Photoinduced Thermal Polymerization Reactions - Macromolecules

Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

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Photoinduced Thermal Polymerization Reactions A.-H. Bonardi,†,‡ F. Bonardi,§ F. Morlet-Savary,†,‡ C. Dietlin,†,‡ G. Noirbent,∥ T. M. Grant,⊥ J.-P. Fouassier,†,‡,# F. Dumur,∥ B. H. Lessard,⊥ D. Gigmes,∥ and J. Lalevée*,†,‡ †

Université de Haute-Alsace, CNRS, IS2M UMR 7361, F-68100 Mulhouse, France Université de Strasbourg, Strasbourg, France § Laboratoire d’Informatique, de Traitement de l’Information et des Systèmes, Normandie Univ, UNIROUEN, UNIHAVRE, INSA Rouen, LITIS, 76000 Rouen, France ∥ Aix Marseille Univ, CNRS, ICR, UMR7273, F-13397 Marseille, France ⊥ Department of Chemical and Biological Engineering, University of Ottawa, 161 Louis Pasteur, Ottawa, Ontario K1N 6N5, Canada # Ecole Nationale Supérieure de Chimie de Mulhouse, Mulhouse, France

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‡

S Supporting Information *

ABSTRACT: The combination of thermally induced and photoinduced free radical polymerization of (meth)acrylic monomers has only been scarcely presented in the literature. In this study, a two-component system with a nearinfrared (NIR) dye combined with a thermal initiator is presented. The dye acts as a very efficient heat generator (heater) upon irradiation with NIR light. Several thermal initiators are presented such as an alkoxyamine (e.g., BlocBuilder-MA), azo derivatives, and (hydro)peroxides. The heat delivered by the dye dissociates the thermal initiator, which initiates the free radical polymerization of (meth)acrylates. Several types of heat generators are presented such as borate-based dyes and a silicon phthalocyanine derivative. For the first time, the effects of the NIR heater concentration, light intensity, and monomer structure on the heat released are studied using thermal imaging studies. NIR light curing is challenging but offers significant advantages: it is safer than shorter wavelength, and it allows a deeper penetration of the light and therefore a better curing of filled samples for a unique access to composites. Systems using a cyanine borate as a dye give high conversion rate of CC for methacrylate monomer under air. Two wavelengths of irradiation are studied: 785 and 850 nm. The presence of additives (phosphines or iodonium salts) can also improve the polymerization profiles.

I. INTRODUCTION Free radical polymerization (FRP) reactions are widely adopted for both academic and industrial production of polymers. There are different methods to perform free radical polymerization, through thermal initiation, redox initiation, or photoinitiation. Currently, the FRP technique is widespread: a vast amount of commercial polymers is produced via thermal radical polymerization, and their applications range from packaging, coatings, paints, rubbers, adhesives, household plastics, electronics, and more.1 Another interesting method of performing FRP is light-induced free radical polymerization (or photopolymerization). This method is relatively more recent: it was developed in the late 1960s2 and immediately found wide application in areas such as coatings, printing industry, dentistry, medicine, inks, paints, and more.3 Most photopolymerization reactions are processed under radiation in the ultraviolet (UV) range of wavelengths.2 These short wavelengths are well-known to be noxious for both eyes and skin of the operator.4 The development of safer systems that polymerized at longer wavelengths is, by consequence, crucial. However, the use of greater wavelengths results in less © XXXX American Chemical Society

energetic photons, and therefore the reaction is much more difficult to perform efficiently. Today, the number of systems that report the successful polymerization of monomers upon visible light irradiation has considerably increased. However, up to now, only a few systems cured upon near-infrared (NIR) irradiation.5−8 Because of the Rayleigh diffusion, NIR light for polymer synthesis has the main advantage to have a deeper penetration in the material (Figure S1 in the Supporting Information). Thus, the curing of thick and/or composite (filled) samples can also be greatly enhanced (better depth of cure) compared to curing with UV or visible light. NIR dyes have been extensively studied over the past decades and found various applications in many fields such as in the biological and medical fields9−19 but also in the photographic industry and the information recording.12 NIR dyes show light absorption between 780 and 1400 nm and involve several family of structures: phthalocyanines/naphReceived: August 13, 2018 Revised: October 13, 2018

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DOI: 10.1021/acs.macromol.8b01741 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Scheme 1. Orthogonal Approach between Photothermal vs Photochemical Redox Processes

Scheme 2. Different Investigated NIR Borate Dyes

conventional thermal processes. Moreover, this allows temporal and spatial resolutions not really accessible with classical thermal curing. Also, the energy consumption for this process is lower because of the low consumption of the irradiation devices used. Thermal curing leads also to emission of volatile organic compounds, which is limited by photopolymerization process. For photothermal curing, the process is also initiated by light irradiation, and all the advantages of photopolymerization can be conserved. Second, photothermal curing presented in this paper also regroups advantages of thermal curing over photopolymerization. Indeed, thermal polymerization is already characterized by a widespread use and with a long history in both academic and industrial fields. Thus, thermal initiators used for the curing are widespread and cheap. The photoinduced thermal polymerization is possible with these thermal initiators. To finish, another advantage of this process over “traditional” photocuring is the higher wavelength used. Curing of filled and/or thick samples is currently performed by thermal curing but can now be accessible with photoinduced thermal curing. The NIR dye acts as a light-to-heat convertor (heater) and transferred enough heat to the system to dissociate the thermal initiator into radicals that can initiate the polymerization. For the first time, the effects of the NIR heater concentration, light intensity, and monomer structure on the heat released are studied using thermal imaging experiments. Other red or NIR chromophore phthalocyanine and more specifically silicon

