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nm) for Free Radical Photopolymerization Processes
Haifaa Mokbel, Bernadette Graff, Frederic Dumur, Jacques Lalevée
To cite this version:
Haifaa Mokbel, Bernadette Graff, Frederic Dumur, Jacques Lalevée. NIR Sensitizer Operating under
Long Wavelength (1064 nm) for Free Radical Photopolymerization Processes. Macromolecular Rapid
Communications, Wiley-VCH Verlag, 2020, pp.2000289. �10.1002/marc.202000289�. �hal-02912860�
NIR Sensitizer Operating under Long Wavelength (1064 nm) for Free Radical
Photopolymerization Processes
Haifaa Mokbel1,2, Bernadette Graff1,2, Frédéric Dumur3, Jacques Lalevée1,2* 1Université de Haute-Alsace, CNRS, IS2M UMR 7361, F-68100 Mulhouse, France
2Université de Strasbourg, France
3Aix Marseille Univ, CNRS, ICR UMR 7273, F-13397 Marseille, France
Corresponding author: jacques.lalevee@uha.fr
Free radical polymerization upon near-infrared (NIR)
1
light is still the subject of intense research efforts and remains
2
a huge challenge particularly for long wavelength (> 1000
3
nm). In this study, a NIR dye operating upon long wavelength
4
(1064 nm) is proposed for an efficient polymerization of
5
acrylate monomers. A new three-component photoinitiating
6
system is developed comprising the NIR dye in combination
7
with an Iodonium salt (Iod) and an amine. Remarkably, the
8
NIR dye (IR 1064) absorbing strongly in all the near infrared
9
region (700-1200 nm) offers the possibility to use a broad
10
range of irradiation wavelengths i.e. examples are provided
11
at 785 nm and 1064 nm. Such long wavelengths are
12
characterized by many advantages such as a deeper
13
penetration of light and therefore a better curing of the
14
monomer but it is also much safer than UV light. Excellent
15
performance is observed for the three-component IR
16
1064/Iod/Amine system under air: high conversion of
17
acrylate functions associated with a fast polymerization time.
18
Three different amines have been examined in this study. The
19
use of IR 1064 as NIR dye with a broad NIR absorption is -
20
to the best of our knowledge – never proposed in the literature.
21
The photoinitiating performances were studied using
Real-22
Time Fourier Transform Infrared Spectroscopy.
23
24
Keywords: NIR dye, NIR irradiation, long
25
wavelength, Free radical polymerization.
26
Light induced polymerization reaction is the subject of
27
intensive research efforts [1-4]. Photopolymerization
28
displays many advantages over conventional thermal
29
processes. Reactions are very fast and take place at room
30
temperature, what decrease the energy demand and reduce
31
thermal runaway hazards. In addition, a spatial and temporal
32
control is easy to achieve with photopolymerization. Most of
33
photopolymerizable formulations are solvent-free,
34
considerably reducing the emission of volatile organic
35
compounds. From this viewpoint, photopolymerization can
36
be considered as an “eco-friendly” polymerization process.
37
Free radical photopolymerization (FRP) upon UV light is
38
now widely used in industry, [5] and FRP notably finds
39
applications in different areas such as coatings, inks, paints,
40
3D printing, medicine etc ... [1].
41
42
At present, most of the photoinitiating systems (PISs)
43
under use in industry are adapted for UV irradiation setups.
44
These short wavelengths are well-known to be harmful and
45
allow only the curing of thin samples [6]. Therefore, the
46
development of photosensitive systems operating under
47
longer and safer wavelengths are actively researched [7-12].
48
When having a look into the photoinitiating systems currently
49
proposed, most of them are capable to initiate a
50
polymerization under visible light [13-16]. More precisely, if
51
an increasing number of systems have been reported during
52
the past 10 years for successful photopolymerizations upon
53
visible light (400-700 nm) exposure, the use of near-infrared
54
(NIR) light irradiation (>750 nm) still remains a challenge
55
due to the low photon energy that decreases the potential
56
photochemical reactivity. NIR photopolymerization strategy
57
could however address the major toxicity and depth of cure
58
issues associated with UV curing. The use of longer
59
wavelengths above 750 nm, specifically, the near-infrared
60
(NIR) wavelength is very convenient and advantageous. NIR
61
light is an ideal spectral range for photopolymerization
62
process as it exhibits several advantages: i) efficiency of the
63
process with low energy consumption and a moderate device
64
cost to proceed contrary to thermal solutions; ii) mild
65
irradiation conditions: safer and cheaper light sources and iii)
66
a deeper light penetration inside the sample allowing the
67
polymerization of thick and highly filled samples [17-19].
