Role of Surface Hydrophobicity in Pretilt Angle Control of Polymer

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The Role of Surface Hydrophobicity in Pretilt Angle Control of Polymer-Stabilized Liquid Crystal Alignment Systems Bang-Yan Liu, and Li-Jen Chen J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp403002d • Publication Date (Web): 11 Jun 2013 Downloaded from http://pubs.acs.org on June 15, 2013

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The Journal of Physical Chemistry

The Role of Surface Hydrophobicity in Pretilt Angle Control of Polymer-Stabilized Liquid Crystal Alignment Systems

Bang-Yan Liu and Li-Jen Chen* Department of Chemical Engineering, National Taiwan University, Taipei 10617, Taiwan *Corresponding author, email address: [email protected].

A certain amount of photocurable monomers biphenyl diacrylate and lauryl acrylate were added to liquid crystal (LC) molecules. The LC alignment was stabilized with a certain pretilt angle after a fixed amount of UV exposure under an applied voltage to the LC cell. The pretilt angle of the LC was successfully adjusted over the whole range from 0.8° to 90° continuously by simply increasing the monomer concentration in the LCs. Polymer bumps formed with a rather uniform size around 250 nm in diameter and chain-like clusters were found on the polymer-coated substrate. The advancing contact angle of water was applied as an index of the surface hydrophobicity. The pretilt angle of the LC monotonically increases along with an increase in the advancing contact angle of water on the polymercoated substrate. The surface hydrophobicity of the substrate plays an essential role in governing the pretilt angle in the polymer-stabilized LC alignment system.

Keywords: Liquid Crystal, Hydrophobicity, Advancing Contact Angle, Pretilt Angle, Polymer Stabilization.

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INTRODUCTION

The alignment of nematic liquid crystal (LC) molecules is of great interest for both industrial applications and fundamental research. Liquid crystal alignment strongly depends on the interface properties. The uniform alignment of LCs is essential for the electro-optical performance in the liquid crystal display (LCD) industry. Particularly, an appropriate pretilt angle of LCs is necessary for the twisted nematic liquid crystal display to prevent reverse tilted disclinations upon exerting an external electric field. Recently, the pretilt angle control of liquid crystal alignment has been intensively developed and investigated.1–11 One of the most important technologies is the introduction of photocurable monomers.12–19 The polymerization of photocurable monomers can be applied to adjust the pretilt angle of LCs. This process is so-called polymer-stabilized LC alignment, which is rather easy and convenient, and already becomes a practical application in the LCD industry, such as: polymer-stabilized vertical alignment.16 Usually photocurable monomers are doped into LC molecules, and injected into the LC cell. A voltage is applied to the LC cell that drives the liquid crystal molecules with a certain pretilt angle. The LC cell is then exposed to UV radiation to trigger the polymerization that stabilizes the LC alignment with a certain pretilt angle even without any applied voltage. The pretilt angle of LCs can be manipulated to a certain extend by adjusting the driving voltage17 or UV dosage.14,18 Despite this technology has been already utilized in the LCD industry, the exact mechanism remains an open question. It has been conjectured that during the UV exposure a polymer film layer is synthesized on the substrate and the polymer network structure is formed to trap and align LC molecules with a prescribed pretilt angle under an applied voltage.13,14,17 It has been found that the monomers tend to locate around cell surface,20 where polymer films were also found.18,19 On the other hand, the control of pretilt angle of LCs has also been investigated by modifying chemical properties of the alignment layer surfaces, for example, the introduction of long linear alkyl ACS Paragon Plus Environment

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side chains or other nonpolar groups to the polymer alignment layers,21,22 the mechanical rubbing of polymers by varying rubbing conditions,1,2,8,23 the alignment layer treatments with UV exposure or O2 plasma,7,21 and doping24,25 or composition adjustment26,27 in alignment layer. All these studies demonstrated a consistent tendency between pretilt angle and surface energy of alignment layer. That is, the pretilt angle of LCs increases along with decreasing surface energy of alignment layer. Moreover, the pretilt angle of LCs has been successfully adjusted over the whole range from 0° to 90° continuously by tuning the surface chemical property of silica alignment layer through the deposition of self-assembled monolayers of organosilanes with different functionalities.6 It has been demonstrated that the pretilt angle increases along with increasing surface hydrophobicity (or advancing contact angle of water), instead of using surface energy.1,5,21,27 It is plausible to conjecture that the pretilt angle of LCs in the polymer-stabilized LC alignment systems is simply driven by the surface hydrophobicity. The purpose of this study is to verify this conjecture by using the advancing contact angle as an index of surface hydrophobicity. Photocurable monomers biphenyl diacrylate and lauryl acrylate are introduced into the LC cell along with LC molecules and the LC molecules are stabilized with a pretilt angle by polymerization under UV radiation. The pretilt angle of LCs can be successfully controlled over a wide range from 0.8° to 90° by varying the monomer concentration in LC molecules. It is found that the pretilt angle is directly related to surface hydrophobicity (or advancing contact angle).

