Photoresponsive Self-Assembled Systems Cheng, Jinling
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Controlling Orientation of Liquid Crystals at
Aqueous-Cholesteric Liquid Crystal Interfaces
The nematic state is the simplest LC phase, which has shown to offer great potential in the development of chemical and biological sensors. To data, the available literature on chemical sensing or biosensing based on other LC phases is extremely scarce. In this chapter, we have examined the orientational ordering of twisted (chiral) nematic or cholesteric liquid crystal (CLC) phases when contacting with aqueous solution. We used molecular motors as chiral dopants in the E7 host to provide the CLC phase and sodium dodecyl sulfate (SDS) surfactant as aqueous solution. We found that the orientation of the LC molecules at the aqueous-LC interface is significantly influenced by two parameters, one is the helicity of the CLC film and the other is the concentration of surfactant. Furthermore, we showed that the photochemical and thermal isomerization of molecular motors in the system can modulate the orientation of the LC molecules at the interface
4.1 Introduction
Supramolecular assemblies, taking advantages of the collective effects of weak non-covalent interactions (e.g., van der Waals interactions, π-π interaction, hydrogen bonds, etc.), are the basis for non-polymer based soft materials that are sensitive to external stimuli.1 When supramolecular assemblies exhibit long-range orientations, such as the ordering within liquid crystalline phases, molecular properties can be amplified into mesoscopic phenomena. Liquid crystals (LCs) have been extensively studied as functional soft materials due to their fluidic and anisotropic properties which are positioned in between liquids and crystals.2–4 Previous studies have established that ordering transitions in liquid crystals (LCs) can be regulated by (i) the nature of the mesogenic molecule itself and (ii) interfaces which confine the material.5 At interfaces, in particular, the ordering transitions of LCs are remarkably delicate depending on, (i) interfacial energetics that controls the phenomenon (typically 10-3 -10-6 J/m2)6,7 and (ii) the distance over which the ordering of LCs can propagate.8 Inspired by the sensitivity of the ordering of LCs to the chemical functionality and organization of molecules at interfaces, recent studies have examined the ordering of LCs at surfaces, which can be classified into three categories : (i) LC- air interfaces,9,10 (ii) LCs- solid interface,5,6,11 (iii) LCs– aqueous interfaces (Figure 4.1).6,12 Among these investigations, the interface between LCs and aqueous solutions is receiving growing attention since the system can be used as sensing material.12–16 It has been revealed that some molecular species can disrupt the interfacial structures that therefore disturb particular LC orientations. In addition, the orientational changes within the bulk of the LC phases in return regulates optical properties enabling the reporting of interfacial events which provide a conventional method to predict the LC molecular orientation.
Figure 4.1 Illustrations of ordering of LCs at different interfaces. (a) Long-range orientational
ordering of mesogens in a nematic LC, 5CB. (b) LC- solid interfaces with planar alignment. (c) LC-solid interface with vertical alignment. (d) LC- air interface. (e) LCs– aqueous interface. Octadecyltrichlorosilane (OTS)-coated substrate induces vertical ordering and rubbed polyimide substrate induces planar ordering.
For instance, it has been reported by Abbott et. al. that thermotropic LCs containing nitrile groups (such as 5CB) can undergo anion-specific ordering transitions at interfaces with aqueous salt solutions as illustrated in Figure 4.2.17 It is revealed that only bulky single charged anions, such as I-, ClO4- and SCN- exhibit positive surface excess concentrations at
isotropic interfaces and are able to modulate the ordering transitions. The 5CB LC undergoes concentration-dependent transition from planar to homeotropic orientation at the aqueous-LCs interface with increasing concentration of the anions in aqueous phase (Figure 4.2a-c). In contrast, small single or multiple-charged anions like F-, Br-, SO42-, Cl-, HPO4-, do not interrupt
orientational transitions as shown in Figure 4.2d.
Figure 4.2 POM images and the corresponding schematic illustration of nematic film of 5CB
with homeotropic substrate at the bottom in contact with aqueous solutions. (a) ultrapure Milli-Q water; (b) 2 M aq. NaClO4; c) 2 M aq. NaSCN and d) 2 M aq. NaCl. The images are
reproduced with permission from Ref 17. Copyright 2002 American Chemical Society. The self-assembly of synthetic surfactants at aqueous-LC interfaces has received particular attention in recent studies.18–22 For instance, lithium dodecyl sulfate (LDS), and dodecylbenzene sulfonate (DBS) can also disturb the organization at the interfaces, as reported by Abbott19-22 and Richmond23. The infrared-visible vibrational sum frequency spectroscopy (VSFS ) was used to characterize the organization of surfactants at water-LC interfaces where the monolayer of surfactant was determined by measuring and comparing of the vibrational spectra of these adsorbed surfactants.23–24 The linear surfactants at the aqueous-LC interfaces pack efficiently that can dictate the orientations of LC molecules at aqueous-LC interfaces. Furthermore, it was observed that classical surfactants like alkyltrimethylammonium halides (CTABs, with longer chains C>8), sodium dodecyl sulfate (SDS), and N,N-dimethylferrocenylalkylammonium bromides (F-CABs, C > 12) adopt linear and tilted orientation at the interface (tilt angle is around 10o), which can give rise to a homeotropic orientation of 5CB at the interfaces (Figure 4.3a and c). The surfactants, which have a bolaform structure like dodecyl-1,12-bis(trimethylammonium bromide) (DBTAB),
((11-hydroxyundecyl)trimethylammonium bromide (HTAB),
11-(ferrocenylundecyl)trimethylammonium bromide (FTMA) adopt a looped configuration which induce planar anchoring of 5CB at the aqueous-LC interface (Figure 4.3b and d).
Figure 4.3 (a and b) Optical images (crossed polarizers) of 5CB confined to a gold grid
supported on an OTS-coated glass slide and in contact with aqueous solutions of SDS and DBTAB. (c and d) The proposed configurations of the surfactants and the orientation of LC at the aqueous-LC interface. The images are reproduced with permission from Ref 19 and 22. Copyright 2002 American Chemical Society and Copyright 2005 American Chemical Society. Several studies to modulate LC orientations at aqueous-LCs interfaces have been performed by exploring a range of bio-molecular interactions, like phospholipids,25,26 enzymes,27,28 peptides,29 proteins30,31 and DNAs32, which demonstrates that the LC film can be used as a transducer element for biosensors. For example, many biologically relevant lipids, such as D-α-dipalmitoyl phosphatidylcholine (D-DPPC) monolayer on the interface of the LCs, resulting in a discontinuous ordering transition in which the interface changes from an orientation that is parallel (planar alignment) to the interface to an orientation that is perpendicular to the interface (homeotropic alignment).27 Further studies have revealed that specific binding can mediate the LC ordering involving protein, nucleic acids, DNA-DNA, DNA-Ligand binding events that can disrupt the interfacial structures as transition to homotopic orientation take place (Figure 4.4).33 It can be seen that the adsorption of octadecyltrimethylammonium bromide (OTAB) surfactant to the LC/aqueous interface resulted in homeotropic LC alignment (Figure 4.4b). Subsequent adsorption of single-stranded DNA (ssDNA) to the surfactant-LC interface modified the interfacial structure, resulting in a reorientation of the LC from homeotropic alignment to an intermediate tilted orientation (Figure 4.4c). Exposure of the ssDNA/OTAB interfacial complex to its ssDNA complement induced a second change in the interfacial organization characterized by the nucleation, growth, and coalescence of lateral regions that induced homeotropic LC alignment (Figure 4.4d)
Figure 4.4 (a) Optical micrographs (crossed polarizers) of an optical cell of 5CB in contact
with water. (b) Adsorption of OTAB at the LC/aqueous interface causes homeotropic alignment of the LC layer. (c) Birefringent regions appear upon exposure of the OTAB-laden interface to an ssDNA probe. (d) Hybridization of 1∙10-9 mmol of ssDNA target to an OTAB/ssDNA interfacial layer. The images are reproduced with permission from Ref 33. Copyright 2013 American Chemical Society.