thalocyanine, quinone dyes, squarylium, diimonium, dithiolene complexes, and cyanines are among a few most common.12 Cyanines used as a photosensitizer to perform curing of the NIR photoinitiating system have already been published.6,13,14 In this first photochemical approach, the initiating radicals are generated by interaction of the excited states of the dye formed after light absorption with additives such as iodonium salts or triazines. However, the role of cyanine as a heat generator has been mentioned, i.e., to melt monomers in powder.5 Light-toheat conversion properties of cyanine are also mentioned in the medical field. In photothermal therapy, the organic compound is irradiated and released heat. The heat produced by the dye can induce hyperthermia of the cells and can treat cancer. It is also used for cell imaging.15−17 Therefore, to propose new NIR photopolymerization systems, it would be ideal to use this photothermal effect using a heat generating NIR dye which, upon irradiation, initiates the thermal polymerization. To our knowledge, such polymerization has never been performed using cyanine dyes but has been recently mentioned using carbon black nanoparticles.18 Polymerization with carbon black nanoparticles and cyanine dye will be compared in this study. This technique has been inspired from both photopolymerization and thermal polymerization and remarkably combined the advantages of both techniques. First, this can be a rapid process. Thanks to the on/off character of light, no preheating and no cooling are necessary and the curing can begin and be stopped whenever the user wants, which is not possible by B


Article

Macromolecules Scheme 3. Structures of Other NIR Dyes

Dichloro-11-(diphenylamino)-3,3′-diethyl-10,12-ethylenethiatricarbocyanine perchlorate (IR140), 2-[2-[2-chloro-3-[(1,3-dihydro-3,3-dimethyl-1-propyl-2H-indol-2-ylidene)ethylidene]-1-cyclohexen-1-yl]ethenyl]-3,3-dimethyl-1-propylindolium iodide (IR-780), and 2-[2-[2-chloro-3-[2-(1,3-dihydro-1,1,3-trimethyl-2H-benzo[e]indol-2-ylidene)-ethylidene]-1-cyclohexen-1-yl]-ethenyl]-1,1,3-trimethyl-1H-benzo[e]indolium p-toluenesulfonate (IR-813) were purchased from Aldrich. Mass spectroscopy was performed by the Spectropole of Aix-Marseille University. ESI mass spectral analyses were recorded with a 3200 QTRAP (Applied Biosystems SCIEX) mass spectrometer. The HRMS mass spectral analysis was performed with a QStar Elite (Applied Biosystems SCIEX) mass spectrometer. Elemental analyses were recorded with a Thermo Finnigan EA 1112 elemental analysis apparatus driven by the Eager 300 software. 1H and 13 C NMR spectra were determined at room temperature in 5 mm o.d. tubes on a Bruker Avance 400 spectrometer of the Spectropole: 1H (400 MHz) and 13C (100 MHz). The 1H chemical shifts were referenced to the solvent peaks CDCl3 (7.26 ppm) and DMSO (2.49 ppm), and the 13C chemical shifts were referenced to the solvent peaks CDCl3 (77 ppm) and DMSO (49.5 ppm). Lithium triphenylbutylborate was synthesized as previously reported in the literature without modifications and obtained in similar yields.14 The soft salt was synthesized as previously reported in the literature by using a biphasic mixture of CHCl3/water and THF to act as a phase transfer agent. The soft salt (the organic salt) was recovered in the organic phase and the inorganic one in the aqueous phase.20,21 From a synthetic point of view, metathesis reactions with NIR dyes21 have been performed with standard conditions classically used for light-sensitive materials. Work has been performed while protecting the fume hood from sunlight as well as the glasswares. All salts have been obtained as powders. Precipitation and filtration of the different precipitates have been performed in the dark. All molecules have been stored in the dark. Lithium triphenylbutylborate (0.770 mmol, 1.2 equiv) in water (20 mL) was added to a solution of IR-140 or IR-780 or IR-813 (0.642

phthalocyanine also show great light-to-heat conversion properties. The present article corresponds to an orthogonal approach compared to our previous work (Scheme 1).13 Indeed, in this latter work, the initiation mechanism corresponds to a redox process (activated by NIR light); no temperature change was observed in these systems. In the present approach, the NIR dye is not active in redox processes but releases heat upon light irradiation. These NIR dyes were called heaters. Remarkably, it will be possible to finely tune the temperature of the polymerizable resin (with the heater concentration or the NIR light intensity) to decompose a thermal initiator (Scheme 1). An outstandingly low concentration of heater will be necessary (0.01% in weight). As the chemical mechanisms are intrinsically different between the photochemically induced redox processes (previous work13) and photothermal approach, the NIR dye structure plays a key role; i.e., IR-140 was the most efficient dye in redox processes,13 but we will show now that IR-780 is a more efficient structure for the photothermal approach. A comparison of the different NIR dyes is provided. Remarkably, for NIR dyes that can be active in both redox and photothermal processes, a combined approach could also be used to still improve the polymerization initiating ability upon NIR mild irradiation conditions.

II. EXPERIMENTAL SECTION Synthesis of NIR Borate Dyes. The synthesis of the different NIR borate dyes (Scheme 2) has been adapted from our previous work.13 All reagents and solvents for the synthesis of IR-140 borate, IR-780 borate, and IR-813 borate were purchased from Aldrich or Alfa Aesar and used as received without further purification. 5,5′C


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Macromolecules Scheme 4. Chemical Structures of Thermal Free Radical Initiators

Scheme 5. Chemical Structures of Monomers

Commercial NIR Dyes and Other Photoinitiators. Indocyanine Green, IR-140 perchlorate, IR-780 iodide, and IR-813 ptoluenesulfonate (Scheme 3) were purchased from Sigma-Aldrich. S0507, S2544, and S2025 (Scheme 3) were obtained from Few Chemicals. Camphorquinone (CQ) and ethyldimethylaminobenzoate (EDB), used as reference two-component CQ/EDB photoinitiating system upon blue light, were purchased from Sigma-Aldrich with the highest purity available. Thermal Free Radical Initiators. Luperox P, Luperox 331M80, dicumyl peroxide, ammonium persulfate, 1,1′-azobis(cyclohexanecarbonitrile), tert-butyl peroxide, and cumene hydroperoxide (Scheme 4) were obtained from Sigma-Aldrich. Peroxan (50 wt % of benzoyl peroxide (BPO) (Scheme 4) on 50 wt % dicyclohexyl phthalate) and BlocBuilder MA were also studied. BlocBuilder MA can also be used for controlled radical polymerization (e.g., nitroxide-mediated polymerization).23 All percentages presented below correspond to weight ratio vs the monomer (abbreviated (wt %)).