EXPERIMENTAL SECTION

ITO glass substrates were cut into the size of 2 x 3 cm, and cleaned by detergent (Extran MA 02, Merck), acetone (Merck) and water in series under sonication. Water was purified by doubledistillation and then followed by a PURELAB Maxima Series (ELGA Labwater) purification system with the resistivity always larger than 18.2 MΩ·cm. After the substrates were blown dry by nitrogen, a ACS Paragon Plus Environment

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polyimide layer (DA-7013, courtesy provided by Daxin Materials Corp., Taiwan) was spin-coated on the conductive side of ITO glass substrates. A rubbing process was followed to align LC molecules. The LC cell was assembled by two polyimide-coated ITO glass substrates separated apart by inserting 6 µm Mylar film (DuPont) at two long edges and the polyimide-coated sides were facing each other in counter rubbing direction. The LC mixture E7 (from Merck) with the nematic-isotropic transition temperature of 59 °C was used in this study. Photocurable monomers biphenyl diacrylate and lauryl acrylate (from SigmaAldrich) were mixed at a mass ratio of 1:2.12,14 The structural formulae of these two monomers are illustrated in Figure 1.A small amount (5% in mass relative to lauryl acrylate) of photo-initiator Irgacure 184 (from BASF) was doped to the monomer mixture. This monomer mixture was then added to the host LC mixture E7 at the concentration ranging from 0% to 3% (in mass) and filled into the LC cells by capillarity. The LC cells were then heated to about 65 °C for 5 min to reach the isotropic phase of LC mixture E7 and were subsequently cooled at room temperature to get into the nematic state. For polymerization of those photocurable monomers, the LC cells were exposed to UV radiation and subject to an applied voltage of 10 Vpp in amplitude with a square wave of 1 kHz, which was supplied from a function generator (33210A, Agilent). The UV source had a peak wavelength at 365 nm with the intensity of 60 mW/cm2 and exposure time was always fixed at 30 min in this study. The UV light was illuminated from both sides of the LC cell, as schematically illustrated in Figure 2, to avoid variation of polymer morphology between two substrates of the LC cell. Note that the polymer morphology of two substrates of the LC cell after the UV exposure from only one direction would be different from each other, consistent with the results of Lyu et al.18 During the UV treatment, the surrounding temperature was controlled always lower than 50 °C to prevent the LC from entering isotropic phase. The crystal rotation method28 was applied to determine the pretilt angle of the as-prepared LC cell by using a homemade system with an accuracy of 0.1°. After the pretilt angle measurement, the LC ACS Paragon Plus Environment

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cells were carefully disassembled and the substrates were dipped into n-hexane (Merck) for 5 sec to remove any residual LC. The advancing contact angle of water on these substrates was measured by a homemade enhanced video-microscopy system incorporated with a digital image analysis. The details of the methodology and experimental setup can be found elsewhere.29 The accuracy of the advancing contact angle measurements is better than ± 0.1°. The surface morphology formed by the polymerization was examined by scanning electronic microscope (SEM, JSM-5600, JEOL) and atomic force microscope (AFM, Digital Instrument, NanoScope IIIa).

