Thermally Stable Nonlinear Optical Moiety-Doped Polyimides for

Chapter 8

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H. K. Kim , H. J. Lee , M . H. Lee , S. K. Han , H. Y. Kim , K. H. Kang , Y. H. Min , and Y. H. Won 2

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Department of Macromolecular Science, Han Nam University, 133 Ojung-Dong Daeduck-Gu, Taejon, Korea 300-791 Photonic Switching Section, ETRI, P.O. Box 106, Yusung, Taejon, Korea 305-600

New NLO moiety-doped polyimides as host-guest systems were developed. NLO moieties based on both dialkyl amino alkyl sulfone stilbenes (DASS) and dialkyl amino nitro stilbenes (DANS) were synthesized as guest chromophores. Owing to the newly introducedflexiblealkyl chains to the NLO chromophore terminal groups, the compatibility between guest chromophores and host polyamic acid in NMP solvent was improved and the solubility of the alkylated-NLO moieties to the host polyimide was increased up to the 40 wt %. These polyimide-based guest-host systems exhibited a significant improvement in the thermal stability at high temperatures exceeding 250 °C by the TGA. The electro-optic coefficient at 632.8 nm is 13 pm/V for the 40 wt % DASS-doped polyimide system poled at the 135 V/mm. These new materials are promising candidates for photonic switching devices with low operating voltage.

Recently, poled NLO (or EO) polymeric materials have been considered potential candidates for high speed integrated electro-optic devices such as Mach-Zehnder modulators and directional couplers*. Compared to inorganic materials, these poled EO polymers offer a number of advantages, such as the subpicosecond response time, the lower power dissipation, ease of processability as thin film with semiconductor technologies . It is strongly noted that poled EO polymeric materials must retain thermal stability and EO thermal stability at both manufacturing and end-use environments, which require long-term stability up to 125 °C and short excursions to the temperature of 300 °C or higher. In many efforts to obtain such materials, two types of poled EO polymers such as side chain EO polymers and guest-host systems were intensively studied. The side chain EO polymer systems are composed of the organic molecule units which are covalently bound to polymer backbones*. In the guest-host system, the EO organic molecules (guest) are dissolved into a polymer matrix. Much attention has been paid on the development of polyimide host-guest systems*. Our main material efforts have been focussed on improving the electro1

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0097-6156/95A)601-0111$12.00A) © 1995 American Chemical Society Lindsay and Singer; Polymers for Second-Order Nonlinear Optics ACS Symposium Series; American Chemical Society: Washington, DC, 1995.

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POLYMERS FOR SECOND-ORDER NONLINEAR OPTICS

optic coefficient of materials by increasing the solubility of EO moieties in a polyimide as well as improving EO thermal stability with high glass transition temperature and no sublimation during curing or poling process. For this purpose, we have chemically modified the EO moieties based on both dialkylamino alkylsulfone stilbenes (DASS) and dialkylamino nitro stilbenes (DANS), yielding new alkylatedEO moieties as guest chromophores. Furthermore, new NLO moiety-doped polyimides as host-guest systems are being demonstrated in making multilayer-stack poled photonic devices. n. EXPERIMENTAL ,

1. Synthesis of 4-(diethylamino)-4-(hexyl sulfonyl) stilbene (DASS 62) (1) Preparation of 4-Hexyl-sulfonyl toluene A two-phase system composed of 1-bromohexane (130 g, 0.79 mol) in toluene (200 mL) and p-toluene sulfonic acid sodium salts (100 g, 0.56 mol) and 10 g of tetrabutylammonium bromide in water (200 mL) was stirred overnight at 80 °C. After the reaction system was cooled to room temperature, water (400 mL) was added, and the organic layer was extracted three times with toluene. The extractor was concentrated after dried with anhydrous MgS04. The product was distilled in vacuo to produce 106 g (79 %, yield) of a viscous liquid, bp 130 °C/0.1 mmHg: iH-NMR ( C D C I 3 ) d 0.80

