Waveguide Spectroscopic Characterization of 3D Anisotropies in

Waveguide Spectroscopic Characterization of 3D Anisotropies in Conventionally Photooriented and Annealed Films of Liquid Crystalline and Amorphous ...
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J. Phys. Chem. B 2005, 109, 7865-7871

7865

Waveguide Spectroscopic Characterization of 3D Anisotropies in Conventionally Photooriented and Annealed Films of Liquid Crystalline and Amorphous Azobenzene Polymers C. Christoph Jung, Michael Rutloh, and Joachim Stumpe* Fraunhofer Institute of Applied Polymer Research, Science Park Golm, Geiselbergstr. 69, D-14476 Potsdam, Germany ReceiVed: October 12, 2004; In Final Form: February 4, 2005

Films of azobenzene-containing polymers were photooriented in the glassy state in such a way that the azobenzene side groups were oriented preferably perpendicular to the electric field vector. In the case of liquid crystalline polymers ,the photoinduced anisotropies generated in the glassy state were modified by thermotropic self-organization due to annealing above Tg. The conventional photoorientation process results in an oblate order. The changes in photoinduced anisotropies brought about by annealing in the liquid crystalline phase were investigated quantitatively for the first time by us for different polymer compositions and experimental conditions. Different biaxial and homeotropic orders result for liquid crystalline polymers, depending on the experimental conditions. Different polymer structures are compared and the influence of the interfaces is investigated. Orientational gradients can be induced by irradiation or annealing and are for the first time determined by the WKB (Wentzel-Kramers-Brillouin) method.

1. Introduction The use of optical anisotropy in thin films is of high importance predominantly in display technologies,1 optical data storage,2 and for optical security elements.3 In display technology, anisotropic thin films are used as optical components such as retarders, polarizers, reflectors, or anisotropic emitters.4-8 These optical layers, including the color filters, are responsible for over 50% of the costs of liquid crystal displays (LCDs). To obtain significant improvement is one of the main causes for the advances in the area of LCDs during the last years. The viewing angle dependence of the contrast and the color of the display can be attenuated by placing a compensator on top of the layer system. For the properties of such compensators precisely defined three-dimensional anisotropies are necessary. Moreover, the compensators should be transparent in the visible range. There are a number of different display designs exhibiting a viewing angle dependence and requiring different compensators with different three-dimensional anisotropies. Light-Induced Anisotropy in Azobenzene-Containing Polymers. Reviews on light-induced orientation in polymers have been published (see refs 9-12). Light-induced anisotropy is advantageous because it can be administered precisely and because structures can be imposed on the film. For digital, holographic, and analogue optical data storage, reversible storage media are of high interest. Multiphoton orientation can be used in order to reach a higher storage density.13 A reversible induction of anisotropy by irradiation with polarized light in a poly(methyl methacrylate) (PMMA) matrix, including the azobenzene dye methyl red was described in ref 14. In ref 15 a first explanation of the phenomenon can be found. The angular selective E/Z photoisomerization of the azobenzene unit (see ref 16) caused by polarized light of the wavelength of 488 nm * Corresponding author. E-mail: [email protected].

and followed by thermal back isomerization is made responsible for this photoorientation process. The azobenzene molecules reorient mainly perpendicular to the (linear) polarization direction of the incident light, because they can have a different orientation after the back isomerization. The molecules, oriented mainly perpendicular to the polarization of the incident light do not absorb and cannot undergo photoisomerization and therefore will not change their orientation anymore. Modification of Anisotropy by Annealing of Liquid Crystalline Polymers. In contrast to amorphous polymers, thin films of liquid crystalline polymers allow an amplification of the light-induced anisotropy in the film plane by heating in the liquid crystalline phase.17 Shorter irradiation times are necessary. As shown in ref 18, this amplified order is in a large range independent of the extent of the light-induced order. In ref 19, even an amplification of the in-plane induced order by annealing just below the glass transition temperature is reported. Determination of the Three-Dimensional Structure of the Photoinduced Order. Different methods allow the determination of the three-dimensional order of polymer films. For films of thicknesses between 1 µm and 10 µm, waveguide spectroscopy is the method of choice. This method uses attenuated total reflection at the base of a prism with high refractive index. Other authors20 even report the development of waveguide microscopy. Anisotropy gradients have been investigated by ref 21. The conventional photoorientation of azobenzene-containing polymers by blue light, leads to an orientation perpendicular to the polarization direction of the irradiation light, as explained above. The resulting three-dimensional indicatrix therefore should be oblate (negatively uniaxial), with two long and one short axis. The (short) optical axis is parallel to the polarization direction of the irradiation light. However, it cannot be assumed that the polymer films are completely isotropic before the irradiation or show indicatrices that deviate from the ideal oblate order because of liquid crystalline properties.

