Article pubs.acs.org/JPCC
Role of Monomer Alkyl Chain Length in Pretilt Angle Control of Polymer-Stabilized Liquid Crystal Alignment System Bang-Yan Liu, Ching-Hsuan Meng, and Li-Jen Chen* Department of Chemical Engineering, National Taiwan University, Taipei 10617, Taiwan S Supporting Information *
ABSTRACT: The pretilt angle of liquid crystal (LC) molecules in LC cells was manipulated by adding a mixture of two photocurable monomers, alkyl acrylate and biphenyl diacrylate, into the LCs after the UV treatment. The hexyl acrylate and octadecyl acrylate were applied to examine the alkyl chain length effect on the pretilt angle control under the condition of a fixed concentration of biphenyl diacrylate. The LC alignment was continuously adjusted from homogeneous to homeotropic alignment in the polymer-stabilized LC system by simply increasing the alkyl acrylate concentration in the LCs. At a given molar concentration of alkyl acrylate, the addition of octadecyl acrylate exhibits higher pretilt angle than that of hexyl acrylate. Both the pretilt angle of the LCs and advancing contact angle of water on the inner surfaces of LC cells simultaneously changed due to surface chemical nature and surface roughness. The pretilt angle increases along with an increase in the advancing contact angle. As a consequence, the surface advancing contact angle acts as an index for pretilt angle control.
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methods.16−21 To control the pretilt angle, monomers are doped into the LC solution. Polymerization is triggered while LC molecules are subjected to electric field, which guides the molecules into uniform alignment. After the treatment, the polymer network helps LCs to “memorize” the uniform configuration, even though the external field is absent. By controlling the degree of reaction,22 amplitude of applied voltage,23 or monomer concentration,24 the pretilt angle can be manipulated continuously. Whereas pretilt angle control with polymer has been comprehensively studied, the mechanism is still disputable. One hypothesis implies that the polymer network traps and “freezes” LC molecules in position during polymerization reaction, and these molecules keep their configuration and affect their neighbors onward.18,22,23 This theory well explains why LC molecules must be aligned before polymerization. Another theory suggests that the polymer film modifies the surface property, and thus the pretilt angle changes. The polymer network usually forms on the alignment layer, which alters the hydrophobicity of substrate. According to the semiempirical Friedel−Creagh−Kmetz (FCK) rule,25 LC molecules exhibit homeotropic alignment on low energy surfaces and homogeneous alignment on high energy surfaces.
INTRODUCTION Control of liquid crystal (LC) alignment is a critical technique for LC applications and has gained a great attention from both industry and fundamental research. Uniform LC alignment is essential for good electro-optic performance, and poor alignment can cause defects like reverse tilted disclinations. Among several LC alignment properties, one of the most important indexes is pretilt angle, which is the angle between LC director and the substrate. Different applications require specific pretilt angle; for example, conventional twisted nematic liquid crystal display (LCD) desires pretilt angle 1), the bead-like aggregation increased in both number and bead size with an increase in the CiA concentration. The grain size rose from 92 to 398 nm for the C6A system and from 72 to 225 nm for the C18A system. It is worth mentioning that the surface morphology at low CiA concentrations seems distinct from that at high CiA concentrations. The surface morphology of the substrate prepared at 0.4 wt % BD without CiA has flake-like texture, as the SEM images show in Figure 7a,a′. The addition of C6A at low concentrations would diminish the flake appearance on the background surface, as shown in Figure 7b′,c′. The AFM image was utilized to determine the mean roughness Ra (i.e., the arithmetic average of the absolute values of the surface height deviations measured from the mean plane and defined by R a =
1 n
n
∑ j=1 |Zj|). The AFM images can be
found in the Supporting Information. Table 1 reports the mean roughness measured by the AFM for those substrates prepared at various CiA concentrations. The mean roughness of the surface gradually decreased with an increase in the C6A D
DOI: 10.1021/acs.jpcc.7b07160 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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Figure 7. SEM images of inner surfaces of LC cells prepared at a fixed BD concentration of 0.4 wt % and at different C6A concentrations: 0.0 wt % (a,a′), 0.2 wt % (b,b′), 0.4 wt % (c,c′), 0.6 wt % (d,d′), 1.2 wt % (e,e′), and 2.0% (f,f′). The image on the right-hand side is the magnification of the image on the left-hand side. The scale bar stands for 1 μm.
