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Banded texture of photo-aligned azobenzene-containing side-chain liquid crystalline polymers
Polymer 45 (2004) 4331–4336 www.elsevier.com/locate/polymer
Banded texture of photo-aligned azobenzene-containing side-chain liquid crystalline polymers Jian Liua,b, Kai Suna,b, Zenchang Lia,b, Jiangang Gaoa,b, Wei Sub, Jun Yangc, Jiangying Zhangc, Pei Wangc, Qijin Zhangb,* a
b
Structure Research Laboratory, University of Science and Technology of China, Hefei, Anhui, 230026, People’s Republic of China Department of Polymer Science and Engineering, University of Science and Technology of China, Hefei, Anhui, 230026, People’s Republic of China c Department of Physics, University of Science and Technology of China, Hefei, Anhui, 230026, People’s Republic of China Received 5 January 2004; received in revised form 30 March 2004; accepted 2 April 2004
Abstract Poly(6-[4-(4-cyanophenylazo)phenoxy]x-methylene methacrylate) (Px, x is 2 or 6) were synthesized and used to study the texture of sidechain liquid crystalline polymers (SCLCPs) after photoinduced orientation. With increasing temperature of irradiated P2 film up to 125 8C, just above its Tg ; 123 8C, a banded texture was observed for SCLCPs under polarized optical microscope (POM). There is no banded texture of irradiated P6 (Tg ; 72 8C) film when heating to 75 8C. The formation of the banded texture is considered to be due to co-existing of in-plane and out-of-plane orientation mesogens in P2 film, which is explained in terms of a molecular model. Based on our experimental results, factors of irradiation light intensity, film thickness, and heating rate on the formation of the banded texture were studied in this work. q 2004 Elsevier Ltd. All rights reserved. Keywords: Side-chain liquid crystalline polymers; Banded texture; Orientation
1. Introduction Azobenzene-containing polymers have attracted considerable attention in the past decades for the possibility of changing the orientation by irradiation with linear polarized laser (LPL) [1 – 6], and the potential applications include reversible optical storage [7], holographic grating [8] and optical switching [9]. During the photo-induced alignment process azobenzene groups would rearrange perpendicular to the laser polarization direction by repeated trans– cis – trans isomerization of the groups. It is worth to note that there are two directions (in-plane and out-of-plane) perpendicular to the polarization direction, and, theoretically, the photoinduced orientation could be obtained at both directions simultaneously. Probability for out-of-plane orientation was controlled by structures of polymers and mesogens, irradiation light, substrate, and film thickness etc., which has not been detected by texture observation although there have been studies concerning the orientation of out-of -plane in the past years [10 –13]. * Corresponding author. Tel./fax: þ 86-5513601704. E-mail address: [email protected] (Q. Zhang). 0032-3861/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2004.04.003
Texture is the observed defects of liquid crystal (LC, generic name given to the imperfection of point, line or wall-like defects, configurations or solitons, etc.) under polarized optical microscope (POM); Friedel suggested classifying the texture structure as nematic-, cholsteric-, and semectic- phase structures according to each LC special optical view under POM [14]. Liquid crystalline polymers have the same topologically stable defects as low molecular weight liquid crystals. Each liquid crystalline phase forms characteristic structures and defects, the textures include information about mesogen distribution, layer structures and optical properties of the liquid crystalline domains [15]. Generally speaking, Tg of SCLCPs is higher than room temperature, freshly made films of SCLCPs under room temperature with no thermal treatment are dark between crossed POM under Tg : Although LPL irradiation can cause birefringence changing, no texture and only blank view can be observed under Tg ; no matter the polymer is the amorphous [16], or the liquid crystalline polymer [17,18]. When sample temperature rises above Tg ; texture can be formed in liquid crystalline phase. The texture is dependent not only on the chemical structure of the polymers, the history of thermal treatment, but also on, in our opinion, the
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Chart 1.
process of LPL irradiation. There are little work concerning the latter aspect, and on the other hand, realizing the effect of photoinduced orientation on the texture of SCLCPs is helpful to understand the mesogen motion driven by LPL. In this work, the texture change of azobenzene containing polymer was examined before and after photoinduced alignment under LPL, factors on the formation of texture, such as light energy density, film thickness, and heating rate, were also discussed.
G500H8) which is calibrated with standard polystyrenes, solvent was THF. Texture observation was carried out under Nikon YS2 POM equipped with a Nikon Coolpix E995 digital camera. Photoinduced anisotropy was introduced in SCLCPs films using LPL (Nd-YAG, 532 nm) and was probed in real-time by a light (3 mW) at 650 nm during irradiation; a standard experimental setup [10] was used. A mask with a 4 £ 4 mm diameter hole was used to select irradiation. Sample films were made by solution casting method, after solvent evaporating, the transparence film was dried under vacuum at room temperature for one week to eliminate the residual solvent. The film with different thickness was obtained by controlling the concentration of polymer solutions. UV – vis absorption spectra were made on Shimadzu UV-2401PC spectrophotometer, and the film thickness is about 20 mm.
