Effect of Position and Structure of the Terminal Moieties in the Side Group on the Liquid Crystal Alignment Behavior of Polystyrene Derivatives

We synthesized a series of polystyrene derivatives containing various side groups, such as the 4-(tert-butyl)-phenoxymethyl, 3-(tert-butyl)-phenoxymethyl, 2-(tert-butyl)-phenoxymethyl, 4-cumyl-phenoxymethyl, and 4-trityl-phenoxymethyl groups, through a polymer modification reaction to examine the liquid crystal (LC) alignment of these derivatives. In general, the vertical LC alignment on polymer films can be affected by the position and structure of the terminal moiety of the polymer side group. For example, the LC cells fabricated with 4-(tert-butyl)-phenoxymethyl-substituted polystyrene having a tert-butyl moiety as a para-type attachment to the phenoxy groups of the polystyrene derivatives exhibited vertical LC alignment, whereas the LC cells prepared from 3-(tert-butyl)- and 2-(tert-butyl)-phenoxymethyl-substituted polystyrene films exhibited planar LC alignment. In addition, the LC cells fabricated from 4-cumyl- and 4-trityl-phenoxymethyl-substituted polystyrene films with additional phenyl rings in the side groups exhibited planar LC alignment, in contrast to the LC alignment of the (tert-butyl)-phenoxymethyl-substituted polystyrene series. The vertical LC orientation was well correlated with the surface energy of these polymer films. For example, vertical LC orientation, which mainly originates due to the nonpolar tertiary carbon moiety having bulky groups, was observed when the surface energy of the polymer was lower than 36.6 mJ/m2.


Introduction
Liquid crystals (LCs), which consist of elongated organic molecules with an uneven charge distribution along their dipoles, are materials with properties between those of ordinary liquids and three-dimensional solids [1][2][3]. LCs have been studied in elementary research and for the development of commercial applications owing to their exceptional anisotropic physicochemical characteristics. Based on the factors that impart the liquid crystalline properties, LCs can be classified into two generic categories: thermotropic and lyotropic mesophases [4,5]. Lyotropic LCs, consisting of amphiphilic molecules, can be subdivided into three phases: lamellar, hexagonal, and cubic phases. The transition among these three phases can be triggered spontaneously, depending on the water content of the aqueous solution. The unique structures of LC molecules, as well as the physicochemical properties of the lyotropic LC systems, render them potential candidates for pharmaceutical applications, such as drug delivery carriers and several other applications [6][7][8][9][10]. For example, the cubic phase has been investigated extensively for drug delivery systems. In addition, the cubic microstructure of the lyotropic LCs provides a release matrix for active materials of varying sizes and polarities owing to the dual polarnonpolar nature [11][12][13][14]. Thermotropic LCs exhibit different liquid crystal phases as a function of temperature. Thermotropic LCs with a rod-like shape can be categorized into layer and the LC molecule is a key factor in improving the LC alignment characteristics and inducing a stable alignment behavior.
In this study, we synthesized a series of polystyrene derivatives having #-(tert-butyl)phenoxymethyl (#TBs) units in the side groups (P#TBs); # indicates the position of the tert-butyl group with respect to the oxymethyl group in disubstituted benzenes ( Figure 1). We also synthesized polystyrene derivatives having 4-cumyl-phenoxymethyl (PCUM) and 4-trityl-phenoxymethyl (PTRI) moieties in the side groups; these groups have an additional phenyl ring in the terminal moieties, in contrast to P#TBs. LC alignment according to the position and structure of the terminal moiety in the side group of the polystyrene derivatives was investigated. The bulk and surface properties of these polymers and the optical characteristics of LC cells fabricated with these polymer films were studied. ers 2021, 13, x FOR PEER REVIEW 3 of 13 layer and the LC molecule is a key factor in improving the LC alignment characteristics and inducing a stable alignment behavior.
In this study, we synthesized a series of polystyrene derivatives having #-(tert-butyl)phenoxymethyl (#TBs) units in the side groups (P#TBs); # indicates the position of the tertbutyl group with respect to the oxymethyl group in disubstituted benzenes ( Figure 1). We also synthesized polystyrene derivatives having 4-cumyl-phenoxymethyl (PCUM) and 4trityl-phenoxymethyl (PTRI) moieties in the side groups; these groups have an additional phenyl ring in the terminal moieties, in contrast to P#TBs. LC alignment according to the position and structure of the terminal moiety in the side group of the polystyrene derivatives was investigated. The bulk and surface properties of these polymers and the optical characteristics of LC cells fabricated with these polymer films were studied.

