Saphiannikova Marina, Neher Dieter
Institute of Physics, University of Potsdam, Am Neuen Palais 10, 14469 Potsdam, Germany.
J Phys Chem B. 2005 Oct 20;109(41):19428-36. doi: 10.1021/jp053249h.
It was discovered 10 years ago that the exposure of an initially flat layer of an azobenzene-containing polymer to an inhomogeneous light pattern leads to the formation of surface relief structures, accompanied by a mass transport over several micrometers. However, the driving force of this process is still unclear. We propose a new thermodynamic approach that explains a number of experimental findings including the light-induced deformation of free-standing films and the formation of surface relief gratings for main inscription geometries. Our basic assumption is that under homogeneous illumination, an initially isotropic sample should stretch itself along the polarization direction to compensate the entropy decrease produced by the photoinduced reorientation of azobenzene chromophores. The magnitude of the elastic stress, estimated by taking the derivative of the free energy over the sample deformation, is shown to be sufficient to induce plastic deformation of the polymer film. Orientational distributions of chromophores predicted by our model are compared with those deduced from Raman intensity measurements.
十年前人们发现，将含偶氮苯聚合物的初始平整层暴露于非均匀光图案下会导致表面浮雕结构的形成，并伴有数微米范围内的质量传输。然而，这一过程的驱动力仍不明确。我们提出了一种新的热力学方法，该方法解释了许多实验结果，包括独立薄膜的光致变形以及主要刻写几何形状下表面浮雕光栅的形成。我们的基本假设是，在均匀光照下，初始各向同性的样品应沿偏振方向拉伸自身，以补偿偶氮苯发色团光致重新取向产生的熵减。通过对样品变形时自由能求导估算出的弹性应力大小，足以引起聚合物薄膜的塑性变形。我们将模型预测的发色团取向分布与从拉曼强度测量推导得出的分布进行了比较。