3. Results and discussion 2. Experiment 2.1. Materials Polymers (Px, x is 2 or 6) were synthesized according to the previous literature [19]. Homopolymers were obtained by polymerization in anhydrous THF at 60 8C for 48 h with AIBN (1 wt%) as initiator. Purification was performed for three times by dissolution in THF and precipitation from methanol, finally dried under vacuum at 50 8C. 2.2. Characterization A Perkin– Elmer DSC 6 was used to determine the phase transition of samples. The heating and cooling rates was 10 8C/min, three times were performed to reduce thermal history. The molecular weights were measured by GPC (Toyo Soda HLC-802; column, GMH6 £ 2 þ G4000H8 þ
Fig. 1. The formation of banded texture of P2: (A) irradiated film at room temperature (25 8C); heating the irradiated film to 120 8C (B); 125 8C (C); 130 8C (D). Heating rate: 5 8C/min. Film thickness: 20 mm.
The structure of Px is shown in Chart 1, donor ( –O – ) and acceptor (– CN) groups at 4 and 40 positions, respectively, of each azobenzene group, with the expectation that electron donor – acceptor interactions may enhance the thermal stability of the liquid crystalline phase of the polymers [20,21]. Molecular weights of two polymers were determined by GPC, the result is also shown in Chart 1. DSC scans were obtained during the second cooling cycle, the glass transitions and clear transitions are G123N175I and G72N169I for P2 and P6, respectively. The LC to isotropic phase transition enthalpy ðDHNI Þ for each polymer is: P2: 7.49 kJ/mol; P6: 19.6 kJ/mol. No other transitions were detected from the DSC scans [19]. 3.1. Banded texture under POM Anisotropy was induced in P2 film by LPL at room temperature, and blank view was observed under POM, there is no any texture structure observable, as shown in Fig. 1(A). Comparing with a dark view image in non-irradiated film (not show here), Fig. 1(A) suggests that mesogens orientated under LPL irradiation. When the irradiated film was heated above Tg ; a banded texture was observed under POM, as shown in Fig. 1(C) and Fig. 1(D), bands run along the direction of electric vector of LPL. Banded texture is a characteristic texture of rigid or semiflexible chain polymers, and the formation of a banded texture results, generally, from shearing of either a lyotropic or thermotropic rigid liquid crystalline polymer [22 –26]. The banded texture indicates one kind of periodic orientation of LC mesogens, which leads to a spatial variation of the effective birefringence, and the molecules in the stripe are perpendicular to the run direction of strip [22, 23]. Compared with non pre-irradiation P2, which shows a nematic texture when heating the film above Tg ; the formation of the banded texture is the result of photoinduced
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Fig. 4. Illustrative representation of mesogens self-assembling for forming periodical ordered micro-domain when heating the irradiated film above Tg :
Fig. 2. Banded textures of P2 under POM when rotating 908 clockwise: (A) rotating 158; (B) 308; (C) 458; (D) 608; (E) 758; (F) 908. Films thickness is 20 mm; a spot was point out by arrow as an indicator. Pp: POM polarizer direction; Pa: POM analyzer direction; Pl: electric vector of LPL.
mesogens orientation as well as the thermal treatment. It is well known that in light induced orientation the mesogens long axis is perpendicular to the electric vector of LPL, and a spatial variation of the effective birefringence is impossible if the photoinduced alignment takes place only in one direction. The formation of the banded texture may be resulted from a biaxial orientation (in-plane and out-ofplane) of P2 sample. It is worth to note that there is no such a banded texture in P6 sample as shown as in Fig. 6(E) for the pre-irradiated film and Fig. 6(F) for the non-irradiated film, respectively, when the film temperature is raised above Tg : When rotating the sample on the stage of POM, the banded texture was still observed as shown in Fig. 2., which
Fig. 3. UV –vis absorption spectra of P2 film, a: casting film before irradiation; b: with LPL irradiation (exposure dose: 20 mW/cm2) to stable transmittance value; c: annealing from T . Tg for pre-irradiation film. Film thickness is 20 mm.