mmol, 1 equiv) in a mixture of CHCl3 (100 mL) and THF (20 mL). The solution was stirred at room temperature while being protected from light for 1 h and then set aside for 10 min. THF was removed under reduced pressure (still while protecting the solution from light), and the solution was transferred in a separating funnel (covered with aluminum foil). The organic phase was separated and dried over magnesium sulfate, and the solvent removed under reduced pressure. Addition of THF (2 mL) followed by pentane precipitated a solid that was filtered off, washed several times with pentane, and dried under vacuum. HRMS (ESI MS) m/z: theor: 679.7428; found: 679.7426 (M+ detected). HRMS (ESI MS) m/z: theor: 299.2375; found: 299.2377 (M− detected). Anal. Calcd for C61H58BCl2N3S2: C, 74.8; H, 6.0; N, 4.3. Found: C, 74.6; H, 6.1; N, 4.4%. Synthesis of (OCT)2-SiPc was synthesized according to the literature and purified by train sublimation prior to polymerization experiments.13,22 D


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Macromolecules Monomers. “Mix-MA” is a mixture of monomers (Scheme 5) consisting of 33.3 wt % of (hydroxypropyl)methacrylate (HPMA), 33.3 wt % of 1,4-butanediol dimethacrylate (1,4-BDMA), and 33.3 wt % of a urethane dimethacrylate monomer (UDMA) (monomers obtained from Sigma-Aldrich). Mix-MA was selected as benchmarked resin due to its low viscosity; i.e., its polymerization will be highly sensitive to the oxygen inhibition. Thus, the polymerization is even more challenging, and the efficiency of the system presented in this paper is well shown. Trimethylolpropane triacrylate (TMPTA) was obtained from Allnex (Scheme 5). Additives and Fillers. 4-(Diphenylphosphino)benzoic acid (4dppba, Scheme 6) was purchased from Sigma-Aldrich. The iodonium

Thermal Imaging Experiments. Thermal imaging experiments were recorded with an infrared thermal imaging camera (Fluke TiX500) with a thermal resolution of about 1 °C and a spatial resolution of 1.31 mrad. The temperature propagation over 1 cm of the sample was recorded, and thermal images were extracted using a Fluke SmartView4.1. Data were then batch processed using a script written in Python. Photolysis Experiments. The photolysis of IR-780 borate under irradiation by laser diode at 785 nm (400 or 2.55 W/cm2) is quantified by UV−vis measurements using a Varian Cary 3 spectrophotometer. Two different solutions were prepared: IR-780 borate alone in acetonitrile (ACN, Sigma-Aldrich) and IR-780 borate and BlocBuilder-MA (10−2 mol/L, ACN).

Scheme 6. Chemical Structures of Additives

III. RESULTS AND DISCUSSION Photoinitiation Ability of the Two-Component System (NIR Dye/Thermal Initiator). Low Intensity NIR Light and 0.1 wt % of NIR Dye. We propose to first investigate IR780 borate to initiate the free radical polymerization of methacrylates upon irradiation by a laser diode at 785 nm (400 mW/cm2). The system used here is a two-component system containing the dye IR-780 borate (as a light-to-heat converter noted heater; 0.1 wt %) and BlocBuilder-MA (2 wt %). Remarkably, this system gives a high polymerization rate under exposure to the NIR light (Figure 2). Without one of the two components (IR-780 borate or BlocBuilder-MA), the polymerization is not possible.

salt bis(4-tert-butylphenyl)iodonium hexafluorophosphate (Ar2I+/ PF6− or also Speedcure 938) was obtained from Lambson Ltd. (Scheme 6). Fillers used in the present paper are silica beads of 400 nm diameter. Irradiation Sources. Several types of irradiation sources were used: NIR LED@850 nm (with an irradiance of 1 W/cm2) from ThorLabs and a NIR laser diode@785 nm with selectable irradiance (ranging from 0 to 2.55 W/cm2) from Changchun New Industries (CNI). Real Time Conversion Measurements upon NIR Light. The double bond CC conversion is follow by real time Fourier transformation infrared (RT-FTIR) spectroscopy using a JASCO 4600. The photosensitive formulations were deposited on a polypropylene film in a mold (thickness of the sample = 1.4 mm) and polymerized under air by irradiation with an LED or a laser diode. The peak followed is at 6100−6220 cm−1 (Figure 1). More details about the conditions of photopolymerization experiments are given in the figure caption. The procedure has been described in detail in ref 24.

Figure 2. Photopolymerization of Mix-MA under air (methacrylates function conversion vs irradiation time) in the presence of (1) no additives, (2) IR-780 borate (0.1 wt %), (3) BlocBuilder-MA (2 wt %), and (4) IR-780 borate/BlocBuilder-MA (0.1 wt %/2 wt %), laser diode@785 nm; 400 mW/cm2; thickness = 1.4 mm, under air. The irradiation starts at t = 17 s.

