- Email: [email protected]
Influence of UV irradiation on the blue and red light photoinduced processes in azobenzene polyesters
Polymer 45 (2004) 6003–6012 www.elsevier.com/locate/polymer
Influence of UV irradiation on the blue and red light photoinduced processes in azobenzene polyesters F.J. Rodrı´gueza, C. Sa´ncheza, B. Villacampaa, R. Alcala´a,*, R. Casesa, M.V. Colladosb, S. Hvilstedc, M. Stranged a
Departamento de Fı´sica de la Materia Condensada, Instituto de Ciencia de Materiales de Arago´n, Universidad de Zaragoza, 50009 Zaragoza, Spain b Departamento de Fı´sica Aplicada, Universidad de Zaragoza, 50009 Zaragoza, Spain c Danish Polymer Centre, Department of Chemical Engineering, Technical University of Denmark, DK-2800 Kgs Lyngby, Denmark d Risø National Laboratory, DK-4000 Roskilde, Denmark Received 7 January 2004; received in revised form 2 June 2004; accepted 15 June 2004
Abstract Birefringence induced in a series of liquid crystalline side-chain azobenzene polyesters with different substituent groups was investigated under irradiation with 488 and 633 nm linearly polarized lights. Two different initial conditions have been used: the effect of a previous irradiation with UV light that yields the films into the isotropic state at room temperature (RT) was compared with the quenching from temperatures above the isotropic transition temperature Ti. UV–visible spectra of the thermally quenched films show the presence of aggregates when measured at RT. We have found that UV light irradiation creates a high concentration of cis isomers and breaks the aggregates, but they are formed again after a few days in dark at RT. Orientation of the chromophores perpendicular to the polarization of the 488 nm light and parallel to the polarization of the 633 nm light was confirmed by dichroism measurements. q 2004 Elsevier Ltd. All rights reserved. Keywords: Azobenzene; Polyesters; Photoinduced optical anisotropy
1. Introduction Photoinduced anisotropy in polymer films containing azobenzene units has been extensively studied [1–16]. Anisotropy is usually induced by illumination with linearly polarised blue or green light (488 or 514 nm) from an ArC laser. Under this illumination, the orientation of the azo units changes through trans–cis–trans isomerization processes and a preferential orientation of the trans moieties, with its axis perpendicular to the polarisation direction of the exciting light, can be achieved. High and stable values of the optical anisotropy, which can be checked by dichroism and birefringence measurements, have been obtained, mainly in films of liquid crystalline polymers (LCP) with the azo units in the side chain. The strong molecular interactions among the azo moieties in LCP seems to * Corresponding author. Tel.: C34-976761333; fax: C34-976761229 E-mail address: [email protected] (R. Alcala´). 0032-3861/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2004.06.036
enhance the photoinduced anisotropy and to improve its stability. Optical anisotropy has also been induced in some azo polymer films under illumination with linearly polarised red light from a He–Ne laser (633 nm), after pre-irradiation with either blue or ultraviolet (UV) light [17–19]. In this case, and depending on the irradiation conditions, the induced orientation of the azo moieties can either be parallel or perpendicular to the polarization direction of the red light. If the films are first irradiated with unpolarised UV light, a subsequent red light irradiation induces, in a first step, a preferential orientation of the trans moieties parallel to the red light polarization [18,20,21]. However, if the red light irradiation goes on for long times, a change in the preferential orientation direction from parallel to perpendicular has been found in some polymers [18]. That perpendicular orientation can also be induced in some azo-polymers under red light irradiation without a previous pre-irradiation with UV light [18,22].
F.J. Rodrı´guez et al. / Polymer 45 (2004) 6003–6012
6004
Pre-irradiation with UV light produces a high concentration of the cis isomer. Subsequent red light irradiation induces the preferential back isomerization of the cis moeities with its axis parallel to the red light polarisation direction, to the trans form. But the presence of high cis content in the films seems to produce some other changes in the properties of the films. Thus, it has been reported that UV irradiation can induce a liquid crystal (LC) to isotropic phase transition at temperatures well below Ti (LC to isotropic transition temperature without irradiation) [23]. This transition is usually accompanied by a strong decrease in the light scattered by the film [24]. UV irradiation is also able to break the aggregates of azobenzene moieties that appear in many polymers and this can play an important role in the photoinduced processes [25–27]. From all these results it is clear that the presence of the cis isomer, induced by UV irradiation, strongly influences the optical properties and the response of azobenzene polymer films to light irradiation. In this paper we present a study of the influence of UV pre-irradiation on the optical properties of a series of side chain LC azobenzene polyesters. The different polymers in this series were chosen to have different terminal groups. In this way the thermal properties as well as the aggregation trend are modified along the series and the influence of those properties on the photoinduced processes can be explored. Optical absorption, dichroism and birefringence have been studied using different irradiation conditions with either blue light (488 nm) or red light (633 nm). The influence of UV pre-irradiation on the photoinduced processes has been analysed.
2. Experimental The liquid crystalline polyesters used in this work were prepared in a K2CO3 catalyzed transesterification of equimolar amounts of diphenyl adipate [28] and the appropriately 4-substituted (X) 2-[6-[4-[(4-Xphenyl)azo]phenoxy]hexyl]1,3-propanediol [29] in the melt under vacuum at elevated temperature [30]. Further details are provided elsewhere [31,32]. The chemical structure of the investigated polyesters P6x4 is given in Fig. 1(a), while the trans and cis configurations of the azo chromophore are given in Fig. 1(b). The five different polyester substituents
Fig. 1. (a) Chemical structure of the P6x4 polymers. (b) trans and cis configurations of the azo moieties.
are shown in Table 1. Their glass transition temperatures (Tg) and their isotropic transition temperatures (Ti) as well as their molecular masses and transition enthalpies are also collected in Table 1. Films were produced by casting a solution of the polymers in chloroform (0.5 mg in 200 ml) onto clean glass substrates. Thickness was measured using a DEKTAK profilometer. Thickness value ranged from 200 to 400 nm. Before performing any experiment, all the films were heated up to 20 8C above Ti for 10 min. Films were then fast quenched to RT by putting them on a metallic plate. A high pressure Hg lamp followed by a band pass UV filter (maximum transmission at 350 nm) was used as the UV light source. The measured light intensity was about 0.5 mW/cm2 in the film. The polymer films have been
Table 1 Transition temperatures and enthalpies and molecular mass of polyesters P6x4 measured heating up the polymers (third run) at 3 8C/min x
R
Tg (8C)
Ti (8C)
DH (cal/g)
Mn
Mw
Mw/Mn
c e fa
OCH3 CF3 CH3
22 44 29
16,000 42,000 55,000
1.6 1.6 1.6
Cl Br
9 24
10.53 5.81 7.37 5.36 2.32 3.99
10,000 27,000 34,000
j k
63 73 61 65 54 81
31,000 35,000
61,000 62,000
2.0 1.8
a
This polymer shows three phase transitions.
