2
Ind. Eng. Chem. Res. 1999, 38, 2-7
APPLIED CHEMISTRY Isocyanate Cross-Linked Polymers for Nonlinear Optics. 2. Polymers Derived from 4-{[2-Methyl-4-nitro-5-(hydroxymethyl)phenyl]azo}N,N-bis(2-hydroxypropyl)-3-methylaniline Cecil V. Francis,* Jacob Liu, and Kenneth M. White 3M Corporation, St. Paul, Minnesota 55144
Prakob K. Kitipichai Siam Cement Public Company Ltd., Bangkok, Thialand
A poled and cross-linked nonlinear optically active polymer was prepared from 4-{[2′-methyl4′-nitro-5′-(hydroxymethyl)phenyl)azo}-N,N-bis(2-hydroxypropyl)-3-methylaniline and multifunctional isocyanates. After an initial 5% loss in nonlinearity of this polyurethane immediately after poling, the polar alignment was unchanged even after 1000 h at 100 °C. The polymer displayed a stable electrooptic coefficient (r33) of 9.4 pm/V. To realize “real world” devices based on organic electrooptic (EO) polymers, several conditions, including large optical nonlinearity, low optical loss in the poled polymer and dipolar orientational stability at elevated temperatures, must be satisfied. The temperature specifications are dictated by the applications. Previous work has demonstrated that long term dipolar orientational stability at elevated temperatures is possible for molcules covalently bound into a crosslinked matrix.1 The higher the glass transition temperature (Tg), the higher the dipolar alignment temperature, providing the nonlinear optical chromophore (NLOphore) is also stable at the desired temperature. We have also shown that the thermal stability of the NLOphore in the polymer can be radically reduced from that of the NLO-phore as a crystal.1a This implies that the thermal stability of the crystalline NLO-phore can be used only as a rough screening tool for high-temperature stability of said molcule in a polymer. This report addresses the design of a thermally stable NLO-phore as well as its dipolar stability (see Figure 1). Several researchers have made significant progress in this area by synthesizing high glass transition temperature (Tg) side chain polymers2 while a few have used cross-linked polymers.1 As previous reports from this lab and Dalton et al. have shown, dipolar orientational stability at elevated temperatures can be obtained with cross-linked polymers, provided the NLO-phore is thermally stable. We now report on a polyurethane system that has displayed high nonlinearity, good thermal stability, and excellent dipolar orientational stability at 100 °C (Figure 2). The salient features of this system comprise the reactivity at both ends of the NLO-phore and the difference in reactivities of these functional groups. The differences in reactivity of the functional groups allow for a stepwise reaction to a cross-linked polymer. After the first reaction, oligomers * E-mail:
[email protected]. Phone: (651) 736-7144. Fax: (651) 736-7080.
capable of being processed into optically clear films of uniform thicknesses are formed. The reaction conditions are subsequently changed to allow the other functionality(ies) to react. This last reaction is generally in the presence of an electric field,1,3 resulting in an optically clear, poled and cross-linked polymer film with uniform thickness. Experimental Section All the chemicals used were purchased from Aldrich Chemical Co. except N-ethyl-N-phenylethanolamine, 69-71% nitric acid, and 95-98% sulfuric acid (all from EM Science) and Tolonate HDT and IPDT from RhonePoluenc. All the chemicals except pyridine were used without further purifications. Pyridine was dried over calcium hydride. Due to the similarities in the syntheses of these molcules only a representative synthesis of one of the target molcules is shown. 2-(Acetamidomethyl)acetanilide. Acetic anhydride (20 g, 0.195 mol) was added dropwise to the solution of 2-aminobenzylamine (10 g, 0.082 mol) in dry pyridine (90 mL) with stirring at room temperature. The resulting solution was left overnight and then acetic anhydride and pyridine left over were evaporated under reduced pressure. The residue was recrystallized from chloroform. White needle crystals were obtained, mp 148 °C (95% yield). 1H NMR (DMSO-d6): δ 1.89 (s, 3H, -CH2NHCOCH3), 2.07 (s, 3H, -NHCOCH3), 4.22 (d, 2H, -CH2NHCOCH3), 7.10, 7.23, 7.68 (t, t, d, 1H, 2H, 1H, Ar-H), 8.52 (broad t, 1H, -CH2NHCOCH3), 9.83 (broad s, 1H, -NHCOCH3). 2-(Acetamidomethyl)-4-nitroacetanilide. A mixture of 69-71% nitric acid (1.10 g, 0.012 mol) and 9598% sulfuric acid (1.24 g, 0.012 mol) was added dropwise with stirring to the solution of 2-(acetamidomethyl)acetanilide (2.45 g, 0.012 mol) in sulfuric acid (3.50 mL) at -8 °C for 1 h. The mixture was stirred at 0-10 °C for 3 h and 40 °C for 1 h. After the reaction was complete, the mixture was poured into cold water with vigorous stirring. The light yellow precipitate was
10.1021/ie970724s CCC: $18.00 © 1999 American Chemical Society Published on Web 01/04/1999
Ind. Eng. Chem. Res., Vol. 38, No. 1, 1999 3
Figure 1. Compounds synthesized.
