Studies on nonlinear optical polyurethanes containing heterocyclic chromophores

Article abstract of DOI:10.1016/j.molstruc.2010.12.010

This paper presents the synthesis of highly stable nitro-substituted thiazole, benzothiazole and thiadiazole chromophores. With these, a series of second-order nonlinear optical (NLO) responsive polyurethanes were successfully synthesized from tolylene-2,

Full text of DOI:10.1016/j.molstruc.2010.12.010

Journal of Molecular Structure 987 (2011) 158–165
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Studies on nonlinear optical polyurethanes containing heterocyclic chromophores
a
a
a
a
b
M.Y. Kariduraganavar a, , S.M. Tambe , R.G. Tasaganva , A.A. Kittur , S.S. Kulkarni , S.R. Inamdar
⇑
a Department of Chemistry and Center of Excellence in Polymer Science, Karnatak University, Dharwad 580 003, India
b Laser Spectroscopy Programme, Department of Physics, Karnatak University, Dharwad 580 003, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 27 July 2010
Accepted 3 December 2010
Available online 13 December 2010
This paper presents the synthesis of highly stable nitro-substituted thiazole, benzothiazole and
thiadiazole chromophores. With these, a series of second-order nonlinear optical (NLO) responsive poly-
urethanes were successfully synthesized from tolylene-2,4-diisocyanate (TDI) and 4,40-methylen-
edi(phenyl isocyanate) (MDI). Molecular structural characterization of these polyurethanes was
achieved by 1H NMR, FT-IR, GPC and analytical data. The weight-average molecular weights (Mw) of
the resulting polyurethanes were determined by GPC and ranged between 22,100 and 26,700. All the
polyurethanes were highly soluble in aprotic solvents such as tetrahydrofuran, cyclohexanone, N,N-
dimethylformamide, N,N-dimethylacetamide, dimethylsulphoxide, N-methyl-2-pyrolidinone, etc. The
thermal behaviour of these polyurethanes was investigated using differential scanning calorimetry
(DSC) and thermogravimetric analysis (TGA). Their glass transition temperatures were in the range
140–165 °C and most of the polymers showed high thermal stability. With an in situ poling and temper-
ature ramping technique, the optimal temperatures (Topts) for corona poling were determined for the
largest second-order NLO response. The second harmonic generation (SHG) coefficients (d33) of the poled
polyurethane films range from 62.21 to 103.11 pm/V at 1064 nm. All the poled films showed outstanding
orientational stability up to 120 °C without any measurable decay in the SHG signal. Of these, the poly-
urethane with nitro-substituted benzothiazole moiety (IIb) showed the best dynamical thermal stability
of the poling-induced dipole alignment up to ꢀ150 °C.
Keywords:
NLO chromophores
Polyurethanes
Corona poling
Order parameter
Thermal stability
SHG coefficient
Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
[8–11]. Among these, organic polymeric materials have advantages
with regard to the ease of structure modifications, processability
The field of nonlinear optics (NLO) has been developing for a
few decades as a promising area of research with important appli-
cations in the domain of photoelectronics and photonics [1–3].
NLO materials can be used to manipulate optical signals in tele-
communication systems and other optical signal processing appli-
cations. NLO activity was first found in inorganic crystals [2,3],
such as LiNbO3, but the range of useful materials is rather limited
either due to low NLO responses or because of limited solubility
which makes processing into thin films and incorporation into mi-
cro-optoelectronic devices difficult. On the other hand, organic
materials exhibit large and fast nonlinearities and are easy to pro-
cess and integrate into optical devices [4,5]. Furthermore, organic
compounds offer the advantage of tailorability: a fine tuning of
the NLO properties can be achieved by rational modification of
the chemical structures as a result, they are ideal in terms of the
ultimate goal of device miniaturization by going to the molecular
level [6,7]. As a consequence, by the mid 1980s, organic materials
emerged as important targets of choice for NLO applications
into thin films, large susceptibilities, high laser damage threshold,
faster response time and others [12–15].
In terms of the NLO chromophore, a wide variety of structural
modifications on the donor, acceptor and
have been carried out [16–19]. Synthetic studies have demon-
strated that replacing the benzene ring of a chromophore -bridge
p-conjugated moieties
p
with easily delocalizable five-membered heteroaromatic rings,
such as thiophene and thiazole, results in an enhanced molecular
hyperpolarizability [20–24]. Theoretical calculations, however,
suggest that heteroaromatic rings play a subtle role in influencing
the second-order NLO response properties of donor–acceptor com-
pounds [25]. While the aromaticity of heteroaromatics affects elec-
tronic transmission between donor and acceptor substituents, the
electron-excessive or electron-deficient nature of the heterocyclic
ring systems may also play a major role in determining the overall
electron-donating and -accepting ability of the substituents. Thus,
attaching a strong electron acceptor to an electron-deficient het-
eroaromatic may yield chromophores with significantly enhanced
NLO responses.
