Semi fluorinated polymers as surface energy controlled layers for liquid crystal alignment

Abstract

The effects of compounded hydrogenated–fluorinated surfaces formed by perfluorocyclobutane (PFCB)-containing polymers on the alignment of 4,4′-octylcycanobiphenyl (8CB), a liquid crystal that exhibits both nematic and smectic phases, were investigated. The inherent segregation between fluorine- and hydrogen-rich segments produced structured interfaces with fluorinated and protonated nano domains. The morphologies of the interfaces and their corresponding surface energies were probed by AFM and the alignment of 8CB was investigated by polarized optical microscopy. We found that varying the fluorine content controlled both the structure and the interfacial morphology. The 8CB layer orientation was strongly affected by the interracial energies. With increasing fluorine content the homeotropic alignment temperature range of 8CB in the nematic phase was increased and that of the smectic phase was hardly affected. The impact observed was attributed to competition between anchoring effects and elasticity of the LC layers.

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1 Introduction

Orientation of liquid crystals (LCs) has been the core of liquid crystalline displays where the orientation of the director with respect to the interface is the key to this technology.¹ The underlining driving forces that control alignment include both chemical and physical interactions of the liquid crystalline molecules with the interface coupled with the conformation that arises from the elasticity of the liquid crystalline layer.^2,3 Interfacial alignment is an inherent characteristic of any liquid crystalline thin film. Achieving specific alignment across large surface areas effective for device applications remains a challenge despite a plethora of studies. Among the most viable technological paths are polymeric layers whose interfaces are modified. Here we probe the effects of structured polymeric interfaces that contain nanometer scale fluorinated domains on the alignment of LCs. We find that small differences in the degree of fluorination at the polymer interface impact the polymer–LC interactions.

Surface alignment has been achieved by orienting the substrate molecules through several methodologies, and the most widely used process is to align LCs on mechanically rubbed polyimides surfaces.^4,5 The LC alignment is determined by the anisotropic interactions of the LC molecules and the oriented polymer backbone, the side chains and the microgrooves formed in the polymer interface by rubbing. Other approaches to alignment include contact-free methods such as photo-alignment⁶ and ion-beam alignment^7,8 have also been developed.

LC alignments are ultimately determined by anisotropic interactions between the LC and the alignment layers which are characterized by a pretilt angle and an anchoring energy.⁶ Depending on the pretilt angle (θ), planar (θ = 0°), homeotropic (θ = 90°), and tilted alignments (0° < θ < 90°) are distinguished.⁶ Precise control of the pretilt angle^9–12 and anchoring energy^13,14 is essential to designing and achieving optimum performance of LCDs. LC cells with homeotropic alignment at zero field has been considered to be one of the most effective modes to increase viewing angles.^15,16 Studies have shown that the pretilt angle can be tuned by varying the surface energy of the alignment layer, where a lower-energy surface leads to a higher pretilt angle.^9,11,17 This finding is consistent with an empirical rule stating that the LCs will be oriented normal to an isotropic substrate if their surface energy is lower than that of the LC's and parallel to them otherwise.¹⁸

With increased complexity of LCs, designing new morphologies that form meso-phases and the desire for fast LC responsive layers, inherently structured interfaces offer a tailorable support.¹⁹ Of particular interests are semifluorinated polymers.²⁰ The degree of fluorination tailors both dielectric constants and refractive indices of the polymers, offering a means to tailor the orienting layers for specific applications.²¹

Here we investigate the effects of a semifluorinated polymer on the alignment of LCs. Tethering protonated and protonated segments into one macromolecule results in incorporating the unique properties induced by fluorine together with processability of hydrocarbon-based polymers.^22–24 These semifluorinated polymers find numerous uses with their applications ranging from marine antifouling coatings and bio technology to controlled dielectric and refractive materials and scratch resistance layers.^17,25–30 The inherent segregation between fluorinated and protonated segments, as well as the weak intermolecular interactions between fluorinated segments, results in surfaces or interfaces that are rich in fluorine.^27,31–35 More recent studies suggest that both the topology of polymers and the rigidity of fluorinated segments affect the interfacial characteristics of semifluorinated polymers.^20,36,37 Polyimides^12,38,39 and polystyrene³⁰ with fluorinated side groups reportedly induce homeotropic alignment on their surfaces due to lower surface energy of the aligning layers. Another way of changing surface energy is to randomly copolymerize monomers with different chemical compositions.⁴⁰ By controlling the relative concentration of these monomers, the surface energy becomes tunable.