10.1021/jp0453360 CCC: $30.25 © 2005 American Chemical Society Published on Web 04/02/2005

7866 J. Phys. Chem. B, Vol. 109, No. 16, 2005

Jung et al.

TABLE 1: Polymer Structures

Figure 1. Configuration of the prism coupler for measuring thin polymer films.

TABLE 2: Thermal Properties of the Polymersa I II III IV V VI

Tg 77 °C g 58 °C sa 115 °C i g 63 °C sa 166 °C i g 74 °C sA 87.5 °C i ? 84 °C n 117.7 °C i g 24°C sx 26°C sa 34 °C n 47 °C i

a Tg, glass transition temperature; g, glassy state; sA, smectic A; i, isotropic; n, nematic; sX, smectic X.

The development of the three-dimensional refractive indices under irradiation using the method of attenuated total reflection (ATR) was studied in refs 22 and 23. They were able to show, that the polarized irradiation of a weak anisotropic film, as expected, leads to the increase of the refractive indices perpendicular to the polarization direction of the irradiation light; simultaneously the parallel refractive index decreased. Similar investigations were carried out as reported in refs 24-26. In the last cited publication, the surface plasmon resonance (SPR) method was also used. Photoorientation in Langmuir-BlodgettKuhn multilayers leading to biaxial orders was investigated using waveguide spectroscopy in ref 27. Changes in the photoinduced three-dimensional order of 1 µm to 10 µm films, brought about by heating above the glass transition temperature of liquid crystalline polymer films, have been investigated for the first time by us and reported in refs 28-29. We intensified these investigations and especially used the WKB method for the first time to determine orientation gradients in the films. 2. Materials and Methods Azobenzene-Containing Polymers. A number of LC polymers and one similar amorphous polymer were used for the investigation. Their synthesis is described elsewhere. The structures are depicted in Table 1 and the thermal properties are contained in Table 2. Polyimide. Liquicoat PI kit ZLI-2650 (Merck) was used. Spin-Coating. The polymers were dissolved in tetrahydrofuran (THF), chloroform, 1,2-dichloroethane, methanol (Merck, UVASOL), cyclopentanone (Aldrich, 99+% (GC)), and toluene (Aldrich, > 99.5% A.C.S. spectrophotometric grade) in different concentrations. Solutions below 50 mg/mL were filtered with 200 µm filters (Nalgene) before spin-coating. The substrate material was glass (Corning, nHeNe ) 1.5025). The substrates

were immersed in concentrated sulfuric acid (for at least 2 h) and then rinsed with deionized water. The spin-coating-system RC 5/8 (Karl Suss Technique S. A., Saint Jeoire, France) was used. Prism Coupler. This measuring technique comprises the coupling of a monochromatic, coherent light beam into the investigated film using a highly refracting prism in total reflection. At certain angles of incidence the film becomes a waveguide, and the detected reflected intensity shows a minimum. These incidence angles have been determined for s-polarization (perpendicular to the plane of incidence) and for p-polarization (perpendicular to the plane of incidence) in two different perpendicular orientations of the sample. The two inplane refractive indices and the film thickness are obtained from the two measurements with s-light by nonlinear least-squares fitting to published equations.30 From the measurements using p-light the refractive index normal to the film plane is obtained. A tilt is detected if the two p-measurements differ significantly. The instrumental configuration for the measurements is shown in Figure 1. The polymer film on a substrate (Corning, nHeNe ) 1.5025) is pressed with the help of a metal sphere to the base of a prism (LaSF N9, Schott, nHeNe ) 1.84489, 90°), whereby the air gap between prism and polymer film becomes so thin that the evanescent field of the measuring light couples into the waveguide modes of the film. This configuration has the advantage over the Kretschmann configuration that it does not include the usage of a metal layer or an immersion liquid. As we use a prism with high refractive index in order to be very sensitive to the refractive index in the z direction when using TM modes, we would need an immersion liquid of a high refractive index. Such liquids are chemically aggressive and, because they have a high vapor pressure, attack the polymer films even if they are not in direct contact with it. Moreover, a metal layer would prevent investigation of the samples by UV/ vis spectroscopy and the subsequent usage of the samples in displays. A frequency stabilized HeNe laser (Coherent, model 200, 1 mW, 632.8 nm) was used. The light was polarized using a GlanThompson prism (B. Halle, Berlin, extinction ratio 10-8). Incidence angle and exit angle could be changed with an accuracy of 0.01° with a Θ/2Θ goniometer (Huber, model 414). In most measurements only 0.1° steps were used. This results in an accuracy of the refractive index of 3 figures after the decimal point. The reflected beam was measured using a photodiode (Spindler & Hoyer), a chopper (Owis GmbH, Staufen i. Br.), and a lock-in amplifier (EG&G Instruments, model 5210). Thereby the influence of fluctuating room light was reduced. Adjustment of the prism was accomplished at 45° and 90° incidence angle to the position of minimum reflected light. The incidence angle of 45° was adjusted using the reflection from the prism side.