All of the studies described above have shown that different adsorptions can disturb the ordering of bulk LCs at interfaces between aqueous phase and nematic LCs. In contrast, in the present study, we shifted the focus to interfaces formed between cholesteric LCs (CLCs) and aqueous solutions. Specifically, we aim to determine if a surfactant can be assembled at the aqueous- CLC interface and trigger an ordering transition in the bulk LC phases. The majority of the experiments in this work were performed using a model system comprised of SDS surfactant which can adsorb at the aqueous interface and CLCs in which the molecular motors are used as chiral dopants into E7 to induced helical superstructures. In this strategy, different concentrations of doped motors in E7 were employed to make different helical pitches for CLC systems. Subsequently, SDS surfactants with different concentrations were used to study the interaction between the aqueous-CLC interfaces. Finally, we investigated whether the photochemical and thermal isomerization of enantiomerically pure molecular motors used as a chiral dopant in the nematic LC material (inducing a CLC phase) can modulate the orientation of LCs at the interfaces. These experiments are significant in guiding the design of dynamic and responsive LC interfaces for the sensing of chemical and biological molecules.
4.2 Results and Discussion
4.2.1 Concept and Design
In our investigation, we employed E7 as the LC host (Figure 4.5a) and enantiomerically pure second generation motor M1 (Figure 4.5b) as the chiral dopant. Of particular relevance to the study reported in this chapter, we recently described the performance of a series of second-generation motors in E7.(more details can be found in Chapter 3). Among these motors, M1 with a phenyl substituent at the stereogenic center possess high helical twisting
power (HTP = 87 μm-1) as a dopant in E7 LC host material. In addition, upon irradiation and subsequently thermal relaxation, the M1 doped LC material presents significant changes in the HTP and helical superstructures. The addition of buffering salts to the aqueous phase was avoided in order to minimize the number of ion types in our experimental system.
Figure 4.5 Molecular structures of (a) nematic E7, (b) chiral dopant M1, (c) surfactant SDS
used in this study. (d) Orientation of the director of the LC at a surface. (e) Schematic illustration of three modes of strain of LCs: (i) splay, (ii) twist, and (iii) bend. (f) Procedure to make the LC –aqueous interfaces.
A film of LC with an approximately flat interface was prepared by filling the pores (300 µm x 300 µm) of a 20 µm-thick copper-coated grid supported on glass slide (Figure 4.5c), following published procedures.34 Briefly, the slides were cleaned and coated with OTS to induce the orientation of LC perpendicular to the LC-glass interface (homeotropic alignment). Immersion of the LC-filled cell under aqueous solution yields a stable interface between the bulk LC phase and the aqueous solution. Additional details on sample preparation are presented in the experimental section (4.4, Experimental Section).
4.2.2 Interfaces Between Nematic Liquid Crystal (E7) and SDS Surfactant.
While several previous studies have reported on the spontaneous adsorption of surfactants like SDS at aqueous-LCs, comprising of 5CB, the adsorption of SDS at the interface using E7 has not been characterized as far as we know. Therefore, we performed a series of measurements to determine the extent at which concentration of SDS surfactant, adsorbed from aqueous solution at interfaces of nematic E7, induces the reorientation of LC molecules at the interface.
In these experiments, a series of E7 interfaces against air, pure water or aqueous solutions containing different concentration of SDS (T = 25 °C) was prepared. The resulting LC interfaces were imaged using Polarized Optical Microscope (POM). Figure 4.6 shows micrographs of different interfaces that were incubated against either air or different aqueous solutions. Literature studies have shown that the system is consistent within a wide range of pH values (0-9),17 and in our case, the pH is determined to be 6.8. Furthermore, ring patterns did not appear during the experiments, suggesting that the film thickness remained constant.12
Figure 4.6 Polarized micrographs (crossed polarizers) and schematic illustrations of nematic
films of E7 in contact with air (a and g) and aqueous solutions of ultrapure Milli-Q water (b and h), 0.1 mM, 0.25 mM, 0.5 mM SDS (c-e, i) 0.75 mM SDS (f, g). Inset in (f) is a conoscopic image confirming homeotropic alignment. In (a−f), the E7 is supported on a OTS-treated glass surface. The size of the each copper grid is ~300 µm x 300 µm and thickness of the sample is ~20 μm.
Figure 4.6a represents a texture with air on top of the film. Both the OTS-coated substrate and the air layer promotes homeotropic orientation of E7 which results in a black image as the orientation of the LC shown in Figure 4.6g. Notice that the orientations of the LCs are influenced by contacting with the grid as bright regions (with planar alignment ) can be observed along the grid frame.17 The region of LCs that is modulated by the frame is less than 10 μm, which is negligible when compared to the grid spacing (~300 µm). Thus, we speculated that the textures of E7 confined within the copper grid are dominated by interactions with the free interfaces of LCs and not the grid surfaces. Under white light illumination (crossed polarizer), the E7 in contacting with the pure water exhibited
interference colors (Figure 4.6b) which is consistent with a LC film that is orientated parallel to the aqueous interface (so-called planar anchoring) and perpendicular to the OTS-coated glass slide (homeotropic anchoring).14,19,20,26,35 The planar anchoring on the top and homeotropic anchoring at the bottom indicate that the bulk of the LC film is thus splayed and bent to accommodate the hybrid boundary conditions as illustrated in Figure 4.6h. We also realize that a prominent feature of the optical appearance of the LC film is the presence of dark brushes. This feature is caused by variation of the azimuthal orientation of the LC within each grid (LC molecules have planar alignment at the interface and homeotropic alignment at the bottom).19,21 In addition, the equilibrium alignment of E7 was observed to occur within few seconds of exposure of the LC films to the aqueous solution which is in good agreement with the time scale in previous studies with nematic 5CB-aqueous interfaces.20
It was demonstrated earlier that aqueous solutions of SDS surfactant were known to cause homeotropic (perpendicular) anchoring of 5CB at high concentrations (1.00 mM) and planar anchoring at low concentrations (≤ 0.50 mM).21 Next, we aim to determine if the presence of SDS surfactant (in a range of 0.10-1.00 mM concentration) can be adsorbed onto the LC interface and further alter the dynamic ordering transition of E7. Upon contacting with aqueous SDS solutions, the parallel orientation of E7 at the aqueous interface is kept unchanged, as the aqueous SDS solutions concentration varied from 0.10 mM to 0.50 mM. However, a comparison of color of b-e in Figure 4.4 reveals that the adsorption of surfactant at the interface of the LC resulted in a slow but continuous progression of interference colors as a tilted orientation at the top interface induced a lower optical retardance. This lower retardance resulted in intense coloration in regions outside of the dark brushes (Figure 4.6d, e and i). Analysis of the interference colors can provide quantitative information (Michel−Levy color chart) regarding the tilt angles of the LC at the interface according to the equation (1),19 as: ∆𝑛𝑒𝑓𝑓 ≈ 1𝑑∫ ( 𝑛0𝑛𝑒 √𝑛02sin𝑧𝜃𝑑 2 +𝑛𝑒2cos𝑧𝜃𝑑 2− 𝑛𝑒)𝑑𝑧 𝑑 0 . (1)
where n0 and ne are the indices of refraction parallel and perpendicular to the optical axis of
E7, respectively, θ is the tilt angle of E7 measured relative to the surface normal (as indicated in Figure 4.5d) and d is the thickness of the LC film, in our case d= 20 µm. The indices of refraction of E7 are taken as constant using the values reported at 25 °C and λ=589 nm, n0 = 1.739 and ne = 1.522.36 The tilt angles take the average of several different random
locations of two independent grids. As calculated, the angles decrease roughly from 85º to 65º upon increasing the concentration of SDS solutions (the error in the tilt angles being ~ 10°) as the birefringence changed from 0.035 to 0.015.