Perfluorocyclobutane (PFCB)-containing polymers are an emerging class of next-generation, solution-processable polymers with proven potential applications, including integrated optics, proton exchange membranes for fuel cells, and light-emissive materials for polymer light-emitting diodes.^41–43 The molecular weights of PFCB-containing polymers can be controlled by optimizing time and temperature parameters and their applications may be tailored by introducing functionalities to the PFCB backbone.⁴⁴

In the present work, a class of semifluorinated polymers containing PFCB-aromatic-ether linkage, obtained by thermal cyclodimerization of aryl trifluorovinylethers (TFVE, Scheme 1),^45–48 was probed as the aligning layer (Fig. 1). The degree of fluorination was controlled by randomly copolymerizing monomers 4 and 5 (Fig. 1) while varying their relative composition. Varying the fluorine content alters the interfacial content of the fluorine and in turn affects the interfacial energy of these films. A well studied liquid crystal 4,4′-octylcycanobiphenyl (8CB), whose bulk phase diagram includes nematic and smectic phase over extended temperature range was used (Fig. 2). With increased temperature, 8CB undergoes phase transition at 21.5 °C from crystalline to smectic-A phase.⁴⁹ Then, transition to a nematic phase occurs at 33.5 °C, and transition to an isotropic liquid occurs at 40.5 °C. The 8CB molecules are paired in dimers with antiparallel dipole moments. In the smectic-A phase, 8CB forms 31.4 Å-spaced layers with dimers aligned perpendicular to these layers.⁴⁹

Scheme 1 Schematics of the preparation of aryl TFVE monomers from functionalized bisphenols.

Fig. 1 PFCB-containing polymers with different aromatic linkages.

Fig. 2 Schematics of the anti-parallel alignment of an 8CB dimer and temperatures of phase transitions in bulk 8CB.⁴⁹

We first discuss the influences of fluorine content on the bulk and interfacial properties of PFCB-containing polymers, followed by the surface alignment of 8CB molecules on PFCB-containing polymer surfaces with variable surface interaction. The current study provides new insight into the interfacial effects of semifluorinated polymers on the alignment of LCs.

2 Experimental section

2.1 Materials, sample preparation and characterization

The liquid crystal 8CB was purchased from BDH Ltd., Poole England and used as received. PFCB-containing semifluorinated polymers were synthesized by the thermal cyclodimerization of aryl TFVE.^47,48 4,4′-Isopropylidenediphenol and 4,4′-biphenol were purchased from Acros and used without further purification. 4,4′-Hexafluoroisopropylidenediphenol is commercially available from Tetramer Technologies, LLC (Pendelton, SC, http://www.tetramertechnologies.com) and distributed by Oakwood Chemicals, Inc. (Columbia, SC, http://www.oakwoodchemical.com). Aryl TFVE-containing monomers (1) are usually prepared in a two step process starting from phenolic precursors (3) via alkylation with 1,2-dibromotetrafluoroethane, followed by zinc mediated dehalogenation of the dibrominated intermediate 2 (Scheme 1).⁴¹

Synthesis of polymers is briefly introduced here. The monomers were added to 30 mL ampoules and were sealed under vacuum. The ampoules were then immersed in an oil bath at 180 °C for 48 h. The bath temperature was increased to 200 °C for 12 h and then increased further to 230 °C for 3 hours. The ampoules were then removed from the bath and allowed to cool. After that, the polymers were dissolved in dichloromethane and precipitated into methanol. The precipitate was dried and characterized. Gel permeation chromatography (GPC) was used for molecular weights and molecular weight distributions of the polymer samples using polystyrene as a reference. ¹H NMR, proton decoupled ¹³C NMR, and ¹⁹F NMR spectra were obtained using a JEOL Eclipse+ 500 or Bruker AF-300 spectrometer system, respectively. Chloroform-d or pyridine-d5 was used as solvent, and chemical shifts reported were internally referenced to tetramethylsilane (0 ppm), CDCl₃ (77 ppm), and CFCl₃ (0 ppm) for ¹H, ¹³C, and ¹⁹F nuclei, respectively. These NMR data were published elsewhere.⁴⁶

