A fluorinated photoinitiator for surface oxygen inhibition resistance

Article abstract of DOI:10.1021/ma2023759

A novel photoinitiator called 2-methyl-2- benzoylethanolpentadecafluorooctanote (1173-F) was synthesized and characterized. 1173-F is an efficient photoinitiator for free radical photopolymerization in the presence of oxygen and has ability of moving to t

Full text of DOI:10.1021/ma2023759

Article
pubs.acs.org/Macromolecules
A Fluorinated Photoinitiator for Surface Oxygen Inhibition
Resistance
Fei Xu, Jin-Liang Yang, Yu-Shan Gong, Gui-Ping Ma, and Jun Nie*
State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing, China, 100029
S
* Supporting Information
ABSTRACT: A novel photoinitiator called 2-methyl-2-
benzoylethanolpentadecafluorooctanote (1173-F) was synthe-
sized and characterized. 1173-F is an efficient photoinitiator
for free radical photopolymerization in the presence of oxygen
and has ability of moving to the surface. The oxygen inhibition
of the photopolymerization surface could be decreased because
the large amount of photoinitiator at the top to be consumed
by oxygen. UV absorption spectroscopy, X-ray photoelectron
spectroscopy, gel permeation chromatography, and EDS-line
mapping were used to prove the migratory of 1173-F in
monomers. Real time FTIR analysis was taken to investigate the ability of 1173-F to overcome oxygen inhibition.
reported a thioxanthone-anthracene (TX-A) photoinitiator,
which is an efficient photoinitiator for free radical polymer-
ization of both acrylate and styrene monomers in the presence
of air. El-Roz et al.18 use silanes, germanes, and stannanes
R3XH as co-initiators and additives to overcome the oxygen
inhibition.
INTRODUCTION
■
Many products are produced via photopolymerization due to
its excellent characteristics as it is economic, environmental
friendly, energy saving, efficient, enabling, and rapid.1−3 It is
widely used in many regions including coatings, adhesives,
dental resins, ink, paints, and microelectronics.1−7 Free radical
photopolymerization dominates most of the interest and
industrial applications due to its high quantum efficiency and
reactivity.1 It is reported that the initiation rate is independently
dictated by the photoinitiator and the light intensity.8 The
generation of reactive species is due to a photoinitiator
absorbing the incident light and cleaving into initiating
radicals.9
However, there are still many limitations of free radical
photopolymerization, such as oxygen inhibition, volume
shrinkage, light shielding.9−12 The existent oxygen can
terminate the polymerization or consume the photoinitiator
by reacting with initiating radicals and growing polymer radicals
to form peroxy radicals.11 The peroxy radicals are stable that
cannot reinitiate the polymerization. The effect of oxygen
inhibition results in an induction period, the slowing down of
the reaction, poor double bond conversion. When the sample is
open to air, the conversion of polymerization is low, especially
at the top of the film. The top layer remains “tacky” due to the
oxygen inhibition.9,12,13 O’Brien et al. studied the impact of
oxygen on photopolymerization kinetics and polymer structure,
indicating sample thickness, initiation rate and oxygen
concentration result photopolymerization kinetics, respec-
tively.8 Several methods have been proposed to reduce the
oxygen inhibition. Some traditional strategies such as using high
intensity light or high cure dosage, blowing inert atmosphere,
adding additives have been developed.13−15 Higashi et
al.16reported an alternating copolymer of alkyl sorbates was
cured by photopolymerization with oxygen. Balta et al.17
Oxygen inhibition could be reduced by increasing the
photoinitiator concentration on the surface but high concen-
tration of photoinitiator would lead a result of light shielding
and volume shrinkage. We want to find such a photoinitiator
which have more content on the surface layer to be consumed
by oxygen and low content in the bulk to initiate monomers.
