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In-silico based development of photoinitiators for 3D printing and composites: Search on the coumarin
scaffold
Mira Abdallah, Hijazi Akram, Bernadette Graff, Jean Fouassier, Frederic Dumur, Jacques Lalevéee
To cite this version:
Mira Abdallah, Hijazi Akram, Bernadette Graff, Jean Fouassier, Frederic Dumur, et al.. In-silico based development of photoinitiators for 3D printing and composites: Search on the coumarin scaf- fold. Journal of Photochemistry and Photobiology A: Chemistry, Elsevier, 2020, 400, pp.112698.
�10.1016/j.jphotochem.2020.112698�. �hal-02887025�
1
In-silico based development of photoinitiators for 3D printing and composites:
Search on the coumarin scaffold
Mira Abdallah1,2,3, Akram Hijazi3, Bernadette Graff1,2, Jean Pierre Fouassier1,2, Frederic Dumur*4, Jacques Lalevée*1,2
1Université de Haute-Alsace, CNRS, IS2M UMR 7361, F-68100 Mulhouse, France
2Université de Strasbourg, France
3EDST, Université Libanaise, Campus Hariri, Hadath, Beyrouth, Liban.
4 Aix Marseille Univ, CNRS, ICR UMR 7273, F-13397 Marseille, France
Corresponding author: jacques.lalevee@uha.fr
ABSTRACT
In the present paper, thirty-one Nitrocoumarins (including 27 structures never reported in the literature) were designed through molecular orbital calculations and synthesized as high performance Near UV-visible light photoinitiators of polymerization for a better understanding of their structure/reactivity/efficiency relationship. Based on their photoinitiating abilities examined during the free radical polymerization of acrylates, the different coumarins examined in this work can be classified into three main categories: (1) very reactive ones (Coum6,7,11,12&26); (2) moderately reactive Nitrocoumarins (Coum1,2,16,20,21,23,25,27&28) and (3) Nitrocoumarins of low reactivity (Coum3,4,5,8,9,10,13,14,15,17,18, 19,22,24,29,30&31). Different techniques were used in order to understand their photoinitiating abilities as well as the associated chemical mechanisms. The real-time FTIR technique has been used to follow the polymerization profiles (reactive function conversion vs. irradiation time). Different two and three-component photoinitiating systems (PISs) based on Nitrocoumarin/Iodonium salt (or N-Phenylglycine (NPG) or ethyl 4-(dimethylamino)benzoate (EDB)) and Nitrocoumarin/Iodonium salt/NPG were
2 examined for the free radical polymerization (FRP) of acrylates or/and the cationic polymerization (CP) of epoxides upon irradiation with the LED at 405 nm as an unharmed and inexpensive irradiation source. Moreover, cyclic voltammetry, fluorescence spectroscopy, UV-visible spectroscopy and electron spin resonance techniques were also used to provide a full picture of the involved chemical mechanisms. Very good polymerization performances (i.e. high final reactive function conversions (FCs) and also great rates of polymerization (Rp)) were achieved using these derivatives. Some applications in 3D printing and composites synthesis are reported to highlight the interest of the newly proposed structures.
KEYWORDS: Cationic polymerization; free radical polymerization; light-emitting diode;
Nitrocoumarins; photopolymerization.
1. INTRODUCTION
Coumarins wich belong to the class of benzopyrones were first isolated in 1820 from the Tonka bean (Dipteryx odorata) and can be abundantly found in various plants such as Apiaceae, Poaceae, Rutaceae, Solanaceae, Asteraceae, Fabaceae, Moraceae and Lamiaceae. Many coumarin derivatives can be bisourced. Coumarins have notably been extensively studied in numerous research fields due to their easiness of synthesis and the versatility of this simple scaffold, enabling to develop numerous derivatives [1-5]. Coumarins are thus commonly used in research fields such as perfumes and cosmetics [6-12], in organic chemistry, but also in medicinal chemistry [13,14].
Among biological properties, their remarkable anti-microbial [15,16], anti-diabetic, analgesic, anti-neurodegenerative, and anti-inflammatory activities [15,17] can be cited as the most relevant ones. Coumarins are also vitamin K antagonists acting as anti-coagulants that are still under use today [18].
3 As far as near UV photoinitiators are concerned, a large series of coumarins and keto- coumarins have been described in previous papers (some examples can be found in [19-25] and references herein). Some of them have been evaluated as photoredox catalysts and used for the preparation of hydrogels paving the way towards the photopolymerization in water using a water- soluble photoinitiator [19]. The outstanding reactivity of some (keto)coumarins allowed their use for 3D printing applications and also for photocomposites synthesis [19,20]. Very good initiating abilities, very high final reactive function conversions and great rates of polymerization were notably obtained with these different coumarins and keto-coumarins. This last point led us to get a deeper insight into the different parameters that could significantly improve their photoinitiating abilities. In this aim, variation of the substituents introduced onto the coumarin core was examined as a possible tool enabling to finely tune their photochemical properties. More precisely, a special focus has been given to a series of coumarins bearing nitro groups as substituents.
In this aim, thirty-one nitrocoumarin derivatives (noted Coum), including 27 structures never reported in the literature, were designed and developed in order to get a better understanding of their structure/reactivity/efficiency relationship. These derivatives were evaluated as high performance photoinitiators (Scheme 1) for the FRP of acrylates or/and the CP of epoxides using a LED emitting at 405 nm, constituting a safe and low-cost light source of irradiation. These derivatives were incorporated into two (Coum/Iod (or NPG or EDB)) and three-component (Coum/Iod/NPG) photoinitiating systems leading to great polymerization ability (high rates of polymerization (Rp) and good final reactive function conversions (FCs). Generally, tack-free polymers were obtained in the presence of these nitrocoumarins without any yellowing.
Depending of their performances (rate of polymerization), nitrocoumarins were divided into three series: (1) very reactive structures (including 5 derivatives), (2) moderately reactive
4 derivatives (containing 9 derivatives) and (3) nitrocoumarins of low reactivity (seventeen derivatives). The classification of coumarins in three main categories is notably based on their reactivity with amines (NPG or EDB) as the majority of the nitrocoumarins tested in this work are poorly effective with the iodonium salt. The discussion of the photoinitiating abilities and the photochemical mechanisms of the investigated compounds are also provided.