F.J. Rodrı´guez et al. / Polymer 45 (2004) 6003–6012
examined in the polarization microscope before and after irradiation with UV light. Fast-quenched films show a random distribution of micro-domains. However, we were not able to identify the type of phase of those domains. In films irradiated with UV light, the domain structure is barely observed and the light scattering produced by the film is drastically reduced. Vertically polarized light beams at 488 and 633 nm were provided by ArC and He–Ne lasers, respectively. The intensity used was 75 mW/cm2 for the 488 nm light and 1 W/cm2 for the 633 nm light. Optical absorption measurements have been performed in a Cary 500 Scan UV–Vis-NIR spectrophotometer. Linear (vertical/horizontal) polarization of the measuring beams was achieved using a Glan-Thomson prism. Birefringence measurements were performed using the setup shown in Fig. 2. The sample was placed between crossed polarizers with their polarization directions at G458 with the vertical axis. The light from a 780 nm diode laser transmitted through the polarizer-sample-polarizer system was measured with a Si detector as a probe of the photoinduced anisotropy. The transmitted intensity I is given by the equation: I Z I0 sin2 ðpjDnjd=lÞ where I0 is the intensity transmitted by the as quenched films between parallel polarizers, d the film thickness, Dn the
6005
birefringence of the sample and l the wavelength of the measuring light (780 nm). In isotropic samples (DnZ0), no transmitted light is expected and thus, birefringence measurements also enable us to detect the transition from an anisotropic (Is0) to an isotropic (IZ0) state. An optical pyrometer was used to check that the temperature of the films did not increase during irradiation.
3. Experimental results and discussion As we said above, the main goal of the present work is to study the influence of UV light pre-irradiation on both the optical properties and the photoinduced optical effects, under several irradiation conditions, in films of side chain liquid crystalline azo-polyesters with different end substitutions. The polyesters generally have relatively high molecular masses. As seen from Table 1 most of the polyesters have Mn>27,000 stemming from DP’s >50 with corresponding polydispersities (Mw/Mn) from 1.6 to 2.0. P6c4 has a somehow lower Mn, however, this has previously not caused significant changes in physical properties for this type of polyesters [30]. Azobenzene units can appear in two isomeric forms: trans and cis. Before any treatment, the azo moieties are in the trans form that is the most stable one. However, the azo moieties show a tendency to form
Fig. 2. Experimental set-up for birefringence measurements.
6006
F.J. Rodrı´guez et al. / Polymer 45 (2004) 6003–6012
different types of aggregates and the photoinduced effects depend on the aggregation state. To check for the presence of these aggregates in our films we have compared the optical absorption spectrum of the different P6x4 polymer measured at RT in a chloroform solution, with the spectra of the polymer films measured at TZTiC10 8C and at RT (after quenching the films from the isotropic phase to RT). The results are given in Fig. 3. It can be seen that independently of the end substitution, all the spectra measured in solution show a main absorption band at about 360 nm and a shoulder at about 450 nm that are associated with the p–p* and n–p* transitions of the azobenzene moieties, respectively. The spectra of the films measured at TZTiC10 8C are similar (although slightly broader) to those in solution but the ones at RT show new bands that are due to the formation of different types of azobenzene aggregates [16,26]. Besides, the strong decrease
in the optical absorption when measured at RT can be related with a tendency of the azobenzene units to be oriented out of the plane of the film. This type of behaviour has been previously reported by several authors [27,33,34]. In conclusion, the RT spectra of the different polymer films show remarkable differences, indicating that the formation of aggregates and their optical absorptions are strongly influenced by the end substitution. In a first step we have studied the time evolution of the birefringence (Dn) induced with 350 nm linearly polarized light in the polymer films quenched from the isotropic phase. The evolution is similar in all the polymers. As an example we give in Fig. 4 the evolution for P6c4. It can be seen that birefringence increases with irradiation time, reaches a maximum and then goes back to zero. The initial growth is associated with the photoinduced trans–cis–trans isomerization. Under irradiation with UV light in the
Fig. 3. Optical absorption spectra of the different polyester measured: .. at RT in CHCl3 solution; .. at TZTiC10 8C in films; — at RT in films quenched from the isotropic phase to RT.
F.J. Rodrı´guez et al. / Polymer 45 (2004) 6003–6012
Fig. 4. Evolution of birefringence with time in a P6c4 film under irradiation at RT with 350 nm linearly polarized light.
absorption band of the trans isomer this can be transformed to the cis form, which can go back to the trans state either by thermal or by photoinduced processes. If the UV light is linearly polarized the photoexcitation of trans moieties is proportional to cos2 q, q being the angle between the molecular axis and the light polarization direction. Thus, the isomerization rate to the cis state is bigger for the trans moieties parallel to the light polarization and decreases when q increases. Through this photoselection (as well as some rotational diffusion) process a preferential orientation of the trans moieties in the plane perpendicular to the polarization direction of the exciting light is induced. Since the trans moieties have a high anisotropy (while that of the cis moieties is much smaller) the trans preferential orientation gives place to the measured birefringence. As the irradiation goes on, the concentration of trans isomers decreases. This can induce a decrease in birefringence since Dn is mainly associated with the trans moieties. On the other hand, while the rod like shape of the trans moieties favours the formation of the liquid crystalline phase, the bent shape of the cis ones tends to destroy that phase and thus the Ti of the LC polymer decreases [23]. If Ti goes below the irradiation temperature, the polymer converts to the isotropic phase. These two effects can account for the decrease in birefringence for long irradiation time. However, since the conversion from trans to cis is not complete, some residual anisotropy should remain in the films, unless that the polymer becomes isotropic. Thus, we conclude that UV irradiation induces a transition from the liquid crystalline phase to the isotropic one. This transition is also accompanied by a decrease in the light scattered by the film. Similar results have been previously reported [23–35]. The optical absorption spectrum of the films has also been measured after irradiation with the unpolarized 350 nm light. The results are given in Fig. 5. A strong decrease is observed in the bands associated with the trans moieties while new bands (mainly two bands at about 310 and 450 nm) appear, that are due to the cis isomer. The spectrum has been measured after several time intervals in dark at RT.
