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Subglass transition and relaxation of oriented chromophores in polyimides for second order nonlinear optics
Synthetic Metals 115 (2000) 245±250
Subglass transition and relaxation of oriented chromophores in polyimides for second order nonlinear optics JeÂroÃme Chauvina, Keitaro Nakatania, Jacques A. Delairea,*, Sylvain Faureb, ReÂgis Mercierc, Bernard Sillionc a
PPSM, UMR 8531 of CNRS, ENS de Cachan, 61, avenue du Prdt Wilson, 94235 Cachan Cedex, France b SMC, ENS de Cachan, 61, avenue du Prdt Wilson, 94235 Cachan Cedex, France c LMOPS, UPR 903 of CNRS, 69390 Vernaison, France
Abstract We synthesized and characterized side-chain polyimides bearing DR1 or NPP groups (PI-DR1 and PI-NPP). These copolymers are soluble in organic solvents and they have a very high glass transition temperature (Tga 250 C). By dielectric loss measurements at different frequencies we put into evidence a subglass transition temperature Tgb around 1208C. We also measured relaxation kinetics of polar order of nonlinear optical (NLO) chromophores by second harmonic generation (SHG) at different temperatures over a wide range p extending from 80 to 1808C. All the decay curves of SHG could be ®tted by a KWW-stretched exponential I 2$ t p I2$ t 0 exp ÿt=tb . An Arrhenius plot of t clearly shows that there is a transition temperature nearby Tgb with two different activation energies, i.e. 140 kJ molÿ1 above 1208C and 28 kJ molÿ1 below 1208C for PI-DR1. An extrapolation of the low-temperature regime to room temperature allows to predict a relaxation time t 1:25 years. Tgb value also coincides with the temperature at which an oriented PI-DR1 ®lm starts to disorient when heated gradually from room temperature. A physical aging procedure is described which allows to increase the thermal stability. Finally, it is shown that photoassisted poling made at temperatures higher than Tgb leads to almost the same stability as thermally assisted poling with lower degradation. # 2000 Elsevier Science S.A. All rights reserved. Keywords: Polyimides; Second order nonlinear optics; Relaxation; Physical aging; Photopoling
1. Introduction One approach for improving the stability of second order nonlinear optical (NLO) polymers is to incorporate NLO chromophores in high Tg matrices like polyimides [1,2]. The orientation of the chromophores is preserved due to the decreased mobility of the host polymer chain. Several authors have developed chromophore-functionalized polyimides [3,4] and we have already described the synthesis and physical characterization of soluble copolymers of NLO chromophores and polyimides [5]. Such copolyimides which have high glass transition temperature (Tga ) give long orientational relaxation times, but the relaxation processes far below Tga are still badly known. By studying the temperature behavior of both dielectric losses and second harmonic generation (SHG) relaxation, we have put into evidence the role played by a secondary glass transition (b transition) in the orientational
relaxation process. Furthermore, this study aims to extrapolate relaxation times measured at elevated temperatures towards room temperature. At last, the in¯uence of different factors (physical aging, method of poling) on the relaxation rates of the SHG signals has been analyzed. 2. Experimental 2.1. Structure and physical characterization of the NLO copolymers The synthesis of two NLO copolyimides (PI-DR1 and PINPP) has been described elsewhere [5]. Their structure is given in Fig. 1. The physico-chemical parameters of these polymers are given in Table 1. 2.2. Preparation and poling of the PI-DR1 and PI-NPP films
*
Corresponding author. Tel.: 33-1-47-40-53-38; fax: 33-1-47-40-24-54. E-mail address: [email protected] (J.A. Delaire).
The copolymers were dissolved in a mixture of tetrahydrofuran (THF)/ N-methyl 2-pyrrolidone (NMP) in a 4:1 v/v
0379-6779/00/$ ± see front matter # 2000 Elsevier Science S.A. All rights reserved. PII: S 0 3 7 9 - 6 7 7 9 ( 0 0 ) 0 0 3 2 0 - 9
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J. Chauvin et al. / Synthetic Metals 115 (2000) 245±250
Fig. 1. Structure of the copolyimides PI-DR1 and PI-NPP.