68
69
Over the past decades, NIR dyes have found applications
70
in various research fields such as in bioimaging,
71
photothermal therapy, theranostic or non-linear optical
72
applications [13-17,20-22], but also in the photographic
73
industry [16]. Obviously, literature data on NIR dyes remains
74
limited and the search for new dyes as good candidates for
75
photopolymerization is a huge challenge [7,9, 23-25]. Most
76
of the near infrared absorbing dyes are based on cyanine.
77
Cyanine dyes have previously been used as photosensitizer
78
for the curing of NIR photoinitiating systems [6,13,14,17,19].
79
From this point of view, cyanine dyes possess high molar
80
extinction coefficients and absorb in the red to near-infrared
81
region, but these dyes are also characterized by a good
82
chemical and photochemical stability [26]. They can be used
83
as heater (light to heat convertor) and they have attracted the
84
interest of researchers due to their application in the medical
85
fields [20-22].
86
The NIR dye can act as a photosensitizer; it absorbs the
87
light emitted by the NIR light source and reacts with a
88
combination of additives to generate reactive species.
89
Recently, Bonardi et al. developed different photoinitiating
90
systems based on NIR dyes absorbing at long
91
wavelengths.[18] In this example, two NIR dyes (IR 1048:
1-92
Butyl-2-[2-[3-[(1-butyl-6-chlorobenz[cd]indol-2(1H)-93
ylidene) ethylidene]-2- chloro-1-cyclohexen-1-yl]ethenyl]-6-
1
chlorobenz[cd]indolium tetrafluoroborate and IR 1061:
4-[2-2
[2-Chloro-3-[(2,6-diphenyl-4H-thiopyran-4 ylidene)
3
ethylidene]-1-cyclohexen-1-yl]
ethenyl]-2,6-diphenylthio-4
pyrylium tetrafluoroborate) with absorption maxima at 1048
5
and 1061 nm respectively, were used to develop a
6
photothermal approach. The four-component photoinitiating
7
system (PIS) containing the NIR dye/Iodonium salt
8
(Iod)/phosphine/thermal initiator combination was able to
9
initiate the FRP of a methacrylate monomer upon exposure
10
to a LD@1064 nm [18]. Besides, this example still showed
11
slow kinetics of polymerization with a high inhibition time
12
(up to 100s) for polymerization under air. Therefore, the
13
design of new PIS, especially for long wavelengths (1064
14
nm), remains a challenging task for mild irradiation
15
conditions and under air.
16
17
In this context, we propose a NIR dye characterized by an
18
original chemical structure and a good absorption in all the
19
700-1200 nm range for the FRP of acrylate monomers.
20
Interestingly here, through a photochemical approach, the IR
21
1064/Iod/amine PIS is proposed as an efficient system for
22
polymerization processes carried out at long wavelength
23
(@1064 nm). IR 1064 presented here (Scheme 1) has an
24
absorption maximum at 1064 nm, but the reactivity was also
25
quite good upon exposure to a LD@785 nm. The frontier
26
orbitals (highest occupied – HOMO – and lowest unoccupied
27
molecular – LUMO – orbitals are depicted in Figure S1 in
28
supporting information) show a clear charge transfer
29
character for the lowest energy transition with HOMO and
30
LUMO located on different moieties of the structure. From
31
our best knowledge, IR 1064 has never been reported in the
32
literature in NIR photosensitive systems. Originality of this
33
work relies on the possibility to change the irradiation
34
wavelength over a wide spectral range. A comparison of the
35
reactivity of different amines is also provided (Scheme 2).