Figure 2. Chemical structures of the dye (AZ1M) and the isocyanate cross-linkers (Tolonate HTD and IPDI) that comprise the NLO polymer.
collected by suction filtration and washed well with cold water and then recrystallized from methanol. Light yellow crystals were obtained, mp 160-1 °C (51% yield). 1H NMR (DMSO-d ): δ 1.94 (s, 3H, -CH NHCOCH ), 6 2 3 2.19 (s, 3H, -NHCOCH3), 4.39 (d, 2H, -CH2NHCOCH3), 8.18 (m, 3H, Ar-H), 8.75 (broad t, 1H, -CH2NHCOCH3), 10.31 (broad s, 1H, -NHCOCH3). 2-(Acetamidomethyl)-4-nitroaniline. 2-(Acetamidomethyl)-4-nitroacetanilide (12.00 g, 0.048 mol) was suspended in 1 N NaOH solution (800 mL). The mixture was refluxed for 45 min and then cooled to room temperature. The yellow precipitate was collected by suction filtration and washed well with cold water until the filtrate was neutral and then recrystallized from ethanol. Bright yellow crystals were obtained, mp 225-6 °C (91% yield). 1H NMR (DMSO-d6): δ 1.90 (s, 3H, -COCH3), 4.15 (d, 2H, -CH2NH-), 6.68 (m, 3H, -NH2-Ar-H), 7.90 (m, 2H, Ar-H), 8.45 (broad t, 1H, -CH2NHCOCH3).
4- {[2-(Acetamidomethyl)-4-nitrophenyl]azo}-Nethyl-N-(hydroxyethyl)aniline. 2-(Acetamidomethyl)4-nitroaniline (10.00 g, 0.0478 mol) was diazotized in concentrated hydrochloric acid (40 mL) by addition of sodium nitrite (3.30 g, 0.0478 mol) in distilled water (10 mL) at 0-5 °C. After addition was complete, the mixture was filtered and a solution of N-ethyl-N-phenylethanolamine (7.90 g, 0.0478 mol) in ethanol (25 mL) was slowly added dropwise to the filtrate with stirring at 0-5 °C. After the addition was complete, 10% sodium hydroxide solution was added until the solution was neutral. A deep red precipitate was collected by suction filtration, washed well with water, and then recrystallized from ethanol. Deep red crystals were obtained, mp 185-6 °C (45% yield). 1H NMR (DMSO-d6): δ 1.17 (m, 3H, -NCH2CH3), 1.92 (s, 3H, -COCH3), 3.60 (broad m, 6H, CH3CH2NCH2-CH2OH), 4.78 (d, 2H, -CH2NH-), 4.90 (broad s, 1H, -OH), 6.91 (d, 2H, Ar-H), 7.72 (d, 1H,
4
Ind. Eng. Chem. Res., Vol. 38, No. 1, 1999
Ar-H), 7.88 (d, 2H, Ar-H), 8.25 (m, 2H, Ar-H), 8.52 (broad t, 1H, -CH2NH-). 4-{[2-(Aminomethyl)-4-nitrophenyl]azo}-N-ethylN-(hydroxyethyl)aniline (DDR1A). 4-{[2′-(Acetamidomethyl)-4′-nitrophenyl]azo}-N-ethyl-N-(hydroxyethyl)aniline (6.85 g, 17.80 mmol), methanol (24 mL), and 6 N hydrochloric acid (26 mL) were heated under reflux with stirring for 16 h. After the reaction was complete, the solution was cooled in an ice-bath and then slowly neutralized by addition of 10% sodium hydroxide solution. The deep red precipitate was collected by suction filtration and then recrystallized from ethanol. Deep red plates were obtained, mp 200 °C (85% yield). 1H NMR (DMSO-d ): δ 1.17 (m, 3H, -NCH CH ), 6 2 3 2.15 (broad s, 2H, -NH2), 3.60 (broad m, 6H, CH3CH2NCH2CH2OH), 4.22 (s, 2H, -CH2NH2), 4.87 (broad s, 1H, -OH), 6.89 (d, 2H, Ar-H), 7.68 (d, 1H, Ar-H), 7.82 (d, 2H, Ar-H), 8.17 (d, 1H, Ar-H), 8.52 (s, 1H, Ar-H). N,N-Bis(2′-hydroxypropyl)-3-methylaniline (1). To a three-necked round-bottomed flask (1000 mL) equipped with dry ice condenser and thermometer were added m-toluidine (107.16 g, 1.0 mol), propylene oxide (127.60 g, 2.20 mol), and the solution of tetrabutylammonium bromide (4.63 g, 0.0144 mol) in ethanol (25 mL) under a nitrogen atmosphere. The resulting solution was stirred at room temperature for 1 h and heated at 45-50 °C for 24 h. The solvent was removed under reduced pressure to afford a dark liquid purified by vacuum distillation to give a colorless liquid, bp 136-8 °C/1 mm (80% yield). 