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The formation of supramolecular liquid-crystal gels for enhancing the electro-optical properties of twisted nematic liquid crystals
Organic Electronics 27 (2015) 24e28
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Organic Electronics journal homepage: www.elsevier.com/locate/orgel
The formation of supramolecular liquid-crystal gels for enhancing the electro-optical properties of twisted nematic liquid crystals Jun-Wei Chen, Yu-Yi Kuo, Chao-Ran Wang, Chih-Yu Chao* Department of Physics, National Taiwan University, Taipei 10617, Taiwan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 1 June 2015 Received in revised form 25 August 2015 Accepted 25 August 2015 Available online xxx
We report that the supramolecular liquid-crystal (LC) physical gel can be formed through the fibrous selfassembly of the polyfluorene-based p-conjugated polymer, poly(9,9-dioctylfluorene-alt-benzothiadiazole) (F8BT), in twisted nematic LC system for the first time. With the utility of alignment layers, the F8BT molecules can be aligned and formed the LC physical gels with the formation of self-assembled supramolecular structures in the twisted nematic LCs. In contrast to conventional LC physical gels, the presence of anisotropic p-conjugated structure makes the twisted nematic LC system exhibit excellent electro-optical properties of driving voltage reduction and contrast ratio enhancement owing to the conjugated polymer having a high p-electron delocalization degree which can efficiently drive LC molecules in much lower operating voltages. The self-assembled supramolecular network has revealed the potential for applying in various LC display devices with the ability of improving their electro-optical performance. © 2015 Elsevier B.V. All rights reserved.
Keywords: Liquid crystal physical gel Electro-optical device p-Conjugated gelator Self-assembled supramolecular structure
1. Introduction The physical gels with fibrous network structures formed by self-assembled gelator molecules in organic solvents have attracted significant attention in this decade [1e6]. The physical formation of fibrous aggregates of the organic gelators is induced by the noncovalent interactions such as hydrogen-bonding, pep stacking and van der Waals force. In recent years, the physical gelation of liquid crystals (LCs) through the formation of fibrous selfassembled network has been developed as soft functional materials - LC physical gels [7e10]. The LC physical gels with the electrically switchable operating property have been widely applied in various electro-optical devices, such as switchable scattering devices [11e14], LC semiconductors with high mobility [15,16], and stabilization of the orientation of ferroelectric LCs [17], etc. In particular, improving the twisted nematic (TN) LC system based on the formation of fibrous self-assembled networks has attracted more and more interests for their potential for enhancing the quality of LC displays [18e20]. In these gels, although the threshold voltages were decreased, however, the contrast ratios became poor in exchange. Therefore, it is not quite suitable for the use of displays if the contrast ratios would be sacrificed.
* Corresponding author. E-mail address: [email protected] (C.-Y. Chao). http://dx.doi.org/10.1016/j.orgel.2015.08.027 1566-1199/© 2015 Elsevier B.V. All rights reserved.
In our previous work, we reported that the electrically switchable scattering device with the properties of high contrast ratio and low operating voltage can be made through the formation of the supramolecular structures in the LC host with anti-parallel alignment [14]. Therefore, we would like to study whether the supramolecular p-conjugated polymer network can improve the TN LC systems. In this paper, we report for the first time that a supramolecular LC gel can be formed in LC host 5CB through the fibrous selfassembly of the polyfluorene-based p-conjugated polymer, poly(9,9-dioctylfluorene-alt-benzothiadiazole) (F8BT), in a LC cell with mutually perpendicular alignment layers, and the presence of anisotropic fibrous supramolecular structures can improve the electro-optical properties of the TN LCs in decreasing threshold voltages and enhancing contrast ratios simultaneously without trade-off. The fantastic electro-optical properties, which are achieved by the use of polyfluorene-based p-conjugated polymer as gelator, provide great applications in various electrically switchable devices.
2. Device fabrication and experiments In our experiment, the LC gels were fabricated through the use of the nematic LC 5CB (clearing point about 35 C) and the gelators molecule, poly(9,9-dioctylfluorene-alt-benzothiadiazole) (F8BT).
J.-W. Chen et al. / Organic Electronics 27 (2015) 24e28
5CB
F8BT Fig. 1. The molecular structures of LC host 5CB and p-Conjugated gelator F8BT.