Fluorinated polymers were widely used due to the excellent
properties such as low surface energy.19 It was reported that the
addition of 1−1.5 wt % of fluorine could reduce the surface
energy from 45 to 20−30 mN/m.20,21 Novel polyurethanes
were prepared with fluorinated side chains attached to hard
blocks. The surface of fluorinated polyurethanes was nonpolar
due to migration of fluorinated side chains to the surface.22
Ravenstein et al. reported low surface energy polymeric films
from fluorinated blocked isocyanates. The fluorinated species
would migrate toward the air interface to minimize the
interfacial energy.23 It was reported that oligoesters end-capped
with a perfluoroalkyl group was cured with a liquid
polyisocyanate cross-linker by self-stratification to prepare
environmentally friendly coating system and low surface energy
films. The fluorine level at the surface was many times higher
than that in the bulk.20,24 Photoinitiator containing fluorine
atom was synthesized to prepare surface active coatings
Received: October 25, 2011
Revised: January 17, 2012
Published: January 27, 2012
© 2012 American Chemical Society
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Figure 1. Scheme of 1173-F for surface oxygen inhibition resistance.
Scheme 1. Preparation of Fluorine-Containing Photoinitiator
before25,26 but related oxygen inhibition resistance was not
reported.
height, and the top surface was exposed to the air. The conversion of
monomers with presence of oxygen was investigated by real time FTIR
with the same conditions above.
In this paper, we combine the advantage of fluorine
molecular and 2-hydroxy-2-methylpropiophenone (1173),
making a fluorinated photoinitiator for free radical photo-
polymerization, which could overcome oxygen inhibition
efficiently without any co-initiators or additives. This photo-
initiator has ability of migratory that result in aggregation of
photoinitiator on the top surface. Monomer with this
photoinitiator has high conversion of double bond with the
presence of air, as shown in Figure 1.
The monomer investigated in the SEM EDS-line mapping was
HDDA. The sample was placed in a mold with 10 mm in length, 10
mm in width, and 3 mm in height. The light intensity was 40 mW/cm2
and the sample was opening to air. The monomer used in GPC
analysis was MMA and the monomer used in XPS test was HDDA.
Both of the monomers were put into a glass tube with a diameter of 4
mm and height of 10 cm. The monomer was irradiated by a high-
pressure mercury lamp with incident light intensity of 40 mW/cm2
(FP-108EX−S1-B, Commonwealth Corporation, Taiwan). The UV
light was a plane light source to guarantee the equal light intensity on
each layer. The glass tube was fixed vertically on a turntable with a
rotate speed of 5 r/min, in order to uniform irradiation of different
location.
Surface Migratory Property. UV Spectrophotometer. A
solution of photoinitiator (3.25 × 10−6 mol) in acetonitrile (25 mL)
was prepared to study the concentration change of 1173-F. The
solution was placed into a quartz cell with 10 mm in length, 10 mm in
width and 12 mm in height. A black sheet with a 1 × 1 mm2 hole was
placed in front of the UV light of the UV spectrophotometer. The UV
light region was controlled in 1 × 1 mm2 by the black sheet. The
thickness of layer observed was 1 mm. The absorbance of top layer
and the bottom layer was investigated respectively by moving the
position of the quartz cell, as shown in Figure 7A. According to the
Lambert formula: A= εcL, where A is the absorbance of the initiator, ε
is the initiator molar absorptivity, c is the initiator concentration, and L
is optical distance. The concentration could be calculated by the A, L,
and ε values. The value of A could be measured by UV absorption
spectroscopy directly.
GPC, XPS, EDS-Line Mapping. The polymer rod initiated by 1173-F
was cut into many sheets. Five segments from top to bottom were
investigated by GPC and eight segments from top to bottom were
observed by XPS. The distribution of fluorine atoms in a polymer
transect initiated by 1173-F was constantly observed by the EDS-line
mapping with transverse direction and vertical direction, respectively.