shows that mesogens are not aligned homogeneously. It is worth to point out that for a sample film only with in-plane alignment a dark view should be observed under POM by rotating to 458 because the long axis direction of mesogens (Pl) is in the same direction of polarizer (Pp), which induce the extinction of aligned film under crossed POM. However, a bright view is still observed as shown in Fig. 2(C), which suggests that there is another kind of alignment of mesogens other than the alignment of in-plane. It has been known that there are two directions (in-plane and out-of-plane) perpendicular to the polarization direction of LPL, and, theoretically, the photoinduced orientation could be obtained at both directions simultaneously. Biaxial orientation (in-plane and out-of-plane) used to be observed in SCLCPs [12,13]. It is easily deduced that biaxial orientation (in-plane and out-of-plane) of P2 has also been induced by LPL because a bright view can be still observed although the extinction of in-plane aligned mesogens and out-ofplane orientated mesogen are responsible to the banded stripe. A further demonstration of out-of-plane orientation can be obtained by UV –vis spectra measurement. In Fig. 3(a), obvious saturated absorption in the thick film before irradiation suggests most mesogens are arranged in plane for the fresh film. The decrease absorption at 363 nm (trans azobenzene p –pp transition) after LPL irradiation (Fig. 3(b)) was resulted from out-of-plane orientation of mesogens [10,12,13]. Further decrease in the absorption of a thermal annealing sample is shown in Fig. 3(c), and the annealing sample film shows a banded texture under POM. The latter result shows that the formation of banded texture induce more mesogens align in the direction out-of-plane by self-assembling for an order domain, which results in the further decrease in the absorption from at 363 nm [10]. According to the analyses above, a possible mechanism of the formation of the banded texture in P2 can be draw out by a molecular model shown in Fig. 4. Under LPL irradiation, biaxial orientation of P2 was induced (Fig. 4, left): some mesogens aligned in the way of in-plane, some mesogens aligned in the tilted way, which is the direction of irradiating direction of LPL. Both of them have their dipolar moment perpendicular to the electric vector of LPL [13]. With the thermal treatment, titled mesogens from out-ofplane orientation and in-plane mesogens will self-organize, respectively with other mesogens around them to form well order LC domains, which results in periodic well-order
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Fig. 5. Photoinduced anisotropy for P2 and P6: P2 a: 10 mW/cm2, b: 4 mW/cm2 , c: 1 mW/cm2 , P6 d: 50 mW/cm 2 , e: 20 mW/cm2 , f: 10 mW/cm2. Films thickness: 20 mm. Probe light: 650 nm, 3 mW/cm2.
regions as shown in Fig. 4 (right). This self-assembly in molecular level would result in a periodic sub-microstructure, which shows the banded texture under POM (Fig. 2.). 3.2. Effect of light irradiation It is well known that the transmittance of SCLCPs film will increase under irradiation of LPL. Generally, the transmittance will increases with irradiation time, and gradually reach a saturation value. Fig. 5 shows the relationship curve between transmittance and irradiation time of P2 and P6 under various irradiation conditions, from which it is clear that general photoinduced orientation process can be obtained for P6 film under light energy density lower than 50 mW/cm2, and for P2 film lower than 1 mW/cm2. However, with the light energy density higher
than 1 mW/cm2, the transmittance of P2 film is increased fast at first, and before reaching the saturation value a decrease in the transmittance can be observed even the sample is still under irradiation. This phenomenon is also observed in other works, which is assigned to homeotropic arrangement of mesogen in the film [27]. This over saturation phenomenon of P2 was thought to be caused by biaxial orientation of mesogens. Taking the observation on textures above into consideration it was found that this special phenomenon is connected with the banded texture. When irradiation was adopted with lower light energy density than the threshold of 1 mW/cm2 for the over saturation orientation result, the irradiated sample of P2 did not show any banded texture after thermal treatment. Fig. 6(B) is the result of such observation with light energy density being 0.5 mW/cm2, and when the sample was irradiated by a light with energy density of 5 mW/cm2, higher than the threshold for the over saturation orientation, a banded texture is observed again, as shown in Fig. 6(C). However, there is no banded texture detected in P6 film even light energy density is as high as 50 mW/cm2 (Fig. 6(E)). This result suggests that no out-of-plane orientation induced in P6 film, and the transmittance curve in Fig. 5(d) also confirms the result, which suggests the length of spacer in SCLCPs is a key role in biaxial orientation. With longer spacer, micro-domain will be formed in P6 solution-casting film, and it is difficult for a whole micro-domain to fulfill out-of-plane orientation. At the same time the orientation degree in P6 is also small than that in P2 with the same irradiated time and light energy. Different from P6, shorter spacer in P2 made interaction between mesogens be weakened by polymer chain in some extent [27], and there is no micro-domain in its solution-casting film. Under LPL irradiation there are more mesogens aligned in the way of out-of-plane. Detailed research work is still being performed in our groups in this aspect. 3.3. Effect of thermal treatments
Fig. 6. POM observation of annealed P2 and P6 film: (A) P2 without preirradiation; (B) P2 with 0.5 mW/cm2 LPL irradiation; (C) P2 with 5 mW/cm2 LPL irradiation; (D) P6 with 50 mW/cm2 LPL irradiation. Film thickness is 20 mm. Thermal treatment: P6 samples, heating at rate of 5 8C/min from 25 to 75 8C, keep at 75 8C for 2 h, and annealing to room temperature. P2 samples, at rate of 5 8C/min from 25 to 130 8C, keep at 130 8C for 2 h, and annealing to room temperature.
