Synthesis and electro-optic properties of novel T-type polyurethane with high thermal stability of second harmonic generation

Article abstract of DOI:10.1080/15421400701548969

2,5-Di-(2'-hydroxyethoxy)benzylidenemalononitrile (3) was prepared and polyadded with 3,3'-dimethoxy-4,4'-biphenylenediisocyanate to yield novel T-type polyurethane 4 containing 2,5-dioxy benzylidenemalononitrile group as a nonlinear optical (NLO)-chromop

Full text of DOI:10.1080/15421400701548969

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Synthesis and Electro-Optic
Properties of Novel T-Type
Polyurethane with High
Thermal Stability of Second
Harmonic Generation
Ju-Yeon Lee a , Won-Taek Jung a & Jin-Hyang Kim a
a Institute of Functional Materials, Department of
Chemistry, Inje University , Obang-dong, Gimhae,
Korea
Published online: 22 Sep 2010.
To cite this article: Ju-Yeon Lee , Won-Taek Jung & Jin-Hyang Kim (2007) Synthesis
and Electro-Optic Properties of Novel T-Type Polyurethane with High Thermal Stability
of Second Harmonic Generation, Molecular Crystals and Liquid Crystals, 471:1,
357-364, DOI: 10.1080/15421400701548969
To link to this article: http://dx.doi.org/10.1080/15421400701548969
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Mol. Cryst. Liq. Cryst., Vol. 471, pp. 357–364, 2007
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400701548969
Synthesis and Electro-Optic Properties of Novel
T-Type Polyurethane with High Thermal Stability
of Second Harmonic Generation
Ju-Yeon Lee
Won-Taek Jung
Jin-Hyang Kim
Institute of Functional Materials, Department of Chemistry,
Inje University, Obang-dong, Gimhae, Korea
2,5-Di-(20-hydroxyethoxy)benzylidenemalononitrile (3) was prepared and
polyadded with 3,30-dimethoxy-4,40-biphenylenediisocyanate to yield novel T-type
polyurethane 4 containing 2,5-dioxy benzylidenemalononitrile group as a non-
linear optical (NLO)-chromophore, which constituted parts of the polymer back-
bones. Polyurethane 4 was soluble in common organic solvents such as acetone
and N,N-dimethylformamide. It showed a thermal stability up to 280ꢀC in thermo-
gravimetric analysis thermogram and the glass-transition temperature (Tg)
obtained from differential scanning calorimetry thermogram was around 146ꢀC.
The second harmonic generation (SHG) coefficient (d33) of poled polymer film at
1064 cmꢁ1 fundamental wavelength was around 5.59 ꢂ 10ꢁ9 esu. The dipole align-
ment exhibited high thermal stability up to Tg, and there was no SHG decay below
140ꢀC because of the partial main-chain character of the polymer structure, which
was acceptable for nonlinear optical device applications.
Keywords: 2,5-Di-(20-hydroxyethoxy)benzylidenemalononitrile; differential scanning
calorimetry (DSC); NLO; poling; polyurethanes; relaxation of dipole alignment; SHG
coefficient; thermogravimetric analysis (TGA)
INTRODUCTION
Recently there has been an extensive research effort in the synthesis
of nonlinear optical (NLO) materials because of their potential applica-
tions in the field of electro-optic devices. Inorganic materials have
This work was supported by Korea Research Foundation Grant.
Address correspondence to Ju-Yeon Lee, Institute of Functional Materials,
Department of Chemistry, Inje University, 607 Obang-dong, Gimhae 621-749, Korea.
E-mail: chemljy@inje.ac.kr
357