And their molecular structures are shown in Fig. 1. They were purchased from Merck and SigmaeAldrich, respectively. The gelator F8BT is a kind of polyfluorene-based p-conjugated molecule with average Mn 10,000e20,000. To prepare the supramolecular LC physical gels, at first, a small amount of F8BT (0.1e1.6 wt%) was added into LC 5CB in glass bottles. Then these materials were uniformly mixed by stirring and gently heating in a hot plate. In the beginning, the LC mixtures of F8BT and 5CB displayed an opaque state in the room temperature. When the mixtures were heated to the temperature about 60 C, which is higher than the clearing point of 5CB, the mixtures became transparent for being isotropic liquid. To make sure the F8BT polymers can be well dispersed in 5CB, the mixtures were further heated at 90 C for 3 h. And then the mixtures were cooled down slowly at a rate of 5 C min1. As the temperature cooled down to required temperature, the mixtures became supramolecular LC gels through the formation of self-assembly fibrous polymers network induced by the pep stacking interactions between F8BT molecules [14,21e23]. To understand the phase transition behavior of the supramolecular LC gels, the 5CB with different concentrations of F8BT polymers were examined. The solegel transition temperatures (Tsol-gel) of F8BT and the isotropicenematic transition temperatures (Tiso-ne) of 5CB were determined by differential scanning calorimetry (DSC) measurements in the cooling process at a rate of 5 C min1. The DSC measurements were performed on a Perkin Elmer Pyris 6 DSC instrument. By confirming the exothermic peaks occur in the DSC data, the transition temperatures of Tiso-ne
25
and Tsol-gel can be obtained. The gels containing different concentrations of F8BT were prepared as described in previous procedures. To obtain the gel in cells, the gels were heated to isotropic liquid state for mixing uniformly and then injected into the cells. Subsequently, the cells were cooled down slowly to required temperature at a rate of 5 C min1, and the supramolecular LC gels were formed in the cells. To confirm that the F8BT molecules can be aligned by the LC 5CB, the gels were injected into the 9 mm thickness glass sandwich cells with anti-parallel rubbed polyimide films. To further verify the main-chain direction of the F8BT polymer in the cells with anti-parallel alignment layers, the polarized photoluminescence (PL) spectra (excitation light lex ¼ 430 nm) were further measured through the instrument of Edinburgh FS 920. By adjusting the polarization of polarizer between the cell and the detector to be parallel or perpendicular with the rubbing direction of the cell, the polarized PL spectra can be obtained. For researching the properties of the supramolecular polymer network in TN LC system, the gels were injected into the 4.2 mm thickness indium tin oxide (ITO) coated glass sandwich cells with mutually perpendicular rubbed alignment layers. To observe the two kinds of structure of the gels in the cells with anti-parallel rubbed alignment layers and mutually perpendicular alignment layers, the dark-field mode of Olympus BX51 optical microscope (OM) was conducted. The detailed electro-optical properties of the supramolecular LC gels with different concentration of F8BT in TN cell were further measured by our experiment system shown in Fig. 2. The system is composed of a HeeNe laser, a pair of polarizers, sample cell mounted in rotatable holder, and the photodetector. An unpolarized HeeNe (632.8 nm) laser was used as an incident light source. At the beginning, the polarizers were tuned to be perpendicular to each other to make sure the transmittance minimum. Then the sample cell with LC gels was placed between the crossed polarizers and tuned to make the transmittance reach maximum. And the sample cell was driven by an AC field (1 kHz, square wave) supplied by the function generator SRS DS360 which was controlled by a computer with LabView programming. Then the transmittances under various applied voltage were recorded by the photodetector. The voltage required for reaching 90% and 10% transmittance relative to the initial light intensity was defined as driving voltage V90 and V10, respectively. The contrast ratio is determined by the initial transmittance to the transmittance under 3 V. And the response time was determined by the data of the rise time ton and the decay time toff through the measuring of the switching times needed for varying the transmittances 90e10% and 10e90% in the electric field on (5 V) and off, respectively. 3. Results and discussions The phase behavior of the gels with different concentrations of F8BT was shown in Fig. 3(a). The transition temperatures of the
Fig. 2. The illustration shows the setup of the electro-optical measurement system. The LC cell is placed between a pair of polarizers with mutually perpendicular polarizing directions for the operation of a normally white TN mode.
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J.-W. Chen et al. / Organic Electronics 27 (2015) 24e28
Fig. 3. (a) Phase diagram of the mixtures of 5CB and F8BT. The mixtures with different concentrations of 0, 0.1, 0.2, 0.5, and 1.0 wt% F8BT were prepared. The Tsol-gel of F8BT and Tiso-ne of 5CB are represented by - and :, respectively. The representative DSC data of 5CB containing (b) 0 wt% and (c) 0.2 wt% of F8BT in the cooling process at a rate of 5 C min1.