Preparation of #-(Tert-butyl)-phenoxymethyl-Substituted Polystyrene (P#TB)
The following procedure was used to synthesize all the #-(tert-butyl)-phenoxymethylsubstituted polystyrenes (# is 4, 3, or 2). The synthesis of 4-(tert-butyl)-phenoxymethylsubstituted polystyrene (P4TB) is discussed as a representative example. A mixture of 4-(tert-butyl)phenol (0.44 g, 2.95 mmol, 150 mol% relative to PCMS) and potassium carbonate (0.50 g, 3.55 mmol) in DMAc (30 mL) was heated to 75 • C. A solution of PCMS (0.30 g, 1.97 mmol) in DMAc (20 mL) was added to the mixture and magnetically stirred at 70 • C for 24 h under a nitrogen atmosphere. The solution mixture was cooled to room temperature and poured into methanol to obtain a white precipitate. The precipitate was further purified by reprecipitation of the DMAc solution in methanol several times, followed by washing with hot methanol to remove any residual salts and potassium carbonate. After drying overnight under vacuum, P4TB was obtained in a yield of over 80%. Other #-(tert-butyl)-phenoxymethyl-substituted polystyrenes having tertiary butyl substituents (P#TB) at different positions were synthesized using a similar procedure, except that 3-(tertbutyl)phenol or 2-(tert-butyl)phenol was used as the reactant. For example, P3TB and P2TB, where the numbers indicate the position of the tert-butyl group containing monomeric units in the polymer, were prepared with 3-(tert-butyl)phenol (0.44 g, 2.95 mmol, 150 mol% relative to PCMS) and 2-(tert-butyl)phenol (0.44 g, 2.95 mmol, 150 mol% relative to PCMS), respectively. Comparison of the integrated peak areas of the oxymethyl and phenyl groups revealed that the degree (%) of substitution from chloromethyl to oxymethyl group was~100%.

Preparation of 4-Cumyl-phenoxymethyl-Substituted Polystyrene (PCUM)
PCUM was synthesized using a procedure similar to that used for preparing P4TB, except that 4-cumylphenol (0.63 g, 2.95 mmol, 150 mol% relative to PCMS) was used instead of 4-(tert-butyl)phenol. The product was obtained in a yield of over 80%. Comparison of the integrated peak areas of the oxymethyl peak in the range 4.6-5.0 ppm and phenyl peaks in the range 6.2-7.5 ppm revealed that the degree (%) of substitution from the chloromethyl to oxymethyl group was~100%.

Preparation of 4-Trityl-phenoxymethyl-Substituted Polystyrene (PTRI)
4-Trityl-phenoxymethyl-substituted polystyrene (PTRI) was synthesized using a procedure similar to that used for preparing P4TB, except that 4-tritylphenol (0.99 g, 2.95 mmol, 150 mol% relative to PCMS) was used instead of 4-(tert-butyl)phenol. The product was obtained in a yield of over 80%. Comparison of the integrated peak areas of the oxymethyl peak in the range 4.6-5.0 ppm and phenyl peaks in the range 6.2-7.5 ppm revealed that the degree (%) of substitution from chloromethyl to oxymethyl group was~100%.

Film Preparation and LC Cell Assembly
Solutions of P4TB, P3TB, P2TB, PCUM, and PTRI were prepared in THF (1 wt.%). The solutions were filtered using a poly(tetrafluoroethylene) membrane with a pore size of 0.45 µm. Polymer thin films were prepared by spin coating (2000 rpm, 90 s) on a glass substrate. The LC cells were fabricated by assembling two polymer films on a glass slide using a 4.25 µm-thick spacer. The cells were filled with nematic LC, MLC-6608. The fabricated LC cells were sealed with epoxy glue.