RESULTS AND DISCUSSION

The pretilt angle of the as-prepared LC cells at various monomer concentrations was determined by using the homemade apparatus based on the crystal rotation method. The pretilt angle of liquid crystal E7 in the LC cell assembled by the bare rubbed polyimide substrates without introducing the photocurable polymers was 0.8°. The results of the pretilt angle as a function of the monomer concentration in the LC/monomer mixtures are shown in Figure 3. It is interesting to observe that the pretilt angle monotonically increases from 0.8° to 90° along with an increase in the monomer concentration from 0 to 2.5 wt%, and the pretilt angle certainly remains 90° for the monomer concentration higher than 2.5 wt%. It is widely believed that there is a polymer network structure to align the liquid crystal molecules with a fixed pretilt.13–19 To understand the effect of additives on LC pretilt angle, the advancing contact angle of water on the substrate was measured as an index of surface hydrophobicity.30–32 Higher advancing contact angle of water corresponds to higher surface hydrophobicity and lower surface energy. Figure 3 also shows variation of the advancing contact angle of water on the substrate as a function of monomer concentration. Figure 3 demonstrates the advancing contact angle increased along with an increase in the monomer concentration. That means the substrate surfaces were modified by the polymer stabilization ACS Paragon Plus Environment

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procedure and the surfaces got a higher hydrophobicity. Both pretilt angle and surface advancing contact angle rose along with increasing total monomer concentration, and these data were applied to plot Figure 4 demonstrating the variation of the pretilt angle as a function of the advancing contact angle. The pretilt angle rose monotonically from 0.8° to 90° along with increasing the advancing contact angle within contact angle window of 30° (ranging from 80° to 110°). This result confirmed that the pretilt angle is highly related to the advancing contact angle (or surface hydrophobicity), consistent with the finding of Chiou and Chen.6 To verify how the polymerization process enhanced surface hydrophobicity, the substrates were examined by using SEM. Figure 5 shows the SEM images of polymer morphology of substrates prepared at various monomer concentrations. When the monomer concentration was lower than 1.5 wt%, no obvious polymer bump was found. While the monomer concentration was as high as 1.5wt%, small aggregation started to appear on the substrates, as shown in Figures 5(a) and 5(b). The number of polymer aggregations increased as the monomer concentration increased, as shown in Figures 5(a), 5(c), 5(e), and 5(g). And the particle size of polymer aggregations was quite uniform with a diameter of around 250 nm. Interestingly, the particles seemed preferring to gather as clusters rather than spread around on the surface, thus the particle coverage on the surface increased rather slowly. The adhesion between polymer clusters and surface was so strong that the clusters still attached on the surface after repetitive rinse by ethanol and acetone (before advancing contact angle measurement). It is also worth to mention that the particles tended to assemble into a long chain structure, and the chain length was as long as several micrometers, as shown in Figure 5(h). It cannot be ruled out the possibility that these long chains might form certain polymer network, such as column structure, inside the bulk liquid crystal phase in-between two substrates. AFM was applied to map materials differences between the surface and aggregated particles by using the tapping mode. The phase imaging (not shown here) confirmed that both the aggregated particles and the surface below were made of identical material. That means the monomers not only ACS Paragon Plus Environment

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polymerized as particles, but also formed a film layer coated on the substrate. Both pretilt angle of LC and advancing contact angle substantially increased at very low monomer concentration 0.5 wt% without the appearance of polymer bumps. It is plausible to conjecture that, the monomers were polymerized to form a film layer on top of the polyimide surface at low monomer concentrations, and aggregations started to occur after the polyimide surface had been fully covered by the polymer film. Biphenyl diacrylate plays a critical role in this process. The benzene structure of biphenyl diacrylate had high affinity with ring structure in polyimide by the π-π interaction. It is plausible to conjecture that biphenyl acrylate would lie on the polyimide surface and lauryl acrylate would extend to the LC bulk phase after polymeriation.14 In addition, a system of adding 0.5 wt% biphenyl diacrylate to LC E7 only without lauryl acrylate was examined and the pretilt angle of LC could be adjusted from 0.8° only up to 34°. This indicated that biphenyl diacrylate can work by itself to form a film layer, which was not hydrophobic enough to adjust the pretilt angle higher than 34°. The introduction of lauryl acrylate to the monomers would dramatically enhance the surface hydrophobicity to drive the pretilt angle up to 90°. It is well understood that the surface hydrophobicity is mainly affected by two factors: surface chemical nature and surface topography (or surface roughness).29,33 Before polymer bumps started to form on the substrates of LC cells, the surface hydrophobicity was governed by the surface chemical property resulted from stacking of polymer film on top of polyimide surface.5 Consequently, surface energy decreased mainly due to the introduction of long alkyl chain of lauryl acrylate on top of polyimide surface. It is obvious that the advancing contact angle (or surface hydrophobicity) of the substrate increased along with increasing the monomer concentration and became level off for the monomer concentrations higher than 1.0 wt%, as shown in Figure 3. As the monomer concentration further increased up to 1.5wt% and beyond, the surface hydrophobicity was also dominated by the surface topography due to the formations of polymer bumps and of long chains of aggregated particles, as mentioned above. Note that the formations of polymer bumps and of long chains of aggregated ACS Paragon Plus Environment