Characterization. The chemical structure of 1173-F was
investigated by FTIR (Thermo Electro Corporation, Waltham, MA),
1H NMR, and 19F NMR (Bruker AV400 unity spectrometer operated
at 400 MHz with CDCl3 as solvent at 23 °C). The photo absorption
and photoinitiator dissociation were studied by UV absorption
spectroscopy (U-3010, Hitachi High-Technologies Corporation,
Tokyo, Japan). The photochemical reactivity was observed by Electron
Paramagnetic Resonance (EPR, Bruker ESP300E, Germany) and laser
flash photolysis experiments (355 nm, LP920, Edinburgh Instruments
Ltd., England). Real time infrared spectroscopy (RTIR, Thermo
Electro Corporation, Waltham, MA) was used to analysis kinetics of
photopolymerization. The migration of 1173-F was studied by X-ray
photoelectron spectroscopy (XPS, Thermo Electron Corporation,
Escalab 250, Germany), gel permeation chromatography (GPC,
Waters 1515, USA), UV Spectrophotometer (Hitachi High-
EXPERIMENTAL DETAILS
■
Materials. 2-Hydroxy-2-methylpropiophenone (1173) was pur-
chased from Changzhou Runtec Chemical Ltd. Pentadecafluoroocta-
noyl chloride (PFOC) was purchased from Alfa Aesar. Triethylamine
(TEA), methylene dichloride, ethyl acetate, petroleum ether, and
acetonitrile were purchased from Sinopharm Group Chemical Reagent
Co. (Beijing, China). The monomers used in this paper were
hexamethylene diacrylate(HDDA) and methyl methacrylate (MMA)
given by Sartomer. All reagents were used as received without further
purification.
Synthesis of 2-Methyl-2-benzoylethanolpentadecafluorooc-
tanote (1173-F). First, 1.64 g (0.01 mol) 2-hydroxy-2-methylpro-
piophenone (1173), 1.012 g (0.01 mol) triethylamine (TEA), and 30
mL methylene dichloride (CH2Cl2) were added into a 100 mL round-
bottom flask and stirred at room temperature by using magnetic
stirring. Mixture of 4.325 g (0.01 mol) of PFOC and 20 mL of CH2Cl2
was dropped into the stirred solution in an ice water bath. The
reaction was monitored by infrared spectroscopy. When the
characteristic peaks of hydroxyl group (OH, 3465.2 cm−1) was
disappeared and carbonyl group (CO, 1773.8 cm−1) was generated,
the reaction was stopped. The primary product was purified by silica
gel column chromatography with ethyl acetate and petroleum ether as
1
elution (1:10 v/v). H NMR and 19F NMR characterizations were
used to observe the chemical structure of final product. The reaction of
synthesis is shown in Scheme 1.
Photopolymerization. The polymerization kinetics was inves-
tigated by real time FTIR. For the sample without oxygen penetration
experiment, a solution of HDDA and initiator was placed in a sealed
mold made from glass slides and spacers with 10 mm in diameter and
1.2 mm in thickness. The solution was irradiated by a medium-
pressure mercury lamp with incident light intensity of 10 mW/cm2
(EXFO S1000-IB, Photonic Solutions Inc., Canada). The light
intensity on the surface of sample was detected by a UV light
radiometer (Beijing Normal University, China). The double bond
conversion of HDDA was monitored by near FTIR spectroscopy. The
absorbance change of peak area from 6100 to 6250 cm−1 was related to
the consumption of monomers. For the sample with oxygen, the
monomers and initiator with different concentrations was put into a
glass container with 10 mm in length, 10 mm in width, 1.2 mm in
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Technologies Corporation, Tokyo, Japan) and mapping energy
dispersive spectroscopy (SEM EDS line mapping, Zeiss Supra 55,
Germany).
RESULTS AND DISCUSSION
■
Synthesis and Photochemical Reactivity. 2-Methyl-2-
benzoylethanolpentadecafluorooctanote (1173-F) was prepared
Figure 4. EPR spectrum of 1173 (A) and 1173-F (B). Photoinitiator
was initiated by 355 nm laser in nitrogen saturated acetonitrile
solutions at 23 °C.
Figure 2. IR spectrum of 1173 (A) and 1173-F (B).
Figure 5. Polymerization kinetics of HDDA with photoinitiator: (a)
1173 with oxygen, (b) 1173 without oxygen, and (c) 1173-F, with
oxygen. All the concentration of photoinitiator is 0.305 mmol/100 g.
The intensity of UV light is 10 mW/cm2.
is 1 cm, the calculated ε of 1173-F is 1040.41 L/(mol·cm).
