Process for a Reactive Monomer Alignment Layer for Liquid ... - MDPI

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Process for a Reactive Monomer Alignment Layer for Liquid Crystals Formed on an Azodye Sublayer Junren Wang 1 , Colin McGinty 1 , Robert Reich 2 , Valerie Finnemeyer 2 , Harry Clark 2 , Shaun Berry 2 and Philip Bos 1, * 1 2

*

Liquid Crystal Institute, Kent State University, Kent, OH 44242, USA; [email protected] (J.W.); [email protected] (C.M.) Lincoln Laboratory, Massachusetts Institute of Technology, Lexington, MA 02421, USA; [email protected] (R.R.); [email protected] (V.F.); [email protected] (H.C.); [email protected] (S.B.) Correspondence: [email protected]





Received: 21 June 2018; Accepted: 10 July 2018; Published: 12 July 2018

Abstract: In this work, the detailed studies of surface polymerization stabilizing liquid crystal formed on an azodye sublayer are presented. The surface localized stabilization is obtained by free-radical polymerization of a dilute solution of a bi-functional reactive monomer (RM) in a liquid crystal (LC) solvent. To optimize the process for surface localized stabilization, we investigate the effects of several process parameters including RM concentration in LC hosts, the types of materials (either RM or LC), the photo-initiator (PI) concentration, ultra-violet (UV) polymerization intensity, and the UV curing temperature. The quality of surface localized stabilization is characterized and/or evaluated by optical microscopy, electro-optical behavior (transmission/voltage curve), the life test, and photo-bleaching. Our results show that, by carefully selecting materials, formulating mixtures, and controlling the polymerizing variables, the RM polymerization can be realized either at the surface or through the bulk. Overall, the combination of surface localized stabilization and photo-alignment offers an elegant and dynamic solution for controlling the alignment for LC, which could play a profound role in almost all liquid crystal optical devices. Keywords: photoalignment; liquid crystals; reactive monomers; azo dye

1. Introduction Well-aligned liquid crystal (LC) layers are required in almost all their electro-optic applications. Currently, the dominant liquid crystal alignment method–mechanically rubbed polyimide suffers several problems such as the creation of contaminating particles, scratches, and electrostatic charges [1]. Therefore, alternative non-contact techniques to align liquid crystals are preferred. As one of the most promising non-contact alignment methods, photo-alignment can utilize the polarized light to generate the anisotropy on the substrate surface, which overcomes the problems mentioned above [2–6]. Photo-alignment based on azo-dye offers an intriguing way to fabricate liquid crystal devices due to the low cost as well as the ability to create complex and precise patterns under mild conditions [5,7]. One drawback of the azo dye photo-alignment layer is its instability to subsequent exposure to light. To stabilize the initial alignment for liquid crystals, solutions including the polymerizable azodyes [8] and passive reactive monomer layer spin-coated on the top of the azo-dye sublayer [9] have been explored. To simplify the process, V. Finnemeyer et al. [6] proposed that a small amount of reactive monomer added to the liquid crystal host could provide excellent stabilization of the alignment by phase separation. Additionally, C. McGinty et al. [10,11] showed that the underlying azodye layer can be photo-bleached to eliminate its visible absorption as well as its ability to re-orient further.

Materials 2018, 11, 1195; doi:10.3390/ma11071195

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Depending on the location where the full polymerization of the reactive monomer (RM) completes from a low molecular weight liquid crystal solvent, the formed structure can be generally classified as a bulk polymer network or a surface localized polymer. Most prior studies were focused on the polymer network that stabilized LC on a rubbed polyimide layer and have shown that some parameters such as solubility parameters of RM in LC [12], the RM concentration in LC [13,14], LC materials [15,16], reactive monomers [17], ultraviolet (UV) curing intensity [18], and the UV curing temperature [19,20] could affect the resulting morphology and subsequent electro-optical behaviors. I. Dierking et al. reported that monomer solubility played a primary role in determining network morphology in the polymer stabilized liquid crystal (PSLC) [12]. Poorly soluble monomers form coarse “rice grain like” structures while soluble monomers yield smooth and continuous networks. R. Yamaguchi et al. reported that the morphology of polymer stabilized liquid crystal cells can be changed by selecting liquid crystal materials. Using an LC with a tolane substance, a “rice grain like” morphology can be obtained and can lower the driving voltage [15]. S. Hudson and L.C. Chien studied the morphology of PSLC when polymerized at different conditions (such as UV intensity and different temperatures) [19,21] and, in the case of different reactive monomers, at isothermal conditions [22]. More work related to PSLC can be found from several review papers by A. Sonin and N. Churochkina [23] and I. Dierking [24–26]. The existence of the polymer network in PSLC can introduce light scattering due to the refractive index mismatch between the bulk polymer network and liquid crystal [18], which will undermine the quality of the display. To eliminate light scattering, surface localized polymerization is preferred. Previous approaches for polymerizing the reactive monomer on the surface instead of in LC bulk include (1) selecting RM materials with high UV absorption [27]; (2) using a phase-separated composite film (PSCOF) [28,29], which is formed due to slow polymerization, phase separation, and fast diffusion of small molecules; and (3) utilizing the electric field to localize RM to the surface [30]. However, many aspects of the process to achieve and to optimize the surface localization of the polymer layer on a photo-alignment sublayer have not been explored. To have a better understanding as well as a better control over the surface localized stabilization on a photo-alignment sublayer, it is necessary to study the process variables such as materials and UV polymerization conditions. In this paper, we begin these investigations in detail. Optical microscopy observations, transmission-voltage curves, life-test, and photo-bleaching are utilized to help characterize and evaluate the quality of the surface stabilized liquid crystals. 2. Experimental 2.1. Sample Preparation At 22 ± 1 ◦ C and 20 ± 3% relative humidity (RH), 1 wt.% brilliant yellow (BY, CAS #: 3051-11-4, obtained from Sigma Aldrich, St. Louis, MO, United States) dissolved in dimethylformamide (DMF, 99.8% anhydrous, from Sigma Aldrich) was spin-coated onto UV/Ozone cleaned indium tin oxide (ITO, Corning Inc., NY, USA) glass substrates (1 inch by 1 inch) at 1500 rpm for 30 s to obtain a uniform and a thin BY layer [31]. After baking at 90 ◦ C for 10 min to evaporate solvent, the BY-coated glass substrates were assembled into cells using spacers of 5 µm thickness. The cell was then sealed on two sides with UV curable optical adhesives (NOA65, Norland Products Inc., Cranbury, NJ, USA) to create a channel for capillary-filling of LC mixtures. Then the assembled cells were exposed to linearly polarized blue light (Luxeon Royal blue LED with peak wavelength of 447 nm) at 25 mW/cm2 for 10 min to obtain a uniform planar alignment. The molecular structure of brilliant yellow is shown in Figure 1a and its absorbance spectra and photo alignment mechanism are explained in a previous research study [31–33]. Its photo-orientations to polarized light [31] and un-polarized light [10] are sketched in Figure 1b,c, respectively.

