Synthesis and properties of novel nonlinear optical polyimide containing nitrophenylazoresorcinoxy group

Article abstract of DOI:10.1080/15421406.2012.710193

New Y-type polyimide (4) containing nitrophenylazoresorcinoxy groups as NLO chromophores, which are components of the polymer backbone, was prepared and characterized. Polyimide 4 is soluble in common organic solvents such as N,N-dimethylformamide and dim

Full text of DOI:10.1080/15421406.2012.710193

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Synthesis and Properties of Novel
Nonlinear Optical Polyimide Containing
Nitrophenylazoresorcinoxy Group
Mi Sung Kim a , Mi Young Song a , Byeongman Jeon a & Ju-Yeon Lee a
a Institute of Basic Science, Department of Chemistry, Inje
University, 607 Obang-dong, Gimhae, 621-749, Korea
Version of record first published: 27 Sep 2012.
To cite this article: Mi Sung Kim, Mi Young Song, Byeongman Jeon & Ju-Yeon Lee (2012): Synthesis
and Properties of Novel Nonlinear Optical Polyimide Containing Nitrophenylazoresorcinoxy Group,
Molecular Crystals and Liquid Crystals, 568:1, 105-110
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Mol. Cryst. Liq. Cryst., Vol. 568: pp. 105–110, 2012
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2012.710193
Synthesis and Properties of Novel Nonlinear Optical
Polyimide Containing Nitrophenylazoresorcinoxy
Group
MI SUNG KIM, MI YOUNG SONG, BYEONGMAN JEON,
AND JU-YEON LEE∗
Institute of Basic Science, Department of Chemistry, Inje University,
607 Obang-dong, Gimhae 621-749, Korea
New Y-type polyimide (4) containing nitrophenylazoresorcinoxy groups as NLO chro-
mophores, which are components of the polymer backbone, was prepared and character-
ized. Polyimide 4 is soluble in common organic solvents such as N,N-dimethylformamide
and dimethylsulfoxide. It shows a thermal stability up to 290◦C in thermogravimet-
ric analysis with glass-transition temperature (Tg) obtained from differential scanning
calorimetry near 128◦C. The second harmonic generation (SHG) coefficient (d33) of
poled polymer film at the 1064 nm fundamental wavelength is around 5.46 × 10−9 esu.
The dipole alignment exhibits a thermal stability up to Tg, and there is no SHG decay
below 130◦C due to the partial main-chain character of polymer structure, which is
acceptable for NLO device applications.
Keywords Differential scanning calorimetry (DSC); dipole alignment; NLO;
polyimide; SHG coefficient; thermogravimetric analysis (TGA)
Introduction
Syntheses of effective nonlinear optical (NLO) materials are very important due to their
potential applications in the fields of optical data storage and optical information process-
ing. NLO polymers have several advantages superior to conventional inorganics, such as
ultrafast response, wide response wave band, high optical damage threshold, and easy fab-
rication [1–3]. In the developments of NLO polymers for electro-optic device applications,
stabilization of electrically induced dipole alignment is one of important considerations; in
this context, two approaches to minimize the randomization have been proposed, namely
the use of cross-linked systems [4,5] and the utilization of polymers with high glass tran-
sition temperature (Tg) such as polyimides [6,7]. Polyimides for NLO applications have
attracted attentions because of their high Tg and high thermal stability. There are two
types of NLO polyimides, which have been used either as host polymers for composite
materials or as matrix polymer backbones for side-chain NLO chromophores. Recently we
prepared novel NLO polyimides containing dioxynitrostilbenyl groups [8] and dioxyben-
zylidenemalononitrile [9], which showed high thermal stability. In this work reported here,
∗Address correspondence to Ju-Yeon Lee, Institute of Basic Science, Department of Chemistry,
Inje University, 607 Obang-dong, Gimhae 621-749, Korea. Tel.: +82-55-320-3221; Fax: +82-55-
321-9718. E-mail: chemljy@inje.ac.kr
[521]/105