6007
From these measurements it has been found that the lifetime of the cis isomers is of several hours in all our polymers. After 1 day, the samples show a recovery of the trans absorption but the 360 nm band is narrower than in the ‘as quenched’ films (see Fig. 3). This indicates that irradiation with 350 nm light produces a breakdown of the azo aggregates. After some days at RT aggregation can be clearly observed in some of the films. The time evolution of birefringence under irradiation with linearly polarized 488 nm light has also been measured in films quenched from the isotropic phase, before and after irradiation with 350 nm light. The results are shown in Fig. 6. It can be seen that the qualitative evolution of Dn with the irradiation time in the P6c4, P6e4 and P6k4 is similar with and without pre-irradiation with 350 nm light. In all the cases birefringence grows up and reaches a saturation value. When the 488 nm light is switched off, a slight decrease in Dn (not shown in the figure) is observed, and then Dn remains stable if the films are kept in dark. However, the saturation values achieved after 350 nm light pre-irradiation are much bigger than those obtained without that preirradiation. The increase is of an order of magnitude in some of the films. In P6f4 and P6j4 samples even the qualitative evolution of Dn under 488 nm light irradiation is different before and after pre-irradiation with 350 nm light. Without preirradiation the time evolution of Dn in these two polymers is similar to that observed in the P6c4, P6e4 and P6k4 films. However, Dn evolution under 488 nm irradiation in preirradiated P6j4 samples shows a faster initial increase, goes through a maximum an then slowly decreases when irradiation time goes on. Besides, the Dn values achieved after pre-irradiation are smaller than those obtained without pre-irradiation, in contrast with the results in P6c4, P6e4 and P6k4 samples. This behaviour could be associated with the low value of Tg for P6j4 and the mutual influence of photoorientation and self-organisation processes [25–27,33,34]. In P6f4 a strong qualitative change is observed in the time evolution of Dn under 488 nm light irradiation after preirradiation with 350 nm light. In this case, the Dn vs. time evolution is much slower in pre-irradiated films, but reaches a saturation value bigger than the one achieved without preirradiation. Besides, another difference in the Dn vs. time evolution under 488 nm light irradiation, before and after preirradiation with 350 nm light, has been observed in all the films. Without pre-irradiation, the growth of photoinduced Dn begins as soon as the 488 nm light is switched on. However, a delay of a few seconds (different delays for different polymers) between the switching on of the 488 nm light irradiation and the increase in Dn has been observed in pre-irradiated films. The induction of birefringence with linearly polarized 488 nm light in samples quenched from the isotropic phase has been extensively reported [5,16]. It is due to photoselection and rotational diffusion of azo moieties
6008
F.J. Rodrı´guez et al. / Polymer 45 (2004) 6003–6012
Fig. 5. Optical absorption spectra of the films taken immediately after long time irradiation at RT with unpolarized 350 nm light and after some periods in dark at RT. The spectra before irradiation are also shown.
through the angle dependent trans–cis–trans isomerization processes. As in the case of irradiation with 350 nm light, trans moieties excited with 488 nm light can experience an isomerization to the cis state, which can be followed by a molecular reorientation before going back to the trans state. Since the excitation probability decreases when the angle between the trans molecular axis and the polarization direction of the exciting light increases, a preferential orientation of the trans moieties in a plane perpendicular to the 488 nm light polarization is achieved. This has been checked in our films by dichroism measurements in the 360 nm band due to the trans moieties. The preferential orientation of the trans moieties gives place to the observed birefringence. However, in contrast with the behaviour obtained with polarized 350 nm light, Dn does not go back to zero when the irradiation with polarized 488 nm light goes on. This can be associated with the lower concentration of cis moieties induced with 488 nm light, as
compared with that induced with 350 nm light (as checked by optical absorption measurements in the 450 nm region). Because of this, the decrease in Ti under blue light irradiation is not enough to reach RT and thus, no transition to the isotropic state has been induced with 488 nm light in our experimental conditions. Concerning the changes induced by pre-irradiation with 350 nm light on the evolution of Dn vs. 488 nm light irradiation time, we can say the following. After UV preirradiation the films have a high concentration of cis isomers and likely are in an isotropic state (Ti!RT). Since both trans and cis isomers absorb in the 488 nm region, irradiation with light of this wavelength produces both trans–cis and cis–trans photoinduced isomerization. Through these processes the concentration of cis isomers is reduced in such a way that, after a few seconds, the Ti of the irradiated polymers becomes higher than RT. This could explain the delay of the increase in Dn growth with respect
F.J. Rodrı´guez et al. / Polymer 45 (2004) 6003–6012
6009
Fig. 6. Birefringence of the different films as a function of time under RT irradiation with linearly polarized 488 nm light. // after quenching from the isotropic phase to RT; — after pre-irradiation with unpolarized 350 nm light.
to the switching on of the 488 nm light irradiation. Then, photo-orientation of trans moieties begins with a process similar to that in the films that have not been pre-irradiated. However, since the initial cis concentration is higher in the pre-irradiated samples Ti is lower. Besides, the azo aggregates that are present before UV irradiation are broken after irradiation. These two facts, the decrease in Ti and the breaking of aggregates, can produce and increase in the mobility of the trans moieties similar to the one achieved by increasing the temperature of the film. It has been reported that the evolution of Dn with irradiation time is strongly dependent on temperature [10,36]. The production efficiency of Dn increases at low temperatures, goes through a maximum and then decreases again to zero at high temperatures. Thus, the effect of UV pre-irradiation could be understood as equivalent to a heating up of the films and this could account for the results shown in Fig. 6. A detailed
study of the temperature and light power dependence of all these processes is under way in our laboratory. Birefringence induced with linearly polarized 633 nm light has also been measured before and after pre-irradiation with UV light. It is known that red light can induce a preferential orientation of the trans moieties that can either be parallel or perpendicular to the polarization of the red light, depending on the polymer and the irradiation conditions. In our case, no birefringence has been induced with 633 nm light in films quenched from the isotropic state. After pre-irradiation with UV light some birefringence has been observed. The results are given in Fig. 7. It can be seen that in all the films, a photoinduced birefringence begins to grow after a delay time. The growth of Dn continues, in some of the films, for several hours, and then reaches a saturation value. When the 633 nm light is switched off Dn remains stable if the samples are kept in dark. The saturation
6010
F.J. Rodrı´guez et al. / Polymer 45 (2004) 6003–6012
Fig. 7. Birefringence of the different films as a function of time under RT irradiation with linearly polarized 633 nm light. The films were pre-irradiated with unpolarized 350 nm light.
values of Dn are, in all the cases, smaller than those obtained with 488 nm light in pre-irradiated films. To check for the kind of molecular orientation that has been achieved under red light irradiation, dichroism measurements have been performed. The sign of the photoinduced dichroism is the same for all the polymers. As an example we give in Fig. 8 the results corresponding to P6c4 and P6j4, which show the highest dichroism values. It can be seen that, after 633 nm light irradiation, the dichroism that appears in the 360 nm band shows that the trans moieties are preferentially oriented parallel to the polarization of the exciting light. As we said above this type of orientation has been observed in several polymers after pre-irradiation [18,20,21]. It has been associated with a photoinduced angle-selective cis–trans isomerization and thus, an initial concentration of cis moieties is needed.
