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22 November 1996

Chemical Physics Letters 262 (1996) 663-667

CHEMICAL PHYSICS LETTERS

Electrical determination of the degree of cross-linking in a poled non-linear optical polymer

S. Bauer-Gogonea a, W. Wirges a, S. Bauer a,l, R. Gerhard-Multhaupt a,l , J. Liang J. Zyss b

a Heinrich-Hertz-lnstitut, Einsteinufer 37. D-10587 Berlin, Germany b France Telecom CNET, Lab. de Bagneux, 196 A. Henri Ravera. F-92220 Bagneux, France

b

Received 11 July 1996; in final form 26 September 1996

Abstract

A comparative investigation of the thermal dipole-orientation stability in a guest-host and a cross-linked non-linear optical polymer with similar structures of the polymer backbone is reported. Cross-linking yields a rigid polymer network with two types of dipoles: those already cross-linked and therefore thermally stable and those not as yet cross-linked and therefore fast relaxing. The combination of isothermal and thermally stimulated measurements allows for a complete characterization of dipole relaxation in the two sample polymers.

1. Introduct ion

Polymers for second-order non-linear optical (NLO) applications are glassy and contain oriented, highly non-linear optically active dipolar molecules [1]. In guest-host polymers, the dipoles are simply dissolved in the amorphous matrix [2], whereas in side-chain polymers the dipoles are chemically at- tached to the matrix via flexible spacer units [3]. In cross-linkable polymers, the dipolar side-groups may furthermore be linked to the polymer by means of one or more additional covalent bonds simultane-

Present address: Institute of Solid State Physics, University of Potsdam, Am Neuen Palais 10, D-14469 Potsdam, Germany.

ously within a possibly subtle combination of chemi- cal and poling processes [4]. In this last case, it is highly desirable to determine the degree of cross-lin- king, e.g. the relative numbers of already cross-lin- ked and of not yet cross-linked dipoles. While it is to be expected that the former are thermally stable, the latter are bound to relax faster.

For the investigation of dipole relaxation pro- cesses, various powerful techniques have been devel- oped. Under isothermal conditions, any physical ef- fect related to the dipolar order, i.e. piezo- or pyro- electricity [5,6], linear electro-optical response [7,8], or second-harmonic generation [9], is suitable for monitoring the relaxation. As it is rather time con- suming to collect the necessary relaxation data with isothermal methods, non-isothermal techniques are to be preferred for a relatively fast investigation. In

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664 S. Bauer-Gogonea et a l . / Chemical Phys'ics Letters" 262 (1996) 663-667

addition to thermally stimulated depolarization (TSD) [7,9,10], in which the current resulting from a ther- mally induced dipole deorientation is measured, any of the above-mentioned physical effects can be mon- itored during non-isothermal experiments in order to study the relevant processes.

Dipole relaxation processes in polymers are char- acterized by a broad distribution of relaxation times, with a strongly temperature-dependent mean relax- ation time r(T). The isothermal dipole relaxation processes can be described by the stretched exponen- tial or Kohlrausch-Williams-Watts (KWW) func- tion [11], which reveals the deviation from an expo- nential decay. In combination with non-isothermal techniques, such as TSD (which yields the product of the stretching parameter and the activation energy of the dipole relaxation process) a complete characteri- zation of the dipole-relaxation process is possible [121.

Here, we report on a comparative investigation of the thermal stabilities of a guest-host and a cross- linked NLO polymer with similar structures as to their polymer backbone. The behavior of the guest- host polymer is described by a single, broad dipole relaxation process, while the behavior of the cross- linked polymer is different, as it contains two vari- eties of dipoles, those already cross-linked and those not yet cross-linked. It is demonstrated that the degree of cross-linking, i.e. the relative amounts of already and not yet cross-linked dipoles, may be inferred from electrical measurements on poled sam- ples.

