Theory of light-induced deformations in azobenzene polymers: structure-property relationship

Abstract

We propose a microscopic theory of light-induced deformation of side-chain azobenzene polymers using a statistical model which takes the chemical architecture of azobenzene macromolecules explicitly into account. The theory provides the values of the light-induced stress larger than the yield stress. This result explains a possibility for the inscription of surface relief gratings in glassy side-chain azobenzene polymers. We show that the photo-elastic behavior of azobenzene polymers is very sensitive to their chemical structure. Depending on the chemical structure, a sample can be either stretched or uniaxially compressed along the polarization direction of the linearly polarized light. For some chemical structures, elongation of a sample demonstrates non-monotonic behavior with the light intensity and can even change its sign (a stretched sample starts to be uniaxially compressed). These results are in agreement with experiments and recent computer simulations.

Theory of Light-Induced Deformation of Azobenzene Elastomers: Effects of the Liquid-Crystalline Interactions and Biaxiality

“…The deformation is caused by the anisotropic character of photoisomerization: maximal probability for a transition from the trans- to cis-state is achieved at an orientation of the rodlike chromophore parallel to the electric vector of the light E . Under visible light irradiation, the rodlike chromophores are reoriented preferably perpendicular to the vector E after multiple trans–cis–trans photoisomerization cycles. − Reorientation of chromophores results in the light-induced mechanical stress and deformation. − Under ultraviolet (UV) irradiation, most of the trans-isomers are transformed to the cis-state and orientation anisotropy appears without the cyclic trans–cis–trans photoisomerisation process. − …”

“…The strength of the orientation potential is proportional to the intensity of the light. Recently, this approach was used to study light-induced deformation of both un-cross-linked glassy azobenzene polymers − and cross-linked azobenzene networks. − Contribution of the light-induced orientation potential into the orientation free energy has been taken into account in ref , and the mechanical stress has been calculated. It was shown that the mechanical stress appearing at typical light intensities used in experiments ( I p ∼ 0.1 W/cm 2 ) is comparable with the yield stress (σ Y ∼ 50 MPa) typical for azobenzene polymers and can be even higher than the yield stress.…”

Modeling of Nonlinear Optical Phenomena in Host-Guest Systems Using Bond Fluctuation Monte Carlo Model: A Review

“…The theoretical approaches, presented in Section 2.1 , which use either angular-dependent kinetics of photoisomerization or the effective orientation potential, were successfully applied to describe the photo-deformation and ordering in azobenzene-containing polymers of various structures, including amorphous polymers [ 114 , 117 , 130 , 131 ], crosslinked isotropic polymer networks [ 132 , 133 ] and anisotropic azobenzene-containing liquid crystalline (LC) polymer networks [ 118 , 134 , 135 , 136 , 137 , 138 ]. The proposed theories can explain various experimental results for broad classes of azobenzene-containing polymers, as summarized briefly below.…”

“…As it was discussed in Section 2.1.3 , the orientation potential provides the mechanical stress of the magnitude 100 MPa—4 GPa at conventional light intensity , the stress being enough to deform irreversibly azobenzene polymers, which are deep in a glassy state [ 111 , 112 , 113 , 114 , 117 , 130 , 131 ]. Depending on the orientation distribution of azobenzene moieties inside polymer chains, preferably perpendicular or along the backbones of polymer chains, azo-polymers demonstrate either expansion or contraction along the polarization vector of the light [ 132 , 133 , 166 , 167 ].…”

“…Note that the effective orientation potential in the form of Equation ( 10 ) was phenomenologically proposed by two independent groups [ 128 , 129 ] and was used later to describe theoretically the photoorientation in broad classes of azo-polymers [ 114 , 118 , 130 , 131 , 132 , 133 , 134 , 135 , 136 , 137 , 138 ]. Thus, the presented theoretical formalism based on the photoisomerization kinetics described by Equations ( 3 )–( 11 ) justifies the introduction of the orientation potential in the form of Equation ( 10 ).…”