The immediate transition (with 10 s) in optical appearance from bright green to a uniformed dark state is observed as the concentration of SDS is increased above 0.75 mM, corresponding to an ordering that changed from planar to homeotropic at the aqueous interface (Figure 4.6f). Additionally, conoscopic imaging (inset of Figure 4.6f) is used to confirm the homeotropic alignment (see the experiment section for details). We note that each of the dark squares in the grid in Figure 4.6f is framed by a bright edge. The bright edge is the result of perpendicular anchoring of the LC on the vertical walls of the grid.17 To
confirm that the bulk phase behavior of E7 is not significantly affected by contacting with the aqueous solutions containing molar concentrations of SDS surfactant, we measured the clearing temperature of E7 under different concentration of the SDS solution.17 The isotropic temperature of pure E7 under free surface was measured to be 58.0 ±0.1 °C; the isotropic temperature of equilibrated E7 with either pure water, aqueous 0.10 M SDS, aqueous 0.50 mM SDS, or 0.75 mM SDS on the top are 57.9 ± 0.1 °C, 57.9 ± 0.1 °C, and 58.0 ± 0.1 °C, respectively. Overall, the results in Figure 4.6 indicated that the presence of SDS in the aqueous phase could trigger an ordering transition of the LC molecules at the nematic E7-aqueous interface. Overall, E7 is a potential platform for measuring the orientational transformation.
4.2.3 Cholesteric Liquid Crystal (E7) with Different Concentration of Motor-Dopants
Next we aimed to demonstrate the interfacial phenomena at the CLCs-aqueous interfaces. The most notable feature of the CLCs is the helical superstructures of the bulk LC phases. A number of previous studies have demonstrated that the morphologies of aqueous-NLCs interfaces were shown to be strongly dependent on the elastic properties of the LC materials.14,25 The bulk elastic energy due to three elastic constants of a nematic LC is:37
𝐹 = ∫ ⌊𝐾11 2 (∇ ∙ 𝒏) 2+𝐾22 2 (𝒏 ∙ ∇ × 𝒏) 2+𝐾33 2 (𝒏 × ∇ × 𝒏) 2⌋ 𝑑 0 (2)
Where, K11, K22 and K33 are Frank elastic constants that pertain to splay, twist and bend elastic deformations of a thermotropic LC (Figure 4.5e). We considered the standard nematic liquid crystal E7 (supplied by Merck), with elastic constants K11 = 12 pN, K22 = 7.3 pN, K33 = 17 pN.38
The elastic splay deformation of the LC can be estimated by the following formula: ∆𝐸𝑠𝑝𝑙𝑎𝑦 =𝐾11
2 ∫(∇⃗⃗ 𝑛⃗ ) 2
𝑑𝑉~𝜋𝐾11(∆𝜑)2𝑑
𝑎𝑣𝑔 (3)
Where 𝑑𝑎𝑣𝑔 is the average thickness of the LC film, 𝑑𝑎𝑣𝑔 ≈ 20 𝜇𝑚 and ∆𝜑s the average
approximate change in director angle due to the splay deformation, ∆𝜑 = 0° for a flat film. A rough estimation shows that the ∆𝐸𝑠𝑝𝑙𝑎𝑦 is of the order of 0.1∙10-16 J, which is negligible in
a flat LC phase.
The elastic bend deformation can be estimated by the following formula:
∆𝐸𝑏𝑒𝑛𝑑 =𝐾33 2 ∫(𝒏⃗⃗ × ∇⃗⃗ × 𝒏⃗⃗ ) 2 𝜋𝑟2𝑑 𝑎𝑣𝑔~𝜋𝐾233(𝑑∆𝜑 𝑎𝑣𝑔) 2𝑟2𝑑 𝑎𝑣𝑔 (4)
A rough estimation shows the ∆𝐸𝑏𝑒𝑛𝑑 is in the order of∙10-16 J, again negligible.
In cholesteric phases, an inherent twist is present with the twist axis perpendicular to the local director exhibits so that the twist deformation should include the helical wave-vector, which is formulated as follows:
∆𝐸𝑡𝑤𝑖𝑠𝑡 =𝐾22 2 ( 2𝜋 𝑝 − 2𝜋 𝑝0) 2𝜋𝑟2𝑑 𝑎𝑣𝑔 (5)
Where 𝑝 is the observed pitch obtained when the CLC film is under different anchoring condition and 𝑝0 is the intrinsic pitch. A rough estimation shows that ∆𝐸𝑡𝑤𝑖𝑠𝑡 is in the order
of -1 pJ, which is the dominant contribution compared with ∆𝐸𝑠𝑝𝑙𝑎𝑦 and ∆𝐸𝑏𝑒𝑛𝑡.
This implies that the free energy of CLC phase can be simply express as 𝐹 ≅ 𝐸𝑡𝑤𝑖𝑠𝑡
So that before using a CLC in experiments, it is important to characterize the used chiral dopant, e.g. its helical pitch (p0) at different concentrations:
𝑝0 =𝑐 ×𝐻𝑇𝑃1 (6)
Here, 𝑐 is the concentration (in % w.t.) of the chiral dopant and HTP is the helical twisting power which is determined by the nature of chiral dopants and the interactions between dopants and hosts. In our system using motor M1 doped in E7, the HTP is 87 µm-1. The concentration 𝑐 and pitch 𝑝 can be found from the analysis of the transmitted light through a wedge cell (as we discussed in Chapter 1 and 3, more details can be found in the Experimental Section). Based on the physical model, we hypothesized that it would be possible to control the orientation of CLCs through doping various concentrations of motors since in principle with a higher concentration of the doped motors, higher free energy of CLCs could be obtained. In this way, by modulating the concentration of motors, we could regulate the free energy of CLCs, and subsequently the ordering transformation when the LC phase is in contact with aqueous solutions can be controlled as well.
