Cholesterol-Based Grafted Polymer Brushes as Alignment Coating

Lviv Academy of Commerce, Samtshuk 9, Lviv 79005 Ukraine. ∥ Vlokh Institute of Physical ... Publication Date (Web): September 30, 2016. Copyright ©...
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Cholesterol-Based Grafted Polymer Brushes as Alignment Coating with Temperature-Tuned Anchoring for Nematic Liquid Crystals Yurij Stetsyshyn,*,† Joanna Raczkowska,*,‡ Andrzej Budkowski,‡ Kamil Awsiuk,‡ Andriy Kostruba,§,∥ Svyatoslav Nastyshyn,⊥ Khrystyna Harhay,† Edward Lychkovskyy,# Halyna Ohar,† and Yuriy Nastishin∥ †

Lviv Polytechnic National University, S. Bandery 12, 79013 Lviv, Ukraine Smoluchowski Institute of Physics, JagiellonianUniversity, Łojasiewicza 11, 30-348 Kraków, Poland § Lviv Academy of Commerce, Samtshuk 9, Lviv 79005 Ukraine ∥ Vlokh Institute of Physical Optics, 23 Dragomanov st., Lviv 79011, Ukraine ⊥ Ivan Franko National University of Lviv, Dragomanov 50, 79005 Lviv, Ukraine # Lviv Danylo Halytsky National Medical University, Pekarska 69, 79010 Lviv, Ukraine ‡

S Supporting Information *

ABSTRACT: Novel alignment coating with temperature-tuned anchoring for nematic liquid crystals (NLCs) was successfully fabricated in three step process, involving polymerization of poly(cholesteryl methacrylate) (PChMa) from oligoproxide grafted to the glass surface premodified with 3-aminopropyltriethoxysilane. Molecular composition, thickness, wettability of the PChMa coating and its alignment action for a NLC were examined with time of flight-secondary ion mass spectrometry, ellipsometry, contact angle measurements, polarization optical microscopy and commercially produced PolScope technique allowing for mapping of the optic axis and optical retardance within the microscope field view. We find that the PChMa coating provides a specific monotonous increase (decrease) in the tilt angle of the NLC director with respect to the substrates normal upon heating (cooling) referred to as anchoring tuning.

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INTRODUCTION Polymer brushes are monomolecular coatings of polymers with their one end grafted to a surface of a solid substrate such that the long axis of the molecule stands out with respect to the surface.1 The involvement of liquid crystal (LC) motifs into the grafted polymer brushes is of great interest in the field of the fabrication of “smart” responsive surfaces.2−7 One example is employment of polymer brushes as alignment layers for liquid crystal molecules. The alignment of LC molecules is described by a nonpolar vector (having equivalent ends) called director n⃡, to which the long axes of the LC molecules tend to be parallel. The presence of liquid crystalline fragments in the grafted polymers brushes implies at least two types of such molecular brushes: polymers with main-chain nematic liquid crystalline blocks and those with liquid crystalline side chains. Main chain liquid crystalline polymer brushes showing nematic orientational order have been studied theoretically by Halperin and Williams.2−4 Most of bare solid substrates induce a tangential alignment of the nematic director. Long molecules such as surfactant and polymer chains, forming a brush-like monolayer on the substrate surface, favor perpendicular (homeotropic) orientation of the director. Development of alignment layers providing uniform tilt of the nematic director along the substrate is a challenging but highly demanded task. In most cases the tilt angle (in liquid crystal terminology called pretilt) © 2016 American Chemical Society

hardly depends on temperature, though in rare cases the alignment can be spontaneously altered at some temperature value called the anchoring transition temperature. In most reported cases the anchoring transition (ATr) takes place near the nematic−isotropic phase transition temperature Tc (called the clearing point). Yet rarely was the pretilt angle reported to change smoothly in some temperature region. The first observations of such a temperature driven pretilt belong to Ryschenkow and Kleman.5 Later this phenomenon was called continuous anchoring transition.6 Side chain-type liquid crystalline polymer brushes synthesized by Peng et al.7,8 as a liquid crystalline alignment layer showed a high polydispersity with random azimuthal orientation, and consequently, unidirectional planar alignment could be achieved only when followed by mechanical rubbing of the substrate. On the other hand, Hamelinck and Huck9 synthesized nematic liquid crystalline polymer brushes, which aligned a LC homeotropically. In the work,10 liquid crystalline polymer brushes containing a mesogenic azobenzene moiety were synthesized on quartz and silicon substrates. The azobenzene side chains of the grafted polymer exhibited a Received: August 8, 2016 Revised: September 13, 2016 Published: September 30, 2016 11029

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Langmuir Scheme 1. Stages of Coating Fabricationa