In addition to chromophore design, stabilization of electrically
induced dipole alignment is also an important parameter to be
considered while designing the new NLO polymers. To minimize
⇑
Corresponding author. Tel.: +91 836 2215286x23; fax: +91 836 2771275.
E-mail address: mahadevappak@yahoo.com (M.Y. Kariduraganavar).
0022-2860/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2010.12.010

M.Y. Kariduraganavar et al. / Journal of Molecular Structure 987 (2011) 158–165
159
the degree of randomization, two approaches have been proposed.
2.3. Second harmonic generation measurements
One is to employ crosslinking [26,27] and the other one is to utilize
high glass transition temperature (Tg) polymers such as polyimides
[28–30]. In an example of the former approach it has been found
that crosslinked polyurethanes bearing azobenzene chromophores
demonstrate enhanced thermal stability [31–33]. This is because,
the polyurethane matrix forms extensive hydrogen bonds between
urethane linkage thereby increasing the rigidity of the matrix and
preventing the relaxation of induced dipoles [33–36]. Polyure-
thanes with NLO chromophores whose dipole moment is aligned
transverse to the main-chain backbone, exhibiting large second-
order nonlinearity with good thermal stability [33,37–39].
Polymers physically crosslinked through hydrogen bonds have
major advantages in terms of homogeneity and in particular
processability relative to the chemically crosslinked systems,
which suffer from significant optical loss and poor processability.
This contribution concerns the synthesis of new polyurethanes
containing nitro-substituted thiazole, benzothiazole and thiadia-
zole chromophores. These polymers have been characterized using
a range of spectroscopic and analytical techniques. Finally the ori-
entational stability and second-order optical nonlinearity is
discussed.
The dried film was heated at a temperature 10 °C below the
glass transition temperature (Tg) and corona poled with an intense
dc electric field as described previously [40,41]. After being poled
for 1 h, the polyurethane film was cooled to room temperature
while keeping the electric field on. The poling conditions were as
follows: voltage, 2–3 kV at the needle point; gap distance
ꢀ0.8 cm; poling current <0.18 mA.
The second harmonic measurements were performed utilizing
the setup described previously [40,41]. A Mode-Locked Nd:YAG la-
ser (Continuum Minilite-I, 6 ns pulse duration, 28 mJ maximum
energy, 10 Hz repetition rate at 1064 nm) was used as a fundamen-
tal light source. The second harmonic signal generated by the
p-polarized fundamental wavelength (1064 nm) was detected by
fast photodiode (FDS010, rise time 0.9 ns, Thorlabs) and an oscillo-
scope (Tektronix TDS 724D, Digital Phosphor Oscilloscope) with a
frequency of 500 MHz. The polyurethane sample was held at a
45° angle to the incident laser beam. A standard potassium dideu-
terium phosphate (KꢁDP) crystal was used as reference sample. The
chromophore alignment in the polyurethane film and the second-
order NLO properties were described by the order parameter (U).
2.4. Synthesis of chromophores
2. Experimental
2.4.1. Synthesis of 4-[2-(5-nitrothiazol-2-yl)diazenyl]benzene-1,3-diol
(a)
2.1. Materials
Dry sodium nitrite (1.04 g, 15 mmol) was slowly added with stir-
ringto coldconcentratedsulphuricacid(10 ml)andwarmedto50 °C
on water bath. The solution was then cooled to 0 °C and a mixture of
glacial acetic acid/propionic acid (30 ml, 5:1) was added dropwise
with stirring so as to maintain the temperature below 15 °C. The
resulting solution was then cooled below 0 °C and to this, 2-ami-
no-5-nitrothiazole (2.17 g, 15 mmol) was added in small portions
whilestirring. Thestirringwasfurthercontinuedatthistemperature
for 2 h. The resulting diazonium salt solution was slowly added to
resorcinol (1.65 g, 15 mmol) dissolved in 50 ml of 10% sodium
hydroxide solution with the reaction temperature was maintained
below 0 °C. The solution was stirred for 2 h and stood for 30 min to
allow the precipitate to settle. The precipitate was filtered and
washed several times with water until the filtrate was neutral. The
product was finally purified by column chromatography over SiO2
with benzene and ethylacetate (1:1) as an eluate to give 1.75 g
(45% yield) of a. M.p.: >295 °C; kmax (DMF): 577 nm. FT-IR (KBr pellet,
cmꢂ1): 3412 (O–H), 1596 (N@N), 1517, 1345 (N@O). 1H NMR
(300 MHz, DMSO-d6): d = 6.46 (t, 2H, ArH), 7.73 (d, J = 9.0 Hz, 1H,
ArH), 8.88 (s, 1H, ArH), 11.41 (s, 2H, OH). Anal. Calcd. for C9H6N4O4S:
C, 40.60; H, 2.25; N, 21.05; Found: C, 40.41%; H, 2.06%; N, 20.86%.