3D Anisotropies in Photooriented and Annealed Films

J. Phys. Chem. B, Vol. 109, No. 16, 2005 7867 TABLE 3: Three-Dimensional Indicatrix of a Film of Copolymer Ia treatment

nx

ny

nz

d/µm

∆nzx

∆nyx Σni/3

freshly 1.637 1.630 1.25 -0.009 1.631 isotropic prepared polarized 1.601 1.659 1.658 1.16 0.057 0.058 1.639 oblate irradiated annealed 1.644 1.644 1.640 1.15 -0.004 0.000 1.643 isotropic a Prepared from 100 mg polymer/mL in THF and spin-coated at 1500 rpm for 60 s. Irradiated at 488 nm, 50 mW/cm2 for 12 h (saturation) and annealed at 90 °C (13 K above Tg) for 4 h.

Figure 2. In-plane WKB mode with parameters m ) 6, n0 ) 1.62, n1 ) 1.55, substrate index ns ) 1.5045, and thickness of the polymer film d ) 5 µm.

Gradients were evaluated using the Wentzel-Kramers-Brillouin (WKB) method31 using eq 1 for a monotonically decreasing gradient from the prism side.

∫0z xn2(z) - nm2 dz ) 4m8- 1 m

(1)

Solution of this equation was done following the method of ref 31. The free parameter n0 ) n(z ) 0) was varied until a smooth curve resulted. Results are the zm defined by n(zm) ) nm, and nm are the measured mode indices. Figure 2 depicts the amplitude of the electric field strength of such a WKB mode (TE) in dependence on the z coordinate. For TM-modes approximately the same equations are valid. The results will be reported using the convention that the z-axis is the film normal and the x-axis is the axis which is in the film plane and parallel to the polarization direction of the irradiation light. 3. Results and Discussion We will report our results in the following way: For each polymer the motivation for the synthesis of the polymer is given, then we report about the investigation of the 3D order of the films in the as-prepared, photooriented, and annealed film states of each polymer. To obtain insight into the structure-property relationship between the polymer composition and the 3D order induced by conventional photoorientation in the steady state of the E/Z photoisomerization and after its thermal amplification by thermotropic self-organization, a number of LC and one amorphous polymethacrylate and one LC polyester with the same azobenzene group were investigated. The polymers were varied with respect to the comesogenic group (4-alkoxy-4′butoxy-benzanilide, 4-alkoxy-4′-cyano-biphenyl, and HEMA), the spacer of the azobenzene and the comesogenic group, and the content and substitution of the 4-alkoxy-azobenzene group (4′-cyano, hydrogen and 4′-butoxy). This variation results in polymethacrylates characterized by SA phases, one with a nematic phase and one amorphous polymer. The polyester shows a much more complicated phase behavior, which is characterized by a nematic, an SA, and a highly ordered SX phase. In addition, this polymer shows a very strong tendency to form J aggregates resulting in a significant bathochromic shift of absorbance, which can be destroyed by the photoisomerization. The azobenzenes are varied concerning the polarity of the 4′-subsitution (4′-CN as acceptor and 4′-butoxy as donor group). but all are characterized by a ππ* transition around 340-370 nm well separated from a nπ* transition at 450 nm. Investigation of the Amorphous Copolymer I. The photochromic side group of the copolymer I is identical to that of the LC homopolymer IV. Although the copolymer contains in addition a side chain with the mesogenic 4′-cyano-biphenyl unit,

