Nonlinear Optical Activity Relaxation Energetics in ... - ACS Publications

Chapter 27

Downloaded by UNIV MASSACHUSETTS AMHERST on October 14, 2012 | http://pubs.acs.org Publication Date: August 11, 1995 | doi: 10.1021/bk-1995-0601.ch027

Nonlinear Optical Activity Relaxation Energetics in a Polyether Thermoplastic Nonlinear Optical Polymer with a High Glass-Transition Temperature 1

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Robert J. Gulotty , Claude D. Hall , Michael J. Mullins , Anthony P. Haag , Stephen E. Bales , Daniel R. Miller , Charles A. Berglund , Marabeth S. LaBarge , Andrew J. Pasztor, Jr. , Mark A. Chartier , and Kimberly A. Hazard 1

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Dow Chemical Company, Central Research and Development, Midland, MI 48674 Dow Chemical Company, Anaytical Sciences, Midland, MI 48667

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A polyether polymer comprised of the 4-nitrophenyl hydrazone of bisphenol K and bisphenol A in a 50/50 comonomer ratio exhibits stable NLO activity at 100°C for years. In an effort to understand the kinetics and energetics of relaxation, isothermal aging studies were done at 150 °C, 175 °C and 200 °C. Williams-Watts stretched exponential and Arrhenius analysis were used to determine the activation energy and prefactors. In addition, polymer relaxations were studied by dynamical mechanical analysis and dielectric spectroscopy. The conclusion from this work is that the energetics of orientational relaxation are that of a sub-Tg relaxation, or beta relaxation, consistent with a crank shaft motion of the chromophore about the polymer axis. However, this relaxation only turns on at temperatures very near the polymer Tg, where enough free volume is developed for the motion to occur. We have termed this a Tg assisted beta relaxation.

A fundamental requirement of nonlinear optical (NLO) polymers for electro-optical (EO) applications is stability at elevated temperatures for long periods of time. For example, an EO modulator for telecommunication applications needs to be stable at -50 °C to 70 °C for many years. Similarly, an EO modulator in a desk top computer may be exposed to temperatures as high as 80 °C during use. It is generally asserted that stabilities at 100 °C for 10 years will be a benchmark for commercial applications. NLO thermoplastics developed at The Dow Chemical Co. have been shown to have excellent thermal stability at 100 °C. For example, a corona poled sample of the 4nitrophenyl hydrazone of bisphenol K/bisphenol A copolyether (NBPE, see structure below) has been shown to exhibit stable NLO activity at 1(X) C in air for a period of 3.8 years, with an estimated 1/e lifetime of 35 years, see Figure 1. 0

0097-6156y95/0601-0368$12.00/0 © 1995 American Chemical Society In Polymers for Second-Order Nonlinear Optics; Lindsay, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1995.

27.

GULOTTYETAL.

NLO Activity Relaxation Energetics

369

The purpose of this study was to obtain a more detailed understanding of the NLO stability and thermal degradative processes affecting the stability of the NBPE.


