Chapter 15

Second-Order Nonlinear Optical Interpenetrating Polymer Networks

S. Marturunkakul1, J. I. Chen1, L. Li1, X. L. Jiang2, R. J . Jeng2,3, S. K. Sengupta1, J . Kumar2, and S. K. Tripathy2

1Molecular Technologies Inc., Westford, MA 01886 2Center for Advanced Materials, Departments of Chemistry and Physics,

University of Massachusetts, Lowell, MA 01854

An interpenetrating polymer network (IPN) incorporating nonlinear optical (NLO) chromophores as a second-order NLO material is reported. This IPN system consists of an NLO active epoxy-based polymer network incorporating a thermally crosslinkable NLO chromophore, and an NLO active phenoxy-silicon polymer network. Characterization of the thermal and optical properties of the IPN samples are reported. The optical nonlinearity of the poled/cured IPN samples is stable after being heated at 100 °C for 168 h. The nonlinear optical coefficient, d33, of a poled/cured IPN sample is measured to be 32 pm/V at 1.064 µm. The electro-optic coefficients, r33, of this sample are determined to be 18 pm/V at 633 nm and 6.5 pm/V at 1.3

µm. Waveguide optical loss measurements have been carried out at 0.83 and 1.3 µm.

Second-order nonlinear optical (NLO) materials are currently of interest to a large number of research groups as they can be used for a number of photonic applications, for example, doubled-frequency laser sources, electro-optic modulation, optical signal processing, optical interconnects, etc. A practical second-order NLO polymer should possess large second-order nonlinearity, excellent temporal stability at elevated temperatures, and low optical loss (1). A number of NLO polymers have been developed to exhibit large second-order NLO coefficients comparable to those of the inorganic NLO materials which are currently in use in devices (2,5). However, the major drawback of NLO polymers is the decay of their electric field induced second-order optical nonlinearities. This decay is a result of the relaxation of the NLO chromophores from the induced noncentrosymmetric alignment to a random configuration. Numerous efforts have been made to minimize this decay through different approaches (4).

Recently, we have reported on an approach to prepare stable second-order NLO polymers using a full interpenetrating polymer network (IPN) structure (5,6).

3Current address: National Chung-Hsing University, Department of Chemical Engineering, Taichung, Taiwan

0097-6156y95/0601-0198$12.00/0 © 1995 American Chemical Society

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In Polymers for Second-Order Nonlinear Optics; Lindsay, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1995.

15. MARTURUNKAKUL ET AL. NLO Interpenetrating Polymer Networks 199

An IPN is a structure in which two or more networks are physically combined (7,8). The IPN is known to be able to remarkably suppress the creep and flow phenomena in polymers. The motion of each polymer in the IPN is reduced by the entanglements between different networks. These properties of the IPN have been shown to be able to restrict the mobility of the aligned NLO chromophores, and greatly enhance the stability of the NLO properties (5,6). The attractive feature of this approach is that it combines high Tg polymers and highly crosslinked networks, elements known to enhance the stability, into a single system. The temporal stability of the IPN is synergistically enhanced as it is better than that of the parent polymers. The second-order NLO properties of an IPN at 110 °C is found to be extremely stable for over 1000 h.

In this paper, investigation of an IPN system with a high degree of crosslinking density and NLO chromophore density is reported. This IPN system consists of two NLO-active networks formed by a phenoxysilicon polymer and a thermally crosslinkable epoxy prepolymer incorporating a thermally crosslinkable NLO chromophore. The synthesis and characterization of the thermal and optical properties of the IPN are described. Waveguide properties of this IPN system are also reported.

Experimental

The epoxy prepolymer (namely BPDOMA, see Figure la) is based on the diglycidyl ether of bisphenol A and 4-(4'-nitrophenylazo)aniline functionalized with thermally crosslinkable methacryloyl groups. The crosslinkable NLO chromophore, namely DRMA (Figure lb, Disperse Red 19 (DR19) functionalized with methacryloyl groups), is synthesized by coupling the corresponding acid chloride with the hydroxyl groups of DR19. The mixture of BPDOMA and DRMA forms a network through a radical addition reaction. The phenoxysilicon polymer, which is prepared through a sol-gel reaction (5), is based on an alkoxysilane dye (ASD) of (3-glycidoxypropyl) trimethoxysilane and 4(4'-nitrophenylazo)aniline, and the multifunctional phenoxyl compound l,l,l-tris(4-hydroxyphenyl)ethane (THPE). The NLO properties and characterizations of the phenoxysilicon polymers have been reported earlier in ref 9. Figure 2 shows the structures of ASD and THPE. The sol-gel process which involves sequential hydrolysis and condensation reactions is shown in Scheme 1.

