Rubbing-induced anisotropy of long alkyl side chains at polyimide surfaces

Molecular organization at polyimide surfaces used as alignment layers in liquid crystal displays was investigated using vibrational sum frequency generation (SFG) spectroscopy. We focus on the orientation of the long alkyl side groups at the polymer surface using polarization-selected SFG spectra of the CH(3)- and CH(2)-stretch modes of the side chain. Mechanical rubbing and baking, an accepted industrial procedure used to produce pretilt of the liquid crystal, was found to induce pronounced azimuthal anisotropy in the orientational distribution of the alkyl side chains. Orientational analysis of the SFG vibrational spectra in terms of the azimuthal and tilt angles (in and out of plane, respectively) of the alkyl side chains shows their preferential tilt along the rubbing direction, with the azimuthal distribution narrower for stronger rubbed polymer samples.     

Novel alignment method for control of high pretilt angle by using a solvent dipping effect on polymer surfaces

A novel alignment method for control of high pretilt angle in nematic liquid crystals (NLC), using a solvent dipping effect on various alignment layers, was successfully investigated. The pretilt angle of a NLC is increased by dipping before rubbing treatment on three kinds of rubbed polyimide (PI) surfaces. The pretilt generated by the dipping after rubbing a PI surface with a short side chain is high compared with a PI surface with a long side chain. The pretilt generated by dipping before rubbing homeotropic layer of a positive type NLC (δε > 0) is lower than that of the negative type NLC (δε < 0). The generated NLC pretilt angle is attributed to the perpendicular component of the permittivity epsilon of the NLC.     

Orientational relationship among main and side chains of a polyimide alignment layer,liquid crystals monolayer and bulk pretilt angle

We have determined the orientational distribution of cyano-substituted side chains of a rubbed polyimide film, and a liquid crystal monolayer adsorbed on the film, by means of optical second harmonic generation. With the orientational distribution of a main chain that was measured in a previous study by means of near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, we have obtained the correlation of all the orientational distributions contributing to the alignment of LC molecules, i.e. pretilt angle. We find that the side chain plays a role in increasing the pretilt angle, but in the case of rubbing strength dependence, the main chain has stronger correlation with the pretilt angle than has the side chain.     

Orientational relationship among main and side chains of a polyimide alignment layer, liquid crystals monolayer and bulk pretilt angle

We have determined the orientational distribution of cyano-substituted side chains of a rubbed polyimide film, and a liquid crystal monolayer adsorbed on the film, by means of optical second harmonic generation. With the orientational distribution of a main chain that was measured in a previous study by means of near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, we have obtained the correlation of all the orientational distributions contributing to the alignment of LC molecules, i.e. pretilt angle. We find that the side chain plays a role in increasing the pretilt angle, but in the case of rubbing strength dependence, the main chain has stronger correlation with the pretilt angle than has the side chain.     

Preliminary Communication : Generation of high pretilt angles of nematic liquid crystal (5CB) on rubbed organic solvent soluble polyimide surfaces with helical backbone structure and trifluoromethyl moieties

High pretilt angles of nematic liquid crystals (NLC) have been successfully generated on rubbed organic solvent soluble polyimide (PI) surfaces with a helical backbone structure and trifluoromethyl moieties. We have found that the pretilt angle of in 4-n-penty-4-cyanobiphenyl (5CB) on rubbed PI surfaces with trifluoromethyl moieties attached to the lateral benzene rings is larger compared to that on rubbed PI surfaces with trifluoromethyl moieties attached to the polymer backbone. The observed pretilt angle of 5CB on unidirectionally rubbed PI surfaces with trifluoromethyl moieties attached to the lateral benzene rings is about 15 degrees in the wide rubbing region. From these results, it is expected that the microscopic surface structure of the polymer strongly contributes to the pretilt angle generation of the LCs at the surface.     

Determination of the molecular orientation of very thin films on solid substrates: surface liquid crystal layers and rubbed polyimide films

The molecular orientation of very thin films on solid substrates can be determined quantitatively by measuring the polarized infrared (IR) absorption spectra of samples as a function of angle of incidence. The quantitative molecular orientation is derived by fitting the incident angle dependence and the dichroic ratio with theoretical calculations. We applied this method to a technologically important system: liquid crystal (LC)/rubbed polyimide film. To understand the alignment mechanism of LC molecules in contact with rubbed polyimide films, we have quantitatively determined the molecular orientation of rubbed polyimide films and a surface LC layer in contact with a rubbed polyimide film. In this paper two relations are discussed: (1) correlation between the inclination angle of polyimide backbone structures in rubbed films and the pretilt angle of bulk LC in contact with them, and (2) relation among the molecular orientation of a rubbed polyimide film and those of surface and bulk LC layers in contact with it.     

