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Manipulating Supramolecular Nanocomposite Self-Assembly via Entropy Modulation

  • Author(s): Bai, Peter Jin
  • Advisor(s): Xu, Ting
  • et al.
Abstract

A grand challenge in materials science is to create materials with complex, rationally designed structures and functionalities. Self-assembly is a promising route to fabricate these materials with nano-scale precision, and efficiency in terms of cost and time. Nanocomposites comprised of block copolymer (BCP)-based supramolecules and nanoparticles (NPs) represent a promising platform to access nanostructures of various structures, chemistries, and functionalities. The thermodynamic landscape, and thus self-assembly behavior, of these supramolecular nanocomposites is dominated by entropic factors. While entropy is often considered a detrimental factor that prevents the formation of ordered phases and the co-assembly of BCPs and NPs, it can also be harnessed to stabilize nanostructures that are not accessible to traditional BCPs. This dissertation will focus on controlling the self-assembly of supramolecular nanocomposites by manipulating the entropy in these systems.

In bulk and thin films of supramolecular nanocomposites, self-assembly is governed by a balance between NP translational entropy and supramolecule chain conformational entropy. By systematically varying the NP size within these systems, this entropic balance can be perturbed, leading to a change in NP spatial distribution within the nanocomposite and the stabilization of cubic and anisotropic microphases that are unique from traditional BCP morphologies, as confirmed by TEM, TEM tomography, and small angle X-ray scattering techniques.

Geometrically confining supramolecular nanocomposites into cylindrical pores introduces boundary conditions that impose additional entropic frustration to the composite system, leading to exotic nanostructures such as NP stacked rings and single and double helices. The single helical structures are also demonstrated, via dark field scattering and finite integration technique simulations, to possess strong chiral plasmonic properties due to the chiral arrangement of NPs and strong plasmonic interparticle coupling.

Finally, replacing alkyl side groups of the supramolecules with liquid crystalline side groups further increases chain rigidity and presents another avenue towards entropic modulation. These modified supramolecules and their associated nanocomposites are thoroughly investigated for their self-assembly behavior. It is observed that the liquid crystalline small molecule side chains introduce hierarchical smectic ordering and thermally triggered, non-reversible order-order transitions to the supramolecular nanocomposite.

These studies demonstrate entropic modulation in supramolecular nanocomposites to be an important tool in creating uniquely structured and functional materials with potential applications in memory storage, energy transport, and metamaterials.

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