(t, 3H),

1.23

(m,

6H),

1.66

(m,

2H),

2.41

(s,

3H),

3.03

(t, 2H),

7.34

(d,

2H), 7.76 (d, 2H). (2) Preparation of 4-Hexyl-sulfone benzyl bromide. 4-Hexylsulfonyl toluene (34.8 g, 0.145 mol) was dissolved in 200 mL of C C I 4 and refluxed under nitrogen atmosphere. NBS (25.81 g, 0.145 mol) and benzoyl peroxide (0.4 g) were added to the solution at reflux for 12 h. The reaction mixture was cooled andfilteredoff the succimide. Thefilteratewashed with water and dried with MgS04. The product mixture was concentrated after the solvent was removed, the residue was a mixture of 30 % unreacted starting material and 70 % brominated product and then used without further purification for later reaction. ^-NMR ( C D C I 3 ) d 0.75 (t, 3H), 1.24 (m, 6H), 1.64 (m, 2H), 3.04 (t, 2H), 4.42 (s, 2H), 7.51 (d, 2H), 7.80 (d, 2H). (3) Preparation of 4-Hexyl-sulfone benzyl phosphonate. The distilled triethyl phosphite (8.3 g, 0.05 mol) was heated at reflux, and 13 g (0. 04 mol) of 4-hexyl-sulfone benzyl bromide was added dropwise while stirring, at such a rate that a gentle reflux was maintained. When the addition was completed, the reaction was refluxed for an additional 2 h. The mixture was cooled to 25 °C, and the product mixture was distilled off the volatile materials. The crude viscous liquid was isolated by cromatography (eluent ethyl acetate : hexane, 5 : 1). 68 % yield. *H-NMR ( C D C I 3 ) d 0.82

(t,

3H),

1.26

(m,

12H),

1.66

(m,

2H),

3.04

(t,

2H),

3.26

(d,

2H),

4.08

(q, 4H), 7.49 (d, 2H), 7.83 (d, 2H). f

(4) Preparation of 4-(diethylamino)-4-(hexyl sulfonyl) stilbene (DASS 62). In a dry 250 mL three-necked, round-bottomed flask equipped with a sealed stirrer, a nitrogen inlet, and a reflux condenser topped with a Drierite tube were placed, under nitrogen, 90 mL of 1,2-dimethoxy ethane (DME) and sodium hydride (2.4 g, 0.06 mol,

Lindsay and Singer; Polymers for Second-Order Nonlinear Optics ACS Symposium Series; American Chemical Society: Washington, DC, 1995.

8.

KIMETAL.

Thermally Stable NLO Moiety-Doped Polyimides

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60 % dispersion in mineral oil). The suspension was stirred for 1 min. To the suspension was added 7.08 g (0.04 mol) of N,N-diethylamino benzaldehyde, and the mixture was stirred for 5 min to ensure complete dissolution. To this solution was added 15 g ( 0.04 mol) of 4-hexyl-sulfone benzyl phosphonate in 30 mL DME, and the reaction mixture was refluxed with vigorous stirring for 5 h. The bright yellow solution was poured over 200 g of crushed ice. And the solid thus formed was washed into the ice mixture. The yellow solid was collected by filtration, washed with cold water, and air dried. Recrystallizationfromethanol / methylene chloride yielded 7.8 g (49 %) of yellow crystals: mp 96 °C ; ^H-NMR ( C D C I 3 ) d 0.80 (t, 3H), 1.17 (t, 6H), 1.23 (m, 6H), 1.67 (m, 2H), 3.05 (t, 2H), 3.36 (q, 4H), 6.66 (d, 2H), 6.90 (d, 1H), 7.20 (d, 1H), 7.41 (d, 2H), 7.59 (d, 2H), 7.82 (d, 2H). 13C-NMR ( C D C I 3 ) d 12.6, 13.9,22. 3, 22.7, 27.9, 31.2, 44.4, 56.5, 111.5, 121.2 123.5, 126.1, 128.4, 128.5, 132.9, 136.0, 143.9,149.1. ,