Experimental Sample Preparation. The NBPE polymer was synthesized in a manner similar to that described elsewhere (1). Two batches were prepared. The first batch had a Tg of 207 °C by DSC and was used in the long term aging study at 100 °C. The second batch had a Tg of 219 °C by DSC and was used for the study of NLO relaxations at elevated temperatures. TG/GC/MS Evolved Gas Analysis. The evolved gas analysis is a thermogravimetry - mass spectrometry (TG/MS) experiment performed simultaneously with a thermogravimetry - gas chromatography - mass spectrometry (TG/GC/MS) experiment. A custom built evolved gas analysis instrument was developed by interfacing a thermogravimetric analyzer (Cahn 131 TGA) with a gas chromatographic mass spectrometer system (Fisions, Trio-1) with heated fused silica capillary transfer lines operated at 280 °C. The thermogravimetric analysis was performed by heating ca. 15 mg of sample contained in a platinum pan to approximately 315 °C in a helium atmosphere. A fraction of the evolving gases was sent to the quadruple mass spectrometer for real time detection of the volatiles during the thermogravimetric analysis (e.g. TG/MS). The mass spectrometer was operated in +EI mode with a scanning range from 12 to 800 amu. Simultaneous with the real time evolved gas analysis, a second small fraction of the evolving gases was directed to a cryotrap (at approximately -180 °C) over the entire experiment. After the thermogravimetric program was completed, these trapped volatiles were analyzed using sub-ambient GC/MS. This latter part of the experiment (e.g. TG/GC/MS) is critical in identifying which components evolved and in correctly interpreting the TG/MS results. Using the TG/GC/MS results, unique ions are selected for each component in order to assign their evolution profiles in the TG/MS experiment to the ion profiles. Using this approach, ions m/z=138, 112, and 49 were assigned to nitroaniline, chlorobenzene, and dichloromethane, respectively. Unfortunately, unique ions which would allow discrimination between the nitrobenzene and an unknown aromatic compound were not found. Therefore, these component evolution profiles are combined and assigned to ion m/z = 123. TGA and DSC Analysis. Thermogravimetric (TGA) measurements of weight loss during heating in air were run on a Dupont Instruments Model 951 Thermogravimetric Analyzer. The Differential Scanning Calorimetry measurements were run on a Dupont Instruments Model 9900 Thermal Analyzer. The scan rates were 20 °C/min and the atmosphere was air. Absorption Spectroscopy. Absorption spectra were recorded using a Shimadzu model UV-3101PC Spectrophotometer. For the isothermal aging data, wavelengths were chosen such that the absorbance values were less than 2 absorption units initially.

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

370

POLYMERS FOR SECOND-ORDER NONLINEAR OPTICS


SHG Measurements. Measurement of the second-order nonlinear optical coefficient, d33, was done by second harmonic generation (SHG) and Maker fringe analysis as described in (2). An excitation wavelength of 1064 nm was used. The parameter values of n© of 1.5677, n2© of 1.5929 and di l quartz of 1.2x10"^ esu were assumed. Dynamic Mechanical Data. Dynamic mechanical measurements were made in torsional shear using a Rheometrics RDS 2 mechanical spectrometer. Rectangular bars about 0.85 x 11.7 x 50 mm were cast from solution and measurements were made at a constant frequency of 1.0 rad/s. Temperature was controlled with a nitrogen gas flow. Strains were 0.2% or less, which was well within the linear viscoelastic regime. Dielectric Spectroscopy. Dielectric measurements were made using a TA Instruments 2970 Dielectric Analyzer, attached to a 2100 Analyzer which controls the instrument, collects data and performs data analyses. The instrument was operated in the single surface mode, and was calibrated according to the instrument manufacturers' instructions. Thin films were fabricated on lxl" ceramic single surface interdigitated electrode plates by spin-coating polymer/cyclopentanone solutions (20 %w/v). Measurements were made on unpoled samples from -150 to 230 °C with a temperature scan rate of 2 °C/min over the frequency range from 10" Hz to 10 Hz at 1 volt applied. 1

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Results and Discussion NLO Activity. The NBPE polymer had an onset Tg of 219 °C measured by DSC. The NLO activity or second-order NLO coefficient (d33) was measured for NBPE films parallel-plate poled at 211, 221 and 231 °C with an applied field of 50 V/|im. Poling near the Tg (221 °C) or above the Tg (231 °C) gave the same result, d33 = 1.0 x 10" esu. Poling at 211 °C (below the Tg) gave 3.7 x 10 esu. Corona poled films gave activities of 20 x 10" esu.

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Thermal Stability. Figure 2 shows a TGA scan (20 °C/min) for the NBPE polymer in air. The sample loses 3% of the weight by 310 °C and shows an extrapolated onset of decomposition at 271 °C. A more detailed understanding of the origin of the weight loss was determined by measuring the decomposition fragments for a related polymer BHPF-NBPE (Tg = 265 °C),

byTG/GC/MS. In Figure 3 it is shown that BHPF-NBPE suffers a weight loss of approximately 5.6% upon heating to 315 °C and holding for ten minutes in an anaerobic (He) atmosphere. The weight loss is principally due to the evolution of nitroaniline, nitrobenzene, and an unidentified aromatic component. These major componerits are observed to evolve during the major weight loss step centered at approximately 30


GULOTTY ET AL.