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Figure 1. (a) Molecular structure of brilliant yellow. (b) When exposed to polarized light, brilliant Figure 1. (a) Molecular structure of brilliant yellow. (b) When exposed to polarized light, brilliant yellow yellow molecules molecules undergo undergo in-plane in-plane re-orientation re-orientation to to aa direction direction of of 90 90 degrees degrees with with respect respect to to the the light light polarization direction. (c) When exposed to un-polarized light, brilliant yellow molecules undergo polarization direction. (c) When exposed to un-polarized light, brilliant yellow molecules undergo out-of-plane along to to the the light light propagation propagation direction. direction. out-of-plane re-orientation re-orientation to to aa direction direction along

Following the photo-alignment, the cells were capillary-filled with mixtures of RM and photoFollowing the photo-alignment, the cells were capillary-filled with mixtures of RM and initiator (PI) dissolved in the LC host at a temperature of 10 °C above◦LC’s clearing points. Then the photo-initiator (PI) dissolved in the LC host at a temperature of 10 C above LC’s clearing points. filled cells were exposed to 365 nm UV light to polymerize the reactive monomer. Then the filled cells were exposed to 365 nm UV light to polymerize the reactive monomer. Two reactive monomers, 2-Methyl-1,4-phenylene bis(4-(3-(acryloyloxy)propoxy)benzoate) Two reactive monomers, 2-Methyl-1,4-phenylene bis(4-(3-(acryloyloxy)propoxy)benzoate) (RM257) from Merck and Bisphenol A dimethacrylate (Bis-MA) from Sigma Aldrich, and two kinds (RM257) from Merck and Bisphenol A dimethacrylate (Bis-MA) from Sigma Aldrich, and two of liquid crystals known as the cyano eutectic liquid crystal (E7, clearing point = 60 °C, birefringence kinds of liquid crystals known as the cyano eutectic liquid crystal (E7, clearing point = 60 ◦ C, Δn = 0.217 at 589 nm and 20 °C) and the super fluorinated liquid crystal (ZLI-4792, clearing point = birefringence ∆n = 0.217 at 589 nm and 20 ◦ C) and the super fluorinated liquid crystal (ZLI-4792, 92 °C, birefringence◦ Δn = 0.0969 at 589 nm and 20 °C) from Merck were chosen to build different clearing point = 92 C, birefringence ∆n = 0.0969 at 589 nm and 20 ◦ C) from Merck were chosen to build RM/LC combinations. In addition, a small amount of 2,2-Dimethoxy-2-phenylacetophenone different RM/LC combinations. In addition, a small amount of 2,2-Dimethoxy-2-phenylacetophenone (Irgacure 651) from Sigma Aldrich was added as a photo-initiator. (Irgacure 651) from Sigma Aldrich was added as a photo-initiator. Effects of six process variables for surface localized stabilization including RM concentration, Effects of six process variables for surface localized stabilization including RM concentration, types of LC, types of RM, PI concentration, UV curing intensity, and the UV curing temperature were types of LC, types of RM, PI concentration, UV curing intensity, and the UV curing temperature were investigated by changing one parameter at a time. Accordingly, different samples/cells were investigated by changing one parameter at a time. Accordingly, different samples/cells were prepared prepared as shown in Tables 1–6. as shown in Tables 1–6. Table Whenstudying studying concentration, mixtures RM (Bis-MA) /LC (ZLI-4792) were Table 1.1.When thethe RMRM concentration, mixtures of RMof (Bis-MA) /LC (ZLI-4792) were prepared prepared In eachthe mixture, addedofamount of PI wasrespect 1% with as below. as In below. each mixture, addedthe amount PI was 1% with to respect the RM.to the RM. Sample RM/LC (Weight Ratio) UV Intensity (mW/cm2)2 UV Curing Time (min) Sample RM/LC (Weight Ratio) UV Curing Time (min) UV Intensity (mW/cm ) RMC1 1.5:98.5 3.5 10 RMC1 1.5:98.5 3.5 RMC2 1:99 3.5 1010 RMC2 1:99 3.5 10 RMC3 0.5:99.5 3.5 10 RMC3 0.5:99.5 3.5 10

UV Curing Temperature (°C) UV Curing Temperature (◦ C) 22 2222 22 22 22

Table 2. For For study study on on the the types typesof ofliquid liquidcrystals, crystals,1.5% 1.5%RM257 RM257was wasadded addedtoto 2 different (either Table 2. 2 different LCLC (either E7E7 or or ZLI-4792). In each mixture, the added amount of PI was 1% with respect to the RM. ZLI-4792). In each mixture, the added amount of PI was 1% with respect to the RM. Sample LC UV Intensity (mW/cm2) UV Curing Time (min) UV Curing Temperature (°C) Sample LC UV Curing Time (min) UV Curing Temperature (◦ C) UV Intensity (mW/cm2 ) LC1 E7 3.5 10 22 LC1 E7 3.5 10 22 LC2 LC2ZLI-4792ZLI-4792 3.5 10 10 3.5 2222

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Table 3. With regard to the types of the reactive monomer, two different reactive monomers—either RM257 or Bis-MA were added to ZLI-4792 at 0.5%. In each mixture, the added amount of PI was 1% with respect to the RM. Sample

RM

UV Intensity (mW/cm2 )

UV Curing Time (min)

UV Curing Temperature (◦ C)

RM1 RM2

RM257 Bis-MA

3.5 3.5

10 10

22 22

Table 4. With regard to the PI concentration, different concentrations of PI (with respect to the RM) were prepared. In each cell, the weight ratio of RM to LC was kept constant (RM257/ZLI-4792 = 0.3/99.7). Sample

PI/RM (Weight Ratio)

UV Intensity (mW/cm2 )

UV Curing Time (min)

UV Curing Temperature (◦ C)

PI1 PI2 PI3 PI4

1:100 50:100 75:100 100:100

3.5 3.5 3.5 3.5

10 10 10 10

22 22 22 22

Table 5. A previous study presented one UV intensity (3.5 mW/cm2 at 365 nm for 10 min) for RM polymerization [34]. To see whether UV intensity will affect the surface localized polymerization, we cured the cells at different UV intensities and different times, which is shown in Table 5. In each mixture, the added amount of PI was 1% with respect to the RM. Sample

RM/LC (Weight Ratio)

UV Intensity (mW/cm2 )

UV Curing Time (min)

UV Curing Temperature (◦ C)