hydrolysis

= SiOR + HOH _ ^ = SiOH + ROH

water condensation

= SiOH+ HOPh- _ w = SiOPh- + HOH

alcohol condensation

= SiOR+ HOPh- ^ ^ = SiOPh- + ROH

Scheme 1. The sol-gel reaction between ASD and THPE.

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In Polymers for Second-Order Nonlinear Optics; Lindsay, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1995.

200 POLYMERS FOR SECOND-ORDER NONLINEAR OPTICS

OR

Figure 1. Chemical structures of (a) BPDOMA and (b) DRMA.

Figure 2. Chemical structures of (a) ASD and (b) THPE.

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In Polymers for Second-Order Nonlinear Optics; Lindsay, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1995.

15. MARTURUNKAKUL ET AL. NLO Interpenetrating Polymer Networks 201

Thin films were prepared as described previously (5). The procedure is summarized in Scheme 2. IPN samples with various compositions were prepared. The network of each polymer was formed simultaneously when the thin films were heated for 1 h at 200 °C on a hot stage. The typical thickness of the cured films was approximately 1 /xm. The reactions of both networks were monitored by FTIR photometer (1760X, Perkin-Elmer). A differential scanning calorimeter (DSC 2910, T.A. Instrument Co.) was used to study the crosslinking reaction and determine thermal properties of the samples.

The corona poling technique (9,10) was employed to align the NLO chromophores. The corona field was applied as the temperature was raised to 80 °C. The temperature was then increased to 200 °C with the corona field on. The corona current was maintained at 2 fiA with a potential of 4 kV while the poling temperature was kept at 200 °C for 60 min. The formation of the networks and the molecular alignment for the poled order proceeded simultaneously during this period. The sample was then cooled down slowly to room temperature before the corona field was removed.

The second-order NLO properties of the poled EPN samples were measured by second harmonic generation from 1.064 /am laser radiation (9,70). Electro-optic coefficients, r^, of the IPN samples were measured at 633 nm and 1.3 /im, using the reflection method (77). The relaxation behavior of the second-order NLO properties was studied by monitoring the decay of the second harmonic (SH) intensity as a function of time at 100 °C after poling and curing. The waveguide optical loss of slab uncladded polymer films were determined at 0.83, 1.064 and 1.3 //m. The measurement scheme has been described in detail elsewhere (72).

Result and Discussion

The design rationale to incorporate the thermally crosslinkable methacryloyl group on the epoxy polymer, BPDOMA, is that a high degree of functionalization is possible (close to 100%). In order to further increase the chromophore- and the crosslinking densities of the IPN system, a thermally crosslinkable NLO chromophore, DRMA, is added. An onset point of an exotherm at 175 °C was observed for the methacryloyl groups from the BPDOMA and the functionalized chromophore by differential calorimetric analysis. As the samples are heated at 200 °C for 1 h during the curing process, inter- and intramolecular crosslinking reactions between the polymer and the functionalized dye take place. The cured materials are no longer soluble in THF which is a good solvent for both the dye and the polymer due to the extensive degree of the reaction. The SH intensity as a function of temperature was monitored for both poled/cured BPDOMA and BPDOMA/DRMA (2:1 by wt) samples. The onset temperature of the decay of the SH intensity for the BPDOMA/DRMA is higher than that of the BPDOMA. This demonstrated that the addition of the thermally crosslinkable dye, DRMA, increased the crosslinking density of the material leading to enhanced stability.