25 20-* ir-*-


0)

15-

o x

10-

y=18.7 e (-0.0284 x)

R=0.68

>

1/e lifetime = 35 years

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2 Time (Years)

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Figure 1. Stability of Corona Poled NBPE in Air at 100°C.

3.748

X

(0.6703

\ ma)

Z97.81"CCX)

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Figure 2. TGA Scan of NBPE in Air.


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minutes and 315 °C (Figure 3). Dichloromethane and chlorobenzene (residual synthesis solvents) also evolved, but are not major contributors to the weight losses as they principally evolve over a region where only minor weight losses are observed. The identification of each of these components are based on the electron impact mass spectra obtained from the TG/GC/MS experiment. Absorption Decay Kinetics and Energetics. We have shown by TG/GC/MS that the decomposition occurs by loss of the nitroaniline group from the chromophore. This loss also results in loss of the charge-transfer absorption band in the visible. For example, Figure 4 shows the change of absorbance before and after aging at 200 °C until no more change occurred in the spectrum. We have used the decrease in absorbance at 470 nm in film samples as a measure of the rate of thermal degradation in isothermal aging studies. Figure 5 shows the isothermal decay plots for samples poled at 221 °C and aged at 150, 175 and 200 °C. The absorption was monitored in an unpoled region of the parallel-plate poled films. The data is well fit to a single exponential rate law with the fit results given above the figure. Figure 6 shows an Arrhenius plot (labeled kdegradation) of the absorbance data for films poled at 221 °C, yielding an activation energy of 26 kcal/mol. The kinetics and activation energies were similar for samples poled at 211 °C and 231 °C. Analysis of the NLO Relaxation Data. Figure 7 shows a typical isothermal decay of NLO activity for a film which was parallel-plate poled with a field of 50 V/|im at 221 °C and aged at 200 °C in air. The data was fit to equation 1, where the degradation rate constant (kdegradation) was determined by the absorbance decay of the same samples during the aging study.

( k

d (t) = d (0) • " ° 33

33

r i e n t a t i o n

e

"

•"

(kde

e

s

radation

"

0

(1)

The Williams-Watts stretched exponential fit (curve a, where 0 is varied) (3) is better than the single exponential fit (curve b, p = 1). The fit parameters for samples poled at 221 °C, aged at 150, 175 and 200 °C and fit using equation 1 in both stretched and single exponential form are listed in Table 1. Note that the 150 °C decay data was single exponential. The samples poled at 231 °C (data not shown) gave similar results. W

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Table 1. Fit parameters for orientational relaxation using Eq. 1. ng < f e

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221

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n

d 3(0) ^orientation (10-9esu) (days ) 3

-1

g

150 175 200

9.11 9.02 12.4 10.0 10.8 9.38

0.0105 0.0126 0.411 0.346 4.78 4.19

kdegradation (days )

r (correlation coefficient)

0.0131 0.0131 0.0831 0.0831 0.328 0.328

0.986 0.985 0.995 0.968 0.998 0.972

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0.836 1 0.423 1 0.501 1

Arrhenius Analysis. The rate constants for orientational decay determined above can be used to estimate the activation energy of orientational relaxation. Here we convert the rate constants measured in the stretched exponential fit (see Table 1) to an average value using equation 2 as in (4).


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60 1

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120 1

Temperature (C) 165 210 252 290 1

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165 210 252 290 Temperature (C)

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315

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Figure 3. TG-GC-MS Data for BHPF-NBPE in He.

Wavelength (nm)

Figure 4. Absorbance of NBPEfilmswith aging at 200°C.


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A

(-0.013143x) R= 0.98402 e (-0.083074x) _R 0.98684 0.99032


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1 0 20 30 40 Time (days)

50 60

Figure 5. Absorbance decay of NBPE at elevated temperatures in Air.

Figure 6. Arrhenius plot of isothermal aging data for NBPE.

Figure 7. NLO Relaxation of NBPE at 200 °C in Air.


27. GULOTTY ET AL.


Nonlinear Optical Activity Relaxation Energetics in ... - ACS Publications

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