1H NMR (DMSO-d6): δ 1.08 (d, 6H, -N (CH2CH(OH)CH3), 2.20 (s, 3H, -CH3), 3.25 (m, 4H, -N(CH2CH(OH)CH3), 3.88 (m, 2H, -N(CH2CH(OH)CH3), 4.69 (d, 1H, -OH), 5.05 (d, 1H, -OH); 6.40 (m, 3H, Ar-H), 6.98 (m, 1H, Ar-H). 3-(Acetyloxymethyl)-6-methylacetanilide (2). Acetic anhydride (96 mL, 1.02 mol) was slowly added dropwise to the solution of 3-amino-4-methylbenzyl alcohol (55.70 g, 0.406 mol) in dry pyridine (450 mL). After the addition was complete (∼30 min), the resulting solution was stirred for 16 h under a nitrogen atmosphere at room temperature. Acetic anhydride and pyridine left over were removed under reduced pressure to give a light yellow solid. Distilled water (600 mL) was added to the crude product with stirring. Precipitate was collected by suction filtration and washed well with water, mp 116-7 °C (98% yield). 1H NMR (DMSO-d ): δ 2.05 (d, 6H, OC(O)CH , NHC6 3 (O)CH3), 2.18 (s, 3H, -CH3), 5.00 (s, 2H, -CH2OC(O)CH3), 7.05 (d, 1H, Ar-H), 7.20 (d, 1H, Ar-H), 7.40 (s, 1H, Ar-H), 9.30 (s, 1H, -NH). Synthesis of 2-Nitro-4-methyl-5-acetamidobenzyl Acetate (3). A 20 g sample of 3-acetamido-4methylbenzyl acetate was dissolved in 50 g of concentrated sulfuric acid with constant stirring at -5 °C, and then the acidic solution was further cooled to -15 °C by an ethylene glycol/dry ice bath. At this point, a mixture of 5.7 g of concentrated nitric acid and 10 g of concentrated sulfuric acid was added dropwise in such a manner that the temperature of the reacting medium did not exceed -10 °C. After the addition was complete (ca. 1 h), the reaction mixture was stirred at -10 °C for an additional 3 h. The resulting solution was poured over ice, with vigorous stirring, and the light brown precipitate obtained was suction filtrated and recrystallized from ethanol to give a pale colored powder. 1H NMR (DMSO-d6): δ 9.54 (s, 1H, NH), 8.02 (s, 1H, Ar-
H), 8.01 (s, 1H, Ar-H), 5.38 (s, 2H, CH2OAc), 2.30 (s, 3H, phCH3), 2.11 (two singlets, 6H, CH3COO-). 3-Amino-4-methyl-6-nitrobenzyl Alcohol (4). A mixture of 69% nitric acid (4.42 g, 0.05 mol) and concentrated sulfuric acid (4.99 g, 0.05 mol) was slowly added dropwise to the solution of 3-(acetyloxymethyl)6-methylacetanilide (10.67 g, 0.0483 mol) in concentrated sulfuric acid (26.00 g) with stirring between -5 and -0 °C. After the addition was complete (∼1 h), the mixture was stirred between -5 and 0 °C for 3 h and at room temperature for 1 h. The resulting solution was poured into ice water with stirring. Light yellow precipitates were collected by suction filtration and washed well with distilled water. The crude product was suspended in 1N aqueous sodium hydroxide solution (100 mL). The resulting mixture was refluxed for 45 min with stirring and cooled with an ice bath. A bright yellow precipitate was collected by suction filtration, washed well with distilled water, and recrystallized from ethanol to give bright yellow crystals, mp 160-1 °C (43% yield). 