Preparation of novel Y-type nonlinear optical polyimides with high thermal stability of second harmonic generation

Article abstract of DOI:10.1080/15421400802330531

2,4-Bis-(3,4-dicarboxyphenylcarboxyethoxy)-1-(2,2-dicyanovinyl)benzene dianhydride (4) was prepared and reacted with 1,4-phenylenediamine and 4,4'-oxydianiline to yield novel Y-type polyimides 5-6 containing 2,4-dioxybenzylidenemalononitrile groups as NLO-chromophores, which constitute parts of the polymer backbone. The resulting polyimides 5-6 are soluble in polar solvents such as dimethylsulfoxide and N,N-dimethylformamide. Polymers 5-6 showed a thermal stability up to 330°C in thermogravimetric analysis thermograms with Tg values obtained from differential scanning calorimetry thermograms in the range 179-188°C. The second harmonic generation (SHG) coefficients (d33) of poled polymer films at the 1064 nm fundamental wavelength were around 6.32 × 10^-9esu. The dipole alignment exhibited exceptionally high thermal stability even at 20°C higher than Tg, there was no SHG decay below 200°C because of the partial main chain character of polymer structure, which is acceptable for nonlinear optical device applications.

Full text of DOI:10.1080/15421400802330531

Mol. Cryst. Liq. Cryst., Vol. 491, pp. 231–239, 2008
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400802330531
Preparation of Novel Y-Type Nonlinear Optical
Polyimides with High Thermal Stability of Second
Harmonic Generation
Dong-Seon Won, Ga-Young Lee, and Ju-Yeon Lee
Institute of Functional Materials, Department of Chemistry,
Inje University, Obang-dong, Gimhae, Korea
2,4-Bis-(3,4-dicarboxyphenylcarboxyethoxy)-1-(2,2-dicyanovinyl)benzene
dianhy-
dride (4) was prepared and reacted with 1,4-phenylenediamine and 4,40-oxydianiline
to yield novel Y-type polyimides 5-6 containing 2,4-dioxybenzylidenemalononitrile
groups as NLO-chromophores, which constitute parts of the polymer backbone.
The resulting polyimides 5-6 are soluble in polar solvents such as dimethylsulfoxide
and N,N-dimethylformamide. Polymers 5-6 showed a thermal stability up to 330ꢀC
in thermogravimetric analysis thermograms with Tg values obtained from differen-
tial scanning calorimetry thermograms in the range 179–188ꢀC. The second
harmonic generation (SHG) coefficients (d33) of poled polymer films at the 1064 nm
fundamental wavelength were around 6.32 ꢁ 10ꢂ9 esu. The dipole alignment exhib-
ited exceptionally high thermal stability even at 20ꢀC higher than Tg, there was no
SHG decay below 200ꢀC because of the partial main chain character of polymer
structure, which is acceptable for nonlinear optical device applications.
Keywords: differential scanning calorimetry (DSC); NLO; polyimides; relaxation of
dipole alignment; SHG coefficient; thermogravimetric analysis (TGA)
INTRODUCTION
Recently, attention has been focused on the synthesis of nonlinear
optical (NLO) polymers because of their potential applications in the
field of electro-optic devices [1–4]. In the developments of NLO
polymers for applications, stabilization of electrically induced dipole
This work was supported by the Korea Research Foundation Grant funded by the
Korean Government (MOEHRD, Basic Research Promotion Fund, KRF-2007-313-
C00498).
Address correspondence to Ju-Yeon Lee, Institute of Functional Materials, Depart-
ment of Chemistry, Inje University, 607 Obang-dong, Gimhae 621-749, Korea. E-mail:
chemljy@inje.ac.kr
231

232
D.-S. Won et al.
alignment is one of the important considerations. Two approaches to
minimize the randomization have been proposed namely the use of
cross-linking methods [5–8] and the utilization of high Tg polymers
such as polyimides [9–12]. Polyimides as NLO materials have many
advantages such as their high Tg and high thermal stability. There
are two types of NLO polyimides, which have been used either as poly-
mer hosts for composite materials or as polymer backbones for side-
chain NLO chromophores. Side-chain polymer systems have the
advantages such as good solubility, homogeneity and high loading
level of NLO chromophores, but they tend to lose nonlinear optical
activity at lower temperature by randomization of NLO chromophores.
Main-chain NLO polymershave good thermal stability of dipole align-
ments, but they often do not dissolve in organic solvents, and their
intractability make them unusable to fabricate stable noncentrosym-
metric films. Recently we reported novel Y-type NLO polyimides con-
taining dioxynitrostilbenyl group [13–14]. In this work we prepared
novel polyimides containing 2,4-dioxybenzylidenemalononitrile groups
as NLO-chromophores. We selected 2,4-dioxybenzylidenemalononitrile
because they have large dipole moments and are rather easy to
FIGURE 1 (a) Main Chain NLO polymers, (b) side chain NLO polymers, and
(c) Y-type NLO polymers.

