Study On The Luminance Efficiency Modulation Mechanism Of Rare Earth Upconversion Particles And Near-infrared Photopolymerization

Near-infrared（NIR）photopolymerization technology uses long-wave NIR light as the irradiation source to initiate the cross-linking reaction of photosensitive resins.The high penetration of NIR light and the low photodamage caused to materials have drawn widespread attention in fields of functional polymer synthesis,composite materials and biomedicine.Upconversion particle-assisted near-infrared photopolymerization（UCAP）has introduced rare-earth doped upconversion particles（UCPs）into photosensitive resins/monomers to induce photopolymerization reactions,using the unique anti-Stokes luminescence property of UCPs to convert the absorbed NIR light into UV-visible light.UCAP has a wide selection of photosensitive systems and is highly versatile.Besides,UCPs have the advantages of high physical/chemical stability,large anti-Stokes shift,designable structure and rich emission spectras.Therefore,UCAP has a promising application in deep light curing,filler systems,additive manufacturing and biosafety.UCPs,as the internal light source of UCAP,is an important part of the photoinitiated system,so it is of great importance to investigate the influence of its basic properties（morphology,crystalline shape,luminous intensity and exotherm）on the polymerization behavior to promote the application of UCAP technology in fields of high-performance materials and functional materials.Based on the above background,this thesis systematically investigated the effect of synthesis parameters of hydrothermal method on the basic properties of upconversion particles which is demonstrated to be well adapted to the UCAP photoinitiation system.Subsequently,the connection between the basic properties of UCPs and the polymerization behavior of UCAP was investigated thoroughly.The study was divided into the following parts:（1）Study on luminance efficiency modulation of upconversion particlesThe first part of the thesis focuses on the investigation of the hydrothermal synthesis parameters and the property modulation factors of the upconversion particles.A series of Na Yb F₄:Tm³⁺upconversion particles were prepared by adjusting the Na F concentration,EDTA concentration,Tm³⁺doping concentration and hydrothermal reaction temperature.The UCPs were characterized by SEM,XRD and fluorescence spectrometer in terms of morphology,crystal shape and upconversion luminescence property.UCPs with best luminous intensity were selected and applied to the UCAP process.Then the effects of photoinitiators（types and concentration）,UCPs concentration and irradiation power on polymerization were investigated.The results of the study are as follows:The increase of Na F concentration will promote the crystalline transformation（cubic phase to hexagonal phase）and increase the particle aspect ratio,the increase of EDTA concentration will decrease the particle size（11.27μm→2.12μm）,and the increase of hydrothermal reaction temperature will improve the crystallinity of UCPs.The synthesis route of UCPs with the highest luminous intensity was:Na F-0.171 mmol·m L^-1,EDTA-0.014 mmol·m L^-1,Tm³⁺-0.5%,220°C.Irgacure 819（photoinitiator）is most suitable for upconversion particle-assisted photopolymerization systems,with an optimal addition concentration of 2 wt.%and 3 wt.%for UCPs,and the irradiated power of 30 W/cm² is nearly saturated for UCAP.（2）Study on the size dependence of upconversion particle luminance efficiency and its materials properties of near infrared photopolymerizationIn the second part of the thesis,upconversion particles with different sizes were prepared and the size dependence of the upconversion luminescence was analyzed using fluorescence spectroscopy.The macroscopic polymerization behavior of UCAP was characterized by a real-time IR-rheological platform,and the double bond conversion rate distribution around the upconversion particles and the polymer growth process were examined by micro-Raman to investigate the influence patterns of particle size on the polymerization process and mechanical properties of photosensitive materials.The results of the study are as follows:The size of UCPs significantly affects the conversion rate at gel point(C_gel)of UCAP,which increases from 17%to 33%as the particle size increases from 5μm to 36μm,while C_gel is found to show an increasing and then decreasing trend with the increase of power（10 W→35 W）or particle concentration（0.1 wt.%→0.9 wt.%）.Under the same irradiation environment,the average conversion rate around one single particle increases（16.7%→71.7%）with the increasing size and all shows a gradient distribution.The increased size of UCPs facilitates the release of shrinkage stresses in the corresponding polymerization system,thus giving the material higher tensile（49.2 MPa）properties and compressive properties（41.15 MPa）.（3）Study on photothermal synergy effect of upconversion particles-assisted photopolymerization and its modulationIn the third part of the thesis,we introduce the thermal initiator into the UCAP system,aiming to utilize the thermal effect of UCAP to achieve photothermal co-polymerization.The effects of sensitizer doping in UCPs on their photothermal conversion efficiency and luminous intensity were characterized by thermal imaging and fluorescence spectroscopy,respectively,and the polymerization kinetics of photosensitized materials under the synergistic effect of light and heat were systematically investigated.The results of the study are as follows:As the sensitizer content increased（20%→80%）,the luminous intensity of UCPs increased,the intensity ratio of UV emission to blue emission increased（0.09→0.78）,and the photothermal conversion efficiency of UCPs increased（2.085→2.608）.After screening commercial thermal initiators,ABVN was found to be the most suitable for upconversion particle-assisted photothermal co-polymerization system,with a decomposition temperature of 89.4^oC at a heating rate of 30^oC/min.Compared with the pure photopolymerization process,the photothermal co-polymerization has a higher polymerization rate;with the increase of sensitizer doping in UCPs,the improve of photothermal conversion efficiency of the particles and polymerization heat release have accelerated the heating rate of the system,which in turn induces the decomposition of the thermal initiator more efficiently,and the photo-thermal co-polymerization can achieve higher polymerization rate at lower NIR light intensity.