From Fig. 1(A) and (B), only slight change can be detected because the motion of mesogens is constrained under temperature below Tg (123 8C) [28]. At above Tg the motion of polymer chain and large mount of mass redistribution are possible [29 – 34], a banded texture of P2 is observed instantly when the temperature is raised above Tg (Fig. 1(C) and (D)). Such a fast formation of a banded texture suggests there is no large mount of polymer main chain diffusion. When the film temperature is raised above Tg ; both the out-of-plane orientated mesogens and in-plane orientated mesogens will self-assembly, respectively with other mesogens around them for forming order domain. The driver force is the potential liquid crystallinity, which makes mesogens have a tendency to form a well-ordered LC micro domain. Cooperative motion induced reorientation of mesogens forming an order domain, and further, the domain that was assembled by in-plane aligned mesogens has its
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Fig. 7. Banded texture of P2 with a stable transmittance after irradiation at light power of 30 mW/cm2 with different heating rate at: (A): 1 8C/min; (B) 10 8C/min; (C) 20 8C/min; (D) 30 8C/min when heating from 25 to 130 8C, keeping at 130 8C for 2 h, annealing to the room temperature. Film thickness: 20 mm.
optical axis in the direction of in-plane and perpendicular to LPL, the domain that was assembled by out-of-plane aligned mesogens has its optical axis in the direction of out-of-plane and also perpendicular to LPL. This alternant order domain leads to a spatial variation of the effective birefringence, which shows a banded texture under POM. It should be pointed out that the formation of the order domain is also affected by the motion of the main chain of polymers. Based on the observation above, a well-ordered LC micro domain can be formed only above Tg with the motion of main chain, although a few mesogens are also assembled at the heating process below Tg : However, the motion of the main chain is constrained by the linked mesogens selfassembled in the order domain when the film temperature is above Tg : This conclusion can be further confirmed by following observations. There is little change in the texture when keeping sample at 130 8C (Fig. 1(D)). Moreover, even when annealing films at any temperature between Tg , Ti ; the banded texture was still observed and the width of the band is not changing, which show the stability of periodic alignment in LC phase. When film was heated above Ti ; a dark view observed under POM, high mobility of main chain above Ti destroys the order LC domain, the polymer is in isotropy phase. The heating rate at the thermal treatment process also has an effect on the formation and the width of the stripes of the
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Fig. 9. Photoiuduced anisotropy of P2 films with various film thicknesses under 20 mW/cm2 LPL irradiation. Probe laser wavelength is 650 nm, 3 mW/cm2.
banded texture. When heating rate is below 20 8C/min, the band width of banded texture is decreased from 5 mm (1 8C/min) to 1 mm (10 8C/min) (Fig. 7(A) and (B)), which suggests that large order domain formed at a slow heating rate. With heating rate of 20 8C/min, only an illegible banded texture can be observed. And when heating rate reached 30 8C/min, no any banded texture can be observed under the observation condition as shown in Fig. 7(D). When the heating rate is fast, out-of-plane aligned or inplane aligned mesogens has no enough time to adjustment each other for forming a periodic order domain. Distinction between the two kind of aligned mesogens vanished, which results in a homogeneous image under POM (Fig. 7(D)). The effect of heating rate within temperature range of room temperature to 130 8C confirms that the formation of the banded texture was a local adjustment of the side chain, because the motion of polymer main chain is restricted within such a temperature range. 3.4. Effect of film thickness In Fig. 8., the film thickness is 1, 10, and 20 mm, respectively, and the banded texture is clearly seen in Fig. 8(B) and (C). According to the transmittance result in Fig. 9, there is no the over saturation phenomenon for the samples with film thickness lower than 1 mm, which suggests that there be no out-of-plane orientation under LPL irradiation, in Fig. 8(A) there is no such a texture observed for 1 mm film. This result is identical with conclusion that film
Fig. 8. Texture of P2 with different thickness: (A) 1 mm; (B) 10 mm; (C) 20 mm. LPL: 20 mW/cm2, heating rate: 5 8C/min.
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thickness also has an effect on the photoinduced orientations in the thin film [35]. The result, shown in Fig. 9, indicates that the out-of-plane orientation is affected by interactions between mesogens and the substrate. When the film is thin, the strong dipole moment interaction between mesogens and the glass substrate makes mesogens be in the state of parallel to the substrate and difficult to achieve the out-ofplane orientation under irradiation of LPL.
4. Conclusions Out-of-plane orientation of azobenzene-containing SCLCPs (P2) plays an important role during photoinduced alignment by LPL at 532 nm. When heating irradiated films above Tg ; at a nematic state, potential liquid crystallinity induces the rearrange of mesogens to form liquid crystal micro domain. In the thermal treatment mesogens in out-ofplane and in-plane orientated states self-assembly mesogens around them to form well order domain, respectively, and each of two kind domains has its own optical axis direction, which results in periodic well-order regions observed as a banded texture under POM. The threshold of light energy density inducing over-saturation for P2 is about 1 mW/cm2. After irradiation of LPL with higher power than the threshold value, banded texture can be observed under POM after thermal treatment. Thickness of sample film has an effect on the banded texture, and the banded texture is clearly seen when the film thickness is 10 and 20 mm. When heating rate is below 20 8C/min, the band width of the banded texture is decreased from 5 to 1 mm with heating rate increasing from 1 to 10 8C/min, showing heating rate is also a factor in forming banded texture.
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Acknowledgements
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This work was supported by the National Science Foundation of China (Grant Nos. 50025309 and 90201016). The authors are grateful for the financial support and express their thanks to the referees for critically reviewing the manuscript and making important suggestions.
