Thermally-controlled deformation of polymeric photonic structures for bandgap engineering

Abstract

Colloidal photonic crystals composed of self-assembled monodisperse particles and their derivatives have been intensively studied and developed during the past two decades owing to their unique optical properties from diffraction and interference of incident lights. A specific range of frequencies or wavelengths cannot propagate through the photonic crystals and are reflected. The range of frequencies or wavelengths are called photonic bandgap and when the bandgap is in a visible range, the photonic crystals show brilliant color. In this thesis, I deal with thermally-controlled deformation of polymeric photonic structures for bandgap engineering. Specifically, I describe a novel preparation of polymeric inverse opal by using a partial coalescence of polymeric shell of the particles, multi-colored micropatterns with spatially addressable and rate controllable of thermally deformation of photonic structures and time-temperature indicator for recording thermal history by using the thermally irreversible deformation of the partial crosslinked inverse opals. In Chapter 2, I report the stable photonic structures created by using colloidal building blocks composed of an inorganic core and an organic shell. The core？shell particles are convectively assembled into an opal structure, which is then subjected to thermal annealing. During the heat treatment, the inorganic cores, which are insensitive to heat, retain their regular arrangement in a face centered cubic lattice, while the organic shells are partially fused with their neighbors

this forms a monolithic structure with high mechanical stability. The interparticle distance and therefore stop band position are precisely controlled by the annealing time

the distance decreases and the stop band blue shifts during the annealing. The composite films can be further treated to give a high contrast in the refractive index. The inorganic cores are selectively removed from the composite by wet etching, thereby providing an organic film containing regular arrays of air cavities. The high refractive index contrast of the porous structure gives rise to pronounced structural colors and high reflectivity at the stop band position. In Chapter 3, I report a lithographic approach that provides a high level of control over the size, shape, and color of a micropattern using the anisotropic shrinkage of inverse opals made of a negative photoresist heated to high temperatures. Shrinkage occurred uniformly across the thickness of the film, leading to a blue shift in the structural color while maintaining a high reflectivity across the full visible spectrum. The rate of shrinkage was determined by the annealing temperature and the photoresist crosslinking density. The rate could, therefore, be spatially modulated by applying UV radiation through a photomask to create multicolor micropatterns from single-colored inverse opals. The lateral dimensions of the micropattern features could be as small as the thickness of the inverse opal. In chapter 4, I report a recording thermal condition device using the anisotropic compression of the partial crosslinked inverse opals made of a negative photoresist. This shrinkage is irreversible, which cannot be back to the original state due to reducing the interfacial energy between air and the negative photoresist. The rate or degree of the compression is determined by UV dose, which can control the crosslinking density of the photoresist. The recording thermal condition device can be fabricated by a consecutive UV irradiation through designed photomasks and provide several different optical codes, which can be converted to the required time for shifting the optical codes at a certain temperature. These different time-temperature functions can provide the decoupled information of heating time and temperature.