Holographic Polymer/liquid Crystal Composites Containing Core-shell Upconversion Nanoparticles

Holographic polymer/liquid crystal(LC)composites containing upconversion nanoparticles(UCNP)are periodically structure-ordered polymer composites,which are fabricated through photopolymerization induced phase separation upon exposure to laser interference patterns.They show a great potential for advanced anti-counterfeiting due to that not only holographic images can be reconstructed but also they can show attractive upconversion photoluminescence.Nevertheless,it still remains challenging to boost the upconversion emission and to enable controllable upconversion emissive images.In this thesis,the core-shell UCNP with controllable surface properties were synthesized,and their effects on the structure and properties of holographic polymer/LC composites were systematically investigated.Eventually,holographic polymer/LC/UCNP composites were fabricated that can display bright holographic images and unconversion emissive images.First,three core-shell UCNP in the size of 12.8±2.2 nm but with different emission colors(e.g.,red,green and blue)were synthesized.The ligand on the UCNP surface was tuned upon treatment with hydrochloric acid(HCl),and it was found that the mass ratio of oleic acid to water(2.41?7.65)on the UCNP surface was critical to form a homogeneous dispersion of UCNP in the monomer.The homogeneous dispersion was the prerequisite for successful holographic patterning.Holographic polymer composites with both high diffraction efficiency and great upconversion emission were simultaneously achieved when loading 20 wt%of the UCNP.Second,by coating a SiO₂ layer in the thickness of 7 nm on the UCNP surface,the UCNP@SiO₂with the blue color emission capability and the methylacrylate group on the surface was synthesized,during which fluorescein isothiocyanate(FITC)was in-situ incorporated.Thus,the emission color of the UCNP@SiO₂could be tuned from blue to yellow green by the F?ster resonance energy transfer from the UCNP to FITC.Holographic polymer/LC/UCNP@SiO₂composites with both high diffraction efficiency and great upconversion emission were achieved by loading 15 wt%of UCNP@SiO₂.Finally,subsequent to the reconstruction of holographic images,upconversion emissive images were patterned in the holographic polymer composites by selectively photobleaching the FITC through a photomask under UV light.The reconstructed holographic image and upconversion emissive image can be identifiable by the naked eye under visible light and 980 nm near-infrared laser,respectively,without noticable crosstalk,which are believed to improve the security of holographic polymer composites for anti-counterfeiting.