Abstract
Complex light-matter interactions in azobenzene polymers have limited our understanding of how photoisomerization induces deformation as a function of the underlying polymer network and form of the light excitation. A unified modeling framework is formulated to advance the understanding of surface deformation and bulk deformation of polymer films that are controlled by linear or circularly polarized light or vortex beams. It is shown that dipole forces strongly respond to polarized light in contrast to higher order quadrupole forces that are often used to describe surface relief grating deformation through a field gradient constitutive law. The modeling results and comparisons with a broad range of photomechanical data in the literature suggest that the molecular structure of the azobenzene monomers dramatically influences the photostrictive behavior. The results provide important insight for designing azobenzene monomers within a polymer network to achieve enhanced photo-responsive deformation.
Highlights
Complex light-matter interactions in azobenzene polymers have limited our understanding of how photoisomerization induces deformation as a function of the underlying polymer network and form of the light excitation
Deformation of azobenzene surface relief grating structures has been characterized by a mass diffusion process where field gradients associated with the spatial variation in intensity of the light beam gives rise to a surface shape change[15,20,34]
Whereas this modeling framework does not directly consider the material microstructure, it matches data considerably well for Gaussian laser beams with both linear and circularly polarized light[20]. These concepts have been extended to optical vortex beams which produce more complex surface texture that depends on the number of topological charges associated with the vortex beam[15]
Summary
Complex light-matter interactions in azobenzene polymers have limited our understanding of how photoisomerization induces deformation as a function of the underlying polymer network and form of the light excitation. Deformation of azobenzene surface relief grating structures has been characterized by a mass diffusion process where field gradients associated with the spatial variation in intensity of the light beam gives rise to a surface shape change[15,20,34] Whereas this modeling framework does not directly consider the material microstructure, it matches data considerably well for Gaussian laser beams with both linear and circularly polarized light[20]. In the vortex beam case, additional phenomenological coupling terms associated with optical absorption are introduced to simulate the polymer surface shape change via similar mass diffusion relations coupled to quasi-static linear momentum These models are predicated on the use of a field gradient driving force (work conjugate to the quadrupole density) that is associated with the light beam to predict azobenzene polymer deformation. Such models will break down when simulating bending and twisting of films exposed to uniform light which contains no time-averaged field gradient on the material surface
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