Finally, the outstanding reactivity showed by some of these derivatives allowed their uses for 3D printing technology applications upon exposure to a laser diode emitting at 405 nm in order to generate thick 3D patterns. The manufacture of glass fibers/acrylates photocomposites with an excellent depth of cure is also outlined using near-UV conveyor (LED at 395 nm).
5 Scheme 1. Chemical Structures of the Different Nitrocoumarin Derivatives Studied.
6 2. EXPERIMENTAL PART
2.1. Synthesis of Nitrocoumarin Derivatives
The full procedure for the synthesis of nitrocoumarins is described below and all the characterizations are given in supporting information (SI).
2.2. Other Commercial Chemicals
All commercial chemicals were selected with the highest purity available and used as received. Their chemical structures are depicted in Scheme 2. Di-tert-butyl-diphenyliodonium hexafluorophosphate (Iod or SpeedCure 938) was obtained from Lambson Ltd. N-Phenylglycine (NPG) and ethyl4-(dimethylamino)benzoate (EDB) were obtained from Sigma Aldrich. (3,4- Epoxycyclohexane)methyl 3,4-epoxycyclohexylcarboxylate (EPOX; Uvacure 1500) and trimethylolpropane triacrylate (TMPTA) were obtained from Allnex. TMPTA and EPOX were selected as benchmarked monomers for radical and cationic polymerization, respectively.
Scheme 2. Other Chemicals Used in this Study.
7 2.3. Light Irradiation Source
The Light Emitting Diode (LED)@405nm (I0 = 110 mW.cm-2; from thorlabs) has been used as light irradiation source for this study.
2.4. Cationic Photopolymerization (CP) and Free Radical Photopolymerization (FRP) followed by Real-Time (RT)-FTIR
In this work, different two-component photoinitiating systems (PISs) based on Coumarin/Iodonium salt (and/or NPG or EDB) (0.2% or 0.5%/1% w/w) couples have been tested for FRP and/or CP. Otherwise, Coumarin/Iod/NPG (0.2%/1%/1% w/w) three-component systems have been also examined for FRP. The weight percent of the different chemicals involved in the photoinitiating systems is calculated from the monomer content (w/w). A BaF2 pellet is used for the CP of EPOX which is realized under air where the photosensitive formulation (thickness ~25
m) is put on it. Whereas, the FRP of thin TMPTA films was done in laminate (the formulation is sandwiched between two polypropylene films to minimize the oxygen inhibition). For thin samples, the evolution of the epoxy group content of EPOX and the double bond content of acrylate functions were followed in real time by FTIR spectroscopy (JASCO FTIR 4100) at about 790 cm-1 and 1630 cm−1, respectively.
For thick TMPTA samples (1.4 mm), the free radical photopolymerization was carried out under air into a rounded plastic mold of ~7 mm diameter and 1.4 mm of thickness. Furthermore, the evolution of the acrylate characteristic peak for the thick samples (1.4 mm) was followed at
∼6160 cm-1 in the near-infrared range. The procedure used to monitor the photopolymerization profiles has been already described in detail in [26-28].
2.5. Redox Potentials
8 Based on cyclic voltammetry experiments, the redox potentials (Eox and Ered) of the investigated coumarin derivatives were evaluated using tetrabutylammonium hexafluorophosphate (0.1 M) as the supporting electrolyte and acetonitrile as the solvent (potential vs. Saturated Calomel Electrode – SCE). The free energy change (Get) for the electron transfer reaction was calculated from the classical equation (eq. 1) [29], where Eox corresponds to the oxidation potential of the electron donor, Ered represents the reduction potential of the electron acceptor, E* (ES1 or ET1) corresponds to the excited state energy and (C) is the coulombic term for the initially formed ion pair, respectively. Here, C is neglected as usually done in polar solvents.
Get = Eox – Ered – E*(S1 or T1) + C (eq 1)
2.6. Electron Spin Resonance - SpinTrapping (ESR-ST) Experiments
The ESR-RT experiments were carried out using an X-Band spectrometer (Magnettech MS400). The generation of radicals was done at room temperature (RT) under N2, in the presence of tert-butylbenzene as the solvent and upon irradiation with the LED at 405 nm. The generated radicals were trapped by phenyl-N-tert-butylnitrone (PBN) as the spin trap agent following a procedure already presented in detail in [27,28]. The ESR spectra simulations were obtained using PEST WINSIM program.
2.7. UV-visible Absorption Experiments
The UV-visible absorption properties of the different compounds examined in this work were studied using a JASCO V730 UV–visible spectrometer.
2.8. Fluorescence Experiments
9 A JASCO FP-6200 spectrofluorimeter was used to study the fluorescence properties of the different compounds.
2.9. Computational Procedure
The frontier orbitals (HOMO and LUMO) were calculated at Density Functional Theory (DFT) level (UB3LYP/6-31G*). The UV-visible absorption spectra were calculated at the time dependant-DFT level. The triplet state energy was evaluated after full geometry optimization of both S0 and T1. The computational procedure was described by us in [30].
2.10. 3D Printing Experiments
3D Printing experiments were performed under air using a laser diode @405 nm (spot size of 50 µm) as light irradiation source. These experiments allow the generation of 3D patterns with different thicknesses which were analyzed by numerical optical microscopy (DSX-HRSU from OLYMPUS Corporation) as presented by us in [31,32].
2.11. Near-UV Conveyor Experiments
A Dymax near-UV conveyor was used for the preparation of photocomposites. This conveyor is equipped with a 120 mm wide Teflon-coated belt (the belt speed was fixed at 2 m.min−1). The prepregs (impregnated glass fibers/organic resin (50%/50% w/w)) are cured with a LED @395 nm (4W/cm²). The distance between the LED and the belt can be manually adjusted (fixed here at 15 mm).
3. RESULTS AND DISCUSSION 3.1 Synthesis of Nitrocoumarins
10 The coumarin scaffold can be found in numerous natural products and different strategies can be developed to access to this structure. In the present case, condensation of salicylaldehyde derivatives with phenylacetic acid or 2-thiopheneacetic acid derivatives was selected as an efficient strategy to prepare the different coumarins (See Scheme 3) [33-36]. The 31 coumarins could be obtained with reaction yields ranging from 72% for Coum26 to 94% for Coum16.
