Direct Observation of Changes in Focal Conic Domains of Cholesteric

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Direct Observation of Changes in Focal Conic Domains of Cholesteric Films Induced by Ultraviolet Irradiation Olga V. Sinitsyna,*,† Alexey Yu. Bobrovsky,† Georgy B. Meshkov,‡ Igor V. Yaminsky,†,‡ and Valery P. Shibaev† †

Faculty of Chemistry, Lomonosov Moscow State University, Leninskie gory, Moscow 119991, Russia Faculty of Physics, Lomonosov Moscow State University, Leninskie gory, Moscow 119991, Russia

‡

S Supporting Information *

ABSTRACT: The helical supramolecular structure of cholesteric liquid crystalline (LC) films predetermines their outstanding optical properties and the unique nanostructure of their surface. The introduction of photochromic dopants in these films opens up an interesting possibility for creation of smart cholesteric materials with photocontrollable optical and photovariable surface properties. Using atomic force microscopy (AFM), we performed in situ measurements of the surface topography of cyclosiloxane LC cholesteric oligomer films during the cholesteric helix twisting caused by their preliminary ultraviolet (UV) irradiation. A chiral-photochromic isosorbidebased dopant was introduced in the films to control the cholesteric helix pitch by UV-irradiation. The initial films are characterized by planar texture with the presence of focal conic domains having the double-spiral relief on their surface. UVirradiation of these films leads to the cholesteric helix twisting resulting in a decrease in the surface relief period, and the enlargement of defect areas between the domains. The detailed mechanisms of the rearrangement of the film surface structure due to the cholesteric helix twisting are suggested. They include the rotation and displacement of cholesteric layers in the bulk, and the nucleation of new ones at the surface in defect regions.

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INTRODUCTION Recently, a special interest has been given to “smart materials”, the properties of which can be changed in a controllable way under an external influence. An example of such materials is cholesteric films containing a chiral-photochromic dopant. Ultraviolet (UV) irradiation of these films changes the twisting power of the dopant that leads to the twisting or untwisting of the cholesteric helix, resulting in changes of the selective light reflection wavelength. The combination of unique optical properties of cholesteric liquid crystals with their phototunability and responsiveness to different external fields makes such systems very useful for different applications in photonics and optoelectronics.1−15 These phototunable cholesteric materials seem to be promising for nanotechnology due to the great capability of liquid crystal media in long-range ordering of nanoparticles at the surface and in the bulk, as has been shown in a number of recent publications.16−20 The optical properties of the liquid crystalline (LC) materials depend not only on the cholesteric helix pitch but also on the local orientation of its axis that is determined by the film preparation technique and surface boundary conditions.21 For example, homeotropic alignment favors focal conic domain (FCD) formation. FCDs were found in cholesterics and smectics, and they can be considered as main building blocks in the assembly of soft microstructures, such as optically selective © XXXX American Chemical Society

photomasks, microlens arrays, and templates for superhydrophobic surfaces.2,22−24 Namely, FCDs are observed on a cholesteric surface as double-spiral relief using an atomic force microscope (AFM).25−29 This relief reflects the supramolecular cholesteric organization of the film in bulk under the surface. According to our knowledge, there are no studies considering the process of the surface relief restructuring due to the changes of the cholesteric helix pitch. How FCDs are altering and what is happening at the interface between them are important but still open questions. To clarify these points, we used AFM to observe in situ the surface changes during the twisting process of the cholesteric helix. The advantages of AFM in comparison with other types of microscopes are high spatial resolution and the sufficient simplicity of the surface relief measurement.30 We studied lefthanded cholesteric cyclosiloxane oligomers SilRed containing a small amount of the right-handed chiral-photosensitive dopant Sorb: Received: February 26, 2017 Revised: April 29, 2017 Published: May 10, 2017 A