it is amorphous because of the short (CH2)2 spacer of this group. This also results in an increase of the Tg value by 19 °C compared to homopolymer II. The order of the freshly prepared films depends on the preparation conditions. When a film was formed from a highly concentrated solution (c ) 100 mg/mL in THF) and spin-coated at high rotation speed (1500 rpm), the film was oriented preferentially in-plane (see Table 3). When a film was formed by casting from a less concentrated solution (c ) 20 mg/mL), subsequent waiting for 5 min, and then spinning at 800 rpm, the refractive index was smaller (1.630) and the order (∆nzx ) nz - nx ) -0.003), too. The resulting thickness was twice as high (2.0 µm). Photoorientation of the first film by linearly polarized irradiation in the nπ* band of the azobenzene unit at 488 nm resulted, as expected, in a decrease of the refractive index in the polarization direction and an increase in the other two directions. The indicatrix was therefore negatively uniaxial or oblate (nx < ny ≈ nz). The small in-plane preference was almost lost after the irradiation. The oblate order could be erased by heating above the glass transition temperature. However, a small in-plane preference persisted. Heating of the second film above the glass transition temperature (1 h 30 min, 85 °C), without preceding photoorientation, resulted in a similar final state. It shows as the first film an increase of the refractive index to 1.644 and the same in-plane preference of ∆nzx ) -0.004. No tilt was detected. The small in-plane preference could either be stable for thermodynamic reasons or the annealing time was too short to erase it. Investigation of a Liquid Crystalline Homopolymer II. In the case of LC polymers annealing above the glass transition temperature is expected to give an amplification of the photoinduced order. The induced oblate of order of the film should be transferred to a uniaxial LC order. However, it is not clear which of the two directions, which are populated most in the oblate order, and which are distinguished by symmetry, will be the optical axis of the resulting uniaxial LC order. Twenty films were prepared from this smectic homopolymer II, which had absolutely the same azobenzene side chain (spacer and azobenzene substitution) as the amorphous polymer I. These films were made by casting from THF solution (c ) 20 mg/mL), waiting for several minutes, and subsequently drying by spinning. Thicknesses between 700 nm and 5.3 µm were obtained. (One layer was made from a solution with c ) 40 mg/mL and was not significantly different.) The averaged absolute refractive index varied around 1.647 with a standard deviation of 0.010. This polymer showed in contrast to I a preference to align outof-plane, e.g., in the z direction with a birefringence ∆nzx of about 0.010 ( 0.008. No correlation between the absolute refractive index and the extent of the preferential orientation could be found. However, a correlation seemed to exist between the thickness and the extent of the variation of the refractive index, in such a way that thicker films varied stronger.

7868 J. Phys. Chem. B, Vol. 109, No. 16, 2005

Jung et al. TABLE 5: Three-Dimensional Indicatrix of a Film Made from Copolymer IIIa treatment

nx

ny

nz

d/µm ∆nzx ∆nyx Σni/3

freshly 1.579 1.580 1.581 2.7 0.002 0.001 1.580 isotropic prepared polarized 1.570 1.586 1.595 2.6 0.025 0.016 1.584 ∼oblate irradiated annealed 1.548 1.556 1.645 2.6 0.097 0.008 1.583 homeotropic a Prepared from THF solution (20 mg/mL) by casting, waiting, and spinning. Irradiation was performed at 488 nm, 56 mW/cm2 for 14 h. Annealed at 90 °C (27 K above Tg) for 4 h.

Figure 3. Refractive indices in z direction (squares) and y direction (triangles) of a 5.3 µm thick film prepared from II, which was irradiated for a short time (1 h, 50 mW/cm2, 488 nm), in dependence on the z coordinate (z ) 0 is the side from which the irradiation was performed, i.e., the air interface).

TABLE 4: Three-Dimensional Indicatrix of a Film Made from the Homopolymer IIa treatment

nx

ny

nz

pretreated 1.650 1.649 1.655 by UV polarized 1.610 1.679 1.673 irradiated annealed 1.565 1.683 1.682

d/µm

∆nzx

∆nyx

Σni/3

1.7

0.005 -0.001 1.651 isotropic

1.8

0.063

0.069 1.654 oblate

1.6

0.117

0.118 1.643 oblate

Prepared from a solution (c ) 20 mg/mL) in THF by casting, waiting, and spinning. The film was pretreated by annealing at T > 115 °C and nonpolarized UV irradiation for 4-5 h using 10 mW/cm2 and a wavelength of 365 nm. Linearly polarized irradiation was performed at 488 nm and 90 mW/cm2 for11 h (saturation). Annealed at 90 °C (13 K above Tg) for 5 min. a

