Second-order nonlinear optical properties and relaxation dynamics of ...

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Vol. 15, No. 1 / January 1998 / J. Opt. Soc. Am. B

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Second-order nonlinear optical properties and relaxation dynamics of aligned crosslinked polyurethanes with hemicyanine-type chromophores Kwang-Sup Lee and Sun-Woong Choi Department of Macromolecular Science, Hannam University, Taejon 300-791, Korea

Han Young Woo, Ki-Jeong Moon, and Hong-Ku Shim Department of Chemistry, Korea Advanced Science and Technology, Taejon 305-701, Korea

Mi-Yun Jeong and Tong-Kun Lim Department of Physics, Korea University, Seoul 136-606, Korea Received March 6, 1997; revised manuscript received September 26, 1997 Three types of different cross-linked polyurethane were prepared by the reaction of poly[(phenylisocyanate)co-formaldehyde] with three different hydroxy-functionalized nonlinear optical chromophores. For poled and cured polymers the x (2) values were between 16.8 and 37.4 pm / V, measured at a wavelength of 1.064 mm. Thermal stability studies performed on a sample having bonding sites of vertical–parallel structure between the chromophore and the liquid polymer matrix indicated, by second-harmonic-generation activity, a minimum decay of the x (2) value owing to lattice hardening of the polyurethane matrix with three bonding sites. The relaxation times of aligned dipoles in vertical–parallel polyurethane were measured by variation of poling and cross-linking conditions. Three decay modes were found to exist: the mode that corresponds to partially bonded or free unbonded chromophores, the mode of thermal relaxation of bonded chromophores, and the mode that is due to the relaxation of an elastically stressed main chain induced by the poling elastic field. © 1998 Optical Society of America [S0740-3224(98)04801-2] OCIS codes: 190.4400, 120.6810, 160.5470, 160.4330.

1. INTRODUCTION In the development of second-order nonlinear optical (NLO) polymers, the ability to stabilize the electric-fieldinduced dipole alignment, particularly at elevated temperatures, has been the critical issue. Improvements over the dye-doped systems have been made by attachment of chromophores as pendants to flexible polymer backbone or by their incorporation as components of the polymer backbone.1–3 For device applications, additional lattice hardening is required. To achieve this, several investigators have utilized cross-linking reactions to stabilize the dipole orientation after electric or corona poling. New types of cross-linked second-order NLO polymers, which use functional groups such as cinnamic groups,4,5 the maleimide group,6 and the ethynyl group,7 appeared after Eich et al. first described the use of thermosetting epoxy polymers.8 In all these systems some degree of stability in dipole alignment was realized. More recently, the reaction of isocyanate group with NLO monomers and polymers has proved useful for preparing second-order NLO polymers that have improved longterm stability.9–15 In the present investigation three types of hydroxy group containing monomers and poly[(phenylisocyanate)co-formaldehyde] (PPICF), with one cross-linking site 0740-3224/98/010393-08$10.00

(2NCO; isocyanate) at every phenylene moiety, were reacted to produce three different cross-linked polyurethanes. PPICF is an amorphous liquid polymer and is thus expected to produce high-optical-quality film through effective and uniform cross-linking reaction. Figure 1 illustrates the structures of three polymers. They are characterized as vertical (V), parallel (P), and vertical–parallel (VP), depending on bonding between the chromophore and the liquid polymer matrix. Each of their physical properties and optical second-order NLO activities, in terms of electrical poling and thermal stability, is presented. In particular, the stability of a polinginduced dipole of polyurethanes derived from three different cross-linkable monomers is compared. In addition, for a cross-linked VP polyurethane (PU-VP), the relaxation dynamics are investigated through aligned dipole relaxation measurement by variation of the poling and the cross-linking conditions.

2. EXPERIMENT A. Materials All reagent-grade chemicals were obtained from the commercial sources. PPICF was purchased from SigmaAldrich (St. Louis, Mo.). N,N 8 -dimethylformamide (DMF) © 1998 Optical Society of America

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Fig. 1.

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Schematic drawing for the cross-linked polyurethanes with three different bonding sites.