1H NMR (DMSO-d ): δ 2.10 (s, 3H, -CH ), 4.77 (d, 6 3 2H, -CH2OH), 5.36 (t, 1H, -OH), 6.48 (s, 2H, -NH2), 7.00 (s, 1H, Ar-H), 7.87 (s, 1H, Ar-H). Synthesis of AZ1M. A 2.5 g sample of 2-nitro-4methyl-5-aminobenzyl alcohol was dissolved in 180 mL of 2 N HCl. The solution was cooled by an ice bath. To the cold solution was added 0.94 g of sodium nitrite in 10 mL of distilled water. The resulting solution was stirred at 0-5 °C for 1/2 h after which the mixture was transferred to a solution of 3.06 g of N,N-bis(2-hydroxypropyl)-m-toluidine in 50 mL of 2 N HCl at 0 °C. The resulting solution was kept stirring at 0-5 °C for 3 h before it was neutralized with 10% sodium hydroxide solution. A red precipitate was collected by suction filtration and further purified by column chromatography using silica gel eluted with ethyl acetate. 1H NMR (DMSO-d6): δ 8.08 (s, 1H, Ar-H), 7.94 (s, 1H, Ar-H), 7.70 (d, 1H, Ar-H), 6.72 (d, 1H, Ar-H), 6.69 (s, 1H, ArH), 5.57 (t, 1H, OH), 5.05 (d, 1H, OH), 4.87 (d, 1H, OH), 4.83 (d, 2H, CH2OH), 3.98 (m, 2H, CH3CHOH), 3.50 (m, 4H, NCH2), 2.68 (s, 3H, Ar-CH3), 2.68 (s, 3H, Ar-CH3), 1.12 (d, 6H, CHCH3). 13C NMR (DMSO), 153.7, 152, 145.9, 142, 141.5, 137.7, 134.5, 127.4, 117.6, 115.0, 112.3, 110.8, 64.2, 60.1, 59.3, 21.3, 18.1,16.7. Polymer preparations, processing, and stability measurements were carried out as in ref 6 and ref 1. The electrooptic coefficient, r33 , was measured in transmission at λ ) 1.3 µm, using a modulating voltage of 5 V rms at 680 Hz and assuming negligible birefringence and r33 ) 3r13. Discussion Choice of Monomer. The monomers synthesized along with the stability as measured by UV-vis are shown in Figure 1. Disperse Red 1 was chosen as the “base” molecule for multifunctional modifications because of its relatively large optical nonlinearity and the tremendous amount of NLO literature on this compound.4 The chosen polymerizable derivatives were such that both the phenyl rings of the molecule contained at least one reactive functionality. As outlined in previous papers from this laboratory,1,6 the reactive functionalities are chosen such that they allow for a stepwise reaction of the functionalities. It is well documented that Disperse Red 1 can display a high nonlinearity.4 Indeed, µβ calculations on these
Ind. Eng. Chem. Res., Vol. 38, No. 1, 1999 5 Scheme 1. Synthesis of AZ1M
derivatives showed that we could expect nonlinearity of about 9 times pNA.7 It was hoped that by utilizing an oligomeric system in which one would expect ion migration to be minimized, the poling field could be increased considerably, thus yielding a poled polymer with an E-O coefficient approaching 20 pm/V, our desired target for these materials. The thermal stability of the molecules synthesized were examined by UV-vis spectrometer. The molecules were dissolved and cured in the desired isocyanate system and also in poly(methyl)methacrylate), and the stability was examined after heating at 150 °C after 1 h. All the molecules with the exception of AZ1M (Scheme 1) and DDR1 showed a significant decrease in the absorption at λmax. The results for DDR1A in the desired poly(urethane urea) as well as in PMMA are displayed in Figure 3a,b, respectively. We interpret the decrease or a shift in absorbance at λmax as an indication of thermal instability of the molcules. Instability could result from a