supramolecular LC gels were determined by the DSC measurements. The representative DSC data of 5CB containing 0 wt% and 0.2 wt% of F8BT are shown in Fig. 3(b,c). For the pure LC 5CB, the peak on 35.0 C exhibits the isotropicenematic transition temperature of 5CB. And for the LC 5CB with 0.2 wt% of F8BT, there are two peaks shown. The peak on 34.7 C exhibits the isotropicenematic transition temperature (Tiso-ne) of 5CB and the peak on 17.3 C exhibits the solegel transition temperature (Tsol-gel) of F8BT. In our experiments, the gels exhibit three states of isotropic liquid, nematic, and nematic LC gel in the phase diagram. And the mixtures undergo the isotropicenematic transition of 5CB first and
then solegel transition of F8BT upon cooling. It means that the F8BT molecules could be aligned through the template effect of the LC materials [14]. To confirm this assumption, the gels with different concentrations of F8BT were filled into a 9 mm empty LC cells with antiparallel or perpendicular rubbed alignment layers in isotropic liquid state. After the cooling process described in the experimental section, we found that the self-assembled fibrous structures were formed in the LC cells through the observations with dark-field optical microscope. Fig. 4(a) shows the OM picture of the cell with the gel containing 1.0 wt% of F8BT. The self-assembled F8BT fibers parallel to the rubbing direction of the cell were observed in this figure. And Fig. 4(b) shows the OM picture of the cell with mutually perpendicular alignment layers injected the gel containing 1.0 wt% of F8BT, the anisotropic polymer network can be observed. The rectangular materials shown in Fig. 4(a) are the spacers made by the material of silicon with the thickness about 9 mm after sealing the cell. And the spacers are not seen in Fig. 4(b), because the cell with 4.2 mm thickness is achieved by only placing the spacers in the sealing place. We consider that he F8BT gelators would be aligned and rotated 90+between mutually perpendicular alignment layers by the LC molecules in the twisted nematic state. It seems that the F8BT fibers are aligned in one direction in Fig. 4(b) in the cell with mutually perpendicular alignment layers. We thought it comes by an average result. The OM focuses on the position with the most fibers which are about the middle of the cell. And that is the position where the F8BT molecules rotate about 45+ from one of the substrates. And the similar results are also shown in Prof. Kato's paper [19]. The environment of the LCs in the homogenous or twisted nematic states which induces the difference of the orientation of F8BT molecules. Therefore, the anisotropic self-assembled structures were formed in the TN cells after aggregation of F8BT molecules as the cells cooled to required temperature. Fig. 5 shows the PL emission spectra (lex ¼ 430 nm) of the LC gel containing 0.1 wt% F8BT in the cell with anti-parallel and mutually perpendicular alignment layers. From Fig. 5(a), the measured PL polarization ratio of parallel to perpendicular intensity in the antiparallel cell at 540 nm is around 3.4. This high PL ratio finding shows that the main-chain direction of F8BT molecules should align with the rubbing direction of cell, that is, the long-axis direction of LC molecules, which is consistent with results of the OM picture and the phase diagram since the Tiso-ne is higher than Tsol-gel. And the polarized PL spectra for samples prepared in cells with mutually perpendicular alignment layers are shown in Fig. 5(b). It can be seen that the polarized PL spectra in two directions are almost the same. This result unambiguously indicates that the F8BT molecules would not have preferred orientation in the TN cells. And then the electro-optical properties of the supramolecular LC gels in TN cells were examined by the HeeNe Laser experimental system shown in Fig. 2. The sample cells filled with the gels in different concentrations of the gelator F8BT were prepared through the previously mentioned experimental procedures. Fig. 6 shows the voltage-transmittance (V-T) curves of the LC gels with different concentrations (0, 0.1, 0.2, and 0.4 wt%) of the gelator F8BT in TN cells. As the electric field is off, the gels present the bright transmission state. When the electric field turns on, the transmittance starts to decrease. From the V-T curves, a bump peak found clearly in transmittance in the case of pure LC 5CB. This bump peak would influence the performance of the TN LC cells which is not the result we expected for the display applications. In our experiments, we found the bump peak could be eliminated due to the formation of self-assembled supramolecular structures of F8BT polymer in LC 5CB. It means that the fibrous structure can be used to stabilize the LC molecules by avoiding them to over-rotate during
J.-W. Chen et al. / Organic Electronics 27 (2015) 24e28
27
Fig. 4. The dark-field optical pictures of F8BT/5CB gels containing 1.0 wt% of F8BT in 5CB LCs in the cell with (a) anti-parallel and (b) mutually perpendicular rubbed alignment layers. The self-assembled fibers of F8BT were found in the cells.
Fig. 5. The polarized PL spectra of a LC gel containing 0.1 wt% F8BT in 5CB in the cell with (a) anti-parallel and (b) mutually perpendicular rubbed alignment layers. The solid and dashed curves in the left figure denote the PL spectrum as the polarizing direction of polarizer was parallel and perpendicular to the rubbing direction of the cell, respectively. And the solid and dashed curves in the right figure denote the PL spectrum in two directions, respectively.
the operating process [17]. In addition, we consider the reason leading to low operating voltages should be caused from the presence of p-electrons. The p-electrons could play as a kind of radical here, since the aromatic rings in the conjugated polymer F8BT have a high p-electron delocalization degree [24]. After the formation of conjugated network through the p-stacking interactions between F8BT molecules, p-electrons would obtain
higher delocalization degree [25]. Therefore, we suggest that these p-electrons could rapidly redistribute around the conjugated network and form localized electric fields which can efficiently drive LC molecules in such lower operating voltages [14]. The electro-optical properties of the supramolecular LC gels with different concentrations of the gelator F8BT are shown in Table 1. In our experiments, the LC gel containing 0.4 wt% F8BT polymer presents the best performance with the lowest driving voltage about 0.83 and 1.28 in V90 and V10, respectively. And the contrast ratio can also be obviously improved. In addition, we can see that as the 0.1 wt% of F8BT gelator was added into LC 5CB, the response time would slightly rise. And as the higher concentrations of F8BT were used, the response times started to decrease. For the concentration of 0.4 wt% of F8BT in LC 5CB, the response time is compatible with the pure 5CB. It is remarkable to note that through the utility of polyfluorene-based p-conjugated polymer F8BT in the TN LC system, the supramolecular LC gel shows the unique property of low driving voltage and high contrast ratios at the same time which are not found in the conventional LC gels formed by the oriented self-assembly of low-molecular-weight gelator [18e20].
Table 1 The electro-optical properties of the supramolecular LC gels with different concentrations of the F8BT gelator.
Fig. 6. The transmittance-voltage curves of the 4.2 mm thick TN LC cells with the supramolecular LC gels containing 0, 0.1, 0.2, and 0.4 wt% of F8BT.