Fig. 2. Schematic diagram of the in situ poling setup for SHG measurement: v, l 5 1064 nm (Q-switched Nd:YAG laser); 2v, l 5 532 nm; ITO, indium tin oxide; TC, temperature controller; V, dc power supply (kV); H, heater; T, thermocoupler.

from Junsei (Tokyo) was purified by distillation under reduced pressure over anhydrous magnesium sulfate and was dried over a molecular sieve. The details of the synthesis of three different monomers are published elsewhere.16 B. Measurement IR spectra of polymers were obtained from a Boemen Michelson series Fourier-transform IR spectrophotometer. UV–visible absorption spectra were measured on a Shimadzu UV-3101PC spectrophotometer. Differential scanning calorimetry and thermogravimetric analysis were performed on a Dupont 9900 analyzer. The secondharmonic-generation (SHG) experiments were performed with a p-polarized beam at the fundamental frequency of a mode-locked Q-switched Nd:YAG laser operating at 500 Hz with 135-ps subpulses in each pulse train. The NLO activity of the polymer films was produced by coronadischarge-induced electric poling. The schematic diagram of the in situ poling setup is presented in Fig. 2. C. Film Preparation PU-V, PU-P, and PU-VP1 (NCO / OH 5 1): To a solution of triol monomer VP-OH (0.24 g) in 0.25 g of anhydrous DMF and 1 g of cyclopentanone was added a 47.6 wt. %

solution (320 ml, d 5 1.06) of PPICF in DMF. Similarly, diol monomers V-OH (0.24 g) and P-OH (0.24 g) were reacted with 220 and 221ml of PPICF in DMF, respectively. The solutions were stirred for 10 min and were then filtered by use of a 0.45-mm microsyringe. A high-quality polymer film sample was obtained by spin casting at 400 rpm. Since the cross-linking reaction occurred even at room temperature, thermal cross-linking was performed under the poling field immediately after spin casting without removal of the solvent. PU-VP2 (NCO / OH 5 2): To achieve higher mobility of the cross-linked polymer matrix and to obtain complete reaction of the monomers, NCO / OH 5 2 samples (PUVP2) were prepared. PU-VP2 film was produced by spin casting at 400 rpm with a solution filtered by a 0.45-mm pore-size microsyringe. Better-quality films were obtained with PU-VP2 as compared with PU-VP1, and the former was used for the aligned dipole relaxation measurements.

3. RESULTS AND DISCUSSION A. Synthesis and Characterization of Monomers and Polymers The synthesis of the novel cross-linkable stilbazolium salt monomer V-OH with tetraphenylborate as the counteranion was achieved by an approach similar to one reported in the literature.17 The synthetic approach to obtain monomers P-OH and VP-OH was previously reported by Moon.16 The synthetic routes for the cross-linked polyurethane systems PU-V, PU-P, and PU-VP are shown in Fig. 3. Dried monomers were reacted with the liquid polymer PPICF. The isocyanate:hydroxy (NCO:OH) ratios were the same except for PU-VP2. Highly viscous liquid PPICF was diluted to approximately 50 wt. % for microsyringe handling. The monomers were dissolved in solvent (DMF: cyclopentanone 5 1:4 by weight), and the diluted PPICF was added and mixed thoroughly for 10 min. The resulting mixture was then filtered with a 0.45-mm pore-size syringe filter and was directly spin coated at 400 rpm. For PU-VP, the solutions did not exhibit the flow characteristics because of fast (30-min) gel formation. The quality of the spin-coated film improved with a cosolvent system compared with the case in which

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only DMF was used. The films were directly poled, and their SHG signal was measured at the in situ poling setup, as shown in Fig. 2. The Fourier-transform IR spectra of the three crosslinked polymer systems are shown in Fig. 4. No significant difference in vibration frequency was observed among the samples. All the polymers exhibited a sharp absorption peak at 1716 cm21 from the carbonyl stretching vibration in the urethane linkage. The absorption peak at 2276 cm21 from the isocyanate group of PPICF was not observed. The results of thermogravimetric analysis and differential scanning calorimetry from the cross-linked polyurethane PU-VP1 are given in Fig. 5. The glass transition was not observed, and the thermal decomposition of this polymer began at approximately 156 °C. This decomposition temperature is somewhat lower than that of other stilbene chromophore systems (;200 °C). The deviation is due to the fact that there is a tetraphenylborate counterion at the center of the hemicyanine chromophore, which is more easily removed.17 The effect of UV exposure time on a cured PU-VP1 system (100 °C, 12 h, vacuum) is given in Fig. 6. The absorption maximum and the absorption edge were observed at 488 nm and at approximately 600 nm,

Fig. 3.