Because of this, the orientation parallel to the polarization of the red light, has only been observed in pre-irradiated films. It has been recently reported that, in some polymers, when the 633 nm light irradiation goes on for long times the orientation slowly changes from parallel to perpendicular with respect to the red light polarization [18]. This second step is associated with the same type of processes that give place to the photoinduced orientation with 488 nm light. However, although we have kept the red light irradiation for several days, no change of the molecular orientation has been observed in our polymers. This can be due to the small absorption of the trans molecules at 633 nm and to the aggregation of azobenzene moieties. Finally we want to comment on the observed delay between the growth of Dn and the beginning of red light irradiation. It has been found that the delay decreases if the
F.J. Rodrı´guez et al. / Polymer 45 (2004) 6003–6012
6011
measured at RT. Molecular interactions among the chromophores modify their absorption spectra. These interactions as well as the transition temperatures depend on the end substitutions. Thus, different polymers show different behaviour. A photoinduced phase transition to the isotropic state can be induced in all the polymer films at RT under irradiation with UV light. This irradiation also breaks the azo aggregates present in the ‘as quenched’ films. When cis population decreases while keeping the films in dark at RT, aggregation appears again at least in some of the films. The kinetic and the saturation values of photoinduced birefringence with 488 nm light are very different if the films are previously irradiated with UV light or fast quenched from the isotropic state. In both cases the orientation of the azobenzene is perpendicular to light polarization. Irradiation with 633 nm light after previous UV irradiation orients the azobenzene parallel to light polarization although no orientation is detected without UV preirradiation.
Acknowledgements
Fig. 8. Dichroism induced at RT with 633 nm light in P6c4 and P6j4 polymer films. Films were pre-irradiated at RT with unpolarized 350 nm light. The polarization of the measuring light is parallel (v) or perpendicular (h) to that of the exciting 633 nm light.
sample is kept in dark for some time after UV preirradiation. As in the case of 488 nm irradiation, we propose that after UV irradiation the film is in the isotropic phase, because of the decrease in Ti induced by the presence of cis isomers. When red light irradiation begins the cis concentration decreases, due to the cis–trans photoinduced isomerization. This gives place to an increase in Ti and, when it becomes higher than RT, the polymer goes back to the liquid crystalline phase. Then, since the photoinduced cis–trans isomerization is angle dependent, the preferential orientation is induced. The decrease in the delay when the sample is kept in dark before red light irradiation can be understood as due to a decrease in the initial cis concentration associated with thermal cis–trans isomerization.
4. Conclusion No aggregation of the azobenzenes was found above Ti in the polyesters studied in this work, but they aggregate in a few seconds after a fast quenching to RT. Although the absorption spectra of the different polymers are quite similar in solution and when the films are measured at T>Ti noticeable differences are observed when the same films are
The financial support from the CICYT, Spain, under project No. MAT2002-04118-02 is gratefully acknowledged. This work has been performed within the COST P8 Action.
References [1] Eich M, Wendorf JH. J Opt Soc A B 1990;94:1428. [2] Anderle K, Birenheide R, Werner MJA, Wendorf JH. Liq Cryst 1991; 9:691. [3] Rochon P, Gosselin J, Natansohn A, Xie S. Appl Phys Lett 1992;60:4. [4] Natansohn A, Rochon P, Gosselin J, Xie S. Macromolecules 1992; 25:2268. [5] Natansohn A, Rochon P. Chem Rev 2002;102:4139. [6] Fischer Th, La¨sker L, Stumpe J, Kostromin SG. J Photochem Photobiol A: Chem 1994;80:453. [7] La¨sker L, Fischer Th, Stumpe J, Kostromin SG, Ivanov S, Shibaev V, Ruhmann R. Mol Cryst Liq Cryst 1994;253:1. [8] La¨sker L, Fischer Th, Stumpe J, Kostromin SG, Ivanov S, Shibaev V, Ruhmann R. Mol Cryst Liq Cryst 1994;246:347. [9] La¨sker L, Stumpe J, Fischer Th, Rutloh M, Kostromin SG, Ruhmann R. Mol Cryst Liq Cryst 1995;261:371. [10] Holme NCR, Ramanujam PS, Hvilsted S. Appl Opt 1996;35:4622. [11] Stumpe J, La¨sker L, Fischer Th, Rutloh M, Kostromin SG, Ruhmann R. Thin Solid Films 1996;284-285:252. [12] Wu Y, Demachi Y, Tsutsumi O, Kanazawa A, Shiono T, Ikeda T. Macromolecules 1998;31:349. [13] Cimrova´ V, Neher D, Kostromin SG, Bieringer Th. Macromolecules 1999;32:8496. [14] Hildebrandt R, Hegelich M, Keller HM, Marowsky G, Hvilsted S, Holme NCR, Ramanujam PS. Phys Rev Lett 1998;81:5548. [15] Zebger I, Rutloh M, Hoffmann U, Stumpe J, Siesler HW, Hvilsted S. J Phys Chem A 2002;106:3454. [16] Shibaev V, Bobrovsky A, Boiko N. Prog Polym Sci 2002;28:729.
6012
F.J. Rodrı´guez et al. / Polymer 45 (2004) 6003–6012
[17] Kulinna Ch, Zebger I, Hvilsted S, Ramanujam PS, Siesler HW. Macromol Symp 1994;83:169. [18] Kempe C, Rutloh M, Stumpe J. J Phys: Condens Matter 2003;15:813. [19] Haosheng H, Wei Z, Wu P, Han L, Zhao Y, Che Y. Opt Lett 1994; 19:411. [20] Sa´nchez C, Alcala´ R, Hvilsted S, Ramanujam PS. Appl Phys Lett 2000;77:1440. [21] Sa´nchez C, Alcala´ R, Hvilsted S, Ramanujam PS. Appl Phys Lett 2001;78:3944. [22] Wu Y, Mamiya JI, Tsutsumi O, Kanazawa A, Shiono T, Ikeda T. Liq Cryst 2000;27:749. [23] Ikeda T, Miyamoto T, Kurihara S, Tsukada M, Tazuke S. Mol Cryst Liq Cryst 1990;182B:373. [24] Lee HK, Kanazawa A, Shiono T, Ikeda T. Chem Mater 1998;10:1402. [25] Fischer Th, Lasker L, Rutloh M, Czapla S, Stumpe J. Mol Cryst Liq Cryst 1997;299:293. [26] Rutloh M, Stumpe J, Stachanov L, Kostromin SG, Shibaev V. Mol Cryst Liq Cryst 2000;352:149.
[27] Meier JG, Ruhmann R, Stumpe J. Macromolecules 2000;(33):843. [28] Hvilsted S, Andruzzi F, Cerrai P, Tricoli M. Polymer 1991;32:127. [29] Hendann C, Siesler HW, Andruzzi F, Kulinna C, Hvilsted S. Mol Cryst Liq Cryst 1998;319:207. [30] Hvilsted S, Andruzzi F, Kulinna C, Siesler HW, Ramanujam PS. Macromolecules 1995;28:2172. [31] Pedersen M, Hvilsted S, Holme NCR, Ramanujam PS. Macromol Symp 1999;137:115. [32] Pedersen M. PhD Thesis New Azobenzene Side-Chain Polyesters for Optical Information Storage, Technical University of Denmark; 1997. [33] Fischer Th, La¨sker L, Czapla S, Ru¨bner J, Stumpe J. Mol Cryst Liq Cryst 1997;298:213. [34] Rosenhauer R, Fischer Th, Czapla S, Stumpe J, Vin˜uales A, Pin˜ol M, Serrano JL. Mol Cryst Liq Cryst 2001;364:295. [35] Sa´nchez C, Alcala´ R, Hvilsted S, Ramanujam PS. J Appl Phys 2003; 93:4454. [36] Song OK, Wang CH, Pauley MA. Macromolecules 1997;30:6913.
