Synthesis and electro-optic properties of novel Y-type polyurethanes having dioxybenzylidenemalononitrile

Article abstract of DOI:10.1080/15421400490506595

3,4-Di-(2′-hydroxyethoxy)benzylidenemalononitrile (3) was prepared and condensed with 2,4-toluenediisocyanate and 3,3′-dimethoxy-4,4′- biphenylenediisocyanate to yield unprecedented novel Y-type polyurethanes (4-5) containing 3,4-dioxybenzylidenemalononitrile group as a NLO-chromophore, which is a part of the polymer main chain. The resulting polyurethanes 4-5 were soluble in common organic solvents such as acetone and DMF. Polymers 4-5 showed a thermal stability up to 280°C in TGA thermograms with T_g values from DSC thermograms in the range of 148-152°C. The SHG coefficients (d₃₃) of poled polymer films were around 5.96 × 10^-9 esu. These poled polymer films exhibited greater thermal stabilities of dipole alignments even at 20°C higher than T_g due to the partial main chain character of the polymer structure, which is acceptable for NLO device applications. Copyright Taylor & Francis Inc.

Full text of DOI:10.1080/15421400490506595

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Synthesis and Electro-
Optic Properties of Novel Y-
Type Polyurethanes having
Dioxybenzylidenemalononitrile
Ju-Yeon Lee a , Han-Bae Bang a & Chang Baek a
a Institute of Basic Sciences, School of Advanced
Materials Engineering , Inje University , Kimhae,
Korea
Published online: 18 Oct 2010.
To cite this article: Ju-Yeon Lee , Han-Bae Bang & Chang Baek (2004)
Synthesis and Electro-Optic Properties of Novel Y-Type Polyurethanes having
Dioxybenzylidenemalononitrile, Molecular Crystals and Liquid Crystals, 425:1,
109-117, DOI: 10.1080/15421400490506595
To link to this article: http://dx.doi.org/10.1080/15421400490506595
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Mol. Cryst. Liq. Cryst., Vol. 425, pp. 109=[387]–117=[395], 2004
Copyright # Taylor & Francis Inc.
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080=15421400490506595
SYNTHESIS AND ELECTRO-OPTIC PROPERTIES
OF NOVEL Y-TYPE POLYURETHANES HAVING
DIOXYBENZYLIDENEMALONONITRILE
Ju-Yeon Lee, Han-Bae Bang, and Chang Suk Baek
Institute of Basic Sciences, School of Advanced Materials Engineering,
Inje University, 607 Obang-dong, Kimhae 621-749, Korea
3,4-Di-(20-hydroxyethoxy)benzylidenemalononitrile (3) was prepared and
condensed with 2,4-toluenediisocyanate and 3,30-dimethoxy-4,40-biphenylene-
diisocyanate to yield unprecedented novel Y-type polyurethanes (4–5) con-
taining 3,4-dioxybenzylidenemalononitrile group as a NLO-chromophore,
which is a part of the polymer main chain. The resulting polyurethanes 4–5
were soluble in common organic solvents such as acetone and DMF. Polymers
4–5 showed a thermal stability up to 280ꢀC in TGA thermograms with Tg values
from DSC thermograms in the range of 148–152ꢀC. The SHG coefficients (d33) of
poled polymer films were around 5.96 ꢁ 10ꢂ9 esu. These poled polymer films
exhibited greater thermal stabilities of dipole alignments even at 20ꢀC higher
than Tg due to the partial main chain character of the polymer structure, which
is acceptable for NLO device applications.
Keywords: 3,4-Di-(20-hydroxyethoxy)benzylidenemalononitrile; AFM; NLO; polyurethanes;
poling; SHG coefficient
INTRODUCTION
Nonlinear optical (NLO) materials based on organic compounds have been
extensively studied in recent years because of their potential applications
in the field of telecommunications, optical switching, etc [1]. One of the cur-
rent challenges is to design novel NLO polymers having optimized proper-
ties. In the developments of NLO polymers for electro-optic device
applications, stabilization of electrically induced dipole alignment is impor-
tant considerations. Two approaches to minimize the randomization have
Received 1 October 2003; accepted 13 January 2004.
This work was supported by the Korea Research Foundation Grant (KRF-2003–041-D20173).
Address correspondence to Ju-Yeon Lee, Institute of Basic Sciences, School of Advanced
Materials Engineering, Inje University, 607 Obang-dong, Kimhae 621-749, Korea. Tel.: þ82-
55-320-3221, Fax: þ82-55-321-9718, E-mail: chemljy@inje.ac.kr
109=[387]