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respectively. Photobleaching on exposure to 365 nm, 55 mW / cm2 UV light did not change the absorption peak position. However, it drastically reduced the intensity of the absorption curve. These results indicate that photobleaching of chromophores used in the present investigation did not occur through a cis–trans transformation but that photodegradation was the main cause of this effect.18 The mechanism for photodegradation is not known at this time; however, we speculate that oxidation of the stilbene group is involved. The stability levels of three cross-linked films, all cured at the same condition (100 °C for 4 h), were determined in a solubility test wherein films were dipped in DMF at room temperature for up to 2 h. The change of normalized UV–visible absorption as a function of dipping time is shown in Fig. 7. For PU-P and PU-V films, 1 h of dipping produced a large reduction in UV absorption. This is probably due to the solvation of the oligomer and to the low-cross-link-density portion of the material in the DMF solvent. No further change was indicated with the increased dipping time of 2 h. For the case of PU-VP1, almost no change in UV absorbance was detected with dipping time. This result indicates that the cross-link density for this system was higher than for PU-P and PU-V systems.

Synthetic scheme of three types of cross-linked polyurethane system.

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current that was passed through the film was large and resulted in deformation of the polymer film. When the substrate was changed to a normal Corning glass, turbidity disappeared, and the transparent film was obtained after poling. For the measurement of macroscopic second-order susceptibility x ( 2 ) on completion of poling, the angular SHG

Fig. 4. Fourier-transform IR spectra of various cross-linked polyurethanes (PU-V, PU-P, and PU-VP1).

Fig. 7. Changes of normalized UV–visible absorbances of the polyurethane films under the same cured condition (100 °C, 4 h) according to the length of dipping time in the DMF solvent.

Fig. 5. Thermogravimetric analysis and differential scanning calorimetry thermograms of PU-VP1 (heating rate, 10 °C / min).

Fig. 8. Comparison of the SHG signal intensities of poled PU-V, PU-P, and PU-VP1 films. A y-cut quartz plate was used as a standard (d 11 value, 0.8 3 10210 esu).

Fig. 6. Change of UV–visible absorption spectra due to photobleaching of cured PU-VP1 sample (365 nm, 55 mW / cm2).

B. Second-Order Nonlinear Optical Properties of Cross-Linked Polyurethanes The NLO polymeric materials for the measurement of SHG are generally spin coated on ITO. The ITO glass was also initially used in the present investigation; however, the films became turbid during poling. Also, underthe ITO glass substrate, the surface charge of the film during poling was small, which implies that the electric

Fig. 9.

Thermal endurance of SHG activity for PU-VP1.

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Table 1. Chromophore Contents and Thermal Stability Data of x ( 2 ) for Cross-Linked Polyurethanes x (2) Value (pm / V)

Bonding Directionb

Chromophore Content (wt. %)

25 °C

75 °C

100 °C

125 °C

PU-V

vertical

69.2

37.4

PU-P

parallel

68.4

16.8

vertical and parallel

69.7

30.2

31.8 (85%)c 15.0 (90%) 30.2 (100%)

20.6 (55%) 9.6 (57%) 30.2 (100%)

4.9 (13%) 3.5 (21%) 26.9 (89%)

Samplesa

PU-VP1

a b c

Sample films were cured at 100 °C for 12 h under vacuum. Bonding directions indicate approximate angle between the dipole of the chromophore and two bonding sites. Each value in parentheses indicates the retained percentage of initial NLO activity at the corresponding temperature.

