Photodeformable Crosslinked Liquid-crystalline Polymers Via Upconversion Based On Triplet-Triplet Annihilation

Photodeformable polymeric materials have been a research focus recently because of their capacity to directly convert light energy into mechanical work. Among these materials, cross-linked liquid-crystal polymers (CLCPs) may represent one of the most studied systems because they exhibit such unique properties as elasticity, anisotropy, stimulus-responsiveness, and molecular-cooperation effects. Recently, in order to extend the use of light-responsive CLCPs in practical applications and possible future biological systems, long wavelength light, such as red light or near-infrared light which penetrate deeper into tissues and has less damage to biosamples, should be a more suitable stimulating source compared to UV and short-wavelength visible light. Therefore, developing the long-wavelength light responsive CLCPs is a very interesting and important research topic nowadays. In this thesis, in one hand, through the utilization of the triplet-triplet annihilation-based upconversion luminescence (TTA-UCL), we have prepared a novel red-light-controllable CLCP driven by low-power excited TTA-UCL. On the other hand, by blending TTA materials、liquid crystalline polymers and polyurethane, a novel oriented blend polymer film with excellent mechanical proformance and controlled structure and composition has been developed. We demonstrate the light-induced bending of oriented blend polymer film using using light of different wavelengths. The main results are as follows:1) To prepare red-light-controllable CLCPs driven by low-power excited TTA-UCL, firstly, a red-to-blue TTA based upconversion system with a high absolute quantum yield of 9.3±0.5% was prepared by utilizing platinum (Ⅱ) tetraphenyltetrabenzoporphyrin (PtTPBP) as the sensitizer and 9,10-bis(diphenylphosphoryl)anthracene (BDPPA) as the annihilator. Its UCL property, photostability and chemical performance were evaluated by the UV-vis absorption spectra, UCL emission spectra and kinetic spectroscopy scan. The results revealed that the upconversion system based on TTA used in our study had a very high absolute UCL quantum yield of 9.3±0.5%, as well as the excellent photostability and high chemical stability. The PtTPBP&BDPPA system was incorporated into a rubbery polyurethane film and then assembled with an azotolane-containing CLCP film. The generating assembly film bent toward the light source when irradiated with 635-nm laser at low power density of 200 mW cm-2. Importantly, it was also observed that the thermal effect caused by the 635-nm laser is minimal, and the assembly film could still bent towards the transmitted light source even when a piece of pork was put between the light source and the assembly film, because of the excellent penetration of red light into biological tissue. Therefore we believe that this red light controllable CLCP is promising for potencial biological applications. This work is not only a significant step forward to develop a novel red-light controllable CLCP based on TTA-UCL but also represents a new strategy on the technological applications of TTA-UCL in solid photonic devices.2) By blending TTA material liquid crystalline polymers and polyurethane, a novel oriented blend polymer film with excellent mechanical proformance and controlled structure and composition has been developed. We demonstrate the light-induced bending of oriented blend polymer film using using light of different. wavelengths. Firstly, a photoresponsive linear polymer containing cross-linkable groups was synthesized by using organic reactions and thermal radical polymerization, whose chemical structures were characterized by 1H-NMR, GPC, DSC, and POM. The results of DSC and POM analysis demonstrate that the linear polymer shows a LC phase in a certain temperature range. The blended film consisted of the linear polymer PMA11ABTN and polyurethane was conveniently prepared via casting technology and post-crosslinking reaction followed by mechanical stretch. The anisotropic orientation of mesogens was evaluated by POM and UV-Vis spectroscopy. The results show that the mesogens are preferentially oriented parallel to the mechanical stretching direction. It was also shown that the TTA-UCL emitted from PtTPBP&BDPPA causes the trans-cis photoisomerization of the azotolane moieties in the linear polymer through emission-reabsorption process and intermolecular energy transfer using UV-vis absorption spectra, UCL emission spectra and phosphorescence lifetime spectrometer. Then, the red light-responsive monodomain CLCP film was prepared by blending the linear polymer with PtTPBP&BDPPA and polyurethane, followed by casting, post-crossl inking reaction, and mechanical stretch. The resulted PtTPBP&BDPPA-containing blend film generated directional bending when irradiated with 635-nm light. Importantly, compared to other photoresponsive CLCPs reported in the previous literature, the red light-responsive CLCP via TTA-UCL could be readily integrated with functional materials, as well as facilitating the enhancement of mechanical properties. This approach provides an efficient way to functionalize the non-light responsive polyurethane with photoresponsive properties. Also, taking into account that the traditional polyurethane used in this study is relatively of low cost and the content of liquid crystal polymers is very low, we envision that this approach demonstrates a simple and economic way to prepare red light responsive soft actuators.