110=[388]
J.-Y. Lee et al.
been proposed. One is to use crosslinking method [2] and the other is to use
high Tg polymers such as polyimides [3]. Polyurethane matrix forms exten-
sive hydrogen bond between urethane linkage and increases rigidity pre-
venting the relaxation of induced dipoles [4]. Polyurethanes with
hemicyanine [5–6] in the side chain, or having transverse NLO chromo-
phore to main chain exhibit enhanced thermal stability [7]. In this work
we prepared novel Y-type polyurethanes (Fig. 1c) containing 3,4-dioxyben-
zylidenemalononitrile groups as NLO-chromophores, which are parts of
polymer backbones. The resulting polymers are mid type of side chain-
and main chain NLO polymers, and are expected to have both of the merits
of the two types; stabilization of dipole alignment and good solubility. After
confirming the structure of the resulting polymers we investigated their
NLO properties and thermal stability. We also compared their properties
with those of benzylidenecyanoacetate derivatives [6].
EXPERIMENTAL
Materials
Synthetic methods of diol 3 and polymers 4–5 are summarized in Scheme 1
ꢀ
1
and Scheme 2. Diol 3: Mp ¼ 108–110 C. H NMR (acetone-d6) d 2.78–2.87
FIGURE 1 Main chain NLO polymers (a), side chain NLO polymers (b), and Y-type
NLO polymers (c).

Polyurethanes Containing 3,4–Dioxybenzylidenemalononitrile
111=[389]
(d, 2H, ꢂOH), 3.87–3.98 (m, 4H, 2 ꢂCH2ꢂOH), 4.09–4.28 (m, 4H, 2
ꢂOꢂCH2ꢂ), 7.21–7.27 (d, 1H, aromatic), 7.63–7.65 (d, 1H, aromatic),
7.76 (d, 1H, aromatic), 8.14 (s, 1H, ꢂPhꢂCH=). IR (KBr) 3337 (s,
OꢂH), 3045 (w, =CꢂH), 2930 (m, CꢂH), 2222 (s, CN), 1607, 1583 (s,
C=C) cmꢂ1. Anal. Calcd for C14H14N2O4: C, 61.31; H, 5.14; N, 10.21. Found:
C, 61.40; H, 5.25; N, 10.28. Polymer 4: ginh ¼0.30 dL=g (c, 0.5 g=dL in
DMSO at 25ꢀC). 1H NMR (DMSO-d6) d) d 2.12–2.21 (m, 3H, ꢂCH3),
3.96–4.49 (m, 8H, 2 ꢂOꢂCH2ꢂCH2ꢂOꢂ), 6.94–8.56 (m, 7H, aromatic),
SCHEME 1 Synthetic method of polymer 4.