DOI: 10.1021/acs.jpcb.7b01886 J. Phys. Chem. B XXXX, XXX, XXX−XXX

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The thickness of the films was determined using profilometer Proton-MIET Model 130. Films were scratched by the sharp blade, and profiles were recorded allowing measurement of film thickness. AFM Measurements. The measurements were performed using a FemtoScan scanning probe microscope in tapping mode in air. The scanning rate was 6 min per frame. We have used MikroMasch cantilevers with a high resonance frequency of about 325 kHz. The samples were heated using a heating stage to observe the topography changes during the cholesteric helix twisting. The temperature of 70 °C was chosen because, under these conditions, the AFM scanning was stable enough and the mesogenic groups had sufficient mobility so the twisting process lasted a few hours. Image processing was carried out using FemtoScan Online software (www.nanoscopy. ru).33 The average period of the surface relief was measured along the fast scan axis using Fourier transform images. For the better visualization of the surface nanostructure, the large elements of the relief were removed from the images by subtracting a coarse spline of the surface. To form it, the surface was divided into rectangles with sides of 16 pixels. The spline was built based on the central points of the rectangles, in which the height was set equal to the average surface height in each rectangle. A movie, based on AFM scans, was created in WSxM software.34

The clearing temperature of this mixture is 163−168 °C. It had been previously shown that the surface of cyclosiloxane oligomer films could be successfully scanned at temperatures above the glass transition point (50 °C) in tapping mode of AFM using cantilevers with a high resonance frequency.31 Under these conditions, these films were in the liquid crystalline state. However, since the interaction time of a rapidly oscillating probe with the surface was less than the relaxation time for the viscous liquid crystal, its surface behaved like a solid. In our experiments, the prepared mixture films were irradiated by UV light at room temperature. This caused E− Z isomerization of the dopant (see Scheme S1 in the Supporting Information) and the decreasing of its twisting power;32 however, the structure of the cyclosiloxane matrix was still frozen in the glassy state and the cholesteric helical structure remained unchanged. In the case of the Sorb which is related to the cinnamoyl derivatives, the isomerization process is irreversible and the back thermal process does not occur under our experimental conditions. Annealing of the UVirradiated samples results in a decrease in the wavelength of selective light reflection associated with the decrease of cholesteric pitch. The direct AFM observation of the surface changes during the annealing allowed us to reveal the main pathways of the FCD transformation during the cholesteric helix twisting.

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RESULTS AND DISCUSSION Changes of the Selective Light Reflection. The shift of the selective light reflection wavelength during heating of the UV-irradiated samples is shown in Table 1. Assuming that the Table 1. Influence of Annealing Time on the Characteristics of the Films Exposed to UV-Irradiation (380 nm, ∼20 mW cm−2, 10 min) annealing time (h), 70 °C 0 1 2 20 a

selective light reflection λmax (nm) 875 800 790 735

± ± ± ±

2 2 2 2

cholesteric pitch P (nm) 547 500 494 459

± ± ± ±

2 2 2 2

average surface relief period d (nm) 576 455 405 356

± ± ± ±

10a 10 10 10a

inclination angle α (deg) 28 33 38 40

± ± ± ±

1 1 2 2

Values were measured at room temperature.

average refractive index of the film n is equal to 1.6 and planar texture is dominating in the depth of the films, we can estimate the pitch of the cholesteric helix P, using the simple eq 1:

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EXPERIMENTAL SECTION Sample Preparation. The samples were prepared as follows: a small amount of the mixture of SilRed and Sorb was heated to 140 °C on a glass substrate. Then, the mixture was covered with another glass plate and annealed for about 30 min. After that, the plates were separated by shearing and cooled to room temperature at a rate of 1 °C/min. The method of the film preparation led to the planar orientation in the depth of the whole films and FCD formation at the boundary between the cholesteric and air.27 The thickness of the films was varied in the range 5−10 μm. They were then irradiated with UV light (380 nm, about 20 mW cm−2) for 10 min. Selective light reflection study was performed by measuring the transmittance spectra using a Hitachi U3400 UV−vis-NIR spectrophotometer equipped with a Mettler TA-400 heating stage.

λmax = ⟨n⟩P

(1)

The calculated data (Table 1) confirm that the major change in the helical pitch takes place in the first hour of heating. However, the question arises: what happens with the orientation of the helix? A cholesteric material in the direction perpendicular to the helix axis can be considered as a stack of nematic layers with a thickness comparable to the size of a mesogen (Figure 1a). The arrangement of cholesteric layers in FCDs are indicated by lines in Figure 1b. The director rotates 180° between one line and an adjacent one. The preparation method provides the layer arrangement parallel to the surface in the depth of the films, which is disrupted near the interface between the cholesteric material and air because of the homeotropic boundary B