The polymers made from monomers 4, 5, and 6 were named BP-PFCB, 6H-PFCB, and 6F-PFCB. Random copolymerization of monomers 5 and 6 gave (6H/6F)-PFCB copolymers with different weight ratios, 25/75, 50/50, and 75/25. The molecular characteristics of 6F-PFCB, 6H-PFCB and the random copolymers are given in Table 1. BP-PFCB had a T_g of 135 °C, M_n of 144, 900 g mol⁻¹, and polydispersity index of 2.75. Polymer films were prepared by spin-casting from 3.0 wt% toluene solutions and dried in a vacuum oven at room temperature. The typical thickness was around 5–7 μm, as measured by a surface profiler. One liquid drop of 8CB was introduced onto the polymer thin film, which was then spin casted to form a thin layer. The typical thickness of 8CB layer was around 1–2 μm.

Table 1 Molecular characteristics of the PFCB-containing polymers

(6F/6H)-PFCB	Mn (g mol^−1)	PDI	Tg (°C)	Tm (°C)
100/0	8300	1.63	103	168
75/25	11 900	1.71	103	155
50/50	16 000	2.38	103	—
25/75	24 600	2.76	103	—
0/100	29 200	3.17	102	—
BP-PFCB	144 900	2.75	135	—

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2.2 X-ray diffraction (XRD)

Wide-angle XRD data were obtained using a Scintag XDS2000 powder diffraction system with Cu Kα camera (λ = 1.54 Å). The scattering intensities (in arbitrary unit) are plotted as a function of q, where q = 4π sin θ/λ and 2θ is the scattering angle. Scans were collected from 2θ = 5 to 50° with step size of 0.03° at 0.8 s per step. Polymer films were prepared as described above. Powder samples were spread evenly across glass substrates before the measurements.

2.3 Atomic force microscopy (AFM)

Surface morphology and force measurements were performed on MultiMode™ SPM(III) (Digital Instruments). The tip used was the standard silicon nitride tip with a diameter less than 10 nm for the morphology measurement and 20–40 nm for the force measurement. The spring constant of the cantilever used in the force measurement was 0.58 N m⁻¹ (characterized by the manufacture Digital Instruments, Inc). The sensitivity of the cantilever was calibrated on bare silicon wafer before each measurement. The tip pull-out was defined by F = k(ΔZ), where k is the spring constant of the tip and ΔZ is a vertical distance from the control point to where the cantilever pulls off the surface.⁵⁰ The forces between a silicon nitride tip and the sample surface were obtained in Contact Mode™. More information on the force measurement is found elsewhere.⁵⁰

2.4 Polarized optical microscopy (POM)

The POM used in this study was type Airplay 2 imaging, Zeiss Inc. equipped with a temperature controlLTS350 stage; Linkam Scientific Instruments Ltd. Samples were heated and cooled at a rate of 0.1 °C min⁻¹.

2.5 Differential scanning calorimetry (DSC)

DSC measurements were carried on a Mettler 8000 DSC, equipped with a liquid nitrogen system, allowing cooling and heating ramps. The apparatus cell was purged with nitrogen at a rate of 30 mL min⁻¹. Samples of approximately 10 mg were introduced to aluminum pans. The samples were heated and cooled at a rate of 1 °C min⁻¹, with an empty aluminum pan as the reference.

3 Results and discussion

3.1 Bulk properties of (6H/6F)-PFCB polymers

The DSC traces of (6H/6F)-PFCB polymers are presented in Fig. 3. The DSC data of pure 8CB is shown as Fig. S1 in ESI.† Powder samples were heated to 135 °C and subsequently cooled to erase previous history. The (0/100)-PFCB sample shows a sharp melting peak on the DSC curve indicative of crystallinity while the (27/75)-PFCB sample exhibits a much weaker crystallinity.⁴⁵ The rest of the samples form glasses and the glass transition temperature of the polymers is hardly affected by the degree of fluorination. Thus, the replacement of –CH₃ group on 6H-PFCB with –CF₃ group leads to a semi-crystalline phase. The introduction of –CF₃ group may have increased the rigidity of 6F-PFCB for the same reason as that of perfluoroalkanes, which renders the packing of 6F-PFCB polymers more efficient.^27,45

Fig. 3 Heat flow as a function of temperature of (6H/6F)-PFCB polymers.