Instrumentation
Various techniques were used for the characterization of the synthesized materials. 1 H nuclear magnetic resonance ( 1 H NMR) spectroscopy was performed on an MR400 DD2 NMR spectrometer (Agilent Technologies, Inc., Santa Clara, CA, USA); differential scanning calorimetry (DSC) was performed on a Q-10 (TA Instruments, Inc., New Castle, DE, USA) calorimeter; polarized optical microscopy (POM) images of the LC cells were acquired on a Nikon Eclipse E600 POL (NIKON, Inc., Tokyo, Japan) instrument equipped with a polarizer and Nikon Coolpix 995 digital camera (NIKON, Inc., Tokyo, Japan). The static contact angles of water on the polymer films were determined using a Krüss DSA10 (KRÜSS Scientific Instruments Inc., Hamburg, Germany) contact angle analyzer equipped with a drop shape analysis software (KRÜSS Scientific Instruments Inc., Hamburg, Germany). Surface energy values were calculated using Owens-Wendt's equation as follows: where γ l is the surface energy of the liquid, γ sl is the interfacial energy of the solid-liquid interface, γ s is the surface energy of the solid, γ l d and γ l p are known for the test liquids, and γ s d and γ s p can be calculated from the measured static contact angles [55]. The contact angles for each sample were measured at least four times for three independently fabricated films, and the average values were used. Ultraviolet (UV) stability test of the LC cells was conducted using a VL-6.LC lamp (λ max = 365 nm, Vilber Lourmat, Paris, France) with intensities of 5, 10, and 15 J/cm 2 to corroborate the reliability to apply severe environment. The exposure dose of irradiated UV light on the LC cells was measured with a UV detector using GT-513 (Giltron, Seoul, Korea). Figure 1 shows the synthetic routes to P2TB, P3TB, P4TB, PCUM, and PTRI. First, poly(4-chloromethylstyrene) was synthesized by the conventional free radical polymerization of 4-chloromethylstyrene with AIBN as the initiator under a nitrogen atmosphere. A series of polymers were obtained through modification reactions using a mixture of poly(4-chloromethylstyrene), phenol derivatives, and potassium carbonate in DMAc, a polar aprotic solvent. The characterization of P4TB is presented as a representative example. Almost complete conversion from chloromethyl to 4-(tert-butyl)-phenoxymethyl was obtained when 150 mol% of 4-(tert-butyl)phenol was used at 75 • C for 24 h, as evident from the 1 H NMR spectra of the 4-(tert-butyl)phenol-containing homopolymer ( Figure 2). The peaks at δ = 6.2-7.4 ppm (peak a) in the 1 H NMR spectrum of P4TB correspond to the phenyl protons. The proton peaks from the tert-butyl side groups (δ = 0.9-1.5 ppm (peak c)) indicate the inclusion of tertiary carbon moieties in the polymer. Comparison of the integrated areas of the oxymethyl peak in the range 4.6-5.0 ppm and phenyl peaks in the range 6.2-7.4 ppm revealed that the degree of substitution from chloromethyl to oxymethyl was~100%. Similar integrations and calculations for P3TB, P2TB, PCUM, and PTRI were performed, and the results were typically within ±10% of the values expected based on the synthesis. A comparison of the integrated peak areas of the oxymethyl peak and phenyl peaks revealed that the degree of substitution from chloromethyl to oxymethyl was~100%. The high degree of substitution in this polymer modification reaction can be attributed to the electrophilicity of the benzylic carbon in poly(4-chloromethylstyrene) and to the structural stability of the phenolate anion as a nucleophile [53]. These polymers are soluble in many low boiling point solvents of medium polarities, such as THF and chloroform, and in polar aprotic solvents, such as N,N -dimethylformamide, N-methyl-2-pyrrolidone, and N,N -dimethylacetamide. The solubility of all the samples in various solvents is sufficient for use as thin-film materials.  The thermal properties of the polymers were studied using DSC at a heating and cooling rate of 10 °C/min under a nitrogen atmosphere. All the polymers were amorphous, and only one glass transition was observed in the DSC thermograms. The glass transition temperatures were determined from the extrapolated intersection of the asymptotes to the glassy and rubbery regions for calculating the enthalpy, as illustrated in Figure 3. A decrease in Tg of polystyrene derivatives having bulky substituents in the side group has been reported before [56]; for example, Tg of P4TB, P3TB, P2TB, and PCUM is lower than that of polystyrene. However, the Tg value increases from 104 °C for PCMS to 149 °C for PTRI. The Tg values of P#TB decrease in the order P4TB > P2TB > P3TB. As expected, the Tg value of P4TB with a tert-butyl group attached to the para position of the phenoxy group of the polystyrene side group is higher than those of P2TB and P3TB