Figure 8. SEM images of inner surfaces of LC cells prepared at a fixed BD concentration of 0.4 wt % and at different C18A concentrations: 0.0 wt % (a,a′), 0.2 wt % (b,b′), 0.4 wt % (c,c′), 0.6 wt % (d,d′), 1.4 wt % (e,e′), and 2.0 wt % (f,f′). The image on the right-hand side is the magnification of the image on the left-hand side. The scale bar stands for 1 μm.
concentration and reduced from 18.09 ± 4.35 nm (C6A concentration of 0 wt %) to 12.29 ± 1.82 nm at C6A concentration of 0.4 wt % (Figure 7b,c). The mean roughness increased to 17.93 ± 2.63 nm due to polymer aggregation appearing on the surface at C6A concentration of 0.6 wt %; however, the surface morphology did not significantly vary onward except for increasing the number of aggregates with further increasing C6A concentration. In contrast, the mean roughness of the substrates prepared at 0.2 wt % C18A concentration was raised up to 25.44 ± 3.58 nm due to small and randomly distributed aggregates on the surface (see Figure 8b′). It is believed that this sudden jump in the mean roughness is responsible to the “unusual” trend in advancing contact angle and pretilt angle. Then, the surface morphology became smooth again with further addition of
C18A and the mean roughness dropped down to 13.95 ± 3.34 and 14.41 ± 2.01 nm at C18A concentration of, respectively, 0.4 and 0.6 wt %, as the SEM images illustrated in Figure 8c′,d′. When the CiA concentration was further increased, the number and density of polymer aggregates on the substrate increased and developed into a polymer network with similar background morphology. The polymer network was composed of an assembly of bead-like structure chains, as the SEM image illustrated in Figure 7d′. With the further increase in C6A concentration, the size of polymer “beads” was enlarged (see Figure 7e′) and the “bead” boundaries melted down and it was hard to distinguish beads (see Figure 7f′). The morphology of C18A + BD polymer aggregates looked like strings of melted beads. The dimension of polymer aggregation networks was also enlarged with an increase in the C18A concentration, and E
DOI: 10.1021/acs.jpcc.7b07160 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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The polymer (or monomer) composition can be the reason why pretilt angle and contact angle behave so differently at low and high alkyl acrylate concentrations. The experimental data in the concentration region n(CiA)/ n(BD) ≥ 1 shown in Figures 5 and 6 are applied to describe the pretilt angle of LCs as a function of the advancing contact angle, as shown in Figure 10. The pretilt angle increased
Figure 9. Grain size of polymer aggregation at various CiA concentrations. These data were obtained from SEM images. Note that the surface morphology of the substrate prepared at 0.2 wt % C18A has flake-like texture instead of bead-like; therefore, grain size datum is unavailable.
Table 1. Mean roughness Ra of the substrates prepared at various CiA concentrationsa,b C6A concn. (wt%) 0.0 0.2 0.4 0.6 0.8
Ra (nm)
C18A concn. (wt%)
± ± ± ± ±
0.0 0.2 0.4 0.6 0.8
18.09 12.19 12.29 17.93 28.22
4.35 1.13 1.82 2.63 4.68
Figure 10. Variation of the pretilt angle of LC mixture E7 as a function of the advancing contact angle of water drops on the inner surface of LC cells prepared by using monomers C6A (blue circle and blue curve) and C18A (orange square and orange curve). The low concentration C18A results (0.2 and 0.4 wt %) showed unusual behavior in both advancing contact angle and pretilt angle, which caused deviation from the general tendency; therefore, these data do not present in this chart. The result of lauryl acrylate (C12A) and BD system24 is also included for comparison. Note that the mass ratio of C12A/BD was fixed at 2/1, distinct from the sample preparation in this study. The lines are only as a guide to the eye.