These results are further correlated with XRD measurements. Fig. 4 introduces the XRD patterns of PFCB-containing polymers. Sharp peaks corresponding to 5.8, 5.5, and 4.8 Å are observed for 6F-PFCB polymer which are consistent with a previous report.⁴⁵ Only a broad feature centered around 5.6 Å are found for all other PFCB-containing polymers. The broad halo seems to come from the short range correlations of chain segments after their long range order is lost. This peak decreases in intensity with decreasing fluorination levels. The broad peak around q = 1.8 Å⁻¹ originates from the glass substrates.⁵¹ Variations in the ratio of 6F-PFCB to 6H-PFCB (1.0, 0.75, 0.50, 0.25, and 0) reduce the degree of correlation within the polymeric layer which in turn affects their interfacial structure. Both DSC and XRD results show that the presence of small number of 6H-PFCB is sufficient to form non-crystalline polymers.

Fig. 4 XRD of (6H/6F)-PFCB polymers.

3.2 Interactions between 8CB and as – cast (6H/6F)-PFCB polymers

To study interactions with 8CB, as-cast (6H/6F)-PFCB polymer thin films were used to exclude the interferences of crystallinity and grain boundaries from annealed films. The surface morphology as observed by AFM is presented in Fig. 5 for three representative films. The surfaces of (0/100)-PFCB (a); (50/50)-PFCB (b) and (100/0)-PFCB (c) all exhibit small domains, but no long range correlations are observed, as shown in the 2D Fourier transform. AFM images of (25/75)-PFCB and (75/25)-PFCB are included in the ESI.† The morphology observed is consistent with the XRD observations which show no long range correlations in polymer films (ESI†).

Fig. 5 AFM height images of (6H/6F)-PFCB polymer thin films and representative cross sections. (a) (0/100)-PFCB; (b) (50/50)-PFCB and (c) (100/0)-PFCB. The inserts are two dimensional Fast Fourier Transform (2D FFT) of the height images.

AFM scanning over several microns reveals homogeneous films with surface roughness of ∼0.5 nm. Roughness anisotropy is deemed responsible for the alignment of LCs on a solid substrate, where the LC director aligns along with the direction of lower roughness.⁵² Fast Fourier transforms of the AFM image (inset in Fig. 5) shows that the surface of the films exhibit isotropic domain distribution. Fig. 6 presents the surface roughness (root mean square of vertical heights) of polymer films as a function of 6F-PFCB concentration. The surface roughness was extracted from areas of different scan sizes. It increases with scan size that is commonly observed for many engineering surfaces.^53–55 This phenomenon has two possible origins. One is related to the dependency of the roughness on the spatial wavelength of the scanned area. Longer wavelength features are included as the scan size is increased.^53,54 The other explanation is that the surface is a fractal surface.^54,55 It is therefore important to compare the observed surface roughness extracted from the same scanned area. With the scan size varying from 1 to 5 μm, the surface roughness exhibits similar trend. It increases with (0/100)-PFCB concentration and peaks for the (50/50)-PFCB sample before the roughness decreases. The variations in roughness are attributed to the different degree of segregation between fluorinated (fluorocarbon) and hydrogenated (hydrocarbon) segments, resulting in different degrees of fluorination at the interfaces.⁵⁶

Fig. 6 Surface roughness of (6H/6F)-PFCB polymer thin films as a function of (0/100)-PFCB fraction. Errors were obtained by averaging roughness over several spots.

While surface roughness reflects the degree of segregation between fluorinated and hydrogenated segments, the surface free energy of polymer films is also expected to be affected by the relative abundances of these segments on surfaces. With increased (0/100)-PFCB concentration and thus fluorine content, thin films made from (6F/6H)-PFCB are expected to show changes in surface free energy. (0/100)-PFCB segments are further expected to preferentially segregate to the surface because of higher fluorine concentration than that of (100/0)-PFCB. The force of adhesion between polymer surfaces and a bare AFM tip was measured.^57,58 The “pull-off” force required separating an AFM tip from a planar surface has been shown to scale with the force of adhesion. The work of adhesion has been previously related to the surface energy and correlated with independently determined surface energies.^25,58–60 Fig. 7 presents AFM force measurements where the force decreases with increased polymer fluorine content. Given that these forces are proportional to surface energy,^25,58 these measurements show that the surface energy or the surface interaction is controlled by varying (0/100)-PFCB content. However, caution need to be taken as the factors that influence the AFM force–distance curves are not fully understood, for example the surface roughness.^25,60 A systematic study of the impacts of PFCB-containing polymer surface roughness on the work of adhesion and surface energy is underway.