Functionalization of glass surface (1) with amino-terminated APTES film (2), followed by grafting of oligoperoxide (3), and subsequently by polymerization of ChMa, initiated by peroxide groups of oligoperoxide, finally forming the PChMa brushes (4). a

the synthesized coatings were analyzed using time of flightsecondary ion mass spectrometry (TOF-SIMS), ellipsometry, and contact angle measurements, respectively. The temperature dependence of light phase retardation for the nematic ZLI-4119 in a cell with PChMa covered substrates, examined with the polarizing optical microscope (POM) and PolScope technique, testifies that the nematic pretilt can be smoothly temperature tuned at the surface covered with grafted polymer brushes. Strictly speaking, such a behavior in a wide temperature region is not typical for phenomena embraced by the term transition (including ATr) and for this reason we further refer to it as the anchoring tuning (ATu). Our experimental results suggest that conformation of the PChMa-grafted brushes is responsible for the observed anchoring behavior.

liquid crystal smectic phase due to parallel orientation of the mesogen fragments such that smectic layers appeared aligned perpendicularly to the substrate. Substrates with poly(methyl methacrylate), poly(styrene), and poly(hexyl methacrylate) grafted as alignment layers for NLC were examined by Osamu Sato et al.,11 and the temperature dependence of anchoring energy was measured. Srinivasarao and coauthors12,13 reported surface anchoring transitions for a polymer-dispersed liquid crystal fabricated by in situ photopolymerization of acrylates with different modified alkyl side chains allowing for tuning nematic alignment from planar to homeotropic in a wide temperature range. Cholesterol-based polymers are capable of creating different self-assemblies in a broad temperature and solvent regimes. Cholesterol polymers can be hydrophobic or amphiphilic, which is determined by the chemical structure of the cholesterol fragment in the polymer.14 Polymers comprising cholesterol self-assembled in the bulk state show cholesteric, smectic, and blue LC phases.14 Each of these mesophases has unique ordering of the cholesterol molecules resulting in specific optical properties. In the present work, for the first time, we synthesized grafted liquid crystalline polymer brushes with cholesterol side chains (poly(cholesteryl methacrylate), PChMa), which shows smooth temperature tuning of surface anchoring for a nematic liquid crystal (ZLI-4119, from EM Industries Inc.). Grafted PChMa brushes were fabricated via polymerization from oligoperoxide grafted to glass premodified with (3-aminopropyl)triethoxysilane (APTES), similarly as it was described in our previous works.15−20 Composition, thickness, and wettability of

2. EXPERIMENTAL SECTION 2.1. Materials. Polyethylene glycol (PEG-9) was purchased from Merck Chem. Co. Cholesterol (Ch), methacryloyl chloride and 3aminopropyltriethoxysilane (APTES) are from Sigma-Aldrich. An approach described by Weissberger et al.21 was used for purification of pyridine and other organic solvents. tert-Butyl hydroperoxide was obtained following the procedure in ref 22; its purification was performed using vacuum rectification. The refractive index nd20 = 1.4002 ± 0.00002 measured for the collected fraction boiling in the temperature range of 45−47 °C (at 1.6 kPa) is in good agreement with the value nd20 = 1.4010 previously reported in ref 22. Synthesis of Pyromellitic Acid Chloride. A 43.6 g (0.2 mol) sample of pyromellitic dianhydride was mixed with 91.6 g (0.44 mol) of PCl5 in a 500 mL round-bottomed flask equipped with a thermometer and a reflux condenser, connected with water scrubber, and then boiled in 11030

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2.3. Water Contact Angle Measurements. To measure the static contact angle (CA) we employ the so-called sessile drop technique using a KrussEasyDrop (DSA15) instrument equipped with the Peltier temperature-controlled chamber. During measurements the sample was stabilized at 20 °C, and the temperature was measured with a thermocouple placed in contact with a sample surface. 2.4. Ellipsometry. We use a commercially produced Nullellipsometer LEF-3M, based on the optical scheme “polarizer− compensator−specimen−analyzer” with a He−Ne single-mode laser (light wavelength λ = 632.8 nm) used as a light source. Accuracy for the determination of angular position of polarizing elements for this device is not worse than 0.01°. The so-called four-zone technique was used to determine the polarization parameters (angles Ψ and Δ) for the light reflected from the sample surface. The light incidence angle ϕ was varied between 55.5° and 57.5° with the step of 0.2°. This range of the ϕ angle corresponds to the region of the principal angle of incidence (where Δ ≈ π/2 or 3π/2) and thus ensures maximal sensitivity. To determine the average thickness d and refractive index n for the APTES and oligoperoxide-graft-PChMa coatings, we employ the iterative procedure for fitting of the (Ψ, Δ)-data recorded at optimal experimental conditions,26 using mono- and two-layer models described in the literature.20,24,25 Raw data for angular dependencies of the ellipsometric parameters for PChMa coating after 12 h of the polymerization are presented in Figure S2. To determine the grafting density of PChMa chains on a peroxided glass surface one needs the value of molecular masses M (g/mol) for grafted polymer brushes. We calculate it from the ellipsometry data and kinetic parameters of the polymerization, using the following equation:

the oil bath. To ensure full homogenization, the mixture was additionally mixed at 130−135 °C for 15−16 h. After replacement of the reflux condenser by a Liebig condenser, POCl3 in the amount of approximately 60−63 g was distilled off during 8 h, and the mixture was heated to 180−185 °C. After recrystallization of the crude product from gasoline we obtained 51.2 g (78.1%) of a colorless crystalline product with the acid number AN = 1373 mg KOH/g (compare to calculated value of 1368 mg KOH/g) and melting point at 67 °C, which is in good agreement with the corresponding value of 68 °C reported in the literature (see ref 23 for example). Synthesis of Oligoperoxide with Residual Acid Chloride Groups. Pyromellitic acid chloride in the amount of 4.6 g (0.014 mol) was dissolved in 15 mL of anhydrous dichlorethane. The obtained solution was mixed with 1.26 g (0.014 mol) of tert-butylhydroperoxide in a three-necked flask equipped with a stirrer. The temperature of the mixture was lowered to 5 °C, and then the solution of 1.1 g (0.014 mol) of pyridine in 10 mL of anhydrous dichlorethane, precooled to the same temperature of 5 °C, was added dropwise. The obtained suspension was mixed during 1 h and then 5.6 g (0.014 mol) of PEG-9 was added, which was then followed by dropwise admixing of the solution of 2.2 g (0.028 mol) pyridine in 10 mL of anhydrous dichlorethane. After the mixture was stirred for another 3 h, it was gradually heated up to room temperature. The precipitate of pyridinium chloride was filtered out, and the solvent was distilled out. Drying of the pellet in vacuum (100−200 Pa) at 40 °C for 3 h results in 8.2 g (81%) of oligoperoxide, which had a yellowish resinlike appearance. Characteristics of the pellet were AN = 163.1 mg KOH/g (compare to the calculated value of 155.3 mg KOH/g), contents of active oxygen and of active chlorine were found to be 1.9% and 5.4%, respectively (compared to the calculated values of 2.2% and 4.9%). Infrared spectra reveal the following characteristic bands located at 1760 and 1752 sm−1 for ν(CO) in Ar−C(O)Cl and for ν(CO) in the ester group, respectively: at 1390, 1365 sm−1 for doublet, which refers to d(C(CH3)3) and at 848 sm−1 for a band of the tert-butoxy group. Cholesteryl Methacrylate (ChMa). ChMa was obtained by the esterification of cholesterol (0.05 mol) with methacryloyl chloride (0.05 mol) in benzene (100 mL) in the presence of triethylamine (0.05 mol) and crystallized. 1H NMR spectroscopy (Figure S1, Supporting Information) data support the chemical structure of the synthesized monomer. The signals of the methyl protons in cholesteryl fragment are assigned to 0.68−1.04 ppm. The signals at 5.40 ppm correspond to the proton near the double bond in the cholesteryl fragment. Multiplet at 4.68 ppm depicts the cholesteryl fragment proton in the α position to the methacrylic fragment. The signals at 6.09 and 5.53 ppm correspond to the proton near the double bond in the methacrylic fragment. Methyl protons in the methacrylic fragment are assigned to 1.94 ppm. The ratio of the intensities of the methyl protons in the methacrylic and cholesteryl fragments is in agreement with that expected for the ChMa structure. 2.2. Fabrication of Coatings. Modification of Glass Surface with Oligoperoxide. Scheme 1 depicts three stages of the procedure of modification. In the first stage, glass plates (1) of 20 × 20 mm in size were immersed in a 0.2% (w/w) APTES solution of methanol and incubated for 24 h. Then, the Soxhlet apparatus was used to remove with methanol the loosely attached silane molecules. In the next stage the plates functionalized with APTES (2) were dipped for 24 h (grafting time) into 1% solution of oligoperoxide in arid dioxin. Similarly, the Soxhlet apparatus was used to remove loosely attached oligoperoxide with dioxan over 1 h and thus, oligoperoxide appears to be grafted to aminated surfaces (3).20 Polymerization of PChMa Brushes (4). Glass plates with grafted oligoperoxide obtained in the previous stage were immersed in the 0.1 M dioxan solution of the monomer ChMa. Thermostabilization at 90 °C under argon atmosphere for a period from 2 to 48 h (polymerization time) leads to formation of the oligoperoxide-graftPChMa coatings. Finally, with the use of the Soxhlet apparatus, the plates coated with polymerized PChMa were washed for 4 h with dioxan and dried.