ratio with a polymer concentration of 6% w/w. Films of polymers with thickness ranging from 0.6 to 0.75 mm were spin cast on glass slides covered by ITO from these solutions after ®ltration through a Millipore ®lter (pore size 0.5 mm). They were ®rst dried at 1008C for 30 min. Physico-chemical characterization of the ®lms after this step shows that they are swollen with residual NMP which plays the role of a plasticizer during poling. Two poling methods have been used: thermo-assisted poling and photo-assisted poling. For thermo-assisted poling the above prepared samples were heated at 2108C under nitrogen atmosphere. Then, an electric ®eld was applied by a corona discharge (electrode at a voltage of 6 kV and a distance of 1 cm from the surface of the ®lm) which was maintained during 10 min at 2108C and then during all the cooling process towards room temperature (rate of cooling: 0.58C minÿ1). For photo-assisted poling the sample was prealably heated under vacuum at 1208C
during 5 h, then placed under the same corona discharge as discussed above and simultaneously irradiated with the circularly polarized light of an Ar ion laser (514 nm, 500 mW cmÿ2) at normal incidence. The sample was placed in a thermoregulated housing allowing on the same time poling at variable temperatures between 20 and 1508C and in situ measurements of SHG. The same housing was used (without the corona discharge and the Ar ion laser beam) for temperature studies of the relaxation processes. 2.3. Dielectric spectroscopy Films for dielectric spectroscopy were prepared as above. The ®lms were covered by a vacuum deposited copper layer (Emitech K950 metallization set up). The thickness of this counter electrode was measured with a quartz balance (Emitech K150) and ®xed at 10 1 nm in order to avoid
Table 1 Physico-chemical characterization of the studied copolyimides Polymer
Chromophoreweight (%)a
Mnb
Z (dl gÿ1)c
Tga (8C)d
Td (8C)e
PI PI-DR1 PI-NPP
0 13.9 12.6
± 33572 ±
0.6 ± 0.6
270 250 255
290 270 270
a
Determined by UV±VIS spectroscopy. Number averaged molecular weights determined by gel permeation chromatography. c Viscosity in NMP at 258C. d Glass transition temperature determined by thermomechanic analysis at 58C minÿ1. e Decomposition temperature determined by thermogravimetric analysis under nitrogen at 58C minÿ1. b
J. Chauvin et al. / Synthetic Metals 115 (2000) 245±250
strong absorption. The ®lm was then ®xed on a heating stage regulated between room temperature and 2508C. An impedance analyser (Hewlett-Packard HP 4194 A) was used to measure the resistance and capacitance of the ®lms between 10 kHz and 5 MHz. The heating rate was around 28C minÿ1. 2.4. SHG measurements The experimental set-up has already been described [6]. A nanosecond Nd:YAG laser (1064 nm, 100 mJ, 10 ns fullwidth at half-maximum (FWHM)) was used as the fundamental beam. This wavelength could be Raman shifted to 1907 nm after passing the collimated beam through a high pressure hydrogen cell. The thermoregulated housing described above allowed in situ SHG measurements during photo-poling at different temperatures or different temperature cycles after thermo-poling or photo-poling (see below). In all cases the SHG signal was recorded and averaged over 10 laser shots for every set of parameters on a fast numerical oscilloscope (Tektronix TDS 620 B) connected with a PC computer. 3. Results and discussion 3.1. Secondary (b) glass transition Fig. 2 presents the temperature variation of the dielectric loss tangent (tan de) for two samples of PI-DR1, one which has been prepared as described for thermo-assisted poling (see above) and the second one prepared as for photoassisted poling (with a supplementary heating step under vacuum). These measurements put into evidence, in addition to the main (a) glass transition, a secondary (b) glass transition which occurs around Tgb 125 C for dry ®lms and 908C for ®lms containing residual NMP. This difference which also occurs for the main transition may be explained by the
247
plasticizing role of residual solvent. An Arrhenius plot of the frequency versus the secondary glass transition temperature Tgb in the dry ®lms gives the activation energy of the relaxation process in¯uenced by the b transition. This activation energy is found equal to 66 kJ molÿ1 for an isotropic dry ®lm and 36 kJ molÿ1 for a poled ®lm. This difference between both ®lms have also been observed by Teraoka et al. [7] for an epoxy polymer. It shows that chromophore dipoles are involved in the b transition and that dipolar interactions which are stronger in the oriented sample, tend to decrease the activation energy of the process. As concern the origin of the b transition one may think of breaking of hydrogen bonds between residual OH groups distributed along the polyimide backbone [8]. 3.2. Relaxation of poled polyimide films measured by second harmonic generation In a ®rst series of experiments PI-DR1 ®lms were previously poled by the thermo-assisted method (see above), then heated quickly (heating rate: 208C minÿ1) to a ®xed temperature above 1208C and then the isothermal decay of the SHG signal was recorded during 10 h. The results are shown in Fig. 3 for a temperature domain ranging from 120 to 1808C which is below Tga. The experimental decays of the second order nonlinear susceptibility deff proportional to the square root of the second harmonic intensity are ®tted with a stretched exponential according to the KWW function [9±11] p p (1) I2$ t I2$ t 0 exp ÿt=tb with 0 < b < 1 and t is an effective relaxation time. The fitting of the experimental curves with Eq. (1) was better than with a biexponential. b values were found in the range of 0.45±0.9 with a tendancy to increase as temperature increases: this evolution probably reflects a narrowing of the size distribution of free volume sites surrounding the chromophores. Isothermal decays in the temperature range
Fig. 2. Dielectric loss tangent as a function of temperature (frequency 100 kHz) for PI-DR1: (a) film with residual NMP (see text); (b) dry film.