2-Amino-5-nitrothiazole, 2-amino-6-nitrobenzothiazole, tolyl-
ene-2,4-diisocyanate (TDI) and 4,40-methylenedi(phenyl isocyanate)
(MDI) were purchased from Sigma–Aldrich. N,N-dimethylformamide
(DMF) was procured from Loba Chemie, Mumbai, India and purified
by distillation over phosphorous pentoxide. Resorcinol, 4-nitroben-
zoic acid, phosphorous oxychloride and thiosemicarbazide were pur-
chased from S.D. Fine Chem. Ltd., Mumbai, India. All of the other
chemicals were of analytical grade purchased from S.D. Fine Chem.
Ltd., Mumbai, India and were used as received unless otherwise
stated.
2.2. Instrumentation
The 1H NMR spectra were recorded on a Bruker Avance
300-MHz spectrometer using tetramethylsilane (TMS) as a refer-
ence. The Fourier transform infrared (FT-IR) spectra were recorded
on a Nicolet-5700 spectrometer. The number- and weight- average
molecular weights (Mn, Mw) of polymers were measured by gel
permeation chromatography (HP/GPC, waters) in tetrahydrofuran
with polystyrene as a standard. Elemental analyses were per-
formed with a Perkin Elmer PE 2400 CHN elemental analyzer.
Ultraviolet–visible (UV–vis) spectra were recorded on a Hitachi
U-2800 spectrophotometer. Differential scanning calorimetry
(DSC) and thermogravimetric analyses (TGA) were respectively
2.4.2. Synthesis of 4-[2-(6-nitrobenzothiazol-2-yl)diazenyl]benzene-
1,3-diol (b)
This chromophore was synthesized according to the procedure
given for a. (55% yield). M.p.: 147–148 °C; kmax (DMF): 530 nm.
FT-IR (KBr pellet, cmꢂ1): 3425 (O–H), 1598 (N@N), 1518, 1337
(N@O). 1H NMR (300 MHz, DMSO-d6): d = 6.49 (t, 2H, ArH), 7.44
(d, 1H, ArH), 7.87 (d, 1H, ArH), 8.09 (d, 1H, ArH), 8.69 (s, 1H,
ArH), 11.50 (s, 2H, OH). Anal. Calcd. for C13H8N4O4S: C, 49.37; H,
2.53; N, 17.72; Found: C, 49.22%; H, 2.40%; N, 17.88%.
performed on
a Mettler-Toledo DSC 822e and Perkin–Elmer
Diamond TGA/DTA thermogravimetric analyzer under nitrogen
atmosphere at a heating rate of 10 °C/min. The melting points were
measured with open capillary tubes on a melting-point apparatus.
Thin polyurethane films were produced from 5 wt.% solution of the
polyurethane in DMF and cast onto an indium–tin oxide (ITO) glass
substrate using spin casting technique at
1500–2000 rpm. Prior to film casting, the polyurethane solution
was filtered through a 0.20 m Teflon membrane filter. All films
a rate of about
2.4.3. Synthesis of 4-[2-(5-(4-nitrophenyl)-1,3,4-thiadiazol-2-
yl)diazenyl]benzene-1,3-diol (c)
l
Prior to the synthesis of c, 2-amino-5-(4-nitrophenyl)-1,3,
4-thiadiazole was prepared using a procedure published else-
where [42]. A mixture of p-nitrobenzoic acid (4.2 g, 0.025 mol),
were dried for 3 h in a vacuum oven at 100 °C to remove residual
solvents. The thickness and refractive index of the polyurethanes
were measured using a SE 850 Ellipsometer.