Fig. 10. In situ poling profiles obtained from (a) preuncured and (b) precured PU-VP2 films. The data represent the average values of the signals, and the standard deviation is shown by the error bar.

dependence was recorded for each film sample removed from the poling stage. Figure 8 shows the SHG profiles of the three different polymers along with that from 1mm-thick, y-cut quartz standard (d 11 5 0.8 3 10210 esu). Relatively high second-order optical activity was shown for the poled and the cured polymer films. The values of x ( 2 ) for PU-V, PU-P, and PU-VP1 were 8.9 3 1028 esu (37.4 pm / V), 4.0 3 1028 esu (16.8 pm / V), and 7.2 3 1028 esu (30.2 pm / V), respectively. The lower x ( 2 ) value of PU-P is indicative of a parallel locking structure known as the main-chain NLO polymeric system.1 Figure 9 illustrates the thermal endurance of SHG activity for poled and cured PU-VP1, measured every 20 min at every constant-temperature interval during a stepwise temperature increase from room temperature to 150 °C. Results for all the samples are also tabulated in Table 1. The decay of thermal stability in PU-VP1 was minimized owing to lattice hardening of polyurethane with three bonding sites. Also, the SHG intensity did not show any reduction until the temperature was raised to 100 °C. For temperatures below 100 °C, the x ( 2 ) value for PU-V and PU-P decreased only to 85–90% of the initial value. The depoling processes were irreversible, and the SHG signal did not recover when the materials were brought back to room temperature.

C. Poling Behavior and Relaxation Dynamics of Dipoles in PU-VP2 Poling profiles of preuncured and precured samples of PU-VP2 (NCO/OH52) are illustrated in Fig. 10. The latter sample was cured at 100 °C for 12 h in vacuum before the poling process such that a random cross-linking was achieved. The SHG signal was monitored under an electric field in a complete temperature cycle between room temperature and 120 °C and back to room temperature, at which time the electric field was removed. The temperature was changed in increments of 15 °C, and at each temperature the signal was recorded until saturation, which corresponded to approximately 10 min. When the poling field is applied at room temperature, chromophores in both samples become effectively aligned; hence poling profiles, although small in difference, take shape. With increasing temperature the signal from the preuncured sample is seen to decay rapidly, whereas, for the precured sample, the initial signal strength is somewhat maintained up to 80 °C, and this effect is followed by a slower decay at higher temperatures. The reason for the difference may be the degree of thermal fluctuation permitted in each sample. In the case of a preuncured sample, unreacted chromophores become largely disoriented, and their friction coefficient decreases with the temperature increase, resulting in a rapid decay. However, with precured samples, when the poling field is applied, chromophores align along the fields, and the main chain to which they are bonded is elastically deformed to accept a new chromophore configuration. Since chromophores are bonded to the main chain, the thermal energy should disorient both the chromophores and the main chain. Therefore the angular fluctuation of bonded chromophores will be much smaller than those of free chromophores, causing higher orientational order. However, when the temperature of the precured sample increases to 90 °C, where a strong relaxation of the main chain is expected, one can predict a rapid decrease in orientational order of the chromophores. This explains the rapid decrease in the SHG signal at 90 °C, as is shown in Fig. 10. The dynamics are similar for two samples during decreasing temperature. This is because the cross-linking has taken place in the initially preuncured sample during a previous temperature rise. By the time the temperature begins to decrease on reaching 120 °C, the crosslinking is almost complete. Thus, for the temperature-

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Fig. 11. Relaxation profiles of (a) preuncured and (b) precured PU-VP2 films at room temperature after corona poling at 120 °C. Curve (c) is the relaxation profile obtained by cold poling directly after film sampling.

Table 2. Fitting Parametersa for Dipole Relaxation at Various Cure Temperatures te (s)

tf (s)

tm (s)

Ae

Af

Am

Baseline

From Data

Precured 44 0 1600 Preuncured 0 320 1980 Free 0 320 2000

4 0 0

0 4 1.3 0.4 0.225 0.35

15.77 19.51 0.422

Fig.10

Curing temperature 30 °C 0 320 1950 90 °C 0 0 2334 120 °C 0 0 2350

0 0 0

0.225 0.35 0 0.418 0 0.619

0.422 0.57 0.692

Fig.13

a A e exp(2t/ t e ) 1 A f exp(2t/ t f ) 1 A m exp(2t/ t m ) 1 B ( t e , elastic relaxation time; t f , free chromophore relaxation time; t m , matrix relaxation time; A e , elastic relaxation amplitude; A f , free chromophore relaxation amplitude; A m , matrix relaxation amplitude; B, baseline).

