Qizhen Zhang

Monolayer behaviors for the photochromic liquid crystalline copolymers with azobenzene and cholesterol side groups

Abstract Monolayer behaviors for a series of newly synthesized photochromic liquid crystalline copolymers were studied. Results showed that the azo-homopolymer would aggregate heavily after spreading and its monolayer was not suitable for deposition of the reversible photochromic LB film. However, the liquid crystallinity, which resulted by introducing mesogenic cholesteric side chains, could reduce monolayer aggregation and improve reversible photoinduced structural change in LB films.

Mesogenic structural effects on monolayer behaviors of the liquid crystalline polysiloxanes at the air/water interface

Abstract The monolayer behavior of three newly synthesized side-chain liquid crystalline polysiloxanes and corresponding monomers have been investigated at the air/water interface. The surface pressure-area isotherms and Brewster angle microscopy (BAM) were used to study the influence by different mesogenic units in the side chains. The results indicate that difference in mesogenic unit structure, such as linkage bond and mesogen length, have marked effects on the liquid crystallinity and self-aggregation tendency of the molecular chains which determine the properties of the monolayers.

Studies on the aggregation behaviour of a side-chain liquid crystalline polymer in Langmuir-Blodgett films

Abstract The aggregation behaviour of a newly synthesized ferroelectric side-chain liquid-crystalline polysiloxane (denoted as PSLC) in Langmuir-Blodgett (LB) films has been investigaated. The surface pressure (π)-area ( A ) isotherm and hysteresis experiments for arachidic acid (AA)/PSLC mixed monolayer confirm existence of aggregation or phase-separation during compression. From the UV-Vis spectra, we can clearly see H-aggregate formation. To study the aggregate structure and molecular packing of PSLC, cyclic voltammetry combined with transmission electron microscopy and scanning electron microscopy morphology observations was used. The exciton model previously advanced by Kasha and coworkers is successfully introduced for a qualitative explanation of the spectra observed in the LB films.

Direct observation of 2-D phase transitions of Chiral liquid crystals at the air/water interface

Monolayer morphology of phase transitions and melting process of three low mass weight chiral liquid crystals have been studied and observed in situ by Brewster angle microscopy (BAM). The liquid crystals (LCs) have benzoate benzoyl or biphenyl groups as mesogens and undecenoyloxy as spacer and 2-methylbutyloxy as chiral end group. Their mesogens have different ester bonds in the chemical structure which lead to different bulk phase transition temperatures and gradual change of monolayer phase transition behaviors at the air/water interface due to the strong anchoring effect of the ester bonds with the water surface. Their phase transition behaviors have been discussed according to classical nucleation theory (CNT). Two-dimensional phase transitions for the molecules with multiple ester bonds via two growth process after initial nuclei is initiated: homogenous growth of single domains and coalescence of adjacent domains. Different sizes of the nucleus also indicates the progressive nucleation. MIII with the biphenyl mesogen has the highest transition temperature and the molecules formed aggregates even initially spread at zero surface pressure, the domains accumulated together to form a condensed monolayer during compressing. Melting process of the monolayers of MII and MIII have also been observed at constant surface concentration of molecules.

Monolayer behavior and BAM observaion of biphenyl liquid crystalline polysiloxane at the A/W interface

Monolayers of a chiral biphenyl liquid crystalline polysiloxane and its monomer have been studied by π–A isotherms and hysteresis measurements. Both of them can form monolayers at different temperatures. The molecular packing in the monolayers are temperature-dependent for the monomer but not for the polymer. The molecular orderings are dominated by the packing of the biphenyl mesogens. Metastable monolayers with different ordering and thickness of domains have been directly observed by Brewster Angle Microscopy (BAM) in monomer monolayers. Both of them show characteristics of rigid monolayers. A broken process of the monolayers can be observed during expansion.

Characterizton on the Structure and Ferroelectricity for the LB Film of a Chiral Liquid Crystalline Polysiloxane

Abstract LB films of an newly synthesized chiral SCLC polysiloxane with biphenyl mesogens is prepared to form unique Y-type layer structure. The ferroelectricity of these LB films are measured in sandwich type electrode pattern by Sawyer-Tower method.

Synthesis, characterization of a chiral liquid crystalline polysiloxane and its monomer

Abstract A liquid crystalline polysiloxane and its monomer have been synthesized and characterized by FT-IR and 1HNMR spectra. Their bulk phase liquid crystallinity were studied. The mesomorphic transition behaviors and the mesophases were observed by Differential Scanning Calorimeter (DSC), Polarized Optical Microscopy (POM) and Wide Angle X-ray Diffraction (WAXD) measurements. The monomer showed SmB and SmA phases which are assured by the corresponding broken fan-shaped or mosaic textures and fan-shaped textures by POM observation and WAXD patterns. The polymer had only one mesophase, SmC phase, which showed characteristic of fan-shaped or Schlieren textures by POM and sharp peaks in low angle region and a diffused peak wide angle region from WAXD.

Studies of casting and LB films of ferroelectric liquid crystalline polysiloxane

Monolayers of side chain ferroelectric liquid crystalline polysiloxane have been successively transferred onto appropriate substrates by vertical dipping method to study the morphology and the orientation of the mesogens and the side chains by UV spectra and polarized FT-IR spectra and X-ray diffraction. The spectra for casting film and LB film are compared. The molecules in LB films are found to be more orientated along the normal of the substrate at an angle of 37° for the mesogens as the side chain as found by polarized FT-IR spectra and transmitting FT-IR spectra than casting film. Layer structure is also confirmed by X-ray diffraction results. The mesogens are found to form H-aggregates structure in bulk phase and LB films from UV spectra. Temperature-dependent UV spectra have revealed the change of the tilt angle of the mesogens inside the layer of both bulk phase and LB films.