Effect of NIR Light Intensity. The NIR laser diode used in this study has a tunable irradiance from 0 to 2.55 W/cm2. The impact of the laser diode power on the maximum temperature reached by the system has been measured with IR780-borate alone (0.1 wt %) in Mix-MA (Figure 3). Obviously, no polymerization occurs without thermal initiator and no heating is observed without IR780-borate, showing that the heat released is not ascribed to polymerization but to the ability of the NIR dye to convert light to heat (heater behavior). Remarkably, through incremental increases of the irradiance on the surface of the sample, the maximal temperature reached by

Figure 1. RT-FTIR spectra of methacrylate monomers between 4500 and 7500 cm−1 (1) before polymerization and (2) after polymerization. Circled in orange is the peak representative of the double bond CC conversion used to calculate the photopolymerization profile (methacrylate function conversion vs irradiation time). E


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(activation) temperature compared to the other initiators. The dissociation temperatures of all thermal initiators1 investigated in this study are summarized in Table 1. For Table 1. Representative Temperatures for Efficient Dissociation of Thermal Initiators from Ref 1 thermal initiators ammonium persulfate Luperox P cumene hydroperoxide BPO BlocBuilder-MA 1,1′azobis(cyclohexanecarbonitrile) Luperox 331M80 dicumyl peroxide tert-butyl peroxide

Figure 3. Temperature profiles of Mix-MA under air (temperature of the sample vs irradiation time) in the presence of IR-780 borate (0.1 wt %). Laser diode@785 nm at (1) 0.4 W/cm2, (2) 0.69 W/cm2, (3) 0.97 W/cm2, (4) 1.26 W/cm2, (5) 1.56 W/cm2, (6) 1.82 W/cm2, (7) 2.1 W/cm2, (8) 2.55 W/cm2; thickness = 10 mm. The irradiation starts at t = 17 s.

representative temperature of dissociation (°C) 120 100−130 115 80 60 80 120 110 115

example, at 400 mW/cm2, polymerization is only observed with BlocBuilder-MA. For greater laser intensity, polymerization is also observed when using other thermal initiators such as 1,1′-azobis(cyclohexanecarbonitrile), cumene hydroperoxide, Luperox 331M80, and Luperox P (Figure 5).

the system also increases. The obtained maximal temperature is 45 °C using 400 mW/cm2 and over 140 °C under 2.55 W/ cm2. We observe that the temperature reached by the sample after 100 s of irradiation at 785 nm is a linearly correlated to the irradiance on the surface of the sample (Figure 4). Therefore,

Figure 5. Photopolymerization of Mix-MA under air (methacrylates conversion vs irradiation time) in the presence of IR-780 borate (0.1 wt %) and (1) tert-butyl peroxide (2 wt %), (2) dicumyl peroxide (2 wt %), (3) Luperox P (2 wt %), (4) Luperox 331M80 (2 wt %), (5) cumene hydroperoxide (2 wt %), (6) 1,1′-azobis(cyclohexanecarbonitrile) (2 wt %); laser diode@785 nm, 2.55 W/cm2, thickness = 1.4 mm. The irradiation starts at t = 17 s.

Figure 4. Temperature reached for Mix-MA under air after 100 s of irradiation at different irradiances (temperature of the sample (°C) vs irradiance (W/cm2)) in the presence of 0.1 wt % IR-780 borate. Laser diode@785 nm; thickness = 10 mm.

Moreover, with these thermal initiators, a bleaching of the dye is observed during polymerization in line with the formation of an alkylated cyanine that has no color2 (see below). This is a great advantage because thicker samples can be polymerized in such conditions as the internal filter effect decreases. However, we noticed that with Peroxan50+ (BPO) and ammonium persulfate the IR-780 borate degraded within 1 h of being in contact with either initiators and resulted in no polymerization under our current setup. When dicumyl peroxide or tert-butyl peroxide was used, low conversion was obtained under similar polymerization conditions.

remarkably, it is possible to modulate the maximum temperature reached by the sample, depending on the application required. This characteristic is essential when tuning the system temperature and when selecting radical initiators with different thermal decomposition temperatures. Other Thermal Initiators. We observed that BlocBuilderMA was the most efficient thermal initiator for our system. This result was expected due to its low dissociation F


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Macromolecules Influence of the NIR Dye Concentration. We would expect that greater dye concentration should lead to a greater maximum temperature and ultimately a faster polymerization. When reducing the concentration of IR-780 borate to below 0.01 wt %, we found this to hold true (Figure 6). However, the

Figure 7. Photopolymerization of Mix-MA under air (methacrylates function conversion vs irradiation time) in the presence of BlocBuilder-MA (2 wt %) and IR-780 borate (1) 0.1 wt % and (2) 0.01 wt %; laser diode@785 nm, 400 mW/cm2, thickness = 1.4 mm. The irradiation starts at t = 17 s. Figure 6. Temperature maximum of Mix-MA under air after 50 s of irradiation with laser diode @785 nm and 2.55 W/cm2 (temperature maximum of the sample (°C) vs concentration of IR-780 borate (wt %)) in the presence of different weight percentages of IR-780 borate; thickness = 10 mm.

maximum temperature of roughly 110 °C is already reached for concentrations as low as 0.01 wt % (Figure 6), showing that for an efficient NIR dye heater only a very low content is required. This property is useful to reduce the overall cost of this new proposed approach to photothermal polymerization.. This maximum temperature indicates that the decomposition of free radical initiator such as BlocBuilder-MA is still possible at low dye loading and low irradiation intensity. The IR-780/BlocBuilder (0.01 wt %/2 wt %) system successfully polymerized a methacrylate monomer mixture (Mix-MA) at 785 nm and 400 mW/cm2, and the conversion was follow by RT-FTIR (Figure 7). With lower concentration of IR-780 borate, the polymerization is still possible, but the polymerization rate is lower. The final conversion of CC is still around 80% with only 0.01 wt % of IR-780 borate. Polymerizations with even lower amount of IR-780 borate were attempted. Using only 0.001 wt % of IR-780 borate, higher light intensities (2.55 W/cm2) at 785 nm are required for the polymerization. This is again interesting because the NIR dye can be the most expensive component of the formulation: less expensive formulations can be made by investing in much more powerful laser diodes. Influence of the Monomer. All previous measurements were done in the benchmarked resin (Mix-MA). The temperature for 0.1 wt % IR-780 borate has also been measured in other monomers(hydroxypropyl)methacrylate (HPMA), urethane dimethacrylate monomer (UDMA), and trimethylolpropane triacrylate (TMPTA)and compared to the temperature reached in Mix-MA (Figure 8). It is important to note that each monomer has a different viscosity (from the less viscous to the most viscous: HPMA < TMPTA < Mix-MA < UDMA). Figure 8 clearly indicates that as the viscosity of the