Figure 4.7 Optical micrographs and schematic illustrations (under crossed polarizers) of E7
with different concentration of doped molecular motors. (a) 2.5 wt %, (b) 1.5 wt% (c) 0.8
wt%, (d), 0.5 wt%, (e) 0.3 wt%, (f) 0.1 wt%, and schematic images for (g) fan-shaped textures,
(h) fingerprint textures and (i) unwound (pseudo nematic) CLC. In (a−f), the CLC films are supported on a OTS-treated glass slide. The size of each copper grid is ~300 µm x 300 µm and thickness of the sample is ~20 μm. In (g-h ) the yellow rods present the nematic E7 and the red rods present the doped motors.
As shown in Figure 4.7a, a freely suspended CLC film in air (on top of OTS-treated glass slide) within TEM grid, a so-called “Fan-shaped texture” is observed.39 The concentration of M1 which was used in the E7 is 2.5 wt %. The texture observation indicates a strongly twisted helical structures coinciding with a short pitch cholesteric phase,39,40 which is measured to be 0.6 μm. When decreasing the concentration of M1 to 1.5 wt%, a broken Fan-shape texture is developed as demonstrated in Figure 4.7b, which indicates a decrease in the helical pitch (~1.3 µm) of the superstructures. Fingerprint textures with distinct periodicity are observed for the samples with low concentration of M1 doped in LC mixtures, which is directly associated with half-pitch of the twisted materials.39 This periodicity gives a pitch length 𝑝 ≈ 2.0 μm when the concentration is reduced to 0.8 wt % (see Figure 4.7c). In addition, the periodicity of the fingerprint texture increased gradually from 2.5 μm for 0.5
wt%, to 3.0 μm for 0.3 wt% and 3.6 μm for 0.1 wt%. We also noticed that when the
concentration of M1 was decreased to 0.1 wt%, the helical orientation reflected in the fingerprint textures is unwound slightly as the d/𝑝 ratio is approached.41 It has been reported that the anchoring energy and the cell thickness influenced the helix unwinding process. For the homeotropic anchoring above the d/𝑝 ratio and the CLC will exhibit isolated
fingerprinted textures (Figure 4.5h), otherwise the pseudo nematic phase can be observed (Figure 4.7i).
4.2.4 Cholesteric Liquid Crystal with Different Concentration of Motor-Dopants under Aqueous Solutions
The previous investigation for aqueous-NLC interfaces are focused on two main anchoring regimes: homeotropic and planar alignment. However, when the system is changed to a CLC in contact with aqueous solutions, the results are more complicated. As discussed above, the adsorption of surfactant at the interface driving the long-range of ordering transformation of the LC is in a manner that minimizes the elastic energy stored in the film of LC. There are two elements that regulate the orientation of LC at the interfaces, one is the elastic energy stored within bulk LC materials which is determined by the helical superstructures, and the other one is the anchoring energy from the aqueous solution. Based on this, we anticipated that in this system the ordering transformation of CLC at the interface when in contact with aqueous solutions can be manipulated by varying the concentration of M1 and the surfactant.
Figure 4.8 Optical images of E7 with 2.5 wt% motor doped as a function of different
interfaces: (a) Air; (b) Pure water; Various bulk SDS concentration: (c) 0.1 mM, (d) 0.25 mM, (e) 0.5 mM and (f) 1 mM. In (a-f), the CLC films are supported on a OTS-treated glass slide. The size of each copper grid is ~300 µm x 300 µm and thickness of the sample is ~20 μm. All of the experiments are performed at room temperature (25 °C).
After obtaining stable CLC films (Figure 4.8a), the films are immersed into different aqueous solutions and left for stabilization until no further changes can be observed. Figure 4.8b-f shows the optical appearance of CLC films confined within grids under aqueous solutions. In the absence of SDS (Figure 4.8b), the optical appearance of CLCs reflects the in-plane birefringence associated with planar anchoring of LC at the interface. The transformation from fan-shaped texture to oily streak texture corresponds to the change in the orientation of CLC phases to a planar geometry. These changes indicate that the interaction of water
layer reorganizes the orientation of CLC director parallel to the aqueous-LC interfaces. This observation is consistent with the previously investigation that nitrile group tends to align nearly parallel to the water surface.17
When contacted with aqueous solutions with increasing concentrations of SDS (0.1 mM and 0.25 mM), a coexistence of planar aligned regions and fan-shaped texture regions of LC, namely broken fan-shaped texture, is observed as shown in Figure 4.6c-d. When the SDS concentrations increases up to 0.5 mM or 1 mM, the fan-shaped texture dominates the whole grid as indicated in Figure 4.6e-f. The results in Figure 4.8 indicate that at low concentration of SDS in aqueous solutions, the surfactant cannot ensure control of the alignment of LC at the interface. Hybrid alignment can be found when the concentration of SDS is in the range of 0.1-0.25 mM. We anticipate that this SDS solution is in the threshold region, above which the interface coverage by SDS surfactant is sufficient to regulate the ordering of LC at the interface, while, concentration below the threshold, a planar alignment of LC at the interface will be observed.
Figure 4.9 Optical images of E7 with 1.5 wt% motor doped as a function of different
interfaces: (a) Air; (b) Pure water; Various bulk SDS concentration: (c) 0.1 mM, (d) 0.25 mM, (e) 0.5 mM and (f) 1 mM. In (a-f), the CLC films are supported on a OTS-treated glass slide. The size of each copper grid is ~300 µm x 300 µm and thickness of the sample is ~20 μm. All of the experiments are performed at room temperature (25 °C).
In order to study the influence of helical superstructures on the orientation of LC materials at CLC-aqueous interfaces, we attempted to increase the helical pitch of the bulk LC phase by decreasing the doping concentration of M1 in E7 systematically. First, a CLC film with a concentration of chiral dopant reduced to 1.5 wt% with a pitch = 1.20 μm was checked. Still, a planar alignment is induced by the water layer (Figure 4.7b). When the SDS concentrations increased up to 0.1 mM (Figure 4.9c), the observed optical texture is reminiscent of oily streaks which is similar to that observed under pure water (Figure 4.9b), indicating in-plane birefringence at the aqueous-LC interface of the LC pattern. As SDS concentrations ranged from 0.25 to 1 mM (Figure 4.9d-f), the optical texture of the LC changed from broken
fan-shaped texture to fan-fan-shaped texture. In general, similar results can be obtained for CLC with 2.5 wt% and 1.5 wt% of motor doped aqueous-LC interfaces, as under a pure water layer, the anchoring alignment of LC molecules is planar, while increasing the concentration of SDS, a fan-shaped texture is more favorable. In addition, increasing the helical pitch of the bulk LC phase from 0.6 µm to 1.3 µm, the reorientation threshold for SDS concentration is increased from 0.1 mM to 0.25 mM.
