Leibniz-Institut für Polymerforschung , Hohe Str. 6 , 01069 Dresden , Germany.
KU-KIST Graduate School of Converging Science and Technology and Department of Biomicrosystem Technology , Korea University , 02841 Seoul , Republic of Korea.
J Phys Chem B. 2019 Apr 18;123(15):3337-3347. doi: 10.1021/acs.jpcb.9b00614. Epub 2019 Apr 4.
To make a polymer-based material photosensitive, it is usually modified by inclusion of azobenzene (azo) chromophores. Their interaction with the light leads to conversion of absorbed energy into mechanical work. The wavelengths ∼500 nm induce cyclic trans-cis isomerization, which results in preferred orientation of the trans-isomers perpendicular to light polarization. This causes reorientation of the polymer backbones to which the azos are attached and appearance of the light-induced stress that dictates a direction of the macroscopic deformation. The directional photodeformations can be explained by an orientation approach, in which the reorientation of azos is described by the effective orientation potential. Here, we show how to calculate the time-dependent orientation state of the polymer backbones and the light-induced stress tensor. For side-chain azopolymers, a tensile stress in the direction of light polarization is predicted. Implementing the stress in a viscoplastic material model of the finite element software ANSYS, we show that a square azopolymer post elongates along the electric field vector for the linearly polarized light and contracts along the propagation direction for the circularly polarized light. These results of viscoplastic material modeling are in accordance with the experiments on light-induced reshaping of microscaled square and cylinder posts. Hence, the orientation approach works rather well for homogeneous illumination. We discuss how this approach can be used to describe surface deformations induced by complex light interference patterns.
为了使聚合物基材料具有光敏性，通常通过包含偶氮苯（偶氮）发色团来进行修饰。它们与光的相互作用导致吸收的能量转化为机械功。波长约为 500nm 诱导循环反式-顺式异构化，导致反式异构体优先沿光偏振方向取向。这会导致与偶氮相连的聚合物主链的重新取向，并出现光致应力，该应力决定了宏观变形的方向。定向光致变形可以通过取向方法来解释，其中偶氮的重取向由有效取向势描述。在这里，我们展示如何计算聚合物主链的时变取向状态和光致应力张量。对于侧链偶氮聚合物，预测沿光偏振方向产生拉伸应力。在有限元软件 ANSYS 的粘塑性材料模型中实现该应力，我们表明，对于线性偏振光，正方形偶氮聚合物柱会沿电场矢量伸长，而对于圆偏振光，它会沿传播方向收缩。这些粘塑性材料模型的结果与微尺度正方形和圆柱形柱的光致形状变化实验相符。因此，这种取向方法对于均匀照明效果很好。我们讨论了如何使用这种方法来描述复杂光干涉图案引起的表面变形。