with ortho-and meta- The thermal properties of the polymers were studied using DSC at a heating and cooling rate of 10 • C/min under a nitrogen atmosphere. All the polymers were amorphous, and only one glass transition was observed in the DSC thermograms. The glass transition temperatures were determined from the extrapolated intersection of the asymptotes to the glassy and rubbery regions for calculating the enthalpy, as illustrated in Figure 3. A decrease in T g of polystyrene derivatives having bulky substituents in the side group has been reported before [56]; for example, T g of P4TB, P3TB, P2TB, and PCUM is lower than that of polystyrene. However, the T g value increases from 104 • C for PCMS to 149 • C for PTRI. The T g values of P#TB decrease in the order P4TB > P2TB > P3TB. As expected, the T g value of P4TB with a tert-butyl group attached to the para position of the phenoxy group of the polystyrene side group is higher than those of P2TB and P3TB with orthoand meta-type attachments, respectively [57]. The decrease in the T g values of P2TB and P3TB can be attributed to the kinking in the molecular chain due to the ortho and meta linkages, which increases the free volume of the polymer [57]. In addition, the ortho linkage increases the steric hindrance and decreases the flexibility, leading to higher T g values than those corresponding to the meta linkage [58]. The T g values of the polystyrene derivatives synthesized with 4-cumylphenol and 4-tritylphenol decrease in Polymers 2021, 13, 2822 7 of 13 the order PTRI > P4TB > PCUM. The decrease in the T g value of PCUM with increasing phenyl ring of the bulky side groups has been previously reported and is ascribed to the increase in the free volume of the polymer because polymers having larger free volume have lower T g values [56]. However, as the number of phenyl rings of the terminal moiety in the side group increases from 1 to 3, the T g value increases from 67 • C for PCUM to 149 • C for PTRI. The increase in the T g value of PTRI can be attributed to the increased molecular interactions, such as π-π and van der Waals interactions, in the side groups [59].

Results and Discussion
creases the steric hindrance and decreases the flexibility, leading to higher T those corresponding to the meta linkage [58]. The Tg values of the polystyren synthesized with 4-cumylphenol and 4-tritylphenol decrease in the order P PCUM. The decrease in the Tg value of PCUM with increasing phenyl ring side groups has been previously reported and is ascribed to the increase in th of the polymer because polymers having larger free volume have lower T However, as the number of phenyl rings of the terminal moiety in the side gr from 1 to 3, the Tg value increases from 67 °C for PCUM to 149 °C for PTRI in the Tg value of PTRI can be attributed to the increased molecular interac π-π and van der Waals interactions, in the side groups [59]. It is known that the molecular orientation of LCs can be affected by composition of the orientation layer, owing to the molecular interactions a between LC molecules and the orientation layer. The orientations of LC mo cells fabricated with polystyrene derivatives and grafted with #-(tert-butyl) thyl (including 4-(tert-butyl)-, 3-(tert-butyl)-, and 2-(tert-butyl)-phenoxymet 4-cumyl-phenoxymethyl, and 4-trityl-phenoxymethyl moieties were observ atically investigate the LC alignment behavior according to the position an the terminal moieties. Figure 4 shows the conoscopic POM images of the L cated from P4TB films onto glass substrates at P4TB weight concentrations 0.05, 0.1, and 1.0 wt.%. Initially, a random planar alignment was observe weight ratio of less than 0.001 wt.% (Figure 4a). When the P4TB weight rati than 0.01 wt.%, vertical alignment was observed, as evident from the Maltese (Figure 4b-e). Therefore, 1 wt.% was selected as the optimum concentration solution for fabricating LC cells using P4TB, P3TB, P2TB, PCUM, and PTRI viously reported [60]. It is known that the molecular orientation of LCs can be affected by the chemical composition of the orientation layer, owing to the molecular interactions at the interface between LC molecules and the orientation layer. The orientations of LC molecules in the cells fabricated with polystyrene derivatives and grafted with #-(tert-butyl)-phenoxymethyl (including 4-(tert-butyl)-, 3-(tert-butyl)-, and 2-(tert-butyl)-phenoxymethyl moieties), 4-cumyl-phenoxymethyl, and 4-trityl-phenoxymethyl moieties were observed to systematically investigate the LC alignment behavior according to the position and structure of the terminal moieties. Figure 4 shows the conoscopic POM images of the LC cells fabricated from P4TB films onto glass substrates at P4TB weight concentrations of 0.001, 0.01, 0.05, 0.1, and 1.0 wt.