Liquid crystal alignment capabilities on rubbed organic solvent soluble polyimide surfaces with trifluoromethyl moieties

High pretilt angles, polar anchoring energy (out of plane-tilt), and surface ordering in the nematic liquid crystal 4-n-pentyl-4'-cyanobiphenyl (5CB) were investigated on rubbed organic solvent soluble polyimide (PI) surfaces with a helical backbone structure and trifluoromethyl moieties. It was found that the pretilt angle of 5CB is about 15° in the wide rubbing region of rubbed soluble PI surfaces with trifluoromethyl moieties attached to the lateral benzene rings. It is suggested that the microscopic surface structure of the polymer contributes to the LC pretilt angle generation at the surface. Also, the polar anchoring energy of 5CB is dependent on the molecular structure of these unidirectionally rubbed soluble PI surfaces. The polar anchoring strength of 5CB on rubbed soluble PI surfaces is as weak with trifluoromethyl moieties attached to the lateral benzene rings weak as when the trifluoromethyl moieties are attached to the polymer backbone. Finally, the polar anchoring energy of 5CB strongly depends on the surface ordering of rubbed soluble PI surfaces.     

Effect of pretilt angle on the polar anchoring energy in NLCs on weakly rubbed polyimide surfaces

The polar anchoring strength and pretilt angle generation in nematic liquid crystals (NLCs), on three kinds of rubbed polyimide (PI) surfaces, were investigated. The pretilt angle generated in 4-n-pentyl-4'-cyanobiphenyl (5CB) is large compared with ZLI-4792 (a fluorinated mixture type NLC) for all rubbed PI surfaces. The high pretilt angle in 5CB is attributed to a much larger birefringence and much larger perpendicular component of permittivity. The polar anchoring energy of ZLI-4792 is high compared with 5CB on all rubbed PI surfaces for a weak rubbing strength. It is suggested that the high anchoring energy of ZLI-4792 may be attributed to the low NLC pretilt angle.     

Liquid crystal alignment capabilities on rubbed organic solvent soluble polyimide surfaces with trifluoromethyl moieties

High pretilt angles, polar anchoring energy (out of plane-tilt), and surface ordering in the nematic liquid crystal 4-n-pentyl-4'-cyanobiphenyl (5CB) were investigated on rubbed organic solvent soluble polyimide (PI) surfaces with a helical backbone structure and trifluoromethyl moieties. It was found that the pretilt angle of 5CB is about 15° in the wide rubbing region of rubbed soluble PI surfaces with trifluoromethyl moieties attached to the lateral benzene rings. It is suggested that the microscopic surface structure of the polymer contributes to the LC pretilt angle generation at the surface. Also, the polar anchoring energy of 5CB is dependent on the molecular structure of these unidirectionally rubbed soluble PI surfaces. The polar anchoring strength of 5CB on rubbed soluble PI surfaces is as weak with trifluoromethyl moieties attached to the lateral benzene rings weak as when the trifluoromethyl moieties are attached to the polymer backbone. Finally, the polar anchoring energy of 5CB strongly depends on the surface ordering of rubbed soluble PI surfaces.     

Relationship between alignment layer polymer surface structures and liquid crystal display parameters

The polar alignment layer (AL) surface provided relatively small liquid crystal (LC) pretilt angles while polyimides with long alkyl side chains gave relatively large LC pretilt angles. The results suggest that LC pretilt angles, in addition to an anchoring effect, are greatly affected by both electronic and steric interactions between LC molecules and a polyimide alignment layer surface. Rubbing with a cotton cloth induces functional groups, side chains, and repeat units at the surface of a liquid crystal polyimide AL to re-orient. It was discovered that rubbing induced polar functional groups and repeat units to re-orient out-of-the-plane of the surface, and it made non-polar aliphatic side chains partially re-orient inwards, toward the bulk of the film.     