Novel Y-Type NLO Polyimides with High Thermal Stability
233
synthesize. Furthermore 2,4-dioxybenzylidenemalononitrile groups
constitute novel Y-type NLO polyimides (Fig. 1c), and these Y-type
NLO polyimides have not yet been presented in the literature. Thus,
we synthesized another new type of NLO polyimide, in which the pen-
dant NLO chromophores are parts of the polymer backbones. These
mid-type NLO polymers are expected to have both the merits of
main-chain and side-chain-NLO polymers namely stable dipole align-
ment and good solubility. After confirming the structure of the resulting
polymers, we investigated their properties such as solubility, second
harmonic generation (SHG) activity, and relaxation of dipole alignment.
We now report the results of the initial phase of the work.
EXPERIMENTAL
Materials
Synthetic methods of dianhydride 4 and polymers 5-6 are summarized
in Schemes 1 and 2. Compound 4: 1H NMR (DMSO-d6) d 4.38–4.82 (t,
8H, 2-O-CH2-CH2-O-), 6.82–6.95 (m, 2H, aromatic), 7.58–7.66 (m, 1H,
benzylic), 7.73–7.82 (m, 2H, aromatic), 8.04–8.30 (m, 4H, aromatic),
=
8.67–8.72 (d, 1H, aromatic). IR (KBr) 3078 (w, C-H), 2962 (w, C-H),
=
=
2226 (s, CN), 1780 (m, C O, dianhydride), 1720 (vs, C O, ester),
1608 (s, C C) cmꢀ1. Anal. Calcd for C32H18N2O12: C, 61.74; H, 2.91;
=
N, 4.50. Found: C, 61.83 H, 2.97; N, 4.58. Polymer 5: Inherent viscosity
(ginh): 0.30 dL gꢀ1 (c ¼ 0.5 g dLꢀ1 in m-cresol at 25ꢁC). 1H NMR (DMSO-
d6) d 4.24–4.82 (m, 8H, 2 -O-CH2-CH2-O-), 6.32–7.25 (m, 5H, benzylic,
aromatic), 7.54–7.68 (s, 3H, aromatic), 7.92–8.55 (m, 6H, aromatic). IR
=
=
(KBr) 3070 (w, C-H), 2953 (w, C-H), 2201 (s, CN), 1779 (m, C O),
SCHEME 1 Synthetic method of compound 4.

234
D.-S. Won et al.
SCHEME 2 Synthetic method of polymers 5-6.
ꢀ1
=
=
1727 (vs, C O), 1610 (s, C C), 1377 (s, C-N), 726 (s, imide ring) cm
.
Anal. Calcd for (C38H22N4O10)n: C, 65.71; H, 3.19; N, 8.07. Found:C,
65.81; H, 3.25; N, 8.16. Polymer 6:ginh:0.28 dL gꢀ1 (c ¼ 0.5 g dLꢀ1 in
m-cresol at 25ꢁC). 1H NMR (DMSO-d6) d 4.23–4.81 (m, 8H, 2 -O-
CH2-CH2-O-), 6.32–7.03 (m, 6H, benzylic, aromatic), 7.05–7.38 (m,
3H, aromatic), 7.40–7.58 (m, 4H, aromatic), 7.96–8.17 (m, 2H, aro-
matic), 8.20–8.49 (m, 3H, aromatic). IR (KBr) 3070 (w, ꢀC-H), 2953
=
=
(w, C-H), 2205 (m, CN), 1779 (m, C O), 1724 (vs, C O), 1610
(s, C C), 1379 (s, C-N), 728 (s, imide ring) cmꢀ1. Anal. Calcd for
=
(C44H26N4O11)n: C, 67.18; H, 3.33; N, 7.12. Found: C, 67.27; H, 3.37;
N, 7.19.
Instrumentation
1
IR, H NMR, and UV-Vis spectra were taken on a Shimadzu FT IR-
8201PC infrared spectrophotometer, Varian 300 MHz NMR spec-
trometer, and Shimadzu UV-3100S spectrophotometer, respectively.
Elemental analyses were performed using a Perkin-Elmer 2400
CHN elemental analyzer. Tg values were measured on a TA 2920 dif-
ferential scanning calorimeter (DSC) in a nitrogen atmosphere.
DuPont 951 thermogravimetric analyzer (TGA) with a heating rate
of 10ꢁC=min up to 800ꢁC was used for the thermal degradation of poly-
mers under nitrogen. The number average molecular weight (Mn) and
weight average molecular weight (Mw) of the polymers were estimated
by gel permeation chromatography (GPC) (columns styragel HR5E4E;

