Photoreaction and photoinduced molecular reorientations in nanoscaled films of poly(methyl 4-(methacryloyloxy)cinnamate) (PMMCi) were investigated through two-dimensional (2D) correlation analyses of their infrared (IR) and ultraviolet (UV) absorption spectra. In the PMMCi film the presence of three distinct chromophore types was evident: isolated trans-isomeric cinnamoyl moieties and parallel and end-to-end cinnamoyl aggregates. Their photoinduced reorientations take place in the following sequence: isolated trans isomeric chromophores f parallel chromophore aggregates f end-to-end chromophore aggregates. It turns out that they react photochemically at different rates. The PMMCi polymer molecules were found to undergo both photoisomerization and photodimerization upon UV irradiation, with the trans-cis isomerization of the isolated trans isomeric cinnamoyl moieties occurring more rapidly than the photodimerization of the cinnamoyl aggregates. Of the photodimerization processes, the reorientation of the parallel cinnamoyl aggregates precedes that of the end-to-end cinnamoyl aggregates. The isolated chromophores may partly involve in the photodimerization, but its fraction is expected small because of the film in a confined geometry. The cinnamoyl moiety was found to be present as two rotational conformers, trans-s-trans and trans-s-cis conformers. Upon UV irradiation, the trans-s-trans conformer changes before the trans-s-cis conformer. The cinnamoyl moieties undergo a cooperative reorientation with the methacryloyloxy units during photoreaction. The photoinduced molecular reorientations occur in the following sequence: trans isomer f phenyl ring f trans-s-trans conformer f trans-s-cis conformer f photodimer.
Nanoscaled films of poly{p-phenylene 3,6-bis [4-(n-butoxy)phenyloxy]pyromellitimide} (C4-PMDA-PDA PI), a well-defined brush polyimide (PI) composed of aromatic-aliphatic bristles set into a fully rodlike polymer backbone (two bristles per chemical repeat unit of the polymer backbone), were studied by atomic force microscopy, optical retardation, prism coupling, and linearly polarized IR spectroscopy before and after mechanical rubbing with velvet fabric, and their liquid crystal (LC) aligning abilities were investigated. Uniform, homogeneous LC alignment was achieved at the rubbed film surfaces of PIs with positively birefringent characteristics. Surprisingly, however, the LC alignment director for this PI is perpendicular to the rubbing direction as well as to the reorientation direction of the polymer main chains. This is the first time that LCs on a PI surface have been induced to align in the direction perpendicular to the rubbing direction, a significant departure from the LC alignment observed for all other PI materials reported so far, for which LC alignment is always parallel to the rubbing direction. This unusual LC alignment is attributed to the strength of the anisotropic intermolecular interactions of the LC molecules with the short bristles attached perpendicularly to the polymer chain, which override the interactions with the main polymer chains and with the microgroove lines with fine grooves that are created along the rubbing direction. The LC pretilt angle is expected to be 25-55°with respect to a direction perpendicular to the rubbing direction, although this angle could not be determined due to the limitations of the crystal-rotation apparatus employed. In particular, the n-butyl end group of the bristle was found to play a critical role in the pretilting of LC molecules, as well as in the perpendicular LC alignment. In addition, the surface morphology of the rubbed PI films was investigated in detail, with particular consideration being given to the polymer property characteristics, the structure of the velvet fabric fibers, and the rubbing conditions.