UVI1 UVI2 UVI3 UVI4

0.3:99.7 0.3:99.7 0.3:99.7 0.3:99.7

0.75 3.5 21 21

70 10 2.5 10

22 22 22 22

Table 6. With regard to the UV curing temperature, cells filled with 1.5% RM257 dissolved in E7 were UV cured at two temperatures (below and above the clearing point of E7), which is shown below. In each mixture, the added amount of PI was 1% with respect to the RM. Sample

RM/LC (Weight Ratio)

UV Intensity (mW/cm2 )

UV Curing Time (min)

UV Curing Temperature (◦ C)

UVT1 UVT2

1.5:98.5 1.5:98.5

3.5 3.5

10 10

22 80

2.2. Alignment and Stabilization Examination The cells filled with mixtures of RM and LC were checked for their alignment by placing them between cross polarizers. The homogeneous (or planar) alignment is assured when the cell appears between alternative dark and bright light under crossed polarizers when the cell is rotated with respect to the polarizer(s) while the homeotropic alignment is secured when the cell appears dark under crossed polarizers regardless of the rotation. The stabilization of LC was checked with the life test, photo-bleaching, and the electro-optical performance (transmission versus applied voltage curves, also called T/V curve). T/V curves were collected with a green light source (550 nm) and compared both before and after UV polymerization of the reactive monomer. The transmission is measured using a photodetector (Thorlabs Inc., Newton, NJ, USA) and the transmission is represented by the voltage shown on the photodetector. Life tests re-expose the LC-filled cells to linearly polarized blue light at 45◦ with respect to the original photo-alignment process. Photo-bleaching re-exposes the cells with un-polarized blue light to eliminate the photosensitivity of the azo dye layer [10,11] because the brilliant yellow molecules re-orient along the un-polarized light propagation direction where no light absorption occurs and the molecules are at rest. If the monomer was not polymerized on (or close to) the surface, the resulting cells would show: (1) re-alignment with life test and/or photo-bleaching, (2) some light scattering observation because of the network through bulk, or (3) increased threshold voltage as indicated by the T/V curve after UV polymerization.

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cells would show: (1) re-alignment with life test and/or photo-bleaching, (2) some light scattering observation because of the network through bulk, or (3) increased threshold voltage as indicated by Materials 2018, 11, 1195 5 of 16 the T/V curve after UV polymerization. 3. Results 3. Results The effects effects of of parameters parameters (including (including RM RM concentration, concentration, types types of of LC, LC, types The types of of RM, RM, photo-initiator photo-initiator concentration, UV curing intensity, and UV curing temperature) on surface localized stabilization concentration, UV curing intensity, and UV curing temperature) on surface localized stabilization were characterized characterized and and evaluated evaluated based based on on macroscopic macroscopic observation, observation, life life test, test, photo-bleaching, photo-bleaching, and and were measured T/V curves. These effects are presented below. measured T/V curves. These effects are presented below. 3.1. Effects of RM Concentration

After photo-alignment with linearly polarized blue light, cells filled with mixtures of different concentrations of RM (Bis-MA) dissolved in LC (ZLI-4792) show planar alignment along the BY observation of alignment both before before and and after after polymerization, polymerization,which whichisisshown shownininFigure Figure2.2.The The observation different colors indicates thatthat the gap thickness of these home-made cells iscells not is very These of different colors indicates the gap thickness of these home-made notuniform. very uniform. ◦ to either cells appeared brightbright when when the BYthe alignment direction was 45° polarizer direction and These cells appeared BY alignment direction wasto45either polarizer direction appeared dark when the BY alignment direction was parallel or perpendicular to one polarizer and appeared dark when the BY alignment direction was parallel or perpendicular to one direction. It is significant that light scattering was observed for cells with a higher RM concentration (both 1% 1% and and 1.5%) 1.5%)while whilethe thelower lowerRM RMconcentration concentration(0.5%) (0.5%)did didnot notshow show the light scattering, which (both the light scattering, which is is shown in Figure 2. shown in Figure 2.

Figure 2. Observation of cells between crossed polarizers after UV polymerization. Cells were filled Figure 2. Observation of cells between crossed polarizers after UV polymerization. Cells were filled with mixtures of different RM (Bis-MA) concentration (0.5%, 1%, or 1.5%) in LC (ZLI-4792). with mixtures of different RM (Bis-MA) concentration (0.5%, 1%, or 1.5%) in LC (ZLI-4792).

Previous studies showed that the formation of the bulk polymer network would lead to light Previous the formation of the[35]. bulkTherefore, polymer we network would lead to light scattering [18] studies and the showed increase that of the threshold voltage believe the observed light scattering [18] and the increase of the threshold voltage [35]. Therefore, we believe the observed scattering is ascribed to the formation of the bulk polymer network. This is also confirmed in the light scattering is ascribed to 3a,b) the formation the bulk polymer network. This is(dash also line confirmed measured T/V curves (Figure where theofcurve peaks and threshold voltages curve) in the measured T/V curves (Figure 3a,b) where the curve peaks and threshold voltages (dash line showed the right-shift after UV polymerization compared with the right-shift before UV curve) showed the right-shift after UV polymerization compared with the right-shift before UV polymerization (solid line curve). For the lower RM concentration (0.5%), no increase in threshold polymerization (solid curve). For thescattering lower RM concentration (0.5%), no increase voltage (Figure 3c) andline no observed light (Figure 2) indicated the formation of in thethreshold polymer voltage (Figure 3c) and no observed light scattering (Figure 2) indicated the formation of the polymer layer at (or near to) the substrate. To further evaluate the localized surface and its stabilization of RM, layer at (or near to) the substrate. To further evaluate the localized surface and its stabilization of RM, we conducted the life test and photo-bleaching experiment on the cell filled with 0.5% RM in LC. we conducted the life test and photo-bleaching experiment on the cell filled with 0.5% RM in LC.

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RMC1

RMC2 1

Detector voltage (V)

Detector voltage (V)

1 0.8 0.6 0.4 0.2 0 0

2

4

6

0.8 0.6 0.4 0.2 0 0

8

2

4

6

8

Applied voltage (V)

Applied voltage (V) 1.5% before UV curing

1% before UV cure

1.5% after UV curing

1% after UV curing

(a)

(b)

RMC3 Detector voltage(V)

1 0.8 0.6 0.4 0.2 0 0

2

4

6

8

Applied voltage (V) 0.5% before UV curing 0.5% after UV curing

(c) Figure 3. Transmission-applied voltage curves of cells filled with mixtures of different concentrations Figure 3. Transmission-applied voltage curves of cells filled with mixtures of different concentrations of RM in LC. (a) 1.5% Bis-MA in LC; (b) 1% Bis-MA in LC; and (c) 0.5% Bis-MA in LC. of RM in LC. (a) 1.5% Bis-MA in LC; (b) 1% Bis-MA in LC; and (c) 0.5% Bis-MA in LC.