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M.Y. Kariduraganavar et al. / Journal of Molecular Structure 987 (2011) 158–165
thiosemicarbazide (2.25 g, 0.025 mol) and phosphorous oxychlo-
ride (10 ml) was refluxed gently for half an hour. It was cooled to
ambient temperature, water was added (25 ml), and the reaction
mixture was further refluxed for 4 h and filtered. The filtrate was
neutralized with 10% of KOH solution and the precipitate thus
obtained filtered and washed several times with water until the fil-
trate was neutral. The product was then crystallized from ethanol.
The analytical and spectral data are in good agreement with the
data reported [42].
This compound was utilized for the preparation of chromo-
phore c as per the procedure adopted for the synthesis of a. (85%
yield). M.p.: 260–261 °C; kmax (DMF): 540 nm. FT-IR (KBr pellet,
cmꢂ1): 3420 (O–H), 1594 (N@N), 1517, 1345 (N@O). 1H NMR
(300 MHz, DMSO-d6): d = 6.48 (t, 2H, ArH), 7.72 (d, J = 9.0 Hz, 1H,
ArH), 8.30–8.41 (q, 4H, ArH), 11.20 (s, 2H, OH). Anal. Calcd. for
ArH), 7.82 (d, 1H, ArH), 7.94 (s, 1H, ArH), 8.24 (s, 1H, ArH), 8.75
(s, 1H, ArH), 9.00 (s, 1H, N–H), 9.20 (s, 1H, N–H). Anal. Calcd. for
C22H14N6O6S: C, 53.88; H, 2.86; N, 17.14; Found: C, 54.02%; H,
2.73%; N, 17.01%.
2.5.3. Polyurethane Ic
FT-IR (KBr pellet, cmꢂ1): 3422 (N–H), 1674 (C@O), 1602 (N@N),
1520, 1344 (N@O). 1H NMR (300 MHz, DMSO-d6): d = 2.08 (br s,
3H, CH3), 6.48 (s, 1H, ArH), 6.75 (d, 1H, ArH), 7.00–7.15 (m, 2H,
ArH), 7.75 (d, 1H, ArH), 7.95 (s, 1H, ArH), 8.31–8.41 (q, 4H, ArH),
9.20 (s, 1H, N–H), 11.20 (s, 1H, N–H). Anal. Calcd. for C23H15N7O6S:
C, 53.38; H, 2.90; N, 18.95; Found: C, 53.13%; H, 3.21%; N, 18.69%.
2.5.4. Polyurethane IIa
FT-IR (KBr pellet, cmꢂ1): 3393 (N–H), 1702 (C@O), 1603 (N@N),
1515, 1346 (N@O). 1H NMR (300 MHz, DMSO-d6): d = 2.49 (br s,
2H, –CH2–), 6.52 (s, 1H, ArH), 7.09–7.30 (m, 10H, ArH), 7.95 (s, 1H,
ArH), 8.54–8.75 (m, 2H, N–H). Anal. Calcd. for C24H16N6O6S: C,
55.81; H, 3.10; N, 16.28; Found: C, 55.98%; H, 2.86%; N, 16.02%.
C14H9N5O4S: C, 48.98; H, 2.62; N, 20.41; Found: C, 48.73%; H,
2.82%; N, 20.22%.
The reaction pathways for the synthesis of chromophores a, b
and c are illustrated in Scheme 1.
2.5. Synthesis of polyurethanes
2.5.5. Polyurethane IIb
FT-IR (KBr pellet, cmꢂ1): 3318 (N–H), 1703 (C@O), 1598 (N@N),
1513, 1335 (N@O). 1H NMR (300 MHz, DMSO-d6): d = 2.49 (br s,
2H, –CH2–), 6.90 (s, 1H, ArH), 7.09–7.33 (m, 12H, ArH), 7.94 (s, 1H,
ArH), 8.21–8.96 (m, 2H, N–H). Anal. Calcd. for C28H18N6O6S: C,
59.36; H, 3.18; N, 14.84; Found: C, 59.13%; H, 2.98%; N, 15.04%.
2.5.1. Polyurethane Ia
A 50 ml two necked round bottom flask containing chromo-
phore a (0.27 g, 1 mmol), was connected to a condenser and
purged with nitrogen for 20 min to remove the atmospheric mois-
ture. To this flask, dry DMF (5 ml) and 1 eq. (0.16 ml, 1.1 mmol) of
tolylene-2,4-diisocyanate were added. The temperature of the
reaction mixture was raised to 110 °C and maintained for 5 h.