Material type
V90 (V)
V10 (V)
ton (ms)
toff (ms)
CR
5CB w/0.1 wt% F8BT w/0.2 wt% F8BT w/0.4 wt% F8BT
1.10 1.00 0.88 0.83
1.58 1.43 1.35 1.28
1.77 2.20 1.87 1.79
10.50 12.68 12.28 10.47
243 332 460 435
28
J.-W. Chen et al. / Organic Electronics 27 (2015) 24e28
4. Conclusion In summary, we have reported that the supramolecular LC physical gel can be formed in nematic LC 5CB through the fibrous self-assembly of the polyfluorene-based p-conjugated polymer F8BT. And the F8BT molecules can be aligned and formed anisotropic fibrous network by utilizing the alignment layers. Opposite to the LC gels formed by low-molecular-weight gelator that need high voltage to drive LCs, the supramolecular LC gels with the presence of anisotropic polymer network makes TN LC 5CB exhibit excellent electro-optical properties such as driving voltage reduction and contrast ratio enhancement. The unique properties of the supramolecular p-conjugated structure with high p-electron delocalization degree can efficiently drive LC molecules in much lower operating voltages. Therefore, we believe that the supramolecular LC gel with the anisotropic p-conjugated polymer network has potential to apply in various electro-optical devices for the ability of improving their performance in operating property. Acknowledgments One of us (C.Y.C.) acknowledges the support from National Taiwan University (grant number: 103R7560 & 104R7560), the Ministry of Science and Technology (grant number: 102-2112-M002-010-MY3), and the National Nano Device Laboratory of Taiwan. We would like to thank K. T. Chen for participation in the early phase of this work. References [1] D.J. Abdallah, R.G. Weiss, Organogels and low molecular mass organic gelators, Adv. Mater. 12 (2000) 1237e1247. € hlen, F. Fages, H. Bouaslaurent, J.-P. Desvergne, A novel [2] T. Brotin, R. Utermo small molecular luminescent gelling agent for alcohols, J. Chem. Soc. Chem. Commun. 6 (1991) 416e418. [3] C. Geiger, M. Stanescu, L.H. Chen, D.G. Whitten, Organogels resulting from competing self-assembly units in the gelator: structure, dynamics, and photophysical behavior of gels formed from cholesterol-stilbene and cholesterolsquaraine gelators, Langmuir 15 (1999) 2241e2245. [4] M. George, R.G. Weiss, Molecular organogels. Soft matter comprised of lowmolecular-mass organic gelators and organic liquids, Acc. Chem. Res. 39 (2006) 489e497. [5] K. Hanabusa, K. Hiratsuka, M. Kimura, H. Shirai, Easy preparation and useful character of organogel electrolytes based on low molecular weight gelator, Chem. Mater. 11 (1999) 649e655. [6] J.H. van Esch, B.L. Feringa, New functional materials based on self-assembling organogels: from serendipity towards design, Angew. Chem. Int. Ed. 39 (2000) 2263e2266.
[7] L. Guan, Y. Zhao, Self-assembly of a liquid crystalline anisotropic gel, Chem. Mater. 12 (2000) 3667e3673. [8] T. Kato, Self-assembly of phase-segregated liquid crystal structures, Science 295 (2002) 2414e2418. [9] T. Kato, Y. Hirai, S. Nakaso, M. Moriyama, Liquid-crystalline physical gels, Chem. Soc. Rev. 36 (2007) 1857e1867. [10] N.M. Sangeetha, U. Maitra, Supramolecular gels: functions and uses, Chem. Soc. Rev. 34 (2005) 821e836. [11] R.H.C. Janssen, V. Stumpflen, D.J. Broer, C.W.M. Bastiaansen, T.A. Tervoort, P. Smith, Electrically controlled light scattering from thermoreversible liquidcrystal gels, J. Appl. Phys. 88 (2000) 161e167. [12] N. Mizoshita, Y. Suzuki, K. Kishimoto, K. Hanabusa, T. Kato, Electrooptical properties of liquid-crystalline physical gels: a new oligo(amino acid) gelator for light scattering display materials, J. Mater. Chem. 12 (2002) 2197e2201. [13] Y. Suzuki, N. Mizoshita, K. Hanabusa, T. Kato, Homeotropically oriented nematic physical gels for electrooptical materials, J. Mater. Chem. 13 (2003) 2870e2874. [14] J.W. Chen, C.C. Huang, C.Y. Chao, Supramolecular liquid-crystal gels formed by polyfluorene-based pi-conjugated polymer for switchable anisotropic scattering device, ACS Appl. Mater. Interfaces 6 (2014) 6757e6764. [15] Y. Hirai, H. Monobe, N. Mizoshita, M. Moriyama, K. Hanabusa, Y. Shimizu, T. Kato, Enhanced hole-transporting behavior of discotic liquid-crystalline physical gels, Adv. Funct. Mater. 18 (2008) 1668e1675. [16] N. Mizoshita, H. Monobe, M. Inoue, M. Ukon, T. Watanabe, Y. Shimizu, K. Hanabusa, T. Kato, The positive effect on hole transport behaviour in anisotropic gels consisting of discotic liquid crystals and hydrogen-bonded fibres, Chem. Commun. 5 (2002) 428e429. [17] C. Tolksdorf, R. Zentel, Reversible physical network stabilized ferroelectric liquid crystals, Adv. Mater. 13 (2001) 1307e1310. [18] N. Mizoshita, K. Hanabusa, T. Kato, Self-aggregation of an amino acid derivative as a route to liquid-crystalline physical gels - faster response to electric fields, Adv. Mater. 