Interestingly, if no reaction occurred between the solvent (acetic anhydride) and Coum4 or Coum10, a different behavior was observed for Coum25, an esterification of the phenolic group occurring specifically for this coumarin. It has to be noticed that this position (R6) is of crucial importance as all natural coumarins contain a hydroxy or a methoxy group at this position.
Notably, aesculetin, scopoletin, umbelliferone, herniarin and daphnetin can be cited as relevant examples [37]. Among the 31 coumarins depicted in this work, 27 of them have never been reported in the literature, demonstrating the novelty of the different structures reported in this work.
11 Scheme 3. Synthetic Route to Coum1-Coum31.
12 3.2. Light Absorption Properties of Nitrocoumarins
Acetonitrile was chosen as the solvent in order to study the absorption properties of the different compounds examined in this research. The UV-Visible absorption spectra of the investigated nitrocoumarins showed that these compounds are characterized by remarkable extinction coefficients in the near-UV and visible range as illustrated in the Figure 1. The absorption properties of these derivatives (@maxand @405 nm that corresponds to the emission wavelength of the LED used in this study) are gathered in the Table 1. The high absorptions of these derivatives in the 270-500 nm spectral range allow a good overlap with the emission spectra of the visible LED (@405 nm) used in this work.
The optimized geometries as well as the frontier orbitals (Highest Occupied Molecular Orbital - HOMO - and Lowest Unoccupied Molecular Orbital - LUMO) for the evaluated nitrocoumarins are depicted in Figure 2 and Figure S1. Both the HOMOs and the associated LUMOs are strongly delocalized all over the π-conjugated system clearly showing a π →π* lowest energy transition. For some derivatives (Coum1,3,6,9,10,14,15,16,18,20,21,23,24,25,27,28,30), a partial charge transfer transition character can also be observed with HOMO and LUMO localized on two different parts of the molecular structure, respectively.
13 Figure 1. Absorption spectra of the Nitrocoumarins Investigated in Acetonitrile: (A): First Series:
Very Reactive Nitrocoumarins;(B): Second Series: Moderately Reactive Nitrocoumarins; (C and D): Third Series: Low-Reactive Nitrocoumarins.
Table 1. Parameters Characterizing the Light Absorption Properties of Nitrocoumarins:
Maximum Absorption Wavelengths (λmax), Extinction Coefficients at λmax(max) and Extinction Coefficients at the Emission Wavelength of the LED@405 nm (@405nm).
PI max (nm) max (M-1.cm-1) @405nm (M-1.cm-1)
First Series: Very reactive Nitrocoumarins
Coum6 340 3220 460
Coum7 310 15170 2400
Coum11 360 17980 3650
Coum12 340 11930 320
Coum26 280 16410 140
280 300 320 340 360 380 400 420
0 2800 5600 8400 11200 14000 16800 19600 22400 25200
28000 (10): Coum17 in ACN
(11): Coum18 in ACN (12): Coum19 in ACN (13): Coum22 in ACN (14): Coum24 in ACN (15): Coum29 in ACN (16): Coum30 in ACN (17): Coum31 in ACN
(17) (16)
(15) (14) (12) (13)
(11)
(10)
(nm)
(M-1 .cm-1 )
275 300 325 350 375 400 425 450
0 1960 3920 5880 7840 9800 11760 13720 15680 17640
19600 (1): Coum3 in ACN
(2): Coum4 in ACN (3): Coum5 in ACN (4): Coum8 in ACN (5): Coum9 in ACN (6): Coum10 in ACN (7): Coum13 in ACN (8): Coum14 in ACN (9): Coum15 in ACN
(9) (8)
(7)
(6) (5)
(3) (4) (1)(2)
(nm)
(M-1 .cm-1 )
275 300 325 350 375 400 425 450 475 500
0 1800 3600 5400 7200 9000 10800 12600 14400 16200 18000
(1): Coum6 in ACN (2): Coum7 in ACN (3): Coum11 in ACN (4): Coum12 in ACN (5): Coum26 in ACN
(5)
(4) (3) (2)
(1)
(nm)
(M-1 .cm-1 )
(A) (B)
(C) (D)
275 300 325 350 375 400 425 450
0 2800 5600 8400 11200 14000 16800 19600 22400 25200
28000 (1): Coum1 in ACN
(2): Coum2 in ACN (3): Coum16 in ACN (4): Coum20 in ACN (5): Coum21 in ACN (6): Coum23 in ACN (7): Coum25 in ACN (8): Coum27 in ACN (9): Coum28 in ACN
(9) (8)
(7) (6)
(5)
(4) (3)
(2)
(1)
(nm)
(M-1 .cm-1 )
14 Second Series:
Nitrocoumarins of moderate
reactivity
Coum1 330 15990 170
Coum2 320 26910 360
Coum16 330 10940 350
Coum20 350 14400 1710
Coum21 330 4650 560
Coum23 320 10100 150
Coum25 350 15190 770
Coum27 310 25490 220
Coum28 280 19150 340
Third Series:
Nitrocoumarins of low-reactivity
Coum3 310 11890 280
Coum4 330 7940 270
Coum5 330 9670 280
Coum8 360 8990 2600
Coum9 320 19560 120
Coum10 290 10860 1650
Coum13 290 13200 420
Coum14 290 15020 280
Coum15 310 13210 390
Coum17 330 1530 150
Coum18 280 27320 360
Coum19 280 16370 90
Coum22 290 15450 90
Coum24 330 18660 170
Coum29 300 18030 310
Coum30 300 770 10
Coum31 280 680 5
15
(A) HOMO LUMO
(Coum6)
(Coum7)
(Coum11)
(Coum12)
(Coum26)
16
(B) HOMO LUMO
(Coum1)
(Coum2)
(Coum16)
(Coum20)
(Coum21)
(Coum23)
(Coum25)
17 (Coum27)
(Coum28)
Figure 2. Contour plots of HOMOs and LUMOs for the (A) First and (B) Second Series of Nitrocoumarins; Structures Optimized at the B3LYP/6-31G* Level of Theory (Isovalue = 0.02).
