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Document Type

Open Access Dissertation

Degree Name

Doctor of Philosophy (PhD)

Degree Program

Polymer Science and Engineering

Year Degree Awarded

2015

Month Degree Awarded

February

First Advisor

Alfred J. Crosby

Subject Categories

Condensed Matter Physics | Polymer and Organic Materials | Statistical, Nonlinear, and Soft Matter Physics

Abstract

Hierarchical structures developed from nanoscale building blocks offer an excellent opportunity to control properties on all length scales, from the molecular level up to the macroscale. Many beautiful examples in Nature have demonstrated the significance of controlling geometry and mechanics on small length scales to control function on an organism-level, shown by the strength of bones, the toughness of a mollusk's shell, or the gecko's ability to climb walls. Inspired by stunning examples in both Nature and common man-made materials and structures, we assemble polymers and inorganic nanoparticles (NPs) with well-defined surface chemistry into long ribbons and fabric-like networks with unprecedented length scales. In particular, we focus on the geometry and mechanics of these structures when released from their underlying substrate, as well as the fabrication methods to create such structures. This thesis describes four concepts in detail: (1) the development of an evaporative assembly method used to prepare polymer and NP mesoscale structures, referred to as flexible blade flow coating, (2) the spontaneous formation of helical ribbons, driven by a 2-phase elastocapillary balance between surface tension and elasticity of an asymmetric geometry, (3) the mechanical stretching properties of NP-based helical ribbons, and (4) the deformation, shape and fluid-structure interactions of small, flexible, polymeric microhelices in viscous flow.

We first describe flexible blade flow coating, a technique that enables the fabrication of polymer, NP and hybrid mesostructures spanning several length scales. By controlling the fabrication parameters, a wide range of materials can be used to create a wide range of geometries, such as ribbons and fabrics. This method relies upon controlled evaporation of a dilute solution confined between a thin polymer film and a flat substrate. By taking advantage of crosslinkable ligand chemistry and the use of a water-soluble sacrificial layer, the structures are liberated from their substrates, affording robust structures floating at the air-liquid interface or fully submerged.

When fully submerged in a fluid with sufficient interfacial tension, like water, we discovered that ribbons spontaneously formed helices. By starting from a general expression that balances the elastic bending energy and surface tension of an asymmetric cross-sectional geometry, we determined that this helical formation is due to the asymmetric reduction of surface area upon bending (serving to lower the system energy). This leads ribbons to bend into helices with a preferred radius governed by both the modulus and interfacial tension, as well as ribbon thickness (R ~ Et2/γ). This universal, geometry-based mechanism provides a new opportunity to create helices from any class of material, which is demonstrated by implementing metallic, ceramic and magnetic NPs, as well as homopolymers.

Upon understanding the mechanics of helical formation, we examined the mechanical properties of NP-based helical ribbons. Through the use of a custom-designed mechanical measurement tool, which is capable of measuring ~nN forces over displacements of 100s of microns or greater, we experimentally measured the force-displacement relationship of these helices. We show that this curve can be predicted through the elastic energy and surface-driven helical shape. Our experiments revealed massive stretchability, where helices are able stretch to their fully straightened contour length, as high as 23 times their original length. At low strains, the helices display stiffness values similar to single polymer chains or biological helices (~10-6 N/m), and when fully stretched, display properties similar to synthetic polymer nanofibers.

Motivated by small, flexible helices in fluids found in Nature, like swimming bacterial flagella, we expand our studies to examine single helices in viscous fluid flow. We fixed one end of a helix while leaving the other free, placed it into a microchannel, and applied a controlled fluid flow rate. Using PMMA as our model polymeric material, we found that the axial deformation is well-described by a nonlinear helix of finite extensibility, defined by a balance between the viscous and elastic forces. From our experiments, we describe the pitch distribution of a deformed helix in flow, as well as calculate a frictional coefficient for the helical geometry. At high flow rates, we qualitatively observed a global-to-local helical shape instability. Finally, we extend the study to show preliminary results on the deformation of NP-based helices.

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