11 (1999) 392e394. [19] N. Mizoshita, K. Hanabusa, T. Kato, Fast and high-contrast electro-optical switching of liquid-crystalline physical gels: formation of oriented microphase-separated structures, Adv. Funct. Mater. 13 (2003) 313e317. [20] G.G. Nair, S.K. Prasad, V. Jayalakshmi, G. Shanker, C.V. Yelamaggad, Fast responding robust nematic liquid crystalline gels formed by a monodisperse dipeptide: electro-optic and rheological studies, J. Phys. Chem. B 113 (2009) 6647e6651. [21] J.-H. Chen, C.-S. Chang, Y.-X. Chang, C.-Y. Chen, H.-L. Chen, S.-A. Chen, Gelation and its effect on the photophysical behavior of poly(9,9-dioctylfluorene-2,7diyl) in toluene, Macromolecules 42 (2009) 1306e1314. [22] Z.-Q. Lin, N.-E. Shi, Y.-B. Li, D. Qiu, L. Zhang, J.-Y. Lin, J.-F. Zhao, C. Wang, L.H. Xie, W. Huang, Preparation and characterization of polyfluorene-based supramolecular p-conjugated polymer gels, J. Phys. Chem. C 115 (2011) 4418e4424. [23] M.H. Rahman, S.-C. Liao, H.-L. Chen, J.-H. Chen, V.A. Ivanov, P.P.J. Chu, S.A. Chen, Aggregation of conjugated polymers in aromatic solvent, Langmuir 25 (2009) 1667e1674. [24] Y. Chen, J. Qian, X.Y. Liu, Q.X. Zhuang, Z.W. Han, Synthesis and photoluminescence properties of polybenzoxazoles containing perylenebisimide functionalized graphene nanosheets via stacking interactions, New J. Chem. 37 (2013) 2500e2508. [25] J. Guo, Y.H. Xu, S.B. Jin, L. Chen, T. Kaji, Y. Honsho, M.A. Addicoat, J. Kim, A. Saeki, H. Ihee, S. Seki, S. Irle, M. Hiramoto, J. Gao, D.L. Jiang, Conjugated organic framework with three-dimensionally ordered stable structure and delocalized p clouds, Nat. Commun. 4 (2013) 2736.
Contents lists available at ScienceDirect
Organic Electronics journal homepage: www.elsevier.com/locate/orgel
The formation of supramolecular liquid-crystal gels for enhancing the electro-optical properties of twisted nematic liquid crystals Jun-Wei Chen, Yu-Yi Kuo, Chao-Ran Wang, Chih-Yu Chao* Department of Physics, National Taiwan University, Taipei 10617, Taiwan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 1 June 2015 Received in revised form 25 August 2015 Accepted 25 August 2015 Available online xxx
We report that the supramolecular liquid-crystal (LC) physical gel can be formed through the fibrous selfassembly of the polyfluorene-based p-conjugated polymer, poly(9,9-dioctylfluorene-alt-benzothiadiazole) (F8BT), in twisted nematic LC system for the first time. With the utility of alignment layers, the F8BT molecules can be aligned and formed the LC physical gels with the formation of self-assembled supramolecular structures in the twisted nematic LCs. In contrast to conventional LC physical gels, the presence of anisotropic p-conjugated structure makes the twisted nematic LC system exhibit excellent electro-optical properties of driving voltage reduction and contrast ratio enhancement owing to the conjugated polymer having a high p-electron delocalization degree which can efficiently drive LC molecules in much lower operating voltages. The self-assembled supramolecular network has revealed the potential for applying in various LC display devices with the ability of improving their electro-optical performance. © 2015 Elsevier B.V. All rights reserved.
Keywords: Liquid crystal physical gel Electro-optical device p-Conjugated gelator Self-assembled supramolecular structure
1. Introduction The physical gels with fibrous network structures formed by self-assembled gelator molecules in organic solvents have attracted significant attention in this decade [1e6]. The physical formation of fibrous aggregates of the organic gelators is induced by the noncovalent interactions such as hydrogen-bonding, pep stacking and van der Waals force. In recent years, the physical gelation of liquid crystals (LCs) through the formation of fibrous selfassembled network has been developed as soft functional materials - LC physical gels [7e10]. The LC physical gels with the electrically switchable operating property have been widely applied in various electro-optical devices, such as switchable scattering devices [11e14], LC semiconductors with high mobility [15,16], and stabilization of the orientation of ferroelectric LCs [17], etc. In particular, improving the twisted nematic (TN) LC system based on the formation of fibrous self-assembled networks has attracted more and more interests for their potential for enhancing the quality of LC displays [18e20]. In these gels, although the threshold voltages were decreased, however, the contrast ratios became poor in exchange. Therefore, it is not quite suitable for the use of displays if the contrast ratios would be sacrificed.
* Corresponding author. E-mail address: [email protected] (C.-Y. Chao). http://dx.doi.org/10.1016/j.orgel.2015.08.027 1566-1199/© 2015 Elsevier B.V. All rights reserved.