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Banded texture of photo-aligned azobenzene-containing side-chain liquid crystalline polymers Jian Liua,b, Kai Suna,b, Zenchang Lia,b, Jiangang Gaoa,b, Wei Sub, Jun Yangc, Jiangying Zhangc, Pei Wangc, Qijin Zhangb,* a
b
Structure Research Laboratory, University of Science and Technology of China, Hefei, Anhui, 230026, People’s Republic of China Department of Polymer Science and Engineering, University of Science and Technology of China, Hefei, Anhui, 230026, People’s Republic of China c Department of Physics, University of Science and Technology of China, Hefei, Anhui, 230026, People’s Republic of China Received 5 January 2004; received in revised form 30 March 2004; accepted 2 April 2004
Abstract Poly(6-[4-(4-cyanophenylazo)phenoxy]x-methylene methacrylate) (Px, x is 2 or 6) were synthesized and used to study the texture of sidechain liquid crystalline polymers (SCLCPs) after photoinduced orientation. With increasing temperature of irradiated P2 film up to 125 8C, just above its Tg ; 123 8C, a banded texture was observed for SCLCPs under polarized optical microscope (POM). There is no banded texture of irradiated P6 (Tg ; 72 8C) film when heating to 75 8C. The formation of the banded texture is considered to be due to co-existing of in-plane and out-of-plane orientation mesogens in P2 film, which is explained in terms of a molecular model. Based on our experimental results, factors of irradiation light intensity, film thickness, and heating rate on the formation of the banded texture were studied in this work. q 2004 Elsevier Ltd. All rights reserved. Keywords: Side-chain liquid crystalline polymers; Banded texture; Orientation
1. Introduction Azobenzene-containing polymers have attracted considerable attention in the past decades for the possibility of changing the orientation by irradiation with linear polarized laser (LPL) [1 – 6], and the potential applications include reversible optical storage [7], holographic grating [8] and optical switching [9]. During the photo-induced alignment process azobenzene groups would rearrange perpendicular to the laser polarization direction by repeated trans– cis – trans isomerization of the groups. It is worth to note that there are two directions (in-plane and out-of-plane) perpendicular to the polarization direction, and, theoretically, the photoinduced orientation could be obtained at both directions simultaneously. Probability for out-of-plane orientation was controlled by structures of polymers and mesogens, irradiation light, substrate, and film thickness etc., which has not been detected by texture observation although there have been studies concerning the orientation of out-of -plane in the past years [10 –13]. * Corresponding author. Tel./fax: þ 86-5513601704. E-mail address: [email protected] (Q. Zhang). 0032-3861/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2004.04.003
Texture is the observed defects of liquid crystal (LC, generic name given to the imperfection of point, line or wall-like defects, configurations or solitons, etc.) under polarized optical microscope (POM); Friedel suggested classifying the texture structure as nematic-, cholsteric-, and semectic- phase structures according to each LC special optical view under POM [14]. Liquid crystalline polymers have the same topologically stable defects as low molecular weight liquid crystals. Each liquid crystalline phase forms characteristic structures and defects, the textures include information about mesogen distribution, layer structures and optical properties of the liquid crystalline domains [15]. Generally speaking, Tg of SCLCPs is higher than room temperature, freshly made films of SCLCPs under room temperature with no thermal treatment are dark between crossed POM under Tg : Although LPL irradiation can cause birefringence changing, no texture and only blank view can be observed under Tg ; no matter the polymer is the amorphous [16], or the liquid crystalline polymer [17,18]. When sample temperature rises above Tg ; texture can be formed in liquid crystalline phase. The texture is dependent not only on the chemical structure of the polymers, the history of thermal treatment, but also on, in our opinion, the
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Chart 1.
process of LPL irradiation. There are little work concerning the latter aspect, and on the other hand, realizing the effect of photoinduced orientation on the texture of SCLCPs is helpful to understand the mesogen motion driven by LPL. In this work, the texture change of azobenzene containing polymer was examined before and after photoinduced alignment under LPL, factors on the formation of texture, such as light energy density, film thickness, and heating rate, were also discussed.
G500H8) which is calibrated with standard polystyrenes, solvent was THF. Texture observation was carried out under Nikon YS2 POM equipped with a Nikon Coolpix E995 digital camera. Photoinduced anisotropy was introduced in SCLCPs films using LPL (Nd-YAG, 532 nm) and was probed in real-time by a light (3 mW) at 650 nm during irradiation; a standard experimental setup [10] was used. A mask with a 4 £ 4 mm diameter hole was used to select irradiation. Sample films were made by solution casting method, after solvent evaporating, the transparence film was dried under vacuum at room temperature for one week to eliminate the residual solvent. The film with different thickness was obtained by controlling the concentration of polymer solutions. UV – vis absorption spectra were made on Shimadzu UV-2401PC spectrophotometer, and the film thickness is about 20 mm.