Ra (nm) 18.09 25.44 13.95 14.41 13.05
± ± ± ± ±
4.35 3.58 3.34 2.01 0.79
a
Ra = arithmetic average of the absolute values of the surface height deviations measured from the mean plane and defined by 1 n R a = n ∑ j = 1 |Zj|. bThe AFM images were applied to determine the surface roughness. At least two substrates prepared at a given CiA concentration were used and at least two different locations on each substrate were chosen to perform the AFM measurement. The mean roughness was determined by averaging over these measurements.
continuously along with an increase in advancing contact angle ranging from 103.8 to 119.9° for the C6A + BD system and from 103.8 to 130.6° for the C18A + BD system. Note that the pretilt angle steeply increased from homogeneous to homeotropic alignment along with increasing advancing contact angle for the C6A + BD system. Similarly, the pretilt angle of LC in the LC cells assembled by the microgrooved silica surfaces modified by organosilanes35 rapidly rose from 10 to 90° within the advancing contact angle window as narrow as 5°. The results illustrated in Figure 10 verified and confirmed that the pretilt angle is highly correlated to the advancing contact angle, consistent with the finding in literature.9,10,20,24,26,35 The pretilt angle of LCs monotonically increases along with an increase in the advancing contact angle. In other words, the advancing contact angle of water drops deposited on the inner surfaces of the LC cells can be used as an index to adjust the pretilt angle of LCs. In this study, the advancing contact angle was proposed and applied as an index for the surface hydrophobicity, which is subject to surface chemical nature and surface roughness.30,36 It is believed that the pretilt angle of LCs can be continuously adjusted by the surface hydrophobicity through the advancing contact angle. That is, the pretilt angle of LCs can be manipulated by the surface chemical nature and surface roughness. (a) The pretilt angle of LCs is manipulated by the surface chemical nature. Recently, the LC alignment on the microgrooved silica surfaces (at a fixed surface roughness) coated with organosilanes (to adjust surface chemical nature) has been examined. The pretilt angle of LCs continuously increased from
the morphology was further transformed into larger and bulky clusters, as the SEM images illustrate in Figure 8e′,f′. It is believed that all of the monomers, BD and CiA, added to the homogeneous LC mixture were completely polymerized to form the polymer network on the inner surfaces of the LC cell. The phase images of AFM demonstrated no material difference between polymer aggregates and background surface, implying that the polymer evenly covered the inner substrates of the LC cells. There was no phase separation or monomer aggregation during polymerization. Note that the BD concentration was fixed at 0.4 wt %, and solely increasing the amount of CiA would change the composition of the polymer and its surface property. Because alkyl chain of CiA is much more hydrophobic than biphenyl group of BD, the surface energy of as-prepared substrates would decrease along with an increase in CiA concentration. The evolution of surface morphology as a function of the CiA concentration discussed above is strongly related to the different compositions of BD and CiA. As mentioned above, n(CiA)/n(BD) = 1 divided pretilt angle and advancing contact angle into fluctuation (n(CiA)/n(BD) < 1) and monotonic increase (n(CiA)/n(BD) > 1) regions. This ratio, n(CiA)/n(BD) = 1, happens to be the watershed of “background morphology change” (n(CiA)/n(BD) < 1) and “bead size growth” (n(CiA)/n(BD) > 1) mechanism of polymerization process, implying that the morphology change is directly related to the polymer (or monomer) composition. F
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BD itself has only limited ability to raise pretilt angle before the occurrence of large polymer aggregations to make the LC cells blurry. The introduction of monomer C6A or C18A with long alkyl chain under the condition of constant BD concentration of 0.4 wt % was proven to raise the pretilt angle by augmenting polymer hydrophobicity to prevent the LC cells from blurring. The pretilt angle was successfully tuned by introducing C18A to a higher value than by introducing C6A with the same molecular number. Solely increasing the amount of CiA with fixed BD concentration resulted in topography change and enhanced surface hydrophobicity of polymer network. At n(CiA)/n(BD) < 1, the addition of CiA changed the surface morphology from flake-like into smooth appearance; at n(CiA)/n(BD) > 1, the grain size of polymer aggregates increased with CiA concentration. The increasing CiA amount also raised the hydrophobic composition in the polymer chain and therefore lowered the surface energy. Both increasing surface roughness and decreasing surface energy were found to give higher pretilt angle and contact angle, which results in the monotonically increasing tendency between contact angle and pretilt angle. Thus the advancing contact angle acts as a convenient index for pretilt angle control by simply manipulating the formula of a mixture of monomers.