In our previous work, we reported that the electrically switchable scattering device with the properties of high contrast ratio and low operating voltage can be made through the formation of the supramolecular structures in the LC host with anti-parallel alignment [14]. Therefore, we would like to study whether the supramolecular p-conjugated polymer network can improve the TN LC systems. In this paper, we report for the first time that a supramolecular LC gel can be formed in LC host 5CB through the fibrous selfassembly of the polyfluorene-based p-conjugated polymer, poly(9,9-dioctylfluorene-alt-benzothiadiazole) (F8BT), in a LC cell with mutually perpendicular alignment layers, and the presence of anisotropic fibrous supramolecular structures can improve the electro-optical properties of the TN LCs in decreasing threshold voltages and enhancing contrast ratios simultaneously without trade-off. The fantastic electro-optical properties, which are achieved by the use of polyfluorene-based p-conjugated polymer as gelator, provide great applications in various electrically switchable devices.
2. Device fabrication and experiments In our experiment, the LC gels were fabricated through the use of the nematic LC 5CB (clearing point about 35 C) and the gelators molecule, poly(9,9-dioctylfluorene-alt-benzothiadiazole) (F8BT).
J.-W. Chen et al. / Organic Electronics 27 (2015) 24e28
5CB
F8BT Fig. 1. The molecular structures of LC host 5CB and p-Conjugated gelator F8BT.
And their molecular structures are shown in Fig. 1. They were purchased from Merck and SigmaeAldrich, respectively. The gelator F8BT is a kind of polyfluorene-based p-conjugated molecule with average Mn 10,000e20,000. To prepare the supramolecular LC physical gels, at first, a small amount of F8BT (0.1e1.6 wt%) was added into LC 5CB in glass bottles. Then these materials were uniformly mixed by stirring and gently heating in a hot plate. In the beginning, the LC mixtures of F8BT and 5CB displayed an opaque state in the room temperature. When the mixtures were heated to the temperature about 60 C, which is higher than the clearing point of 5CB, the mixtures became transparent for being isotropic liquid. To make sure the F8BT polymers can be well dispersed in 5CB, the mixtures were further heated at 90 C for 3 h. And then the mixtures were cooled down slowly at a rate of 5 C min1. As the temperature cooled down to required temperature, the mixtures became supramolecular LC gels through the formation of self-assembly fibrous polymers network induced by the pep stacking interactions between F8BT molecules [14,21e23]. To understand the phase transition behavior of the supramolecular LC gels, the 5CB with different concentrations of F8BT polymers were examined. The solegel transition temperatures (Tsol-gel) of F8BT and the isotropicenematic transition temperatures (Tiso-ne) of 5CB were determined by differential scanning calorimetry (DSC) measurements in the cooling process at a rate of 5 C min1. The DSC measurements were performed on a Perkin Elmer Pyris 6 DSC instrument. By confirming the exothermic peaks occur in the DSC data, the transition temperatures of Tiso-ne
25
and Tsol-gel can be obtained. The gels containing different concentrations of F8BT were prepared as described in previous procedures. To obtain the gel in cells, the gels were heated to isotropic liquid state for mixing uniformly and then injected into the cells. Subsequently, the cells were cooled down slowly to required temperature at a rate of 5 C min1, and the supramolecular LC gels were formed in the cells. To confirm that the F8BT molecules can be aligned by the LC 5CB, the gels were injected into the 9 mm thickness glass sandwich cells with anti-parallel rubbed polyimide films. To further verify the main-chain direction of the F8BT polymer in the cells with anti-parallel alignment layers, the polarized photoluminescence (PL) spectra (excitation light lex ¼ 430 nm) were further measured through the instrument of Edinburgh FS 920. By adjusting the polarization of polarizer between the cell and the detector to be parallel or perpendicular with the rubbing direction of the cell, the polarized PL spectra can be obtained. For researching the properties of the supramolecular polymer network in TN LC system, the gels were injected into the 4.2 mm thickness indium tin oxide (ITO) coated glass sandwich cells with mutually perpendicular rubbed alignment layers. To observe the two kinds of structure of the gels in the cells with anti-parallel rubbed alignment layers and mutually perpendicular alignment layers, the dark-field mode of Olympus BX51 optical microscope (OM) was conducted. The detailed electro-optical properties of the supramolecular LC gels with different concentration of F8BT in TN cell were further measured by our experiment system shown in Fig. 2. The system is composed of a HeeNe laser, a pair of polarizers, sample cell mounted in rotatable holder, and the photodetector. An unpolarized HeeNe (632.8 nm) laser was used as an incident light source. At the beginning, the polarizers were tuned to be perpendicular to each other to make sure the transmittance minimum. Then the sample cell with LC gels was placed between the crossed polarizers and tuned to make the transmittance reach maximum. And the sample cell was driven by an AC field (1 kHz, square wave) supplied by the function generator SRS DS360 which was controlled by a computer with LabView programming. Then the transmittances under various applied voltage were recorded by the photodetector. The voltage required for reaching 90% and 10% transmittance relative to the initial light intensity was defined as driving voltage V90 and V10, respectively. The contrast ratio is determined by the initial transmittance to the transmittance under 3 V. And the response time was determined by the data of the rise time ton and the decay time toff through the measuring of the switching times needed for varying the transmittances 90e10% and 10e90% in the electric field on (5 V) and off, respectively. 3. Results and discussions The phase behavior of the gels with different concentrations of F8BT was shown in Fig. 3(a). The transition temperatures of the
Fig. 2. The illustration shows the setup of the electro-optical measurement system. The LC cell is placed between a pair of polarizers with mutually perpendicular polarizing directions for the operation of a normally white TN mode.