number of processes or combinations thereof, including isomerization, which is well documented for azo compounds,5 as well as chemical degradation of the molecule. The instability observed for DDR1A and DDR1C was not attributed solely to trans-cis isomerization. This was attributed mainly to chemical degradation of the molecule. When DDR1A was heated in pyridine, a new product similar to that observed from the molecule being heated in poly(methyl methacrylate) (PMMA) was observed for the product. Thin-layer chromatography on the heated solution in pyridine also revealed a new compound along with the parent compound. NMR analysis of the reaction product suggests the formation of a 2,3-dihydro-1,2,3-benzotrianine, as shown in Figure 4. This structure was supported by observation of a para-substituted aromatic ring (doublets, J ) 9.1 Hz at 6.84 and 7.84 ppm), a 1,2,4-trisubstituted aromatic ring (8.82 ppm, d, 2.3 Hz; 8.03 ppm, dd, 2.2 and 9.4 Hz; 7.86 ppm, d, 9.0 Hz), a very sharp singlet at 9.26 ppm, and the usual absorptions for the Et-N-CH2CH2OH. A very sharp singlet at 9.26 ppm suggested a CHdN; the line width was much less than that expected for an NH. Supporting this assignment was the fact that there was no evidence for the aliphatic CH2 connected to the NH2, as found in the starting material.
Figure 3. (a). UV-vis spectrum of a film of DDR1A reacted with Tolonate HDT/IPDI. The solid line represents unheated sample at room temperature, and the dotted line represents the sample after being heated at 150 °C for 1 h. (b) UV-vis spectrum of a film of DDR1A dissolved in PMMA. The solid line represents unheated sample at room temperature, and the dotted line represents the sample after being heated at 150 °C for 1 h.
The 13C NMR spectrum indicated the presence of four inequivalent CH aromatic carbons in addition to the pair of doubly intense aromatic CH from the aminophenyl ring. Electron impact and chemical ionization direct probe studies did show a molecular ion, and mass spectra analysis suggested the sample decomposed on heating in the probe. A Fourier transform mass spectrum showed a peak at m/z 341 corresponding to the molecular ion of this unusual dihydro-1,2,3-benzotriazine and a peak at 295 from loss of NO2. Interestingly, when the primary amine was converted to the urea, similar to that expected when it is polymerized with isocyanates, or an amide, the degradation was noticeably slowed, but not eliminated. DDR1C also showed degradation, as observed by UV-vis. Due to decoloration observed with both DDR1A and DDR1C it was thought that the conjugation at the azo linkage was being destroyed. These observations led us to conclude that reactive hydrogens, separated by only one carbon atom, on the phenyl ring bearing the electron withdrawing nitro group were undesirable. Preliminary results in our lab on a compound similar to DDR1C, in which the hydroxyl is spaced two carbon atoms from the phenyl ring, show excellent thermal stability under identical conditions at 100 °C. In contrast, Hubbard et al. have reported that a similar compound with a 5-methylamino group, rather than at the 2 position, was stable at 85 °C. However, the unexpectedly poor tem-
6
Ind. Eng. Chem. Res., Vol. 38, No. 1, 1999
Figure 4. Chemical degradation of DDR1A.