112=[390]
J.-Y. Lee et al.
SCHEME 2 Synthetic method of polymer 5.
8.92–9.11 (m, 2H, NꢂH). IR (KBr) 3383 (s, NꢂH), 2953, 2883 (m, CꢂH),
2222 (m, CN), 1711, 1699 (s, C=O), 1562, 1534, 1512 (vs, C=C) cmꢂ1
.
UV=Vis (DMSO) kmax¼371. Anal. Calcd for (C23H20N4O6)n: C, 61.60; H,
4.50; N, 12.49. Found: C, 61.68; H, 4.58; N, 12.58. Polymer 5:
ginh ¼ 0.25 dL=g (c, 0.5 g=dL in DMSO at 25ꢀC). 1H NMR (DMSO-d6) d
3.79–4.48 (m, 14H, 2 ꢂOCH3, 2 ꢂOꢂCH2ꢂCH2ꢂOꢂ), 7.15–8.69 (m, 10H,
aromatic), 8.91–9.06 (m, 2H, NꢂH). IR (KBr) 3383 (m, NꢂH), 2953,
2848 (m, CꢂH), 2222 (m, CN), 1726, 1711, 1699 (s, C=O), 1583, 1512
(vs, C=C) cm ꢂ 1
.
UV=Vis (DMSO) kmax ¼362. Anal. Calcd for
(C30H26N4O8)n: C, 63.15; H, 4.59; N, 9.82. Found: C, 63.25; H, 4.66; N, 9.91.
Instrumentation
IR spectra were taken on a Shimadzu FT IR-8201PC infrared spectrophot-
1
ometer. H NMR spectra were obtained on a Varian 300 MHz NMR spec-
trometer. UV-Vis absorption spectra were measured on a Shimadzu
UV-3100S spectrophotometer. Elemental analyses were performed using
a Perkin-Elmer 2400 CHN elemental analyzer. The glass transition tem-
peratures (Tg) were measured on a TA 2920 differential scanning calor-
imeter in a nitrogen atmosphere. DuPont 951 thermogravimetric analyzer
with a heating rate of 10ꢀC=min up to 800ꢀC 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
polymers were estimated by gel permeation chromatography (GPC) (col-
umns styragel HR5E4E; solvent THF). The alignment of the NLO-chromo-
phore of the polymers was carried out by corona poling method (6.5 kV,
160ꢀC, 30 min). Atomic force microscopy (AFM) images were recorded
with a Park Science Instrument Autoprobe CP, operated in a contact mode,
which measures topography. Second harmonic generation (SHG) measure-
ments were made using a Maker fringe technique [7].

Polyurethanes Containing 3,4–Dioxybenzylidenemalononitrile
113=[391]
RESULTS AND DISCUSSION
Synthesis and Characterization of Polymers 4–5
3,4-Di-(20-vinyloxyethoxy)benzylidenemalononitrile (2) was prepared by
the condensation reaction of 1with malononitrile. Compound 2 was hydro-
lyzed to yield acetaldehyde and diol 3. Polymers 4-5 were prepared by
polyaddition reaction between a diol 3 and TDI and DMBPI, respectively
(Scheme 1–2). The structural features of these polymers are that they have
pendant NLO chromophores, which are part of their polymer backbones.
Thus we found a new type of side chain NLO polyurethanes. These mid-
type NLO polymers are expected to have both of the merits of main chain-
and side chain NLO polymers. The chemical structures of the compounds
1
were confirmed by H NMR, IR spectra, and elemental analysis. The signal
at 8.92–9.11 ppm of the 1H NMR spectra assigned to the amine proton indi-
cates the formation of urethane linkage. The IR spectra of the same poly-
mer samples also show a strong carbonyl peaks near 1726, 1711,
1705 cmꢂ1 indicating the presence of urethane bond. These results are
consistent with the proposed structures. The number average molecular
weight (Mn) of the polymers was determined to be 15300 (Mw=Mn ¼ 2.04)
for polymer 4. Having well defined polyurethanes (4–5), we investigated
their properties.
Electro-Optic Properties of Polymers 4–5
The polymers 4–5 was soluble in common solvents such as acetone, DMF
and DMSO, but were not soluble in methanol and diethyl ether. The
inherent viscosities were in the range of 0.25–0.30 dL=g. Polymers 4–5
showed strong absorption near 380 nm by the NLO-chromophore dioxyni-
trostilbenyl group. The thermal behaviors of the polymers were investi-
gated by thermogravimetric analysis (TGA) and differential scanning
calorimeter (DSC) to determine the thermal degradation pattern and glass
transition temperature (Tg). The results are summarized in Table I. Poly-
mers 4–5 showed a thermal stability up to 280oC from their TGA thermo-
grams. Glass transition temperature (Tg) values of the polymers 4–5
measured by DSC were around 148–152ꢀC. According to our experimental
results, these Tg values are higher than those of cyanoacetate derivatives.
To induce noncentrosymmetric polar order, the spin-coated polymer
films were corona-poled. As the temperature was raised to 160ꢀC, 6.5 kV
of corona voltage was applied and kept 160ꢀC for 30 min. The UV-Vis
absorption spectra of the polymer sample 4 before and after the poling
are presented in Figure 2. After the electric poling, the dipole moments
of the NLO-chromophores were aligned and UV-Vis spectrum of polymer