2. Isothermal and non-isothermal dipole relax- ation processes

In the following, it is assumed that the isothermal relaxation of the dipole orientation can be described by the KWW function q~(t)= exp(-[t/r]) t3, with the mean relaxation time r(T) and the stretching parameter /3 accounting for the deviation from a single exponential decay. If furthermore the time- temperature superposition principle holds (which ne- cessitates a temperature-independent parameter /3), the polarization P(T) and the current density j(T)

during heating with a constant rate 1/h can be expressed as [12]

(1)

and

j ( T ) = /3P(T) df ]8-'

(2)

respectively, where P0 is the frozen polarization at the starting temperature T O of the TSD experiment, and ~-(T) is the temperature-dependent mean relax- ation time, which in the glassy state usually follows an Arrhenius equation [13]:

(3)

As shown in Ref. [12], fitting of the TSD curves is insensitive to the stretching parameter/3 and one can only determine the product of the activation temperature T A and the KWW stretching parameter

/3. For a complete characterization of the dipole-re-

laxation process it is thus necessary to further mea- sure the isothermal relaxation of the dipole orienta- tion at a suitable high temperature in addition to the TSD response. From the isothermal-relaxation data, the stretching parameter /3 and the mean relaxation time ~-(T) at the chosen temperature are obtained, while the TSD data independently yield the product of the stretching parameter /3 and of the activation temperature T g.

The frozen polarization contains a reversible tem- perature-dependent part which contributes to the ob- served TSD currents [14]. With poled amorphous polymers, thermal expansion of the polymer is the main source of this reversible temperature-dependent polarization [14]. The situation is well described by the experimental pyroelectric coefficient p at a ( ~ + 2 )P /3 , where ~ is the relative thermal expansion coefficient and e~ the unrelaxed dielectric constant, and forms the basis of the pyroelectrical thermal analysis (PTA) technique, which has been reported in detail elsewhere [5,6] and which is used in the

S. Bauer-Gogonea et al . / Chemical Physics Letters 262 (1996) 663-667 665

~ H3

E-i'---CH2-3-

0 0 I

+CH 3

HO .-,.C2H4\ / C 2H 5 N + N II N

N~

~H3 ~ H3

-E- CH,--~ ax r" CH,--~--~y C~O C~---O I I 0 0 I I CH 2 I ~ 2H4

/CH N--C2H 5 + N II N ~ COOH

NO 2

Fig. 1. Sample materials: (top) guest-host polymer, (bottom) cross-linking polymer.

following for the measurement of the isothermal decay of the dipole orientation.

3. Sample materials and preparation

The following experiments were performed on the two NLO polymers depicted in Fig. 1. Fig. 1 (top) shows a guest-host polymer based on PMMA and 12 wt.% of the azo dye 4-[ethyl(2-hydroxy- ethyl)amino]-4-nitrobenzene (usually called Disperse Red 1 or DR1), with a glass transition temperature of 80°C [7]. This polymer was mainly chosen because

of the similarity of its backbone with that of the cross-linkable polymer shown in Fig. 1 (bottom). It must, however, be noted that polyimide-based guest-host materials with much higher glass transi- tion temperatures were reported in the literature [15].

Cross-linking of the NLO dipoles with the poly- mer host is an attractive approach to stabilize NLO polymers. In the polymer of Fig. 1 (bottom), known as Red-Acid Magly, thermal cross-linking is possible between a carboxylic acid function (COOH) located on the non-linear chromophore and an epoxy side group, while a chemical reaction between the chro- mophores is not possible [16].

For the present experiments, approximately 1 /xm thick films were prepared from suitable solutions of the polymers by means of spin coating onto ITO- coated glass substrates. After vacuum evaporation of a top aluminium or gold electrode, the polymer films were poled under an electric field of approximately 50 V/ixm.

4. Experimental results

Fig. 2a shows isothermal decay measurements of the pyroelectric response for the guest-host polymer of Fig. 1 (top), while Fig. 2b shows the correspond- ing TSD scan. Fig. 2a also shows fitting curves based on the KWW function (Eq. (2)), together with the respective fitting parameters (stretching coeffi- cient /3 and temperature-dependent mean relaxation time ~-(T)). From Fig. 2b, an activation energy /3E A = k T A -~ (1.1 + 0.4) eV is obtained.