poral stability of their epoxy cross-linked system was unexplained.1b It is interesting that the usual technique utilized by dye chemists to stabilize azo dyes is having a hydrogen bonding group ortho to the azo linkage, albeit in most cases the hydrogen bonding group is on the phenyl ring bearing the donor groups. It should be pointed out that the stabilities of those dyes were never reported under such adverse conditions as being carried out here and DDR1A and DDR1C display excellent stability at temperatures slightly above room temperature. These molcules are also very stable when used in the crystalline form. On the basis of the excellent thermal stability of DDR1, it was decided to keep the methyl group ortho to the azo linkage and modify the molecule at the position meta to the azo linkage. This would discourage intramolecular interaction between the methylol group and the azo linkage. As predicted, this compound, AZ1M (Scheme 1), did indeed display excellent thermal stability at 100 °C. From the excellent thermal stability observed for DDR1 and AZ1M it seems reasonable that the methyl group is creating a steric barrier such that the normal trans-cis thermal isomerization (degradation) route allowable to these molecules, like DR1, is being restricted. An interesting feature of AZ1M is that the gain in thermal stability over DR1 was not at the expense of the nonlinearity. Also noteworthy are the facts that the relative nonlinearities of compounds bearing dihydroxyethyl groups at the donor nitrogen (DDR1, AZ1, and AZ1M) are slightly lower than the compound with a monohyrdoxyalkyl group (DDR1A), and the compound with the primary amine (DDR1C) displays a slightly higher µβ than the other compounds. Our studies of other families of NLO compounds reveal a similar trend, indicating that the differences observed in the µβ values, though small, are real and not due to experimental errors. One possible explanation for the slight increase in the µβ value may be due to donation of electron density by the primary amine group (DDR1C) via the inductive effect. This results in the dialkyl nitrogen being a slightly better donor for the conjugated NLO framework. On the other hand, introduction of the dihydroxy functionalities withdraws electron density from the donor nitrogen through the inductive effect of the oxygen. This diminishes the electron density available to the conjugated system and thus lowers the µβ value. Thus, it is desirable to have the reactive nitrogen connected to the donor nitrogen, but such that upon reacting it will not deplete the electron density on the donor nitrogen atom. In this case, however, the reaction of this primary amine with the isocyanate cross-linker was very fast and thus made characterization of the reacting medium difficult. Choice of Isocyanate. Oligomeric multifunctional isocyantes were chosen such that one could easily synthesize a prepolymer with good film-forming proper-
Figure 5. Thermal stability of poled AZIM film at 100 °C.