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Fig. 3. Isothermal decays of the second order susceptibility of poled films of PI-DR1 fitted with Eq. (1) (see text).
80±1108C were also performed in a different way: depending on the decay duration the poled films were placed in a thermoregulated drying oven for several weeks and months, and the SHG signals were periodically measured and compared with the SHG reference signal of a Y-cut quartz cristal. These decays still obeyed Eq. (1). The Arrhenius plot of the relaxation times t is shown in Fig. 4 for all the temperature range 80±1808C. It shows two linear parts which intersect near 1158C, this temperature almost coincides with Tgb . Moreover, the activation energy is lower below Tgb (28 kJ molÿ1) than above (141 kJ molÿ1) and it is nearly the activation energy found for b relaxation by dielectric measurements (36 kJ molÿ1). Both techniques measure dipole relaxation but dielectric measurements are sensitive to all dipoles, conversely SHG relaxation signals are only sensitive to orientation of NLO chromophores. However, from the proximity of both activation energies we conclude that the relaxation phenomena responsible for b transition are also at the origin of relaxation of polar order at temperatures below Tgb. The extrapolation of the above Arrhenius plot to room temperature allows to determine a relaxation time of t 10727 h (1.25 years). Extrapolation from measurements near Tga would have given more than 5 104 years!
Fig. 4. Arrhenius plot of the relaxation times of SHG signals in PI-DR1.
The SHG signal of a poled PI-DR1 sample measured during heating of the ®lm at a constant rate of 108C minÿ1 from room temperature to 2008C is shown in Fig. 5. Interestingly it shows a threshold for the decay near 1208C which corresponds to Tgb . So, chromophore relaxation seems to be governed by the same molecular librations or rotations of chain fragments as those who explain b transition. 3.3. Relaxation of PI-NPP copolymer The SHG signal of a poled PI-NPP ®lm was also measured during heating of the sample (cf. Fig. 5 for PIDR1) and again the threshold for relaxation was found near 1208C. As concerns isothermal decays above Tgb they were also ®tted by a stretched exponential and relaxation times t at temperatures above Tgb also obey an Arrhenius relationship with an activation energy of 186 kJ molÿ1, which is near the value obtained for PI-DR1 in the same temperature range.
Fig. 5. Decay of SHG signal of a PI-DR1 film (fundamental at 1064 nm) during heating of the film (heating rate: 108C minÿ1).
J. Chauvin et al. / Synthetic Metals 115 (2000) 245±250
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3.4. Influence of physical aging of the poled film on the relaxation of polar order Physical aging of a polymer [12,13] corresponds to a rearrangement of the polymer chains which tend to increase their interactions in order to minimize the total energy. This is accompanied by an increase of density (decrease of free volume) of the polymer. Temperature can accelerate this spontaneous process. In the case of NLO polymers physical aging is generally performed during the poling process. It consists in keeping the ®lm under electric voltage during the cooling process at an intermediate temperature which must be between room temperature and Tga . Although, this `aging temperature' is still badly known Torkelson et al. [14] realized physical aging at Tga ÿ 15 C (during several tenths of hours). For PI-DR1 ®lms we have compared the relaxation of three ®lms poled according to three different procedures (see Fig. 6). The relaxation curves are similar for ®lms 1 and 2 which show that physical aging after poling is inef®cient. On the contrary, there is a substantial increase in stability when the isothermal step is applied during the cooling process under an applied electric ®eld. Two effects contribute to slow down relaxation: ®rst the material becomes more dense during the plateau at 1308C and secondly the global cooling time is increased. This slow cooling allows the ®lm to reach a structure which is near the thermal equilibrium and minimizes energy and free volume [15]. The rigidity of the matrix is then increased and the orientational stability of chromophores is improved.