3.3. Cationic Photopolymerization (CP) of Epoxides
Good efficiency was showed for the CP of epoxides using EPOX as the benchmarked monomer in thin films (25 m) under air, in the presence of the two-component Coum/Iod (0.5%/1% w/w) photoinitiating systems upon irradiation with the LED @405 nm as a safe irradiation source (e.g. FC= 51% with 0.5% Coum11 (w/w); Figure 3A, curve 4; Table 2). The same holds true but with slightly lower efficiency when lower concentrations of coumarins are used (e.g. curve 2 for (0.5% Coum7) vs. curve 1 for (0.2% Coum7) in Figure 3A, respectively; see also Table 2). Therefore, the performance of the PIS increases when increasing the coumarin concentration i.e. a higher concentration of photoinitiator generates more reactive species (radical cations) to initiate the photopolymerization reaction.
For these irradiation conditions, no polymerization occurs using iodonium salt alone, showing the crucial effect of the coumarins on the initiating ability. Therefore, these derivatives can be selected as good photoinitiators for a photo-oxidation process when combined with iodonium salt as an additive (see the chemical mechanisms in part 3.7).
18 During the photopolymerization reaction, a new peak that corresponds to the formation of the polyether network arises at ~1080 cm-1 (see the FTIR spectra in the 750-1150 cm-1 range in Figure 3B for 0.5% Coum11 (w/w)).
Taking into account the efficiency trend for the CP of epoxides using a LED at 405 nm, the following order could be determined: Coum11>> Coum7 >> Coum26. This behavior is directly related to the absorption properties (@405nm) of these compounds. Notably, among all the investigated structures, Coum11 is the most efficient photoinitiator and this latter exhibits the highest molar extinction coefficient at 405 nm compared to Coum7 and Coum26 (e.g., @405nm is
⁓3650 M-1.cm-1 for Coum11 compared to ⁓2400 M-1.cm-1 for Coum7 and ⁓140 M-1.cm-1 for Coum26). In fact, when the extinction coefficient of the compound increases, this latter can absorb more light which allows a better photoinitiating ability.
Besides, a significant difference of reactivity between the radical cations (Coum●+) can also affect the initiating ability of the system. Furthermore, the formation of the Brønsted acid as initiating species have also an effect on the system’s efficiency.
Moreover, the other derivatives have been tested as photoinitiators of polymerization and no polymerization occured for the CP of thin epoxides films (25 m) under air (upon irradiation with the LED at 405 nm), in the presence of Coum/Iod systems. This may be associated to a lower interaction of coumarins with iodonium salt or a lower reactivity of the generated radical cations to initiate the photopolymerization reaction.
19 Figure 3.(A): Polymerization profiles (epoxy function conversion vs. irradiation time) for EPOX under air (thickness = 25 μm) upon irradiation with the LED@405 nm, in the presence of different two-component photoinitiating systems: (1):Coum7/Iod (0.2%/1% w/w); (2):Coum7/Iod (0.5%/1% w/w);(3):Coum11/Iod (0.2%/1% w/w); (4): Coum11/Iod (0.5%/1% w/w); and (5):
Coum26/Iod (0.5%/1% w/w). The irradiation starts for t = 10s. (B): IR spectra registered before and after polymerization for Coum11/Iod (0.5%/1% w/w) using LED@405 nm.
Table 2. Final Reactive Epoxy Function Conversion (FC) for EPOX using Different Two- component PISs after 800s of Irradiationwith the LED @405 nm.
% epoxy function conversion (FC) (at t = 800 s) Coum/Iod (thickness = 25 m) under air
Coum7/Iod Coum11/Iod Coum26/Iod
58%a 63%b
64%a
51%b 55%b a: Coum/Iod: (0.2%/1% w/w)
b: Coum/Iod: (0.5%/1% w/w)
3.4. Free Radical Photopolymerization of Acrylates (TMPTA)
3.4.1. First Series: Very Reactive Nitrocoumarins (Coum6,7,11,12&26)
(B) (A) @ t=10s
700 750 800 850 900 950 1000 1050 1100 1150 0.4
0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0
1081 cm-1 790 cm-1
(2) (1)
(cm-1)
O.D
(1) Before polymerization (2) After polymerization
0 100 200 300 400 500 600 700 800
0 10 20 30 40 50 60 70
(5) (4) (3)
Time (s)
(2) (1)
Conversion %
20 Remarkably, the nitrocoumarins of this series showed very high photoinitiating abilities for the FRP of thin TMPTA films (25 m), in laminate when irradiated with the LED at 405 nm. This is illustrated by very high final acrylate function conversions (FCs) but also by high rates of polymerization (Rp) in the presence of the Coum/Iod, Coum/NPG, Coum/Iod/NPG or Coum/EDB combinations (Figure 4). The FCs are gathered in Table 3. No or very low polymerization was observed for nitrocoumarins, Iod, NPG or EDB alone showing that the presence of these derivatives plays a very important role on the initiating ability. This will be discussed later in the chemical mechanisms part.
Taking into consideration the photo-reduction process using an amine (NPG or EDB) as an additive, we notice that Coum11 has a superiority over the other derivatives studied in this series to initiate the FRP of acrylates in thin films (curve 3 in Figures 4B (with NPG) and 4D (with EDB), respectively). This can be ascribed to the highest extinction coefficient of Coum11 at 405 nm compared to the other compounds ((Coum11) = 3650 M-1.cm-1 @405 nm, see also Table 1). Note that all the other derivatives showed also great photoinitiating abilities in the presence of NPG or EDB as shown in the Figures 4B and 4D, respectively, highlighting the excellent efficiency of such derivatives for a photo-reduction process using an amine.
While for the photo-oxidation process, only Coum12 (or Coum11)/Iod systems (curves 4 &
3 in Figure 4A, respectively) had the ability to initiate the FRP of thin TMPTA films. Coum12 was selected as the best photoinitiator with Iod, proposing a more suitable oxidation process for this latter compound. This behavior can be related to the more favorable free energy changes of the electron transfer (Get) with iodonium obtained with Coum12 compared to Coum11 (Get(S1)
= -2.57 eV for Coum12/Iod compared to -2.24 eV for Coum11/Iod; see below in Table 5), highlighting that the photochemical reactivity can also play a key role on the photointiating ability.