%. Initially, a random planar alignment was observed for a P4TB weight ratio of less than 0.001 wt.% (Figure 4a). When the P4TB weight ratios were more than 0.01 wt.%, vertical alignment was observed, as evident from the Maltese cross pattern (Figure 4b-e). Therefore, 1 wt.% was selected as the optimum concentration of the coating solution for fabricating LC cells using P4TB, P3TB, P2TB, PCUM, and PTRI films, as previously reported [60]. 0.05, 0.1, and 1.0 wt.%. Initially, a random planar alignment was observed for a P4 weight ratio of less than 0.001 wt.% (Figure 4a). When the P4TB weight ratios were mo than 0.01 wt.%, vertical alignment was observed, as evident from the Maltese cross patte (Figure 4b-e). Therefore, 1 wt.% was selected as the optimum concentration of the coati solution for fabricating LC cells using P4TB, P3TB, P2TB, PCUM, and PTRI films, as p viously reported [60].   Photographic images of the LC cells made from P4TB, P3TB, P2TB, PCUM, and PTRI films are shown in Figure 5. The vertical LC alignment in the LC cells fabricated from P4TB films was considerably uniform over the entire area and was maintained for at least several months, whereas LC cells fabricated from P3TB, P2TB, PCUM, and PTRI films showed planar LC alignment. The effect of the position and structure of the terminal moieties in the side groups on the LC alignment behavior was investigated based on the POM images of the LC cells made from PTRI, PCUM, P2TB, P3TB, and P4TB films. Furthermore, the orthoscopic and conoscopic POM images of LC cells fabricated from these polymers were studied ( Figure 6) for a more accurate analysis of the LC orientation behavior. The conoscopic POM images of LC cells fabricated with PTRI, PCUM, P2TB, and P3TB films revealed a planar LC alignment. On the other hand, the dark orthoscopic POM images and the Maltese cross pattern in the conoscopic POM image confirmed the vertical LC alignment of LC cells fabricated using the P4TB film. According to the photograph and POM images, only the P4TB film provided a stable uniform vertical orientation layer. The vertical LC alignment is related to the surface energy of the alignment layer surface and/or the steric repulsion between LC molecules and the alignment layer [61]. For example, nonpolar and bulky polyimide derivatives such as pentylcyclohexylbenzene [62] and 4-(n-octyloxy)phenyloxy [63] show vertical alignment behavior. Therefore, we attempted to analyze the LC alignment behavior of the P#TB, PCUM, and PTRI films with different positions and structures of the terminal moieties using several surface characterization techniques, including surface energy measurement of the polymer films. Figure 7 and Table 1 provide the surface energy values obtained by Furthermore, the orthoscopic and conoscopic POM images of LC cells fabricated from these polymers were studied ( Figure 6) for a more accurate analysis of the LC orientation behavior. The conoscopic POM images of LC cells fabricated with PTRI, PCUM, P2TB, and P3TB films revealed a planar LC alignment. On the other hand, the dark orthoscopic POM images and the Maltese cross pattern in the conoscopic POM image confirmed the vertical LC alignment of LC cells fabricated using the P4TB film. Photographic images of the LC cells made from P4TB, P3TB, P2TB, PCUM, and PTRI films are shown in Figure 5. The vertical LC alignment in the LC cells fabricated from P4TB films was considerably uniform over the entire area and was maintained for at least several months, whereas LC cells fabricated from P3TB, P2TB, PCUM, and PTRI films showed planar LC alignment. The effect of the position and structure of the terminal moieties in the side groups on the LC alignment behavior was investigated based on the POM images of the LC cells made from PTRI, PCUM, P2TB, P3TB, and P4TB films. Furthermore, the orthoscopic and conoscopic POM images of LC cells fabricated from these polymers were studied ( Figure 6) for a more accurate analysis of the LC orientation behavior. The conoscopic POM images of LC cells fabricated with PTRI, PCUM, P2TB, and P3TB films revealed a planar LC alignment. On the other hand, the dark orthoscopic POM images and the Maltese cross pattern in the conoscopic POM image confirmed the vertical LC alignment of LC cells fabricated using the P4TB film. According to the photograph and POM images, only the P4TB film provided a stable uniform vertical orientation layer. The vertical LC alignment is related to the surface energy of the alignment layer surface and/or the steric repulsion between LC molecules and the alignment layer [61]. For example, nonpolar and bulky polyimide derivatives such as pentylcyclohexylbenzene [62] and 4-(n-octyloxy)phenyloxy [63] show vertical alignment According to the photograph and POM images, only the P4TB film provided a