2. Synthesis of 4-(Dihexylamino)-4-nitrostilbene (DANS 6) (1) Preparation of 4-(dihexylamino)benzaldehyde. A mixture of 25 g (0.2 mol), 37 g (0.2 mol) of dihexylamine, 21.3 g (0.2 mol) of sodium carbonate, 150 mL of dimethylsulfoxide, and 0.5 g of distearyldimethylammonium chloride was heated under nitrogen while stirring at 110 °C for 113 h. The reaction mixture was cooled and poured into 1.5 L of water, and the resulting solution was extracted with ethyl ether (4 x 300 mL). The combined extracts were washed with water and dried with anhydrous MgS04, and the solvent was removed at reduced pressure. The residue wasfractionallydistilled in vacuo to provide 32.4 g (56 %) of a pale yellow oil: bp 162 - 185 oC/0.1mmHg ; *H-NMR ( C D C I 3 ) d 0.89 (t, 6H), 1.23 (m, 12H), 1.60 (m, 4H), 3.31 (t, 4H), 6.63 (d, 2H), 7.69 (d, 2H), 9.67 (s, 1H). (2) Preparation of 4-(Dihexylamino)-4'-nitrosulbene (DANS 6) A mixture of 12.1 g (0.042 mol) of 4-(dihexylamino)benzaldehyde, 8.2 g (0.045 mol) of 4-nitrophenylacetic acid, 3.57 g (0.042 mol) of piperidine and 100 mL of xylene was heated while stirring at reflux or 20 h with continuous removal of water by using a Dean-Stark apparatus. Approximately half of the xylene was distilled, 30 mL of heptane was added, and the residue was cooled to -30 °C. Red crystals separated which were collected and recrystallizedfromtoluene / heptane to yield 4.8 g (28 %) of a red solid: mp 82 °C ; ^H-NMR ( C D C I 3 ) d 0.89 (t, 6H), 1.30 (m, 12H), 1. 58 (m, 4H), 3.28 (t, 4H), 6.62 (d, 2H), 6.90 (d, 1H), 7.21 (d, 1H), 7.40 (d, 2H), 7.54 (d, 2H), 8.17 (d, 2H). 13c.NMR ( C D C I 3 ) d 14.0, 22.7, 26.8, 27.3, 31.7, 51.0, 111.5, 120. 8,123.4,124.1,125.9,128.6,133.8,145.2,145.7,148.9. 3. Thin film preparation Guest chromophores (DASS-62 & DANS-6) with the content of 20 wt % and 40 wt % were mixed in Amoco Ultradel 3112/4212 polyimides (U-PI3112/4212) or Hitachi isoindoroquinazorin-dione polyimide (PIQ-2200) using a magnetic stirrer. The solutions in all cases were filtered using a 0.45 |im teflon membrane filter. Thin films were prepared by spin-coating the solution on various substrates such as a silicon wafer, indiumtinoxide (ITO) coated glass, quartz and NaCl discs. The thinfilmswere soft-baked for 5 minutes at 120 °C and imidized (cured) for several hours at various