36
Amine compounds are known to be cheaper and more stable
37
than phosphines. To monitor the polymerization kinetics,
38
Real-Time Fourier Transform (RT-FTIR) spectroscopy was
39
used as an appropriate technique.
40
41
42
Scheme 1. Chemical structure of IR 1064 “N1,N1,N4,N4–
43
tetrakis(4-(dibutylamino)phenyl)benzene-1,4-diaminium
44
hexafluoroantimonate” NIR dye purchased from Lotchem
45
Ltd. (China).
46
47
Scheme 2. Chemical structures of additives investigated in
48
this work.
49
50
Photoinitiating ability of IR 1064 for the FRP: Effect of the
51
amine structure
52
IR 1064 has been used in this study to initiate the free radical
53
polymerization of a benchmark acrylate monomer (PETIA)
54
upon irradiation with a LD@1064 nm. The three-component
55
photoinitiating system containing IR 1064 (IR
56
1064/Iod/Amine; Scheme 2) leads to high polymerization
57
rates. High final acrylate function conversions (FC close to
58
90%) were obtained with the three amines proposed.
Tack-59
free samples were obtained within 20-100 seconds of
60
irradiation (Figure 1A). These results are in full agreement
61
with the IR spectra recorded before and after light activation
62
of the IR 1064 based PIS (Figure 1B), where the characteristic
63
pic of the acrylate function at 6160 cm-1 drastically decreased
64
after irradiation. Without the dye, no polymerization is
65
observed.
66
67
68
Figure 1. (A) Photopolymerization profiles of PETIA
69
(Acrylate function conversion vs. irradiation time) using IR
70
1064/Iod/Amine (0.1/2/3 %wt) where amine is (1) MHPT,
71
(2) NPG and (3) DABA; (B) RT-FTIR sepctra of PETIA
72
monomer using IR 1064/Iod/MHPT (0.1/2/3 %wt) recorded
73
before and after irradiation, the peak representative of
74
acrylate function is indicated, upon exposure to LD@1064
75
nm (I=1.6 W/cm2), thickness = 1.4 mm, under air conditions.
76
The irradiation starts after 10 seconds.
77
78
However, as shown in the Figure 1A, major differences
79
can be found concerning the efficiency of the different
80
amines: obviously, the polymerization is delayed as a
81
function of the amine used. The polymerization reactions
82
were performed under air and the formed initiating radicals
83
react more rapidly with oxygen than with the monomer. By
84
taking into account the time necessary to reach high final
85
conversions, the efficiency trend followed the order shown in
86
Figure 1A: 3-(Dimethylamino)benzyl alcohol (DABA) >
2-87
(N-methyl-p-toluidino)ethanol (MHPT) > N-phenylglycine
88
(NPG). The inhibition time is clearly longer in the case of
89
NPG compared to the two other amines, evidencing the
90
dramatic influence of the amine structure on the
91
polymerization process. Therefore, the interaction existing
92
0 20 40 60 80 100 120 140 0 20 40 60 80 100 3 2 1 C o n v e rs io n ( % ) Time (s) 6100 6150 6200 6250 0,0 0,5 1,0 1,5 O .D . (cm-1 ) Before irradiation After irradiation (A) (B)between the dye and the amine is an important parameter
1
governing the formation of initiating radicals. The low
2
reactivity of NPG vs. MHPT and DABA can be probably
3
ascribed to the low production of initiating radicals in this
4
case, rendering the associated PIS less efficient to overcome
5
the oxygen inhibition. Tertiary amine (such as DABA and
6
MHPT in this work) are more efficient than the secondary
7
one (NPG in this work), as already mentioned in the literature
8
[27]. In all cases, an excellent reactivity of the different
9
amines examined in this work could be evidenced, suggesting
10
that the NIR approach was an elegant strategy for fast curing
11
of thick samples upon long wavelength irradiation, what is
12
beneficial for numerous applications.