The viscous solution produced was cooled to ambient temperature
and then poured into ice-cold water. The resulting precipitate was
filtered and washed several times with water. It was then dissolved
in DMF and reprecipitated by adding methanol to remove mono-
mer, low molecular weight oligomers. The deep red polyurethane
Ia was filtered and dried in vacuum desiccator. FT-IR (KBr pellet,
cmꢂ1): 3413 (N–H), 1653 (C@O), 1610 (N@N), 1536, 1341 (N@O).
1H NMR (300 MHz, DMSO-d6): d = 2.08 (br s, 3H, CH3), 6.48
(t, 2H, ArH), 6.75 (d, 1H, ArH), 7.15 (m, 2H, ArH), 7.42 (s, 1H,
ArH), 7.69 (d, 1H, ArH), 7.94 (s, 1H, N–H), 8.30 (s, 1H, N–H), 8.86
(s, 1H, ArH). Anal. Calcd. for C18H12N6O6S: C, 49.09; H, 2.73; N,
19.09; Found: C, 48.90%; H, 2.89%; N, 18.93%.
2.5.6. Polyurethane IIc
FT-IR (KBr pellet, cmꢂ1): 3388 (N–H), 1679 (C@O), 1600 (N@N),
1516, 1340 (N@O). 1H NMR (300 MHz, DMSO-d6): d = 2.49 (br s,
2H, –CH2–), 6.48 (s, 1H, ArH), 7.09–7.33 (m, 9H, ArH), 7.74 (d,
1H, ArH), 7.94 (s, 1H, ArH), 8.32–8.40 (q, 4H, ArH), 8.51 (s, 2H,
N–H). Anal. Calcd. for C29H19N7O6S: C, 58.68; H, 3.20; N, 16.52;
Found: C, 58.38%; H, 3.01%; N, 16.35%.
The chemical reaction routes of all the polyurethanes are pre-
sented in Scheme 2.
3. Results and discussion
3.1. Synthesis and characterization of chromophores
A similar procedure was followed for the synthesis of polyure-
thanes Ib, Ic, IIa, IIb and IIc.
The new NLO chromophores containing thiazole, benzothiazole
and thiadiazole were respectively synthesized by azo-coupling reac-
tion of 2-amino-5-nitrothiazole, 2-amino-6-nitrobenzothiazole and
2-amino-5-(4-nitrophenyl)-1,3,4-thiadiazole with resorcinol (see
Scheme 1). The chromophores were satisfactorily purified
with column chromatography. The chemical structure of the
2.5.2. Polyurethane Ib
FT-IR (KBr pellet, cmꢂ1): 3381 (N–H), 1702 (C@O), 1604 (N@N),
1525, 1336 (N@O). 1H NMR (300 MHz, DMSO-d6): d = 2.08 (br s,
3H, CH3), 6.75 (d, 1H, ArH), 7.06–7.18 (m, 3H, ArH), 7.40 (d, 1H,
Scheme 1. Synthetic reaction routes for chromophores.

M.Y. Kariduraganavar et al. / Journal of Molecular Structure 987 (2011) 158–165
161
Scheme 2. Synthetic reaction routes for polyurethanes.
chromophores was confirmed with FT-IR, 1H NMR, UV–vis and ele-
mental analysis. The FT-IR spectra illustrated in Fig. 1 clearly show
characteristic bands at around 3380 and 1600 cmꢂ1 which are as-
signed to hydroxyl groups and azo groups respectively. The strong
bands appearing at around 1520 and 1330 cmꢂ1 are respectively as-
signed to asymmetric and symmetric stretchings of a nitro group,
signifying the presence of an azo group in the structure. All the as-
signed peaks in 1H NMR are consistent with the proposed structures.
The DMF solutions of chromophores a, b and c showed strong UV
absorption maxima at around 577, 530 and 540 nm, respectively.
All the analytical data also confirmed the expected chemical struc-
tures. These chromophores are soluble in many common organic
solvents such as acetone, THF, cyclohexanone, DMF, N,N-dimethyl-
acetamide (DMAc), dimethylsulphoxide (DMSO) and N-methyl-2-
pyrrolidinone (NMP).
These NLO chromophores are the push–pull type compounds
(see Scheme 1) with a hydroxyl group as the electron donor and
a nitro group as the electron acceptor linked through an azo conju-
gated bridge.