Photoorientation by short irradiation (1 h) of a film with the thickness of 5.3 µm resulted in a gradient of the refractive indices perpendicular to the polarization direction (Figure 3). The refractive indices decreased exponentially from the irradiation side, probably as a result of the filter effect that is exhibited by the absorbing polymer film. Photoorientation of a thinner (1.7 µm) film for 11 h with 90 mW/cm2 resulted in a birefringence in the film that was twice as high (∆nyx ) 0.069 instead of 0.035), although the film had been pretreated by heating above the clearing point and irradiation with UV light. The data of this film are contained in Table 4. Annealing of this photooriented film for only 5 min in the liquid crystalline phase resulted in an amplification of the oblate order. The alignment occurs in all directions of the photoinduced orientational plane perpendicular to E. A macroscopic uniaxial LC order is not established. This can be explained by the formation of microdomains or cooperativity of the side-chains, which is effective only locally under these conditions. The film scattered after annealing, which resulted in flat minima of the waveguide modes. This should be caused by the aggregation or the formation of microdomains, whose spatial extent was close to or higher than the red measuring wavelength. It was found that LC homopolymers tend to aggregate.32 The scattering was anisotropic, which means that the aggregates were anisotropic, as well. Annealing for 5 min of a nonpretreated, photooriented film (for 12 h, the photoorientation resulted in ∆n ) 0.068 and an almost perfectly oblate order) of the same thickness resulted in an amplification of the z direction, which was stronger than the amplification of the y direction. The refractive indices were nz ) 1.750 > ny ) 1.655 > nx ) 1.558. (This was detected by the compensation method of Ehringhaus for the in-plane birefringence under the assumption that the average refractive index did not change. The x direction was measured via prism

coupling, other than the y- and z- directions, which could not be measured in this way due to strong light scattering.) Annealing of an extremely short (8 s) photooriented film for 18 min resulted in a 2.0 µm film in an amplification of the in-plane birefringence from 0.011 to 0.022, but the film was almost completely homeotropic (nz ) 1.716 . ny ) 1.579 > nx ) 1.557). This could have been due to the fact that the z direction was the preferred direction already after photoorientation (nz ) 1.641 > ny ) 1.622 > nx ) 1.611). Investigation of the Liquid Crystalline Copolymer III. The polymer III has the same photochromic 4′-cyano-azobenzene unit as the polymers II and I, which were discussed in the previous subsections. But it has a hexyl-oxy spacer. The photochromic unit is contained in this copolymer only to a proportion of 1:9 with respect to the comesogenic unit. The comesogenic side chain is formed by a 4-hexoxy-4′-butoxy benzanilide group. The lower azobenzene content causes a higher transparency in the visible range, which makes the polymer interesting for some applications. The copolymer III has a smectic phase as well, and the thermal properties are quite similar to those of II. It is known that the nonphotochromic units orient cooperatively to the photochromic units under polarized irradiation, even below the glass transition temperature.33 An amplification of the in-plane photoinduced orders by annealing in the liquid crystalline phase has been reported for these polymers in relatively thin films (around 200 nm). The freshly prepared films showed a different behavior from the homopolymer II. Six films with thicknesses between 2.8 µm and 5.8 µm were prepared from a THF solution (c ) 20 mg/mL) by casting, waiting, and drying by spinning. The prism coupler measurement showed very sharp minima in contrast to the homopolymer II. This could be caused by the lower apparent extinction coefficient at 632.8 nm caused by a lower tendency to aggregate. The capability for H bonding of the benzanilide group reduces the tendency of ππ stacking. The films had an average refractive index of 1.580 ( 0.002, showing a lower standard deviation than the homopolymer II. When cyclopentanone was used as solvent for three films, a significantly lower averaged refractive index of 1.559 ( 0.001 resulted, which hints to a lower packing density. The preferential orientation was ∆nzx ) 0.003 ( 0.002 and therefore lower, but the z direction was also more strongly populated. Photoorientation of one of the films made from THF solution resulted in a higher orientation in the z direction, as was expected from the values of the fresh film. This might be due to the selfordering tendency already during the irradiation. Annealing resulted in a completely homeotropic film (Table 5). Photoorientation for only 1 h (at 100 mW/cm2) of a film of III also resulted in an orientation with a strong population of the z direction (∆nzx ) 0.014 > ∆nyx ) 0.008). Annealing gave also a homeotropic order. Annealing without photoorientation of films with thicknesses of 2.8 µm, 3.3 µm, and 3.7 µm resulted in homeotropic orientations, as well.

