Optical Erasing And Turning Of Surface Wrinkles On Azo-containing Polymer Films

Surface wrinkling of a soft matter with a hard skin is ubiquitous in both nature and engineering.On the one hand,surface wrinkling may pose a limit on the performance of materials and structures and is often thought to be a nuisance to be avoided.On the other hand,the characteristics of wrinkles have found a wide variety of applications ranging from characterizing mechanical properties of ultrathin films,fabricating functional surfaces,to designing flexible electronics and advanced photovoltaics.However,how to erasure the surface wrinkles or create desired wrinkling patterns on thin films resting on a compliant substrate remains a challenging issue.In this thesis,we incorporate the photo-responsive component into surface film,for constructing responsive or adaptive film/substrate wrinkling system based on the surface wrinkling mechanism,to dynamically tune and/or erase the surface wrinkles.According to the different configuration of the film/substrate wrinkling system,the main contents of this thesis are divided into two parts:photomodulation of surface wrinkles on bilayer wrinkling system and photomodulation of surface wrinkles on multilayer wrinkling system.In the bilayer wrinkling system,we report a simple yet powerful method to dynamically tune and/or erase the surface wrinkles over large areas on an azocontaining poly(disperse orange 3)(PDO3)film bonded to a poly(dimethylsiloxane)(PDMS)substrate with visible light.Upon the visible light irradiation,reversible photoisomerization of azobenzene in the wrinkled PDO3 film takes place,leading to the dynamically evolution of the wrinkle morphology on the film surface.Meanwhile,we systematically explore the relation of the erasure time with light density,light wavelength,film thickness and wrinkle amplitude.The results indicate that the continuous stress/strain release due to the reversible photoisomerization of azobenzene in the film decreases the wrinkle amplitude and finally erases the wrinkling patterns in the exposed region,which is further supported by numerical simulations.Subsequently,we fabricate different highly ordered wrinkle patterns with well-defined hierarchical microstructures over large areas by selective exposure via differnt copper grids or photomasks.Finally,this optically erasable wrinkling system has been adopted for application as reversible optical read-in/erasure of classifiedinformation.In the multilayer wrinkling system,we incorporate the azobenzene-containing PDO3 film as an intermediate layer,and the polystyrene(PS)film as the top-layer,to design a photo-tunable multilayer wrinkling system(PDMS/PDO3/PS)for hierarchically patterning the films in a simple and inexpensive way.Laminated multilayer films comprised of the intermediate layer of photosensitive PDO3 film were deposited on PDMS substrate and then wrinkled upon heating/cooling.When irradiated with visible light,surface wrinkles on the PDO3-based multilayer films can be photo-modulated over large area due to the reversible photoisomerization of azobenzene groups in the intermediate PDO3 film.Our analysis indicates that photosoftening effect in the PDO3 film due to the reversible photoisomerization of azobenzene plays a key role here.When selective exposure is employed,surface wrinkles in the exposed region are modulated with wavelength obviously decreasing due to the photosoftening of the azobenzene-containing polymer layer and wrinkles in unexposed region are evolved with their orientation perpendicular to the exposed/unexposed boundary because of the effect of the boundary conditions.Further more,the dimension of the resultant wrinkle patterns could be controlled conveniently by adjusting the the experimental parameters,such as the film thickness of the photosensitive intermediate layer and the photo-inert outmost layer,selective exposure size and geometry and so on.More importantly,this azobenzene-containing multilayer wrinkling system is versatile with different composite functional materials as the stiff layer.On the other hand,we can apply it as an independent simple method for gaining insight into the photo-induced change of the mechanical properties of the azobenzene-containing polymer in the laminated multilayer system.Our analysis showed that,the elastic modulus of the PDO3 film decreased by about one order of magnitude.Further more,we incorporate the photoisomerizable azobenzene into surface film with two different photomodulatable film/substrate wrinkling systems(the PDMS/PS/PDO3 system and the PDMS/SiOx/PDO3 system)for desirable patterned surface formation.The designed photomodulatable wrinkling systems show distinguishing wrinkle evolution process upon irradiation when different films were incorporated as the intermediate layer.The inherent mechanism of the photomodulated wrinkle evolutions on azobenzene-containing multilayer films via visible light irradiation and the influences of the experimental parameters(e.g.,thefilm thickness,the composition of the intermediate layer,or selective exposure size and geometry)on the resultant microstructures have been systematically investigated.Our analysis showed that upon the visible light irradiation,the elastic modulus of the PDO3 film decreases plays a key in the wrinkles wavelength decrease on the PDMS/PS/PDO3 system and the stress/strain releasing due to the reversible photoisomerization of azobenzene in the film plays a key role in the wrinkle erasion on the PDMS/SiOx/PDO3 system.Further more,we fabricate different highly ordered wrinkle patterns with well-defined hierarchical microstructures over large areas on these two different multilayer systems through selective exposure via differnt copper grids combing with the variation of related experiment parameters,such as the films thickness and so on.