Abstract
Azobenzene-containing polymer networks are unique compounds that are able to change their shape in response to light, which makes them prospective materials for photocontrollable nano-templates, sensors, microrobots, artificial muscles, etc. In present work, we study the kinetics of light-induced ordering and deformation in two-component polymer networks containing optically inert liquid crystalline (LC) mesogens and azobenzene chromophores. By this, we generalize our previous theory [J. Phys. Chem. Lett. 2017, 8, 1094–1098] devoted to the kinetics of photoizomerization in one-component azo-polymers without mesogenic inclusions. The kinetic equations of photoisomerization are used, taking into account the angular selectivity of the photoisomerization with respect to the polarization direction of the light E. After multiple trans-cis-trans photoisomerization cycles, the azobenzenes are reoriented preferably perpendicular to the vector E. This changes the ordering of the mesogens due to the orientational LC interactions between the components. The light-induced reordering is accompanied by network deformation. Time evolution of ordering and deformation is found as a function of the intensity of light and structural parameters of the LC azo-networks, which define the viscosity, the strength of the LC interactions between the components, the volume fraction of the azobenzene moieties, and the angular distribution of azobenzenes in polymer chains. Established structure-property relationships are in agreement with a number of experimental data.
Highlights
Liquid crystalline (LC) polymer materials are well-known for their versatility when it concerns a stimuli-triggered mechanical response
A thermomechanical response in mono-domain liquid crystalline elastomers (LCE) produced by a two-step crosslinking technique [1] was reported by the group of Finkelmann [2]
We study the photomechanical properties of the two-component polymer networks which are in the LC state at the absence of light with preferable orientation of the mesogens and azobenzenes along the LC director, n, Figure 1a
Summary
Liquid crystalline (LC) polymer materials are well-known for their versatility when it concerns a stimuli-triggered mechanical response. The direction of photoinduced bending of the polymer network with azobenzene LC moieties can be reversed by switching the polarization of the laser beam in orthogonal directions [23]. The effects of bent cis-isomers were accounted for in the study [39], which allowed us to consider both the reorientation and dilution effects induced in the azobenzene LC elastomers by the polarized laser beam. We consider the photoinduced dynamics in a two-component LC network possessing a realistic chemical structure, that is, a structure containing optically inert mesogens and trans- and cis-isomers of the azobenzenes. A very rich time-dependent behavior of ordering as represented by LC order parameters and of network deformation has been found for the two-component azobenzene-containing LC polymer networks in agreement with multiple experiments
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