M=

Cp Co(1 − e−kτ )n

(1)

where Co = 2.3·10−6 mol/m2 is the initial concentration of peroxide groups of the oligoperoxide initiator on the surface, the concentration Cp (g/m2) of PChMa on the surface is determined from Figure 2, the parameter k = 8.1·10−6 s−1 is the first order constant of the initiation at 90 °C, τ(s) is the polymerization time, n is the polymerization effectiveness on the surface, which according to our measurements is ∼0.1. 2.5. Time of Flight-Secondary Ion Mass Spectrometry (TOFSIMS). The surface chemistry of the oligoperoxide-graf t-PChMa coatings was examined with the TOF-SIMS technique using the TOF.SIMS 5 (ION-TOF GmbH) apparatus, which is equipped with 30 keV bismuth liquid metal ion gun. To ensure static mode conditions, the Bi3-clusters were used as the primary ions at the ion dose density lower than 1012 ion/cm2. Charge compensation was achieved with the pulsed low energy electron flood gun. Four different, nonoverlapping spots (each of 200 μm × 200 μm area) were used for acquisition of high mass resolution spectra for each sample. Mass resolution (m/Δm) at the C4H5 (m/z = 53) peak was not worse than 5500 for all collected spectra. H, H2, CH, C2H2, and C4H5 peaks were used for mass calibration. 2.6. Liquid Crystal (LC) Cells and Polarizing Optical Microscopy Observations. LC cells were assembled from two glass substrates coated with a PChMa layer fabricated as described in section 2.2 (a polymerization time of 48 h). The substrates were faced against each other by their coatings. The gap thickness between the substrates was fixed with spherical spacers of calibrated thickness dispersed in an UV-sensitive glue (NORLAND 64) applied along the cell sides. After UV-polymerization of the glue, the gap thickness of the empty cell was measured with the light interference technique. The cell was filled by capillary action with a nematic ZLI-4119 preheated to the temperature, which is at least 5 °C above the temperature of the nematic−isotropic phase transition (T = 79.5 °C according to our observations). The cell was slowly cooled down to room temperature and after several hours was used for textural observations with a Nikon polarizing optical microscope (POM) at crossed polarizers and characterized with the PolScope technique27 in the temperature range from 2 to 78 °C (using a hot-stage from Linkam). 11031

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Figure 1. Typical positive ion TOF-SIMS spectra of an APTES-modified glass surface registered after subsequent steps of peroxide grafting during 24 h (a) and PChMa polymerization for 48 h (b). The signals, which are specific for the coating following each fabrication step are labeled with the names of the secondary ion and encircled by ellipses. PolScope is an optical technique based on an optical microscope equipped with electrically tunable elliptical polarizer (a liquid-crystal (LC) universal compensator developed by Oldenburg and Mei) in the illumination system and a circular analyzer between the objective and CCD-camera (see ref 27 for details). Computer controlled software operates the electrically tunable LC elliptical polarizer and records images of the sample at five different states of the elliptical polarizer, and then the algorithm given in ref 27 is used to compute the distribution of the orientation of the optic axis and retardance (product of the sample effective birefringence and thickness) for each CCD-camera pixel within the microscope field view. We used the PolScope Abrio Imaging System produced by Cambridge Research & Instrumentation, Inc. (CRi).

procedure was analyzed using TOF-SIMS technique, operated in the static mode, which is known to show superior chemical specificity and surface sensitivity, characterized by sampling depth not larger than 2 nm. Typical positive ion TOF-SIMS spectra, registered for glass surface preliminary modified with APTES after grafting of oligoperoxide for 24 h and subsequent polymerization of oligoperoxide-graf t-PChMa layer are presented in Figure 1 panels a and b, respectively. TOF-SIMS spectrum measured for oligoperoxide layer (Figure 1a) depicts series of peaks of CH3O+, C2H5O+, C3H7O+, and C4H9O+, which are specific features for molecules with high abundance of oxygen and, therefore, unique for oligoperoxide molecules. In turn, in the second spectrum, recorded for the oligoperoxide-graf t-PChMa coating (Figure 1b), the series of peaks of C4H7+,C5H7+, C5H9+, and C6H7+ specific for PChMa showed up. It is clear from the spectra that the secondary ions containing oxygen disappear or are strongly reduced (cf. Figure 1a), confirming coverage of the surface by the grafted PChMa layer. To monitor the thickness of grafted layers we employed ellipsometry. Typical thickness of APTES measured at conditions of glass functionalization was about 0.5 nm, which is in good in agreement with the literature data.25 After the 48 h grafting, the average thickness of the PChMa coating layer increased up to 34 nm (Figure 2a) and correspondingly the refractive index increased from 1.49 to 1.506 (Figure 2b). Grafting density σ (in chains/nm2 units) of PChMa chains on peroxided glass surface was calculated using the following expression:

3. RESULTS AND DISCUSSION A three-step process, shown in Scheme 1 (details are given in the Experimental Section), was employed for fabrication of the oligoperoxide-graf t-PChMa coatings attached to glass: (Step 1) the native glass surface was cleaned with organic solvents and then modified with APTES; (Step 2) grafting of the oligoperoxide due to the interaction between amino groups of the APTES layer and pyromellitic chloroanhydride fragments in oligoperoxide molecules; (Step 3) fabrication of the PChMa brushes using polymerization “‘from the surface”’ of oligoperoxide. Composition, wettability, and thickness of the resulting PChMa coatings were respectively characterized using TOFSIMS, contact angle measurements, and ellipsometry. To examine alignment of elongated molecular PChMa fragments with respect to the surface, as well as to get information on tuning of the anchoring of the nematic liquid crystal at PChMa coatings at different temperatures, the PChMa-coated glass plates were used as substrates for cells filled with the nematic ZLI-4119, which were inspected using PolScope and POM. 3.1. Composition, Thicknesses, Refractive Indices and Wettability of Oligoperoxide-graf t-PChMa Coatings (ToF-SIMS, Contact Angle, and Ellipsometry). Chemical composition of PChMa coatings after two steps of fabrication

σ=

hρNA M

(2)

where h is the thickness (nm) of a dry layer as measured by ellipsometry, NA is the Avogadro’s number, M (g/mol) is the molecular mass of the PChMa brushes grafted onto the surface, 11032

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surface modification is accompanied by surface hydrophobization. A relatively higher rate of the modification at the initial stage, is most probably due to the bulk nature of cholesterol. Contact angles Θ, measured in course of grafting time, changes similarly to those registered for an amino-terminated glass surface (Θperoxided glass = 67°) and for pure PChP coatings (ΘPChMa = 79°, as determined for a reference thick PChMa layer). We are led to conclude that the partial PChMa modification of the peroxided glass surface is characterized by intermediate Θ values. Using the Cassie relation28,29 and our experimental data for Θ, we have determined the effective fraction of the modified peroxide glass surface (Figure 3b), which characterizes the effective PChMa coverage. Similar wetting was demonstrated for a surface modified by cholesterolcontaining self-organized monolayers in ref 30, which together with TOF-SIMS and ellipsometric data are conclusive evidence for complete modification of surfaces with PChMa-grafted brushes. 3.2. Temperature-Tuned Anchoring of a Nematic at the Surface Modified by PChMa Coating (POM and PolScope). Alignment of the nematic director at PChMa substrates was examined using a polarization optical microscope. Between crossed polarizers for a broad temperature range between 0 °C and room temperatures, the cell assembled with PChMa substrates and filled with the nematic ZLI-4119 shows an extinguished texture similar to that corresponding to T = 24 °C in Figure 4. Rotation of the cell in the azimuthal plane around the microscope axis does not result in visible brightening of the texture. The latter certifies that the zenithal angle θ of the nematic director defined with respect to the cell normal is small, corresponding to homeotropic or quasihomeotropic alignment of the director. Touching the cell with a needle leads to flash-like brightening of the texture, which is an additional indication of the homeotropic (or quasi-homeotropic) alignment. The textural changes are hardly visible up to temperatures roughly close to 40 °C, but the texture gradually brightens on further heating (see Figure 4). Brightening of the cell unambiguously indicates tilting of the director. For all the pictures in Figure 4 the polarizer and analyzer are crossed (PxA) being oriented respectively along the long and short sides of photographs. The last photograph (at right bottom

Figure 2. Average thicknesses (a) and refractive indices (b) of the PChMa layers grafted to peroxided glass, measured by ellipsometry as a function of grafting time. Solid lines are guides to eye.

and the PChMa bulk density ρ = 1. For polymerization times of 12 and 48 h we find the calculated molecular masses to be 178 000 and 190 000, respectively, and the corresponding grafting densities of 0.04 and 0.11 chains/nm2, respectively. It is worth noticing that the latter values match well with the values (0.1−0.4 chains/nm2) previously reported in the literature.16,17 Receding water contact angle, characterizing the wettability of PChMa coatings grafted from peroxided glass, is shown as a function of grafting time in Figure 3a. One can expect that the

Figure 3. Receding contact angles Θ for water (a) and effective coverage (b), as determined from the Cassie relation for peroxided glass surfaces modified with PChMa coating as a function of grafting time. Solid lines are a guide to eye.

