Novel photocrosslinking nonlinear optical polymer systems

Article abstract of DOI:10.1080/15421400500366837

We designed and synthesized novel photocrosslinking guest-host nonlinear optical (NLO) polymer systems. In the system, guest NLO molecules bearing two photocrosslinking groups such as cinnamate or acrylate double bonds were homogeneously dispersed in the host photocrosslinking polymer matrix. The NLO molecules are then aligned by using the corona poling technique at an elevated temperature and the system was subsequently crosslinked by irradiating UV light. We expect the photocrosslinking keeps the NLO molecules from relaxing to random orientation. Formation of network structure was confirmed by comparing the absorbances of the NLO moiety in the crosslinked film before and after washing the film with methanol or acetone, exhibiting little leaching out of the NLO moiety from the polymer matrix.

Full text of DOI:10.1080/15421400500366837

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Novel Photocrosslinking
Nonlinear Optical Polymer
Systems
Dong Kyo Seo a , Hyun Soon Lim a , Jun Young Lee a
& Whan Gun Kim b
a School of Applied Chemistry and Chemical
Engineering, Sungkyunkwan University, Suwon, Korea
b Department of Applied Chemistry, Seokyeong
University, Seoul, Korea
Version of record first published: 23 Aug 2006.
To cite this article: Dong Kyo Seo , Hyun Soon Lim , Jun Young Lee & Whan Gun Kim
(2006): Novel Photocrosslinking Nonlinear Optical Polymer Systems, Molecular Crystals
and Liquid Crystals, 445:1, 323/[613]-330/[620]
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Mol. Cryst. Liq. Cryst., Vol. 445, pp. 323=[613]–330=[620], 2006
Copyright # Taylor & Francis LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400500366837
Novel Photocrosslinking Nonlinear Optical
Polymer Systems
Dong Kyo Seo
Hyun Soon Lim
Jun Young Lee
School of Applied Chemistry and Chemical Engineering,
Sungkyunkwan University, Suwon, Korea
Whan Gun Kim
Department of Applied Chemistry, Seokyeong University,
Seoul, Korea
We designed and synthesized novel photocrosslinking guest-host nonlinear optical
(NLO) polymer systems. In the system, guest NLO molecules bearing two photo-
crosslinking groups such as cinnamate or acrylate double bonds were homoge-
neously dispersed in the host photocrosslinking polymer matrix. The NLO
molecules are then aligned by using the corona poling technique at an elevated
temperature and the system was subsequently crosslinked by irradiating UV light.
We expect the photocrosslinking keeps the NLO molecules from relaxing to random
orientation. Formation of network structure was confirmed by comparing the
absorbances of the NLO moiety in the crosslinked film before and after washing
the film with methanol or acetone, exhibiting little leaching out of the NLO moiety
from the polymer matrix.
Keywords: corona poling; guest-host system; network structure; nonlinear optical
polymer; photocrosslinking
1. INTRODUCTION
Polymers are most promising materials for practical applications in
the fields of optics and electrooptics, which can possess various kinds
This work was supported by the Korea Research Foundation Grant (KRF-2004-005-
D00063).
Address correspondence to Jun Young Lee, School of Applied Chemistry and Chemi-
cal Engineering, Sungkyunkwan University, 300 Chunchun-dong, Jangan-gu, Suwon
440-746, Kyunggi-do, Korea. E-mail: jylee7@skku.edu
323=[613]

324=[614]
D. K. Seo et al.
FIGURE 1 Schematic diagram of photocrosslinking NLO guest-host system.
of functionalities as well as can be easily processed to a desired shape.
For second order nonlinear optical (NLO) polymer system, stable and
high second order NLO response is required. Since the last decades, a
number of researches have been focused on improvement of the NLO
stability by obtaining high Tg polymers such as NLO polyimides [1,2]
or by creating a thermo- or photocrosslinked polymer networks during
poling [3,4].
Photocrosslinking for NLO applications has been achieved by either
[2 þ 2] cycloaddition of cinnamates [5,6] or radical reaction of (meth)a-
crylates [7,8]. Photocrosslinkable NLO polymers can be directly pat-
terned to a desired shape by irradiation of UV light through a mask,
possibly fabricating channel waveguide and other integrated optical
devices. Photocrosslinking NLO system can be processed within a very
short time (about 20 minutes) even with weak light intensity.
In this study, design and syntheses of novel photocrosslinking second
order NLO polymer systems are reported, where guest NLO molecules
are aligned in one direction by applying an electric field and crosslinked
to the polymer matrix by UV irradiation as shown in Figure 1. We
expect the photocrosslinking keeps the NLO molecules from relaxing
to random orientation. Formation of network structure was confirmed
by comparing the absorbances of the NLO moiety in the crosslinked film
before and after washing the film with methanol or acetone.
2. EXPERIMENTAL
The photocrosslinking host matrix polymers containing cinnamate or
methacrylate groups were synthesized through two-step reactions.
Figure 2(a) shows the typical synthesis scheme of the photocrosslin-
king polymer and the structures of the epoxy resins used in the study.