0 to 90° along with an increase in the advancing contact angle of water drops on the organosilane coated flat silica surfaces ranging from 80 to 100°.9,35 (b) The pretilt angle of LCs is manipulated by the surface roughness. According to Wenzel model,36 the contact angle of hydrophobic surfaces would increase along with increasing surface roughness. The system with a fixed mass ratio of C12A/ BD at 2/1 has been applied to prepare the polymer-stabilized LC alignment system.24 The surface chemical nature should remain the same throughout the whole monomer concentration range due to a fixed composition. The polymer aggregated into bead-like structure as well, but the bead size was rather uniform and independent of monomer (C12A + BD) concentration. The number of beads increased to form chainlike clusters to further enhance the surface roughness with an increase in monomer (C12A + BD) concentration.24 For this C12A + BD prepared polymer stabilized LC alignment system, the surface chemical nature was fixed and the surface roughness was chosen as the sole effect on the surface hydrophobicity. It is interesting to find out that the pretilt angle also increased continuously and monotonically along with an increase in advancing contact angle window ranging from 79.9 to 109.7°,24 which is also included and plotted in Figure 10 for comparison. The reasonable explanation is that with the roughness rise, the contact area between polymer surface and LC is also increased; therefore, the pretilt angle control is enhanced. (c) The pretilt angle of LCs is manipulated by the surface chemical nature and surface roughness simultaneously. In this study, both surface chemical nature and surface roughness changed during solely increasing CiA concentration under the condition of a fixed BD concentration of 0.4 wt %. In this case, the surface chemical nature became more hydrophobic as more CiA was added to the system. However, the surface roughness was not monotonically increasing along with an increase in CiA concentration due to surface morphology change, especially at the concentration region n(CiA)/n(BD) < 1. Both the surface chemical nature and surface roughness would contribute to the advancing contact angle enhancement to a certain extent. This also explains the reason why different contact angle windows for pretilt angle control were consistently observed, as illustrated in Figure 10 due to the interplay of surface chemical nature and surface roughness. The pretilt angle of LCs increased continuously and monotonically along with an increase in advancing contact angle, as illustrated in Figure 10. It should be noted that the FCK rule claims that LC alignment is governed by the relationship between surface energy of substrate (γs) and surface tension of LCs (γLC): when γLC < γs, homogeneous alignment (pretilt angle = 0°); when γs < γLC, homeotropic alignment (pretilt angle = 90°). All experimental results discussed above confirm that the advancing contact angle of water drops deposited on the inner surfaces of the LC cells, instead of surface energy and surface tension of LC, can be used as an index to continuously and systematically adjust the pretilt angle of LCs. Note that the pretilt angle of LCs can be “continuously” manipulated by the advancing contact angle, in contrast to the limitation of homogeneous (pretilt angle = 0°) and homeotropic (pretilt angle = 90°) alignment predicted by the FCK rule.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b07160. Additional information about AFM images, surface roughness, and mean roughness of substrates prepared at low monomer concentrations. (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Li-Jen Chen: 0000-0003-3565-551X Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank Professor S. L. Wu of Tatung University for his technical assistance and guidance of synthesizing the chemical biphenyl diacrylate used in this study and Professor J. Y. Lee of National Taiwan University of Science and Technology for his technical assistance. This work was supported by the Ministry of Science and Technology of Taiwan.
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REFERENCES
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CONCLUSIONS Pretilt angle control by photocurable monomers with different side chain lengths was demonstrated. According to the FCK rule, LC prefers homeotropic alignment on low-energy surfaces. G
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