Influence of UV irradiation on the blue and red light photoinduced processes in azobenzene polyesters F.J. Rodrı´gueza, C. Sa´ncheza, B. Villacampaa, R. Alcala´a,*, R. Casesa, M.V. Colladosb, S. Hvilstedc, M. Stranged a
Departamento de Fı´sica de la Materia Condensada, Instituto de Ciencia de Materiales de Arago´n, Universidad de Zaragoza, 50009 Zaragoza, Spain b Departamento de Fı´sica Aplicada, Universidad de Zaragoza, 50009 Zaragoza, Spain c Danish Polymer Centre, Department of Chemical Engineering, Technical University of Denmark, DK-2800 Kgs Lyngby, Denmark d Risø National Laboratory, DK-4000 Roskilde, Denmark Received 7 January 2004; received in revised form 2 June 2004; accepted 15 June 2004
Abstract Birefringence induced in a series of liquid crystalline side-chain azobenzene polyesters with different substituent groups was investigated under irradiation with 488 and 633 nm linearly polarized lights. Two different initial conditions have been used: the effect of a previous irradiation with UV light that yields the films into the isotropic state at room temperature (RT) was compared with the quenching from temperatures above the isotropic transition temperature Ti. UV–visible spectra of the thermally quenched films show the presence of aggregates when measured at RT. We have found that UV light irradiation creates a high concentration of cis isomers and breaks the aggregates, but they are formed again after a few days in dark at RT. Orientation of the chromophores perpendicular to the polarization of the 488 nm light and parallel to the polarization of the 633 nm light was confirmed by dichroism measurements. q 2004 Elsevier Ltd. All rights reserved. Keywords: Azobenzene; Polyesters; Photoinduced optical anisotropy
1. Introduction Photoinduced anisotropy in polymer films containing azobenzene units has been extensively studied [1–16]. Anisotropy is usually induced by illumination with linearly polarised blue or green light (488 or 514 nm) from an ArC laser. Under this illumination, the orientation of the azo units changes through trans–cis–trans isomerization processes and a preferential orientation of the trans moieties, with its axis perpendicular to the polarisation direction of the exciting light, can be achieved. High and stable values of the optical anisotropy, which can be checked by dichroism and birefringence measurements, have been obtained, mainly in films of liquid crystalline polymers (LCP) with the azo units in the side chain. The strong molecular interactions among the azo moieties in LCP seems to * Corresponding author. Tel.: C34-976761333; fax: C34-976761229 E-mail address: [email protected] (R. Alcala´). 0032-3861/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2004.06.036
enhance the photoinduced anisotropy and to improve its stability. Optical anisotropy has also been induced in some azo polymer films under illumination with linearly polarised red light from a He–Ne laser (633 nm), after pre-irradiation with either blue or ultraviolet (UV) light [17–19]. In this case, and depending on the irradiation conditions, the induced orientation of the azo moieties can either be parallel or perpendicular to the polarization direction of the red light. If the films are first irradiated with unpolarised UV light, a subsequent red light irradiation induces, in a first step, a preferential orientation of the trans moieties parallel to the red light polarization [18,20,21]. However, if the red light irradiation goes on for long times, a change in the preferential orientation direction from parallel to perpendicular has been found in some polymers [18]. That perpendicular orientation can also be induced in some azo-polymers under red light irradiation without a previous pre-irradiation with UV light [18,22].
F.J. Rodrı´guez et al. / Polymer 45 (2004) 6003–6012
6004
Pre-irradiation with UV light produces a high concentration of the cis isomer. Subsequent red light irradiation induces the preferential back isomerization of the cis moeities with its axis parallel to the red light polarisation direction, to the trans form. But the presence of high cis content in the films seems to produce some other changes in the properties of the films. Thus, it has been reported that UV irradiation can induce a liquid crystal (LC) to isotropic phase transition at temperatures well below Ti (LC to isotropic transition temperature without irradiation) [23]. This transition is usually accompanied by a strong decrease in the light scattered by the film [24]. UV irradiation is also able to break the aggregates of azobenzene moieties that appear in many polymers and this can play an important role in the photoinduced processes [25–27]. From all these results it is clear that the presence of the cis isomer, induced by UV irradiation, strongly influences the optical properties and the response of azobenzene polymer films to light irradiation. In this paper we present a study of the influence of UV pre-irradiation on the optical properties of a series of side chain LC azobenzene polyesters. The different polymers in this series were chosen to have different terminal groups. In this way the thermal properties as well as the aggregation trend are modified along the series and the influence of those properties on the photoinduced processes can be explored. Optical absorption, dichroism and birefringence have been studied using different irradiation conditions with either blue light (488 nm) or red light (633 nm). The influence of UV pre-irradiation on the photoinduced processes has been analysed.
2. Experimental The liquid crystalline polyesters used in this work were prepared in a K2CO3 catalyzed transesterification of equimolar amounts of diphenyl adipate [28] and the appropriately 4-substituted (X) 2-[6-[4-[(4-Xphenyl)azo]phenoxy]hexyl]1,3-propanediol [29] in the melt under vacuum at elevated temperature [30]. Further details are provided elsewhere [31,32]. The chemical structure of the investigated polyesters P6x4 is given in Fig. 1(a), while the trans and cis configurations of the azo chromophore are given in Fig. 1(b). The five different polyester substituents
Fig. 1. (a) Chemical structure of the P6x4 polymers. (b) trans and cis configurations of the azo moieties.
are shown in Table 1. Their glass transition temperatures (Tg) and their isotropic transition temperatures (Ti) as well as their molecular masses and transition enthalpies are also collected in Table 1. Films were produced by casting a solution of the polymers in chloroform (0.5 mg in 200 ml) onto clean glass substrates. Thickness was measured using a DEKTAK profilometer. Thickness value ranged from 200 to 400 nm. Before performing any experiment, all the films were heated up to 20 8C above Ti for 10 min. Films were then fast quenched to RT by putting them on a metallic plate. A high pressure Hg lamp followed by a band pass UV filter (maximum transmission at 350 nm) was used as the UV light source. The measured light intensity was about 0.5 mW/cm2 in the film. The polymer films have been
Table 1 Transition temperatures and enthalpies and molecular mass of polyesters P6x4 measured heating up the polymers (third run) at 3 8C/min x
R
Tg (8C)
Ti (8C)
DH (cal/g)
Mn
Mw
Mw/Mn
c e fa
OCH3 CF3 CH3
22 44 29
16,000 42,000 55,000
1.6 1.6 1.6
Cl Br
9 24
10.53 5.81 7.37 5.36 2.32 3.99
10,000 27,000 34,000
j k
63 73 61 65 54 81
31,000 35,000
61,000 62,000
2.0 1.8
a
This polymer shows three phase transitions.