Figure 8. Temperature profiles for IR-780 borate (0.1 wt %) in different monomers under air (temperature maximum of the sample vs irradiation time): (1) Mix-MA, (2) TMPTA, (3) UDMA, and (4) HPMA; laser diode@785 nm at 2.55 W/cm2; thickness = 10 mm. The irradiation starts at t = 17 s.

monomer increases, the greater the temperature would reach under illumination. By increasing the viscosity of the monomer, the heat dissipation ability by the system is decreased, which leads to greater measured temperatures. In-Depth Polymerization of Thick Samples Using NIR Light: A Unique Access to Composites. The polymerization of composites by photopolymerization is usually very difficult due to the very low light penetration in the sample (especially for the UV light). Indeed, the strong diffusion of the UV light by the fillers strongly reduces the light penetration and therefore the potential depth of cure. In this work, different types of fillers were explored: silica, glass fibers, and carbon fibers. NIR light offers a good penetration in the samples and especially in the presence of fillers. Using the twoG


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cyanine/iodonium salts were proposed as photoinitiating system where the heat is not involved (pure photochemical processes). Interestingly, the presence of iodonium salt decreases the inhibition time ascribed to the oxygen inhibition, i.e., the lag times being 300 and 150 s using IR-780 borate/ BlocBuilder MA (Figure 10, curve 1) and IR-780 borate/

component initiating systems, the polymerization has been successful (final methacrylate functions conversion between 50 and 60% in formulations with loading of silica beads from 25 to 75 wt %). Tack-free, 1.9 cm thick films containing up to 50 wt % of fillers have been successfully polymerized using NIR light (Figure 9, sample 1). As a comparison, when using visible light

Figure 10. Photopolymerization of Mix-MA under air (methacrylates function conversion vs irradiation time) in the presence of IR-780 borate/Blocbuilder (0.01 wt %/2 wt %) and (1) no other photochemical additives, (2) Ar2I+PF6− (1.5 wt %), and (3) 4dppba/Ar2I+PF6− (0.5 wt %/1.5 wt %); laser diode@785 nm, 400 mW/cm2, thickness = 1.4 mm. The irradiation starts at t = 17 s.

Figure 9. Photopolymerization of Mix-MA under air in the presence of 50 wt % fillers and (1) IR 780-borate (0.1 wt %) and Blocbuilder (2 wt %); laser diode at 785 nm, 2.55 W/cm2; thickness = 19 mm; (ref) camphorquinone (1 wt %) + Amine EDB (2 wt %), irradiation for 10 min under LED@470 nm; thickness = 6 mm.

BlocBuilder MA/Ar2I+PF6− (Figure 10, curve 2), respectively. This is in agreement with the ability of iodonium salt to react with the excited states of the dye to generate additional initiating radicals.13 These polymerizations were also performed under air even though the methacrylates are sensitive to oxygen. Initiating and propagating radicals and oxygen react together, leading to the creation of peroxyl radicals (Scheme 8). These radicals are too

camphorquinone/EDB as a reference system for photopolymerization (Scheme 7) at 470 nm,3 a maximum thickness Scheme 7. Chemical Structures of the Photoinitiators for the Reference System

Scheme 8. Mechanisms of Free Radical Scavenging by O2 and Conversion of Stable Peroxyls (ROO·) to Initiating Alkoxyl (RO·) Using Phosphine

of ∼6 mm was obtained (Figure 9; sample noted “ref”). This comparison suggests that greater film thicknesses can be obtained by employing longer wavelength photoinitiating systems. Typically, for samples that are photopolymerized in the UV range, the maximal thickness that can be polymerized is roughly 100 μm.3 Use of Photochemical Additives to Enhance the Polymerization. We have shown that polymerization using a two-component system (NIR dye/thermal initiator) is already very efficient upon irradiation at 785 nm (400 mW/cm2) using a laser diode. The final methacrylate conversion is roughly 60− 70% for multifunctional monomers. As cyanine excited states can also be involved in photochemical processes,13 an iodonium salt (Ar2I+PF6−) was investigated as additive; i.e.,

stable to initiate the polymerization leading to an oxygen inhibition of the free radical polymerization process. Phosphines, such as 4-(diphenylphosphino)benzoic acid (4dppba), react with the formed peroxyl radicals to generate alkoxyl radicals (RO·) that can initiate the polymerization.3,13,25,26 We explored the addition of 4-dppba but found that the presence of the phosphine did not increase the final conversions but with much better results in terms of polymerization rates (Figure 10, curve 3). These results show that the combination of photothermal (NIR dye/thermal initiator) with pure photochemical initiation (NIR dye/ 4dppba/Ar2I+PF6−)13 can improve the global performance. H


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Figure 11. (A) Photolysis of IR 780-borate in acetonitrile upon laser diode at 785 nm, 2.55 W/cm2: UV−vis spectra for different irradiation times. (B) Photolysis of IR 780-borate + BlocBuilder-MA in acetonitrile upon laser diode at 785 nm, 2.55 W/cm2: UV−vis spectra for different irradiation times. (C) Photolysis of IR 780-iodide in ACN upon laser diode at 785 nm, 2.55 W/cm2: UV−vis spectra for different irradiation times. (D) Photolysis of IR 780-iodide + BlocBuilder-MA in acetonitrile upon laser diode at 785 nm, 2.55 W/cm2: UV−vis spectra for different irradiation times.