As shown in the microscopic images of Figure 4, in the afterlife test, the BY realigned and the shown in the microscopic imagesappearance) of Figure 4,between in the afterlife the BY(Figure realigned cells As showed some brightness (blue/green crossed test, polarizers 4b). and The the cells showed some brightness (blue/green appearance) between crossed polarizers (Figure birefringence magnitude of this BY (tens of nanometer thickness [34]) is considerably less than 4b). the The birefringence thisFollowed BY (tensby of photo-bleaching nanometer thickness [34])to is un-polarized considerablylight), less than aligned nematic LCmagnitude (close to 5 of µm). (exposed the the aligned nematic LC (close to 5 µm). Followed by photo-bleaching (exposed to un-polarized BY was realigned perpendicularly to the substrates and the cell appeared dark between crossed light), the (shown BY was in realigned perpendicularly toto the substrates and the cellbefore appeared dark between polarizers Figure 4c), which is similar the original appearance the life-test (Figure crossed polarizers (shown in Figure 4c), which is similar to the original appearance before thecell life-test 4a). Although the BY layer showed some realignment, the alignment of liquid crystal in the was (Figure 4a). Although the BY layer showed some realignment, the alignment of liquid crystal in the not changed, which is shown in Figure 4d,e. This is the same as the cell before the life-test and photocell was not changed, which is shown in Figure 4d,e. This is the same as the cell before the life-test and bleaching (0.5% cell shown in Figure 2). Combining Figure 4 and the T/V curve in Figure 3, we can photo-bleaching (0.5% shown in Figure 2). Combining Figure 4 and the T/Vsurface curve in 3, conclude that: (1) the cell polymerization occurred at (or close to) the substrate inFigure the cell we can conclude (1) the polymerization at (or to) concentration the substrate surface in the containing lower that: concentration (0.5%) of RM occurred in LC while theclose higher (1%, 1.5%) ledcell to containing lower concentration (0.5%) of RM in LC while the higher concentration (1%, 1.5%) led the the formation of the bulk polymer network; (2) the formed surface polymer layer in to0.5% formation of the polymer (2) thelayer formed surfaceBY polymer layer in 0.5% concentration concentration cellbulk worked likenetwork; an alignment between and LC, which preserved the LC cell workedregardless like an alignment layer between alignment of the reorientation of BY BY.and LC, which preserved the LC alignment regardless of the reorientation of BY.

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Figure 4. Polarized microscope images of the cell (0.5% cell in Figure 2): (a) before life-test; (b) after Figure4. 4. Polarizedmicroscope microscopeimages imagesof ofthe thecell cell(0.5% (0.5%cell cellin inFigure Figure2): 2): (a)before beforelife-test; life-test;(b) (b)after after Figure Polarized (a) life test; and (c) after photo-bleaching. The BY alignment was represented by a red dash arrow with lifetest; test;and and(c) (c)after afterphoto-bleaching. photo-bleaching.The TheBY BYalignment alignmentwas wasrepresented representedby byaared reddash dasharrow arrowwith withaaa life dynamic re-orientation direction after the life-test and photo-bleaching with respect to the polarizer polarizer dynamicre-orientation re-orientationdirection directionafter afterthe thelife-test life-testand andphoto-bleaching photo-bleaching withrespect respectto tothe the dynamic with polarizer and/or analyzer direction. Macroscopic observation of cells after photo-bleaching when the original and/oranalyzer analyzerdirection. direction.Macroscopic Macroscopicobservation observationofofcells cellsafter afterphoto-bleaching photo-bleachingwhen whenthe theoriginal original and/or LC alignment was 45° to polarizer (d) or parallel to one polarizer (e). ◦ LC alignment was 45 to polarizer (d) or parallel to one polarizer (e). LC alignment was 45° to polarizer (d) or parallel to one polarizer (e).

3.2. Effects of Types of LC 3.2.Effects EffectsofofTypes Types of of LC LC 3.2. Depending on the applications, liquid crystals with specific properties (such as proper Depending on on the the applications, applications, liquid liquid crystals crystals with with specific specific properties properties (such (such as as proper proper Depending birefringence, dielectric constant, clearing points, ion solubility, etc.) would be preferred. In this birefringence, dielectric dielectric constant, constant, clearing clearing points, points, ion ion solubility, solubility, etc.) would would be be preferred. preferred. In In this this birefringence, section, two different liquid crystals were chosen for the study. One is a common cyano eutectic liquid section, two different liquid crystals were chosen for the study. One is a common cyano eutectic liquid section, two different liquid crystals were chosen for the study. One is a common cyano eutectic liquid crystal mixture–E7 and the other one is super fluorinated liquid crystal–ZLI-4792. Two cells filled crystal mixture–E7 mixture–E7and and the the other other one one is is super super fluorinated fluorinated liquid liquid crystal–ZLI-4792. crystal–ZLI-4792. Two Two cells cells filled filled crystal with mixtures contain 1.5% RM257 dissolved in either E7 or ZLI-4792. These two cells were processed withmixtures mixtures contain contain 1.5% 1.5% RM257 RM257 dissolved dissolved in in either eitherE7 E7or orZLI-4792. ZLI-4792.These Thesetwo twocells cellswere wereprocessed processed with through polymerization at the same conditions. throughpolymerization polymerization at at the the same same conditions. conditions. through The cell filled with RM257 (1.5%)/E7 showed no light scattering and no increase in the threshold Thecell cellfilled filledwith withRM257 RM257 (1.5%)/E7 (1.5%)/E7showed showedno nolight lightscattering scatteringand andno noincrease increasein inthe thethreshold threshold The voltage, which is illustrated in the T/V curves of Figure 5a. The cell filled with RM257 (1.5%)/ZLIvoltage, which is Figure 5a.5a. The cellcell filled withwith RM257 (1.5%)/ZLI-4792 voltage, is illustrated illustratedininthe theT/V T/Vcurves curvesofof Figure The filled RM257 (1.5%)/ZLI4792 showed light scattering and some increase in the threshold voltage (Figure 5b). It may be showed light scattering and some in the threshold voltage (Figure It may be It concluded 4792 showed light scattering and increase some increase in the threshold voltage5b). (Figure 5b). may be concluded that 1.5% RM257 formed surface localized stabilization in the E7 cell while it formed a that 1.5% RM257 formed surface localized stabilization in the E7 cellinwhile it formed a bulk network concluded that 1.5% RM257 formed surface localized stabilization the E7 cell while it formed a bulk network in the ZLI-4792 cell. The difference indicates that the types of LCs counts when using in the ZLI-4792 cell. The difference indicates that the types of LCs counts when using the reactive bulk network in the ZLI-4792 cell. The difference indicates that the types of LCs counts when using the reactive monomer to stabilize the alignment. With regard to the analysis, we will propose a monomer to monomer stabilize thetoalignment. With regard to the analysis, a possible the reactive stabilize the alignment. With regardwe towill thepropose analysis, we will explanation propose a possible explanation in the Discussion section. in the Discussion section. possible explanation in the Discussion section.