Figure 4.10 Schematic representations showing the orientation of LC molecules under
different interfaces: (a) Air layer; (b) Water layer and (c) Surfactant/salt aqueous solutions. Notation: P, planar orientation; H, homeotropic (perpendicular) orientation. The images are reproduced with permission from Ref 42. Copyright 2017 American Chemical Society.
It is known that aqueous solutions of SDS could form homeotropic (perpendicular) anchoring for nematic LC films at high concentrations and planar anchoring at low concentrations.14,19,20,26 Recent theoretical studies have confirmed these experimental investigations.42–45 From the standpoint of mechanics, the surfactant-driven NLC anchoring is related to the free energy of bulk LC film (γLC) and surfactant adsorbed substrate surface
energy (γs). The perpendicular orientation of NLCs results for γs < γLC, while parallel
orientation occurs for γs > γLC. The mechanisms can be further extended to the CLC system,
where the free energy of the LC phase is mainly determined by HTP. This means that the surfactant-driven CLC anchoring is related to the helicity of the LC and the surface tension of the aqueous solution, which is further determined by the concentration of chiral dopants and surfactant, respectively. In other words, by varying the concentration of chiral dopants and the concentration of surfactant, the anchoring of CLC at the interface is controlled, as γs >
γCLC, the planar anchoring energy from the aqueous solution is more pronounced, while
when γs < γCLC, the orientation from the CLC film will be the dominant element.
However, at the molecule level, the physical principles that lead to the collective reorientation of the LC upon the additions of even simple salt or surfactant remained poorly understood. One of accepted theories is that the spontaneous flux of water across the interface, triggers a transient reorientation of the LC.42,45 The free energy cost that must be overcome by a water molecule to reach the LC phase is calculated to be 4.08 kcal/mol which is due to its favorable electrostatic and van der Waals interaction between water and LC
molecules (5CB used for the calculation). Upon addition of salts or other surfactants, the water activity of the solution decreases from unity leading to higher free energy costs for transfer of water molecules into the LC phases. In other words, a higher free energy for the transfer of water molecule into LC phase must be overcome (7.62 kcal/mol). Our experimental results are in good agreement with the principles that in CLCs systems, aqueous solution of SDS induces planar anchoring at low concentration as the free energy for water to overcome to reach the LC phase is remained low. However, when immersing the LC phase into contact with higher concentration of SDS, the water molecules are prevented from entering into the bulk LC phase and the initial fan-shape textures of CLCs are displayed. We also noted that with higher concentration of motor doped CLCs systems, the initial fan-shape textures were easier to achieved. The observation can be explained as followed: upon doping with a higher concentration of M1 dopant in the E7 mixture, a highly twisted helical superstructure with high elastic energy can be generated. As a result, high water activity is needed for the water molecule transferring into the bulk LC phase, which dictates that a low concentration of SDS solution is needed.
Figure 4.11 Optical images of E7 with 0.8 wt% motor M1 doped as a function of different
interfaces: (a) Air; (b) Pure water; Various bulk SDS concentration: (c) 0.1 mM, (d) 0.25 mM, (e) 0.5 mM and (f) and (g) 1 mM. (h) Photographs and optical micrograph of the exoskeleton of the beetle C. gloriosa. In (a-f), the CLC films are supported on a OTS-treated glass slide. The size of each copper grid is ~300 µm x 300 µm and thickness of the sample is ~20 μm. Next we performed experiments using samples with lower concentration of doped motor
M1 (0.8 wt%) with initial helical pitch increasing to 2.0 μm. The results of these experiments
(Figure 4.11) show that the anchoring of the CLCs induced by adsorption of SDS surfactant at the interfaces changed dramatically as compared to the samples having with small pitches as
shown in Figure 4.8 and 4.9. Under low concentration of SDS (0 mM-0.1 mM) solutions, oily streaks texture still can be observed. However, when the concentration of SDS solution increased to 0.25 mM, the texture did not recover to fingerprinted textures nor showed to be oily streaks texture, but instead it changed to an intermediate state as focal conic patterns were observed (Figure 4.11d). Each focal conic domain (FCD) contains green stripes and a bright yellow nucleus, which is similar to the pattern that found in Jeweled Beetles as shown in Figure 4.11h.46 By characterizing the extent of spatial structures, we also note that the structure of FCDs seems to be constructed mainly by polygons (inset in Figure 4.11g as red), as there exist large numbers of heptagons (blue) and hexagons (yellow). The average radii (R) of the cell is around 8 μm.
Figure 4.12 A cholesteric phase with homeotropic boundary conditions: (a) Scheme of
cholesteric mesophase structure on OTS substrate. (b) Schematic image indicates the director of CLC molecules on OTS substrate. (c) Schematic image indicates the director of CLC molecules at water interface. (d) Schematic image indicates the director of CLC molecules at surfactant aqueous interfaces. (e) Model of the FCDs; note that the figure shows a cut along the center of the cone and the exact structure of the center is neglected in this sketch. (f-g) Model of the two defect curves for adjacent FCDs. The sketches are all based on Ref 46-48. Copyright 2007 AAAS and Copyright 2017 American Chemical Society.
Although the structure of FCDs textures and the resulting optical responses have been investigated before,47–49the observed FCDs texture under aqueous solution have not been reported as far as we know. Because limited knowledge of the structural parameters necessary for determining the patterns is available, we can put forward only phenomenological arguments. It has been reported that cholesteric phases show a great manifold of possible surface structures depending on several parameters, such as the thickness of the liquid crystal layer, the thermal treatment of the sample, the boundary conditions, etc.48 The topology of the cholesteric structure plus homeotropic boundary condition requires the introduction of disclamations. These can be arranged in several ways leading to a large number of different structures. In particular, a FCDs texture can be generated.47,48 In our case, for brevity, cholesteric materials in the direction perpendicular to the helix axis can be considered as a stacked layer with a thickness comparable to the size of a mesogen (Figure 4.12a). In addition, the water layer at the interface can promote parallel arrangement for the LC phase in a long-range order as indicated in Figure 4.12c. With the help of the SDS surfactant, a certain amount of water molecules transfers into the LC phase which leads to the boundary conditions at the interface are altered in order to allow for an intermediate state, as the LC layer will keep a homeotropic alignment in the depth of LC phase, but disrupted to a twist alignment (out of plane) near the interface between the cholesteric material and SDS solution. At this point, the equilibrium relief FCDs from the competition between the surface energy induced from interface and the elastic energy from bulk LC phase. The proposed structure is sketched in Fig. 4.12 e-g.47,48 The two adjacent FCDs are illustrated in Figure 4.12f. The model cuts out a part of an infinite FCD, the defect line can be seen in the actual sample while the circle lies outside of the region that is covered by the domain. Overall, it can be considered as a cholesteric helix twisted accompanied by an additional rotation of layers near the interface. The inclination angle α between the surface and the layer (Figure 4.12g) can be calculated as follows:
α = cot−1 𝑑
𝑅 eq.7
where d is the surface relief period measured under microscopy, according to the color change between the green stripe and the yellow nucleus, the d is around 2 µm; R is the radius of FCDs. As calculated, the additional rotation of layers near the surface was about 14°.