21 Furthermore, the high photoinitiating ability of these derivatives in photo-reduction processes was also showed for the FRP of thick TMPTA (1.4 mm) samples under air upon the same irradiation conditions (LED@405 nm), where Coum/NPG or EDB couples (0.2%/1% w/w) lead to very high final reactive acrylate function conversions (FCs) but also polymerization rates (Rp) as seen in Figures 5B & 5D - see also in Table 3, highlighting again the importance of these derivatives as Type II photoinitiators when combined with an amine as co-initiator (electron transfer from NPG or EDB to coumarin). Again, nitrocoumarins, Iod, NPG or EDB alone were tested and no polymerization occurred, showing the huge effect of these derivatives on the global performance of the system. Remarkably, these derivatives are able to overcome the oxygen inhibition usually observed in FRP under air.
Coum12 and Coum11 present excellent photoinitiator behaviors for the FRP of thick acrylate samples through a photo-oxidation process in combination with an iodonium salt using the LED at 405 nm (curve 4 for Coum12 and curve 3 for Coum11 in Figure 5A).
In the case of thick samples, Coum12 was chosen as the best photointiator for both processes:
the photo-oxidation (using an iodonium salt) and the photo-reduction (using an amine: NPG or EDB). This behavior can be related to the low value of the absorbance at 405 nm for Coum12 which is calculated according to Beer-Lambert law (A(@405 nm) = (@405 nm).l.C) (A(Coum12) = 0.31 at 405 nm; see also in Table 4). In fact, a low absorbance allows a better penetration of the light inside the sample enabling to overcome the inner filter effect taking place during the photopolymerization process for thick samples. In addition, we note that the efficiency trend of these derivatives is not only connected to this inner filter effect (in the case of thick samples) but also to the free energy changes for the electron transfer process and the solubility of such compounds.
22 Otherwise, when these derivatives are incorporated into three-component (Coum/Iod/NPG) PISs, the performance is clearly increased in terms of FC and Rp compared to the two-component systems e.g.the maximum acrylate function conversion (FC) of TMPTA attained was about 49%
with Coum11/Iod/NPG after 12s of irradiation compared to only 24% with Coum11/Iod (curve 3 in Figure 4C vs. Figure 4A, respectively; see also in Table 3).This enhancement is also observed for the other nitrocoumarins (Figure 4C vs. Figure 4A; Table 3). To compare, a very low polymerization profile process is shown for the two-component Iod/NPG (1%/1% w/w) system for the FRP of thin acrylate samples upon irradiation with the LED at 405 nm (curve 6 in Figure 4C).
The same behavior is observed for thick samples where the efficiency rises up to reach 83%
for Coum7/Iod/NPG after 12s of irradiation compared to the non-efficiency of Coum7/Iod at this time of irradiation (curve 2 in Figure 5C vs. Figure 5A, respectively; see also in Table 3). We note that the two-component Iod/NPG (1%/1% w/w) system was tested and a good polymerization profile for the FRP of acrylate in thick samples upon irradiation with the LED at 405 nm was observed (Figure 5C, curve 6) but a low rate of polymerization was showed compared to those obtained by adding the coumarin derivatives (FC = 84%, 86%, 83%, 83%&69% for Coum6, Coum7, Coum11, Coum12& Coum26 , respectively after 20s of irradiation compared to Iod/NPG (1%/1% w/w) for which no polymerization occurs at this time of irradiation (curves 1-5 vs. curve 6 in Figure 5C at t = 20s, respectively).
23 Figure 4. (A):Polymerization profiles(acrylate function conversion vs. irradiation time) of TMPTA (thickness = 25 m, in laminate; the irradiation starts for t = 10s) upon exposure to the LED@405 nm in the presence of different two and three-component photoinitiating systems using very reactive nitrocoumarins:(A): PI/Iod (0.2%/1% w/w):(1):Coum6/Iod; (2):Coum7/Iod;
(3):Coum11/Iod; (4): Coum12/Iod; and (5):Coum26/Iod.(B): PI/NPG (0.2%/1% w/w): (1):
Coum6/NPG; (2): Coum7/NPG; (3):Coum11/NPG; (4): Coum12/NPG; and (5): Coum26/NPG.
(C): PI/Iod/NPG (0.2%/1%/1% w/w): (1): Coum6/Iod/NPG; (2): Coum7/Iod/NPG;
(3):Coum11/Iod/NPG; (4): Coum12/Iod/NPG;(5): Coum26/Iod/NPG;and (6): Iod/NPG (1%/1%
w/w). (D): PI/EDB (0.2%/1% w/w): (1): Coum6/EDB; (2): Coum7/EDB; (3): Coum11/EDB;
(4): Coum12/EDB; and (5): Coum26/EDB.
(A) (B)
(C) (D)
WITH 1% IOD WITH 1% NPG
WITH 1% IOD + 1% NPG WITH 1% EDB
10 20 30 40 50 60 70 80 90 100 110
0 5 10 15 20 25 30 35
40 (4)
(2)
(5) (3) (1)
Time (s)
Conversion %
0 10 20 30 40 50 60 70 80 90 100 110
0 5 10 15 20 25 30 35 40 45
(1) (4)
(5) (3) (2)
Time (s)
Conversion %
0 10 20 30 40 50 60 70 80 90 100 110
0 5 10 15 20 25 30 35 40 45 50 55
(1) (5) (2)
(6) (4) (3)
Time (s)
Conversion %
10 20 30 40 50 60 70 80 90 100 110
0 5 10 15 20 25 30 35 40 45 50
(5) (4) (2) (3)
(1)
Time (s)
Conversion %
24 Figure 5. Polymerization profiles (acrylate function conversion vs. irradiation time) of thick TMPTA samples (1.4 mm, under air, using LED@405 nm, the irradiation starts for t = 10s) in the presence of different two and three-component photoinitiating systems using very reactive nitrocoumarins:(A): PI/Iod (0.2%/1% w/w): (1): Coum6/Iod; (2): Coum7/Iod; (3): Coum11/Iod;
(4): Coum12/Iod; and (5): Coum26/Iod.(B): PI/NPG (0.2%/1% w/w): (1): Coum6/NPG; (2):
Coum7/NPG; (3): Coum11/NPG; (4): Coum12/NPG; and (5): Coum26/NPG. (C): PI/Iod/NPG (0.2%/1%/1% w/w): (1): Coum6/Iod/NPG; (2): Coum7/Iod/NPG; (3): Coum11/Iod/NPG; (4):
Coum12/Iod/NPG; (5): Coum26/Iod/NPG; and (6): Iod/NPG (1%/1% w/w). (D): PI/EDB (0.2%/1% w/w): (1): Coum6/EDB; (2): Coum7/EDB; (3): Coum11/EDB; (4): Coum12/EDB; and (5): Coum26/EDB.