13
14
Monomer effect on the initiation of FRP using IR 1064
15
In the previous section, photopolymerization measurements
16
were done in PETIA monomer. The efficiency of NIR dye
17
has also been demonstrated in
18
di(trimethylolpropane)tetraacrylate (TA) as a benchmark
19
monomer (Scheme 3). It is important to note that both
20
monomers have very different viscosity (1000 mPa.s for
21
PETIA and 160 mPa.s for TA). Therefore, the viscosity of the
22
monomers is expected to have a major impact on the oxygen
23
inhibition: The higher the viscosity of the monomer is, the
24
less the oxygen inhibition is. In other words, the
25
polymerization of viscous monomers is faster under air that
26
for the fluid ones.
27
As anticipated, Figure 2 clearly indicates that a fast
28
polymerization process could be obtained using the PETIA
29
monomer whereas a slower polymerization kinetic was
30
observed in TA. Besides, in the two cases, a similar final
31
monomer conversion could be determined in the two
32
monomers. The time necessary to reach a high final
33
conversion is 30 seconds in PETIA, whereas this value
34
increased to 55 seconds in TA. However, the final acrylate
35
conversion is roughly 85 − 90% for the two multifunctional
36
monomers. Tack-free samples were obtained with the two
37
monomers. These data show the high reactivity of IR 1064 in
38
terms of efficiency in different acrylate monomers.
39
40
Figure 2. Photopolymerization profiles (Acrylate function
41
conversion vs. irradiation time) using IR 1064/Iod/DABA
42
(0.1/2/3 %wt) in (1) PETIA and (2) TA; upon exposure to
43
LD@1064 nm (I=1.6 W/cm2), thickness = 1.4 mm, under air
44
conditions. The irradiation starts after 10 seconds.
45
Effect of irradiation wavelengths
46
Influence of the irradiation wavelength on the
47
polymerization rate and the final monomer conversion has
48
been checked in this section. The NIR dye IR 1064 allowed a
49
satisfying polymerization under very different irradiation
50
conditions. At 785 nm, the polymers obtained were tack-free
51
using different amines. However, the polymerization was
52
slower compared to the results obtained upon exposure to a
53
LD@1064 nm. Using the two irradiation wavelengths, FCs
54
approaching 90% could be determined. Meanwhile, the time
55
necessary to reach high final conversions was considerably
56
higher using the LD@785 nm in the presence of different
57
photoinitiating systems: from 150 seconds upon LD @785
58
nm vs. 30 seconds upon LD @1064 nm for the
three-59
component system IR1064/Iod/DABA as shown in Figure
60
3A. Modification of the inhibition time is directly related to
61
the absorption properties of IR 1064 at these two
62
wavelengths. Indeed, IR 1064 exhibits a better absorption at
63
1064 nm than at 785 nm (Figure 3B). An improved
64
photoactivation was thus evidenced upon irradiation at 1064
65
nm, even if good efficiencies were still obtained at 785 nm.
66
67
0 20 40 60 80 100 0 20 40 60 80 100 Time (s) C o n v e rs io n ( % ) 2 1 LD@ 785 --LD@ 1064 -- -- --LD@ 785 --LD@ 1064 -- -- --LD@ 785 --LD@ 1064 0 20 40 60 80 100 120 140 160 T im e n e c e s s a ry t o r e a c h h ig h F C ( s ) Irradiation wavelength (A) 600 700 800 900 1000 1100 0,0 0,2 0,4 0,6 0,8 1,0 1,2 O.D. (nm) 785 nm 1064 nm (B)Figure 3. (A) Representative time necessary to reach high
1
acrylate function conversion (FC = 90%) using IR
2
1064/Iod/Amine (0.1/2/3 %wt) upon exposure to LD@1064
3
nm (I=1.6 W/cm2) and LD@785 nm (I=0.9 W/cm2),
4
thickness = 1.4 mm, under air conditions. The irradiation
5
starts after 10 seconds; (B) UV-visible-NIR absorption
6
spectrum of IR 1064 in ACN.
7
8
In all cases, IR 1064 can be assumed as a versatile NIR
9
dye that can be activated in all the 700-1200 nm spectral
10
range (Figure 3B) ensuring a fairly good overlap with the
11
emission spectra of the many NIR LEDs or laser diodes of
12
academic or industrial interest.
