Flexible mechano-responsive nanocomposites with structure-controlled sensitivity for multifunctional applications

Abstract

Mechano-responsive nanocomposites are attractive for their capability to change the electrical, optical, and chemical properties when external forces are applied, and are promising in emerging applications such as stretchable electrodes and flexible displays. Because of the fabrication difficulties to control the structure of materials, most studies have so far focused on exploiting emerging materials to tailor the mechano-responsive behaviors while the role of material structures remains largely unknown. However, with the development of micro/nano fabrication techniques that allow building materials with well-defined architectures, a better understanding is now available on structural effects. In this thesis, several design approaches are proposed to fabricate ordered structures, and the relationship between the structure and the strain sensitivity of mechano-responsive nanocomposites is established. Based on the understanding of the relationships, the sensitivities are carefully controlled to realize multidimensional strain sensing and high-contrast optical modulation. Anisotropic graphene aerogels are developed through unidirectional freeze-casting for in-plane sensing. The aligned structure is found to endow the aerogel-based nanocomposites with strain sensitivity highly dependent on loading directions. Combining this in-plane sensor with a pressure sensor insensitive to lateral tension, the integrated sensor can differentiate mechanical stimuli including in-plane tension, normal pressure, and shear, and presents an outstanding selectivity of 3.68 to in-plane strain directions. Based on such multidimensional sensing capability, a smart device is demonstrated to monitor sports performance in real-time. Periodic three-dimensional (3D) heterogeneous structure is fabricated by introducing modulus mismatch between the two interdigitated phases. It is revealed that the strain concentrations at the interfaces ameliorate the debonding at the interface to generate numerous nanogaps as light scattering sites. The optimized light scatterer exhibits high transmittance modulation achievable at a low strain of 15% and a remarkable maximum contrast of 82%. The same 3D heterogeneous design strategy is successfully extended to improve the sensitivity of strain-induced coloration, proving its wide applicability. Inclined 3D porous structures are proposed from optical simulation and realized for the first time by 3D patterning with a slanted exposure angle. Based on the structure, a novel light scatterer is presented which changes from opaque to transparent under normal compression because of the closing of the light-scattering pores. The pore closure is found to be facilitated when the pores are inclined to the normal direction, resulting in the 3D inclined scatterer showing an unprecedentedly high transmittance contrast of 96% and an extremely low lateral activation strain of ~1% for a meaningful contrast. Such performance makes the scatterer highly promising for smart windows with large sizes. These results may open new horizons for the structural designs of mechano-responsive nanocomposites that are practical in different application scenarios.