Theory of Light‐Induced Deformation of Azobenzene Elastomers

Abstract

Summary: A microscopic theory is proposed to describe light‐induced deformation of photo‐sensitive elastomers bearing azobenzene chromophores in their strands. We use an orientation approach in which it is assumed that the light‐induced deformation is caused by reorientation of azobenzene chromophores with respect to the electric vector of the linearly polarized light, E, due to the trans‐cis‐trans photoisomerizaion process whose efficiency depends on the orientation of the chromophores with respect to the vector E. In the framework of the Gaussian approximation for elasticity of network strands it is shown that the value of the light‐induced deformation depends on the chemical structure of network strands, namely, on the orientation distribution of chromophores around the main chains which is related to the length and elasticity of spacers. Depending on the chemical structure, azobenzene elastomers can demonstrate expansion or uniaxial contraction along the vector E, as well as non‐monotonic deformation with increasing light intensity (expansion at small light intensities and contraction at high ones).

Light-induced deformation of polymer networks containing azobenzene chromophores and liquid crystalline mesogens

“…[19][20][21] The reorientation of chromophores under light irradiation is described here, as in ref. [22][23][24][25][26][27][28][29][30][31], by introducing an orientation potential, which acts on each chromophore:…”

“…Maximal probability of the transformation of the rodlike trans-isomer to the bent cis-isomer takes place at the parallel orientation of the long axis of the trans-isomer with respect to the vector E. [19][20][21] Thus, aer the multiple trans-cistrans isomerization process under visible light irradiation the orientation distribution of the chromophores becomes anisotropic with preferable orientation of the chromophores perpendicular to the vector E. [19][20][21] To describe the light-induced anisotropy, an effective orientation potential, which reorients the chromophores perpendicular to the vector E, has been introduced independently by Chigrinov et al 22 and one of the authors. 23 The orientation approach, based on this potential, was recently applied to describe the light-induced deformation of azobenzene-containing polymers of different structures, including uncrosslinked amorphous polymers, [24][25][26] crosslinked isotropic polymer networks [27][28][29] and anisotropic polymer networks with orientation interactions between the chromophores. 30,31 The proposed theories are able to explain experimental results, as summarized briey below.…”

“…(i) The orientation potential provides the mechanical stress, which is enough to deform azobenzene polymers in the glassy state. [24][25][26][32][33][34] (ii) Depending on the orientation distribution of chromophores inside polymer chains, which is related to the chemical structure, azo-polymers can demonstrate either expansion or contraction along the polarization vector E. [24][25][26][27][28][29][30][31][35][36][37][38] (iii) Light-induced deformation of azobenzene networks depends on the degree of cross-linking and on the concentration of azobenzene chromophores, [27][28][29] the theoretically predicted dependences being in a good agreement with experimental data for the light-induced bending deformation of azo-networks. 29 (iv) Liquid crystalline (LC) interactions between rodlike chromophores result in the biaxial order and biaxial deformation of azobenzene containing polymers.…”

“…In the present study the orientation approach is used to describe the light-induced deformation of a two-component polymer system consisting of a LC polymer network with included azobenzene chromophores. By this, we develop previous theories, [26][27][28][29][30][31] which dealt with one-component photosensitive polymers containing only azobenzene chromophores in their structures. At the same time, many experimental works were devoted to the synthesis and investigation of photomechanical properties of two-component azobenzene polymers, which include LC mesogens and azobenzene chromophores.…”

Enhancement of Mechanical Properties of Photo‐Responsive Polymers

“…On the other hand, the cis‐azobenzene returns to its trans form after irradiation with visible light (400–500 nm) or storage in the dark . Such stimuli induced changes in this class of materials are the focus of many studies …”