Figure 4. POM textures demonstrating temperature tuning of anchoring for the nematic director at the PChMa coating. The length of the long sides of the photographs is 770 μm. 11033

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Langmuir corner of Figure 4) shows the texture observed at T = 78 °C with the red-lambda (530 nm) plate being inserted before the analyzer such that its slow axis is in quadrants I and III formed by crossed polarizers. At such an optical geometry (PxA+λ) for an optically positive nematic (ZLI-4119 in our experiment), the green (yellow) color on the corresponding photograph visualizes places where the projection of the nematic director is oriented along the diagonals of quadrants I and III (respectively, II and IV). The temperature-induced textural changes described above are reversible upon the heating−cooling cycling. To set these observations on a quantitative basis we measure the temperature dependence of the tilt angle θ of the nematic director at the PChMa substrates, using the PolScope technique. The motivation for the choice of the nematic ZLI-4119 was 2-fold. First, it has wide temperature range [−30;+80] °C of the nematic phase, which is important regarding possible temperature induced molecular conformations of the studied brushcoating layer. Second, the nematic ZLI-4119 has relatively low birefringence, which is desirable for examination of the optical phase retardation with PolScope technique. The idea of the experiment is as following. PolScope technique provides a map of the retardance R = Δneffd between the extraordinary and ordinary waves propagating through a nematic layer of the thickness d, measured at the wavelength λ = 546 nm as a function of the in-plane coordinates within the microscope view field. Here Δneff = neff(θ) − no is the effective birefringence of the nematic at a given zenithal angle θ of the optic axis (read the nematic director) with respect to the light propagation direction, which is along the cell normal for the PolScope geometry. neff(θ) is the effective refractive index for the extraordinary light wave, which is related to the angle θ by the following equation: none neff = 2 2 no sin θ + ne 2 cos2 θ (3)

sphere spacers dispersed in UV-glue) was measured with the light interference technique to have a thickness d = 2.3 μm. After UV-curing of the glue, the cell was filled with the nematic ZLI-4119. Using POM and PolScope observations we have assured that no anchoring transition is observed for the nematic ZLI-4119 at PI255 substrates. Temperature dependence of the retardance measured with PolScope for the cell with substrates coated by PI255, divided by the cell thickness, gives the temperature dependence Δn(T), which is shown in Figure 5.

Figure 5. Temperature dependence of the birefringence for the nematic ZLI-4119 in the cell with PI 2555 substrates, measured with PolScope.

Similar PolScope measurements of R(T) were performed for a cell assembled using glass substrates with PChMa brushes grafted to their surfaces and filled with the same nematic ZLI4119. Figure 6 shows a temperature dependence of the zenithal angle θ of the nematic director at the PChMa substrates, calculated using eq 5 from data R(T), measured for the cell with PChMa substrates and using data for Δn(T), shown in Figure 5.

where ne and no are refractive indices for extraordinary and ordinary light waves at θ = 0ο and θ = 90ο, respectively. In most practical cases it is safe to use an approximated expression Δneff = Δn sin 2 θ

(4)

where Δn = ne − no is the birefringence value, which for the studied nematic ZLI-4119 is Δn = 0.06 at room temperature. Therefore, if the zenithal angle θ of the nematic director is uniform through the cell thickness, then its value can be calculated from measured temperature dependence R(T) as

θ = arcsin

R (T ) Δn(T )d

(5)

It is seen from eq 5 that the temperature dependence of the birefringence Δn(T) for the used nematic is needed to determine θ from R(T) data. To measure Δn(T) we have assembled a cell using substrates covered with the polymer PI2555, which is widely used for planar alignment of conventional nematics with rod-shape molecules (including ZLI-4119). Azimuthal orientation of the nematic director was set by unidirectional rubbing of the substrates with velvet cloth. In principle, the Polscope technique does not require uniform in-plane alignment of the director along the view field. Nevertheless, we rubbed the substrates to avoid any complications which might be caused by the possible presence of singular defects. The gap between substrates (fixed with silica

Figure 6. Temperature dependence of the zenithal angle θ [deg] of the nematic director at the PChMa-coated substrates for the nematic ZLI-4119. PolScope images of the studied cell, from which data points for θ [deg] were calculated, are inserted above the plot for some temperatures, which are indicated by arrows. The color legend for OPD values depicted in PolScope images is inserted above the plot. 11034