F.J. Rodrı´guez et al. / Polymer 45 (2004) 6003–6012
examined in the polarization microscope before and after irradiation with UV light. Fast-quenched films show a random distribution of micro-domains. However, we were not able to identify the type of phase of those domains. In films irradiated with UV light, the domain structure is barely observed and the light scattering produced by the film is drastically reduced. Vertically polarized light beams at 488 and 633 nm were provided by ArC and He–Ne lasers, respectively. The intensity used was 75 mW/cm2 for the 488 nm light and 1 W/cm2 for the 633 nm light. Optical absorption measurements have been performed in a Cary 500 Scan UV–Vis-NIR spectrophotometer. Linear (vertical/horizontal) polarization of the measuring beams was achieved using a Glan-Thomson prism. Birefringence measurements were performed using the setup shown in Fig. 2. The sample was placed between crossed polarizers with their polarization directions at G458 with the vertical axis. The light from a 780 nm diode laser transmitted through the polarizer-sample-polarizer system was measured with a Si detector as a probe of the photoinduced anisotropy. The transmitted intensity I is given by the equation: I Z I0 sin2 ðpjDnjd=lÞ where I0 is the intensity transmitted by the as quenched films between parallel polarizers, d the film thickness, Dn the
6005
birefringence of the sample and l the wavelength of the measuring light (780 nm). In isotropic samples (DnZ0), no transmitted light is expected and thus, birefringence measurements also enable us to detect the transition from an anisotropic (Is0) to an isotropic (IZ0) state. An optical pyrometer was used to check that the temperature of the films did not increase during irradiation.
3. Experimental results and discussion As we said above, the main goal of the present work is to study the influence of UV light pre-irradiation on both the optical properties and the photoinduced optical effects, under several irradiation conditions, in films of side chain liquid crystalline azo-polyesters with different end substitutions. The polyesters generally have relatively high molecular masses. As seen from Table 1 most of the polyesters have Mn>27,000 stemming from DP’s >50 with corresponding polydispersities (Mw/Mn) from 1.6 to 2.0. P6c4 has a somehow lower Mn, however, this has previously not caused significant changes in physical properties for this type of polyesters [30]. Azobenzene units can appear in two isomeric forms: trans and cis. Before any treatment, the azo moieties are in the trans form that is the most stable one. However, the azo moieties show a tendency to form
Fig. 2. Experimental set-up for birefringence measurements.
6006
F.J. Rodrı´guez et al. / Polymer 45 (2004) 6003–6012
different types of aggregates and the photoinduced effects depend on the aggregation state. To check for the presence of these aggregates in our films we have compared the optical absorption spectrum of the different P6x4 polymer measured at RT in a chloroform solution, with the spectra of the polymer films measured at TZTiC10 8C and at RT (after quenching the films from the isotropic phase to RT). The results are given in Fig. 3. It can be seen that independently of the end substitution, all the spectra measured in solution show a main absorption band at about 360 nm and a shoulder at about 450 nm that are associated with the p–p* and n–p* transitions of the azobenzene moieties, respectively. The spectra of the films measured at TZTiC10 8C are similar (although slightly broader) to those in solution but the ones at RT show new bands that are due to the formation of different types of azobenzene aggregates [16,26]. Besides, the strong decrease
in the optical absorption when measured at RT can be related with a tendency of the azobenzene units to be oriented out of the plane of the film. This type of behaviour has been previously reported by several authors [27,33,34]. In conclusion, the RT spectra of the different polymer films show remarkable differences, indicating that the formation of aggregates and their optical absorptions are strongly influenced by the end substitution. In a first step we have studied the time evolution of the birefringence (Dn) induced with 350 nm linearly polarized light in the polymer films quenched from the isotropic phase. The evolution is similar in all the polymers. As an example we give in Fig. 4 the evolution for P6c4. It can be seen that birefringence increases with irradiation time, reaches a maximum and then goes back to zero. The initial growth is associated with the photoinduced trans–cis–trans isomerization. Under irradiation with UV light in the
Fig. 3. Optical absorption spectra of the different polyester measured: .. at RT in CHCl3 solution; .. at TZTiC10 8C in films; — at RT in films quenched from the isotropic phase to RT.
F.J. Rodrı´guez et al. / Polymer 45 (2004) 6003–6012
Fig. 4. Evolution of birefringence with time in a P6c4 film under irradiation at RT with 350 nm linearly polarized light.
absorption band of the trans isomer this can be transformed to the cis form, which can go back to the trans state either by thermal or by photoinduced processes. If the UV light is linearly polarized the photoexcitation of trans moieties is proportional to cos2 q, q being the angle between the molecular axis and the light polarization direction. Thus, the isomerization rate to the cis state is bigger for the trans moieties parallel to the light polarization and decreases when q increases. Through this photoselection (as well as some rotational diffusion) process a preferential orientation of the trans moieties in the plane perpendicular to the polarization direction of the exciting light is induced. Since the trans moieties have a high anisotropy (while that of the cis moieties is much smaller) the trans preferential orientation gives place to the measured birefringence. As the irradiation goes on, the concentration of trans isomers decreases. This can induce a decrease in birefringence since Dn is mainly associated with the trans moieties. On the other hand, while the rod like shape of the trans moieties favours the formation of the liquid crystalline phase, the bent shape of the cis ones tends to destroy that phase and thus the Ti of the LC polymer decreases [23]. If Ti goes below the irradiation temperature, the polymer converts to the isotropic phase. These two effects can account for the decrease in birefringence for long irradiation time. However, since the conversion from trans to cis is not complete, some residual anisotropy should remain in the films, unless that the polymer becomes isotropic. Thus, we conclude that UV irradiation induces a transition from the liquid crystalline phase to the isotropic one. This transition is also accompanied by a decrease in the light scattered by the film. Similar results have been previously reported [23–35]. The optical absorption spectrum of the films has also been measured after irradiation with the unpolarized 350 nm light. The results are given in Fig. 5. A strong decrease is observed in the bands associated with the trans moieties while new bands (mainly two bands at about 310 and 450 nm) appear, that are due to the cis isomer. The spectrum has been measured after several time intervals in dark at RT.
