Bioinspired Nanocomposite Adhesives Based on 3D Microarchitectures and 1D Nanomaterials for Advanced Thermal and Electrical Applications

Abstract

Functional adhesives are essential components in a variety of application fields from daily life to high-tech industries, including precision manufacturing, aerospace, flexible electronics, and wearable devices. However, conventional functional adhesives based on chemically reactive, hot-melt, and viscoelastic adhesive materials generally form uncontrollable mechanical contact, producing bulky, contaminated, or damaged contact interfaces. To address these issues, bioinspired adhesive architectures exhibiting robust, reversible, and residue-free adhesion properties have been proposed. The extraordinary adhesion properties are due to the presence of nano- or micro-hair arrays with protruding tips that maximize van der Waals interactions between surfaces. The photolithography process followed by the replica-molding process has allowed the production of bioinspired artificial adhesives with robust adhesion and high structural stability in a simple, precise, and highly reproducible way. Nevertheless, the manufacturing process narrows the selection of materials to thermal- or UV-curable polymers whose inherently poor thermomechanical and electrical properties hinder the application of bioinspired adhesives in advanced industrial fields. One-dimensional (1D) nanomaterials including carbon nanotubes (CNTs), metallic nanowires, and nanorods have been actively studied as nanofillers to enhance the mechanical, electrical, and thermal properties of polymeric materials. Yet, the existing methods for the application of nanomaterials are not suitable for fabricating three-dimensional (3D) microarchitectures since the high viscosity of nanomaterial–polymer mixtures inhibits the successful formation of the structures. Furthermore, the rough morphology of the nanomaterials hinders the formation of intimate contact interfaces resulting in low adhesion strength. In this dissertation, we present novel design strategies for bioinspired nanocomposite adhesives, in which 1D nanomaterials are integrated into 3D microarchitectures. The strategies include microarchitecture designs, nanomaterial selections, and optimization of integration processes that allow microarchitectures to have enhanced thermal or electrical properties while maintaining superior adhesion performance. In Chapter 2, we propose high-temperature compatible adhesives based on an integration of mushroom-shaped microarchitectures and CNT-based nanocomposites. The nanocomposite microarchitectures are prepared by a photolithography process followed by replica-molding techniques in which polydimethylsiloxane (PDMS) matrices are reinforced with CNT fillers. The excellent thermomechanical properties of the CNTs enable the mushroom-shaped adhesive architectures to have exceptionally enhanced thermomechanical stability compared to pristine PDMS. Moreover, the manufactured adhesives exhibit robust adhesion performances even when exposed to elevated temperatures of ~350 °C; thus, they could be utilized as versatile high-temperature compatible adhesives with high reversibility. In Chapter 3, we propose a flexible, transparent, and electrically conductive adhesive composed of tentacle-like adhesive architectures and selectively coated percolating silver nanowires (AgNWs). The integrated design provides robust mechanical and low-resistance electrical contacts by forming intimate contact interfaces. The contact interfaces enable efficient electrical connections with active electrodes through attachment without the use of additional contact processes such as mechanical clamping, chemical adhesives, or vacuum deposition, with the contact remaining stable even when highly bent. The superior features of bioinspired conductive adhesives are demonstrated in self-attachable transparent heaters that can form a direct, seamless contact between its AgNWs and the target substrates, providing direct heat-transfer pathways for precise temperature control of the substrate while minimizing energy loss.