(A) (B)
(C) (D)
WITH 1% IOD WITH 1% NPG
WITH 1% IOD + 1% NPG WITH 1% EDB
10 20 30 40 50 60 70 80 90 100
0 10 20 30 40 50 60 70 80
(1) (4) (3)
(5) (2)
Time (s)
Conversion %
0 10 20 30 40 50 60 70 80 90 100
0 10 20 30 40 50 60 70 80
(1) (3) (2)
(5) (4)
Time (s)
Conversion %
0 10 20 30 40 50 60 70 80 90 100
0 10 20 30 40 50 60 70 80 90
(5)(6) (4) (3)(2) (1)
Time (s)
Conversion %
0 10 20 30 40 50 60 70 80 90 100
0 10 20 30 40 50 60 70 80
(1) (2) (4)
(5) (3)
Time (s)
Conversion %
25 3.4.2. Second Series: Moderately Reactive Nitrocoumarins (Coum1,2,16,20,21,23,25,27&28) Moderate reactivity is noted with the series of nitrocoumarins for the FRP of TMPTA in thin films (25 m, in laminate, using LED@405 nm), in the presence of the Coum/Amine (NPG or EDB) couples where typical acrylate function conversion-time profiles are depicted in Figures 6B
&6D, respectively (the FCs are gathered in Table 3). In addition, it should be noted that these nitrocoumarins, NPG or EDB alone are not able to initiate the photopolymerization of acrylates under the same conditions, highlighting the role of these coumarins for the global performance of the system.
In this series, Coum25 can be selected as the best Type II photoinitiator through a photo- reduction process using an amine as a co-initiator (NPG or EDB) since it presents the best performance as shown in Figures 6B & 6D (curve 7). When using NPG, the polymerization rates for the FRP thin films follow the order: Coum25> Coum2 ⁓ Coum16> Coum1 > Coum21>
Coum20> Coum28> Coum23 ⁓ Coum27, and for Coum/EDB couples, it follows the following order: Coum25> Coum1 > Coum21> Coum2 ⁓ Coum16> Coum20> Coum28> Coum23>
Coum27. The efficiency of Coumarin/Amine couples seems to be conveniently connected to both the absorption properties of the coumarins and their solubility. Otherwise, the difference of reactivity between Coum/NPG and Coum/EDB systems can be related to a difference between the electron transfer quantum yield with NPG vs. EDB (see below).
The compounds studied in this series are considered as quite poor for a photo-oxidation process using an iodonium salt as an additive for the FRP of thin TMPTA films excepted Coum16 which can moderately initiate the photopolymerization reaction (Figure 6A). Therefore, a higher yield of electron transfer can be envisaged with Coum16 which can lead to a higher initiating radical yield to initiate the photopolymerization process.
26 Whereas, a good reactivity (with a slight inhibition time compared to the first series studied in part 3.4.1) is shown with these derivatives for the FRP of thick TMPTA samples (1.4 mm) under air, in the presence of Coum/Amine (NPG or EDB) systems upon irradiation with the LED at 405 nm (Figures 7B (with NPG) and 7D (with EDB)).
When using Coum/NPG couples for thick samples, the polymerization rates follow the order:
Coum1 > Coum25> Coum2 > Coum16> Coum21> Coum28> Coum27> Coum23> Coum20, whereas the following order was found for Coum/EDB couples: Coum1 > Coum2 > Coum25>
Coum21> Coum16> Coum23> Coum20> Coum28> Coum27. The efficiency trend of Coum/Amine combinations can be related to the inner filter effect phenomenon observed during the photopolymerization reaction. Indeed, here it is necessary to take into account the absorbance values at 405 nm which are calculated according to the Beer-Lambert law (A(@405 nm) = (@405 nm).l.C) and presented in the Table 4. As an example, Coum1 which is considered as the best Type II photoinitiator for a photo-reduction process in this series is characterized by the lowest absorbance at 405 nm compared to the other derivatives(A(Coum1) = 0.15 at 405 nm; see also in Table 4) leading to a better penetration of light and then to a better efficiency.
When replacing amine by Iod, only Coum16 (or Coum25)/Iod (curves 3 & 7in Figure 7A, respectively) showed moderate performance for the FRP of thick TMPTA samples upon exposure to the LED at 405 nm. This can be attributed to their low initiating radical yields that are not able to overcome the oxygen inhibition generally noticed in FRP under air.
In summary, the photo-reductive process shows a better efficiency over the photo-oxidation process.
Obviously, very high rates of polymerization and also great final reactive acrylate function conversions (FCs) were obtained when NPG was incorporated as the third component in Coum/Iod
27 systems leading to a better efficiency with the three-component PISs compared to the almost non- reactivity of the two-component systems for the FRP thick samples using LED at 405 nm. FC increases up to 76% with Coum16/Iod/NPG after 18s of irradiation compared to the non-efficiency of the two-component Coum16/Iod system at this time of irradiation (curve 3 in Figure 7C vs.
Figure 7A, respectively). The performance increase with the three-component PISs is also remarkable with the other derivatives compared to the two-component systems for which no or moderate polymerization occurs (Figure 7C vs. Figure 7A; Table 3). The two-component Iod/NPG (1%/1% w/w) system shows a rather good polymerization profile for FRP thick samples (curve 10 in Figure 7C) but it exhibits lower rates of polymerization compared to the Coum/Iod/NPG systems, highlighting the huge effect of these derivatives on increasing the photopolymerization rates.
Furthermore, the same enhancement is also noted for the FRP of TMPTA in thin films where the performance of the three-component system increases in terms of FC and Rp compared to the two-component ones for which almost no polymerization took place (Figure 6C vs. Figure 6A, respectively).