6007
From these measurements it has been found that the lifetime of the cis isomers is of several hours in all our polymers. After 1 day, the samples show a recovery of the trans absorption but the 360 nm band is narrower than in the ‘as quenched’ films (see Fig. 3). This indicates that irradiation with 350 nm light produces a breakdown of the azo aggregates. After some days at RT aggregation can be clearly observed in some of the films. The time evolution of birefringence under irradiation with linearly polarized 488 nm light has also been measured in films quenched from the isotropic phase, before and after irradiation with 350 nm light. The results are shown in Fig. 6. It can be seen that the qualitative evolution of Dn with the irradiation time in the P6c4, P6e4 and P6k4 is similar with and without pre-irradiation with 350 nm light. In all the cases birefringence grows up and reaches a saturation value. When the 488 nm light is switched off, a slight decrease in Dn (not shown in the figure) is observed, and then Dn remains stable if the films are kept in dark. However, the saturation values achieved after 350 nm light pre-irradiation are much bigger than those obtained without that preirradiation. The increase is of an order of magnitude in some of the films. In P6f4 and P6j4 samples even the qualitative evolution of Dn under 488 nm light irradiation is different before and after pre-irradiation with 350 nm light. Without preirradiation the time evolution of Dn in these two polymers is similar to that observed in the P6c4, P6e4 and P6k4 films. However, Dn evolution under 488 nm irradiation in preirradiated P6j4 samples shows a faster initial increase, goes through a maximum an then slowly decreases when irradiation time goes on. Besides, the Dn values achieved after pre-irradiation are smaller than those obtained without pre-irradiation, in contrast with the results in P6c4, P6e4 and P6k4 samples. This behaviour could be associated with the low value of Tg for P6j4 and the mutual influence of photoorientation and self-organisation processes [25–27,33,34]. In P6f4 a strong qualitative change is observed in the time evolution of Dn under 488 nm light irradiation after preirradiation with 350 nm light. In this case, the Dn vs. time evolution is much slower in pre-irradiated films, but reaches a saturation value bigger than the one achieved without preirradiation. Besides, another difference in the Dn vs. time evolution under 488 nm light irradiation, before and after preirradiation with 350 nm light, has been observed in all the films. Without pre-irradiation, the growth of photoinduced Dn begins as soon as the 488 nm light is switched on. However, a delay of a few seconds (different delays for different polymers) between the switching on of the 488 nm light irradiation and the increase in Dn has been observed in pre-irradiated films. The induction of birefringence with linearly polarized 488 nm light in samples quenched from the isotropic phase has been extensively reported [5,16]. It is due to photoselection and rotational diffusion of azo moieties
6008
F.J. Rodrı´guez et al. / Polymer 45 (2004) 6003–6012
Fig. 5. Optical absorption spectra of the films taken immediately after long time irradiation at RT with unpolarized 350 nm light and after some periods in dark at RT. The spectra before irradiation are also shown.
through the angle dependent trans–cis–trans isomerization processes. As in the case of irradiation with 350 nm light, trans moieties excited with 488 nm light can experience an isomerization to the cis state, which can be followed by a molecular reorientation before going back to the trans state. Since the excitation probability decreases when the angle between the trans molecular axis and the polarization direction of the exciting light increases, a preferential orientation of the trans moieties in a plane perpendicular to the 488 nm light polarization is achieved. This has been checked in our films by dichroism measurements in the 360 nm band due to the trans moieties. The preferential orientation of the trans moieties gives place to the observed birefringence. However, in contrast with the behaviour obtained with polarized 350 nm light, Dn does not go back to zero when the irradiation with polarized 488 nm light goes on. This can be associated with the lower concentration of cis moieties induced with 488 nm light, as
compared with that induced with 350 nm light (as checked by optical absorption measurements in the 450 nm region). Because of this, the decrease in Ti under blue light irradiation is not enough to reach RT and thus, no transition to the isotropic state has been induced with 488 nm light in our experimental conditions. Concerning the changes induced by pre-irradiation with 350 nm light on the evolution of Dn vs. 488 nm light irradiation time, we can say the following. After UV preirradiation the films have a high concentration of cis isomers and likely are in an isotropic state (Ti!RT). Since both trans and cis isomers absorb in the 488 nm region, irradiation with light of this wavelength produces both trans–cis and cis–trans photoinduced isomerization. Through these processes the concentration of cis isomers is reduced in such a way that, after a few seconds, the Ti of the irradiated polymers becomes higher than RT. This could explain the delay of the increase in Dn growth with respect
F.J. Rodrı´guez et al. / Polymer 45 (2004) 6003–6012
6009
Fig. 6. Birefringence of the different films as a function of time under RT irradiation with linearly polarized 488 nm light. // after quenching from the isotropic phase to RT; — after pre-irradiation with unpolarized 350 nm light.
to the switching on of the 488 nm light irradiation. Then, photo-orientation of trans moieties begins with a process similar to that in the films that have not been pre-irradiated. However, since the initial cis concentration is higher in the pre-irradiated samples Ti is lower. Besides, the azo aggregates that are present before UV irradiation are broken after irradiation. These two facts, the decrease in Ti and the breaking of aggregates, can produce and increase in the mobility of the trans moieties similar to the one achieved by increasing the temperature of the film. It has been reported that the evolution of Dn with irradiation time is strongly dependent on temperature [10,36]. The production efficiency of Dn increases at low temperatures, goes through a maximum and then decreases again to zero at high temperatures. Thus, the effect of UV pre-irradiation could be understood as equivalent to a heating up of the films and this could account for the results shown in Fig. 6. A detailed
study of the temperature and light power dependence of all these processes is under way in our laboratory. Birefringence induced with linearly polarized 633 nm light has also been measured before and after pre-irradiation with UV light. It is known that red light can induce a preferential orientation of the trans moieties that can either be parallel or perpendicular to the polarization of the red light, depending on the polymer and the irradiation conditions. In our case, no birefringence has been induced with 633 nm light in films quenched from the isotropic state. After pre-irradiation with UV light some birefringence has been observed. The results are given in Fig. 7. It can be seen that in all the films, a photoinduced birefringence begins to grow after a delay time. The growth of Dn continues, in some of the films, for several hours, and then reaches a saturation value. When the 633 nm light is switched off Dn remains stable if the samples are kept in dark. The saturation
6010
F.J. Rodrı´guez et al. / Polymer 45 (2004) 6003–6012
Fig. 7. Birefringence of the different films as a function of time under RT irradiation with linearly polarized 633 nm light. The films were pre-irradiated with unpolarized 350 nm light.
values of Dn are, in all the cases, smaller than those obtained with 488 nm light in pre-irradiated films. To check for the kind of molecular orientation that has been achieved under red light irradiation, dichroism measurements have been performed. The sign of the photoinduced dichroism is the same for all the polymers. As an example we give in Fig. 8 the results corresponding to P6c4 and P6j4, which show the highest dichroism values. It can be seen that, after 633 nm light irradiation, the dichroism that appears in the 360 nm band shows that the trans moieties are preferentially oriented parallel to the polarization of the exciting light. As we said above this type of orientation has been observed in several polymers after pre-irradiation [18,20,21]. It has been associated with a photoinduced angle-selective cis–trans isomerization and thus, an initial concentration of cis moieties is needed.
Because of this, the orientation parallel to the polarization of the red light, has only been observed in pre-irradiated films. It has been recently reported that, in some polymers, when the 633 nm light irradiation goes on for long times the orientation slowly changes from parallel to perpendicular with respect to the red light polarization [18]. This second step is associated with the same type of processes that give place to the photoinduced orientation with 488 nm light. However, although we have kept the red light irradiation for several days, no change of the molecular orientation has been observed in our polymers. This can be due to the small absorption of the trans molecules at 633 nm and to the aggregation of azobenzene moieties. Finally we want to comment on the observed delay between the growth of Dn and the beginning of red light irradiation. It has been found that the delay decreases if the
F.J. Rodrı´guez et al. / Polymer 45 (2004) 6003–6012
6011
measured at RT. Molecular interactions among the chromophores modify their absorption spectra. These interactions as well as the transition temperatures depend on the end substitutions. Thus, different polymers show different behaviour. A photoinduced phase transition to the isotropic state can be induced in all the polymer films at RT under irradiation with UV light. This irradiation also breaks the azo aggregates present in the ‘as quenched’ films. When cis population decreases while keeping the films in dark at RT, aggregation appears again at least in some of the films. The kinetic and the saturation values of photoinduced birefringence with 488 nm light are very different if the films are previously irradiated with UV light or fast quenched from the isotropic state. In both cases the orientation of the azobenzene is perpendicular to light polarization. Irradiation with 633 nm light after previous UV irradiation orients the azobenzene parallel to light polarization although no orientation is detected without UV preirradiation.