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particles would introduce and enhance the surface roughness that makes hydrophobic surface more hydrophobic, known as the Wenzel model.34 Consequently, the advancing contact angle took off around the monomer concentration of 1.5 wt% to further increase along with increasing the monomer concentration, as shown in Figure 3. Therefore, this study confirms that photocurable monomers can be applied to modify both chemical nature of surface and surface topography to induce a rise in LC pretilt angle. It should be pointed out that Chen and Chu14 demonstrated that the pretilt angle of LCs can also be controlled to a certain extent (44°-88°) by varying the UV dosage (exposure time) at a fixed monomer (both monomers biphenyl diacrylate and lauryl acrylate were used) concentration. It was found that the pretilt angle increases along with increasing the UV exposure time (shown in Figure 3 of Ref.14). It is plausible to understand that increasing the UV exposure time would enhance the extent of polymerization and increase the surface hydrophobicity. Consequently, this experimental result14 also confirms that the pretilt angle can be successfully manipulated by simply adjusting the surface hydrophobicity.

CONCLUSION

In this study, the relationship between surface hydrophobicity and LC pretilt angle is investigated in polymer stabilized LC alignment system. LC pretilt angle increased with increasing doped monomer amount after UV treatment. Polymer layer formed on inner cell surfaces at low monomer concentrations. While the concentration reaches 1.5 wt% and beyond, polymer bumps started to form with uniform size around 250 nm in diameter. When the monomer concentration further increased, the number of aggregated particles raised and chain-like clusters were found on the surface. The advancing contact angle was measured and used as an index of surface hydrophobicity. It was observed that the pretilt angle strongly depends on the surface hydrophobicity (or the advancing contact ACS Paragon Plus Environment

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angle). In spite of slight influence from the change of surface morphology, the tendency remained. Surface hydrophobicity was found to govern the LC pretilt angle in polymer stabilized LC alignment system.

ACKNOWLEDGEMENTS The authors would like to thank Professor S. L. Wu of Tatung University for his technical assistance and guidance of synthesizing the chemical biphenyl diacrylate used in this study, Professor J. Y. Lee of National Taiwan University of Science and Technology for his technical assistance, and Professor G. S. Liou of National Taiwan University for meaningful discussion. This work was supported by the National Science Council of Taiwan.

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Figure captions

Figure 1. Structural formula of photocurable monomers: (a) biphenyl diacrylate, (b) lauryl acrylate. Figure 2. Schematic illustration for experimental setup. A LC cell containing photocurable monomers was subject to a voltage 10 Vpp and 1 kHz, meanwhile this LC cell was exposed to UV light from both sides to trigger the polymerization. Figure 3. Variation of pretilt angle (triangle) of LC E7 and the advancing contact angle (circle) of water on the substrate as a function of total monomer concentration. Figure 4. Variation of the pretilt angle of LC mixture E7 as a function of the advancing contact angle of water. Figure 5. SEM images of LC cell inner surfaces prepared at different total monomer concentrations: 1.5 wt%, (a) and (b); 1.8 wt%, (c) and (d); 2.5 wt%, (e) and (f); 3 wt%, (g) and (h).


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Figure 1.

Figure 1. Structural formula of photocurable monomers: (a) biphenyl diacrylate, (b) lauryl acrylate.


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Figure 2.

Figure 2. Schematic illustration for experimental setup. A LC cell containing photocurable monomers was subject to a voltage 10 Vpp and 1 kHz, meanwhile this LC cell was exposed to UV light from both sides to trigger the polymerization.


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Figure 3.

Figure 3. Variation of pretilt angle (triangle) of LC E7 and the advancing contact angle (circle) of water on the substrate as a function of total monomer concentration.


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Figure 4.

Figure 4. Variation of the pretilt angle of LC mixture E7 as a function of the advancing contact angle of water.


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Figure 5.

Figure 5. SEM images of LC cell inner surfaces prepared at different total monomer concentrations: 1.5 wt%, (a) and (b); 1.8 wt%, (c) and (d); 2.5 wt%, (e) and (f); 3 wt%, (g) and (h).


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Role of Surface Hydrophobicity in Pretilt Angle Control of Polymer

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