When the CLC film was brought in contact with a 0.5 mM~1 mM SDS solution, the double spiral of FCDs unwound to uniformly fingerprint textures as shown in Figure 4.11e-f. The optical texture of LCs suggests that the director of the LC is parallel to the interfaces by the anchoring energy from the interface as a monolayer of SDS is formed.
Figure 4.13 Optical images of E7 with 0.5 wt% motor doped as a function of different
interfaces: (a) Air; (b) Pure water; Various bulk SDS concentration: (c) 0.1 mM, (d) 0.25 mM, (e) 0.5 mM and (f) 1 mM. In (a-f), the CLC films are supported on a OTS-treated glass slide. The size of each copper grid is ~300 µm x 300 µm and the thickness of the sample is ~20 μm.
Figure 4.14 Optical images of E7 with 0.3 wt% motor doped as a function of different
interfaces: (a) Air; (b) Pure water; Various bulk SDS concentration: (c) 0.1 mM, (d) 0.25 mM, (e) 0.5 mM and (f) 1 mM. In (a-f), the CLC films are supported on a OTS-treated glass slide. The size of each copper grid is ~300 µm x 300 µm and the thickness of the sample is ~20 μm. Upon decreasing the concentration of motor dopants to 0.5 wt% and with an initial pitch around 3 µm, the texture variations are presented in Figure 4.13. As contacting with low concentration of SDS surfactant solutions in the range from 0 to 0.25 mM, the LC shows oily streak textures associated with planar alignment of the LC phase at the interface. When the concentration of the SDS increased to 0.5 mM, FCDs textures can be obtained. The average
radii for the FCDs is around 10 μm. Similar orientational behavior of CLC can be observed for LC system with 0.3 wt% motor dopants with initial pitch ~ 5 µm as indicated in Figure 4.14. The FCDs textures can be observed at moderated concentration (0.5 mM) and fingerprint texture at high surfactant concentrations. In addition, the radii of FCDs is around 15 μm. We also compared the three FCDs that were constructed under different conditions. As discussed above, the features of surface topography in FCDs is correlated with the helix pitch of the cholesteric supramolecular structures. According to eq. 7, the additional rotation of layers (as indicated in Figure 4.12g) near the surface was calculated as 14°, 11° and 8° for that of CLC with helical pitch as 2 µm, 3µm and 5 µm, respectively (we assumed d=20 µm). Based on the calculations, we speculated that the increasing of helix pitches resulted in increasing of both diameter of FCDs and inclination angle α. The variation in helical pitch, determines the equilibrium FCD configuration, and the increase of the helical pitch resulted in an increase in the tilt of the cholesteric helical axis near the surface.
Figure 4.15 Optical images of E7 with 0.1 wt% motor doped as a function of different
interface: (a) Air; (b) Pure water; Various bulk SDS concentration: (c) 0.1 mM, (d) 0.25 mM, (e) 0.5 mM and (f) 1 mM. In (a-f), the CLC films are supported on a OTS-treated glass slide. The size of each copper grid is ~300 µm x 300 µm and the thickness of the sample is ~20 μm. When the concentration of motor dopants decreased to 0.1 wt%, the optical texture of the LC again appeared to be different from those observed at higher concentrations of motors as shown in Figure 4.15. In contacting with pure water or low concentration of SDS solutions (0-0.25 mM), the Schlieren texture is observed (Figure 4.15 b-d). As mentioned before, the fingerprint texture under homeotropic alignment can be slightly unwound, while when the CLC film was immersed under aqueous solution, the anchoring from the aqueous solution on the top and the glass substrate at the bottom work together to induce the planar alignment of the pseudo nematic LC phase. However, when the concentration of SDS solution increased to 0.5mM, random lines appeared as a result of the low helical strain and anchoring energy from the surfaces (Figure 4.15 e). At concentration of SDS up to 1 mM, the helical spirals appeared as periodic fingerprint textures with 30 µm periodicity consistent with homeotropic alignment of LCs (Figure 4.15 f).
To summarize this section, the concentration of chiral motors and the concentration of SDS solution have a significant influence on the ordering of LC molecules at the interface. Higher concentration of chiral motors induced higher elastic energy of CLC materials at the interface that it is more difficult for the aqueous solution to reorientate the orientation of LC molecules at the aqueous-CLC interface.
4.2.5 Reversibility
Based on the mechanism that the anchoring transition of LC molecules at the interfaces is controlled by the surface tension, which is further controlled by the SDS concentrations, we hypothesized, that the orientational behavior of CLC shown in Figure 4.10~4.16 should be reversible when contacting with SDS solutions with different concentrations. The procedures of a typical experiment are presented in Figure 4.16. The CLC with 2 μm helical pitch in the grid was exposed to a sequence of aqueous solution with increasing concentration of SDS in the range of 0, 0.1, 0.25, 0.5 and 1 mM. The resulting observations are as follows: first an oily streak texture is observed under the water layer. As the concentration of SDS is increased, the texture is changed to FCDs textures (0.25 mM) and further changed to fingerprint texture (1 mM). The evolution is in good agreement with our models as upon increasing the concentration of the SDS solutions, the water molecules that are embedded in the LC phase are eventually “squeezed out”. As demonstrated that the content of water in the LC phase that controls the orientation of LC molecules.42 In an attempt to confirm the reversibility of reorientation, the exchange of solution from 1 mM SDS to 0.1 mM SDS and pure water was performed. In each case, the optical appearance of the LC is identical with the images shown in Figure 4.16 which has the same concentration of SDS, thus confirming that the orientational behavior of the LC is reversible.
Figure 4.16．Optical images of E7 with 0.1 wt% motor doped as a function of different
interfaces and corresponding LC orientation: (a) Air; (b) Pure water; Various bulk SDS concentration: (c) 0.1 mM, (d) 0.25 Mm, (e) 0.5 mM and (f) 1 mM. Inset in (a), (d) and (f) are the zoom-in images. Notation: P, planar orientation; H, homeotropic (perpendicular) orientation; T, titled orientation. In (a-f), the CLC films are supported on a OTS-treated glass slide. The size of each copper grid is ~300 µm x 300 µm and the thickness of the sample is ~20 μm.