1 1 0.5 0.5 0 0

before UV curing before UV curing after UV curing after UV curing

LC2 LC2

1.5 1.5

Detector Detectorvoltage voltage(V) (V)

1.5 1.5

Detector Detectorvoltage voltage(V) (V)

before UV curing before UV curing after UV curing after UV curing

LC1 LC1

1 1

0.5 0.5

0 0

2 2

4 4

Applied voltage (V) Applied voltage (V) (a) (a)

6 6

8 8

0 0

0 0

2 2

4 4

Applied voltage (V) Applied voltage (V) (b) (b)

Figure 5. T/V curves of (a) 1.5% RM257 in E7 and (b) 1.5% RM257 in ZLI-4792. Figure Figure 5. 5. T/V T/Vcurves curvesofof(a) (a)1.5% 1.5%RM257 RM257ininE7 E7and and(b) (b)1.5% 1.5%RM257 RM257ininZLI-4792. ZLI-4792.

6 6

8 8

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3.3. Effects Effects of of RM RM Types Types 3.3. As shown shown above, above, the the types types of of LCs To see see whether whether the the types types of of reactive As LCs showed showed a a difference. difference. To reactive monomer matter or not, we conducted experiments with a different RM in the same LC. The results monomer matter or not, we conducted experiments with a different RM in the same LC. The results were curves, which which is is shown shown in in Figure Figure 6. 6. were compared compared and and explained explained using using T/V T/V curves,

RM1

0.5% after UV curing

4 2 0 0

2

4

Applied voltage (V) (a)

6

8

0.5% before UV curing 0.5% after UV curing

0.8

Detector voltage (V)

6

Detector voltage (V)

RM2

0.5% before UV curing

0.6 0.4 0.2 0 0

2

4

6

8

Applied voltage (V) (b)

Figure 6. T/V curves of cells containing different monomers in the same LC. (a) 0.5% RM257 in ZLIFigure 6. T/V curves of cells containing different monomers in the same LC. (a) 0.5% RM257 in 4792; (b) 0.5% Bis-MA in ZLI-4792. ZLI-4792; (b) 0.5% Bis-MA in ZLI-4792.

In Figure 6a, the T/V curve (dash line) after polymerization shifted to the right when compared 6a, the T/V polymerization curve (dash line)(solid after line). polymerization shifted right with In theFigure T/V curve before This showed that,to forthe the cell when filled compared with 0.5% with the T/V curve before polymerization (solid line). This showed that, for the cell filled with after 0.5% RM257 in ZLI-4792, there was a small increase in the threshold voltage and weak light scattering RM257 in ZLI-4792, there was a small increase in the threshold voltage and weak light scattering after polymerization. These indicate that there were some formations of continuous networks through polymerization. These indicate that there were some formations of continuous networks through bulk. bulk. T/V curves (Figure 6b) for cells filled with 0.5% Bis-MA in ZLI-4792 did not show an increase T/V (Figure 6b) for filledalmost with 0.5% Bis-MA in ZLI-4792 show an increase in the in thecurves threshold voltage andcells showed no light scattering, whichdid wasnot macroscopically observed. threshold voltage and showed almost no light scattering, which was macroscopically observed. This is This is shown in the 0.5% cell in Figure 2. With these results, we can conclude that, similar to the shownofinLC, thethe 0.5% cell of in RM Figure Witheffects these results, we can conclude similar to the types of LC, types types also2.have on the localized surface that, stabilization. the types of RM also have effects on the localized surface stabilization. 3.4. Effects of Photo-Initiator (PI) Concentration 3.4. Effects of Photo-Initiator (PI) Concentration The major role of the photo-initiator in radical polymerization is to create reactive species (free The major role of the photo-initiator in radical polymerization is to create reactive species radicals) under mild conditions (such as UV light exposure, room temperature) and promote radical (free radicals) under mild conditions (such as UV light exposure, room temperature) and promote reactions. In order to investigate its effects, cells filled with mixtures of 0.3% RM257 in ZLI-4792 with radical reactions. In order to investigate its effects, cells filled with mixtures of 0.3% RM257 in ZLI-4792 different amounts of added PI were studied. with different amounts of added PI were studied. As shown in Figure 7a,b, at a lower PI concentration (PI/RM = 1/100 and 50/100, respectively), As shown in Figure 7a,b, at a lower PI concentration (PI/RM = 1/100 and 50/100, respectively), T/V curves almost kept the same before and after UV polymerization. As the PI concentration T/V curves almost kept the same before and after UV polymerization. As the PI concentration increased, the threshold voltage increased slightly and weak light scattering was observed, which is increased, the threshold voltage increased slightly and weak light scattering was observed, which is shown in the T/V curves in Figure 7c,d. These results may indicate that, at a lower PI concentration, shown in the T/V curves in Figure 7c,d. These results may indicate that, at a lower PI concentration, the RM tends to polymerize on the surface and the RM may possibly polymerize with some the RM tends to polymerize on the surface and the RM may possibly polymerize with some protrusion protrusion to the bulk at a higher PI concentration. to the bulk at a higher PI concentration.

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(c)

8

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2

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Applied voltage (V) 100/100 before UV 100/100 after UV

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Figure 7. T/V curves of cells containing mixtures with different concentrations of photo-initiator (PI):

Figure 7. T/V curves of cells containing mixtures with different concentrations of photo-initiator (PI): (a) PI/RM = 1/100; (b) PI/RM = 50/100; (c) PI/RM = 75/100; and (d) PI/RM = 100/100. (a) PI/RM = 1/100; (b) PI/RM = 50/100; (c) PI/RM = 75/100; and (d) PI/RM = 100/100.

3.5. Effects of UV Curing Intensity

3.5. Effects of UV Curing Intensity

To investigate the effects of UV light intensity on the surface localized stabilization, we