A soluble photoreactive polyimide (PSPI) with side-group cinnamate (CI) chromophores that forms good quality films through conventional solution spin-casting and drying processes was successfully synthesized with a reasonably high molecular weight: poly(3,3‘-bis(cinnamoyloxy)-4,4‘-biphenylene hexafluoroisopropylidenediphthalimide) (6F-HAB-CI). This 6F-HAB-CI PSPI is thermally stable up to 340 °C, its glass transition temperature (T g) is 181 °C, and it was determined by prism coupling to be positively birefringent. The photochemical reactions of the PSPI in solution and in films, as well as the molecular orientations of the PSPI that are induced by exposure of its films to linearly polarized ultraviolet light (LPUVL), were investigated in detail by nuclear magnetic resonance, ultraviolet−visible, infrared, and Raman spectroscopies and by dissolution testing and optical retardation measurements. It was confirmed that the CI chromophores of the PSPI undergo both photoisomerization and photodimerization. LPUVL exposure was found to induce anisotropic orientations of the polymer main chains and of the CI side groups in the films. Moreover, the LPUVL-irradiated films homogeneously aligned nematic liquid crystal (LC) molecules along a direction at an angle of 107° with respect to the polarization of the LPUVL, which coincides with the orientation direction of the PSPI polymer chains. This result shows that the LC alignment process is principally governed in irradiated PSPI films by the orientations of the polymer main chains and the unreacted CI side groups. Along the director of the LC alignment in the cell, the pretilt angle was measured to be 0−0.15°, depending on the exposure energy and the exposure method. The LC alignment was found to be thermally stable up to 200 °C, which is 20 °C higher than the T g of the film. In summary, the LC alignment characteristics of the 6F-HAB-CI PSPI make it a promising candidate material for use as the LC alignment layer in advanced LC display devices with in-plane switching modes that require as low as possible LC pretilt angles.
We have investigated the surfaces of rubbed polystyrene (PS) films in detail using atomic force microscopy and have discovered a previously unknown surface feature: submicroscale meandering groove-like structures composed of gravel-like grooves in tens of nanometers are present, oriented perpendicular to the rubbing direction. This unusual surface morphology is a significant departure from the surface topographies observed so far for rubbed PS and other polymer films, for which grooves are usually only found parallel to the rubbing direction. We also conclude from retardation analysis and linearly polarized infrared (IR) spectroscopy that the phenyl side groups of the PS chains are reoriented perpendicular to the rubbing direction, with para-directions that are nearly normal to the film plane, whereas the vinyl backbones of the PS chains are reoriented along the rubbing direction. This is the first time IR spectroscopic techniques have been used to determine the three-dimensional reorientation geometry of phenyl rings on the surface of rubbed PS films. Uniform, homogeneous alignment of liquid crystal (LC) molecules is achieved at rubbed PS film surfaces, but the alignment director is usually perpendicular to the rubbing direction. This perpendicular LC alignment was found to have very low anchoring energy (less than 3 × 10-7 J/cm2) and to persist only for a limited time (less than 1 day), indicating that the interaction of LCs with rubbed PS surfaces is very weak. Collectively, the results lead to the conclusion that the unusual well-developed groove topography oriented perpendicular to the rubbing direction plays a critical role in governing the alignment of LC molecules that weakly interact with the perpendicularly reoriented phenyl side groups, overriding the effects of the parallel reoriented vinyl main chains. In addition, the zero degree pretilting behavior of LCs on PS surfaces was discussed, in particular with respect to the rubbing-induced reorientations of the PS polymer segments and their anisotropic interactions with the LC molecules.
The FeSe nematic phase has been the focus of recent research on iron-based superconductors (IBSs) due to its unusual properties, which are distinct from those of the pnictides. A series of theoretical/experimental studies were performed to determine the origin of the nematic phase. However, they yielded conflicting results and caused additional controversies. Here, we report the results of angle-resolved photoemission and X-ray absorption spectroscopy studies on FeSe detwinned by a piezo stack. We fully resolved band dispersions with orbital characters near the Brillouin zone (BZ) corner, and revealed an absence of any Fermi pocket at the Y point in the 1-Fe BZ. In addition, the occupation imbalance between d xz and d yz orbitals was the opposite of that of iron pnictides, consistent with the identified band characters. These results resolve issues associated with the FeSe nematic phase and shed light on the origin of the nematic phase in IBSs.
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