Figure 3 shows the IR spectra of an IPN sample before and after curing at 200 °C. The THPE/ASD network was formed through a sol-gel process. After the sample was heated at 200 °C for 1 h, the broad O-H stretching band at 3383 cm"1 and the bands at 1456 and 1376 cm-1 for the O-CH3 bending decreased significantly due to the reaction of THPE with ASD. In addition, a new band at 948 cnr1 for the Si­OPh stretching in the cured sample confirms the formation of phenoxysilicon bonds (9). The epoxy-based polymer network was formed through a free radical polymerization of the methacryloyl groups. The C=C stretching at 1677 cm-1

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In Polymers for Second-Order Nonlinear Optics; Lindsay, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1995.

2 0 2 POLYMERS FOR SECOND-ORDER NONLINEAR OPTICS

Polymer I: BPDOMA/DRMA (Radical Addition)

curing

Polymer II: ASD/THPE

(Sol-gel reaction) ^ >

poling

^ NLO active chromophores

Scheme 2. Schematic diagram for the formation of an IPN.

4000

T r 1800 1400 1200 2000 1

Wavenumber (cm-i)

- r 1000 400

Figure 3. Infrared spectra of the IPN sample before (top) and after (bottom) curing.

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15. MARTURUNKAKULETAL. NLO Interpenetrating Polymer Networks 203

disappeared in the cured sample. The strong carbonyl stretching band at 1735 cm-1

due to conjugation with the C=C group reduced after curing. The results from the IR spectra indicate that these two polymers each form their own networks through different reaction mechanisms upon heating.

The poled and cured IPN samples showed excellent optical quality. The homogeneity of this full IPN system is also suggested by the single To at 141 °C observed from the DSC thermogram with a 10 °C/min scanning rate. The Tg of the cured phenoxysilicon polymer (9) and BPDOMA were determined to be 110 and 145 °C respectively. The single Tg of the IPN sample implies that phase separation did not occur in this molecular composite. Optical microscopy reveals a clear transparent featureless film.

The temporal stability of the second-order optical nonlinearity for the poled/cured IPN, phenoxysilicon polymer, and BPDOMA samples are shown in Figure 4. The SH intensity of the sample is stable after a small initial decay as the sample is subjected to thermal treatment at 100 °C for up to 168 h, whereas the BPDOMA and the phenoxy-silicon samples show faster initial reduction of the SH intensity at the same heat treatment condition. The synergistically enhanced stability of the poled order in this IPN system is on account of the permanent entanglement within the interpenetrating network structure. The second-order NLO coefficient, J33, of the poled/cured IPN samples are summarized in Table I. The IPN sample with 1:1 weight ratio of each network component exhibits a ^33 of 32 pm/V at 1.064 /zm. The 7-33 values of this sample were determined to be 18 pm/V at 633 nm and 6.5 pm/V at 1.3 fim.

Table I. Optical properties of the poled and cured IPN samples processed at 200 °C for 1 h.

Phenoxysilicon (ASD/THPE)

Polymer-11/ DRMA

(2:1 w/w)

d33 at 1.064 pm

Weight ratio

1 1 32.0 Weight ratio 2 1 26.0

BPDOMA exhibits relatively low optical loss even when the processing conditions have not been optimized. Measurements on unclad slab waveguides gave values of 4.7 dB/cm at 830 nm and 2 dB/cm at 1.3 pm. Loss measurement on the IPN samples was performed at 1.3 jUm. The waveguide optical loss was determined to be 4 dB/cm.

Conclusion

A new second-order NLO IPN system has been developed and the samples exhibit large and stable optical nonlinearities. The nonlinearity of this IPN system is affected by the compositions. Waveguide optical loss measurement was possible on this system. Further improvements of the linear and nonlinear optical properties of this class of materials is currently underway from the perspective of practical device applications.

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204 P O L Y M E R S F O R S E C O N D - O R D E R N O N L I N E A R O P T I C S

1.0

0.8-!!

0.6-I I 0.4-1

0.2-

0.0

? 2 A

T o

IPN (2:1) Phenoxysilicon BPDOMA/DRMA

50 100 Time (hours)

150 200

Figure 4. Temporal Stability of the IPN sample (Phenoxysilicon/(BPDOMA/ DRMA), 2:1), phenoxysilicon and BPDOMA/DRMA (2:1) samples as subjected to thermal treatment at 100 °C.

Acknowledgments

Funding from NSF and ONR at UMass Lowell, and AFOSR (Contract # F49620-93-C-0039) at MTI is gratefully acknowledged.

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RECEIVED January 30, 1995

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