Novel Y-Type NLO Polyimides with High Thermal Stability
235
solvent THF). The alignment of the NLO-chromophore of the polymers
was carried out by corona poling method. The refractive index of the
polymer sample was measured by the optical transmission technique
[15]. Second harmonic generation (SHG) measurements were made
using a Maker fringe technique [16].
RESULTS AND DISCUSSION
Synthesis and Characterization of Polymers 5-6
Synthetic method of monomer 4 is summarized in Scheme 1. The
polymers 5-6 were synthesized by reacting aromatic dianhydride
monomer
4
containing NLO-chromophore with stoichiometric
amounts of the corresponding aromatic diamine in m-cresol (Scheme 2).
The polymerization yield was 88–92%. The resulting polymers were
purified by Soxhlet extraction with diethyl ether as a solvent. The
chemical structures of the resulting polymers were confirmed by
1H NMR, IR spectra, and elemental analysis. 1H NMR spectra of
the polymers showed a signal broadening due to polymerization,
but the chemical shifts are consistent with the proposed polymer
structures. The IR spectra of the polymer samples showed peaks
near 1779 and 726 cm–1 that are characteristic bands of imide asym-
metric carbonyl stretching and imide ring deformation, respectively.
IR spectra of the same polymer samples also showed strong absorp-
tion peaks near 1727 cmꢀ1 and 1377 cm–1 due to a symmetric car-
bonyl stretching and C-N stretching of imide ring, respectively.
These results are consistent with the proposed structures, indicating
that the NLO-chromophore remained intact during the imidization
process. The number-average molecular weight (Mn) was determined
by GPC to be 21600 (Mw=Mn ¼ 1.75) for polymer 5. Polydispersities
were in the range of 1.67–1.79. The polyimides 5-6 were soluble in
common solvents such as DMF and DMSO, but were not soluble in
methanol and diethyl ether. The inherent viscosities were in the
range of 0.28–0.30 dL=g. Polymers 5-6 showed strong absorption near
382 nm by the NLO-chromophore dioxybenzylidenemalononitrile
group. The striking feature of these polymers was that they had pen-
dant NLO chromophores that were parts of the polymer main chains.
Thus, we obtained a new type of NLO polyimide with side chain and
main chain characteristics. These mid-type NLO polymers were
expected to have the advantage of both main-chain and side-chain
NLO polymers. We now have well defined polymers 5-6 and investi-
gate their properties.

236
D.-S. Won et al.
TABLE 1 Thermal Properties of Polymers 5-6
ꢁ
Degradation temp, Cb
Residueb at
800ꢁC, wt%
Polymer
Tga;ꢁC
5 wt%-loss
20 wt%-loss
40 wt%-loss
5
6
188
179
376
342
436
427
542
498
41.8
35.6
aDetermined from DSC curves measured on a TA 2920 differential scanning calor-
imeter with a heating rate of 10ꢁC=min under nitrogen atmosphere.
bDetermined from TGA curves measured on a DuPont 951 thermogravimetric analy-
zer with a heating rate of 10ꢁC=min under nitrogen atmosphere.
Thermal Properties of Polymers
The thermal behavior of the polymers was investigated by TGA and
DSC to determine the thermal degradation pattern and Tg. The
results are summarized in Table 1. Polymers 5-6 showed thermal
stability up to 330ꢁC from their TGA thermograms as shown in
Table 1. Tg values of the polymers 5-6 measured by DSC were in the
range 179–188ꢁC. These are high values compared to those of poly-
imides containing 2,3-dioxynitrostilbene [13,14], but are rather low
values compared to those of common rigid polyimides, which can prob-
ably be attributed to the flexibility of the polymer backbone containing
ether linkages. The TGA and DSC studies showed that the decompo-
sition temperatures of the polyimides 5-6 were higher than the corres-
ponding Tg values. This indicates that high-temperature poling for a
short term is feasible without damaging the NLO chromophore.
Nonlinear Optical Properties of Polymers
The NLO properties of polymers were studied by the SHG method.
The spin-coated polymer films were corona-poled to induce noncentro-
symmetric polar order. UV-Vis absorption spectra of the polymers
before and after poling were recorded. After electric poling, the dipole
moments of the NLO-chromophores were aligned and the UV-Vis spec-
tra of polymers exhibited a slight blue shift and a decrease in absorp-
tion due to birefringence. The estimated order parameter value U was
equal to 0.16 for polymer 6 (U ¼ 1 ꢀ A1=A0, where A0 and A1 are the
absorbances of the polymer film before and after poling). The decrease
in absorbance after poling is an indicator of the dipole alignment. The
refractive index of the sample was measured by the optical trans-
mission technique [14]. SHG measurements were performed at a

