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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

May

First Advisor

Alfred Crosby

Subject Categories

Polymer and Organic Materials

Abstract

Adhesives have long been designed around a trade-off between adhesive strength and releasability. Within this spectrum, specialized materials have been designed to maximize adhesive ability for a given application. To overcome this trade-off, a new adhesive paradigm is required. Biologically inspired adhesives have been of interest over the past two decades, because organisms are seen using their adhesive pads to achieve high adhesive forces, while maintaining releasability and reusability. Many biological organisms possess microscopic fibrillar features on their toe-pads, which enables climbing. While much effort has been spent attempting to mimic these features, ultimately high force capacities have not been achieved. Recently, a new framework has been introduced which states that a specific surface morphology is not necessary for creating high force capacity, easy release adhesives. This framework states that for shear adhesives to achieve high force capacity, the ratio of contact area to compliance in the loading direction, A/C, must be increased. In this thesis we focus on expanding this framework to quantitatively understand both compliance and area, for a wide range of adhesive materials and geometries, and across a wide range of substrates with varying roughness. To increase the functionality of high strength, reusable adhesives, we have developed a new adhesive configuration which supports normal loading as well as shear loading. Finally, we expand to a new field, biological prosthetic materials, and develop fabric-based composites which are extremely tough, strong, and flexible, while containing water.

The foundation of the work presented in this thesis is based upon an analytical model developed to calculate the compliance of fabricated adhesives (Chapter 2). Combining this knowledge with the previously developed scaling theory allows a high degree of accuracy in calculating force capacity. While this method works well for smooth surfaces such as glass, it assumes that the nominal pad area is equal to the true area of contact, which is not true on rough surfaces. A model is developed to calculate the true area of contact based on surface roughness and adhesive materials properties (Chapter 3). The results of this model demonstrate that there is an optimum pad modulus for any given surface roughness to achieve maximum stress capacity. In some situations, high strength and easy release adhesives are required in normal loading situations. We develop a new adhesive configuration which enables shear adhesives to support normal loads (Chapter 4). This method results in a six-fold increase in normal force capacity. This provides tolerance in adhesives applications, greatly improving the commercial utility of these adhesives. Finally, we use techniques learned from the fabrication of adhesives to develop composites from polyampholyte gels and glass fiber fabrics (Chapter 5). These materials exhibit enhanced properties over the controls, including extremely high toughness and strength, while maintaining flexibility and containing water. A general mechanism is explained that results in these improved properties, opening up opportunities to develop enhanced composites from fabrics and soft materials in other fields.

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