Research On Photo-induced Morphology And Inverse Design Of Polydomain Glassy Nematic Sheets

Contactless control of morphing structures is important for many modern technology fields including sensing and actuation,microsystems,biomedical and aerospace engineering,and thus has become one of the most concerned research frontier in recent years.Azobenzene-containing nematic sheets show attractive application prospects in photocontrolled morphing structures due to their sensitive photo-responsivity.Upon illumination by ultraviolet light of specific wavelength,this kind of nematic sheets contracts and expands respectively in the directions parallel and perpendicular to the director.The deformation can be recovered by heating or visible light irradiation.Therefore,a variety of recoverable deformation configurations can be formed by designing the director distribution reasonably.The photoresponsivity of glassy nematic polymers has been deeply studied and significant progress has been made.Especially,with the development of synthetic technique,accurate control of local director orientation is possible.For further device application,the key is to quantitatively understand the relationship between director distributions and deformation modes.However,this is seriously challenged by the structural complexity of glassy nematic sheets and the geometric nonlinearity of deformation.Most of the existing theoretical analysis focus on the monolayer sheets with continuously varying director fields in the plane,and often ignore the influence of sheet thickness and in-plane deformation.Although the relevant research results can provide important clues,it also makes many important problems difficult to clarify,and may even lead to conclusions inconsistent with experiments.In view of the above reasons,in this thesis,we are intended to study photo-induced large deflections of glassy nematic sheets based on the rigorous continuum theory,to establish reliable theoretical models,and to develop efficient numerical methods.Our attention is focused mainly on monolayers and orthogonally plyed bilayers with periodic director domains.The effects of incident light intensity,director distribution,domain geometry,domain boundary and vertex,and elastic anisotropy on the deformation mode and 3D morphology of the sheets are systematically explored.Moreover,making use of the unique light responsivity of the orthogonally plyed bilayers,a methodology of inverse design is developed to determine the director field in a sheet with specified 3D deformation morphology.The details are as follows:1)Taking into account of elastic anisotropy and intensity attenuation,the governing equations and boundary conditions are established for large deflection of arbitrary glassy nematic bilayers,and the corresponding numerical methods are developed.The equations involve the mid-plane deflection and stress function as the fundamental unknowns,and apply to all kinds of bilayer combinations that may appear in practical applications.2)Photo-induced large deflections of glassy nematic monolayers and orthogonally plyed bilayers with unidirectionally and bidirectionally periodic director domains are analyzed,special attention is paid to the roles of the domain boundaries and vertices in regulating the deformation morphology.In the meantime,the influences of light intensity and its attenuation,elastic anisotropy,domain geometry and free boundary effect are quantitatively examined.3)Photo-induced deformations of circular glassy nematic monolayers and orthogonally polyed bilayers with multi-fold rotationally symmetric director domains are studied.Analytical solutions in closed form and numerical solutions to the resulting 3D morphologies are obtained in small and large deflections respectively,and the effects of symmetric multiplicity,vertex and domain boundary on the local curvature of the deformed surface are quantified.4)A methodology is proposed to realize inverse design of director distribution for specified 3D morphologies of orthogonally plyed bilayer sheets under arbitrary boundary constraints.Analytical expressions of director distribution are obtained for unidirectionally variable morphology of an infinite sheet and axisymmetrically variable morphology of a disc.The variation range of the geometric and material parameters of the sheets are estimated according to the 3D morphologies as defined.