Novel Y-Type NLO Polyimides with High Thermal Stability
237
FIGURE 2 Angular dependence of SHG signal in a poled film of polymer 6.
fundamental wavelength of 1064 nm using a mode locked Nd-YAG
laser. In order to determine the microscopic second-order suscepti-
bility of the polymer, the angular SHG dependence was recorded.
Figure 2 shows the angular dependence of SHG signal in a poled
polymer 5. The SHG values were compared with those obtained from
a Y-cut quartz plate. To calculate the d31 and d33 values, both s-polar-
ized and p-polarized IR laser were directed to the samples and
recorded. Nonlinear optical properties of polymers 5-6 are summarized
in Table 2. The values of d31 and d33 for polymer 6 were 2.28 ꢂ 10ꢀ9
and 6.32 ꢂ 10ꢀ9 esu, respectively. These values are somewhat higher
TABLE 2 Nonlinear Optical Properties of Polymers 5-6
Film thicknessd (mm) d31 (esu)
d33=d31
a
b
c
b
Polymer kmax (nm) d33 (esu)
/
5
6
382
381
5.48 ꢂ 10ꢀ9 0.24
6.32 ꢂ 10ꢀ9 0.16
0.32
0.30
1.94 ꢂ 10ꢀ9
2.82
2.77
2.28 ꢂ 10ꢀ9
aPolymer film after corona poling.
bSHG coefficients (d33) were derived from the analysis of measured Maker-fringes
[16].
cOrder parameter U ¼ 1 ꢀ A1=A0, where A0 and A1 are the absorbances of the polymer
film before and after corona poling, respectively.
dFilm thickness was determined by the optical transmission technique [15].

238
D.-S. Won et al.
FIGURE 3 Normalized SHG signal of polymer 6 as a function of temperature
at a heating rate of 10ꢁC=min.
than those of polyimides containing 2,3-dioxynitrostilbene as NLO
chromophores. Since the second harmonic wavelength was at
532 nm, which is not in the absorptive region of the resulting polymer,
there was not resonant contribution to this d33 value. To evaluate the
high-temperature stability of the polymers, we studied the temporal
stability of the SHG signal. In Figure 3, we present the dynamic
thermal stability study of the NLO activity of the film 6. To investigate
the real time NLO decay of the SHG signal of the poled polymer films
as a function of temperature, in situ SHG measurements were per-
formed at a heating rate of 10ꢁC=min from 30 to 250ꢁC. The poled film
of polymers 5-6 exhibited a thermal stability even at 20ꢁC higher than
Tg and no significant SHG decay was observed below 200ꢁC, which is
acceptable for NLO device applications. These thermal stability of
SHG are higher than those of polymers with 2,3-dioxynitrostilbenyl
groups. Side-chain NLO polymers usually lose thermal stability of
dipole alignment below Tg. Stabilization of dipole alignment is a
characteristic of main-chain NLO polymers. The exceptionally high
thermal stability of second harmonic generation of polymers 5-6 was
probably due to the stabilization of dipole alignment of NLO chromo-
phore, which stemmed from the partial main chain character of the
polymer structure. Thus, we obtained a new type of NLO polyimide
having the advantage of both main chain- and side-chain NLO poly-
mers: stabilization of dipole alignment and good solubility.

Novel Y-Type NLO Polyimides with High Thermal Stability
239
CONCLUSIONS
Novel, Y-type polyimides (5-6) with pendant NLO chromophores as
parts of polymer main chains were synthesized. These mid-type
NLO polyimides 5-6 are soluble in common organic solvents and
showed thermal stability up to 320ꢁC and Tg values in the range
179–188ꢁC. The SHG coefficients (d33) of corona-poled polymer films
were around 6.32 ꢂ 10ꢀ9 esu. The striking feature of these polymers
is that they exhibit SHG stability up to 20ꢁC higher than Tg and no
SHG decay was observed below 200ꢁC. The high thermal stability of
optical nonlinearity stems from the stabilization of dipole alignment
of the NLO chromophore, which was a part of the polymer backbone.
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