Fig. 7. Decay of SHG signals of a PI-DR1 films (fundamental at 1064 nm) during heating of the films (heating rate: 108C minÿ1). Film 1±4 are photopoled at room temperature 90, 120 and 1508C, respectively and film 5 is poled thermally at 2108C as described in the experimental part.
3.5. Relaxation after photo-assisted and thermo-assisted poling We and others have shown that photo-poling at room temperature of azo copolymers could be very ef®cient [16,17]. In the case of polyimides [18] it can be a method to prevent or decrease degradation of chromophores which occurs during thermally assisted poling at temperatures above 2008C (DR1 starts to decompose at 1908C). Fig. 7 compares relaxation curves of SHG signals of ®ve samples poled in different conditions: thermo-assisted poling at 2108C (®lm 5) and photo-assisted poling at different temperatures below Tga (®lms 1±4). Indeed we have shown that absolute values of deff increase with the temperature of photopoling [18]. According to Fig. 7 we want to point out another effect of the poling temperature: if the temperature is higher than Tgb then the stability is increased and is comparable with that of a thermally poled ®lm. This effect also shows the role played by the b transition on the free volume induced by the photochemical trans±cis reaction at the origin of photo-assisted poling. 4. Conclusion
Fig. 6. Decay of SHG signals of three PI-DR1 films (fundamental at 1064 nm) during heating of the films (heating rate: 108C minÿ1). The films are poled under three different protocoles: Film 1, thermo-assisted poling as described in the experimental part; Film 2, once cooled at room temperature (1 day after), the film is heated again at T 1308C and kept at this temperature during 2 h then cooled again; Film 3, during the cooling process (with the external electric field on), the temperature is kept at a plateau value of 1308C during 2 h then cooled at room temperature.
From our studies we have compared isothermal relaxation rates at different temperatures and Arrhenius plot shows two different relaxation regimes, one below and another above Tgb . It has been possible to extrapolate the relaxation time of poled ®lms towards room temperature and a value of 1.25 year has been determined for PI-DR1. A secondary (b) glass transition occurring around 1208C has been put into evidence in PI-DR1. This transition could be due to the breaking of hydrogen bonds between hydroxyle groups
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distributed along the polymer backbone. It plays an important role both in the orientational relaxation and in the poling procedure. Thermal analysis of orientational stability shows a signi®cant disorientation above Tgb . Orientational stability of samples photo-oriented below Tgb is lower than for thermally poled ones near Tga, whereas samples photooriented above Tgb exhibit the same stability as thermally poled samples near Tgb. Physical aging of the polymer ®lm allows to increase signi®cantly the orientational stability. References [1] D.M. Burland, R.D. Miller, C.A. Walsh, Chem. Rev. 94 (1994) 31. [2] D. Yu, A. Gharani, L. Yu, J. Am. Chem. Soc. 117 (1995) 11680. [3] T. Verbiest, D.M. Burland, M.C. Jurich, V.Y. Lee, R.D. Miller, W. Volksen, Macromolecules 28 (1995) 3005. [4] T.A. Chen, A.K.-Y. Jen, Y. Cai, J. Am. Chem. Soc. 117 (1995) 7295. [5] C. Marestin, R. Mercier, B. Sillion, J. Chauvin, K. Nakatani, J.A. Delaire, Synth. Metals 81 (1996) 143. [6] R. Loucif-SaõÈbi, K. Nakatani, J.A. Delaire, M. Dumont, Z. Sekkat, Chem. Mater. 5 (1993) 229.