Zigzag defect-free alignment of surface stabilized ferroelectric liquid crystal cells with a polyimide irradiated by polarized UV light

Zigzag defect-free surface stabilized ferroelectric liquid crystal (SSFLC) cells were prepared using a photodegradable polyimide (PI) having a cyclobutane ring in the backbone. The PI layers were irradiated by polarized ultraviolet light (PUVL) at normal incidence to the surface, and characterized by UV and FTIR spectroscopy. The anisotropy originates from preferential cleavage of PI chains oriented parallel to the polarization direction of the irradiating PUVL. After the polarized UV light irradiation, the PI surface was much flatter than that after rubbing, but it induced a similar order parameter of dye-doped nematic LC molecules to that for a rubbed cell. Alignment of both the FLC molecules and the layer structure is important in SSFLC. After 40 min irradiation, the FLC molecules were well aligned homogeneously, and the FLC cells showed a uniform texture without zigzag defects which also indicates a well aligned layer structure. Zigzag defect-free alignment may result from the flatter surface, the much smaller and more constant pretilt angles, and the bigger cone angle than those achieved by rubbing.     

Laser‐induced periodic surface structure on a polymer surface: The effect of a prerubbing treatment on the surface structure

The influence of the mechanical rubbing of a polyimide (PI) film on the laser‐induced periodic structure (LIPS) was demonstrated. The periodicity and amplitude of LIPS were greater when the rubbing direction was parallel to the laser polarization direction. The amplitude became small and the periodicity of LIPS did not show an obvious change when the rubbing direction was perpendicular to the laser polarization direction. The effect of the rubbing pretreatment on LIPS was explained on the basis of the wave‐guide effect of rubbing‐induced microgrooves on LIPS formation. The orientation of PI chains induced by mechanical rubbing was relaxed after laser irradiation, and a new orientation of PI chains was formed during the LIPS formation. When the rubbing direction was perpendicular to the laser polarization direction, the orientation of PI chains remained in the rubbing direction. The laser‐irradiated, perpendicularly rubbed PI surface could be used to verify the effects of surface morphologies and intermolecular interactions on liquid‐crystal alignment. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 1273–1280, 2003     

Zigzag defect-free alignment of surface stabilized ferroelectric liquid crystal cells with a polyimide irradiated by polarized UV light

Zigzag defect-free surface stabilized ferroelectric liquid crystal (SSFLC) cells were prepared using a photodegradable polyimide (PI) having a cyclobutane ring in the backbone. The PI layers were irradiated by polarized ultraviolet light (PUVL) at normal incidence to the surface, and characterized by UV and FTIR spectroscopy. The anisotropy originates from preferential cleavage of PI chains oriented parallel to the polarization direction of the irradiating PUVL. After the polarized UV light irradiation, the PI surface was much flatter than that after rubbing, but it induced a similar order parameter of dye-doped nematic LC molecules to that for a rubbed cell. Alignment of both the FLC molecules and the layer structure is important in SSFLC. After 40 min irradiation, the FLC molecules were well aligned homogeneously, and the FLC cells showed a uniform texture without zigzag defects which also indicates a well aligned layer structure. Zigzag defect-free alignment may result from the flatter surface, the much smaller and more constant pretilt angles, and the bigger cone angle than those achieved by rubbing.     

Surface morphology at nanometer level and optical phase retardation of rubbed alignment films

The surface morphology of rubbed polyimide LC aligning films has been studied by scanning force microscopy. We examined three types of alignment film: HT-210, AL-1051 (main chain type) and AL-8044 (side chain type) polyimide surfaces. The rubbed polyimide surfaces show anisotropic alignment of the polymer chain cluster, and a microgroove between the ripple pattern streaks. Also, they are periodically oriented along the rubbing direction. The optical phase retardation of the rubbed surface does not increase with increased cumulative time of rubbing, On the other hand, it increases monotonically with increasing rubbing depth.     

Surface morphology at nanometer level and optical phase retardation of rubbed alignment films

The surface morphology of rubbed polyimide LC aligning films has been studied by scanning force microscopy. We examined three types of alignment film: HT-210, AL-1051 (main chain type) and AL-8044 (side chain type) polyimide surfaces. The rubbed polyimide surfaces show anisotropic alignment of the polymer chain cluster, and a microgroove between the ripple pattern streaks. Also, they are periodically oriented along the rubbing direction. The optical phase retardation of the rubbed surface does not increase with increased cumulative time of rubbing, On the other hand, it increases monotonically with increasing rubbing depth.     