114=[392]
J.-Y. Lee et al.
TABLE I Thermal Properties of Polymers 4–5
ꢀ
Degradation temp, Cb
ꢀ
Polymer
Tga, C
5%-loss
20%-loss
40%-loss
Residueb at 800ꢀC, %@
4
5
152
148
284
308
327
346
383
393
17.2
11.7
a Determined from DSC curves measured on a TA 2920 differential scanning calorimeter
with a heating rate of 10ꢀC=min under nitrogen atmosphere.
bDetermined from TGA curves measured on a DuPont 951 thermogravimetric analyzer with
a heating rate of 10ꢀC=min under nitrogen atmosphere.
4 exhibited a decrease in absorption due to birefringence. From the absor-
bance change, the order parameter of the poled film could be estimated,
which is related to the poling efficiency. The estimated order parameter
value, U, was 0.13 (U ¼ 1-A1=A0, A0 and A1 are the absorbances of the
polymer film before and after poling) for the polymer 4.
Domain structures of NLO-chromophores for the poled thin-film sam-
ples were obtained using atomic force microscopy (AFM). AFM images
FIGURE 2 UV-Vis absorption spectra of a film of polymer 4 before and after poling.

Polyurethanes Containing 3,4–Dioxybenzylidenemalononitrile
115=[393]
TABLE II Nonlinear Optical Properties of Polymers 4–5
Uc
d33=d31
a
b
d33 (esu)
b
d31 (esu)
Polymer
kmax (nm)
4
5
6
371
376
370
5.96 ꢁ 10ꢂ9
6.84 ꢁ 10ꢂ9
4.76 ꢁ 10ꢂ9
0.13
0.21
0.11
2.03 ꢁ 10ꢂ9
2.36 ꢁ 10ꢂ9
1.63 ꢁ 10ꢂ9
2.94
2.90
2.92
a Polymer film after corona poling.
b SHG coefficients (d33) were derived from the analysis of measured Maker-fringes [10].
c Order parameter U ¼ 1-A1=A0, A0 and A1 are the absorbances of the polymer film before
and after corona poling, respectively.
show that the surface of the film sample is extremely flat and clean. How-
ever, the morphology of the film was dramatically changed after poling,
resulting in numerous hills and valleys in the surface structure, which were
aligned the poling direction. The NLO properties were studied by the SHG
method. SHG measurements were performed at a fundamental wavelength
of 1064 nm using a mode locked Nd-YAG laser [8]. The refractive index of
FIGURE 3 Normalized SHG intensity of polymer 4 as a function of temperature at
a heating rate of 10ꢀC=min.

116=[394]
J.-Y. Lee et al.
the polymer sample was measured by the optical transmission technique
[9]. In order to determine the microscopic second-order susceptibility of
the polymer, the angular SHG dependence was recorded. 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 to the samples and recorded. The SHG coefficient d33 was calcu-
lated through the method developed by Herman et al. [10]. The SHG mea-
surements revealed a d33 value of 5.96 ꢁ 10ꢂ9 esu for polymer 4. NLO
properties of polymers 3–5 are summarized in Table II. Since the second
harmonic wavelength was at 532 nm, which is not in the absorptive region
of the resulting polyurethane, there was not resonant contribution to this
d33 value. According to our experimental results, these d33 values are a
bit lower than those of benzylidenecyanoacetate derivatives. To evaluate
the high-temperature thermal 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 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 heating rate of 10ꢀC=min from 30ꢀC to 250ꢀC. The poled film of polymer
4 exhibited greater thermal stability and there was little decay of the d33
signal up to 175ꢀC, as shown in Figure 3. This excellent thermal stability
of optical nonlinearity is due to the stabilization of dipole alignment of
NLO chromophore. In general, side chain NLO polymers loose thermal
stability of dipole alignment around Tg. On the contrary of these common
side chain NLO polymers, Y-type NLO polymer 4 showed a thermal stability
of SHG even at 20oC above Tg. Stabilization of dipole alignment is a charac-
teristic of main chain NLO polymers. Thus, we obtained novel Y-type NLO
polymers having both of the merits of main chain- and side chain NLO poly-
mers; stabilization of dipole alignment and good solubility. These good tem-
poral thermal stabilities are probably due to the partial main chain
character of the polymer structures. We are now in the process of extend-
ing the polymerization system to the synthesis of other type of NLO poly-
mers and the results will be reported elsewhere.
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