Initiated by 355 nm laser, the photoinitiator could produce
radicals. The radical signal was detected by adding
dimethylpyridine N-oxide (DMPO) as radical scavenger, as
shown in Figure 4. The EPR spectra of radicals produced by
1173 and 1173-F were similar and both had 12 lines, which
indicated the cleavage mechanism of 1173-F was the same as
1173. Since the triplet lifetime of 1173-F was shorter than the
instrument response, an indirect method of triplet quenching
was used to study the triplet lifetime by adding naphthalene.27
The triple lifetime of 1173-F was 13 ns (Figure 2S, Supporting
Information). The rate of photoinitiator dissociation (Rd) was
calculated according to the following equation:
Figure 3. Absorption spectra of 1173 (dashed line) and 1173-F (solid
line) in acetonitrile solution with the same concentration (5 × 10−5
mol/L) at 23 °C.
by reacting the OH of 1173 with the COCl of PFOC. The peak
of hydroxyl (3645.2 cm−1) disappeared after fluorination;
instead a new peak of carbonyl group was shown at 1773.8
cm−1 (Figure 2). The change of peak at 1000 cm−1 to 1300
1
cm−1 was due to the stretching vibration of fluorine atom. H
NMR and 19F NMR were used to monitor the chemical
1
structure of 1173-F. H NMR (δ, 400 MHz, ppm): 7.945−
7.413 (5H, Ar−H), 1.849 (6H, C(CH3)2); 19F NMR (δ, 400
MHz, ppm): −80.774, −117.757, −121.535, −121.969,
−121.969, −122.703, −126.091. (Figure S1, Supporting
Information) The UV absorption spectroscopy of 1173 and
1173-F were demonstrated in Figure 3. It could be found that
1173-F had the same UV absorption peak as 1173 at 245 and
280 nm. According to the Lambert formula: A = εcL and the L
⎛
⎜
⎝
⎞
−δ[PI]
δt
[PI] δ[PI]
R =
= −
⎟
d
Ab
δt
⎠
(1)
0
where Ab0 is the absorbance of 1173 and 1173-F at 245 nm
before exposure to the UV light. The Rd of 1173-F was 1.46 ×
10−7 molL−1s−1 and Rd of 1173 was 2.11 × 10−7 molL−1s−1.
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Figure 6. (A) XPS spectrum of HDDA with photoinitiator 1173-F (5 wt %). (B) Fluorine content of different layers. No. 1 is the top layer and no. 8
is the bottom layer.
Table 1. GPC Data for Different Layers Based on
Polymerization of MMA
Mn(1173- Mw(1173- D(1173-
layer
F)
F)
F)
Mn(1173) Mw(1173) D(1173)
1
2
3
4
5
7544
24 457
26 012
39 392
3.4
1.6
1.4
2.5
3.1
14 836
13 348
13 203
12 650
13 457
40 076
42 580
40 849
38 541
39 255
2.7
3.2
3.1
3.0
2.9
79 696
113 643
249 292
338 510
100 839
107 870
(Figure 3S, Supporting Information) The quantum yield was
related to the dissociation rate of photoinitiator according to
the equation below:28
R = I φε[I]l = φI
a
(2)
d
0
where I0 is the intensity of incident light, φis quantum yield for
production of radicals, ε is molar absorption of photoinitiator,
[I] is the initiator concentration, l is the path length and Ia is
absorbed intensity. According to eq2, the quantum yield of
1173-F was similar to 1173 due to the uniform magnitude order
of dissociation rate in the same conditions.
Photopolymerization Kinetic. As shown in Figure 5, the
monomers were initiated by 1173 and 1173-F, respectively.
Because of the oxygen inhibition of free radical photo-
polymerization, conversions of HDDA with 1173 were different
when the system was with or without oxygen. It was clear that
the oxygen inhibited the polymerization at the first 50 s and
finally got a conversion of 45% (Figure 5a), compared to the
polymerization without oxygen that the conversion got to 75%
in 1 min and to 90% finally (Figure 5b). However, it was found
that the conversion of HDDA with 1173-F in air (Figure 5c)
was higher than HDDA with 1173 in the absence of oxygen.