114


temperatures. The thin films were analyzed by IR (Bomem MB-100) and UV spectroscopy (Shimadzu UV-3100S). The film thickness was measured by a surface profilometer (Tencor instruments, Alpha Step 300). 4. Thermal stability The thermal stability of guest chromophore-doped polyimide host-guest systems were studied by thermal gravimetric analysis (TGA) (Dupont TGA 9900). The weight loss as a function of temperature was followed by TGA at 10 °C/minute under a nitrogen purge of 50 cc/minute. 5. Optical property measurements The refractive index of guest chromophore-doped polyimide host-guest systems at the wavelength of 632.8 nm or 1300 nm was measured with the prism coupling method. The He-Ne laser beam ( or the laser diode) was coupled into and out of the polymer films spin-coated on Si-wafers with a Gadolinium Gallium Garnet (GGG) prism. The electro-optic coefficients of samples were measured using the simple reflection technique reported by Teng and Man? m. RESULTS AND DISCUSSION Host-guest system design: The EO moiety such as dimethylaminonitrostilbene (DANS-1), Disperse Red (DR1), etc., was iniatially mixed into polymethylmethacrylate) (PMMA) as a host polymer8,9. These host-guest systems had limited thermal stability and limited miscibility. The concentration of the EO moiety was kept below 15 %. Recently, it was reported that polyimide based-host guest systems were suitable for multilayerstack poled polymer electro-optic devices * . The host polymers in these systems used Amoco Ultradel 3112/4212 polyimides (U-PI 3112/4212) and Hitachi PIQ-2200 polyimide isoindoroquinazorindione (PIQ-2200). These polyimides exhibited thermal stability to withstand electronic processing conditions without structural deterioration and optical transparency at the operating wavelength of the devices. However, the thermal stability of the EO moiety and the compatibilty of the EO moiety with polyimides are still needed to be improved. We have synthesized long alkylated diethylamino hexylsulfone stilbene (DASS-; 62) and dihexylaminonitro stilbene (DANS-6) as guest chromophores to increase the solubility of EO moieties in a polyimide and to enhance the electro-optic coefficient of the materials as well as improve the EO termal stability with neither sublimation nor decomposition during curing or poling process. The main synthetic routes of these long alkylated guest chromophores are shown in Scheme 1. The relative EO coefficient for EO moieties and the maximum solubility of various DANS and DASS moieties mixed into either an U-PI 3112/4212 or a PIQ-220 polyimide are summarized in Table 1. The similar values of $\xg were theoretically obtained for DANS and DASS moieties, where P is the hyperpolarizability (X= 1907nm) and jig is the ground state dipole momentiS02Na

NBS c

*

a

/=\

H 0/TBAB

+

H

2

ft

BrH2C-^_^-|-Hex

P(OCH CH ) 2

3

ft

3

C

j? , ,

- \ > | -

H

8

X

/=v ft

(H3CH2CO)2PH2C-^_^-|-Hex

CHO

NaH/DME

Hex

K,CO.

v

DMSO

Hex^

H0 CCH2—Q-N0 2

2

Xylene/Piperidine

Hex^ He*T

Hex

N

H

e

/ - i i " ^ Q - N 0

2

PANS 6)

Scheme 1. Synthetic Routes of DASS-62 and DANS-6 as Guest Chromophores. The curing condition and thermal stability of guest chromophore-doped polyimide systems were studied by TGA. Polyimide PIQ-2200 has two distinct stages in the curing process with different temperature ranges. The first stage is the imidization stage in which the polyamic acid condenses to form the imideringsat the temperature of lower than 200 °C and the second is a densification stage in which the polyimide iminizes to form a bicyclic analogue at the temperature of 210 °C (for at least 4 hrs).


116


Table 1. Relative EO Coefficient (r*) for NLO Moieties. NLO moiety (M.W.)

(X.= I907nm)

max. sol. (wt%)

^relative r* (r/rnANs.i)

DANS-1 (268)

135

15

1

DANS-6 (408)

135

30

1.30

DASS-11 (301)

113

15

0.84

DASS-62 (399)

113

50

1.87

#The relative EO coefficient (r*) was estimated as a standard of which is calculatedfromthe following equation: r oc p n / (M.W.) x maximum solubility (wt %) /100.

TDANS-I

g

However, U-PI 3112/4212 has one-step process of imidization at 200 °C for at least 3 hours. IR spectroscopy indicates that the characteristic peaks of the carbonyl groups in polyamic acid disappeared around 1660 cm-i and two new peaks corresponding to the characteristic peaks of the carbonyl group in polyimide appeared around 1775 cmand around 1723 cm-i. The thermal stability of these host-guest systems is summarized in Table 2. TGA traces show that 40 wt% DASS-62 and 40 wt% DANS-6 in polyimides are all thermally stable at high temperature exceeding 250 °C. Practically, however, 40 wt% DASS-62 in a PIQ-220 polyimide is thermally stable up to 210 °C and 40 wt% DANS-6 in a PIQ-220 polyimide was thermally stable up to 190 °C with no sublimation. Experimentally, the glass transition temperature (T ) was estimated during the poling process. The T of 20 wt% DASS-62 is ~ 170°C and that of 40 wt% DASS-62 is - 160 °C. 1