358
J.-Y. Lee et al.
many disadvantages such as slow response time and degradative
photorefractive effects, which limit their applications. NLO polymers,
on the other hand, are considered candidate materials, mainly because
they offer many advantages such as light weight and good processabil-
ity to form optical devices [1]. Stabilization of electrically induced
dipole alignment is important considerations in the developments of
NLO polymers. Two approaches have been proposed to minimize the
randomization. One is to use crosslinking method [2] and the other
is to utilize high Tg polymers such as polyimides [3,4]. Polyurethanes
with a NLO chromophore, whose dipole moment is aligned transverse
to the main chain backbone, showed large second-order nonlinearity
with enhanced thermal stability [7,8]. Main-chain NLO polymers [5]
have good thermal stability of dipole alignments, but they often do
not dissolve in organic solvents. Side-chain NLO polymers [6] have
the advantages such as good solubility, homogeneity and high level
of NLO chromophore, but they often suffer from poor stability of dipole
alignments at high temperatures. In this work we have prepared novel
T-type polyurethanes containing the dioxybenzylidenemalononitrile
groups as NLO-chromophores. We selected 2,5-dioxy benzylidenemalo-
nonitrile groups as NLO-chromophores because they have a large
dipole moment and are rather easy to synthesize. Furthermore, the
2,5-dioxybenzylidenemalononitrile groups constitute novel T-type NLO
polyurthanes (see Fig. 1c), in which the pendant NLO-chromophores
are part of the polymer backbones. These mid-type NLO polymers are
expected to have the advantages of both main-chain and side-chain
NLO polymers; stabilization of dipole alignment and good solubility.
EXPERIMENTAL
Materials
Synthetic methods of diol 3 and polymer 4 are summarized in Schemes
ꢀ
1
1 and 2. Compound 3: Mp ¼ 138–140 C. H NMR (acetone-d6) d 3.74–
3.94 (m, 4H, 2 ꢁCH2ꢁOH), 3.94–4.12 (m, 2H, ꢁOH), 4.12–4.22 (m, 4H,
2 ꢁOꢁCH2ꢁ), 7.15–7.32 (m, 2H, aromatic), 7.75 (s, 1H, aromatic), 8.58
=
=
(s, 1H, ꢁPhꢁCH ). IR (KBr) 3516, 3233 (s, OꢁH), 3045 (m, CꢁH),
2941 (m, CꢁH), 2233 (m, CN), 1576 (s, C C) cmꢁ1. Anal. Calcd for
=
C14H14N2O4: C, 61.31; H, 5.14; N, 10.21. Found: C, 61.41; H, 5.22;
N, 10.28. Polymer 4: Inherent viscosity (ginh) ¼ 0.29 dL gꢁ1
(c ¼ 0.5 g dLꢁ1 in m-cresol at 25ꢀC). 1H NMR (DMSO-d6) d 3.88 (s,
6H, 2 ꢁOCH3), 4.17–4.53 (q, 8H, 2 ꢁOꢁCH2ꢁCH2ꢁOꢁ), 7.16–7.39
(d, 6H, aromatic), 7.57–7.76 (d, 2H, aromatic), 8.20 (d, 1H, aromatic),
8.47 (s, 1H, aromatic), 8.59–8.66 (d, 1H, N-H), 9.02 (s, 1H, NꢁH).

Novel T-Type NLO Polyurethane with High Thermal Stability
359
FIGURE 1 (a) Main-chain NLO polymers, (b) side-chain NLO polymers, and
(c) T-type NLO polymers.
=
IR (KBr) 3420 (m, NꢁH), 2941 (m, CꢁH), 2233 (m, CN), 1728 (s, C O),
1583 (s, C C) cmꢁ1. Anal. Calcd for (C30H26N4O8)n: C, 63.15; H, 4.59;
=
N, 9.82. Found: C, 63.25; H, 4,64; N, 9.88.
SHG Measurements
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 [9].
SCHEME 1 Synthetic method of diol 3.

360
J.-Y. Lee et al.
SCHEME 2 Synthetic method of polymer 4.
Second harmonic generation (SHG) measurement was carried out one
day after poling. A continuum PY61 mode-locked Nd:YAG laser
(k ¼ 1064 nm) with pulse width of 40 ps and repetition rate of 10 Hz
was used as the fundamental light source and Y-cut quartz was used
as reference. A 3-mm-thick Y-cut quartz crystal (a piece of quartz plate
whose plane is perpendicular to the crystalline y-axis and the thick-
ness of the plate is 3 mm and d11 ¼ 0.3 pm=V) was used as a reference
for determining the relative intensities of the SH signals generated
from the samples.
The Maker Fringe pattern was obtained from measuring the SHG
signal at 0.5ꢀ intervals using a rotation stage. SHG coefficients (d33)
were derived from the analysis of measured Maker-fringes [10].
RESULTS AND DISCUSSION
Synthesis and Characterization of Polymer 4
The synthetic route for compound 3 is presented in Scheme 1. Polymer
4 was prepared by the polyaddition reaction between a diol 3 and 3,30-
dimethoxy-4,40-biphenylenediisocyanate (DMBPI) in a dry DMF
solvent (see Scheme 2). We obtained polyurethanes with similar mol-
ecular weights and compositions. The striking feature of this polymer-
ization system is that it gives unprecedented novel T-type NLO
polymers, in which the pendant NLO chromophores are part of the
polymer backbones. In this way the mid-type NLO polymers can have
the advantages of both main-chain and side-chain NLO polymers sim-
ultaneously. The chemical structure of the polymer was identified by
1
1H NMR and IR spectra, and elemental analysis. H NMR spectrum
of the polymer showed a signal broadening due to polymerization,
but the chemical shifts are consistent with the proposed polymer
structures. The signal at 8.59–8.66 ppm assigned to the amine proton
indicates the formation of urethane linkage. The IR spectra of the same
polymer samples also show a strong carbonyl peak near 1728 cmꢁ1
indicating the presence of urethane bond. The number-average