28 Figure 6. (A):Polymerization profiles (acrylate function conversion vs. irradiation time) of TMPTA (thickness = 25 m, in laminate; the irradiation starts for t = 10s) upon exposure to the LED@405 nm in the presence of different two and three-component photoinitiating systems using moderately reactive nitrocoumarins: (A): PI/Iod (0.2%/1% w/w): (1): Coum1/Iod; (2):
Coum2/Iod; (3): Coum16/Iod; (4): Coum20/Iod; (5): Coum21/Iod; (6): Coum23/Iod; (7):
Coum25/Iod; (8): Coum27/Iod; and (9): Coum28/Iod.(B): PI/NPG (0.2%/1% w/w):(1):
Coum1/NPG; (2): Coum2/NPG; (3): Coum16/NPG; (4): Coum20/NPG; (5): Coum21/NPG; (6):
Coum23/NPG; (7): Coum25/NPG; (8): Coum27/NPG; and (9): Coum28/NPG. (C): PI/Iod/NPG (0.2%/1%/1% w/w): (1): Coum1/Iod/NPG; (2): Coum2/Iod/NPG; (3): Coum16/Iod/NPG; (4):
Coum20/Iod/NPG; (5): Coum21/Iod/NPG; (6): Coum23/Iod/NPG; (7): Coum25/Iod/NPG; (8):
Coum27/Iod/NPG; (9): Coum28/Iod/NPG; and (10): Iod/NPG (1%/1% w/w). (D): PI/EDB (0.2%/1% w/w): (1): Coum1/EDB; (2): Coum2/EDB; (3): Coum16/EDB; (4): Coum20/EDB; (5):
Coum21/EDB; (6): Coum23/EDB; (7): Coum25/EDB; (8): Coum27/EDB; and (9): Coum28/EDB.
(A) (B)
(C) (D)
WITH 1% IOD WITH 1% NPG
WITH 1% IOD + 1% NPG WITH 1% EDB
0 10 20 30 40 50 60 70 80 90 100 110
0 5 10 15 20 25 30 35 40 45
(9) (6)(8) (4)
(7)(1) (3) (5)
(2)
Time (s)
Conversion %
10 20 30 40 50 60 70 80 90 100 110
0 5 10 15 20 25 30 35 40 45 50
(10) (6) (9) (8)(4) (7)
(1) (3) (5) (2)
Time (s)
Conversion %
0 10 20 30 40 50 60 70 80 90 100 110
0 5 10 15 20 25 30 35 40 45 50
(9) (8) (7) (6) (3)
(4) (1) (2) (5)
Time (s)
Conversion %
0 10 20 30 40 50 60 70 80 90 100 110
0 5 10 15 20 25 30 35 40 45
(9) (6)(8)
(4) (7) (1)
(3)(5) (2)
Time (s)
Conversion %
29 Figure 7. Polymerization profiles (acrylate function conversion vs. irradiation time) of thick TMPTA samples (1.4 mm, under air, using LED@405 nm; the irradiation starts for t = 10s) in the presence of different two and three-component photoinitiating systems using moderately reactive nitrocoumarins: (A): PI/Iod (0.2%/1% w/w): (1): Coum1/Iod; (2): Coum2/Iod; (3): Coum16/Iod;
(4): Coum20/Iod; (5): Coum21/Iod; (6): Coum23/Iod; (7): Coum25/Iod; (8): Coum27/Iod; and (9):
Coum28/Iod.(B): PI/NPG (0.2%/1% w/w): (1): Coum1/NPG; (2): Coum2/NPG; (3):
Coum16/NPG; (4): Coum20/NPG; (5): Coum21/NPG; (6): Coum23/NPG; (7): Coum25/NPG; (8):
Coum27/NPG; and (9): Coum28/NPG. (C): PI/Iod/NPG (0.2%/1%/1% w/w): (1):
Coum1/Iod/NPG; (2): Coum2/Iod/NPG; (3): Coum16/Iod/NPG; (4): Coum20/Iod/NPG; (5):
Coum21/Iod/NPG; (6): Coum23/Iod/NPG; (7): Coum25/Iod/NPG; (8): Coum27/Iod/NPG; (9):
Coum28/Iod/NPG; and (10): Iod/NPG (1%/1% w/w). (D): PI/EDB (0.2%/1% w/w): (1):
Coum1/EDB; (2): Coum2/EDB; (3): Coum16/EDB; (4): Coum20/EDB; (5): Coum21/EDB; (6):
Coum23/EDB; (7): Coum25/EDB; (8): Coum27/EDB; and (9): Coum28/EDB.
(A) (B)
(C) (D)
WITH 1% IOD WITH 1% NPG
WITH 1% IOD + 1% NPG WITH 1% EDB
0 10 20 30 40 50 60 70 80 90 100 110
0 6 12 18 24 30 36 42 48 54 60
(9) (6)(8)
(4) (7)
(1) (3)
(5) (2)
Time (s)
Conversion %
10 20 30 40 50 60 70 80 90 100 110
0 10 20 30 40 50 60 70 80
(8) (6) (9) (4)
(2) (3) (1) (7) (5)
Time (s)
Conversion %
0 10 20 30 40 50 60 70 80 90 100 110
0 10 20 30 40 50 60 70 80
(6) (9) (4)
(8) (7)(1)
(3) (5) (2)
Time (s)
Conversion %
0 10 20 30 40 50 60 70 80 90 100 110
0 10 20 30 40 50 60 70 80 90
(10) (6) (4)
(8) (9)(2) (1) (5) (3)(7)
Time (s)
Conversion %
30 3.4.3. Third Series: Low Reactive Nitrocoumarins (Coum3,4,5,8,9,10,13,14,15,17,18,19, 22,24,29,30&31)
In this section, another series of nitrocoumarins is reported for the FRP of TMPTA both in thin (25m, in laminate) and thick (1.4 mm, under air) samples upon irradiation with the LED at 405 nm in the presence of two and three-component photoinitiating systems based on Coum/Iod, Coum/NPG or Coum/EDB couples (0.2%/1% w/w) and Coum/Iod/NPG combinations (0.2%/1%/1% w/w), respectively (Figure S2 for thin samples and Figure S3 for thick samples;
Table S1).
