Photopatterning of Azobenzene-Containing Liquid Crystalline Triblock Copolymers: Light-Induced Anisotropy and Photostabilization.

Photochromic liquid crystalline block copolymers (PLCBCs) are currently playing a significant role as light-responsive materials because of their light controllable features over multiple length scales. Herein, a study of the photoinduced optical anisotropy derived by the combination of orientation phenomena at molecular and supramolecular levels in a novel kind of side-chain PLCBCs with mesogenic phenyl benzoate groups and pyridine units that is hydrogen bonded with azobenzene-containing phenol is reported. Based on the polymeric architectures and composition, the supramolecular configuration self-organizes in different microphases that affect the material response to the external stimuli. Simple, 1D, polarization holograms are recorded to evaluate the photoinduced birefringence. The first step, light patterning, involves the orientation of the azobenzene units and precedes a thermal treatment that amplifies the induced anisotropy through the cooperative orientation of the mesogenic units. By selective extraction, the azobenzene units can be removed, making the material transparent to the visible light. Excellent photostability of the material birefringence is obtained, whose final value is strongly affected by the block copolymer's architecture. The versatility in the molecular design, the fine control of the photoinduced features by external parameters, and, finally, the possibility to achieve photostability make these materials of great potential for developing optical and photonic devices.

Direct Observation of Changes in Focal Conic Domains of Cholesteric Films Induced by Ultraviolet Irradiation.

The helical supramolecular structure of cholesteric liquid crystalline (LC) films predetermines their outstanding optical properties and the unique nanostructure of their surface. The introduction of photochromic dopants in these films opens up an interesting possibility for creation of smart cholesteric materials with photocontrollable optical and photovariable surface properties. Using atomic force microscopy (AFM), we performed in situ measurements of the surface topography of cyclosiloxane LC cholesteric oligomer films during the cholesteric helix twisting caused by their preliminary ultraviolet (UV) irradiation. A chiral-photochromic isosorbide-based dopant was introduced in the films to control the cholesteric helix pitch by UV-irradiation. The initial films are characterized by planar texture with the presence of focal conic domains having the double-spiral relief on their surface. UV-irradiation of these films leads to the cholesteric helix twisting resulting in a decrease in the surface relief period, and the enlargement of defect areas between the domains. The detailed mechanisms of the rearrangement of the film surface structure due to the cholesteric helix twisting are suggested. They include the rotation and displacement of cholesteric layers in the bulk, and the nucleation of new ones at the surface in defect regions.

Surface Relief Changes in Cholesteric Cyclosiloxane Oligomer Films at Different Temperatures.

The development of new approaches for the surface topography control is an important topic as the relief significantly affects physical and chemical properties of surfaces. We studied cholesteric cyclosiloxane oligomeric films on which surface focal conic domains with double-helix pattern were observed by means of AFM. In situ investigation of the dependence of the films topography on temperature showed that the surface relief formation can be effectively managed by varying conditions of thermal treatment. Obtained structures can be frozen by cooling the films below glass-transition temperature.

Polarization holographic recording in amorphous polymer with photoinduced linear and circular birefringence.

Polarization grating recording in an amorphous and nonchiral azo copolymer has been investigated. The reported study shows that the amorphous polymeric film undergoes a light-guided inhomogeneous supramolecular modification as a consequence of the illumination with proper polarized light patterns, acquiring new functionalities. Both linear and circular, spatially modulated, photoinduced birefringences occur, attaining their peak values in the linearly and circularly polarized regions of the light pattern, respectively. The photoinduced anisotropic structures strongly affect the polarization state of the light propagating through them, and the characterization of their optical diffraction enables measurement of the amplitude of the linear and circular birefringences. The recorded gratings show long-time stability and full reconfigurability functional to the multiple holographic recording.

Order and dynamics of a liquid crystalline dendrimer by means of 2H NMR spectroscopy.

A complete Deuterium NMR study performed on partially deuterated liquid crystalline carbosilane dendrimer is here reported. The dendrimer under investigation shows a SmA phase in a large temperature range from 381 to 293 K, and its mesophasic properties have been previously determined. However, in this work the occurrence of a biphasic region between the isotropic and SmA phases has been put in evidence. The orientational order of the dendrimer, labeled on its lateral mesogenic units, is here evaluated in the whole temperature range by means of (2)H NMR, revealing a peculiar trend at low temperatures (T < 326 K). This aspect has been further investigated by a detailed analysis of the (2)H NMR spectral features, such as the quadrupolar splitting, the line shape, and the line-width, as a function of temperature. In the context of a detailed NMR analysis, relaxation times (T(1) and T(2)) have also been measured, pointing out a slowing down of the dynamics by decreasing the temperature, which determines from one side the spectral changes observed in the NMR spectra, on the other the observation of a minimum in the T(1).

Liquid crystal codendrimers with a statistical distribution of phenolic and mesogenic groups: behavior as Langmuir and Langmuir-Blodgett films.

The first series of carbosilane liquid crystal codendrimers with groups of different polarity has been synthesized. The chemical structure of the newly synthesized materials and the composition of the codendrimers were studied by NMR spectroscopy and MALDI-TOF MS. It was found that the codendrimers tend to form stable Langmuir films at the air-water surface. The influence of composition and generation number on surface pressure-surface area isotherms and film stability was studied. Brewster angle microscopy confirmed the different phase behavior for monolayers of different codendrimer composition and generation number. It was found that side groups of fifth-generation codendrimers do not segregate, unlike those of lower generations. Langmuir-Blodgett films on solid substrates were obtained by the vertical dipping method. X-ray diffraction showed that the codendrimers with 75% of hydrophobic mesogenic terminal groups formed ordered layers parallel to the substrate.