PASSIVE AND ACTIVE CELLULAR MICRO-STRUCTURED MATERIALS FOR MORPHING APPLICATIONS

Abstract

Morphing materials represent an emerging field since they have the potential to improve performance of the materials/structures/devices in which they are integrated. These materials have the capability of passively or actively providing radical shape changes to adapt to a varying environment. For this reason, they can be extremely beneficial in several applications including optics, three-dimensional biological scaffolds, airplane skin, micro-air vehicles, controlled drug delivery and smart textiles etc. In this thesis we design, fabricate, test and analyze novel passive and active cellular micro-structured films for morphing applications to provide ultra-light weight, non-invasive, highly stretchable and synclastic shape changes with the addition of 2D and 3D local/global morphing when the films are also active. One of the key aspects in the proposed material design approach is that it is possible to shape the microstructure of a material in such a way that, by strategically adding to the material microstructure extra elements of another forming material, it is possible to switch the material from a passive to an intrinsically active (temperature driven in this case study) response. For instance, in this thesis polyimide based micro-structured unit cells were designed to form an auxetic film (passive response with synclastic shape changes), but polyimide-based microstructured unit cells with a light integration of poly (vinyl alcohol) microelements, behave as a temperature activated film that can morph in 2D and its intrinsic degree of morphing can be tuned. This is possible because the latter film is structured in such a way to exploit a negative, and indeed tunable, coefficient of thermal expansion. In this case the highest ever presented negative coefficient of thermal expansion for a polymer material is presented and the beauty is that this is done by using only materials with a positive coefficient. Now, if the latter film is sandwiched with another film that has a positive coefficient of thermal expansion, then 3D morphing can be exploited whose response can also be magnified as needed. Another interesting aspect of this research activity is that if the miniaturized cells are properly designed, then it is possible to tune the material properties also in terms of isotropy, anisotropy, energy dissipation, ultra-high stretchability etc. With this in mind, polyimide-based auxetic films with a novel miniaturized structure as well as polyimide-based temperature activated morphing films, were studied in depth. The designed and tested auxetic film provides enhanced auxeticity despite its higher stretchability if compared with existing materials, auxetic anisotropy, hysteretic and repeatable loading/unloading cycles even if performed after auxetic failure which is here for the first time introduced. The active film, that finds its actuation capability in its intrinsic microstructure, is capable to provide tunable and up-to- ultra-low negative coefficient of thermal expansion, isotropic response, in-plane and out-of plane morphing (in the latter case in a multilayer configuration) which, if used in a pixelling fashion can be used to induce local/global, radical and unprogrammed shape changes. Moreover, the latter design is not affected by orders of magnitude stiffness losses as may occurs in other existing morphing materials, and is capable to provide fast responses. The results of this thesis are all based on numerical models (Comsol Multiphysics) that were used to design and predict the material response. They are also based on the implementation and optimization of the fabrication process that was carried-out constantly verifying the resulting microstructure and its influence on the overall material response (tested at the unit cell level as well at the global level).