Such a combined mechanism will be explored more in details in the future. Chemical Mechanisms for IR-780 Borate. Steady-state photolysis experiments of IR-780 borate alone, IR-780 iodide alone, IR-780 borate/BlocBuilder-MA, and IR-780 iodide/ BlocBuilder-MA at 785 nm (2.55 W/cm2; Figure 11) show that (i) the degradation of the dye is pretty slow even under high light intensity, (ii) there is no influence of the counterion in IR-780 borate vs IR-780 iodide alone (Figure 12, curves 2 and 3), (iii) an acceleration of the degradation in the presence of BlocBuilder-MA occurs (likely through a redox process) (Figure 12, curve 1 vs curve 2), and (iv) in that case an effect of the anion (the more electron-donating borate allows an electronic rearrangement between the cyanine and the anion; Figure 12, curves 1 and 4). Heat is released by the dye and the temperature of the monomer is high enough to dissociate the thermal free radical initiator (see again the decomposition temperatures in Table 1). The initiator decomposition leads to the generation of free radicals which can initiate the polymerization (Scheme 9). The temperatures of the monomer in the presence of a thermal initiator (2 wt %) without light and when irradiated at 785 nm and 2.55 W/cm2 were measured and released no heat (Figure 13). Thus, this confirms that no polymerization is possible in the absence of the NIR dye used as a heater. Photothermal Initiation Ability with Other NIR Dyes Used as Heater. Effect of Structure on Photoinitiation at 785 nm. A wide range of commercially available dyes (whose

Figure 12. Photolysis (conversion of the dye determined through its peak at 780 nm vs irradiation time) upon laser diode at 785 nm, 2.55 W/cm2 of (1) IR-780 borate + BlocBuilder-MA, (2) IR-780 borate, (3) IR-780 iodide, and (4) IR-780 iodide + BlocBuilder-MA in acetonitrile.

absorption properties are summarized in Table 2) have been characterized as potential NIR dyes (irradiation at 785 nm; 400 mW/cm2). Of those characterized, some were particularly interesting: S0507, S2025, IR-140 (with a counterion borate), I


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Macromolecules Scheme 9. Proposed Chemical Mechanisms for the Photoinitiated Thermal Polymerization

Figure 14. Temperature profiles of Mix-MA under air (temperature of the sample vs irradiation time) in the presence of 0.1 wt % (1) IR-780 borate, (2) (OCT)2-SiPc, (3) S0507, (4) IR-140 borate, (5) S2025, (6) IR-813 borate, and (7) Indocyanine Green. Laser at 785 nm at 2.55 W/cm2; thickness = 10 mm. The irradiation starts at t = 17 s.

Figure 13. Temperature profiles of Mix-MA under air (temperature of the sample vs irradiation time) in the presence of (1) monomer alone, (2) cumene hydroperoxide (2 wt %), (3) Luperox P (2 wt %), (4) BlocBuilder-MA (2 wt %), (5) BPO (2 wt %), (6) ammonium persulfate (2 wt %), (7) dicumyl peroxide (2 wt %), and (8) Luperox 331M80 (2 wt %); laser diode@785 nm at 2.55 W/cm2; thickness = 10 mm. The irradiation starts at t = 17 s.

IR-813 (borate too), and Indocyanine Green. Another really interesting dye is the novel silicon phthalocyanine (SiPc)based dye ((Oct)2-SiPc, Scheme 3). Silicon phthalocyanines are stable and inexpensive dyes which have recently found application as the photoactive component in organic photovoltaics27,28 and organic light-emitting diodes (OLEDS).29,30 Under 2.55 W/cm2 irradiation, for these selected dyes, all generated temperatures above 100 °C (Figures 14 and 15) demonstrating their ability to thermally initiate the polymerization and to emphasize their role of conversion of light to heat (heater behavior). Thus, for all these dyes, the thermal initiation with thermal initiator is possible. Molar extinction coefficients (ε) at 785 nm were determined for each dye, and the results are tabulated in Table 2 (UV spectra of the different dyes in acetonitrile are given in the Supporting Information, Figure S3). It is important to note that ε for SiPcs are often >20000 L/(mol cm) when irradiated

Figure 15. Temperature of Mix-MA with 0.1 wt % of NIR dye under air after 5 0s under irradiation at 785 nm (2.55 W/cm2) vs the absorbed light.

at their maximum absorbance: 680−690 nm. However, these measurements were done at 785 nm to simulate a photopolymerization scenario.31 Intensity absorbed has also been calculated and compared to temperature reached after 50 s of irradiation upon 785 nm (2.55 W/cm2) (Figure 15). We observed no apparent correlation between the temperature reached and the molar extinction coefficient at 785 nm: this suggests that the light absorption properties do not govern alone the heat released that can be affected by other

Table 2. Absorption Properties of Different NIR Dyes and Heater Behavior dye

IR780 borate

IR 813 borate

IR 140 borate

(OCT)2-SiPc

S2025

S0507

Indocyanine Green

ε (785 nm) L/(mol cm) T (°C) after 30 s T (°C) after 100 s T (°C) after 200 s

18200 79 128 145

24800 83 133 146

110900 71 116 129

7700 60 103 120

195800 78 133 143

186100 76 136 143

11300 80 135 145

J


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Macromolecules

electron-donating component, the reaction with BlocBuilderMA seems to be enhanced (as already mentioned above). Photothermal Initiation Abilities upon NIR@850 nm. As the penetration of light is better when the wavelength increases, it would be interesting to use systems operating at greater wavelengths than 785 nm. Examples are provided in Table 3 for the polymerization Mix-MA under air in the

