Synthesis and Properties of Novel NLO Polyurethane
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of the polymer has a signal broadening due to polymerization, but the chemical shifts are
consistent with the proposed polymer structure. The signal at 8.92–9.18 ppm assigned to
the amine proton indicates the formation of urethane linkage. The IR spectrum of polymer
3 shows strong carbonyl peaks near 1729 cm−1 indicating the presence of urethane bond.
The spectrum also shows strong absorption peak near 1589 cm−1 due to azo group and
absorptions at 1516 and 1341 cm−1 due to nitro group indicating the presence of nitroa-
zobenzene unit. These results are consistent with the proposed structure, indicating that
the NLO chromophores remained intact during the polymerization. The molecular weights
were determined using GPC with polystyrene as the standard and THF as the eluent. Mn of
the polymer 3, determined using GPC, is 14,200 g mol−1 (Mw/Mn = 1.98). The polymer
3 is soluble in common solvents such as acetone, DMF, and DMSO, but is not soluble
in methanol and diethyl ether. The inherent viscosity is 0.31 dL g−1. Polymer 3 shows
strong absorption near 384 nm due to the 4-(4-nitrophenylazo)catecholic group NLO chro-
mophore. The striking feature of this polymerization system is that it gives unprecedented
Y-type NLO polymers, in which the pendant NLO chromophores are part of the polymer
backbone. These Y-type NLO polymers are expected to have the advantages of both main-
chain and side-chain NLO polymers. Having obtained the well defined Y-type polyurethane
3, we investigated its properties.
Thermal properties of polymer
The thermal behavior of the polymer was investigated using TGA and DSC to determine
the thermal degradation pattern and Tg. Polymer 3 shows a thermal stability up to 270◦C
according to its TGA thermogram. The Tg value of the polymer 3 measured using DSC is
around 96◦C. This Tg value is lower than that of the polyurethane containing dioxynitros-
tilbene, which is near 143◦C [8] or dioxybenzylidenemalononitrile, which is near 146◦C
[9].
Nonlinear optical properties of polymer
The NLO properties of polymer were studied using the SHG method. To induce noncen-
trosymmetric polar order, the spin-coated polymer film was corona-poled. As the tempera-
ture 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. After electric poling, the dipole moments
of the NLO chromophores were aligned and UV-visible absorption of polymer 3 exhibits
a slight blue shift and a decrease in absorption due to birefringence. SHG measurements
were performed at a fundamental wavelength of 1064 nm using a mode locked Nd-YAG
laser. NLO properties of polymer 3 are summarized in Table 1. 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 3. 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 [11]. The values of d33
and d31 for polymer 3 are 4.58×10−9 and 1.45×10−9 esu, respectively. These SHG values
are smaller than those of the polyurethanes containing 3,4-dioxynitrostilbene [8] or 2,4-
dioxybenzylidenemalononitrile [9], as shown in Table 1. The lower SHG values of polymer
3 are probably due to the easy cis-trans isomerization of diazo linkage.
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