[7] I. Teraoka, D. Jungbauer, B. Reck, D.Y. Yoon, R. Twieg, C.G. Willson, J. Appl. Phys. 69 (1991) 2568. [8] P. Cortes, S. Montserrat, J. Polym. Sci. Part B: Polym. Phys. 36 (1998) 113. [9] A. Dhinojwala, G.K. Wong, J.M. Torkelson, J. Chem. Phys. 100 (1994) 6046. [10] P. Kaatz, P. PreÃtre, U. Meier, U. Stalder, C. Bosshard, P. Gunter, B. Zysset, M. StaÈhelin, M. Alheim, F. Lehr, Macromolecules 29 (1996) 1666. [11] H.-J. Winkelhahn, D. Neher, Th.K. Servay, H. Rengel, M. Pfaadt, C. BoÈffel, M. Schulze, Poled polymers and their applications to SHG and EO devices, in: S. Miyata, H. Sasabe (Eds.), Advances in Nonlinear Optics, Vol. 4, Gordon and Breach, London, 1997, p. 179. [12] E. Andrejew, J. Baschnagel, Phys. A 233 (1996) 117. [13] C.Y. Stacey, H.S. Lackritz, Macromolecules 29 (1996) 3470. [14] A. Dhinojwala, J.C. Hooker, J.M. Torkelson, J. Non-Cryst. Solids 172 (1994) 286. [15] J.M.G. Cowie, S. Harris, I.J. McEwen, Macromolecules 31 (1998) 2611. [16] Z. Sekkat, M. Dumont, Appl. Phys. B 54 (1992) 486. [17] J.A. Delaire, Y. Atassi, R. Loucif-SaõÈbi, K. Nakatani, Nonlinear Optics 9 (1995) 317. [18] J. Chauvin, K. Nakatani, J.A. Delaire, C. Marestin, R. Mercier, B. Sillion, SPIE 2998 (1997) 205.
Subglass transition and relaxation of oriented chromophores in polyimides for second order nonlinear optics JeÂroÃme Chauvina, Keitaro Nakatania, Jacques A. Delairea,*, Sylvain Faureb, ReÂgis Mercierc, Bernard Sillionc a
PPSM, UMR 8531 of CNRS, ENS de Cachan, 61, avenue du Prdt Wilson, 94235 Cachan Cedex, France b SMC, ENS de Cachan, 61, avenue du Prdt Wilson, 94235 Cachan Cedex, France c LMOPS, UPR 903 of CNRS, 69390 Vernaison, France
Abstract We synthesized and characterized side-chain polyimides bearing DR1 or NPP groups (PI-DR1 and PI-NPP). These copolymers are soluble in organic solvents and they have a very high glass transition temperature (Tga 250 C). By dielectric loss measurements at different frequencies we put into evidence a subglass transition temperature Tgb around 1208C. We also measured relaxation kinetics of polar order of nonlinear optical (NLO) chromophores by second harmonic generation (SHG) at different temperatures over a wide range p extending from 80 to 1808C. All the decay curves of SHG could be ®tted by a KWW-stretched exponential I 2$ t p I2$ t 0 exp ÿt=tb . An Arrhenius plot of t clearly shows that there is a transition temperature nearby Tgb with two different activation energies, i.e. 140 kJ molÿ1 above 1208C and 28 kJ molÿ1 below 1208C for PI-DR1. An extrapolation of the low-temperature regime to room temperature allows to predict a relaxation time t 1:25 years. Tgb value also coincides with the temperature at which an oriented PI-DR1 ®lm starts to disorient when heated gradually from room temperature. A physical aging procedure is described which allows to increase the thermal stability. Finally, it is shown that photoassisted poling made at temperatures higher than Tgb leads to almost the same stability as thermally assisted poling with lower degradation. # 2000 Elsevier Science S.A. All rights reserved. Keywords: Polyimides; Second order nonlinear optics; Relaxation; Physical aging; Photopoling
1. Introduction One approach for improving the stability of second order nonlinear optical (NLO) polymers is to incorporate NLO chromophores in high Tg matrices like polyimides [1,2]. The orientation of the chromophores is preserved due to the decreased mobility of the host polymer chain. Several authors have developed chromophore-functionalized polyimides [3,4] and we have already described the synthesis and physical characterization of soluble copolymers of NLO chromophores and polyimides [5]. Such copolyimides which have high glass transition temperature (Tga ) give long orientational relaxation times, but the relaxation processes far below Tga are still badly known. By studying the temperature behavior of both dielectric losses and second harmonic generation (SHG) relaxation, we have put into evidence the role played by a secondary glass transition (b transition) in the orientational
relaxation process. Furthermore, this study aims to extrapolate relaxation times measured at elevated temperatures towards room temperature. At last, the in¯uence of different factors (physical aging, method of poling) on the relaxation rates of the SHG signals has been analyzed. 2. Experimental 2.1. Structure and physical characterization of the NLO copolymers The synthesis of two NLO copolyimides (PI-DR1 and PINPP) has been described elsewhere [5]. Their structure is given in Fig. 1. The physico-chemical parameters of these polymers are given in Table 1. 2.2. Preparation and poling of the PI-DR1 and PI-NPP films
*
Corresponding author. Tel.: 33-1-47-40-53-38; fax: 33-1-47-40-24-54. E-mail address: [email protected] (J.A. Delaire).