Fig. 1. FT-IR spectra of (a) chromophore b, (b) polyurethane Ib.
the results are presented in Table 2. The elemental analyses data
also fit the polyurethane structures. All the polyurethanes were
soluble in many common aprotic solvents such as acetone, THF,
cyclohexanone, DMF, DMAc, DMSO and NMP. These polyurethanes
offer good processability and hence, optical quality thin films could
be easily prepared from their solutions.
3.2. Synthesis and characterization of polyurethanes
The newly synthesized chromophores a, b and c were con-
densed with tolylene-2,4-diisocyanate (I) and 4,40-methylen-
edi(phenyl isocyanate) (II) in
a dry DMF solvent to yield
polyurethanes (see Scheme 2). The structures of the resulting poly-
urethanes were confirmed with FT-IR, 1H NMR, GPC and elemental
analysis. In the FT-IR spectra of polyurethanes (see Fig. 1), a char-
acteristic band appeared near 1700 cmꢂ1 which was attributed to
C@O stretching, indicating the presence of urethane bond. A strong
band appeared at around 3400 cmꢂ1 which can be assigned to N–H
stretchings. These N–H stretchings are broad, suggesting that the
urethane linkages experience hydrogen bonding. The characteristic
bands of symmetrical and asymmetrical stretching vibrations of
the nitro groups were also evident at approximately 1525 and
1340 cmꢂ1 respectively, establishing the presence of the NLO chro-
mophores in the polyurethanes. 1H NMR spectra of polyurethanes
showed a signal broadening due to polymerization, but the chem-
ical shifts were consistent with the proposed polyurethane struc-
tures. The molecular weights of polyurethanes were measured in
tetrahydrofuran using GPC (with polystyrene as a standard) and
3.3. Thermal properties of polyurethanes
To determine the glass transition temperature (Tg) and the ther-
mal degradation pattern, the thermal behaviour of the polyure-
thanes was investigated using differential scanning calorimetry
(DSC) and thermogravimetric analysis (TGA), respectively. The re-
sults are summarized in Table 1. The DSC and TGA thermograms
are respectively presented in Figs. 2 and 3. From Fig. 2, it is ob-
served that Tg of the polyurethanes containing thiazole and benzo-
thiazole chromophores were in the range of 140–165 °C. Among
the polyurethanes, chromophores containing benzothiazole moi-
ety (Ib and IIb) exhibited relatively higher Tg values compared to
those of chromophores containing thiazole moiety (Ia and IIa). This
presumably reflects the more rigid and bulky nature of benzothia-
zole compared to thiazole moiety, which relatively hinders the

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M.Y. Kariduraganavar et al. / Journal of Molecular Structure 987 (2011) 158–165
Table 1
Physical properties of polyurethanes.
a
Polyurethanes
Tg (°C)
Topt (°C)
Field applied (kV)
Td (°C)
Ia
IIa
Ib
IIb
Ic
IIc
140
147
155
165
—
140
145
150
170
180
205
2.0
3.0
3.0
3.0
2.5
3.0
207
221
227
273
250
276
—
a
Decomposition temperature measured with TGA at 10 wt.% under the protec-
tion of N2 at a scan rate of 10 °C/min.
Table 2
NLO properties of polyurethanes.
kmax (nm) U c
ls
(lm) d33
a
a
b
d
Polyurethanes Mn
Mw
(pm/V)
Ia
IIa
Ib
IIb
Ic
IIc
12,500 23,800 491
11,700 24,300 462
13,000 22,100 470
12,000 24,000 449
13,800 26,200 484
12,700 26,700 451
0.10 0.07
0.14 0.08
0.13 0.07
0.20 0.10
0.15 0.08
0.28 0.08
62.21
65.13
62.70
94.73
90.44
103.11
Fig. 2. DSC thermograms of polyurethanes at a heating rate of 10 °C/min under a
nitrogen atmosphere.
a
b
c
Measured by GPC in THF using polystyrene as a standard.
Polymer film after corona poling.
Order parameter.
d
Film thickness.
degree of free-rotation. Consistent with this it was found that no Tg
could be observed for the polyurethanes containing thiadiazole
chromophores before they decomposed, implying that the Tg is
above the decomposition temperatures (Td).