stable uniform vertical orientation layer. The vertical LC alignment is related to the surface energy of the alignment layer surface and/or the steric repulsion between LC molecules and the alignment layer [61]. For example, nonpolar and bulky polyimide derivatives such as pentylcyclohexylbenzene [62] and 4-(n-octyloxy)phenyloxy [63] show vertical alignment behavior. Therefore, we attempted to analyze the LC alignment behavior of the P#TB, PCUM, and PTRI films with different positions and structures of the terminal moieties using several surface characterization techniques, including surface energy measurement of the polymer films. Figure 7 and Table 1 provide the surface energy values obtained by measuring the static contact angle of water and diiodomethane. The total surface energy was calculated using Owens-Wendt's equation, which is a summation of the polar and dispersion contributions [55]. We also found that the vertical LC alignment could be affected by the critical surface energy of the polymer films. The total surface energy of P4TB exhibiting vertical LC alignment is lower than 36.6 mJ/m 2 , whereas P3TB, P2TB, PCUM, and PTRI, with a total surface energy of higher than 43.3 mJ/m 2 , do not show vertical LC alignment. These results indicate that para-type attachment of the tert-butyl side groups induced vertical LC alignment behavior, whereas orthoand meta-type attachments of the same group could not. Different types of possible steric repulsions and/or interactions between LC molecules and the surfaces of the P4TB, P3TB, and P2TB films result in different LC alignment behaviors. PCUM and PTRI have additional phenyl groups in the terminal moieties, and the surface energies of these polymers are relatively high because the additional phenyl side groups increase the aromatic ring-aromatic ring interactions between LC molecules and polymer surfaces [64]. Therefore, the ability of P4TB to exhibit vertical alignment can be attributed to the increased steric repulsion between LC molecules and the polymer surfaces due to the incorporation of the nonpolar and bulky tertiary butyl moieties into the para-type attachment of the polystyrene side groups and the low surface energy (<36.6 mJ/m 2 ) resulting from the unique chemical structure of the nonpolar carbon group. ers 2021, 13, x FOR PEER REVIEW 9 of exhibiting vertical LC alignment is lower than 36.6 mJ/m 2 , whereas P3TB, P2TB, PCU and PTRI, with a total surface energy of higher than 43.3 mJ/m 2 , do not show vertical L alignment. These results indicate that para-type attachment of the tert-butyl side grou induced vertical LC alignment behavior, whereas ortho-and meta-type attachments of t same group could not. Different types of possible steric repulsions and/or interactions b tween LC molecules and the surfaces of the P4TB, P3TB, and P2TB films result in differe LC alignment behaviors. PCUM and PTRI have additional phenyl groups in the termin moieties, and the surface energies of these polymers are relatively high because the add tional phenyl side groups increase the aromatic ring-aromatic ring interactions betwe LC molecules and polymer surfaces [64]. Therefore, the ability of P4TB to exhibit vertic alignment can be attributed to the increased steric repulsion between LC molecules an the polymer surfaces due to the incorporation of the nonpolar and bulky tertiary bu moieties into the para-type attachment of the polystyrene side groups and the low surfa energy (<36.6 mJ/m 2 ) resulting from the unique chemical structure of the nonpolar carb group.    The reliability of LC cells fabricated using the polymer films was investigated through a stability test of the LC alignment under severe conditions such as high temperatures and UV energies. Thermal and UV stabilities of the LC cell fabricated using the P4TB film were measured from the POM images obtained after heating at 100, 150, and 200 • C for 10 min and UV irradiation at 5, 10, and 15 J/cm 2 , respectively. As shown in Figure 8, no distinct differences in the vertical LC orientation on the P4TB films were observed through the Maltese cross pattern in the conoscopic POM images, indicating that the vertical LC orientation was maintained in the P4TB LC cell even at a high temperature and UV energy. For the P4TB films, the total surface energy obtained from the static contact angle of water and diiodomethane was also measured after heating and UV irradiation. The total surface energy, a characteristic of the P4TB film, was maintained in the range of 36-37 mJ/m 2 even when the temperature and UV energies were increased to 200 • C and 15 J/cm 2 , respectively. Therefore, based on these results, P4TB, which exhibits thermal and UV stabilities, is a potential candidate as next-generation LC alignment films for diverse applications.