Fig. 3a shows isothermal decay measurements at 90°C on the cross-linkable polymer shown in Fig. 1 (bottom) after different times of thermal cross-lin- king at 130°C. From Fig. 3a, the enhanced stability after increased cross-linking times is obvious. The experiments indicate the presence of two types of dipoles in the cross-linked polymer: already cross- linked (and thus thermally stable) dipoles and not yet cross-linked (and thus thermally unstable) dipoles. The fitting curves thus consist of superpositions of two relaxation processes according to ~ ( t ) = q~j e x p [ - ( t / T j ) ] t3' + c192 e x p [ - ( t / r 2 ) ] t32, where q~ and ~2 are the respective fractions of not cross- linked and cross-linked dipolar chains. For the fast

666 S. Bauer-Gogonea et a l . / Chemical Physics Letters 262 (1996) 663-667

relaxation process of the not cross-linked dipoles, it is possible to derive the parameters r~ and ~ , while for the slow relaxation process, the time ~'2 is much larger than the time window of our experiment. The stretching parameter and the mean relaxation time for the fast, not cross-linked dipoles are comparable with the relaxation time for the dipoles in the guest-host polymer which has to be expected from their similar structures. Furthermore, the fraction 4 2 is a direct measure for the number of cross-linked dipolar chains. After the fast, not cross-linked dipoles had relaxed, the polymer was cooled down to room temperature and subjected to the TSD scan, whose result is shown in Fig. 3b. From Fig. 3b, a high activation energy fiE A = (3.2 __+ 0.6) eV is obtained,

0 o

o

c

o u) I-

[ 0 r-

1.0.

0.8.

0.6.

0., t 0.2

0

1.o. Ib)

0.8.

0.6-

0.4- ~

0.2- ° ° ° ° ° o o ,oo °

o

0.0 30 6,0 9'0

temperature (°C)

r

(a) ~=o.~ ~ T=60*C : 220min

~ x = 9 0 m i n

T:80*C z : 16min

is 5'0 temperature (°C)

i i

/ c

D/,

~ o o Du

120

Fig. 2. (a) Isothermal relaxation of the pyroelectric response, together with fitting curves and parameters based on the stretched exponential function, and (b) TSD response and fitting curve of the guest-host polymer depicted in Fig. 1 (top).

c Q

'6

E o 8 (J

es

o

1.0.

0.8.

(a) i i i i t

g =0.35 x = 11min

0.6. ~ " ~ - , 1 ~ - ~ 2 ~ - - - ~ - ~ - - ~ - ~ _ ~ _

0.2.0"4 ~ ' ~ ~ ~ 5 6 1 / * cross-linked dipolar chains

30 60 90 120 150 180 temperature (*C)

10 (b) ~I 0.8- °

~ 0 . 6 - O ffl p . ,

~ 0.4-

~ 0.2.

o oo

0 . 0 . . . . . , .

50 80 110 140 temperature (*C)

Fig. 3. (a) Isothermal relaxation at 90°C of the pyroelectric response after different times of cross-linking at 130°C, together with fitting curves and parameters as above, and (b) TSD response and fitting curve of the cross-linked polymer depicted in Fig. 1 (bottom).

which demonstrates the rigidity of the cross-linked dipole polymer network.

5. Conclusions

For a complete characterization of dipole relax- ation processes in NLO polymers, a combination of isothermal and non-isothermal techniques is re- quired. While the behavior of guest-host polymers is described by a single broad relaxation process, the situation is more complex in the case of cross-linked polymers: cross-linking yields a rigid dipole polymer network with a high activation temperature of the

S. Bauer-Gogonea et al. / Chemical Physics Letters 262 (1996) 663-667 667

dipole orientation, but at the same time some not (yet) cross-linked and therefore fast relaxing dipoles could still be observed.

Acknowledgements

We are indebted to Dr. A. Rousseau (E.N.S.C.M. Montpellier, France) for providing the cross-linked Red-acid Magly polymer and to S. Yilmaz, W. Brinker and W.-D. Molzow (all at HHI Berlin, Ger- many) for stimulating discussions.

References

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