polymerized cells filled with 0.3% ZLI-4792 with different intensities. Their effects were To investigate the effects ofRM257 UV in light intensity on theUVsurface localized stabilization, illustrated in T/V curves (Figure 8) before and after polymerization. we polymerized cells filled with 0.3% RM257 in ZLI-4792 with different UV intensities. Their effects Note that look(Figure somewhat different when before and after UV curing. This were illustrated in the T/Vplots curves 8) before and aftercompared polymerization. may be due to the non-uniform thickness of the home-made cells. A different area may be Note that the plots look somewhat different when compared before and after UV curing. This may characterized before and after UV curing, which leads to the difference of the measured retardation be due to the non-uniform thickness of the home-made cells. A different area may be characterized (detector voltage). Despite the small difference of the T-V curve, the threshold voltage from the T-V beforecurves and after UV curing, leads to the difference of the measured retardation (detector (measured beforewhich and after curing) offer more important information to tell the location of voltage). the Despite small difference T-V curve, the threshold from before the T-V curves (measured RMthe formed polymer layer.of If the the threshold voltages remain atvoltage similar levels and after curing, beforethis and after curing) the offer more important information to tell Ifthe location of the RM formed polymer can determine localized surface RM polymerization. the threshold voltages are clearly it means the polymer network through LC bulk The plots inthis Figure show layer.different, If the threshold voltages remain at forms similar levelsthe before and[35]. after curing, can8determine that no effect is found on the threshold after polymerization, which different, indicates that UV the the localized surface RM polymerization. If voltage the threshold voltages are clearly it means intensity does not greatly affect the surface localized stabilization. This may be because the UV on polymer network forms through the LC bulk [35]. The plots in Figure 8 show that no effect is found intensity does not affect the production of radical species of photo-initiator or does not affect theaffect the threshold voltage after polymerization, which indicates that UV intensity does not greatly chain reaction rate. To further understand the effects in detail, the high-resolution scanning electron the surface localized stabilization. This may be because the UV intensity does not affect the production microscopy (SEM) may be used to check the formed polymer morphology. of radical species of photo-initiator or does not affect the chain reaction rate. To further understand the effects in detail, the high-resolution scanning electron microscopy (SEM) may be used to check the formed polymer morphology.

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Applied voltage (V) UVI2 before UV UVI2 after UV

(a)

(b)

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Figure 8. T/V curves of cells cured at different UV exposure intensity and time: (a) 0.75 mW/cm2 for

Figure 8. T/V curves of cells cured at different UV exposure intensity and time: (a) 0.75 mW/cm2 for 70 min; (b) 3.5 mW/cm2 for 10 min; (c) 21 mW/cm2 for 2.5 min; and (d) 21 mW/cm2 for 10 min. 70 min; (b) 3.5 mW/cm2 for 10 min; (c) 21 mW/cm2 for 2.5 min; and (d) 21 mW/cm2 for 10 min. 3.6. Effects of UV Curing Temperature

3.6. Effects The of UV UVCuring curingTemperature temperature has an effect on the long-range order of the LC solvent. To see whether the stabilization is affected by theon disorder in the LC order system, experiments of The UV curing temperature has an effect the long-range ofwe theconduct LC solvent. To see whether polymerizing cells (filled of 1.5% RM257 in E7) at different temperatures. the stabilization is affected by the disorder in the LC system, we conduct experiments of polymerizing As shown in Figure 9a, the T/V curves before and after polymerization are similar for the cell cells (filled of 1.5% RM257 in E7) at different temperatures. cured at a low temperature (22 °C). For the cell cured at a high temperature (80 °C), the T/V curves As shown in Figure 9a, the T/V curves before and after polymerization are similar for the cell show a difference in speed before and after polymerization. However, the threshold voltage is almost ◦ ◦ cured the at asame, low temperature (22 inC). For the cured at curve a highmeasurement temperatureafter (80 photo-bleaching C), the T/V curves which is shown Figure 9b. cell Another T-V show a(exposed difference in speed before and after polymerization. However, the threshold voltage is almost to un-polarized light for 8 h) appears similar to the one before photo-bleaching and doesn’t showwhich an increased threshold voltage.9b. Macroscopic crossed polarizers revealed the same, is shown in Figure Anotherobservation T-V curvebetween measurement after photo-bleaching thatto the cell cured at 80 °C for kept8 its no photo-bleaching light scattering observed, (exposed un-polarized light h) original appearsalignment similar toand thethere one was before and doesn’t which is shown in Figure 10. All these indicate that RM localized near the surface and stabilized the show an increased threshold voltage. Macroscopic observation between crossed polarizers revealed liquid crystal alignment regardless of the re-orientation of the azodye sublayer. Furthermore, it that the cell cured at 80 ◦ C kept its original alignment and there was no light scattering observed, would be interesting to point out the stabilization of the cell cured at 80 °C in terms of where the whichtemperature is shown inisFigure All the these indicate that localized surface actually10. above clearing point of RM E7 (which is 60near °C).the Even thoughand thestabilized UV the liquid crystal alignment regardless of the re-orientation of the azodye sublayer. Furthermore, polymerization occurred at an isotropic state, the original LC alignment was preserved. This indicates ◦ C in terms of where it would to point were out the stabilization of the atby80the thatbe RMinteresting monomers/oligomers diffused to the surface andcell werecured aligned azodye sublayer ◦ while undergoing the polymerization process. point of E7 (which is 60 C). Even though the UV the temperature is actually above the clearing polymerization occurred at an isotropic state, the original LC alignment was preserved. This indicates that RM monomers/oligomers were diffused to the surface and were aligned by the azodye sublayer while undergoing the polymerization process.

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Figure 9. T/V curves of cells cured at different temperatures: (a) 22 °C (b) 80 °C. ◦ C and ◦ C. Figure 9. 9. T/V T/Vcurves curvesofofcells cellscured curedatatdifferent differenttemperatures: temperatures:(a) (a)22 22°C and(b) (b)80 80°C. Figure and

Figure 10. Macroscopic observation of the cell before photo-bleaching (a) and after photo-bleaching Figure 10. Macroscopic observation of the before photo-bleaching (a) and after photo-bleaching Figure 10. original Macroscopic observation the cell cellto before photo-bleaching (a)to and photo-bleaching (b,c). The LC alignment wasofparallel one polarizer (b) or 45° oneafter polarizer (c). Half of ◦ (b,c). The original LC alignment was parallel to one polarizer (b) or 45° to one polarizer (c). Half of (b,c). The original LC alignment was parallel to one polarizer (b) or 45 to one polarizer (c). Half of the the cell (highlighted in the dashed line box of b, c) was masked during the exposure. Therefore, a the cell (highlighted in the dashed line box of b, c) was masked during the exposure. Therefore, a cell (highlighted in the dashed lineindicates box of b,stable c) wasalignment. masked during the exposure. Therefore, a uniform uniform dark state across the cell uniform dark statethe across the cell indicates stable alignment. dark state across cell indicates stable alignment.