Fig. 7 Surface force of (6H/6F)-PFCB polymer thin films measured by AFM.

Following an insight into the structure and interfacial behavior of the polymer film, the orientation of 8CB molecules was studied by POM. Similar to the X-ray results, no long range order was detected by microscopy for any of the thin polymeric films coated on glass. A drop of melted 8CB was allowed to spread across the boundary between the bare glass and the area coated with the polymers. The images of 8CB on the bare glass were used for comparison. These multilayers were studied in both the nematic and smectic phases. The POM images of 8CB coated on the bare glass and on the (6F/6H)-PFCB polymers measured at 29 °C (in smectic-A phase) were collected. Each sample is divided diagonally, where on the left 8CB is coated on glass and on the right on the polymers. Representative images of 8CB on (0/100)-PFCB and on (100/0)-PFCB polymer surfaces are shown in Fig. 8. The left side of each image corresponds to 8CB on glass surface, and the right side corresponds to 8CB on top of the polymer films. POM images of 8CB on glass slides in the smectic phase show characteristic smectic textures corresponding to a homogeneous alignment. The slight variation in textures is attributed to the difference in thicknesses of 8CB layers. No light is transmitted through 8CB layers on top of all (6H/6F)-PFCB polymers films. To ensure that the liquid crystalline structure was not distracted, the multilayer was studied by XRD revealing a clear peak that corresponds to the smectic layer spacing (inset in Fig. 8b). Therefore these dark images indicate a homeotropic alignment of 8CB molecules on top of PFCB-containing polymers films.

Fig. 8 POM images of 8CB on: (a) (0/100)-PFCB, (b) (100/0)-PFCB (at 29 °C in smectic A phase). The images are divided diagonally; where on the left 8CB is coated on glass and on the right on the polymers. Insert: XRD of 8CB on (6H/6F)-PFCB polymer films at room temperature.

Similar experiments were carried out at higher temperatures to probe the nematic phase. We find that the alignment of the nematic phase is strongly affected by degree of fluorination of the polymers.

The patterns of 8CB on (100/0)-PFCB surfaces and (75/25)-PFCB surfaces are shown in Fig. 9. In contrast to the smectic phase, while the patterns of the LCs are different on top of glass and the polymers, the nematic phase exhibits homogenous orientation on the PFCB-containing polymers over the whole nematic phase range. As the samples are further cooled down to the smectic phase is formed. Fig. 9b and d are recorded at temperatures above the nematic-to-smectic-A transition temperature (33.5 °C). Beyond this point, homeotropic alignment dominates in the smectic-A phases, as previously observed.

Fig. 9 POM images of 8CB on (100/0)-PFCB (a, b); on (75/25)-PFCB (c, d) films at the indicated temperatures. The left side of each image corresponds to PFCB coated on glass and the right side, coated on the polymers. All images were recorded upon cooling from the isotropic phase.

Increasing the degree of fluorination impacts the orientation of the director in the nematic phase. POM images of (50/50)-PFCB, (25/75)-PFCB and (0/100)-PFCB surfaces in the nematic phase, recorded upon cooling, are shown in Fig. 10, respectively. Upon cooling from the isotropic phase, the dark image characteristics of the isotropic phase become lightly illuminated, though featureless near the isotropic to the nematic transition temperature (41.5 °C). This is attributed to director fluctuations at the phase transition. No light is transmitted through all films coated on the polymers throughout most of the nematic phase range (33.5–40.5 °C). The nature of the phase is verified by slightly shearing the top cover slip. This result in light transmission that disappears as the samples are relaxed demonstrating that the images correspond to homeotropic alignment.

Fig. 10 POM images of 8CB on (a) and (b) (50/50)-PFCB; (c) and (d) (25/75)-PFCB; (e) and (f) (0/100)-PFCB surfaces in the nematic phase. The left side of each image corresponds to PFCB coated on glass and the right side, coated on the polymers. Images were recorded upon cooling.

The homeotropic alignment temperature range as a function of 6F-PFCB content is shown in Fig. 11. With increased (0/100)-PFCB content, the homeotropic alignment range increases. We therefore attribute the homeotropic alignment to interfacial interactions between the polymeric surface and the LC.^9,11 Though the interactions decrease the 8CB/polymer multilayers remain stable and the LC does not dewet.