The conversion of HDDA with 1173-F reached to more than
90% though in the condition of opening to air and the
polymerization started at the very beginning. The results
indicated that the photoinitiator 1173-F had good ability to
overcome the oxygen inhibition. It was initially inferred to the
fluorine group that contributed to the oxygen inhibition
resistance. Because of the low surface tension and low surface
energy of fluorine atoms, the photoinitiator containing fluorine
tended to move to the surface and gathered on the top. This
movement caused high photoinitiator concentration on the top
to consume the oxygen and prevented the further penetration
of oxygen. The polymerization rate of monomer with 1173-F
Figure 7. (A) Scheme of photoinitiator concentration in different
layers by UV spectrophotometer. (B) Concentration of 1173 and
1173-F in acetonitrile solution. All the initial concentration is 1.3 ×
10−4 mol/L.
was slower than that of 1173 due to the low concentration of
initiator in the bulk.
Surface Migratory of 1173-F. The migratory aptitude was
further supported by XPS tests with the monomer HDDA and
initiator 1173-F (5 wt %). As shown the integrated spectra
Figure 6A, the peak at 689.08 eV represented of fluorine.
Because the only source of F in this system is 1173-F, the
content of photoinitiator all through the sample could be
tracked by F signal. It could be found that the fluorine atom
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Figure 8. EDS-line mapping of HDDA with photoinitiator 1173-F, 5% wt: (A) SEM graph of polymer with 3 mm in thickness, (B) vertical direction
from top to bottom, (C) transverse direction of the top layer, and (D) transverse direction of the bottom layer.
Figure 10. Polymerization kinetics of HDDA with different photo-
initiator: (1) 1173, 0.305 mmol/100 g, (2) 1173, 0.1525 mmol/100 g
and 1173-F, 0.1525 mmol/100 g, and (3) 1173-F, 0.305 mmol/100 g.
The intensity of UV light is 40 mW/cm2. All samples were investigated
without oxygen.
Figure 9. Polymerization kinetics of HDDA with different photo-
initiator: (1) 1173, 0.305 mmol/100 g, (2) 1173, 0.1525 mmol/100 g
and 1173-F, 0.1525 mmol/100 g, and (3) 1173-F, 0.305 mmol/100 g.
The intensity of UV light is 10 mW/cm2. All samples were investigated
without oxygen.
gradient distribution was attributed to the low surface tension
and the resulted movement of the fluorine atoms.
content in the first layer was 1.53%, while the fluorine atom
content in the eighth layer was only 0.05%, as shown in Figure
6B. No signal for fluorine element could be detected when
running XPS to the very bottom layer of the sample. The
distribution of F element through the sample indicated that the
initiator containing fluorine had a good migratory ability
leading the aggregation of the initiator to the surface. The
Another proof showing the migratory of 1173-F was the
resultant molecular weight of polymer. Because of gradient
distribution of 1173-F, the surface layer had more initiator than
the bottom layer. As known to us, the molecular weight was
closely related to the content of initiator. The higher of
photoinitiator concentration, the lower the molecular weight is.
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a
fluorine on the top was more than that in the bottom. The
trend of migration accorded with the results of XPS. Some
fluorine atoms detected in the bottom were due to the
molecular diffusion and Brownian motion.
Table 2. Molar Ratio of 1173 and 1173-F in HDDA
1173/1173-F 1173/10−4 mol 1173-F/10−4 mol total/10−4 mol total/g
1173
4/1
3.05
2.44
1.525
0.61
0
3.05
3.05
3.05
3.05
0.05
0.61
1.525
2.44
0.074
0.11
Oxygen Inhibition. To further investigate the properties of
photopolymerization with 1173-F, polymerization kinetics were
used to investigate both in the presence of oxygen and absence
of oxygen. As shown in Figure 9, all the monomers were
irradiated without oxygen and the intensity of light was 10
mW/cm2. All the samples with different photoinitiator had
same molar concentration, 0.305 mmol/100 g of HDDA. It was
found that the induction period of 1173-F was longer probably
due to the lower concentration of initiator in the bulk and the
inhomogenous photopolymerization, as show in Figure 9c.