4. Discussion 4. 4.Discussion Discussion As seen from the results above, the concentrations of reactive monomers and the types of liquid As from the results above, the of and the types of Asseen seen theeffects resultson above, theconcentrations concentrations ofreactive reactivemonomers monomers and types ofliquid liquid crystals showfrom larger the surface localized polymerization compared to the other parameters. crystals show larger effects on the surface localized polymerization compared to other parameters. crystals show larger effects onphysical the surface localized compared to other parameters. These show some hints on the properties of polymerization RMs and LCs such as the solubility limit [12]. These show some hints on the physical properties of RMs and LCs such as the solubility limit TheseThe show some hints on the physical properties of RMs and LCs such as the solubility limit [12]. [12]. solubility limit can be compared through the solubility parameters. For a material with a The solubility limit can be compared through the solubility parameters. For aamaterial with aa The solubility limit can be compared through the solubility parameters. For material known molecular structure, its solubility parameter can be calculated based on the with group known molecular structure, its solubility parameter can be calculated based on the group known molecular structure, its solubility parameter can be based on the group contribution contribution theory. In our case, most materials (either RMcalculated or LC) have known molecular structures contribution theory. In ourmaterials case, most(either materials (either RM orknown LC) have known molecular structures theory. In our case, most RM or LC) have molecular structures except for except for ZLI-4792, which is not released by Merck. However, based on the molecular structures of except for which ZLI-4792, which is notbyreleased by Merck. based However, based on the structures molecularof structures of ZLI-4792, is not released Merck. However, on the molecular some super some super fluorinated liquid crystals similar to ZLI-4792, we performed some estimated calculations some super fluorinated liquid crystals similar to ZLI-4792, we performed some estimated calculations fluorinated for RM and liquid LC. crystals similar to ZLI-4792, we performed some estimated calculations for RM for and LC. andRM LC. According to the components in the molecular structure, the cohesive energy, and molar volume According to in molecular structure, the energy, and According tothe thecomponents components inthe the molecular thecohesive cohesive andmolar molarvolume volume obtained from the Polymer Handbook [36], solubilitystructure, parameters (δ) of two energy, fluorinated liquid crystals obtained from the Polymer Handbook [36], solubility parameters (δ) of two fluorinated liquid crystals obtained from the Polymer Handbook [36], solubility parameters (δ) of two fluorinated liquid crystals (FLC, as shown in Figure 11) were calculated using group contribution theory, which is shown in (FLC, as shown in Figure 11) were calculated using group contribution theory, which isis shown in (FLC, as shown in Figure 11) were calculated using group contribution theory, which shown in Tables 7 and 8, respectively. Tables Tables 77 and and 8, 8, respectively. respectively.

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Table 7. Calculation process of the solubility parameter based on the molecular structure of FLC#1. Components for FLC #1

Cohesive Energy = Ej (J/mol)

Molar Volume = Vj (cm3 /mol)

# of Group = Nj

Cohesive Energy Density Ec,j = Ej/Vj (J/cm3 )

-CH2 6 atom ring Phenylene C-F3 CH3 -

4940 1050 31,940 4270 4710

16.1 16 52.4 57.5 33.5

4 2 1 1 1

306.8 65.6 609.5 74.3 140.6

Volume = Vj × Nj (cm3 /mol) 64.4 32 52.4 57.5 33.5 Volume sum = 239.8

Volume Fraction = Vf = Vj × Nj/(Volume Sum)

Component Solubility Parameter = Ec,j0.5 × Vf (J/cm3 )0.5

0.27 0.13 0.22 0.24 0.14

4.7 1.1 5.4 2.1 4.2 Solubility parameter sum δ = 17.5

Table 8. Calculation process of the solubility parameter based on the molecular structure of FLC#2. Components for FLC #2

Cohesive Energy = Ej (J/mol)

Molar Volume = Vj (cm3 /mol)

# of Group = Nj

Cohesive Energy Density = Ec,j = Ej/Vj (J/cm3 )

Volume = Vj × Nj (cm3 /mol)

Volume Fraction = Vf = Vj × Nj/(Volume Sum)

Component Solubility Parameter = Ec,j0.5 × Vf (J/cm3 )0.5

-CH2 -Ophenylene (tetrasubstituted) phenylene -CF2 F- (trisubstituted) F- (disubstituted) CH3 -

4940 3350

16.1 3.8

2 1

306.8 881.6

32.2 3.8

0.11 0.01

2.0 0.4

31,940

14.4

2

2218.1

28.8

0.10

4.8

31,940 4270 2300 3560 4710

52.4 23 22 20 33.5

1 1 3 2 1

609.5 185.6 104.5 178 140.6

52.4 23 66 40 33.5 Volume sum = 279.7

0.18 0.08 0.24 0.14 0.12

4.6 1.1 2.4 1.9 1.4 Solubility parameter sum δ = 18.6

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Figure 11. 11.Molecular Molecularstructures structures of two fluorinated crystals (FLC) to chosen to the estimate the Figure of two fluorinated liquidliquid crystals (FLC) chosen estimate solubility solubility parameters for ZLI-4792. parameters for ZLI-4792.

The estimated solubility parameters of RMs and LCs used in experiments are presented in Table The estimated solubility parameters of RMs and LCs used in experiments are presented in 9. We can see that the fluorinated liquid crystals have the smallest solubility parameter values. The Table 9. We can see that the fluorinated liquid crystals have the smallest solubility parameter values. smaller difference in the solubility parameters between two materials refers to higher solubility while The smaller difference in the solubility parameters between two materials refers to higher solubility the larger difference refers to lower solubility. while the larger difference refers to lower solubility. Table 9. Solubility parameters of RM and LC materials used in the experiments. Table 9. Solubility parameters of RM and LC materials used in the experiments. Materials RM257 Bis-MA E7 ZLI-4792 Solubility Materials RM257 Bis-MA E7 ZLI-4792 22.81 [37] 18.35 [37] 22.19 [38] 17.5–18.6 parameters Solubility 3)0.5 (J/cm parameters 22.81 [37] 18.35 [37] 22.19 [38] 17.5–18.6 (J/cm3 )0.5

The sketch in Figure 12 may explain the possible mechanism of the surface localized polymerization of RM from the LC solvent. Before UV polymerization or exposure, the RM and PI The sketch in Figure 12 may explain the possible mechanism of the surface localized are homogeneously dissolved in LC (Figure 12a). At the initial UV exposure (polymerization), photopolymerization of RM from the LC solvent. Before UV polymerization or exposure, the RM and initiator molecules absorb UV photons and produce radical species. The free radicals will promote PI are homogeneously dissolved in LC (Figure 12a). At the initial UV exposure (polymerization), several bi-functional RM molecules to form oligomers by using a chain reaction. The oligomers are photo-initiator molecules absorb UV photons and produce radical species. The free radicals will not very soluble to some extent and would decline when separated from the LC solvent (Figure 12b). promote several bi-functional RM molecules to form oligomers by using a chain reaction. The oligomers As the oligomer increase its molecular weight through further chain reactions, the insolubility would are not very soluble to some extent and would decline when separated from the LC solvent (Figure 12b). repel the LC solvent by diffusing to the substrates while the chain reaction continues. If the diffusion As the oligomer increase its molecular weight through further chain reactions, the insolubility would rate dominated, the oligomer would be localized at (or close to) surface (Figure 12c), which is repel the LC solvent by diffusing to the substrates while the chain reaction continues. If the diffusion followed by a further polymerization reaction. If the polymer chain reaction dominated, the oligomer rate dominated, the oligomer would be localized at (or close to) surface (Figure 12c), which is followed that would not have enough time to move the surface would undergo another chain reaction at its by a further polymerization reaction. If the polymer chain reaction dominated, the oligomer that would initial location, which would form the bulk polymer network (Figure 12d). In both cases, the final not have enough time to move the surface would undergo another chain reaction at its initial location, polymer would have preferred alignment because of the bi-functionality of the reactive monomer which would form the bulk polymer network (Figure 12d). In both cases, the final polymer would and the long-range order in the LC solvent aligned by the azodye sublayer. Previous studies on the have preferred alignment because of the bi-functionality of the reactive monomer and the long-range polymer stabilized liquid crystal have described the effect of the diffusion rate versus the reaction order in the LC solvent aligned by the azodye sublayer. Previous studies on the polymer stabilized rate in the formation of the polymer strand morphology [19,21,27]. A more bulk network will be liquid crystal have described the effect of the diffusion rate versus the reaction rate in the formation of formed if the reaction rate dominates over the diffusion rate. In our case, a similar result to the one the polymer strand morphology [19,21,27]. A more bulk network will be formed if the reaction rate proposed in Figure 12 should be discovered. dominates over the diffusion rate. In our case, a similar result to the one proposed in Figure 12 should be discovered.