photophysical and photochemical processes; i.e., in the Perrin−Jablonski diagram the reactivity of the excited states can also occur through other pathways than heat released such as fluorescence, phosphorescence or even electron transfer (e.g., see r1 in Scheme 9). Examples of photopolymerization profiles under air upon exposure to the 785 nm laser (400 mW/cm2) are displayed in Figure S2. IR-780 borate still appears as the most efficient NIR dye. The performance of (OCT)2-SiPc is also interesting because relatively high conversions were obtained even with such low ε at 785 nm; this suggests that improved performance could be obtained with a better overlap of the irradiation of the laser and the absorption profile of the silicon phthalocyanine dye. In the recent literature, free-radical polymerization with nanoparticles of carbon black has been reported.18 Carbon black is used as light-to-heat convertor like in our study. In this latter paper, a multiwavelength source is used to perform the polymerization equal to wavelength range of the sun. For comparison, we tried the polymerization under our conditions with carbon black particles. Using 0.01 or 0.1 wt % of carbon black in our resin does not lead to any exothermicity. Accordingly, the polymerization of Mix-Ma using 0.1 wt % of carbon black and 2 wt % of Blocbuilder-MA does not work with 400 mW/cm2, contrary to the IR-780 borate developed system. This comparison with the NIR dyes proposed here clearly show their better ability to work as heater for polymer synthesis than carbon black. Influence of the Counterion. We compared two dyes with two different counterions: IR-780 iodide/IR-780 borate and IR-813 p-toluenesulfonate/IR-813 borate (Figure 16). We

Table 3. Polymerization Performance of Mix-MA under Air in the Presence of Different NIR Dyes (0.1 wt %) and Blocbuilder MA (2 wt %) upon Exposure to LED at 850 nm (1 W/cm2) for 500 s (Thickness = 1.4 mm)a dye

polymerization

conversion (%)

IR-780 borate IR-140 borate IR-813 borate Indocyanine Green S2544 S2425 Ni1 Ni2

yes no yes yes yes yes yes yes

69 5 74 69 69 59 66 70

a

Yes: polymerization observed; no: no polymerization observed. The conversion of CC is measured by RT-FTIR.

presence of different NIR dyes upon exposure to LED with wavelength of irradiation of 850 nm (1 W/cm2). Not only is cyanine a good dye for polymerization at this wavelength, but also squarylium dyes such as S2425 (provided by Few Chemicals) or dithiolene nickel (named here Ni1 or Ni2, synthesized such as described in the literature in refs 32 and 33) show good polymerization profiles. Chemical structures of these dyes are represented in the Supporting Information (Scheme S1). IR-780 Borate/BlocBuilder vs a Known System for Polymerization. In the patent published by Shimada et al.,14 a heat-sensitive composition was described. This system comprised a sulfinate salt as the thermal initiator and a light to heat converting agent. In our work, using IR-780 borate (0.1 wt %) as the best heater developed (see above) and in the presence of sodium p-toluenesulfinate (2 wt %) (depicted in Scheme 10), no polymerization was observed while a color Scheme 10. Chemical Structure of Sodium pToluenesulfinate

Figure 16. Photopolymerization of Mix-MA under air (methacrylates function conversion vs irradiation time) in the presence of BlocBuilder (2 wt %) and (1) IR-780 iodide (0.1 wt %), (2) IR780 borate (0.1 wt %), (3) IR-813 p-toluenesulfonate (0.1 wt %), and (4) IR-813 borate (0.1 wt %) laser diode at 785 nm; 400 mW/cm2, thickness = 1.4 mm. The irradiation starts at t = 17 s.

change from green to orange upon irradiation at 785 nm (400 mW/cm2) was noted. Therefore, the heat-sensitive compositions using sulfinates as described in this patent14 cannot be used under our mild polymerization conditions (much lower light intensity in the present work).

observed in both case that the borate-based dye is better. The first advantage of the borate as a counterion has already been mentioned above: it helps the discoloration of the dye by alkylation of the cyanine. A second advantage of the use of a borate counterion is that the solubility of the dye in the monomer is enhanced. Moreover, as the borate ion is an

IV. CONCLUSION In this study, a new polymerization process for NIR curing of (meth)acrylate monomers has been proposed. It consists of a combination between two polymerization processes: free radical thermal polymerization and photopolymerization. The reaction using NIR light (at both 785 and 850 nm irradiation) is successfully initiated by several types of dyes including a K