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the surface topography changes is demonstrated in an AFM movie (Supporting Information). Before the heating, the entire surface consists of bright concentric spiral ribbons separated by dark grooves (Figure 2a). These surface height variations of 1−3 nm are the result of the periodic mesogenic groups’ twist under the surface35 and correspond to the presence of a cholesteric supramolecular helix. The ribbons form domains with double-spiral relief in the surface. The domain diameter is 6−9 μm, and the height is from −4 to 30 nm. The height of domains with double-spiral relief was measured as the average difference between the heights of the domain center and its periphery. The space gap between them is filled by defect areas. The cholesteric helix twisting after UV-irradiation leads to a decrease in the surface relief period (the ribbon width) from 576 ± 10 to 397 ± 10 nm during the first 3 h of heating at 70 °C (Figure 3). Let us assume that the rate of the surface relief

Figure 1. (a) Scheme of cholesteric mesophase structure (P - helix pitch); (b) scheme of the layer arrangement in the focal conics domain of cholesteric film. For simplicity, cyclic siloxane groups and spacers are omitted.

conditions. The inclination angle α between the surface and the layers can be calculated as follows P α = arcsin (2) 2d where d is the surface relief period measured by the AFM method. If the layers are oriented perpendicular to the surface, then the surface relief period would be exactly equal to the half pitch of the cholesteric helix, since equivalent cholesteric structures are formed after 180° rotation. In the investigated films, α is less than 90°, and therefore, the surface relief period is more than the half pitch of the cholesteric helix. The data of Table 1 demonstrate that the cholesteric helix twisting is accompanied by an additional rotation of layers near the surface by about 10°. This result is in accordance with previous observations for different cholesteric films: systems with a small cholesteric helix pitch are characterized by the larger inclination angles.27 Surface Nanorelief Rearrangement. AFM images of the irradiated cholesteric film surface before and during heating at 70 °C are shown in Figure 2. In more detail, the dynamics of

Figure 3. Experimental dependence of the surface relief period on time (black squares) and the fitting curve (red line).

period d decreasing is proportional to the difference between the current value of d and its equilibrium value deq, which will be achieved when all of the relaxation processes, related to a change in the twisting power of the dopant Sorb after UVirradiation, will be completed in the cyclosiloxane matrix: dd = −C(d − deq), dt

C = Const

(3)

Then, for the fitting of the time dependence of the surface relief period, we should use the following function d = (d0 − deq)e−Ct + deq

(4)

Figure 2. AFM images of the surface of the cholesteric mixture after UV irradiation: at room temperature before heating (a); at 70 °C after 41 (b) and 142 (c) min of heating. The size of the images is 20 × 20 μm2. Rough relief was subtracted (actual topography is shown in Figure S1). Four domains, discussed below, are denoted by numbers 1−4. The film thickness is ca. 6 μm. C

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The Journal of Physical Chemistry B where d0 is the initial surface relief period before heating. The experimental data and the fitting curve are shown in Figure 3. The fitting result gives the value of deq equal to 394 ± 6 nm. Additional heating time (about 20 h in total) results in a further reduction of the period up to 356 ± 10 nm. We assume that the prolonged thermal treatment may lead to macroscopic changes in the shape of the film itself. The cholesteric mixture film poorly wets the glass substrate, and the preformed flat film tends to turn into a drop. The variation in film thickness, which determines the equilibrium FCD configuration, results in a decrease in the tilt of the cholesteric helical axis near the surface. In addition, the surface relief period diminishes. Details of the process are considered in refs 25 and 31. The reduction of the surface relief period should lead to an increase in the density of the groove lines on the surface. The experimental data revealed that new grooves formed in the domain centers and in the defect regions between FCDs. Rotation of the Domain Centers. It can be seen in Figure 2b and c and most clearly in the movie (Supporting Information) that the centers of the domains rotate anticlockwise during the thermal treatment. Let θ denote the rotation angle of a domain center (Figure 4). The time

bounding a double-spiral and hyperbola passing through the center of the domain deep into the film. The double-spiral rotates anticlockwise because new layers flow up in the domain center from the depth of the film to the surface, pushing adjacent layers to the domain periphery (Figure 4). The flow up of a stack of layers with the thickness equal to the helix pitch is accompanied by the domain center rotation at 180°. This mechanism may lead to the material flow from the depth to the surface in the domain centers. It explains less than 50% of the surface relief period reduction. Other sources of the groove generation were found between the domains in the defect areas. Generation of New Groove Lines in the Defect Areas. Let us consider a region between four domains 1−4 in Figure 2 and the movie in the Supporting Information. In this area, the nucleation of a double-spiral rotating anticlockwise is observed. In its center, there is a pair of λ+ disclinations, the ends of two branches of the double-spiral.36 During heating, they start to grow straight (Figure 2b), pushing the neighboring domains 2 and 3 in the growth direction. Encountering resistance from the surroundings, the ends of the branches turn and start to grow in the perpendicular direction (Figure 2c), which leads to the repulsion of domains 1 and 4. The antiphase changes of the distance between the pairs of domains are clearly seen in Figure 6. It is interesting to note that each FCD can move as a single unit.