Novel Photocrosslinking NLO Polymer Systems
325=[615]
FIGURE 2 Synthetic schemes of (a) photocrosslinking matrix polymer and
(b), (c) photocrosslinking NLO chromophores.
In the first step, XP2030=CA (or XP2030=MA) was prepared by reac-
tion between XP2030 and cinnamic acid (or methacrylic acid). The
reaction was carried out using tetramethyl ammonium chloride as
a catalyst in MEK at 80ꢀC for 120 h. In the second step, cinnamate
(or acrylate) double bond was incorporated into XP2030=CA (or
XP2030=MA) matrix polymer by the reaction between hydroxyl group
of XP2030=CA and cinnamoyl (or methacroyl) chloride, resulting in
XP2030=CA=CA (or XP2030=MA=MA). The reaction was carried out
using triethylamine as a catalyst in THF at room temperature for 12 h.
Photocrosslinking NLO molecules were synthesized as shown in
Figures 2(b) and 2(c). 0.12 mol of 2-amino-5-nitrophenol was dissolved
in 75 ml of 0.78 M sulfuric acid aqueous solution and cooled down to
0ꢀC, to which 100 mL of 0.8 M sodium nitrite ice-cold aqueous solution
was slowly added. To the above solution, 115 mL of 2 M sodium
hydroxide ice-cold solution containing 0.12 mol of 3-diethylaminophe-
nol was added while stirring. The reaction was carried out for 12 h,
where the temperature was slowly increased to room temperature.
The precipitate, DADNAB was filtered, purified by column chromato-
graphy and dried under vacuum, giving rise to a deep purple powder.

326=[616]
D. K. Seo et al.
Cinnamate (or methacrylate) double bond was incorporated into
DADNAB by the reaction between hydroxyl group of DADNAB and
cinnamoyl chloride. The reaction was carried out following the same
procedure as that for matrix polymer. Disperse Red 19 (DR19) was
purchased and also employed as NLO moiety.
The matrix polymer and guest NLO molecules were completely dis-
solved in THF and the solution was filtered through 0.2 mm PTFE
membrane filter and spin-coated on a glass substrate at 1500 rpm
for 60 s. The concentration of the NLO molecules was 20 wt% with
respect to the polymer. The polymer film was then dried at 70ꢀC for
30 min, leading to an excellent quality of film with a thickness of about
1 mm. NLO molecules were aligned by the corona poling technique at
about 10ꢀC below Tg of the corresponding polymers. Photocrosslinking
of the polymer film was carried out by exposing the film to 254 nm UV
light with the intensity of 9 mW=cm2 on the surface of the film for
about 20 min.
Photocuring behaviors of the acrylate and cinnamate double bonds
with UV exposure time were quantitatively studied by using FT-IR
and UV-vis spectroscopies, respectively. Photocrosslinking between
polymer matrix and NLO molecules was confirmed by comparing the
absorbances of the NLO moiety in the crosslinked film before and after
washing the film with methanol or acetone.
3. RESULTS AND DISCUSSION
We confirmed the chemical structures of the synthesized photocross-
linking matrix polymers and photocrosslinking NLO molecules by
1H-NMR and FT-IR spectroscopies. In the 1H-NMR spectra of XP2030=
CA=CA and XP2030=MA=MA, the peak at 2.7–2.9 ppm corresponding
to the epoxide ring proton disappeared and new peaks at 6.48–7.95,
5.6, and 6.1 ppm corresponding to the proton of the double bond
appeared. In the FT-IR spectra of XP2030=CA=CA and XP2030=
MA=MA, new characteristic absorption peaks were observed at
ꢁ1
ꢁ1
=
1720 cm (carbonyl group), 1620 cm (C C).
In the 1H-NMR spectra of DADNAB=CA, new peaks at 6.48–
7.95 ppm corresponding to the proton of the double bond appeared.
In the FT-IR spectra of the DADNAB=CA, new characteristic absorp-
tion peaks were observed at 1720 cmꢁ1 (carbonyl group), 1518 cmꢁ1 and
1330 cmꢁ1 (nitro group).
Photocrosslinking behaviors of the cinnamate and methacrylate
double bonds were studied by monitoring change of the absorbances
at 275 nm of UV-Vis spectrum and at 810 cmꢁ1 of FT-IR spectrum,
respectively. We confirmed very effective photocuring of the cinnamate

