Photo-reconfiguration of azopolymer toward biomimetic 3D micro/nano patterning

Abstract

Azobenzene polymer, briefly called as azopolymer, exhibits extraordinary light-matter interaction phenomena, which is ‘photo-fluidization’. This peculiar property gives unprecedented opportunities to fabricate unique 3D micro/nano structures based on directional movement. The azopolymer contains azobenzene groups grafted to the main chain, which undergo a repetitive photo-isomerization between trans- to cis-isomers until the long axis of the azobenzene is perpendicularly aligned to the polarization direction of the incident light. This photo-induced molecular reaction brings about the directional macroscopic motion of the azopolymer following the polarization direction of incident light even under the glass transition temperature (T$_g$) or melting temperature (T$_m$). For example, when linearly polarized light is irradiated to azopolymer, the mass migration only occurs following the polarization direction of incident light. Such polarization-dependent, fluidic behavior of azopolymer can be usefully used in numerous fields including micro/nano structuring. With azopolymer, inspired by living creatures in nature, many blue-prints for new innovations have been given to scientists and engineers in various fields since they have evolved naturally through rigorous competition to survive natural selection over millions of years. Thus, much effort has been given to mimic designs and materials of living creatures, which efficiently adapt to the harshest of environments. Against this backdrop, biomimetic 3D micro/nano structuring through photo-reconfiguration, exploiting azopolymer’s directional photo-fluidization, will be carefully discussed. This thesis begins with some brief on azobenzene polymer and their behavior of photo-fluidization in Chapter 1

i) light-responsive behavior of azopolymer, ii) basic mechanisms and iii) the relevant application or patterning techniques (i.e. directional photo-fluidization lithography). After that, I describe a facile and pragmatic way of fabricating sub-micron sized bent pillar structure with programmable and systematic morphology controllability in Chapter 2. The fabricated structure in this work further extends to application in uni-directional wetting and directional adhesion. In Chapter 3, producing shark skin-mimetic denticle structure is presented, where interference light of two-incident light is exploited to form slant riblet geometry onto top of azopolymer pillar. The fabricated structure in this work paves a novel way to mimic complex denticle structure and suggests potential in study of structure-fluidic behavior in micro/nano scale. In chapter 4, brief summary of this thesis is presented.