From TGA traces (Fig. 3), it can be seen that polyurethanes con-
taining benzothiazole and thiadiazole chromophores exhibited
better thermal stability than those of polyurethanes containing
the thiazole chromophore. Of these more stable systems at higher
temperatures, polyurethanes containing the thiadiazole moiety ap-
peared to be the more thermally stable. This is in good agreement
with the DSC data. Thermal decomposition temperatures (Td) were
calculated at the loss of 10 wt.% and the values so obtained are in-
cluded in Table 1. From both the TGA and DSC patterns, it is ob-
served that decomposition temperatures of all the polyurethanes
except Ic and IIc, were higher than their respective Tg values. In
view of this and since the poling was done below Tg, no thermal
decomposition was expected to occur for the majority of the sam-
ples during the poling process. However, in case of Ic and IIc, the
poling temperature was fixed depending on the maximum second
harmonic generation (SHG) results. This was determined by mea-
suring the SHG as a function of temperature at a constant electric
field.
Fig. 3. TGA thermograms of polyurethanes at a heating rate of 10 °C/min under a
nitrogen atmosphere.
3.4. Nonlinear optical properties of polyurethanes
In order to check the largest second-order NLO response in the
polyurethanes, the selection of optimum poling temperature is
crucial so as to align the molecules to a greater extent along the
direction of the applied field. The in situ poling and temperature
ramping technique were used to select the optimal temperature
(Topt). As the set poling temperature increased step by step, the
SHG signal was detected at a different temperature after the tem-
perature was held for 10–15 min. In this way, the Topts of these
polyurethanes were measured and for representation the data of
Ib are given in Fig. 4. For all polyurethanes, the SHG signals rose
steadily before the Topts were reached, and then the SHG signals
decreased rapidly due to damage to the films. The Topt’s data of
these polyurethanes are presented in Table 1. From the data, it is
Fig. 4. The SHG signals of polyurethane Ib (which select Topts) as a function of
temperature.

M.Y. Kariduraganavar et al. / Journal of Molecular Structure 987 (2011) 158–165
163
clear that optimum temperatures of the polyurethanes are close to
their glass transition temperatures. Similarly it is important to
optimize the poling voltage to obtain a maximum SHG signal.
The poling time was fixed for 10–15 min and the poling voltage
varied between 1.0 and 4.0 kV and the SHG signals recorded for
all the polyurethanes. The resulting optimized applied voltages
are included in Table 1. A typical profile of SHG on the polyure-
thane containing nitro-substituted thiadiazole (Ic) is shown in
Fig. 5. It is observed that the SHG signal increased with the applied
voltage up to a point and then decreased steadily. This is again ow-
ing to the damage of polyurethane films. Based on these observa-
tions, the applied voltage was selected and used as standard
condition for the measurement of SHG signals for all the
polyurethanes.
The typical UV–vis absorption spectra of the polyurethane Ia
before and after poling are presented in Fig. 6. After poling, a de-
crease of absorbance in the spectrum was observed. A similar
behaviour was also observed for the other polyurethanes, and it
can be ascribed to two different mechanisms: (1) a partial align-
ment of molecules along the direction of the poling field, i.e., per-
pendicular to the plane of the film, which reduces the absorption
cross-section for light at normal incidence [43], or (2) a possible
partial degradation of the chromophores during the poling process.
As for as these polyurethanes, the decomposition temperatures of
the chromophores are well above the poling one, the second mech-
anism can be ruled out. From the change of absorbance, the order
parameter of the poled film was estimated, which is related to the
poling efficiency. The estimated order parameter for polyurethane
Fig. 6. UV–vis absorption spectra of polyurethane Ia before and after poling.
(11.4 lm); ls is the thickness of the polyurethane sample; and F is
the correction factor, which is equal to 1.2 approximately when
lc,k ꢃ ls. The calculated d33 values of these polyurethanes at the
wavelength of 1064 nm and films thicknesses are included in
Table 2.
From Table 2, it can be seen that all the polyurethanes exhibited
d33 values in the range of 62.21 to 103.11 pm/V, signifying that
polyurethanes showed enhanced resonance effect due to high
absorption at 532 nm with reference to a 1.0 mm thick KꢁDP
crystal. The d33 values reported here are much superior to the data
reported for dialkylaminonitrostilbene (DANS)-substituted NLO
polyurethane systems [33,37]. From the results, we observe that
the polyurethanes having MDI backbone (IIa, IIb and IIc) demon-
strated an excellent d33 values compared to TDI backbone irrespec-
tive of the chromophores attached to polyurethane matrix. This
suggests that MDI backbone favours better molecular alignment
during poling compared to TDI backbone. This is because of flexible
group (–CH2–) present in the MDI backbone. It is also noticed that
among the polyurethanes, chromophores containing thiadiazole
moiety exhibited larger d33 values. This may be due to higher
aromaticity of thiadiazole moiety as compared to thiazole and
benzothiazole moieties. The order of d33 of chromophores is:
thiadiazole > benzothiazole > thiazole.
