfactant counterparts, is markedly increased, allowing assemblies of the particles to withstand an in-plane (interfacial) compressive force, enabling the liquid domains on which the Janus nanoparticles are assembled, to be shaped. For hard Janus nanoparticle, the point at which the particles jam at the interfacial area is decreased defines the point at which further changes in the interfacial area are arrested. In this thesis we address the influence of the hardness or softness of the nanoparticles on the manner in which the assemblies are jammed and what role if any, does the softness or deformability of the nanoparticles have on the relaxation of the assemblies upon jamming. In particular, the Janus nanoparticles used in this thesis are formed from multiple polymer chains emanating from the nanoparticle core, where two distinct immiscible polymers are used. We address issues associate with the molecular weight of the polymer chains comprising this “fuzzy” nanoparticle, the length of the polymer chains relative to the core volume, and changes in the configuration of the polymer chains when they are assembled at the interface and relaxations of the polymer chains after the compressive force is applied. By the dissolution of salts in the aqueous phase (one of the two immiscible liquids) the binding energy was changed and the influence of this change on the interface activity was examined. The advantages and disadvantages on the “softness” of the particles is discussed relative to specific applications.

Advisor: Xu, Ting