3. Results and discussion 2. Experiment 2.1. Materials Polymers (Px, x is 2 or 6) were synthesized according to the previous literature [19]. Homopolymers were obtained by polymerization in anhydrous THF at 60 8C for 48 h with AIBN (1 wt%) as initiator. Purification was performed for three times by dissolution in THF and precipitation from methanol, finally dried under vacuum at 50 8C. 2.2. Characterization A Perkin– Elmer DSC 6 was used to determine the phase transition of samples. The heating and cooling rates was 10 8C/min, three times were performed to reduce thermal history. The molecular weights were measured by GPC (Toyo Soda HLC-802; column, GMH6 £ 2 þ G4000H8 þ
Fig. 1. The formation of banded texture of P2: (A) irradiated film at room temperature (25 8C); heating the irradiated film to 120 8C (B); 125 8C (C); 130 8C (D). Heating rate: 5 8C/min. Film thickness: 20 mm.
The structure of Px is shown in Chart 1, donor ( –O – ) and acceptor (– CN) groups at 4 and 40 positions, respectively, of each azobenzene group, with the expectation that electron donor – acceptor interactions may enhance the thermal stability of the liquid crystalline phase of the polymers [20,21]. Molecular weights of two polymers were determined by GPC, the result is also shown in Chart 1. DSC scans were obtained during the second cooling cycle, the glass transitions and clear transitions are G123N175I and G72N169I for P2 and P6, respectively. The LC to isotropic phase transition enthalpy ðDHNI Þ for each polymer is: P2: 7.49 kJ/mol; P6: 19.6 kJ/mol. No other transitions were detected from the DSC scans [19]. 3.1. Banded texture under POM Anisotropy was induced in P2 film by LPL at room temperature, and blank view was observed under POM, there is no any texture structure observable, as shown in Fig. 1(A). Comparing with a dark view image in non-irradiated film (not show here), Fig. 1(A) suggests that mesogens orientated under LPL irradiation. When the irradiated film was heated above Tg ; a banded texture was observed under POM, as shown in Fig. 1(C) and Fig. 1(D), bands run along the direction of electric vector of LPL. Banded texture is a characteristic texture of rigid or semiflexible chain polymers, and the formation of a banded texture results, generally, from shearing of either a lyotropic or thermotropic rigid liquid crystalline polymer [22 –26]. The banded texture indicates one kind of periodic orientation of LC mesogens, which leads to a spatial variation of the effective birefringence, and the molecules in the stripe are perpendicular to the run direction of strip [22, 23]. Compared with non pre-irradiation P2, which shows a nematic texture when heating the film above Tg ; the formation of the banded texture is the result of photoinduced
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Fig. 4. Illustrative representation of mesogens self-assembling for forming periodical ordered micro-domain when heating the irradiated film above Tg :
Fig. 2. Banded textures of P2 under POM when rotating 908 clockwise: (A) rotating 158; (B) 308; (C) 458; (D) 608; (E) 758; (F) 908. Films thickness is 20 mm; a spot was point out by arrow as an indicator. Pp: POM polarizer direction; Pa: POM analyzer direction; Pl: electric vector of LPL.
mesogens orientation as well as the thermal treatment. It is well known that in light induced orientation the mesogens long axis is perpendicular to the electric vector of LPL, and a spatial variation of the effective birefringence is impossible if the photoinduced alignment takes place only in one direction. The formation of the banded texture may be resulted from a biaxial orientation (in-plane and out-ofplane) of P2 sample. It is worth to note that there is no such a banded texture in P6 sample as shown as in Fig. 6(E) for the pre-irradiated film and Fig. 6(F) for the non-irradiated film, respectively, when the film temperature is raised above Tg : When rotating the sample on the stage of POM, the banded texture was still observed as shown in Fig. 2., which
Fig. 3. UV –vis absorption spectra of P2 film, a: casting film before irradiation; b: with LPL irradiation (exposure dose: 20 mW/cm2) to stable transmittance value; c: annealing from T . Tg for pre-irradiation film. Film thickness is 20 mm.