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and repulsive steric interaction, which tend to align LMCL tangentially. Below we discuss in detail these and other related surface anchoring mechanisms. Several molecular mechanisms of the of LC alignment at a surface have been considered in the literature, namely, (1) steric interactions (contributing to entropy) between LC molecules and surface as well as between LC molecules near the surface; (2) van der Waals interaction (dispersion forces); (3) elastic distortion of the LC director field at rough surfaces (including unidirectionally buffed and randomly etched surfaces). It was shown that elastic distortions might be mediated by pretransitional 1D positional ordering of the LC forming smectic layers near the surface35,36 approaching the temperature transition to the smectic phase and as a result the TATr might have a continuous character; (4) electrostatic interactions between charged molecular groups, forming transverse or longitudinal electric dipole moments of the LC molecules interacting with charged groups on the surface; (5) covalent and hydrogen bonding of the LC molecules to the surface, encompassed by terms of epitaxy and pseudoepitaxy. Therefore, at present the common view is that there are two main types of the director alignment: homeotropic and planar, driven by their corresponding intermolecular forces, and the oblique alignment is considered to be a result of the competition between these forces. Concerning planar alignment it is worth noticing that to get uniform planar alignment, special action has to be performed to fix the azimuthal angle of the director, which otherwise appears to be azimuthally degenerated or randomly fixed. The difference between the two states is that azimuthally degenerated planar alignment takes place at the interfaces of thermotropic LCs with sof t surfaces such as isotropic hydrophilic liquids (water, glycerol, ethyleneglycol, etc.)37−39 or some polymers,40,41 where the azimuth of the director freely floats along the surface. At hard surfaces the director azimuth is usually fixed, though it can be random. Azimuthal randomness of the director at a hard surface can be removed by mechanical buffing,42 anisotropic patterning using lithography,43−45 scratching with atomic force microscopy (AFM),35 irradiation of photosensitive polymers by polarized light,46,32,33 electron beam sputtering,47 oblique SiOx evaporation,48−50 and other actions. It is also worth noticing that, there is no universal rule according to which a given substrate would be expected to produce specific alignment for all LCs or vice versa. The rule is that the type of the alignment is specific for a given pair of LC− substrate. The molecular mechanisms of LC alignment might be temperature dependent, and their competition might lead to TATr. Although, even for the case of a LMLC at a smooth hard surface, the understanding of the surface alignment phenomena is far from being complete, it is well accepted now that van der Waals interactions (also called dispersion forces) promote homeotropic alignment,51 whereas steric interactions of the rod-like LM nematic with the surface as well as between the LMLC molecules, favor tangential alignment of the long axes of nematic molecules near a smooth hard surface.52−54 Dispersion forces are temperature-dependent, weakening on heating, whereas steric interactions hardly depend on temperature. As a results one expects that due to these two competing factors at TATr a LM nematic can show a discontinuous homeotropic-toplanar TATr on heating or in the opposite direction on cooling.52−54 Both discontinuous55 and continuous5,6,35,36 direc-

PolScope images inserted in Figure 6 for some temperature points indicated by arrows are in qualitative agreement with the POM textures (Figure 5). Namely, textural changes are hardly visible in the temperature range below and slightly above room temperatures, but with PolScope the corresponding retardance values are well measurable for the studied temperature range. It has to be noticed that the PolScope technique is recommended for measurements of the retardance values smaller than 273 nm, which is the half-wavelength of the probing light. When the real retardance values exceed this limit, a special algorithm has to be applied to deduce the real retardance values from those displayed by PolScope. Quasi-homeotropic alignment observed in the temperature region between 0 °C and room temperature results in retardance values which are well below 273 nm. When heat is added, the retardance values increase and at temperatures above 76 °C, but still below the temperature Tc (called clearing point) of the nematic−isotropic phase transition, the retardance overcomes the value of 273 nm. Additional complications might take place in the pretransitional temperature region, where birefringence decreases rapidly upon heating and where possible temperature gradients might lead to the coexistence of the nematic and isotropic phases through the cell thickness. For these reasons we have excluded from consideration the temperature region above 76 °C. Therefore, we conclude that at a substrate with PChMagrafted polymer brushes the nematic liquid crystal shows temperature tuning of the director tilt angle in a wide temperature region from 0 °C to almost 80 °C at the temperature of the phase transition to the isotropic phase. In liquid crystal literature the phenomenon of temperatureinduced realignment of the director at a substrate surface is called the temperature anchoring transition (TATr). Anchoring and TATr at surfaces with grafted polymer brushes are of particular interest since they are claimed to be potentially capable of tuning the surface alignment with external actions such as temperature, light irradiation, electric, or magnetic fields. To achieve this, it was proposed31 to use LC-side-chain polymers grafted to the surface such that the mesogenic moieties forming brushes are exposed to contact a low molecular (LM) LC in a cell assembled of such substrates. Several experimental results have been reported concerning such a possibility.7−10,32,33 It is believed that homeotropic alignment realizes at low anchoring strength values34 and when the alignment layer is hydrophobic.9 If the latter is true then the LC molecules are expected to stick to the surface by the ends of their aliphatic chains and consequently the anchoring energy should be indeed rather low. In their innovative paper Halperin and Willams31 have predicted that sparsely grafted chains will be aligned tangentially to the surface. At high enough density of LC brushes the dispersion forces between them tend to induce their homeotropic alignment with respect to the surface, which in its turn opens a possibility for penetration of LMLC molecules in between the brushes, thereby swelling them. We believe that the latter model is well applicable to the system under study, which also comprises the LM nematic aligned by the surface modified with the LC-brushed polymer. Owing to swelling by the LM nematic the polymer brushes become subject to orientational elastic torque from the distorted LMLC director field. In such a case the LMLC-swollen brush layer has to be considered as a LC system and the observed anchoring tuning can be considered a result of the competition between the dispersion forces favoring homeotropic director orientation 11035

