3D–printing AIE stereolithography resins with real−time monitored printing process to fabricate fluorescent objects
Graphical abstract
Introduction
3D printing as known as additive manufacturing, refers to the printing of three−dimensional objects from computer aided design, usually by continuously adding material layer by layer [1]. In the past decade, digital light processing (DLP) (Fig. 1 A) has been used to produce models, patterns and production parts [2,3]. During the printing process, the stereolithography resin (SLR) was polymerized by an ultraviolet (UV) laser or other light sources [4]. New 2D layers were successively printed on top of previous layers, a coherent 3D object was then finally fabricated [5]. So, it is imagined that the visualization of the process of 3D printing is very important for artificial intelligence manufacturing [6], bio−printing [7], aerospace components [6,8], manufacturing and tooling [9,10]. However, monitoring the process of 3D printing remains as a formidable challenge.
Aggregation−induced emission (AIE) luminogens emit intense fluorescence in aggregated state or in solid state, which was pioneered by Tang et al. [11,12] These AIE molecules exhibit stronger fluorescence as the concentration is increased [13,14]. A variety of AIE luminogens have been discovered or prepared. In addition, a great deal of efforts have also been directed to elucidate the mechanism of AIE effects, such as restriction of intramolecular rotations (RIR) [15], restriction of intramolecular vibrations (RIV) [16], and restriction of intramolecular motions (RIM) [15], twisted intramolecular charge transfer (TICT) [12,16] and excited−state intramolecular proton transfer (ESIPT) [17,18]. In contrast to AIE probes, traditional fluorescent dyes such as pyrene, perylene, coumarin, rhodamine and 1,1,2,3,4,5−hexaphenylsilole (HPS) usually exhibit significant fluorescence in solution, not in a solid state due to aggregation−caused quenching (ACQ) phenomenon [11,13,19]. Kim et al. [20] first utilized the pyrene as a fluorescent probe to detect the glass transition temperature (Tg) of the polymers. This method was hindered by weak fluorescence emission, because pyrene is an ACQ luminogen [21,22]. In recent decades, the applications of AIE luminogens have been explored in various areas, such as fluorescent probes [23,24], bio−imaging [25], optoelectronic devices [26] and so on [27,28]. Bao et al. [23] has successfully doped polystyrene (PS) with an AIE luminogen tetraphenylethene (TPE), which acts as a fluorescent probe to detect the Tg of PS based on increased intensity of fluorescence below Tg because of the restriction of intramolecular rotations. Our research group [24] reported a facile measurement of the intrinsic topology freezing transition temperature (Tv) of vitrimers, which was difficult to measure before. Taking advantage of the high quantum yield of AIE luminogens, the Tv of polyurethane vitrimer has been determined. In addition, AIE luminogens have been frequently used as fluorescent markers to study the properties of the polymer materials. Li et al. [28] reported that the functional groups in epoxy resin could be opened by benzylamino of 2CH2NH2–TPE according to electrophilic addition mechanism. The AIE luminogens trapped inside the polymer network emit fluorescence as the motions are restricted. To the best of our knowledge, SLR combined with AIE molecules and used as 3D printing material has not been reported before.
Inspired by these work, here we present two different kinds of AIE photo-curable resins for the first time, the 3D printing AIE stereolithography material system that allow us to monitor the entire printing process. The thermoset photo–curable resin with fluorescence after solidifying is synthesized by covalent crosslinking of tetrakis (4–aminophenyl) ethene, bisphenol F epoxy resin (BPF) and methylacrylic acid (MAA). The thermoplastic resin with “ON–OFF” fluorescence response is also reported. The AIE luminogens are just simply mixed with polymers rather than covalently crosslinking. The models printed by the DLP 3D printer with these two AIE photopolymers not only possess outstanding mechanical properties [29], but also provide visual warnings based on the changes of the fluorescence in industrial production. Therefore, those two different kinds of AIE stereolithography resins are applicable for 3D printing.
Section snippets
Materials
Bisphenol F epoxy resin was bought from Jiangyin Nanhui Chemical New Material Factory (Jiangsu province, China). Tetrakis (4–aminophenyl)ethene (TPE−4NH2) was purchased from Yanshen Technology Co., Ltd. (Jilin province, China). Hydroquinone, triphenylphosphine, phenylbis (2,4,6–trimethylbenzoyl) phosphine oxide (PI 819), diphenyl (2,4,6–trimethylbenzoyl) phosphine oxide (PI TPO), 2–(2–ethoxyethoxy)ethyl acrylate, salicylaldehyde, N,N–dimethyl–1,4–phenylenediamine and benzophenone were bought
Effect of the curing time to the fluorescence intensity
In order to figure out the relationship between the curing time and photoluminescence intensity, the equation (1) similar to adsorption kinetic is used. A time–dependent process of curing shown in Fig. 2 and Fig. S7 shows the change of the fluorescence intensity during the curing process, the letters “THU” (the abbreviation of Tsinghua University) are invisible in fluid phase state under the 365 UV laser and are visible after curing. The AIE photo–curable resins in the liquid state do not emit
Conclusion
In summary, two different kinds of AIE photopolymers are prepared, and are successfully used in 3D printing field. The AIE molecules are covalently crosslinked into the thermoset photo−curable resin, which is different from the fluorescent thermoplastic photo−curable resin doped AIE molecules. Furthermore, the fluorescence emission of the 3D printing of photopolymers can be continuously monitored for the first time. Before printing, the solution is non−fluorescent, during the printing process,
CRediT authorship contribution statement
Jiujiang Ji: Conceptualization, Methodology, Software, Investigation, Writing - original draft. Mengshi Wang: Validation, Formal analysis. Miaomiao Hu: Software, Design the 3D printing model. Liucheng Mao: Software, Visualization. Qigang Wang: Curing mechanism analyses, Writing - review & editing. Wuyi Zhou: Writing - review & editing. Mei Tian: Writing - review & editing. Jinying Yuan: Conceptualization, Methodology, Writing - review & editing. Kun Hu: Conceptualization, Writing - review &
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors thank the following programs for the financial support: The National Science Foundation of China (No. 21788102). Science and Technology Major Project Foundation of Fujian Province (No.2015YZ0003) and Beijing Municipal Education Commission Scientific Research Project Funding−3D printing composite gel material for cartilage repair research (No.KM201910015009). We are most grateful to Dr. Changshi Lao of Shenzhen Longer Technology Co., Ltd for providing the Equipment, to Prof. Xiaosong
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