reduction region, thermal fluctuations in both samples are expected to be similar, as shown. The competing effect of electric field and temperature on the equilibrium of the aligned dipole is also depicted in Fig. 10. At lower temperatures the aligned dipole portion is larger because the electric field has a greater effect than the temperature. Similarly, for higher temperatures, the opposite becomes true, and the aligned dipole portion decreases. This effect explains the symmetry of the SHG signal profile between temperature-increase and -decrease regions, which is observed for the precured sample. Such symmetry does not exist for the preuncured sample because, under the poling-induced alignment in the direction of the dc field, greater cross-linking exists in the temperaturereduction region than in the rise region. Figure 11 shows the relaxation behavior of preuncured [Fig. 11(a)], precured [Fig. 11(b)], and un-cross-linked [Fig. 11(c)] samples, and the regression that results from fitting a double-exponential function is given in Table 2. The dynamics are measured immediately after the electric field is removed, at room temperature and after the temperature cycle. For preuncured [Fig. 10(a)] and precured [Fig. 10(b)] samples poling was performed as shown

in Fig. 10. One can see from this curve that a large portion of both samples was already cross-linked by the time the dynamics were measured. The difference between samples (a) and (b) in Fig. 10 is that, while cross-linking was random for sample (b), oriented cross-linking existed for sample (a), as chromophores were aligned during poling. Both samples show double-exponential decay with characteristic slow and fast time scales. While the preuncured sample shows slow and fast time scales of 320 and 1980 s, respectively, the corresponding time scales for the precured sample are 44 and 1600 s. The slow time scales of the two samples are similar within the range of experimental error, and we speculate that they are related to the thermal relaxation behavior of the main chain to which the chromophores are bonded. The fast time behavior of the preuncured sample is believed to be due to the thermal orientational relaxation of the partially bonded or unbonded free chromophores. The fast decay of 44 s for the precured sample can be interpreted as follows. When the electric field is on, the chromophores cross-linked to the main chain become subjected to an orienting torque along the electric field. Since the chromophores are attached to the main chain, this condition in turn applies force to the main chain and causes the main chain to be deformed elastically. When the field is removed, the recovery of elastic deformation facilitates main-chain relaxation, and, as a result, the time scale is much faster than that of the diffusive motion of the chromophores. To confirm this we measured the dynamics of the sample in which chromophores are free from the main-chain motion. The relaxation time of this sample [Fig. 11(c)] is similar to the fast time behavior of the preuncured sample. This result justifies our interpretation that there are three different relaxation mechanisms of chromophores: the first corresponds to partially bonded or free unbonded chromophore relaxation, the second is the thermal relaxation of bonded chromophores, and the third is the relaxation of an elastically stressed main chain by the poling electric field. D. Cold Poling of Randomly Cross-Linked Polymers It was previously shown that the SHG signal was greater for the precured sample than for the preuncured sample

Fig. 12. SHG signal intensity obtained from preuncured and precured films by variation of the corona poling voltage at room temperature.

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Fig. 13. In situ cold poling profiles of precured and preuncured films that were left at room temperature (RT) for the times listed in the figure. vac., vacuum.

(see Fig. 10). To confirm such an unexpected behavior, precured and preuncured samples were subjected to various electric-field strengths at room temperature, and the degree of alignment was monitored. As shown in Fig. 12, under the same condition, a larger SHG signal is seen for the precured sample as compared with the preuncured one. This result is opposite those obtained from crosslinked polymers in general, and the reason for this can be found by consideration of thermal fluctuation and polymer backbone flexibility. That is, since the rigidity of the polymer matrix is low because of low cross-link density, a substantial chain mobility exists in the polymer chains. Thus the alignment begins at the poling voltage of 2.9 kV and hence yields larger SHG than the preuncured sample. The SHG signal reflects the competing effects of electric-field and thermal fluctuation on alignment, and the signal observed is the component of thermally fluctuating dipoles in the direction of the electric field. Hence the larger SHG signal obtained for the precured sample indicates that more dipoles are aligned in the direction of the electric field. In other words, the moment of inertia of the angular motion of chromophores in the precured samples is large, as the chromophores’ movements are restricted by the polymer matrix, thus producing a smaller degree of orientational fluctuation. This results in a larger degree of dipole alignment in the field direction as compared with the preuncured sample, in which the angular fluctuation is greater owing to a smaller moment of inertia, which is caused by chromophores’ being freed from the polymer matrix. The above speculation regarding the competing effects on the orientation of the chromophores by the thermal energy and the poling field is confirmed by the experiments shown in Fig. 13. For these experiments we prepared the samples to be partially cross-linked to a different degree. We obtained partially cross-linked samples by leaving the sample for various times at room temperature. We then measured the SHG signal from samples while the poling field was applied. As was expected, the more a sample was cross-linked, the larger the SHG signal that appeared, and we believe that this occurs because the thermal disorientation of the chromophores was