The copolymers were dissolved in a mixture of tetrahydrofuran (THF)/ N-methyl 2-pyrrolidone (NMP) in a 4:1 v/v
0379-6779/00/$ ± see front matter # 2000 Elsevier Science S.A. All rights reserved. PII: S 0 3 7 9 - 6 7 7 9 ( 0 0 ) 0 0 3 2 0 - 9
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J. Chauvin et al. / Synthetic Metals 115 (2000) 245±250
Fig. 1. Structure of the copolyimides PI-DR1 and PI-NPP.
ratio with a polymer concentration of 6% w/w. Films of polymers with thickness ranging from 0.6 to 0.75 mm were spin cast on glass slides covered by ITO from these solutions after ®ltration through a Millipore ®lter (pore size 0.5 mm). They were ®rst dried at 1008C for 30 min. Physico-chemical characterization of the ®lms after this step shows that they are swollen with residual NMP which plays the role of a plasticizer during poling. Two poling methods have been used: thermo-assisted poling and photo-assisted poling. For thermo-assisted poling the above prepared samples were heated at 2108C under nitrogen atmosphere. Then, an electric ®eld was applied by a corona discharge (electrode at a voltage of 6 kV and a distance of 1 cm from the surface of the ®lm) which was maintained during 10 min at 2108C and then during all the cooling process towards room temperature (rate of cooling: 0.58C minÿ1). For photo-assisted poling the sample was prealably heated under vacuum at 1208C
during 5 h, then placed under the same corona discharge as discussed above and simultaneously irradiated with the circularly polarized light of an Ar ion laser (514 nm, 500 mW cmÿ2) at normal incidence. The sample was placed in a thermoregulated housing allowing on the same time poling at variable temperatures between 20 and 1508C and in situ measurements of SHG. The same housing was used (without the corona discharge and the Ar ion laser beam) for temperature studies of the relaxation processes. 2.3. Dielectric spectroscopy Films for dielectric spectroscopy were prepared as above. The ®lms were covered by a vacuum deposited copper layer (Emitech K950 metallization set up). The thickness of this counter electrode was measured with a quartz balance (Emitech K150) and ®xed at 10 1 nm in order to avoid
Table 1 Physico-chemical characterization of the studied copolyimides Polymer
Chromophoreweight (%)a
Mnb
Z (dl gÿ1)c
Tga (8C)d
Td (8C)e
PI PI-DR1 PI-NPP
0 13.9 12.6
± 33572 ±
0.6 ± 0.6
270 250 255
290 270 270
a
Determined by UV±VIS spectroscopy. Number averaged molecular weights determined by gel permeation chromatography. c Viscosity in NMP at 258C. d Glass transition temperature determined by thermomechanic analysis at 58C minÿ1. e Decomposition temperature determined by thermogravimetric analysis under nitrogen at 58C minÿ1. b
J. Chauvin et al. / Synthetic Metals 115 (2000) 245±250
strong absorption. The ®lm was then ®xed on a heating stage regulated between room temperature and 2508C. An impedance analyser (Hewlett-Packard HP 4194 A) was used to measure the resistance and capacitance of the ®lms between 10 kHz and 5 MHz. The heating rate was around 28C minÿ1. 2.4. SHG measurements The experimental set-up has already been described [6]. A nanosecond Nd:YAG laser (1064 nm, 100 mJ, 10 ns fullwidth at half-maximum (FWHM)) was used as the fundamental beam. This wavelength could be Raman shifted to 1907 nm after passing the collimated beam through a high pressure hydrogen cell. The thermoregulated housing described above allowed in situ SHG measurements during photo-poling at different temperatures or different temperature cycles after thermo-poling or photo-poling (see below). In all cases the SHG signal was recorded and averaged over 10 laser shots for every set of parameters on a fast numerical oscilloscope (Tektronix TDS 620 B) connected with a PC computer. 3. Results and discussion 3.1. Secondary (b) glass transition Fig. 2 presents the temperature variation of the dielectric loss tangent (tan de) for two samples of PI-DR1, one which has been prepared as described for thermo-assisted poling (see above) and the second one prepared as for photoassisted poling (with a supplementary heating step under vacuum). These measurements put into evidence, in addition to the main (a) glass transition, a secondary (b) glass transition which occurs around Tgb 125 C for dry ®lms and 908C for ®lms containing residual NMP. This difference which also occurs for the main transition may be explained by the
247