Novel Photocrosslinking NLO Polymer Systems
327=[617]
FIGURE 3 (a) UV-visible absorption spectra and (b) photocure conversions of
the photocrosslinking polymers with UV exposure time.
and acrylate double bonds in the matrix polymers as shown in Figure 3.
=
The absorption peak at 275 nm corresponding to cinnamate C C bond
decreased with UV irradiation time as shown in Figure 3(a). We also
figured out photocrosslinking of the cinnamate and methacrylate
double bonds took place extremely rapidly. Figure 3(b) shows the
photocure conversion obtained from the change of absorbances of
cinnamate and acrylate double bonds normalized with respect to the
absorbance of the double bonds before UV irradiation. In all polymers
prepared in this study, photocrosslinking occurred rapidly within the
initial 300 s, even though a little difference in the reaction rate and
the final conversion were observed.
We also confirmed the effective photocrosslinking of the polymer
systems from glass transition behavior. Tg of XP2030=CA=CA and
XP2030=MA=MA were found to be 90 and 87.5ꢀC, respectively. Even
though Tg decreased with the amount NLO molecules, the plasticizing
effect of NLO molecule is not so pronounced as that in other common
plasticizers. The polymer system irradiated by UV for 20 min did not
show any observable glass transition behavior, confirming the forma-
tion of network structure with high degree of crosslinking.
During the radiation of UV light, two different types of photoreac-
tion can occur in the NLO molecules. One is the photocrosslinking
reaction of photoreactive units and the other is the photoinduced
degradation of the NLO chromophore. The absorbance of NLO moiety
slightly decreased with irradiation time as shown in Figure 4, possibly
because of the photoinduced degradation of the NLO chromophore.
We confirmed the formation of network structure, which was
formed by crosslinking between NLO molecules and polymer matrix.
Before UV irradiation the NLO molecules could be completely leached

328=[618]
D. K. Seo et al.
FIGURE 4 UV-vis spectra of the pristine film, the photocrosslinked film and the
film photocrosslinked and washed with acetone. (a) XP2030=CA=CA þ DADNAB=
CA, (b) XP2030=CA=CA þ DR19=CA, (c) XP2030=MA=MA þ DADNAB=MA and
(d) XP2030=MA=MA þ DR19=MA.
out from the polymer matrix by dipping the film in acetone or meth-
anol, while after UV irradiation the NLO molecules could not be
washed out from the polymer matrix. This implies the NLO molecules
are employed as the part of the network structure.
The formation of crosslinked network was also confirmed by UV-vis
spectra as shown in Figure 4. The absorbances of NLO molecules chan-
ged a little even after washing the photocrosslinked film with acetone
or methanol, indicating no leaching out of the NLO molecules from the
polymer matrix. As shown in Figure 4, it seemed that cinnamate double
bonds produced better network structure than acrylate double bonds.
The crosslinked system bearing cinnamate double bonds exhibited no
change in the absorbance after washing, while the system bearing
acrylate double bonds showed a little decrease in the absorbance after
washing. We, therefore, strongly believe the photoinduced crosslinking
can keep the NLO molecules from relaxing to random orientation. The

Novel Photocrosslinking NLO Polymer Systems
329=[619]
FIGURE 5 TGA curves of the photocrosslinked polymers.
photocrosslinked polymer showed good thermal stability as shown in
Figure 5, exhibiting little weight loss up to about 250ꢀC.
4. CONCLUSION
Novel photocrosslinking guest-host nonlinear optical (NLO) polymer
systems were designed and synthesized. In the system, guest NLO
molecules bearing two photocrosslinking groups were homogeneously
dispersed in the host photocrosslinking polymer matrix. The NLO
molecules are then aligned by using the corona poling technique at
an elevated temperature and the system was subsequently crosslinked
by irradiating UV light. We confirmed formation of network structure
by comparing the absorbances of the NLO moiety in the crosslinked
film before and after washing the film with methanol, exhibiting little
leaching out of the NLO moiety from the polymer matrix. Therefore,
we believe the photocrosslinking can keep the NLO molecules from
relaxing to random orientation.
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