ties. The higher molecular weight prepolymer would also assist in increasing the electrical breakdown potential of the film. This specific combination of Tolonate IPDT and HDT, though not optimized, was found to give superior performance in film toughness over pure Tolonate IPDT and higher film depoling temperature over pure Tolonate HDT. Film Formation and Poling/Curing. Typically, the films were made by using a 50/50 equivalent mixture of the two different isocyanate cross-linkers, HDT based on the trimerization of 1,6-hexamethylene diisocyanate and IPDT based on the trimerization of isophorone diisocyanate. This combination of flexible and rigid cross-linkers ensured the films would be substantially rigid to maintain polar alignment and at the same time sufficiently flexible to prevent cracking when stored at elevated temperatures.6 The total quantity of isocyanate used was calculated on the basis of the amount required to react with the primary alcohol and one secondary alcohol along with a 10% excess. The solvent was freshly distilled dry pyridine. Films of typical thickness 2 µm were made and poled via contact poling. (For further details see ref 1). Phase contrast microscopy of the poled films revealed some-poling induced surface roughness. No roughness was detected in the unpoled regions of the cured films. Identical poling-induced “film damage” has also been reported for a side chain NLO polymer that was corona poled. 8 Poling Stability. As shown in Figure 5, within 24 h of poling a decrease of approximately 5% was observed in the E-O coefficient when the film was kept at 100 °C. This initial 5% decrease may be due to molecules bonded at one attachment point undergoing randomization. Supporting this is the fact that for similar systems bearing an amino group and a hydroxyl functionality FT-IR reveals remnants of free hydroxy groups even though the band at 2270 cm-1 corresponding to the free isocyante group disappears completely.1a After this initial 5% decrease the E-O coefficient was stable even after 1000 h at 100 °C. The temperature of the film was increased to 125 °C and the stability of the dipolar orientation observed. As shown in Figure 6, a linear decay in r33 vs log t was observed over three decades. The UV-vis of an identical unpoled film held at 125 °C displayed a 10% decrease in absorbance (λmax 490) over
Ind. Eng. Chem. Res., Vol. 38, No. 1, 1999 7
Literature Cited (1) (a) Francis, C. V.; White, K. M.; Boyd, G. T.; Moshrefzadeh, R. S.; Mohapatra, S. K.; Radcliffe, M. D.; Trend, J. E.; Williams, R. C. Chem. Mater. 1993, 5, 506. (b) Eich, M.; Reck, B.; Yoon, D. Y.; Wilson, C. G.; Bjorklund, J. Appl. Phys. 1989, 66, 3241. (c) Dalton, L. R.; Harper, A. W.; Ghosn, R.; Steier, W. H.; Ziari, M.; Fetterman, H.; Shi, Y.; Mustacich, R. V.; Jen, A. K-Y.; Shea, K. J. Chem. Mater. 1995, 7, 1060. (d) Kitipachi, P.; La Peruta, R.; Korenowski, G. M.; Wnek, G. E. J. Polym. Sci. Part A 1993, 31, 1365. (2) (a) Dai, D.; Marks, T. J.; Yang, J.; Lundquist, P. M.; Wong, G. K. Macromolecules 1990, 23, 1891. (b) Chen, T. A.; Jen, A. K-Y.; Cai, Y. Chem. Mater. 1996, 8, 607. Figure 6. Thermal stability of poled AZIM film at 125 °C.
42 days. Thermally stimulated discharge current (TSC) revealed a depoling temperature (Td) of 149 °C (χ2)/2 point) when heated at 2.5 °C/min. Thus, with the poled system being held at approximately 25 °C below its Td coupled with the UV-vis results it would seem to indicate that the instability in the system at 125 °C may be due primarily NLO-phore instability rather than the cross-linked system falling apart. Through a method of systematic designs and synthesis we have arrived at an azo compound with excellent thermal stability in a cross-linked polyurethane system. This NLO-phore is also easily processed when combined with the appropriate isocyanates to yield optical quality films. Acknowledgment The authors thank Dr. Richard Newmark and the 3M Analytical lab staff for the NMR work.
(3) Hubbard, M. A.; Marks, T. J.; Lin, W.; Wong, G. K. Chem. Mater. 1992, 4, 965. (4) Chen, M.; Dalton, L. R.; Yu, L. P.; Shi, X. Q.; Steier, W. H. Macromolecules 1992, 25, 4032. (5) Shi, Y.; Steier, W.; Yu, L. P.; Chen, M.; Dalton, L. R. Appl. Phys. Lett. 1991, 58, 1131. (6) White, K. M.; Kitipichai, P. K.; Francis, C. V. Appl. Phys. Lett. 1995, 66, 3099. (7) Leung, P. C. W.; Francis, C. V.; Harelstad, R. E.; Stevens, J.; Gerbi, D. J.; Spiering, M. O.; Tiers, G. V. D.; Trend, J. E.; Boyd, G. T.; Ender, D. A.; Williams, R. C. SPIE Proc. 1989, 1147, 48. (8) Hill, R. A.; Knoesen, A.; Mortazavi, M. A. Appl. Phys. Lett. 1994, 65, 1733.
Received for review October 17, 1997 Revised manuscript received September 25, 1998 Accepted November 2, 1998 IE970724S