4.2.6 Morphology Changes under Photo-Irradiation and Thermal Isomerization
The preceding discussion is to provide insight into the reorientation of LC phase as the variation of the amount of motor dopant and the concentration of SDS solutions. In this section, we describe the investigation focused on the influence of photochemical and thermal isomerization of molecular motors on the orientation changes at the interface. As was discussed in previous chapters, photochemical and thermal isomerization of the motor can induce HTP changes in the motor doped CLC systems. In brief, initial UV irradiation induces the formation of the unstable isomer with opposite helicity, due to the
photochemical conversion from stable form to unstable form. At a certain stage, when the stable and unstable isomers are present in equal amount, the nematic LC is formed. Further irradiation with UV light to the photostationary state (PSS) induces a twisted nematic LC as the dopant is predominantly present as the unstable isomer with opposite helicity. Consequently, the cholesteric helix is reversed. The thermal process causes the reappearance of the stable form with the cholesteric helix in the opposite direction. With this in mind, the aim was to establish if the isomerization can also induce the interfacial changes. An aqueous-CLC system with 5 µm helical pitch was selected as representative sample.
Morphologic Changes under Photo-irradiation
We first performed experiments to determine whether the photoisomerization of doped motors could influence the morphology of the interface. In our first approach, a 1mM aqueous solution of SDS was layered onto the LC film. As demonstrated in Figure 4.17, initially, fingerprint textures were observed, indicating the homeotropic alignment of CLC molecules at the interface. Upon irradiation with ultraviolet light (365 nm) for 20s, the increasing of striped period over the entire sample area was observed until it complete disappeared as the fingerprint texture transformed into a dark state (Figure 4.17a-b). The dark state might correspond to a compensated nematic phase in which both HTPs of stable form and unstable form of M1 compensate each other resulting in complete helix pitch unwinding. Upon prolonged irradiation to 40 s, the fingerprint textures were regenerated and accompanied by decreasing of periodicity of the fingerprint texture until the system reached the PSS state as shown in Figure 4.17d-f. It should be noted that the hybrid-oriented CLC layer is the key element to transform the rotation of chiral motors to the LC scale.50,51 Unlike previous studies in which the fingerprint textures rotated under UV irradiation,50–52 the rotation of the texture is insignificant in this case. The photoresponsive unwinding and rewinding of fingerprint texture without significant rotation suggests that the LC molecules at the interfaces might maintain a homeotropic alignment during the irradiation (Figure 4.17g-i). Firstly, the high concentration of SDS solutions (1mM) and the OTS substrate induces a stable homeotropic anchoring for the CLC phase (γs). In addition, the free energy
(γLC) of the LC phase is determined by the helical structure which is determined by the HTP of
the motor dopant. During the irradiation, the value of HTP decreased initially and increased subsequently, and is smaller than the initial value. As a result, the energy of surface tension
γs is always larger than the γLC’ during the process. As a whole, the LC molecules at the
interface maintain the homeotropic alignment during UV irradiation. Based on the results, it can be concluded that when contacting with higher concentration of SDS solution, although the photoisomerization of motor control the changes of helical pitch of the LC system, these changes have negligible effects on the orientation of LC at the interface.
Figure 4.17 Light-induced control of orientation of aqueous (1 mM SDS)/CLC (initial pitch = 5
μm) interface. (a-f) Evolution of the optical texture of interface during UV irradiation to the
PSS state; (g-h) Illustration of light-induced three-dimensional control over the direction of the helical axis. The CLC films are supported on a OTS-treated glass slide. The size of each copper grid is ~300 µm x 300 µm and the thickness of the sample is ~20 μm.
Figure 4.18 Light-induced control of orientation of aqueous (0.5 mM SDS)/CLC (initial pitch =
5 μm) interface. (a-f) Evolution of the optical texture of interface during UV irradiation to the PSS state; (g-k) Illustration of light-induced three-dimensional control over the direction of the helical axis. The CLC films are supported on a OTS-treated glass slide. The size of each copper grid is ~300 µm x 300 µm and the thickness of the sample is ~20 μm.
When an aqueous solution with lower concentration SDS (0.5 Mm) was used, the photoresponsive behavior was more pronounced. As illustrated in Figure 4.18, initially a FCDs texture is observed. After 5 s UV irradiation, the size of the FCDs are clearly increased, indicating an elongation of the cholesteric helical pitch (Figure 4.18b and h). After prolonging the irradiation to 8s, oily streaks textures can be observed which means that a planar alignment is induced (Figure 4.18c and i). The photoisomerization of molecular motor increases the helical pitch of the LC material which releases the elastic energy of the bulk LC phase, therefore, the water molecules are more capable to transfer into the bulk LC phase resulting in a planar alignment of LC molecules at the interface. Further irradiation of the sample during 15s, the stripes reappeared demonstrating the alignment of the LC at the interface which is transferred into a homeotropic alignment again (Figure 4.18d). Surprisingly, another 10s irradiation the regeneration of FCDs is seen, and subsequently the FCD are changed to a fingerprint texture (Figure 4.18e-f), which means the titled orientation of the bulk LC transfers to a homeotropic alignment as demonstrated in Figure 4.18j and k. The rotation of the stripes occurs by unwinding and rewinding of the texture, which further demonstrates that the reorientation of LC molecules at the interface is happening. These results are consistent with the prediction by our model, which shows that under UV irradiation, the overall HTP of LC phase first decreases, the water activity increases relatively, and more water molecules transfer into the LC phase, consequently, the titled orientation of LC molecules transfers into planar orientation. Subsequent upon irradiation to the PSS state, the overall HTP reestablished, as a result, the homeotropic alignment is observed. Based on the observation, it is seen that the HTP at the PSS state is lower than before irradiation and at the PSS state a homeotropic alignment is observed under the aqueous interface.
Figure 4.19 Light-induced control of orientation of aqueous (0.1 mM SDS)/CLC (initial pitch =
5 μm). (a-c) Evolution of the optical texture of interface during UV irradiation to PSS state; (d-f) Illustration of light-induced three-dimensional control over the direction of the helical axis. The CLC films are supported on a OTS-treated glass slide. The size of each copper grid is ~300 µm x 300 µm and the thickness of the sample is ~20 μm.
Finally, a CLC contacting with low concentration of SDS surfactant solution was examined and the results are shown in Figure 4.19. Initially, oily streaks textures are observed and upon irradiation with UV light (365 nm) for 10s, some stripes appear (Figure 4.19b and e). Upon prolonged irradiation to the PSS state, the initial textures reappeared with few more defect in the grid (Figure 4.19c and f). The above experimental observation reveals that at low concentration of surfactant, the morphology of the LC at the interface maintains a
planar alignment although the helical pitch of the LC phase varies during the irradiation. We assume that photoisomerization of molecular motor in the LC phase has insignificant influence on the LC orientation at aqueous–LC interface when the LC film in contact with a surfactant solution with low concentration of SDS surfactant.
Based on these data, it can be concluded that the orientation of LC at the interface is based on two parameters, the elastic energy of the LC phase which is controlled by UV irradiation and the anchoring energy at the surface which is altered by the concentration of the surfactant. When the magnitude of an anchoring potential at the interface is closed to that in the LC phase, the reorientation of the LC at the interface can be controlled by UV irradiation, while, when the anchoring energy from the solution is either too large or too small, the reorientation of LC at the interface cannot be effected by light irradiation using a photoresponsive molecular motor as chiral dopant.
Morphologic Changes During Thermal Isomerization
After irradiation the sample to reach the PSS state, we studied whether morphology changes at the interface can be driven reversibly by the thermal isomerization of doped motors in the LC.