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J.-W. Chen et al. / Organic Electronics 27 (2015) 24e28
Fig. 3. (a) Phase diagram of the mixtures of 5CB and F8BT. The mixtures with different concentrations of 0, 0.1, 0.2, 0.5, and 1.0 wt% F8BT were prepared. The Tsol-gel of F8BT and Tiso-ne of 5CB are represented by - and :, respectively. The representative DSC data of 5CB containing (b) 0 wt% and (c) 0.2 wt% of F8BT in the cooling process at a rate of 5 C min1.
supramolecular LC gels were determined by the DSC measurements. The representative DSC data of 5CB containing 0 wt% and 0.2 wt% of F8BT are shown in Fig. 3(b,c). For the pure LC 5CB, the peak on 35.0 C exhibits the isotropicenematic transition temperature of 5CB. And for the LC 5CB with 0.2 wt% of F8BT, there are two peaks shown. The peak on 34.7 C exhibits the isotropicenematic transition temperature (Tiso-ne) of 5CB and the peak on 17.3 C exhibits the solegel transition temperature (Tsol-gel) of F8BT. In our experiments, the gels exhibit three states of isotropic liquid, nematic, and nematic LC gel in the phase diagram. And the mixtures undergo the isotropicenematic transition of 5CB first and
then solegel transition of F8BT upon cooling. It means that the F8BT molecules could be aligned through the template effect of the LC materials [14]. To confirm this assumption, the gels with different concentrations of F8BT were filled into a 9 mm empty LC cells with antiparallel or perpendicular rubbed alignment layers in isotropic liquid state. After the cooling process described in the experimental section, we found that the self-assembled fibrous structures were formed in the LC cells through the observations with dark-field optical microscope. Fig. 4(a) shows the OM picture of the cell with the gel containing 1.0 wt% of F8BT. The self-assembled F8BT fibers parallel to the rubbing direction of the cell were observed in this figure. And Fig. 4(b) shows the OM picture of the cell with mutually perpendicular alignment layers injected the gel containing 1.0 wt% of F8BT, the anisotropic polymer network can be observed. The rectangular materials shown in Fig. 4(a) are the spacers made by the material of silicon with the thickness about 9 mm after sealing the cell. And the spacers are not seen in Fig. 4(b), because the cell with 4.2 mm thickness is achieved by only placing the spacers in the sealing place. We consider that he F8BT gelators would be aligned and rotated 90+between mutually perpendicular alignment layers by the LC molecules in the twisted nematic state. It seems that the F8BT fibers are aligned in one direction in Fig. 4(b) in the cell with mutually perpendicular alignment layers. We thought it comes by an average result. The OM focuses on the position with the most fibers which are about the middle of the cell. And that is the position where the F8BT molecules rotate about 45+ from one of the substrates. And the similar results are also shown in Prof. Kato's paper [19]. The environment of the LCs in the homogenous or twisted nematic states which induces the difference of the orientation of F8BT molecules. Therefore, the anisotropic self-assembled structures were formed in the TN cells after aggregation of F8BT molecules as the cells cooled to required temperature. Fig. 5 shows the PL emission spectra (lex ¼ 430 nm) of the LC gel containing 0.1 wt% F8BT in the cell with anti-parallel and mutually perpendicular alignment layers. From Fig. 5(a), the measured PL polarization ratio of parallel to perpendicular intensity in the antiparallel cell at 540 nm is around 3.4. This high PL ratio finding shows that the main-chain direction of F8BT molecules should align with the rubbing direction of cell, that is, the long-axis direction of LC molecules, which is consistent with results of the OM picture and the phase diagram since the Tiso-ne is higher than Tsol-gel. And the polarized PL spectra for samples prepared in cells with mutually perpendicular alignment layers are shown in Fig. 5(b). It can be seen that the polarized PL spectra in two directions are almost the same. This result unambiguously indicates that the F8BT molecules would not have preferred orientation in the TN cells. And then the electro-optical properties of the supramolecular LC gels in TN cells were examined by the HeeNe Laser experimental system shown in Fig. 2. The sample cells filled with the gels in different concentrations of the gelator F8BT were prepared through the previously mentioned experimental procedures. Fig. 6 shows the voltage-transmittance (V-T) curves of the LC gels with different concentrations (0, 0.1, 0.2, and 0.4 wt%) of the gelator F8BT in TN cells. As the electric field is off, the gels present the bright transmission state. When the electric field turns on, the transmittance starts to decrease. From the V-T curves, a bump peak found clearly in transmittance in the case of pure LC 5CB. This bump peak would influence the performance of the TN LC cells which is not the result we expected for the display applications. In our experiments, we found the bump peak could be eliminated due to the formation of self-assembled supramolecular structures of F8BT polymer in LC 5CB. It means that the fibrous structure can be used to stabilize the LC molecules by avoiding them to over-rotate during
J.-W. Chen et al. / Organic Electronics 27 (2015) 24e28
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Fig. 4. The dark-field optical pictures of F8BT/5CB gels containing 1.0 wt% of F8BT in 5CB LCs in the cell with (a) anti-parallel and (b) mutually perpendicular rubbed alignment layers. The self-assembled fibers of F8BT were found in the cells.