Figure 4. Mechanism of a domain center rotation. The surface and the perpendicular cut through the domain center are shown in the top and in the bottom, respectively, (a) before and (b) after heating. A dashed line shows a new fragment of the double-spiral in part b.

dependencies of θ for four selected domains 1−4 (Figure 2) are shown in Figure 5. They are almost linear. Small negative angle

Figure 6. Dependence of the distance between the centers of FCDs 1−4 and 2−3 on heating time.

The heating of the film for 20 h results in a quite perfect double-spiral twisted anticlockwise. According to the scheme (Figure 7), its development requires the generation of new cholesteric layers on the sample surface. Previously, a similar structure, consisting of a tetragonal lattice of cholesteric FCDs and double-spirals twisted in the opposite direction between them, was described in ref 37. The

Figure 5. Dependencies of the rotation angle (θ) on heating time for domains 1−4 (see Figure 2).

values at the beginning of heating can probably be explained by the rise of the temperature of the sample from ambient value to 70 °C. To explain the domain center rotation, the changes in three-dimensional structure of FCDs should be taken into account. In FCDs, cholesteric layers are bent around two centers of curvature: ellipse lying in the surface plane and

Figure 7. Formation of a double-spiral rotating anticlockwise. Before (a) and after (b) heating. D

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of photochromic dopant Sorb in cyclosiloxane oligomer SilRed allowed us to change in a controllable manner the cholesteric helix pitch by means of UV-irradiation followed by the film annealing, and thereby to manage the optical properties and the surface topography of the material. An annealing of the irradiated films results in helix twisting and transformation of surface topography. In the case of polygonal texture, we revealed three main peculiarities of the film microstructure rearrangement caused by the cholesteric helix twisting: (1) increase in the inclination angle of the cholesteric layers near the surface, which contributes to the reduction of the surface relief period (2) flow up of the cholesteric layers from the depth of the film to the surface in the centers of FCDs, that results in the growth of the groove length within the domains and the rotation of their centers (3) nucleation of new cholesteric layers on the film surface in the defect regions (in the case of the tetragonal arrangement of adjacent domains, it leads to the formation of double-spirals twisted in opposite direction). The domination of the last mechanism leads to an increase in the area of the defect regions. Thus, a mixed structure, comprised of the elements of polygonal and “fingerprint” textures, arises. The results obtained in our paper are significant for the development of nanotechnology, where liquid crystalline systems are particularly important.

tetragonal symmetry is important for new double-spiral formation in interdomain spaces, because in a less symmetrical environment and in the case of “fingerprint” texture only simple disclinations are generated. Examples are presented in Figure 8 and Figure S3 in the Supporting Information.

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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.jpcb.7b01886. AFM movie of the surface changes during heating after UV-irradiation (AVI) Scheme of the isomerization process of the dopant Sorb, AFM images with actual topography for Figure 2 and Figure 8, and AFM images of an area with “fingerprint texture” (PDF)

Figure 8. AFM images obtained at room temperature before (a) and after (b) heating at 70 °C within 100 min. The sample was previously irradiated with ultraviolet light. Three neighboring domains are circled. The size of the images is 20 × 20 μm2. Rough relief was subtracted (actual topography is shown in Figure S2).

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New ribbons intrude the space between the domains and squeeze them (Figure 9). Thus, the area of the defect regions significantly increases.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +7(495)939-10-09. E-mail: [email protected].

CONCLUSIONS In the present paper, we have discovered and described a new relationship between the changes of the cholesteric helix pitch and the microstructure rearrangement in cyclosiloxane oligomer films containing a photochromic dopant. The addition

ORCID

Olga V. Sinitsyna: 0000-0003-3381-6156 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS O.V.S., A.Yu.B., and V.P.S. acknowledge support from the Russian Science Foundation (14-13-00379). REFERENCES

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Figure 9. Schemes of a defect area formation between FCDs. Before heating (a) and after (b) heating. E

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