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Figure 12. Sketched mechanism polymerizationof of RM RM from (a)(a) RMRM dissolved in in LC Figure 12. Sketched mechanism for for thethe polymerization fromLC LCsolvent: solvent: dissolved LC before UV exposure; (b) Oligomer formed and phase separated from LC after initial UV before UV exposure; (b) Oligomer formed and phase separated from LC after initial UV polymerization; polymerization; (c) Surface localized polymerization and (d) bulk network polymerization. (c) Surface localized polymerization and (d) bulk network polymerization.

Considering the effects of RM concentration and the effects of different LCs (such as E7 versus ZLI-4792) and different (suchconcentration as RM257 versus Bis-MA), the underlying reason for (such the differing Considering the effectsRMs of RM and the effects of different LCs as E7 versus final morphology appears to be the solubility limit. If the monomer has a smaller solubility, thediffering ZLI-4792) and different RMs (such as RM257 versus Bis-MA), the underlying reason for the corresponding oligomer would have a smaller solubility limit. In this case, the oligomer may be phase final morphology appears to be the solubility limit. If the monomer has a smaller solubility, separated in the bulk to have local areas of higher density that would promote polymerization in the the corresponding oligomer would havelimits a smaller solubility limit. In this the bulk. Essentially, the higher solubility may not have a high enough local case, density in oligomer the bulk to may be phase separated in thethe bulk to have higher density that would polymerization react. Therefore, oligomer can local diffuseareas to theof surface where it achieves a higherpromote density and is able to realize surface localized polymerization. in the bulk. Essentially, the higher solubility limits may not have a high enough local density in the bulk to react. Therefore, the oligomer can diffuse to the surface where it achieves a higher density and 5. Conclusions is able to realize surface localized polymerization. We have shown that process parameters (such as RM concentration, types of LC, types of RM, photo-initiator concentration, UV curing intensity, and UV curing temperature) could somewhat 5. Conclusions influence the surface localized stabilization. By tuning these parameter(s), RM polymerized

Wemorphology have shown (suchtoasthe RM concentration, LC, types of RM, can that vary process from the parameters surface localization bulk network. Usingtypes opticalofmicroscopy, photo-initiator concentration, UV curing intensity,theand UV curing could somewhat T/V curves, the life test, and photo-bleaching, morphology cantemperature) be further characterized, distinguished, and localized confirmed. Both types of materials (especially LCs)parameter(s), and concentrations of RM in influence the surface stabilization. By tuning these RM polymerized LC show drastic differences. These may be ascribed to the solubility limit of the monomer (or morphology can vary from the surface localization to the bulk network. Using optical microscopy, T/V oligomer) in the LC host, which has a control over the diffusion rate or the reaction rate. Overall, we curves, the life test, and photo-bleaching, the morphology can be further characterized, distinguished, hope that the results presented in this paper could help improve understanding of the mechanism and confirmed. typesstabilization of materials (especially LCs)the andfeasibility concentrations of RM in LC show drastic for surfaceBoth localized and further extend to combine surface localized differences. Theseand may be ascribed for to the solubility limitdevice of the monomer (or oligomer) in the LC stabilization photo-alignment liquid crystal optical fabrication. host, which has a control over the diffusion rate or the reaction rate. Overall, we hope that the Contributions: Conceptualization, J.W., P.B.; Investigation, J.W., C.M.; Methodology, J.W., C.M., V.F., results Author presented in this paper could help improve understanding of the mechanism for surface and P.B.; Supervision, R.R., P.B.; Validation, J.W., C.M., and P.B.; Visualization, J.W., C.M. and P.B.; Formal localized stabilization to combine localized stabilization and analysis, J.W., C.M.,and R.R., further V.F., H.C.,extend S.B., andthe P.B; feasibility Writing—original draft, J.W.;surface Writing—review & editing, J.W., C.M., R.R., V.F., S.B.,crystal and P.B; optical Funding acquisition, R.R.; photo-alignment forH.C., liquid device fabrication. Funding: We thank the Army Research Office Program Manager Michael Gerhold for support (Grant Number:

Author Contributions: Conceptualization, J.W., P.B.; Investigation, J.W., C.M.; Methodology, J.W., C.M., V.F., W911NF-14-1-0650). and P.B.; Supervision, R.R., P.B.; Validation, J.W., C.M., and P.B.; Visualization, J.W., C.M. and P.B.; Formal analysis, Conflicts of Interest: The authors declare no conflict of interest. J.W., C.M., R.R., V.F., H.C., S.B., and P.B; Writing—original draft, J.W.; Writing—review & editing, J.W., C.M., R.R., V.F., H.C., S.B., and P.B; Funding acquisition, R.R.; References

Funding: We thank the Army Research Office Program Manager Michael Gerhold for support (Grant Number: W911NF-14-1-0650). 1. Yaroshchuk, O.; Reznikov, Y. Photoalignment of liquid crystals: Basics and current trends. J. Mater. Chem. 2012, 22, 286–300. Ichimura, K. Photoalignment of Liquid-Crystal Systems. Chem. Rev. 2000, 100, 1847–1873.

Conflicts of Interest: The authors declare no conflict of interest. 2.

References 1. 2.

Yaroshchuk, O.; Reznikov, Y. Photoalignment of liquid crystals: Basics and current trends. J. Mater. Chem. 2012, 22, 286–300. [CrossRef] Ichimura, K. Photoalignment of Liquid-Crystal Systems. Chem. Rev. 2000, 100, 1847–1873. [CrossRef] [PubMed]

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