Pretilt angle for BTDA,PMDA and CPDA series polyimides with various side chain lengths

Polyamic acid precursors were prepared by mixing dianhydride of 3,3',4,4'-benzophenone-tetracarboxylic dianhydride (BTDA), 1,2,3,4-benzene-tetracarboxylic dianhydride (pyrromellitic dianhydride PMDA), cis-1,2,3,4-cyclopentane-tetracarboxylic dianhydride (CPDA), the diamine (alkyl 3,5-diaminobenzoate) with side chain, and 4,4'-oxydianiline (ODA) without side chain. Copolyimide films with various side chain lengths were prepared by thermal imidization of polyamic acid precursors. The roughness of rubbed polyimide surface increased with increase in the side chain length. The pretilt angle for the BTDA and PMDA series polyimide (PI) increased exponentially with increase in side chain length. Various pretilt angles were obtained on the synthesized polyimides. In the case of CPDA series PI, the pretilt angle was nearly constant at 0 until the alkyl side chain length reached 12 (C12) and then increased markedly at C18. Models of pretilt angle generation were tested.     

Relationship between surface anchoring strength and surface ordering on weakly rubbed polyimide surfaces

We have investigated the relationship between the polar anchoring strength and surface ordering in a nematic liquid crystal on two kinds of weakly rubbed polyimide (PI) surfaces. The polar anchoring strength of 5CB on weakly rubbed PI surfaces, both with and without side chains, increases with rubbing strength and with decreasing temperature. The surface order parameter of 5CB on these surfaces increases with rubbing strength, suggesting that the polar anchoring strength on rubbed PI surfaces is related to the surface order parameter.     

Relationship between surface anchoring strength and surface ordering on weakly rubbed polyimide surfaces

We have investigated the relationship between the polar anchoring strength and surface ordering in a nematic liquid crystal on two kinds of weakly rubbed polyimide (PI) surfaces. The polar anchoring strength of 5CB on weakly rubbed PI surfaces, both with and without side chains, increases with rubbing strength and with decreasing temperature. The surface order parameter of 5CB on these surfaces increases with rubbing strength, suggesting that the polar anchoring strength on rubbed PI surfaces is related to the surface order parameter.     

Pretilt angle of liquid crystals and liquid-crystal alignment on microgrooved polyimide surfaces fabricated by soft embossing method

In this study, the soft embossing method is proposed to fabricate periodical microgrooved structure on polyimide surfaces. These microgrooved polyimide surfaces are assembled to form liquid-crystal cells. It is found that the director of liquid crystals uniformly aligns along the groove direction even when the groove width is as high as 3 microm. The anchoring energy of these microgrooved polyimide surfaces is higher than that of the typical rubbed surfaces. The pretilt angle of liquid crystals is adjusted by tuning the surface polarity of the polyimide alignment layer, which is identified by the advancing contact angle of water. The surface polarity of polyimide alignment layers is manipulated by simply mixing two kinds of polyimide: a more hydrophilic one and a more hydrophobic one. It is found that the pretilt angle of liquid crystals increases along with the advancing contact angle of water on the alignment layer under the condition of a fixed surface topography.     

Asymmetrically tilted alignment of rigid-rod helical polysilanes on a rubbed polyimide surface

The homogeneous alignments of helical rod-like polysilanes on a rubbed polyimide alignment layer were investigated by polarized optical microscopy (POM) and atomic force microscopy (AFM) analyses. The POM and AFM observations determined that polysilanes with a series of aliphatic side chains helically arranged around the main chains were tilted to the right and left by 33° from the rubbing direction when the handedness of the side-chain helical array is left and right, respectively. It is interesting to note that the side-chain arrays run perpendicular to the rubbing direction on the polyimide surface, which is different from intuitive "knob and hole" packing of the extended polyimide chain and the helical grooves between the side-chain arrays surrounding the polysilane backbone. More surprisingly, both right- and left-tilting smectic domains were simultaneously observed with an equal probability for an achiral polysilane, which apparently has the interconverting right- and left-handed helical segments separated by helical reversals. This might be the first observation of the chiral segregation of dynamic helical polymers.