13
14
15
Conclusion:
16
In this paper, IR 1064 was proposed as an interesting NIR
17
dye for the development of new high performance PIS upon
18
near-infrared irradiation. Excellent efficiencies were found
19
for the free radical polymerization of multifunctional acrylate
20
monomers such as PETIA and TA (Scheme 3). The
three-21
component photoinitiating systems IR 1064/Iod/Amine were
22
very reactive: different amines can be excellent candidates.
23
The most interesting aspect of this work is clearly the
24
possibility to photoactivate our PIS at two different
25
wavelengths separated from each other by almost 300 nm. A
26
control of the polymerization time with the selected light
27
source was also demonstrated, what is of crucial importance
28
for practical applications. For the forthcoming studies, new
29
photoinitiating systems offering a wider photoactivation
30
window will be presented. More particularly, the
31
development of other NIR dyes for absorption > 1000 nm will
32
be presented in forthcoming papers.
33
34
35
36
Experimental part:37
38
Chemical compounds39
Chemical structures of the different additives used in this
40
study are given in Scheme 2.
Bis(4-tert-41
butylphenyl)iodonium hexafluorophosphate (Iod) were
42
obtained from Lambson Ltd. (UK). N-phenyl glycine (NPG)
43
and 3-(dimethylamino)benzyl alcohol (DABA) were
44
purchased from Sigma-Aldrich.
2-(N-methyl-p-45
toluidino)ethanol (MHPT) was obtained from Albemarle.
46
All chemical compounds were selected with highest purity
47
available and used as received.
48
Efficiency of the different PISs was checked with a
49
benchmarked multifunctional monomer containing acrylate
50
functions (See Scheme 3). Pentaerythritol triacrylate
51
(PETIA) and Di(trimethylolpropane)tetraacrylate (TA) were
52
received from Allnex.
53
54
55
Scheme 3. Benchmark acrylate monomers used in this work.
56
57
Irradiation device
58
59
Two laser diodes (LD) with different wavelengths were used
60
throughout the study: LD@785 nm with a tunable irradiance
61
(from 0 to 2.55 W/cm2) and LD@1064 nm with a tunable
62
irradiance (from 0 to 2.55 W/cm2), both from Changchun
63
New Industries (CNI).
64
65
Photopolymerization experiments
66
67
Kinetics of polymerization of the photosensitive formulations
68
were followed through the double bond C=C conversion of
69
acrylate function vs. time. The peak followed by Real-Time
70
Transform Infrared (RT-FTIR) Spectroscopy (JASCO 6600)
71
is located in the near infrared region at 6100-6220 cm-1.
72
Photosensitive formulations were deposited on a
73
polypropylene film in a mold (thickness of the sample = 1.4
74
mm) and polymerized under air upon irradiation with NIR
75
laser diodes. The light was turned on 10 seconds after the first
76
spectrum measurements. More details about the
77
photopolymerization experiments are given in the figure
78
captions. All the experiments are carried out at room
79
temperature (RT) and under air.
80
81
82
UV-visible absorption spectroscopy
83
84
The UV-visible-NIR absorption spectrum of IR 1064 in
85
acetonitrile were acquired using a Varian Cary 3
86
spectrometer.87
88
Page Numbers
89
DO NOT input any page numbers.
90
References and Notes
91
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Graphical Abstract
Textual Information
A brief abstract (required)
In this study, a NIR dye operating upon long wavelength (1064 nm) is proposed for an efficient polymerization of acrylate monomers. As unique originality, IR 1064 is used here in NIR photoinitiating systems. A new three-component photoinitiating system is developed comprising the NIR dye in combination with an Iodonium salt (Iod) and an amine. Remarkably, the NIR dye (IR 1064) absorbing strongly in all the near infrared region (700-1200 nm) offers the possibility to use a broad range of irradiation wavelengths i.e. examples are provided at 785 nm and 1064 nm.
Title(required)
NIR dye Operating under Long Wavelength (1064
nm) for Free Radical Photopolymerization processes
Authors’Names(required)
Haifaa Mokbel1,2, Bernadette Graff1,2, Frédéric Dumur3, Jacques Lalevée1,2*