3D Anisotropies in Photooriented and Annealed Films

J. Phys. Chem. B, Vol. 109, No. 16, 2005 7869

TABLE 6: Three-Dimensional Indicatrix of a Film of Copolymer IV on Rubbed Polyimidea treatment

nx

ny

nz

d/µm ∆nzx

∆nyx Σni/3

freshly 1.580 1.580 1.580 4.6 0.000 0.000 1.580 isotropic prepared polarized 1.568 1.603 1.598 4.4 0.030 0.035 1.590 ∼oblate irradiated annealed 1.540 1.588b 1.679 (2.7) 0.139 0.048* 1.602 ∼homeotropic

Prepared from a THF solution (c ) 20 mg/mL), by casting, waiting, and spinning. Irradiation at 488 nm, 50 mW/cm2 for 9 h. Annealed at 85 °C (11 K above Tg) for 17 d. b Determination by an Ehringhaus measurement. a

Polarized irradiation of a film, which had been pretreated by annealing for 2 days, did not induce any detectable birefringence after 1 h at 100 mW/cm2. This could be caused by aging and aggregation. The reflectivity minima were slightly broader, which could be due to aggregation, as well. It should be noted that the annealing times (> 2 h) were much longer than for the homopolymer II and that this might probably cause the stable homeotropic orders. In addition, the tendency to establish an out-of-plane orientation already during photoorientation was not present for the homopolymer II, whereas the freshly prepared films from II had a slightly higher preference for the z direction. Investigation of the Liquid Crystalline Copolymer IV without 4′-Azobenzene Substitution. A second possibility to enlarge the transparency in the visible range consists of a change of the substitution in the 4′-position of the azobenzene unit. Thus, 4-alkoxy-azobenzene absorbs at shorter wavelengths than 4-alkoxy-4′-cyano-azobenzene (Azo-CN > Azo-H). The polymer IV has a maximum of the ππ* transition of the azobenzene at 345 nm compared to 370 nm for the previously discussed copolymer III. It contains the same nonphotochromic unit, but with a proportion of 70%. The spacer of the photochromic unit has been shortened to only two CH2-units. This increases the Tg value and reduces the existing range of the smectic A mesophase. Six freshly prepared films of copolymer IV from a THF solution with c ) 20 mg/mL by waiting and spinning had an average refractive index of 1.588 ( 0.003 and thicknesses between 870 nm and 4.1 µm. The preferred orientation was in this case in the plane of the film, however, was almost isotropic with ∆nzx ) -0.001 ( 0.001. When two films (4.6 µm and 5.0 µm) were prepared on rubbed polyimide to reach planar homogeneous orders, they had averaged refractive indices of 1.581 and 1.580 and were measured to be completely isotropic. When a film was prepared using the high spinning speed of 1500 rpm from a 125 mg/mL solution, the density was significantly higher, as is reflected in a refractive index of 1.594. The in-plane preference, however was only slightly higher (∆nzx ) -0.003). Photoorientation resulted in nearly oblate orders with a small preference for the in-plane direction. However, annealing of the films after photoorientation gave a large preference of the outof-plane orientation. This was the case for films that had been prepared directly on glass and even for films, which were prepared on rubbed polyimide. For these films on rubbed polyimide the out-of-plane orientation (∆nzx) was even higher by a factor of 3. This might be caused by the longer annealing times for these films (17 d compared to 23 h). Table 6 shows the data for a film, which was prepared on rubbed polyimide. Here a small amplification of the photoinduced in-plane order could be brought about by annealing in contrast to the films, which had been prepared directly on glass. A linear gradient of the refractive index nz along the z direction was detected for the irradiated film on glass which

Figure 4. Refractive index in the z direction of a nonirradiated film of IV on rubbed polyimide. Annealed for 17 d at 85 °C.