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Graphic Industry. Dyes and Chromophores in Polymer Science 2015, 213−249. (8) Karatsu, T.; Yanai, M.; Yagai, S.; Mizukami, J.; Urano, T.; Kitamura, A. Evaluation of Sensitizing Ability of BarbiturateFunctionalized Non-Ionic Cyanine Dyes; Application for Photoinduced Radical Generation System Initiated by near IR Light. J. Photochem. Photobiol., A 2005, 170 (2), 123−129. (9) Daehne, S.; Resch-Genger, U.; Wolfbeis, O. S. Near-Infrared Dyes for High Technology Applications; Springer Science & Business Media: 2012; Vol. 52. (10) Matsuoka, M. Infrared Absorbing Dyes; Springer Science & Business Media: 2013. (11) Berezin, M. Y. Nanotechnology for Biomedical Imaging and Diagnostics: From Nanoparticle Design to Clinical Applications; John Wiley & Sons: 2014. (12) Fabian, J.; Nakazumi, H.; Matsuoka, M. Near-Infrared Absorbing Dyes. Chem. Rev. 1992, 92 (6), 1197−1226. (13) Bonardi, A.; Dumur, F.; Grant, T.; Noirbent, G.; Gigmes, D.; Lessard, B.; Fouassier, J.-P.; Lalevée, J. High Performance NearInfrared (NIR) Photoinitiating Systems Operating under Low Light Intensity and in the Presence of Oxygen. Macromolecules 2018, 51 (4), 1314−1324. (14) Shimada, K.; Sorori, T.; Yagihara, M. Photosensitive Composition and Planographic Printing Plate Precursor; Patent US6908727B2, 2005. (15) Guha, S.; Shaw, S. K.; Spence, G. T.; Roland, F. M.; Smith, B. D. Clean Photothermal Heating and Controlled Release from NearInfrared Dye Doped Nanoparticles without Oxygen Photosensitization. Langmuir 2015, 31 (28), 7826−7834. (16) Zhou, B.; Li, Y.; Niu, G.; Lan, M.; Jia, Q.; Liang, Q. NearInfrared Organic Dye-Based Nanoagent for the Photothermal Therapy of Cancer. ACS Appl. Mater. Interfaces 2016, 8 (44), 29899−29905. (17) Lei, T.; Fernandez-Fernandez, A.; Manchanda, R.; Huang, Y.C.; McGoron, A. J. Near-Infrared Dye Loaded Polymeric Nanoparticles for Cancer Imaging and Therapy and Cellular Response after Laser-Induced Heating. Beilstein J. Nanotechnol. 2014, 5, 313. (18) Steinhardt, R. C.; Steeves, T. M.; Wallace, B. M.; Moser, B.; Fishman, D. A.; Esser-Kahn, A. P. Photothermal Nanoparticle Initiation Enables Radical Polymerization and Yields Unique, Uniform Microfibers with Broad Spectrum Light. ACS Appl. Mater. Interfaces 2017, 9 (44), 39034−39039. (19) Hu, S.; Sarker, A. M.; Kaneko, Y.; Neckers, D. C. Reactivities of Chromophore-Containing Methyl Tri-n-Butylammonium Organoborate Salts as Free Radical Photoinitiators: Dependence on the Chromophore and Borate Counterion. Macromolecules 1998, 31 (19), 6476−6480. (20) Dumur, F.; Nasr, G.; Wantz, G.; Mayer, C. R.; Dumas, E.; Guerlin, A.; Miomandre, F.; Clavier, G.; Bertin, D.; Gigmes, D. Cationic Iridium Complex for the Design of Soft Salt-Based Phosphorescent OLEDs and Color-Tunable Light-Emitting Electrochemical Cells. Org. Electron. 2011, 12 (10), 1683−1694. (21) Nasr, G.; Guerlin, A.; Dumur, F.; Beouch, L.; Dumas, E.; Clavier, G.; Miomandre, F.; Goubard, F.; Gigmes, D.; Bertin, D.; et al. Iridium (III) Soft Salts from Dinuclear Cationic and Mononuclear Anionic Complexes for OLED Devices. Chem. Commun. 2011, 47 (38), 10698−10700. (22) Lessard, B. H.; Grant, T. M.; White, R.; Thibau, E.; Lu, Z.-H.; Bender, T. P. The Position and Frequency of Fluorine Atoms Changes the Electron Donor/Acceptor Properties of Fluorophenoxy Silicon Phthalocyanines within Organic Photovoltaic Devices. J. Mater. Chem. A 2015, 3 (48), 24512−24524. (23) Grishin, D. F.; Grishin, I. D. Mechanisms of Polymer Polymerization. In Polymeric Materials for Clean Water; Springer: 2019; pp 7−58. (24) Lalevée, J.; Blanchard, N.; Tehfe, M.-A.; Peter, M.; MorletSavary, F.; Fouassier, J. P. A Novel Photopolymerization Initiating System Based on an Iridium Complex Photocatalyst. Macromol. Rapid Commun. 2011, 32 (12), 917−920.

silicon phthalocyanine, borate cyanine, squarylium dye, and dithiolene complex. The system operates with only 400 mW/ cm2, but other systems can be designed for higher power densities. One very important aspect of the work developed here is the potentiality of having a tunable heat released. By changing the concentration of the dye, the power of the light delivered, and the monomer used or the dye, it is easy to modulate the temperature of the system and ultimately control the polymerization rate. The use of this photothermal effect with the advantage of spatial control will be presented in forthcoming papers for 3D printing applications.

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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.8b01741.

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Figure S1: diffusion of light vs wavelength; Figure S2: photothermal polymerization with NIR dyes for 400 mW/cm2; Figure S3: UV−vis spectra of the different NIR dyes; Scheme S1: other NIR dyes (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

F. Dumur: 0000-0003-4872-094X B. H. Lessard: 0000-0002-9863-7039 J. Lalevée: 0000-0001-9297-0335 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Dr. Jérémie Fournier and Gwendoline Lejeune from SATT Conectus for fruitful discussions on NIR initiating systems. The authors thank the ANR agency for the financial support for the ANR project “FastPrinting”. We are very grateful for financial support from the NSERC Discovery Grant to B.L. The authors thank Dr. Emmanuel Lacôte and Dr. Fréderic Le Quemener (University of Lyon) for the latex preparation for Figure S1.

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REFERENCES

(1) Moad, G.; Solomon, D. H. The Chemistry of Radical Polymerization; Elsevier: 2006. (2) Gibson, I.; Rosen, D. W.; Stucker, B. Photopolymerization Processes. In Additive Manufacturing Technologies; Springer: 2010; pp 78−119. (3) Fouassier, J.-P.; et al. Photoinitiators for Polymer Synthesis: Scope, Reactivity, and Efficiency; John Wiley & Sons: 2012. (4) Moseley, H. Ultraviolet and Laser Radiation Safety. Phys. Med. Biol. 1994, 39 (11), 1765. (5) Schmitz, C.; Strehmel, B. Photochemical Treatment of Powder Coatings and VOC-Free Coatings with NIR Lasers Exhibiting LineShaped Focus: Physical and Chemical Solidification. ChemPhotoChem. 2017, 1 (1), 26−34. (6) Schmitz, C.; Halbhuber, A.; Keil, D.; Strehmel, B. NIRSensitized Photoinitiated Radical Polymerization and Proton Generation with Cyanines and LED Arrays. Prog. Org. Coat. 2016, 100, 32−46. (7) Strehmel, B.; Brömme, T.; Schmitz, C.; Reiner, K.; Ernst, S.; Keil, D. NIR-Dyes for Photopolymers and Laser Drying in the L


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Photoinduced Thermal Polymerization Reactions - Macromolecules

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