The relaxation process of dipole orientation is directly related to
the glass transition temperature (Tg). A higher Tg implies a higher
orientational stability. To probe this stability, we have monitored
the temporal and thermal stability of the second harmonic gener-
ation signals for all the polyurethanes and the data generated as a
function of time are presented in Fig. 7. All the polyurethanes were
stable at room temperature and there was no decay in the intensity
of the SHG signals. However, at 100 °C in air, after an initial decay
to ꢀ89% of the original signal, more than 87% of the SHG signal was
remained stable after 70 h for polyurethane Ia as can be seen from
Fig. 7. Almost the same situation was noticed in the polyurethanes
having TDI backbone. In contrast, there was no noticeable decay in
the polyurethanes containing MDI backbone and remained stable
up to 70 h. This further indicates that the MDI backbone favours
better stability compared to TDI backbone.
Ia was found to be 0.10. The estimated order parameters (
U = 1–
A1/A0, where A0 and A1 are the absorbances of the polymer film be-
fore and after poling, respectively) of the remaining polyurethane
films are given in Table 2. All these data ascertain that polyure-
thanes were efficiently poled and as a result, the chromophores
were effectively aligned along the direction of applied field.
To evaluate the NLO activity of the poled polyurethane films,
thin films were prepared for the SHG measurement. The second-
order NLO coefficient (d33) for the poled polyurethane was calcu-
lated based on the following equation [44].
sffiffiffiffi
d33;s vðs2Þ
d36;k
Is l
Ik ls
¼
¼
c;k F
vðk2Þ
where d36,k is the d36 of KꢁDP crystal, which is 0.40 pm/V; Is and Ik
are the SHG intensities of the polyurethane sample and the KꢁDP
crystal, respectively; lc,k is the coherence length of the KꢁDP crystal
To evaluate the high-temperature stability of the polyure-
thanes, we have studied the temporal stability of the SHG signal
for all the polyurethanes. Fig. 8 represents the dynamic thermal
stability of the NLO activity of polyurethane IIb. To investigate
the real time NLO decay of the SHG signal of the poled
polyurethane films as a function of temperature, in situ SHG mea-
surements were performed at a heating rate of 5 °C/min from 25 to
240 °C. The poled film of polyurethane IIb exhibited the highest
Fig. 5. The SHG signals of polyurethane Ic (which select optimum poling voltage) as
a function of applied field.

164
M.Y. Kariduraganavar et al. / Journal of Molecular Structure 987 (2011) 158–165
the analytical and spectral data. The resulting polyurethanes
exhibited very good solubility in most of the common aprotic sol-
vents and as a consequence, these could be easily processed into
high quality optical thin films. The polyurethanes exhibited high
glass transition temperatures in the range of 140–165 °C, which
facilitates a stable NLO response at elevated temperatures. How-
ever, no Tg was observed for the polyurethanes containing thiadia-
zole chromophores, implying that Tgs could be higher than their
corresponding decomposition temperatures (Td). This clearly re-
flects that thiadiazole moiety enhanced the stability of the poly-
urethane matrix due to hindrance in its degree of free-rotation.
All the poled polyurethanes exhibited high thermal endurance.
This was explained on the basis of formation of extensive hydrogen
bonds between urethane linkages, which are known to increase the
rigidity of the polymer matrix preventing the relaxation of induced
dipoles. These materials exhibited relatively large d33 values rang-
ing from 62.21 to 103.11 pm/V due to high absorption at 532 nm.
All the poled films showed outstanding orientational stability up
to 120 °C without any measurable decay in SHG signal. In particu-
lar, the polyurethane with nitro-substituted benzothiazole moiety
(IIb) showed the best dynamical thermal stability of the poling-in-
duced dipole alignment up to ꢀ150 °C. On the whole, the versatil-
ity of the reaction scheme and the ease of processing ensure this
series of materials for further utilization in optical device
applications.
Fig. 7. Temporal stability of the SHG signals of polyurethanes at 100 °C in air.
Acknowledgements
The authors gratefully acknowledge the Department of Physics,
Indian Institute of Science, Bangalore for providing Ellipsometer to
measure the refractive index and film thickness. We gratefully
thank Mr. James R. Mannekutla, Department of Physics, Karnatak
University, Dharwad for his assistance in measuring the SHG activ-
ities of the samples.
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