Polymers 2021, 13, x FOR PEER REVIEW The reliability of LC cells fabricated using the polymer films was inve through a stability test of the LC alignment under severe conditions such as high atures and UV energies. Thermal and UV stabilities of the LC cell fabricated u P4TB film were measured from the POM images obtained after heating at 100, 200 °C for 10 min and UV irradiation at 5, 10, and 15 J/cm 2 , respectively. As s Figure 8, no distinct differences in the vertical LC orientation on the P4TB films served through the Maltese cross pattern in the conoscopic POM images, indica the vertical LC orientation was maintained in the P4TB LC cell even at a high tem and UV energy. For the P4TB films, the total surface energy obtained from the st tact angle of water and diiodomethane was also measured after heating and UV tion. The total surface energy, a characteristic of the P4TB film, was maintaine range of 36-37 mJ/m 2 even when the temperature and UV energies were increase °C and 15 J/cm 2 , respectively. Therefore, based on these results, P4TB, which exhi mal and UV stabilities, is a potential candidate as next-generation LC alignment diverse applications.

Conclusions
A series of #-(tert-butyl)-phenoxymethyl-substituted polystyrenes (P#TB, wh dicates the position of the tert-butyl group with respect to the oxymethyl group stituted benzenes), 4-cumyl-phenoxymethyl-substituted polystyrene (PCUM) trityl-phenoxymethyl-substituted polystyrene (PTRI) were synthesized, and th crystal (LC) alignment behavior of these films was investigated. The LC alignmen ior can be influenced by the position and structure of the tertiary carbon moiety to the phenoxy units in the side group of polystyrene. For example, LC cells fa using the P4TB film having tertiary butyl moieties as a para-type attachment to noxy groups of polystyrene exhibited vertical LC alignment, whereas LC cells p using P3TB and P2TB films having tertiary butyl moieties as meta-and ortho-typ ments, respectively, to the phenoxy groups of polystyrene exhibited planar LC al

Conclusions
A series of #-(tert-butyl)-phenoxymethyl-substituted polystyrenes (P#TB, where # indicates the position of the tert-butyl group with respect to the oxymethyl group in disubstituted benzenes), 4-cumyl-phenoxymethyl-substituted polystyrene (PCUM), and 4-trityl-phenoxymethyl-substituted polystyrene (PTRI) were synthesized, and the liquid crystal (LC) alignment behavior of these films was investigated. The LC alignment behavior can be influenced by the position and structure of the tertiary carbon moiety attached to the phenoxy units in the side group of polystyrene. For example, LC cells fabricated using the P4TB film having tertiary butyl moieties as a para-type attachment to the phenoxy groups of polystyrene exhibited vertical LC alignment, whereas LC cells prepared using P3TB and P2TB films having tertiary butyl moieties as metaand ortho-type attachments, respectively, to the phenoxy groups of polystyrene exhibited planar LC alignment. LC cells prepared from the PCUM and PTRI films with one and three phenyl rings, respectively, in the terminal moieties on polystyrene, exhibited planar LC alignment owing to the ππ and van der Waals interactions between LC molecules and the alignment layer. The vertical LC alignment was well correlated to the steric repulsion between LC molecules and polymer surfaces due to the nonpolar and bulky moieties attached to the side groups of the polymer and to the total surface energy of the polymer being lower than 36.6 mJ/m 2 . This provides the basic information for designing an LC alignment layer based on polymer films containing different terminal moieties.