Charge Transfer Complexes as Pan-Scaled Photoinitiating Systems

Dec 20, 2017 - Photopolymerization profiles (methacrylate C═C function conversion vs irradiation time) measured in RT-FTIR for the resin 1; 1.4 mm t...
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Charge Transfer Complexes as Pan-Scaled Photoinitiating Systems: From 50 μm 3D Printed Polymers at 405 nm to Extremely Deep Photopolymerization (31 cm) Patxi Garra, Bernadette Graff, Fabrice Morlet-Savary, Céline Dietlin, Jean-Michel Becht, Jean-Pierre Fouassier, and Jacques Lalevée* Institut de Science des Matériaux de Mulhouse IS2M, UMR CNRS 7361, UHA, 15, rue Jean Starcky, Cedex 68057 Mulhouse, France S Supporting Information *

ABSTRACT: Charge transfer complexes (CTC) between Naromatic amines (donors) and iodonium salts (acceptors) are used here as photoinitiating systems (PIS) for the polymerization of clear methacrylate formulations under a 405 nm LED irradiation. Outstandingly, a complete spatial and temporal resolution is kept for 50 μm resolved 3D printed photopolymers at 405 nm (50 μm being the size of the printing laser used here). Photocuring of a high thickness (31 cm) is also possible. The photopolymerization propagation is rationalized and interpreted from both experimental (using thermal imaging experiments) and predicted data. An experimental/molecular modeling study also attempts to rationalize the CTC structure/reactivity/efficiency relationships. These systems are commercially available, stable, and metal-free and have a low toxicity. of these compounds, the photocuring of 2.9 cm thick filled materials and ∼10−15 cm thick clear varnishes has been claimed. Nevertheless, long irradiation times and UV irradiation sources were necessary for the curing of very thick samples thanks to the photobleaching observed with acylphosphine oxides (i.e., 25 min to cure a 8.5 cm of clear acrylate formulation21). Developing systems active under near-infrared irradiation sources22−26 is a recent answer (to light penetration issues in photopolymerization) particularly relevant when fillers are causing some light diffusion enhancing inner filter effect, e.g., in composites.27−30 Very recently, another advanced report stated the use of upconversion particles to produce the free radical photopolymerization of acrylate samples up to 13.7 cm.31 These upconversion particles (0.3 wt % particles in the resin) re-emitted blue light (allowing photoinitiation by a titanocene photoinitiator) upon a 980 nm high-intensity (9.4 W/cm2) laser beam. The evaluation of the spatial control of the method was not presented. Other recent different strategies involve the presence of latent species created under a primary irradiation that will diffuse through the entire polymerizable media, e.g., in controlled photopolymerization under air,32−36 UV-induced photopolymerization triggered by photocaged amines,37−39 a process based on a diffusion by a leuco form of the dye,40 or in

1. INTRODUCTION Light-induced reactions are very interesting as they require small energy amounts for reactions at mild temperatures without significant emissions of VOC. Most of the time, they result in excellent spatial and temporal controls. Nevertheless, a huge limitation of them is the inner filter effect: light is mainly absorbed by layers close to the light source, and a deep light penetration is not possible (as due to the Beer−Lambert law1). As a result, most applications of photochemistry are occurring close to the surface receiving the actinic light, for example in solar cells2 or in continuous-flow photochemistry.3 This is particularly true for free radical photopolymerization:4−12 most of the current applications are often limited to thicknesses 55% in the first 4.5 cm and slightly decreases between 4.5 and 9 cm (43% at 8.5 cm). The conversions obtained for this multifunctional resin 2 are comparable with the 1.4 mm samples photocured by a commercial PIS implemented with additives (50% with a four-component system camphorquinone (CQ)/Am3/Iod1/ J

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Figure 7. Photopolymerization experiments, in resin 2, under air. Initiating system: Am2 = 0.3 wt %/Iod2 = 0.9 wt %/4dpps = 2.0 wt %. (A) 2D evolution of the temperature vs irradiation time followed by infrared thermal imaging camera (Fluke TiX500). Irradiation at t = 0 s by LED@405 nm (1.1 W/cm2). (A′) 31 cm thick sample before breaking the graduated pipet. (A″) 31 cm thick sample obtained after breaking the surrounding glass. (B) Raman conversion as a function of the distance from the top surface (irradiated zone). (C) Profilometry of the photopolymer obtained from laser (405 nm) writing of an about 200 μm layer (see further images in Figure S4).