decreased from 1.626 at the air interface to 1.564 at the glass interface. A subsequent UV irradiation (1 h 40 min, 30 mW/ cm2, 365 nm) could be shown to decrease the homeotropic birefringence ∆nzx from 0.031 to 0.014. Annealing of nonirradiated films on polyimide resulted in an even stronger gradient of the refractive index nz (Figure 4). The birefringence in the plane of the film was very small (∆nyx ) 0.005). Nonirradiated films (1.7 µm and 3.1 µm thick) that had been prepared on glass were annealed and showed after 23 h also an out-of-plane orientation. The extent was again with ∆nzx ) 0.034 and ∆nzx ) 0.028 by a factor of 3 smaller than for the films on rubbed polyimide, which had been annealed for 17 d. The experiments performed with the copolymer IV clearly demonstrated that the order before annealing is not the only decisive element for the order after annealing. In this polymer especially the air interface was responsible for a homeotropic order. Investigation of the Liquid Crystalline Copolymer V Containing HEMA and 4-Ethoxy-4′-butoxy-azobenzene Side Groups. The investigations that have been described up to this point could lead to the conclusion that the benzanilide side group is the cause for the homeotropic alignment of the polymers after annealing in the liquid crystalline phase. V is a copolymer with a different nonphotochromic side chain, the 2-hydroxyethylmethacrylate (HEMA) group, which would be expected to interact strongly with the glass interface and which increases the polarity of the backbone. The butoxy substitution of the azobenzene unit shifts the maximum of absorbance to 360 nm. Six films prepared from a chloroform solution of V (c ) 20 mg/mL) by casting, waiting and spinning were indeed all preferentially oriented in the film plane apart from one and had an out-of-plane birefringence ∆nzx of -0.010 ( 0.006. The films had thicknesses from 730 nm to 3.7 µm and refractive indices of 1.573 ( 0.007. When films were prepared by spin-coating from highly concentrated solutions (c ) 200 mg/mL) without waiting before spinning the films also had an orientation preferentially in the plane of the film. So, a film prepared from a cyclopentanone solution (1.7 µm) had a refractive index of 1.567 and ∆nzx ) -0.008. A film prepared from a THF solution (8.2 µm) had a refractive index of 1.563 and ∆nzx ) -0.003. The in-plane orientation was amplified from 0.015 for a 1.3 µm film (chloroform) to a value of 0.020 by annealing without irradiation. Photoorientation of the films with linearly polarized light resulted in indicatrices, where the y direction was significantly higher populated than the z direction. This is illustrated by the following example. A 2.9 µm thick film had before irradiation a preferential orientation of ∆nzx ) -0.007 and had after irradiation for 20 s at 260 mW/cm2 a birefringence in the plane of the film of ∆nyx ) 0.021. The difference between the

7870 J. Phys. Chem. B, Vol. 109, No. 16, 2005

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TABLE 7: Three-dimensional Indicatrix of a Film of Copolymer Va treatment

nx

ny

nz

freshly 1.575 1.576 1.564 prepared polarized 1.534 1.603 1.587 irradiated annealed 1.527 1.603 1.581

d/µm

∆nzx

∆nyx Σni/3

3.7

-0.011 0.001 1.572 isotropic

3.8

0.053 0.069 1.575 ∼oblate

3.3

0.054 0.076 1.570 ∼oblate

a Prepared from a chloroform solution (c ) 20 mg/mL) by casting, waiting, and spinning. Irradiated at 488 nm, 88 mW/cm2 for 12 h. Annealed at 73 °C for 12 d.

TABLE 8: Three-Dimensional Indicatrix of a Film of Polymer VIa treatment

nx

ny

nz

irradiated+ 1.577 1.640 1.549 annealed a

d/µm 1.2

∆nzx

∆nyx

Σni/3

-0.028 0.063 1.589 biaxial

Irradiation see text.

refractive indices of the two less populated directions was measured to be only ∆nzx ) 0.002. Annealing of this nearly in-plane prolate film (positively uniaxial) for 50 min at 73 °C, however, led to a decrease of the birefringence in the film plane. It is possible, that the reason for this behavior could be the short irradiation time and the filter effect caused by the high volume density of the azobenzene group leading to a gradient in the film. An example for a film, which had been irradiated for a longer time, is shown in Table 7. Here annealing at 73 °C, resulted in a small amplification of the photoinduced biaxial order. Annealing at 85 °C or above led to orders that were characterized by a highest population of the z direction for films of thicknesses of 1.2 µm and 1.7 µm. This behavior was independent of the initial order of the freshly prepared films. The birefringences in the plane were only weakly amplified. But annealing of a 730 nm thin film at 83 °C after photoorientation finally resulted in a preferential population of the in-plane y direction, and the order was between an oblate and a prolate in-plane orientation. The irradiation conditions were with 25 s and 250 mW/cm2 similar to the above-mentioned 2.9 µm film. The difference is the higher initial preference for the in-plane directions in the thinner film (∆nzx ) -0.015 instead of -0.007), which was maintained after annealing. Investigation of the Liquid Crystalline Polyester VI. The polymer VI with a 4-alkoxy-4′-cyano-azobenzene group contains an octyl spacer in the side chain and an undecyl spacer in the main chain. It is known that this polymer has a high inclination toward microphase separation and aggregation.34 The polymer shows a pronounced tendency to form J aggregates resulting in a significant bathochromic shift of the absorbance, which can be destroyed by the photoisomerization.35 Photoorientation of a 1.2 µm thick film at a power density of 35 mW/cm2 for 5 min resulted in an order in which the order perpendicular to the film plane was even least populated (Table 8). An amplification of the induced order was already accomplished during the irradiation as the glass transition was close to the room temperature and further decreased by the photo-isomerization. The birefringence in the film plane had a value of ∆nyx ) 0.063 and the difference between the refractive indices in the z and x direction was ∆nzx ) -0.028. (It was not possible to change the orientation of the sample after the first prism coupler measurement, because the polymer sticks to the prism. Therefore only nx and nz were measured, ny was determined from these values and the known36 averaged