Fig. 11 Homeotropic alignment range of 8CB as a function of the (0/100)-PFCB fraction. The alignment range is calculated as the temperature difference between 33.5 °C and the point where POM images become totally dark.

The effects of (6F/6H)-PFCB polymers on the alignment of 8CB strongly depend on the degree of fluorination of the interface. In contrast the smectic phase exhibits homeotropic alignment independent of the degree of fluorination. As the degree of fluorination of the interface changes with the ratio of (6F/6H)-PFCB, however this reduction in surface interaction cannot solely explain the homeotropic alignment either in the nematic or the smectic phases.

We venture that the interfacial structure of the PFCB with (CH₃)₂ and (CF₃)₂, two relatively dynamic groups at the interface, provide sufficient drive to align the director of the 8CB parallel to the polymer surface with sufficiently large director fluctuations in the nematic phase at low (CF₃)₂ concentrations and to drive homogenous alignment away from the interface at high (CF₃)₂ concentrations.

To test the hypothesis of the role of the dynamic groups at the interface, the experiments were repeated on BP-PFCB, which does not contain the dynamic bridge. The results are shown in Fig. 12. Both nematic and smectic phases exhibit homogenous alignment on this polymer, though the textures are markedly different than those observed on the (6H/6F)-PFCB series. The surface roughness of BP-PFCB film is similar to that of (6H/6F)-PFCB polymer films, and the surface force (0.45 nN) is lower than that of (100/0)-PFCB (0.52 nN). However, (100/0)-PFCB induces homeotropic alignment in smectic-A phase, whereas BP-PFCB does not.

Fig. 12 POM images of 8CB on the BP-PFCB layers. (a) In the smectic A phase and (b) in the nematic phase. The left side of each image corresponds to PFCB coated on glass and the right side, coated on the polymers.

A low-energy surface favors homeotropic alignment, but the chemical structure of is also important to design low-energy-surface materials. Three moieties are identified in terms of their contributions to homeotropic alignment, i.e., the PFCB, –CF₃, and –CH₃ groups. The PFCB group plays a minimal role because BP-PFCB exhibits no homeotropic alignment in nematic and smectic phases. Considering the homeotropic alignment conditions in PFCB-containing polymers, both –CF₃ and –CH₃ groups contribute to the homeotropic alignment in the smectic phase. In the nematic phase, –CF₃ prefers homeotropic alignment, and –CH₃ prefers nonhomeotropic alignment. These preferences are also coupled with a lower surface energy of surfaces rich in –CF₃ groups.

The orientation of –CF₃ and –CH₃ groups on the surface was not studied in this work. Surface orientation studies of –CF₃ group on a flexible side chain suggest that these groups are more or less normal to the surface.^27,61 Most likely, –CF₃ groups adopt a vertical orientation to cover the surface more efficiently, which in turn causes the homeotropic alignment of 8CB molecules.

Both rigid side chains and flexible alkyl side chains have been introduced onto polyimides. Studies have shown that polyimide alignment layers contain out-of-plane units can generate high pretilt angles.^62–65 In the recent study of dependence of pretilt angle on the side chain orientation and conformation in polyimide surfaces, a vertical orientation of the side chain has been correlated with high pretilt angles including 90°.⁶⁴ A parallel orientation of the side chains leads to homogeneous alignment in that study.⁶⁴ Increasing rigidity of the side chains improves rubbing resistance of polyimide films.⁶⁶ In addition to the orientation of the side chains, surface energy which affects LC alignment can also be changed the same time after incorporation of side chains.^30,67,68 The coupling effects between side chain orientation and surface energy determine the resultant alignment of LC molecules.

4 Conclusions

The replacement of –CH₃ group on amorphous (100/0)-PFCB polymers with –CF₃ group leads enhanced crystallinity in (0/100)-PFCB polymers. Increased segregation between fluorinated and hydrogenated segments results in structured surfaces with increased surface roughness. A systematic variation in interfacial interactions leads to an orientation transition in 8CB. The trifluoromethyl group is suggested to prefer homeotropic alignment, and the methyl group, a nonhomeotropic alignment in the nematic phase. In the smectic phase, both groups are necessary to induce homeotropic alignment.

Acknowledgements

We acknowledge the support of NSF (DMR0203660). Gang Cheng acknowledges China Scholarship Council (201606885004).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra10730c

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