Acknowledgements
Fig. 8. Dichroism induced at RT with 633 nm light in P6c4 and P6j4 polymer films. Films were pre-irradiated at RT with unpolarized 350 nm light. The polarization of the measuring light is parallel (v) or perpendicular (h) to that of the exciting 633 nm light.
sample is kept in dark for some time after UV preirradiation. As in the case of 488 nm irradiation, we propose that after UV irradiation the film is in the isotropic phase, because of the decrease in Ti induced by the presence of cis isomers. When red light irradiation begins the cis concentration decreases, due to the cis–trans photoinduced isomerization. This gives place to an increase in Ti and, when it becomes higher than RT, the polymer goes back to the liquid crystalline phase. Then, since the photoinduced cis–trans isomerization is angle dependent, the preferential orientation is induced. The decrease in the delay when the sample is kept in dark before red light irradiation can be understood as due to a decrease in the initial cis concentration associated with thermal cis–trans isomerization.
4. Conclusion No aggregation of the azobenzenes was found above Ti in the polyesters studied in this work, but they aggregate in a few seconds after a fast quenching to RT. Although the absorption spectra of the different polymers are quite similar in solution and when the films are measured at T>Ti noticeable differences are observed when the same films are
The financial support from the CICYT, Spain, under project No. MAT2002-04118-02 is gratefully acknowledged. This work has been performed within the COST P8 Action.
References [1] Eich M, Wendorf JH. J Opt Soc A B 1990;94:1428. [2] Anderle K, Birenheide R, Werner MJA, Wendorf JH. Liq Cryst 1991; 9:691. [3] Rochon P, Gosselin J, Natansohn A, Xie S. Appl Phys Lett 1992;60:4. [4] Natansohn A, Rochon P, Gosselin J, Xie S. Macromolecules 1992; 25:2268. [5] Natansohn A, Rochon P. Chem Rev 2002;102:4139. [6] Fischer Th, La¨sker L, Stumpe J, Kostromin SG. J Photochem Photobiol A: Chem 1994;80:453. [7] La¨sker L, Fischer Th, Stumpe J, Kostromin SG, Ivanov S, Shibaev V, Ruhmann R. Mol Cryst Liq Cryst 1994;253:1. [8] La¨sker L, Fischer Th, Stumpe J, Kostromin SG, Ivanov S, Shibaev V, Ruhmann R. Mol Cryst Liq Cryst 1994;246:347. [9] La¨sker L, Stumpe J, Fischer Th, Rutloh M, Kostromin SG, Ruhmann R. Mol Cryst Liq Cryst 1995;261:371. [10] Holme NCR, Ramanujam PS, Hvilsted S. Appl Opt 1996;35:4622. [11] Stumpe J, La¨sker L, Fischer Th, Rutloh M, Kostromin SG, Ruhmann R. Thin Solid Films 1996;284-285:252. [12] Wu Y, Demachi Y, Tsutsumi O, Kanazawa A, Shiono T, Ikeda T. Macromolecules 1998;31:349. [13] Cimrova´ V, Neher D, Kostromin SG, Bieringer Th. Macromolecules 1999;32:8496. [14] Hildebrandt R, Hegelich M, Keller HM, Marowsky G, Hvilsted S, Holme NCR, Ramanujam PS. Phys Rev Lett 1998;81:5548. [15] Zebger I, Rutloh M, Hoffmann U, Stumpe J, Siesler HW, Hvilsted S. J Phys Chem A 2002;106:3454. [16] Shibaev V, Bobrovsky A, Boiko N. Prog Polym Sci 2002;28:729.
6012
F.J. Rodrı´guez et al. / Polymer 45 (2004) 6003–6012
[17] Kulinna Ch, Zebger I, Hvilsted S, Ramanujam PS, Siesler HW. Macromol Symp 1994;83:169. [18] Kempe C, Rutloh M, Stumpe J. J Phys: Condens Matter 2003;15:813. [19] Haosheng H, Wei Z, Wu P, Han L, Zhao Y, Che Y. Opt Lett 1994; 19:411. [20] Sa´nchez C, Alcala´ R, Hvilsted S, Ramanujam PS. Appl Phys Lett 2000;77:1440. [21] Sa´nchez C, Alcala´ R, Hvilsted S, Ramanujam PS. Appl Phys Lett 2001;78:3944. [22] Wu Y, Mamiya JI, Tsutsumi O, Kanazawa A, Shiono T, Ikeda T. Liq Cryst 2000;27:749. [23] Ikeda T, Miyamoto T, Kurihara S, Tsukada M, Tazuke S. Mol Cryst Liq Cryst 1990;182B:373. [24] Lee HK, Kanazawa A, Shiono T, Ikeda T. Chem Mater 1998;10:1402. [25] Fischer Th, Lasker L, Rutloh M, Czapla S, Stumpe J. Mol Cryst Liq Cryst 1997;299:293. [26] Rutloh M, Stumpe J, Stachanov L, Kostromin SG, Shibaev V. Mol Cryst Liq Cryst 2000;352:149.
[27] Meier JG, Ruhmann R, Stumpe J. Macromolecules 2000;(33):843. [28] Hvilsted S, Andruzzi F, Cerrai P, Tricoli M. Polymer 1991;32:127. [29] Hendann C, Siesler HW, Andruzzi F, Kulinna C, Hvilsted S. Mol Cryst Liq Cryst 1998;319:207. [30] Hvilsted S, Andruzzi F, Kulinna C, Siesler HW, Ramanujam PS. Macromolecules 1995;28:2172. [31] Pedersen M, Hvilsted S, Holme NCR, Ramanujam PS. Macromol Symp 1999;137:115. [32] Pedersen M. PhD Thesis New Azobenzene Side-Chain Polyesters for Optical Information Storage, Technical University of Denmark; 1997. [33] Fischer Th, La¨sker L, Czapla S, Ru¨bner J, Stumpe J. Mol Cryst Liq Cryst 1997;298:213. [34] Rosenhauer R, Fischer Th, Czapla S, Stumpe J, Vin˜uales A, Pin˜ol M, Serrano JL. Mol Cryst Liq Cryst 2001;364:295. [35] Sa´nchez C, Alcala´ R, Hvilsted S, Ramanujam PS. J Appl Phys 2003; 93:4454. [36] Song OK, Wang CH, Pauley MA. Macromolecules 1997;30:6913.