The morphology variations under aqueous solution of SDS at 1 mM concentration was checked firstly as shown in Figure 4.20. During this process, the textures faded out to a dark state in 5 min (Figure 4.20b), producing a nematic LC phase and further UV irradiation led to the appearance of the fingerprint texture again (Figure 4.20c and d). The rotation of the stripes during the thermal process is also negligible which means the orientation of the LC molecules remain the same.
Figure 4.20. Thermal-induced control of orientation of aqueous (1 mM SDS)/CLC (initial pitch
= 5 μm) interface. The CLC films are supported on a OTS-treated glass slide. The size of each copper grid is ~300 µm x 300 µm and the thickness of the sample is ~20 μm.
Decreasing the surfactant concentration to 0.5 mM, the reversible thermal process is also complicated. After halting the irradiation, the fingerprint texture fade away which is companied by an increasing distance between the stripes until the oily streak texture appeared, indicating the generation of planar alignment of the LC phase (Figure 4.21b). Afterward, it takes another 11 min to resume the FCDs textures (Figure 4.21c). In this process rotation in opposite direction was observed. We noted that the thermal process were much slower than the photoisomerization process, as it takes 60 min to fully restore the FCDs texture via thermal isomerization.
Figure 4.21. Thermal-induced control of orientation of aqueous (0.5 mM SDS)/CLC (initial
pitch = 5 μm) interface. The CLC films are supported on a OTS-treated glass slide. The size of each copper grid is ~300 µm x 300 µm and the thickness of the sample is ~20 μm.
The morphology change during thermal isomerization of the LC phase at low concentration of SDS solution is not so pronounced when observed with POM as described in Figure 2.22. At the PSS state, an oily streak texture is initially observed (Figure 4.22a), and then the defect disappeared (Figure 4.22b). The new generated homogenous bright state might originate from the planar alignment of the nematic LC at the interface. After 18 min, the former oily streak texture was resumed fully.
Figure 4.22. Thermal-induced orientation controls over aqueous (0.1 mM SDS)/CLC (initial
pitch = 5 μm) interface. The CLC films are supported on a OTS-treated glass slide. The size of each copper grid is ~300 µm x 300 µm and the thickness of the sample is ~20 μm.
These experimental results led us to conclude that the thermal isomerization of the motors doped in the LC phase could induce morphology changes and the recovery of the initial orientation of LC at the aqueous-LC interface. However, the thermal process takes more time than the photoisomerization process.
4.3 Conclusion
We have studied the anchoring of motor-doped CLC phase in contact with aqueous solutions containing surfactants that have the ability to control the ordering of LC phase at the LC-aqueous interface. Our results lead us to several conclusions.
Figure 4.23. Summary of the alignment of LC molecules at the aqueous-LC interface and
corresponding ordering transition under UV irradiation.
First, the concentration of chiral motors has a major influence on the order of LC phase at the aqueous-LC interface as higher concentration of the doped molecular motors induced a higher elastic energy of LC phase, which consequently induced a higher threshold for the ordering transition induced by surfactant solutions, and vice versa.
Second, aqueous solutions exhibit the ability to trigger ordering transitions at the interface. Water and aqueous solutions with low concentration of surfactant contribute to a planar alignment of the LC at the interface, whereas a high concentration surfactant significant decrease the water activity, so that the ordering of LC at the interface are mainly controlled by the elastic energy of the LC phase at the interface.
Third, the FCDs texture is a result of the combination of two competing factors: the anchoring energy from the aqueous-LC interface and the elastic energy from bulk LC phase. Finally, all of the structures at the aqueous-LC interface can be erased and driven reversibly with UV irradiation and thermal isomerization of the photoresponsive dopants. Since the photochemical and thermal isomerization of motors can alter the helical structure of LC bulk phase, which determines the elastic energy of LC material, and consequently the interfacial orientation at the interface can be controlled by the UV irradiation and thermal isomerization.
Our work is a step towards the realization of complex, light-activated smart self-assembled and reconfigurable morphologies at distinct liquid crystal-aqueous interfaces.
4.4 Experimental Section
Materials. Sodium dodecyl sulfate (SDS) at 99+% purity was obtained from Sigma and used
without further purification. Octadecyltrichlorosilane (OTS) was obtained from Fisher Scientific. Deionization of a distilled water source was performed using a Milli-Q system (Millipore, Bedford, MA) to give water with a resistivity of 18.0 MΩ cm. Glass microscope slides were obtained from Fisher. Copper TEM grids (thickness is around 20 µm, the pores size is around 300 µm) were purchased from Electron Microscopy Sciences. The details of synthesis procedure and the characterizations of molecular motor M1 can be found in Chapter 3.
Treatment of Glass Microscope Slides with OTS.
Glass microscope slides were cleaned according to a procedure detailed in an earlier chapter (Chapter 2). Briefly, the slides were immersed into a piranha solution (70% (v/v) sulfuric acid and 30% (v/v) hydrogen peroxide) for 1 h at approximately 80 °C. (Caution! Piranha solution
is highly corrosive and reactive toward organics). The slides were then rinsed with water,
ethanol, and methanol and dried nitrogen flow followed by heating to ∼120 °C for >2 h prior to deposition.
Treatment of glass substrate with octadecyltrichlorosilane (OTS) is according to a published procedure.19,22,34 A 0.5 mM OTS solution was prepared by adding OTS to heptane that was dried by an aluminum oxide column. The slides were immersed in the 0.5 mM OTS in heptane solution for 30 min at room temperature. They were then rinsed with methylene chloride and dried under nitrogen. The quality of the OTS slides was tested by forming a sandwich of two treated OTS slides; E7 was introduced between the slides and the resulting optical texture was examined using polarized light to confirm homeotropic anchoring.
Preparation of Optical Cells. The copper specimen grids were cleaned sequentially in
methylene chloride, ethanol, and methanol, dried under nitrogen, and then heated at 110 °C for 24 h. The grids were then placed onto the surface of treated glass. One microliter of LC material was dispensed onto each grid, and the excess LC was removed by contacting a 5 µL capillary tube (Fisher) with the LC droplet on the grid. This procedure led to the formation of a stable film of LC within the grid. The optical cell was heated to ∼60 °C and then cooled to room temperature at a speed of 1 °C/min.
Optical Cells Exposed to an Aqueous Phase. The LC film impregnated grid supported on a
solid glass surface was quickly (<2 s) immersed in and withdrawn from deionized (DI) water to promote further removal of excess LC. Then the slides immediately immersed in the aqueous solution of interest (4 mL) held at room temperature (25 °C) in a glass petri dish. A schematic representation of this system is shown in Figure 4.3.
Optical Examination of LC Textures. The orientation of LC was examined by using
plane-polarized light in transmission mode on an LV100 POL polarizing microscope (Nikon). The cells were placed on a rotating stage located between the polarizers. Homeotropic alignments for E7 were determined by first observing no transmission of light over a 360°
rotation of the stage. Insertion of a condenser below the stage and a Bertrand lens above the stage allowed conoscopic examination of the cell.
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