g

g

Refractive index measurement The refracrive indices for the polyimide host-guest systems at both 632.8 nm and 1300 nm were measured with the prism coupler. The laser light was coupled into and out of the polymer films spin-coated on Si-wafers with a Gadolinium Gallium Garnet (GGG) prism. In this topology, the Si wafer acts as the substrate with a higher refractive index than that of the polymerfilm.As a result, the polymerfilmforms a leaky quasi-waveguide, since the light is guided by thefilm-airinterface while the reflection at thefilm-substrateinterface is leaky * . The beam is totally reflected except for the coupling conditions to waveguiding modes of both TM and TE modes which determine the guiding mode angles. A calculation based on the theory given by Tien et. al. is used to determine thefilmrefractive index and thicknessfromthe measured guiding mode angles. The measured refractive indices of polyimide hostguest system cured at 200 °C for 4 hours are presented in Table 3. 12

13

14


8. KIM ET AL.


117

Table 2. Thermal Stability of NLO Moiety-doped Polyimides for NLO Applications Guest

Host

subl. temp. (°C)*

DANS-6

PIQ-2200

190

DASS-62

PIQ-2200

210

this work

DCM

PIQ-2200

190

exciton

Lophine 1

PIQ-2200

200

IBM

Product of Guest this work

#

* No sublimation of the guest chromophores was observed at the given temperature. * The decomposition temperature was detected at 250 °C by TGA.

Table 3. Refractive index measurement of polyimide host-guest systems cured at 200 °C for 2 hours on a hot plate at the wavelength of 1300 nm. Host

Guest

PIQ

-

n

TE

1.6460 (1.6860)*

1.6431 (1.6822)

PIQ

DASS-62 (40wt%)

1.6469 (1.6904)

1.6439 (1.6871)

UPI-3112

-

1.5858

1.5784

UPI-3112

DASS-62 (40 wt%)

1.6018

1.5959

* indices of refraction were measured at the wavelength of 632.8 nm.


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Determination of the electro-optic coefficients: The electro-optic coefficients of a DASS-62/PIQ-2200 system were presently measured using the simple reflection technique reported by Teng and Man». A thin film was spun on ITO coated glass adjusting the spin speed to produce the film thickness of ~ 2 \xm. For the poling and the reflection, another electrode was fabricated by thermal evaporation to form an over 0.1 jim - thick film of aluminum on top of the polymer layer. For poling, the sample was heated to about 160 ~ 170 °C in a convection oven and a voltage was applied between Al and ITO electrodes measuring the current. The results of the measurements are plotted in Figure 1. The filled symbols present the electro-optic coefficients which are calculated with the maximum modulated signals. However, the signal is complicated by the interference effects between the reflected beamsfromthe two sides of the polymer film. To reduce the spurious modulating effects by the interference, the magnitudes of the modulated signals biased at the +90° and -90° phase retardation between the s- and p-waves are averaged. These results are also plotted with open symbols in Figure 1. As expected, the electro-optic coefficient (r 3) increases with increasing the amount of guest chromophores and the intensity of the poling field. The maximum electro-optic coefficient in the present study is ~ 13 pm/V for the 40 wt% DASS-62 poled at the 135 V/jim. Similarly, for the sample of 40 wt% DASS-62 in a PIQ-2200 polyimide, the measured value of the electro-optic coefficient (r 3) with a polingfieldof 40 V/^im at the wavelength of 1300 nm was about 5.2 pm/V. At the present, the electro-optic coefficient (r 3) as a function of the polingfieldis being measured. 3

3

3

25

•

20

1

1

o

• •

1

i

i

1

i1

4 0 wt55 D A S S - 6 2 20 wtX

1

DASS-62

15

CM

oo

c

CO 10

a

-

•

-

1

20

40

-

o

B

e

0

•

60

80

I

1

1

100

120

140

Poling Field (V//xm)

Figure 1. The electro-optic coefficients ( r ) at the wavelength of 632.8 nm as a function of poling field. 33


8.