shows that mesogens are not aligned homogeneously. It is worth to point out that for a sample film only with in-plane alignment a dark view should be observed under POM by rotating to 458 because the long axis direction of mesogens (Pl) is in the same direction of polarizer (Pp), which induce the extinction of aligned film under crossed POM. However, a bright view is still observed as shown in Fig. 2(C), which suggests that there is another kind of alignment of mesogens other than the alignment of in-plane. It has been known that there are two directions (in-plane and out-of-plane) perpendicular to the polarization direction of LPL, and, theoretically, the photoinduced orientation could be obtained at both directions simultaneously. Biaxial orientation (in-plane and out-of-plane) used to be observed in SCLCPs [12,13]. It is easily deduced that biaxial orientation (in-plane and out-of-plane) of P2 has also been induced by LPL because a bright view can be still observed although the extinction of in-plane aligned mesogens and out-ofplane orientated mesogen are responsible to the banded stripe. A further demonstration of out-of-plane orientation can be obtained by UV –vis spectra measurement. In Fig. 3(a), obvious saturated absorption in the thick film before irradiation suggests most mesogens are arranged in plane for the fresh film. The decrease absorption at 363 nm (trans azobenzene p –pp transition) after LPL irradiation (Fig. 3(b)) was resulted from out-of-plane orientation of mesogens [10,12,13]. Further decrease in the absorption of a thermal annealing sample is shown in Fig. 3(c), and the annealing sample film shows a banded texture under POM. The latter result shows that the formation of banded texture induce more mesogens align in the direction out-of-plane by self-assembling for an order domain, which results in the further decrease in the absorption from at 363 nm [10]. According to the analyses above, a possible mechanism of the formation of the banded texture in P2 can be draw out by a molecular model shown in Fig. 4. Under LPL irradiation, biaxial orientation of P2 was induced (Fig. 4, left): some mesogens aligned in the way of in-plane, some mesogens aligned in the tilted way, which is the direction of irradiating direction of LPL. Both of them have their dipolar moment perpendicular to the electric vector of LPL [13]. With the thermal treatment, titled mesogens from out-ofplane orientation and in-plane mesogens will self-organize, respectively with other mesogens around them to form well order LC domains, which results in periodic well-order
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Fig. 5. Photoinduced anisotropy for P2 and P6: P2 a: 10 mW/cm2, b: 4 mW/cm2 , c: 1 mW/cm2 , P6 d: 50 mW/cm 2 , e: 20 mW/cm2 , f: 10 mW/cm2. Films thickness: 20 mm. Probe light: 650 nm, 3 mW/cm2.
regions as shown in Fig. 4 (right). This self-assembly in molecular level would result in a periodic sub-microstructure, which shows the banded texture under POM (Fig. 2.). 3.2. Effect of light irradiation It is well known that the transmittance of SCLCPs film will increase under irradiation of LPL. Generally, the transmittance will increases with irradiation time, and gradually reach a saturation value. Fig. 5 shows the relationship curve between transmittance and irradiation time of P2 and P6 under various irradiation conditions, from which it is clear that general photoinduced orientation process can be obtained for P6 film under light energy density lower than 50 mW/cm2, and for P2 film lower than 1 mW/cm2. However, with the light energy density higher
than 1 mW/cm2, the transmittance of P2 film is increased fast at first, and before reaching the saturation value a decrease in the transmittance can be observed even the sample is still under irradiation. This phenomenon is also observed in other works, which is assigned to homeotropic arrangement of mesogen in the film [27]. This over saturation phenomenon of P2 was thought to be caused by biaxial orientation of mesogens. Taking the observation on textures above into consideration it was found that this special phenomenon is connected with the banded texture. When irradiation was adopted with lower light energy density than the threshold of 1 mW/cm2 for the over saturation orientation result, the irradiated sample of P2 did not show any banded texture after thermal treatment. Fig. 6(B) is the result of such observation with light energy density being 0.5 mW/cm2, and when the sample was irradiated by a light with energy density of 5 mW/cm2, higher than the threshold for the over saturation orientation, a banded texture is observed again, as shown in Fig. 6(C). However, there is no banded texture detected in P6 film even light energy density is as high as 50 mW/cm2 (Fig. 6(E)). This result suggests that no out-of-plane orientation induced in P6 film, and the transmittance curve in Fig. 5(d) also confirms the result, which suggests the length of spacer in SCLCPs is a key role in biaxial orientation. With longer spacer, micro-domain will be formed in P6 solution-casting film, and it is difficult for a whole micro-domain to fulfill out-of-plane orientation. At the same time the orientation degree in P6 is also small than that in P2 with the same irradiated time and light energy. Different from P6, shorter spacer in P2 made interaction between mesogens be weakened by polymer chain in some extent [27], and there is no micro-domain in its solution-casting film. Under LPL irradiation there are more mesogens aligned in the way of out-of-plane. Detailed research work is still being performed in our groups in this aspect. 3.3. Effect of thermal treatments
Fig. 6. POM observation of annealed P2 and P6 film: (A) P2 without preirradiation; (B) P2 with 0.5 mW/cm2 LPL irradiation; (C) P2 with 5 mW/cm2 LPL irradiation; (D) P6 with 50 mW/cm2 LPL irradiation. Film thickness is 20 mm. Thermal treatment: P6 samples, heating at rate of 5 8C/min from 25 to 75 8C, keep at 75 8C for 2 h, and annealing to room temperature. P2 samples, at rate of 5 8C/min from 25 to 130 8C, keep at 130 8C for 2 h, and annealing to room temperature.