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less severe for the chromophores bonded to the main chain than for the unbonded chromophore. Figure 14 shows further evidence that there is mainchain relaxation as well as orientational relaxation of chromophores. The samples for this experiment were left at room temperature for 2 days to cause partial random cross-linking. The temperature of the samples was raised to 30, 90, 120 °C, under the poling field for 1.5 h. With the poling field still on, the samples were cooled down to room temperature. The relaxation dynamics were then measured, immediately after the field was removed. The analysis results of the relaxation dynamics are given in Table 2. For the sample poled at 30 °C the data were fitted with a double-exponential function. The fast time scale was assigned to the relaxation of partially bonded or still unbonded free chromophores, and the slow decay time was assigned to the relaxation of the crosslinked main chain. For the sample poled at 90 and 120 °C, it is believed that all the chromophores were cross-linked and that only the relaxation motion of the main chain was observed from these samples. Table 2 illustrates decay times and amplitudes of the exponential functions (single, double, or triple) that are fitted to the experimental data of samples poled at various temperatures. The slowest time scale t m is considered to be related to the relaxation of a cross-linked matrix, and t f is interpreted as the fast decay times of partially bonded or unbonded free chromophores. These decay times agree well with the data shown in Fig. 10. More cross-linking occurred as the sample experienced higher temperatures (Fig. 14). We observed no elastic-type time scale (44 s), but this effect was seen in the completely precured sample (Fig. 11). This result indicates that the samples are partially cross-linked, so that the whole matrix rotates easily under the external force, and that during the poling process a complete cross-linking occurs, thereby

Fig. 14. Relaxation behavior or the effect of poling temperature on the relaxation behavior of aligned dipoles. Fitting parameters are given in Table 2.

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and by the Korean Science and Engineering Foundation.

REFERENCES 1.

2. 3.

4. Fig. 15. Relaxation dependence of aligned dipoles on the rate of temperature rise: (a) 17 °C / min, (b) 3.1 °C / min, (c) 1.2 °C / min.

causing no elastic deformation. The increase in the baseline of the fitted data with the increase in poling temperature indicates that more chromophores are oriented and cross-linked along the poling field. Illustrated in Fig. 15 is the effect of the rate of temperature increase on room-temperature relaxation. The samples were left at room temperature for 2 days to cause partial cross-linking. They were then subjected to an electric field, and the temperature was raised to 120 °C, with rates of 1.2, 3.1, and 17 °C / min. After the samples were cooled to room temperature their relaxation was measured. The oriented cross-linked portion is shown to clearly increase as the rate of temperature rise decreases. That is, the lower the rate of temperature rise, the more time there is for the chromophores to find sites for subsequent formation of stable orientation. In conclusion, three types of second-order nonlinear optical cross-linked polyurethane from PPICF and hemicyanine-type derivatives having bulky tetraphenylborate anion were prepared and were then simultaneously poled and thermally cross-linked. The three resulting materials exhibited high second-order activities as given by their x ( 2 ) values between 16.8–37.4 pm / V. The poled and thermally cured polymeric films demonstrated the improved temporal stability of their nonlinear optical properties with increased number of cross-linking sites and parallel locking. Three relaxation times were found on samples having electric-field-induced chromophore alignment. The slowest time scale of ;2000 s is related to the relaxation of the cross-linked main chain, and the intermediate time scale of 320 s is related to the relaxation motion of partially bonded or unbonded chromophores. The fastest time scale of 45 s is related to the elastic recovery of the elastically deformed matrix. Also, with higher curing temperature (120 °C) and slower rate of temperature rise, the best poling efficiency was achieved and resulted in the highest oriented cross-linked portion.

ACKNOWLEDGMENTS This work was supported by the Korean Ministry of Education Research Fund for Advanced Materials in 1996

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