refractive index of 1.589. Obviously, the interaction of the azobenzene side chains with the glass substrate is enabled by the long main chain spacer. 4. Conclusions A great variety of three-dimensional orders can be generated by photoorientation and annealing in the liquid crystalline state of azobenzene containing polymer films. This type of investigation was performed for the first time by us, and the orders include biaxial, homeotropic, in-plane prolate, and oblate orders, depending on the irradiation and annealing conditions. The determined initial orders before irradiation were small and mainly dependent on the polymer structure. The order after photoorientation was nearly oblate and dependent on the polymer structure and the initial order. Small deviations from this order were found. Anisotropy gradients could be induced in thick films exploiting the filter effect of the polymers and were for the first time investigated by us using the WKB method. Annealing for short times or at low temperatures led to a local amplification of the photoinduced orders. Annealing for longer times or at higher temperatures led for thicker films to a homeotropic order and for thinner films to an amplification of the in-plane birefringence. For the homeotropic order in one of the polymers the air interface could be made responsible. An exception was the polymer VI, which is preferentially oriented parallel to the film plane, which is probably caused by specific design and specific ordering processes including J aggregation. The results clearly demonstrate the dependence of the threedimensional order after annealing and conventional photoorientation on irradiation time, annealing time, temperature, initial order, interfacial interactions, and chemical structure of the polymers. These key parameters have been identified for the first time by us. By controlled variation of these parameters, defined three-dimensional indicatrices can be obtained. Finally, we would like to note, that recently a new method (ITE: immersion transmission ellipsometry37) has been developed for the investigation of the three-dimensional anisotropy of films, which are too thin to be investigated by waveguide spectroscopy. Acknowledgment. We are grateful for the synthesis of the polymers to S. Kostromin, Bayer AG, Leverkusen (I and II), S. Czapla, Technical University of Berlin38 (III), R. Ruhmann, microresist technology GmbH, Berlin (IV, V) and S. Hvilsted, Technical University of Denmark, Lyngby (VI). Note Added after ASAP Publication. This article was published ASAP on April 2, 2005. Changes were made to page numbers in refs 16 and 23. The correct version was posted on April 6, 2005. References and Notes (1) Fuhrmann, T.; Wendorff, J. H. Intern. J. Polymeric Mater. 2000, 45, 621. (2) Haarer, D.; Bieringer, T.; Eisenbach, C.; Fischer, K.; Ruhmann, R.; Wuttke, R.; Stumpe, J.; Fischer, T.; La¨sker, L.; Rutloh, M.; Claussen, U. German patent 1994, DE 4431823 A1. (3) Moia, F.; Seiberle, H.; Schadt, M. Proc. SPIE, 2000, 3973, 196. (4) Broer, D. J.; van Haaren, J. A. M. M.; Bastiaansen, C. W. M. http:// www.e-polymers.org/papers/broer_301201.pdf, 2001, 23. (5) Rosenhauer, R.; Fischer, T.; Stumpe, J. Proc. SPIE 2004, 5213, 169. (6) Schmidt, H.-W. AdV. Mater. 1989, 7, 218. (7) Seiberle, H.; Benecke, C.; Bachels, T. SID 03 Digest 2003, 1162. (8) Broer, D. J.; Lub, J.; Mol, G. N. Nature 1995, 378, 467. (9) Ichimura, K. Chem. ReV. 2000, 100, 1847.

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