After adding 1173 to 1173-F (n(1173)/n(1173-F) = 1/1), the
induction period of mixed photoinitiator decreased and the
conversion was higher, as shown in Figure 9b. The molar
concentration of photoinitiator in Figure 10 was the same as
Figure 9 and the intensity of light was 40 mW/cm2. But the
conversion of double bond became higher due to increase of
light intensity. In Figure 10, the induction period of 1173-F
(Figure 10c) was decreased but was still longer than 1173
(Figure 10a). It was speculated that the inhomogenous
distribution of 1173-F prevented fast polymerization of low
layer. Subsequently the mixture of 1173 and 1173-F was used
to initiate monomer in the presence of air. All the samples had
an equal molar concentration with different ratio, as shown in
Table 2. According to Figure 11, it was found that the
conversion increased obviously when the ratio of 1173/1173-F
was 1/4 and its induction period was less than other ratios. The
results indicated that the existence of 1173-F could decrease the
oxygen inhibition effect. Moreover there was a requirement of
content of 1173-F to overcome the oxygen inhibition. Low
concentration of 1173-F could not satisfy the consumption of
oxygen on the surface.
0.11
1/4
0.147
a
Above HDDA = 100 g. The concentration of PI is 3.05 × 10−4 mol in
100 g monomer.
Figure 11. Conversion of HDDA with varying molor ratio of 1173 and
1173-F. The intensity of UV light is 10 mW/cm2. All the samples are
open to air.
The data from GPC was accordance well with the result from
XPS, shown in Table 1. The molecular weights of the top layers
were much lower than the bottom layers and increasing
gradually, contrasting with the distribution of F in the system.
However, the molecular weight of sample with 1173 has a
uniform value from top to bottom.
CONCLUSION
■
In order to investigate the migration behaviors of the 1173-F,
the distribution of concentration of 1173-F in the acetonitrile
solution were characterized by the UV spectrophotometer.
According to Figure 7, the concentration of top solution
increased while the concentration of bottom solution
decreased. Compared to 1173-F, the concentration of 1173
had no change all the time. The change of concentration
occurred simultaneously both in top and bottom. Because of
Brownian Motion, not all the 1173-F molecules could aggregate
on the surface. But it was found that the concentration of the
bottom was stable and the concentration of the top still
increased. It might be explained that most fluorine atoms in
different layers had the trend to move to the top surface with
different rate. The fluorine atoms in the bottom had advantage
to compete in the migratory due to attraction of low surface
energy. Although the concentration of the top changed at the
very beginning, it could increased because of more and more
fluorine atoms moving to the top until the saturation state.
The surface migratory was investigated constantly by the
EDS-line mapping, as shown in Figure 8. The height of Y axis
indicated the intensity of fluorine signal. It was found that the
amount of fluorine decreased along with the EDS-line from the
top to bottom in Figure 8B. Parts C and D of Figure 8 show the
amount of fluorine atom along with transverse direction on the
top and bottom, respectively. It was found that the content of
fluorine atom tended to be the same in the same layer. The
A fluorinated photoinitiator promoted a method to overcome
oxygen inhibition due to its excellent migratory. The results
indicated that this fluorinated photoinitiator had better qualities
in surface photopolymerization than traditional photoinitiator
and could decrease oxygen inhibition effectively without any
other additives or co-initiators. It dramatically increased the
final conversion of HDDA to 90% with oxygen in a condition
of a low light intensity and thin sample. The surface migratory
was confirmed by XPS, EDS-line mapping, GPC and UV
spectrophotometer. This method of relating the migratory was
especially attractive to be utilized in oxygen inhibition
resistance since it was simple and valid.
ASSOCIATED CONTENT
■
S
* Supporting Information
1H NMR and 19F NMR spectra of final product, triplet time of
1173-F by flash laser photolysis, and dissociation rate of 1173
and 1173-F by UV-abs spectroscopy. This material is available
free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: niejun@mail.buct.edu.cn. Telephone: +86 010
64421310 Fax: +86 010 64421310.
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors are grateful to the Program for Open Fund from
State Key Laboratory of Chemical Resource Engineering,
Beijing University of Chemical Technology.
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