106/[522]
M. S. Kim et al.
(a)
(b)
(c)
Figure 1. Main chain NLO polymers (a), Side chain NLO polymers (b), and Y-type NLO
polymers (c).
we prepared new Y-type polyimide containing nitrophenylazoresorcinoxy groups as NLO
chromophores. We selected the latter as NLO chromophores because they were expected
to have high optical nonlinearities. Furthermore, these nitrophenylazoresorcinoxy groups
constitute novel Y-type NLO polyimides (Fig. 1(c)), and these Y-type NLO polyimides
have not yet been reported in the literature. This Y-type NLO polymer is expected to have
the merits of both main-chain (Fig. 1(a)) and side-chain (Fig. 1(b)) NLO polymers, namely
stable dipole alignment and good solubility. After confirming the structure of the resulting
polymer, we investigated its NLO properties. We now report the results of the initial phase
of the work.
Experimental
Materials
Polyimide 4 was synthesized through the reaction of monomer 3 containing NLO chro-
mophore with equivalent amount of 1,4-phenylenediamine in m-cresol. Synthetic method
of polymer 4 is summarized in Scheme 1. The polymer was further purified by extrac-
tion in a Soxhlet extractor with methanol and dried under vacuum to give 6.90 g (90%
yield) of polymer 4. Inherent viscosity (ηinh): 0.30 dL g−1 (c, 0.5 g dL−1 in DMSO at
25◦C). 1H NMR (DMSO-d6) δ 4.08–4.86 (m, 8H, 2 O CH2 CH2 O ), 6.34–7.45 (m,
6H, aromatic), 7.56–7.68 (m, 2H, aromatic), 7.75–8.56 (m, 9H, aromatic). IR (KBr) 2960
(w, C H), 1779 (m, C O), 1723 (vs, C O), 1612 (s, C C), 1369 (s, C N), 729 (s, imide
ring) cm−1. Anal. Calcd for (C40H25N5O12)n: C, 62.58; H, 3.28; N, 9.12. Found: C, 62.67;
H, 3.36; N, 9.20.

Synthesis of Novel NLO Polyimide
[523]/107
OH
HO
O
O
HO
OH
O
O
O
O
CH2=CHOCH2CH2Cl
K2CO3, DMF
H2O, DMF
HCl
N
N
N
N
N
N
2
1
NO2
NO2
NO2
O
O
O
C
O
C
O
O
O
O
O
O
O
O
O
O
O
ClOC
N
N
Pyridine, DMF
3
NO2
O
O
O
C
O
C
N
N
n
O
O
O
O
O
O
H2N
NH2
N
N
4
m-Cresol, N2, 190 o
C
NO2
Scheme 1. Synthetic scheme and structure of polymer 4.
Measurements
1
IR, H NMR, and UV-visible spectra were obtained with a Shimadzu FT IR-8201PC in-
frared spectrophotometer, Varian 300 MHz NMR spectrometer, and Shimadzu UV-3100S
spectrophotometer, respectively. Tg values were measured on a TA 2920 differential scan-
ning calorimeter (DSC) in a nitrogen atmosphere. TA Q50 thermogravimetric analyzer
(TGA) with a heating rate of 10◦C/min up to 800◦C was used for the thermal degradation
study of polymers under nitrogen. The number average molecular weight (Mn) and weight
average molecular weight (Mw) of the polymer were estimated by gel permeation chro-
matography (GPC) (columns styragel HR5E4E; solvent THF). The alignment of the NLO
chromophore of the polymer was carried out by corona poling method. The refractive index
of the polymer sample was measured by the optical transmission technique [10]. Second
harmonic generation (SHG) measurements were made using a Maker fringe technique [11].
Results and Discussion
Synthesis and Characterization of Polymer 4
The synthetic route for polymer 4 is presented in Scheme 1. The resulting polymer was
purified by Soxhlet extraction with methanol as a solvent. The polymerization yield was
90%. The chemical structure of the polymer was identified using 1H NMR, IR spectra, and
elemental analysis. Elemental analysis results fit the polymer structures. 1H NMR spectrum

