Study On Upconversion Materials-assisted Near-infrared Photoinitiated Mechanism And High-efficiency Photoinitiation System

Photopolymerization technology using ultraviolet light,electron beam and near-infrared light,etc.as energy sources has the characteristics of high efficiency,energy saving,environment protection,and time-space controllability.Among them,near-infrared（NIR）photopolymerization technology has received extensive attention in the research of biomedical materials,coating materials and multi-material additive manufacturing due to the high penetrability and low biotoxicity of near-infrared light.Upconversion materials（UCm）can emit ultraviolet-visible light through upconversion process after absorbing near-infrared light,thereby exciting the photoinitiator to generate active species to induce the photopolymerization of the photosensitive materials.The upconversion material-assisted photopolymerization technology（UCAP）based on the above process is applicable and versatile for commercialized ultraviolet photopolymerization material systems and has a wide selection of raw materials.In recent years,it has been extensively used in the fields of dental materials,deep photopolymerization,functional polymer synthesis and additive manufacturing.However,the upconversion efficiency of UCm,the thermal effect of near-infrared light and its decay behavior in photosensitive materials will all have important impacts on the photoinitiated process of UCAP.The illustration of corresponding photoinitiated mechanism will effectively promote the UCAP photoinitiation-system research,materials-performance regulation and application-technology innovation.Based on the above background,this dessertation took aims to investigate the free radical photoinitiated system of the upconversion materials,and explored its photoinitiation mechanism under near-infrared light irradiation.The upconversion material-assisted near-infrared photopolymerization processes of filler-containing and black pigments-containing systems were further studied.And an efficient near-infrared photoinitiated system was successfully designed and synthesized,which improved the photopolymerization efficiency of UCAP.The specific research includes the following parts:1.Study on the photoinitiated mechanism of upconversion materials-assisted near-infrared photopolymerizationThe upconversion materials（UCm）were selected to form near-infrared photoinitiation systems with commercial type I and type II radical photoinitiators.The spectral matching between UCm and the photoinitiators was determined by fluorescence（FL）and ultraviolet-visible（UV-vis）spectroscopy.The photolysis kinetics and polymerization kinetics were used to characterize the initiation activity and polymerization ability of the photoinitiated systems,respectively.An evaluation method for the UCAP photoinitiated system was established,and the UCAP photoinitiated mechanism was proved.The research results showed that the UCAP photoinitiated system realized the efficient polymerization of photosensitive materials under the light-heat synergy of near-infrared light,and the photoinitiated system releases free radical active species after absorbing near-infrared light,and the accompanying thermal effect effectively improved the initiating activity and polymerization ability of the species.In addition,the thermal effect delayed the generation of the polymerized gel point of the photosensitive materials,which enabled lower polymerization stress and better mechanical properties.2.Study on upconversion material-assisted near-infrared photopolymerization processes of filler-containing systemsBased on the photosensitive material system containing silica and glass fiber as the research object and Beer-Lambert law,the near-infrared light intensity meter was used to measure and fit the light-attenuation gradient,and the effect of filler components on the near-infrared light attenuation behavior was studied.Further characterization of filler size,polymerization kinetics and materials properties by scanning electron microscopy（SEM）,attenuated total reflection infrared spectroscopy（ATR-FTIR）and nanoindentation tests revealed the influence of filler components on the photo-initiated and polymerized processes of UCAP.The research results showed that the spontaneous aggregation caused by the increase of filler content can improve the utilization rate of near-infrared light,and this filling aggregation-induced extinction mechanism can keep the polymerization system relatively stable near-infrared light penetration and effective photopolymerization depth.The scattering and refraction of the fillers on the near-infrared light can enhance the 15%of polymerization conversion and 30%of the mechanical properties of the local micro region in the cross-section of the system.Based on the regulation effect of fillers on the distribution of near-infrared light intensity,a variety of gradient materials were established and a UCAP manufacturing method of 92 wt.%ultra-high filler content materials was realized.3.Study on upconversion material-assisted near-infrared photopolymerization processes of black pigments-containing systemsPhotosensitive material system contained black pigments as the research object,and carried out the research on the photopolymerization process of UCAP.Melanin was synthesized by oxidized dopamine,and the attenuation degree of near-infrared light in the system of melanin and carbon black was studied by FL and UV-vis spectroscopies.The research results showed that the synthesized melanin has a low NIR/UV absorption ratio,which provided a"light transmission window"for UCAP.According to this,the direct photopolymerization of centimeter-thick black photosensitive materials was successfully achieved,and the curing depth reached 23 mm at 0.5 wt.%melanin content.A prediction model of the maximum curing depth of melanin-containing photosensitive materials under different near-infrared light doses was established,which overcame the difficulty of photopolymerization of traditional black photosensitive material systems.The 3D printing technology of macroscale black photosensitive materials was further established by UCAP,and it was found that the UV absorption characteristics and hydrogen bond enhancement of melanin can improve the aging resistance and mechanical properties of the cured materials.In addition,after 1000 h of aging experiments,the performance of photosensitive materials with0.5 wt.%melanin is basically unchanged.4.Research on synthesis and properties of a novel high-efficiency near-infrared photoinitiation systemIn order to further improve the utilization efficiency of near-infrared light,a novel high-efficiency near-infrared photoinitiation system was developed based on the UCAP photoinitiation mechanism.A series of macromolecular photoinitiators,polyoxime-ester coumarin（POEC）,with different molecular weights were synthesized by mercapto-alkyne click chemistry from oxime-ester coumarin monomers with triple bonds and dithiols,and then they were connected to double-bond-modified upconversion materials through mercapto-ene coupling reaction.The core-shell structure of the near-infrared photoinitiated system UCm@POEC was prepared.The successful synthesis of UCm@POEC was confirmed by UV-vis spectroscopy,IR spectroscopy,transmission electron microscope（TEM）,nuclear magnetic resonance（NMR）,and energy dispersive X-ray spectroscopy（EDAX）.The grafting rate of UCm@POEC was measured by UV standard curve and thermogravimetric analysis（TGA）,and the photolysis kinetics and photopolymerization kinetics of UCm@POEC were systematically explored.The research results showed that the incorporation of macromolecular photoinitiator effectively reduced the energy transfer distance between UCm and photoinitiator,and increased the spatial density of active groups on the surface of UCm,thus increasing the utilization of light energy effectively.Compared with the physically blended UCm&POEC system,the increased light energy utilization efficiency of the UCm@POEC system effectively increased the final conversion by 16.6%,which improved the photoinitiation and polymerization efficiency.UCm@POEC photoinitiation system greatly improved the printing throughput of near-infrared photocuring DIW printing,reaching1017 mm³ min^-1,showing its application potential of UCAP technology.