Fig. 5. The polarized PL spectra of a LC gel containing 0.1 wt% F8BT in 5CB in the cell with (a) anti-parallel and (b) mutually perpendicular rubbed alignment layers. The solid and dashed curves in the left figure denote the PL spectrum as the polarizing direction of polarizer was parallel and perpendicular to the rubbing direction of the cell, respectively. And the solid and dashed curves in the right figure denote the PL spectrum in two directions, respectively.
the operating process [17]. In addition, we consider the reason leading to low operating voltages should be caused from the presence of p-electrons. The p-electrons could play as a kind of radical here, since the aromatic rings in the conjugated polymer F8BT have a high p-electron delocalization degree [24]. After the formation of conjugated network through the p-stacking interactions between F8BT molecules, p-electrons would obtain
higher delocalization degree [25]. Therefore, we suggest that these p-electrons could rapidly redistribute around the conjugated network and form localized electric fields which can efficiently drive LC molecules in such lower operating voltages [14]. The electro-optical properties of the supramolecular LC gels with different concentrations of the gelator F8BT are shown in Table 1. In our experiments, the LC gel containing 0.4 wt% F8BT polymer presents the best performance with the lowest driving voltage about 0.83 and 1.28 in V90 and V10, respectively. And the contrast ratio can also be obviously improved. In addition, we can see that as the 0.1 wt% of F8BT gelator was added into LC 5CB, the response time would slightly rise. And as the higher concentrations of F8BT were used, the response times started to decrease. For the concentration of 0.4 wt% of F8BT in LC 5CB, the response time is compatible with the pure 5CB. It is remarkable to note that through the utility of polyfluorene-based p-conjugated polymer F8BT in the TN LC system, the supramolecular LC gel shows the unique property of low driving voltage and high contrast ratios at the same time which are not found in the conventional LC gels formed by the oriented self-assembly of low-molecular-weight gelator [18e20].
Table 1 The electro-optical properties of the supramolecular LC gels with different concentrations of the F8BT gelator.
Fig. 6. The transmittance-voltage curves of the 4.2 mm thick TN LC cells with the supramolecular LC gels containing 0, 0.1, 0.2, and 0.4 wt% of F8BT.
Material type
V90 (V)
V10 (V)
ton (ms)
toff (ms)
CR
5CB w/0.1 wt% F8BT w/0.2 wt% F8BT w/0.4 wt% F8BT
1.10 1.00 0.88 0.83
1.58 1.43 1.35 1.28
1.77 2.20 1.87 1.79
10.50 12.68 12.28 10.47
243 332 460 435
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J.-W. Chen et al. / Organic Electronics 27 (2015) 24e28
4. Conclusion In summary, we have reported that the supramolecular LC physical gel can be formed in nematic LC 5CB through the fibrous self-assembly of the polyfluorene-based p-conjugated polymer F8BT. And the F8BT molecules can be aligned and formed anisotropic fibrous network by utilizing the alignment layers. Opposite to the LC gels formed by low-molecular-weight gelator that need high voltage to drive LCs, the supramolecular LC gels with the presence of anisotropic polymer network makes TN LC 5CB exhibit excellent electro-optical properties such as driving voltage reduction and contrast ratio enhancement. The unique properties of the supramolecular p-conjugated structure with high p-electron delocalization degree can efficiently drive LC molecules in much lower operating voltages. Therefore, we believe that the supramolecular LC gel with the anisotropic p-conjugated polymer network has potential to apply in various electro-optical devices for the ability of improving their performance in operating property. Acknowledgments One of us (C.Y.C.) acknowledges the support from National Taiwan University (grant number: 103R7560 & 104R7560), the Ministry of Science and Technology (grant number: 102-2112-M002-010-MY3), and the National Nano Device Laboratory of Taiwan. We would like to thank K. T. Chen for participation in the early phase of this work. References [1] D.J. Abdallah, R.G. Weiss, Organogels and low molecular mass organic gelators, Adv. Mater. 12 (2000) 1237e1247. € hlen, F. Fages, H. Bouaslaurent, J.-P. Desvergne, A novel [2] T. Brotin, R. Utermo small molecular luminescent gelling agent for alcohols, J. Chem. Soc. Chem. Commun. 6 (1991) 416e418. [3] C. Geiger, M. Stanescu, L.H. Chen, D.G. Whitten, Organogels resulting from competing self-assembly units in the gelator: structure, dynamics, and photophysical behavior of gels formed from cholesterol-stilbene and cholesterolsquaraine gelators, Langmuir 15 (1999) 2241e2245. [4] M. George, R.G. Weiss, Molecular organogels. Soft matter comprised of lowmolecular-mass organic gelators and organic liquids, Acc. Chem. Res. 39 (2006) 489e497. [5] K. Hanabusa, K. Hiratsuka, M. Kimura, H. Shirai, Easy preparation and useful character of organogel electrolytes based on low molecular weight gelator, Chem. Mater. 11 (1999) 649e655. [6] J.H. van Esch, B.L. Feringa, New functional materials based on self-assembling organogels: from serendipity towards design, Angew. Chem. Int. Ed. 39 (2000) 2263e2266.
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