108/[524]
M. S. Kim et al.
of the polymer 4 has a signal broadening due to polymerization, but the chemical shifts
are consistent with the proposed polymer structure. The IR spectrum of polymer 4 shows
peaks near 1779 and 729 cm−1 that are characteristic bands of imide asymmetric carbonyl
stretching and imide ring deformation, respectively. The spectrum also shows strong ab-
sorption peaks near 1723 cm−1 and 1369 cm−1 due to symmetric carbonyl stretching and
C N stretching of imide ring, respectively. These results are consistent with the proposed
structures, indicating that the NLO chromophores remained intact during the polymeriza-
tion. The molecular weights were determined by GPC using polystyrene as the standard
and THF as an eluent. The number average molecular weight (Mn) of the polymer 4, deter-
mined by GPC, was 14,800 g mol−1 (Mw/Mn = 1.78). The polymer 4 is soluble in common
solvents such as DMF and DMSO, but is not soluble in methanol and diethyl ether. The
inherent viscosity is 0.30 dL g−1. The striking feature of this polymerization system is that
it gives unprecedented Y-type NLO polymer, in which the pendant NLO chromophores are
parts of the polymer backbone. Having obtained the well defined Y-type polyimide 4, we
investigated its properties.
Thermal Properties of Polymer
The thermal behavior of the polymer was investigated by TGA and DSC to determine
the thermal degradation pattern and glass transition temperature. Polymer 4 has a thermal
stability up to 290◦C according to its TGA thermogram. The Tg value of the polymer 4
measured using DSC is near 128◦C. This Tg value is pretty lower than that of the polyimide
containing dioxynitrostilbene, which is near 162◦C [8], or dioxybenzylidenemalononitrile,
which is near 188◦C [9].
Nonlinear Optical Properties of Polymer
The NLO properties of polymers were studied using the SHG method. To induce non-
centrosymmetric polar order, the spin-coated polymer films were corona-poled. As the
temperature was raised gradually to 5–10◦C higher than Tg, a corona voltage of 6.5 kV
was applied and this temperature was maintained for 30 min. SHG measurements were
performed at a fundamental wavelength of 1064 nm with a mode locked Nd-YAG laser and
OPO. In order to determine the microscopic second-order susceptibility of the polymer,
the angular SHG dependence was recorded. Figure 2 shows the angular dependence of
SHG signal for a poled sample of polymer 4. The SHG values were compared with those
obtained from a Y-cut quartz plate. To calculate the d31 and d33 values, both s-polarized and
p-polarized IR laser were directed at the samples. The NLO properties of polymer 4 are
summarized in Table 1. SHG coefficients (d33) were derived from the analysis of measured
Table 1. Nonlinear Optical Properties of Polymer 4
d33
d31
film thicknessb
a
a
c
c
Polymer
(esu)
(esu)
(µm)
n
Mn
14,800
Mw
26,300
4
5.46 × 10−9
1.72 × 10−9
0.54
1.682
aSHG coefficients (d33) were derived from the analysis of measured Maker-fringes [11].
bFilm thickness was determined by the optical transmission technique [10].
cMeasured by gel permeation chromatography (GPC) in THF using polystyrene standard.

Synthesis of Novel NLO Polyimide
[525]/109
0.12
0.10
0.08
0.06
0.04
0.02
0.00
s-wave
p-wave
-80 -60 -40 -20
0
20
40
60
80
Incident Angle [o]
Figure 2. Angular dependence of SHG signal for a poled film of polymer 4.
Maker-fringes with the Pascal fitting program according to the literature procedure [11].
The values of d33 and d31 for polymer 4 are 5.46 × 10−9 and 1.72 × 10−9 esu, respectively.
This d33 value is higher than that of the polyimide containing dioxynitrostilbene [8], but is
similar with that of polyimide with dioxybenzylidenemalononitrile [9]. However, the d33
value is rather smaller than those of conventional polymers [1,2].
To evaluate the high-temperature stability of the polymer, we studied the temporal
stability of the SHG signal. Figure 3 shows the dynamic thermal stability study of the NLO
activity of a film of polymer 4. To investigate the real time NLO decay of the SHG signal
of the poled polymer film as a function of temperature, in situ SHG measurement was
performed at a heating rate of 4◦C min−1 from 30 to 200◦C. The polymer film exhibits a
thermal stability up to Tg, and no significant SHG decay is observed below 130◦C. However,
1.2
1.0
0.8
0.6
0.4
0.2
0.0
0
50
100
150
200
Temperature (oC)
Figure 3. Normalized SHG signal of polymer 4 as a function of temperature at a heating rate of
4◦C/min.

110/[526]
M. S. Kim et al.
the SHG thermal stability is lower than that of the polyimide containing dioxynitrostilbene,
which is in the range 180–190◦C [8] or dioxybenzylidenemalononitrile, which is in the
range 190–200◦C [9]. This lower SHG thermal stability of the polymer 4 is probably due to
the lower Tg value. In general, side-chain NLO polymers lose the thermal stability of dipole
alignment below Tg. Stabilization of dipole alignment is a characteristic of main-chain
NLO polymers. The enhanced thermal stability of second harmonic generation of polymer
4 is due to the stabilization of dipole alignment of NLO chromophores, which stems from
the partial main-chain character of the polymer structure. Thus, we obtained a new type of
NLO polyimide with quite stable dipole alignment and good solubility.
Conclusions
New Y-type polyimide 4 with pendant NLO chromophores as parts of the polymer main
chain was synthesized. This Y-type NLO polyimide 4 is soluble in common organic solvents
and displayed a thermal stability up to 290◦C with a Tg of 128◦C. The SHG coefficient
(d33) of corona-poled polymer film is 5.46 × 10−9 esu. Polymer 4 exhibits SHG stability
up to glass transition temperature (Tg), and no SHG decay is observed below 130◦C. This
enhanced thermal stability of optical nonlinearity stems from the stabilization of dipole
alignment of the NLO chromophores, which are parts of the polymer backbone.
Acknowledgements
This research was supported by Basic Science Research Program through the National
Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and
Technology (No. 2011-0026534).
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