From Fig. 1(A) and (B), only slight change can be detected because the motion of mesogens is constrained under temperature below Tg (123 8C) [28]. At above Tg the motion of polymer chain and large mount of mass redistribution are possible [29 – 34], a banded texture of P2 is observed instantly when the temperature is raised above Tg (Fig. 1(C) and (D)). Such a fast formation of a banded texture suggests there is no large mount of polymer main chain diffusion. When the film temperature is raised above Tg ; both the out-of-plane orientated mesogens and in-plane orientated mesogens will self-assembly, respectively with other mesogens around them for forming order domain. The driver force is the potential liquid crystallinity, which makes mesogens have a tendency to form a well-ordered LC micro domain. Cooperative motion induced reorientation of mesogens forming an order domain, and further, the domain that was assembled by in-plane aligned mesogens has its
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Fig. 7. Banded texture of P2 with a stable transmittance after irradiation at light power of 30 mW/cm2 with different heating rate at: (A): 1 8C/min; (B) 10 8C/min; (C) 20 8C/min; (D) 30 8C/min when heating from 25 to 130 8C, keeping at 130 8C for 2 h, annealing to the room temperature. Film thickness: 20 mm.
optical axis in the direction of in-plane and perpendicular to LPL, the domain that was assembled by out-of-plane aligned mesogens has its optical axis in the direction of out-of-plane and also perpendicular to LPL. This alternant order domain leads to a spatial variation of the effective birefringence, which shows a banded texture under POM. It should be pointed out that the formation of the order domain is also affected by the motion of the main chain of polymers. Based on the observation above, a well-ordered LC micro domain can be formed only above Tg with the motion of main chain, although a few mesogens are also assembled at the heating process below Tg : However, the motion of the main chain is constrained by the linked mesogens selfassembled in the order domain when the film temperature is above Tg : This conclusion can be further confirmed by following observations. There is little change in the texture when keeping sample at 130 8C (Fig. 1(D)). Moreover, even when annealing films at any temperature between Tg , Ti ; the banded texture was still observed and the width of the band is not changing, which show the stability of periodic alignment in LC phase. When film was heated above Ti ; a dark view observed under POM, high mobility of main chain above Ti destroys the order LC domain, the polymer is in isotropy phase. The heating rate at the thermal treatment process also has an effect on the formation and the width of the stripes of the
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Fig. 9. Photoiuduced anisotropy of P2 films with various film thicknesses under 20 mW/cm2 LPL irradiation. Probe laser wavelength is 650 nm, 3 mW/cm2.
banded texture. When heating rate is below 20 8C/min, the band width of banded texture is decreased from 5 mm (1 8C/min) to 1 mm (10 8C/min) (Fig. 7(A) and (B)), which suggests that large order domain formed at a slow heating rate. With heating rate of 20 8C/min, only an illegible banded texture can be observed. And when heating rate reached 30 8C/min, no any banded texture can be observed under the observation condition as shown in Fig. 7(D). When the heating rate is fast, out-of-plane aligned or inplane aligned mesogens has no enough time to adjustment each other for forming a periodic order domain. Distinction between the two kind of aligned mesogens vanished, which results in a homogeneous image under POM (Fig. 7(D)). The effect of heating rate within temperature range of room temperature to 130 8C confirms that the formation of the banded texture was a local adjustment of the side chain, because the motion of polymer main chain is restricted within such a temperature range. 3.4. Effect of film thickness In Fig. 8., the film thickness is 1, 10, and 20 mm, respectively, and the banded texture is clearly seen in Fig. 8(B) and (C). According to the transmittance result in Fig. 9, there is no the over saturation phenomenon for the samples with film thickness lower than 1 mm, which suggests that there be no out-of-plane orientation under LPL irradiation, in Fig. 8(A) there is no such a texture observed for 1 mm film. This result is identical with conclusion that film
Fig. 8. Texture of P2 with different thickness: (A) 1 mm; (B) 10 mm; (C) 20 mm. LPL: 20 mW/cm2, heating rate: 5 8C/min.
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thickness also has an effect on the photoinduced orientations in the thin film [35]. The result, shown in Fig. 9, indicates that the out-of-plane orientation is affected by interactions between mesogens and the substrate. When the film is thin, the strong dipole moment interaction between mesogens and the glass substrate makes mesogens be in the state of parallel to the substrate and difficult to achieve the out-ofplane orientation under irradiation of LPL.
4. Conclusions Out-of-plane orientation of azobenzene-containing SCLCPs (P2) plays an important role during photoinduced alignment by LPL at 532 nm. When heating irradiated films above Tg ; at a nematic state, potential liquid crystallinity induces the rearrange of mesogens to form liquid crystal micro domain. In the thermal treatment mesogens in out-ofplane and in-plane orientated states self-assembly mesogens around them to form well order domain, respectively, and each of two kind domains has its own optical axis direction, which results in periodic well-order regions observed as a banded texture under POM. The threshold of light energy density inducing over-saturation for P2 is about 1 mW/cm2. After irradiation of LPL with higher power than the threshold value, banded texture can be observed under POM after thermal treatment. Thickness of sample film has an effect on the banded texture, and the banded texture is clearly seen when the film thickness is 10 and 20 mm. When heating rate is below 20 8C/min, the band width of the banded texture is decreased from 5 to 1 mm with heating rate increasing from 1 to 10 8C/min, showing heating rate is also a factor in forming banded texture.
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Acknowledgements
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This work was supported by the National Science Foundation of China (Grant Nos. 50025309 and 90201016). The authors are grateful for the financial support and express their thanks to the referees for critically reviewing the manuscript and making important suggestions.
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