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changes in hydrophobic−hydrophobic interactions of the cholesterol fragments of the PChMa, thereby opening the possibility for swelling the PChMa layer with LM nematic molecules, making their interaction more temperature sensitive and thus providing competition between dispersion and steric forces to tune the zenithal director angle with temperature toward planar orientation upon heating and reversibly on cooling.

tor realignment has been reported in the literature. Discontinuous TATr implies that switching from homeotropic to planar director alignments (or in opposite direction) takes place at TATr. If at TATr the director realigns to an intermediate oblique orientation, such that above (or below) TATr the director tilt angle with respect to the surface normal smoothly changes with temperature from 0° to 90° (or in the opposite direction), then one says that TATr is continuous. In such a (2) case one is led to introduce two values T(1) ATr and TATr bounding the temperature region, where the director tilt is between 0° and 90°. A model explaining discontinuous TATr between tangential and homeotropic alignment upon cooling as a result of competition between the excluded volume entropy contribution due to steric interaction favoring planar alignment and adsorption energy contribution favoring normal alignment was proposed by Sharlow and Gelbart.54 In the approach developed by Kimura and Nakano51,56,57 the TATr is a result of competition between steric hard-core repulsive interaction and attractive dispersion forces. It was concluded in ref 51 that depending on the material parameters the TATr can be either discontinuous or continuous. In particular, it was shown that the theory51 agrees with experimental observations58,59 for cyanobiphenyls. The nematic ZLI-4119 used in our studies is also a mixture of cyanobiphenyls, showing continuous increase of the director tilt angle on heating, and here there are two aspects to be considered. First, a TATr can be driven by the LMLC and due to swelling the corresponding elastic torque can be transferred to polymer brushes, dragging them to realign. Second, the polymer brushes themselves form soft matter and have been recognized to be capable for high enough orientational order.60 The latter implies that the notion of TATr can be applied to the grafted brushes in the same vein as it concerns the LM nematics. Therefore, regarding both aspects alluded to above one can expect homeotropic-to-planar TATr on heating (or in opposite direction on cooling) for grafted PChMa brushes under study. Namely, (1) nematic alignment at the surface can be realigned due to at least two competing mechanisms driven by the attractive dispersion forces favoring homeotropic alignment and by repulsive steric interaction, which through the minimization of the excluded volume tend to align LMLC tangentially. (2) At high enough density of brushes they might interact through the same two competing mechanisms of dispersion forces and repulsive steric interaction, and similarly at lower temperatures where dispersion forces are stronger, the homeotropic alignment of the brushes is expected and at higher temperatures it can be switched to tangential alignment favored by steric interaction. Results reported in this paper can be compared with our previous work30 where a multifunctional cholesterol-based peroxide layer coated onto amino-terminated glass surfaces produces quasi-homeotropic and homeotropic nematic alignment without temperature tuning of the anchoring. To shed light on the influence of the PChMa coatings at different temperatures on the tilt angle of anchoring in a nematic liquid crystal, AFM analysis was employed (Figure S3). AFM images demonstrate considerable changes in morphology of the PChMa coatings at increasing temperature. It suggests that the conformation of the PChMa grafted brushes is responsible for the observed anchoring behavior of the nematic crystals. We can assume that temperature-induced changes in conformation of the PChMa grafted brushes are attributed to

4. SUMMARY AND CONCLUSIONS Grafted polymer brushes with cholesterol side chains attached to glass were synthesized for the first time in a three-step process involving polymerization from oligoproxide grafted to the APTES-modified glass surface. The conclusion on the final composition of the coated layer is supported by data from different characterization techniques such as static TOF-SIMS, ellipsometry, and wetting contact angle measurements. Our experiments show that glass plates with PChMa-grafted polymer brushes, used as substrates for a nematic liquid crystal, align elongated nematic molecules quasi-homeotropically with respect to the surface coated with the PChMa layers at temperatures close to 0 °C. The nematic director undergoes gradual temperature-tuned tilting toward a planar orientation in a wide temperature region from 0 °C up to bulk nematic phase transitions. Another significant aspect is that anchoring behavior is reversible with temperature cycling. The conformation of the PChMa-grafted brushes is proposed as a possible mechanism, responsible for the observed anchoring behavior. New alignment layers capable of tuning the director tilt angle are greatly needed for progress in fundamental studies of liquid crystals as well as in their application. We have demonstrated that the fabricated and characterized PChMa coating is such a fairly new, highly promising material. The temperature tuning of the nematic anchoring can be used for fabrication of liquid crystalline cells with desired orientation of the director in a wide range of temperatures, which is highly demanded in liquid crystalline display production.

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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.langmuir.6b02946. 1 H NMR spectra of PChMa; angle dependences of ellipsometric parameters; AFM images of the PChMa coating at different temperatures (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

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