plasticizing role of residual solvent. An Arrhenius plot of the frequency versus the secondary glass transition temperature Tgb in the dry ®lms gives the activation energy of the relaxation process in¯uenced by the b transition. This activation energy is found equal to 66 kJ molÿ1 for an isotropic dry ®lm and 36 kJ molÿ1 for a poled ®lm. This difference between both ®lms have also been observed by Teraoka et al. [7] for an epoxy polymer. It shows that chromophore dipoles are involved in the b transition and that dipolar interactions which are stronger in the oriented sample, tend to decrease the activation energy of the process. As concern the origin of the b transition one may think of breaking of hydrogen bonds between residual OH groups distributed along the polyimide backbone [8]. 3.2. Relaxation of poled polyimide films measured by second harmonic generation In a ®rst series of experiments PI-DR1 ®lms were previously poled by the thermo-assisted method (see above), then heated quickly (heating rate: 208C minÿ1) to a ®xed temperature above 1208C and then the isothermal decay of the SHG signal was recorded during 10 h. The results are shown in Fig. 3 for a temperature domain ranging from 120 to 1808C which is below Tga. The experimental decays of the second order nonlinear susceptibility deff proportional to the square root of the second harmonic intensity are ®tted with a stretched exponential according to the KWW function [9±11] p p (1) I2$ t I2$ t 0 exp ÿt=tb with 0 < b < 1 and t is an effective relaxation time. The fitting of the experimental curves with Eq. (1) was better than with a biexponential. b values were found in the range of 0.45±0.9 with a tendancy to increase as temperature increases: this evolution probably reflects a narrowing of the size distribution of free volume sites surrounding the chromophores. Isothermal decays in the temperature range
Fig. 2. Dielectric loss tangent as a function of temperature (frequency 100 kHz) for PI-DR1: (a) film with residual NMP (see text); (b) dry film.
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J. Chauvin et al. / Synthetic Metals 115 (2000) 245±250
Fig. 3. Isothermal decays of the second order susceptibility of poled films of PI-DR1 fitted with Eq. (1) (see text).
80±1108C were also performed in a different way: depending on the decay duration the poled films were placed in a thermoregulated drying oven for several weeks and months, and the SHG signals were periodically measured and compared with the SHG reference signal of a Y-cut quartz cristal. These decays still obeyed Eq. (1). The Arrhenius plot of the relaxation times t is shown in Fig. 4 for all the temperature range 80±1808C. It shows two linear parts which intersect near 1158C, this temperature almost coincides with Tgb . Moreover, the activation energy is lower below Tgb (28 kJ molÿ1) than above (141 kJ molÿ1) and it is nearly the activation energy found for b relaxation by dielectric measurements (36 kJ molÿ1). Both techniques measure dipole relaxation but dielectric measurements are sensitive to all dipoles, conversely SHG relaxation signals are only sensitive to orientation of NLO chromophores. However, from the proximity of both activation energies we conclude that the relaxation phenomena responsible for b transition are also at the origin of relaxation of polar order at temperatures below Tgb. The extrapolation of the above Arrhenius plot to room temperature allows to determine a relaxation time of t 10727 h (1.25 years). Extrapolation from measurements near Tga would have given more than 5 104 years!
Fig. 4. Arrhenius plot of the relaxation times of SHG signals in PI-DR1.
The SHG signal of a poled PI-DR1 sample measured during heating of the ®lm at a constant rate of 108C minÿ1 from room temperature to 2008C is shown in Fig. 5. Interestingly it shows a threshold for the decay near 1208C which corresponds to Tgb . So, chromophore relaxation seems to be governed by the same molecular librations or rotations of chain fragments as those who explain b transition. 3.3. Relaxation of PI-NPP copolymer The SHG signal of a poled PI-NPP ®lm was also measured during heating of the sample (cf. Fig. 5 for PIDR1) and again the threshold for relaxation was found near 1208C. As concerns isothermal decays above Tgb they were also ®tted by a stretched exponential and relaxation times t at temperatures above Tgb also obey an Arrhenius relationship with an activation energy of 186 kJ molÿ1, which is near the value obtained for PI-DR1 in the same temperature range.
Fig. 5. Decay of SHG signal of a PI-DR1 film (fundamental at 1064 nm) during heating of the film (heating rate: 108C minÿ1).