of interest to simulate more accurately light penetration for new sizes of samples up to several tens of centimeters. 3.4.2. 31 cm Deep Samples. It appeared (Figures 5 and 6) that a higher light irradiance allows deeper and faster photopolymerization (different parameters influencing the depth of cure are given in a recent review14). Therefore, in Figure 7A, a more powerful LED@405 nm was used with irradiance of 1.1 W/cm2 at the surface of the sample. Resin 2 was chosen as the propagation was faster than in resin 1 (Figure 6A). Impressively, the photopolymerization propagation was possible throughout the entire 31 cm thick resin formulation (containing Am2/Iod2/4dpps) after 416 s (Figures 7A′ and 7A″). About 15.5 cm already exhibited an exothermicity after only 93 s! The reached temperatures are slightly lower than in Figure 5 (likely as a result of a smaller inner diameter of the mold (Pasteur pipet) that contains the resin). The conversion throughout the entire sample slowly decreases from 56% at the surface to 46% at 30.5 cm (Figure 7B). These conversions are close to those obtained in thinner samples (50−55% for 1.4 mm samples cured by reference PIS; see above). The 31 cm thick performance is outstanding as the previous reference stated 13.7 cm thick clear acrylate polymer for a 9000 mW/cm2 irradiation (about 8 times the LED@405 nm light intensity) for 10 min (slower than the present report with 7 min for 31 cm).31

compared to 6 cm after 871 s (resin 1). As expected, for both resins, photopolymerization is faster close to the light source (particularly the 0−4 cm range) and gradually slower in depth, where penetrating irradiances are gradually lower. There is also a visible light absorption due to [Am2− Iod2]CTC in resin 1 as in DCM (Figure 6B). The original optical density at 405 nm (0.13 in a 1 cm cell) can be bleached: after 1 min 30 s, the remaining absorption is coming from the resin 1 itself. Finally, from the Abs@405 nm (see eq 1), the predicted light penetration (irradiance for a given depth) of the LED@405 nm is compared to the time to reach the maximum temperature in resin 1 (Figure 6C). For all the depths characterized by an irradiance >75 mW/cm2, the polymerization is achieved in less than 300 s. For lower irradiances, a longer irradiation time is necessary. In Figure 4C, the maximum conversion (and thus temperature) was reached after 92 s under a 65 mW/cm2 irradiation using higher concentrations of the reactants (1 wt %/2 wt %/1.5 wt % vs 0.3 wt %/0.9 wt %/2.0 wt %). These results are consistent: the most concentrated formulation reaches a maximum exothermicity faster. More complex models of light penetration taking in account other issues of light diffusion (for example, due to shrinkage and formation of oxygen bubbles; see previous report44), but also other phenomena such as photobleaching should likely be K

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3.4.3. Laser Write Experiments: Micrometric Resolved (50 μm) Photopolymerization. Finally, the same formulation than for the extremely deep sample was used in a laser write application in order to check the spatial control (Figure 7C and Figure S4). A well-resolved pattern of about 200 μm thick was obtained by free radical photopolymerization which shows that the system is still able to face oxygen inhibition for application in coatings or in 3D printing resins. Also, the step width (Figure S4B) between the unpolymerized and polymerized resin was of about 54 μm, which is close to the actual size of the laser beam (50 μm) showing a very good spatial resolution.

ACKNOWLEDGMENTS ̈ Bonardi The authors thank Fabien Bonardi and Aude-Héloise for the programming of the Python script described in section 2.4 . The authors thank the Agence Nationale de la Recherche for the funding of the project “FASTPRINTING”. This work was granted access to the HPC resources of the Mesocentre of the University of Strasbourg.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b02185. Figure S1: ESR spectra in toluene/DCM (75/25 v/v) under air after 20 s irradiation by LED@405 nm of an Am1/Iod1/phosphine; Figure S2: photopolymerization profiles (methacrylate CC function conversion vs irradiation time) measured in RT-FTIR for the resin 1, 1.4 mm thick samples, in air, in resin 1; Figure S3: 9 cm deep photopolymerization experiments, in air, upon LED@405 nm irradiation; Figure S4: numerical optical microscope (DSX-HRSU) observations of 3D photopolymer obtained by laser write experiments (PDF)

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REFERENCES

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4. CONCLUSION Here, the interaction between electron donors (mostly Naromatic amines) and electron acceptors (mostly iodonium salts) was studied. It appears that when these two types of molecules get closer, they generate a charge transfer complex (CTC) characterized by longer wavelength absorption properties. The structure/reactivity/efficiency relationships of the different CTCs were studied from both experiments and molecular modeling data. Such CTCs show very interesting photopolymerization initiating capabilities, particularly for the photopolymerization of extremely deep samples (9 cm using a LED@405 nm at 230 mW/cm2). The photopolymerization propagation was studied from thermal imaging data allowing rationalizing the propagation/light penetration relationships from experimental data. Outstandingly, a photopolymerizable methacrylate resin containing an electron donor, an electron acceptor, and a phosphine derivative was able to produce 31 cm thick clear polymer materials (LED@405 nm (1.1 W/cm2)) and also 50 μm resolved 3D printed polymers in laser write experiments (405 nm laser). The performance of these stable CTCs based on metal-free, low toxic, commercially available compounds is attributed to the low optical density of the absorbing species coupled with an outstanding reactivity of the PIS.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (J.L.). ORCID

Jacques Lalevée: 0000-0001-9297-0335 Notes

The authors declare no competing financial interest. L

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