Novel T-Type NLO Polyurethane with High Thermal Stability
361
molecular weights (Mn) of the polymers were determined to be 14600
(Mw=Mn ¼ 1.93) for polymer 4. The polymer 4 was soluble in common
solvents such as acetone, DMF, and DMSO, but was not soluble in
methanol and diethyl ether. The inherent viscosity value was around
0.29 dL=g. Polymer 4 showed strong absorption near 407 nm by the
NLO-chromophore 2,5-dioxybenzylidenemalononitrile group. We now
have well defined T-type polyurethanes, and we investigate their
properties in the following section.
Thermal Properties of Polymers
The thermal behavior of the polymers was investigated by TGA and
DSC to determine the thermal degradation pattern and Tg. Polymer
4 showed a thermal stability up to 280ꢀC according to their TGA
thermograms. The initial weight loss in the polymers begins at
294ꢀC. The Tg values of the polymer 4 measured by DSC were around
146ꢀC. This is relatively high value compared to those of common poly-
urethanes and probably attributed to the rigid biphenyl unit in the
polymer backbone. The TGA and DSC studies showed that the
decomposition temperature of the polymer 4 was higher than the cor-
responding Tg. This indicates that high-temperature poling for a short
term is feasible without damaging the NLO chromophore.
Electro-Optic Properties of Polymers
The electro-optic properties of polymers were studied by the SHG
method. To induce noncentrosymmetric polar order, the spin-coated
polymer films were corona-poled. As the temperature was raised
gradually to 155ꢀC, 6.5 kV of corona voltage was applied and kept that
temperature for 30 min. After the electric poling, the dipole moments
of the NLO-chromophores were aligned and UV-Vis spectrum of poly-
mer 4 exhibited a slight blue shift and a decrease in absorption due
to birefringence. The estimated order parameter value U was 0.14
(U ¼1ꢁA1=A0, where A0 and A1 are the absorbances of the polymer
film before and after poling, respectively). The decrease in absorbance
after poling was an indicator of the dipole alignment. The refractive
index of the sample was measured by the optical transmission tech-
nique [9]. The transmittance of thin film includes on the information
of the thickness, refractive index and extinction coefficient of that.
Thus, we can determine these parameters by analyzing the transmit-
tance. SHG measurements were performed at a fundamental wave-
length of 1064 nm using a mode locked Nd-YAG laser. Nonlinear
optical properties of polymer 4 are summarized in Table 1. In order

362
J.-Y. Lee et al.
TABLE 1 Nonlinear Optical Properties of Polymer 4
a
b
b
kmax
d33
(esu)
d31
(esu)
film thicknessd
Polymer
(nm)
Uc
(mm)
n
4
407
(5.59 ꢃ 0.28) ꢂ 10ꢁ9 0.14 0.51 (2.28 ꢃ 0.16) ꢂ 10ꢁ9 n1 ¼ 1.58
n2 ¼ 1.64
aPolymer film.
bSHG coefficients (d33) were derived from the analysis of measured Maker-fringes
[10].
cOrder parameter U ¼ l-A1=A0, where A0 and A1 are the absorbances of the polymer
film before and after corona poling, respectively.
dFilm thickness was detennined by the optical transmission technique [9].
to determine the microscopic second-order susceptibility of the poly-
mer, the angular SHG dependence was recorded. Figure 2 shows the
angular dependence of SHG signal in a poled polymer 4. The SHG
values were compared with those obtained from a Y-cut quartz plate.
SHG coefficients (d33) were derived from the analysis of measured
Maker-fringes with Pascal fitting program according to the literature
procedure [10]. The values of d33 and d31 for polymer 4 were
(5.59 ꢃ 0.28) ꢂ l0ꢁ9 and (2.28 ꢃ 0.16) ꢂ l0ꢁ9 esu, respectively. Since
the second harmonic wavelength was at 532 nm, which is not in the
absorptive region of the resulting polyurethane, there was not res-
onant contribution to this d33 value. To evaluate the high-temperature
FIGURE 2 Angular dependence of SHG signal in a poled film of polymer 4.

Novel T-Type NLO Polyurethane with High Thermal Stability
363
FIGURE 3 Normalized SHG signal of polymer 4 as a function of temperature
at a heating rate of 10ꢀC=min.
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 of polymer 4. 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 performed at a heat-
ing rate of 10ꢀC=min from 30ꢀC to 250ꢀC. The polymer film 4 exhibited
a thermal stability up to Tg and no significant SHG decay was
observed below 140ꢀC. In general side chain NLO polymers lose ther-
mal stability below Tg. The stabilization of the dipole alignment is a
characteristic of main-chain NLO polymers. The high thermal stab-
ility of second harmonic generation of polymer 4 was due to the stabi-
lization of dipole alignment of NLO chromophore, which stemmed
from the partial main chain character of the polymer structure. Thus,
we obtained a new type of NLO polyurethane having the advantages of
both main-chain and side-chain NLO polymers: stabilization of dipole
alignment and good solubility.
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