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3.4. Influence of physical aging of the poled film on the relaxation of polar order Physical aging of a polymer [12,13] corresponds to a rearrangement of the polymer chains which tend to increase their interactions in order to minimize the total energy. This is accompanied by an increase of density (decrease of free volume) of the polymer. Temperature can accelerate this spontaneous process. In the case of NLO polymers physical aging is generally performed during the poling process. It consists in keeping the ®lm under electric voltage during the cooling process at an intermediate temperature which must be between room temperature and Tga . Although, this `aging temperature' is still badly known Torkelson et al. [14] realized physical aging at Tga ÿ 15 C (during several tenths of hours). For PI-DR1 ®lms we have compared the relaxation of three ®lms poled according to three different procedures (see Fig. 6). The relaxation curves are similar for ®lms 1 and 2 which show that physical aging after poling is inef®cient. On the contrary, there is a substantial increase in stability when the isothermal step is applied during the cooling process under an applied electric ®eld. Two effects contribute to slow down relaxation: ®rst the material becomes more dense during the plateau at 1308C and secondly the global cooling time is increased. This slow cooling allows the ®lm to reach a structure which is near the thermal equilibrium and minimizes energy and free volume [15]. The rigidity of the matrix is then increased and the orientational stability of chromophores is improved.
Fig. 7. Decay of SHG signals of a PI-DR1 films (fundamental at 1064 nm) during heating of the films (heating rate: 108C minÿ1). Film 1±4 are photopoled at room temperature 90, 120 and 1508C, respectively and film 5 is poled thermally at 2108C as described in the experimental part.
3.5. Relaxation after photo-assisted and thermo-assisted poling We and others have shown that photo-poling at room temperature of azo copolymers could be very ef®cient [16,17]. In the case of polyimides [18] it can be a method to prevent or decrease degradation of chromophores which occurs during thermally assisted poling at temperatures above 2008C (DR1 starts to decompose at 1908C). Fig. 7 compares relaxation curves of SHG signals of ®ve samples poled in different conditions: thermo-assisted poling at 2108C (®lm 5) and photo-assisted poling at different temperatures below Tga (®lms 1±4). Indeed we have shown that absolute values of deff increase with the temperature of photopoling [18]. According to Fig. 7 we want to point out another effect of the poling temperature: if the temperature is higher than Tgb then the stability is increased and is comparable with that of a thermally poled ®lm. This effect also shows the role played by the b transition on the free volume induced by the photochemical trans±cis reaction at the origin of photo-assisted poling. 4. Conclusion
Fig. 6. Decay of SHG signals of three PI-DR1 films (fundamental at 1064 nm) during heating of the films (heating rate: 108C minÿ1). The films are poled under three different protocoles: Film 1, thermo-assisted poling as described in the experimental part; Film 2, once cooled at room temperature (1 day after), the film is heated again at T 1308C and kept at this temperature during 2 h then cooled again; Film 3, during the cooling process (with the external electric field on), the temperature is kept at a plateau value of 1308C during 2 h then cooled at room temperature.
From our studies we have compared isothermal relaxation rates at different temperatures and Arrhenius plot shows two different relaxation regimes, one below and another above Tgb . It has been possible to extrapolate the relaxation time of poled ®lms towards room temperature and a value of 1.25 year has been determined for PI-DR1. A secondary (b) glass transition occurring around 1208C has been put into evidence in PI-DR1. This transition could be due to the breaking of hydrogen bonds between hydroxyle groups
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distributed along the polymer backbone. It plays an important role both in the orientational relaxation and in the poling procedure. Thermal analysis of orientational stability shows a signi®cant disorientation above Tgb . Orientational stability of samples photo-oriented below Tgb is lower than for thermally poled ones near Tga, whereas samples photooriented above Tgb exhibit the same stability as thermally poled samples near Tgb. Physical aging of the polymer ®lm allows to increase signi®cantly the orientational stability. References [1] D.M. Burland, R.D. Miller, C.A. Walsh, Chem. Rev. 94 (1994) 31. [2] D. Yu, A. Gharani, L. Yu, J. Am. Chem. Soc. 117 (1995) 11680. [3] T. Verbiest, D.M. Burland, M.C. Jurich, V.Y. Lee, R.D. Miller, W. Volksen, Macromolecules 28 (1995) 3005. [4] T.A. Chen, A.K.-Y. Jen, Y. Cai, J. Am. Chem. Soc. 117 (1995) 7295. [5] C. Marestin, R. Mercier, B. Sillion, J. Chauvin, K. Nakatani, J.A. Delaire, Synth. Metals 81 (1996) 143. [6] R. Loucif-SaõÈbi, K. Nakatani, J.A. Delaire, M. Dumont, Z. Sekkat, Chem. Mater. 5 (1993) 229.
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