As shown in Figures S2&S3, it is quite obvious that the two-component (Coum/Iod, Coum/NPG and Coum/EDB) PISs are not able to initiate the FRP of thin or thick samples. This shows that these derivatives can be considered as poor photoinitiators in photo-oxidation process using an iodonium salt but also in photo-reduction process in combination with an amine. The non- efficiency of such compounds can be related to (i) the low absorption properties (the case of Coum19,22,30&31; see also Table 1), (ii) the low solubility (for Coum18,19,22,24,29& 31; see also Table 5), but also (iii) a low initiation yield that can’t overcome the oxygen inhibition for the FRP of TMPTA in the case of thick samples in the presence of the Coum/Iod (or NPG or EDB) systems. It should be noted that most nitrocoumarins with moderate (section 3.4.2) or low reactivity have fairly high oxidation potentials (Eox > 1 V), and thus lower ΔG(Coum/Iod). Therefore, it seems to lead to moderate or low photointiating abilities.
On the other hand, a rather good performance is observed for some of these derivatives when an amine (NPG) was added to the system as a third component e.g. with an increase of the FC = 43% for Coum14/Iod/NPG (thin films) compared to Coum14/Iodfor which no polymerization is observed (curve 8 in Figure S2C vs. Figure S2A, respectively). The same behavior is noticed for
31 thick samples e.g. FC achieved after 100s of irradiation is about 74% for the Coum15/Iod/NPG system compared to the non-efficiency ofthe two-component (Coum15/Iod) system (curve 9 in Figure S3C vs. Figure S3A, respectively; Table S1).
Table 3.Final Reactive Acrylate Function Conversion (FC) for TMPTA using Different Two(0.2%/1% w/w) and Three-component (0.2%/1%/1% w/w) PISs (First and Second Series of
Nitrocoumarins) after 100s of Irradiation with the LED @405 nm.
Thin sample (25m) in laminate Thick sample (1.4 mm) under air
Two-component PIS
Three- component
PIS
Two-component PIS
Three- component
PIS
+Iod +NPG +EDB +Iod/NPG +Iod +NPG +EDB +Iod/NPG
First Series:
Very reactive Nitrocoumarins
Coum6 n.p. 36% 39% 45% 59% 72% 73% 85%
Coum7 n.p. 34% 31% 46% 50% 72% 73% 87%
Coum11 27% 43% 46% 54% 64% 72% 72% 84%
Coum12 39% 42% 41% 50% 72% 72% 75% 84%
Coum26 n.p. 33% 34% 40% n.p. 67% 58% 79%
Second Series:
Moderately reactive Nitrocoumarins
Coum1 n.p. 32% 31% 44% n.p. 72% 73% 82%
Coum2 n.p. 42% 33% 41% 22% 64% 63% 80%
Coum16 36% 38% 33% 47% 59% 70% 66% 82%
Coum20 n.p. 29% 22% 36% n.p. 64% 66% 81%
Coum21 n.p. 42% 42% 47% n.p. 60% 59% 84%
32
Coum23 n.p. 18% 26% 33% n.p. 54% 53% 74%
Coum25 n.p. 32% 30% 45% 38% 71% 69% 83%
Coum27 n.p. 19% 22% 35% n.p. 63% 59% 81%
Coum28 n.p. 25% 24% 39% n.p. 57% 50% 77%
n.p.: no polymerization
Table 4. Extinction Coefficients and Absorbances at the Emission Wavelength of the LED@405 nm for a thick sample.
PI @405nm (M-1.cm-1) A@405nm
First Series: Very reactive Nitrocoumarins
Coum6 460 0.39
Coum7 2400 2.35
Coum11 3650 3.58
Coum12 320 0.31
Coum26 140 0.13
Second Series:
Moderately reactive Nitrocoumarins
Coum1 170 0.15
Coum2 360 0.3
Coum16 350 0.34
Coum20 1710 1.47
Coum21 560 0.48
Coum23 150 0.13
Coum25 770 0.75
Coum27 220 0.15
Coum28 340 0.29
33 3.5. Laser Write Experiments for the Access to 3D Generated Patterns using Coum7/NPG, Coum12/EDB or Coum2/Iod/NPG Systems
The great performances observed with different nitrocoumarins reported in this work make them ideal candidates for laser write experiments with aim at generating 3D patterns. These experiments were successfully achieved under air by means of irradiation of a laser diode at 405 nm, in the presence of two (Coum7/NPG and Coum12/EDB) or three-component (Coum2/Iod/NPG) PISs using TMPTA as an organic resin which showed a good reactivity for acrylates polymerization (see above). In a very short time of irradiation (< 1 min) and with a very high spatial resolution (only limited by the size of the laser diode beam: spot of 50 m), very thick 3D generated patterns were obtained. These latter were characterized by a numerical optical microscopyand the results are shown in Figure 8.
Figure 8. Laser write experiments in TMPTA with a laser diode at 405 nm: Characterization of the written 3D patterns by numerical optical microscopy:(A)Coum12/EDB (0.06%/0.3% w/w) (thickness = 2590m); (B)Coum7/NPG (0.06%/0.3% w/w) (thickness = 2870m); and (C)Coum2/Iod/NPG (0.06%/0.3%/0.3% w/w) (thickness = 2620m); respectively.
3.6. LED Conveyor Experiments for the Preparation of Photocomposites (B)
(A) (C)
34 The good photoinitiating abilities noticed for several nitrocoumarins examined in this work during the FRP of acrylates make them ideal canditates for their use as photoinitiators for the synthesis of photocomposites for which the mechanical properties could be greatly improved, as required for industrial applications. In this study, the prepregs were prepared (50% glass fibers/50% resin w/w) and then irradiated with the LED at 395 nm in order to prepare the photocomposites (one layer of glass fibers; thickness = 2 mm); TMPTAwas used as the organic resin. Interestingly, nitrocoumarins were capable to fully cure composites and very fast curing polymerizations were noticed since only one pass of irradiation at the surface but also for the bottom of the sample is required to be tack-free (for 2 m/min belt speed). Moreover, no significant change of the color of the initial composition was observed after irradiation as shown in the Figure 9.
35 1)
2)
3)
4)
5)
Tack-free at the Surface and
on the Bottom after ONLY one Pass of Irradiation with the LED
@395 nm Pictures before
irradiation
Pictures after irradiation
6)
