Influence of side-chain structure and irradiation condition on photoalignment of ladder-like polysiloxane films

In this paper we consider the photo-induced aligning capability of various ladder-like polysiloxane-based photoalignment films-which could be used in liquid-crystal displays-bearing different photoreactive side chains, i.e., laterally grafted cinnamate/azobenzene-based dual photoreactive side chains with a short or longer spacer, and terminally fixed coumarin-containing side chains. Results from polarized optical microscopy (POM), Fourier-transform infrared (FTIR) spectroscopy, surface-enhanced Raman scattering (SERS), atomic force microscopy (AFM), etc., are integrated to elucidate the influence of side-chain structure and the irradiation conditions on the photoalignment of ladder-like polysiloxane films. It is demonstrated that the film containing the dual photoreactive group with a longer spacer exhibits better alignment properties. Reasonably, the concerted photoreactions of the dual photoreactive group and the longer spacer are beneficial to the cooperative motion of chromophores at the "domain level", resulting in improved alignment facility and stability. The complicated effects of irradiation conditions and moderate annealing are also discussed. High-quality alignment of the polysilsesquioxane (LPS)-based photoalignment film LPS-CA11. with a longer spacer between the LPS main chain and cinnamoyl/azobenzene side chains can be achieved only within an optimal range of exposure (5-8 J cm(-2)), while the pretilt angles can be adjusted in the range 0.5degrees-7degrees by varying the incident light intensity. Additionally, moderate annealing before and after illumination can markedly improve the alignment uniformity by self-healing of defects.