KIMETAL.


119

IV. SUMMARY New NLO moiety-doped polyimides as host-guest systems are being demonstrated in making photonic devices. Long alkylated-NLO moieties based on both dialkylamino alkyl sulfone stilbenes (DASS) and dialkylamino nitro stilbenes (DANS) exhibited a significant improvement in the thermal stability at high temperatures exceeding 250°C. They retain remarkably high solubility in polyimides, improving the electro-optic coefficients( r 3) of these polyimide-based guest-host systems. The r value in the present study has been increased to 13 pm/V for the 40 wt% DASS62-doped polymer system poled at the 135 V/\im. However, further increase up to 25 pm/V may easily be achieved by increasing the amount of guest moieties and/or the intensity of the polingfield.Currently, various DASS analogues were systematically designed and synthesized, and were chemically attached to the polymer backbone of PMMA, providing side chain NLO polymers. These NLO polymeric materials are being characterized. 3

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V. REFERENCES 1. E. van Tomme, P. P. van Daele, R.G. Baets, and P. E. Lagasse, IEEE Journal of Quantum Electronics, 27, 778-787 (1991). 2. G. H. Cross, A. Donaldson, R. W. Gymer, S. Mann, N. J. Parsons, D. K. Haas, H. T. Man, and H. N. Yoon, SPIE, 1177, 79 (1989). 3. A. K. M. Rahman, B. K. Mandal, X. F. Zhu, J. Kumar, and S. K. Tripathy, Mat. Res. Soc. Symp. Proc., 214, 67 (1991). 4. C. P. J. M. van der Vorst, W. H. G. Horsthuis, and G. R. Mohlman, "Polymers for Lightwave and Integrated Optics: Technology and Applications", edited by L. A. Hornak, pp365-395, Marcel Dekker, Inc. New York, 1992. 5. J. W. Wu, E. S. Binkley, J. T. Kenny, R. Lytel, and A. F. Garito, J. Appl. Phys., 69, 7366 (1991). 6. S. Ermer, J. F. Valley, R. Lytel, G. F. Lipscomb, T. E. Eck, and D. G. Girton, Appl. Phys. Lett.,61,2272 (1992). 7. C. C. Teng and H. T. Man, Appl. Phys. Lett., 56, 1734 (1990). 8. (a) K. D. Singer, W. R. Holland, M. G. Kuzyk, G. L. Wolk, H. E. Katz, M. L. Schilling, and P. A. Cahill, SPIE, 1147, 233 (1989); (b) P. Pantelis and J. R. Hill, "Polymers for Lightwave and Integrated Optics : Technology and Applications", edited by L. A. Hornak, pp. 343-363, Marcel Dekker, Inc. New York, 1992. 9. H. L. Hampsch, J. Yang, G. K. Wong, and J. M. Torkelson, Macromolecules, 21, 526 (1988). 10. A. Ulman, C. S. Willand, W. Kohler, D. R. Robello, D. J. Williams, and L. Handley, J. Am. Chem. Soc., 112, 7083 (1990). 11.S. Stahelin, D. M. Burland, M. Ebert, R. D. Miller, B. A. Smith, R. J. Twieg, W. Volksen, and C. A. Walsh, Appl. Phys. Lett., 61, 1626 (1992). 12. T. N. Ding and E. Garmire, Appl. Opt., 22, 3177 (1983). 13. Y. Shuto, M. Amano, and T. Kaino, Jpn. J. Appl. Phys., 30, 320 (1991). 14. P. K. Tien, R. Ulrich and R. J. Martin, Appl. Phys. Lett., 14, 291 (1969). RECEIVED January 30, 1995


Thermally Stable Nonlinear Optical Moiety-Doped Polyimides for

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