Francesco Vita

Cybotaxis dominates the nematic phase of bent-core mesogens: a small-angle diffuse X-ray diffraction study

New temperature dependent X-ray diffraction (XRD) data on the bent-core mesogens based on the nonlinear 2,5-bis(p-hydroxyphenyl)-1,3,4-oxadiazole (ODBP) unit enable a consistent interpretation of the supramolecular structure in this class of liquid crystals. Strong evidence for cybotaxis in the high temperature phase explains the small-angle four-spot pattern and calls into question prior XRD interpretations. We find that the data can be satisfactorily explained by skewed cybotaxis, a stratified arrangement of tilted, bent-core mesogens (BCMs). We also observe the temperature-induced evolution of skewed cybotaxis to normal cybotaxis—strata wherein the long axes of the BCMs are normal to the layer fluctuations. Our XRD interpretation is compatible with the NMR data that exhibit biaxiality in the nematic phase of ODBP mesogens.

Low nematic onset temperatures and room temperature cybotactic behavior in 1,3,4-oxadiazole-based bent-core mesogens possessing lateral methyl groups

As part of our efforts to access the biaxial nematic phase at low temperatures, we prepared a series of 1,3,4-oxadiazole-based bent-core mesogens that possess either one or three lateral methyl groups. The phase behavior of these derivatives was characterized using polarizing microscopy and differential scanning calorimetry. Target compounds containing one methyl group showed large nematic ranges but relatively high clearing points. The derivatives with three methyl substituents showed lower nematic onset temperatures and two of these compounds supercooled in the nematic phase to room temperature. X-ray diffraction experiments confirmed the presence of cybotactic clusters in the nematic phase, as observed in other bent-core mesogens. However, for two of these derivatives, cybotaxis persists at room temperature, due to the formation of a glassy nematic phase at low temperatures. These results suggest that these materials could be promising candidates in the search for low temperature biaxial thermotropic nematics.

Fine Tuning of Lithographic Masks through Thin Films of PS-b-PMMA with Different Molar Mass by Rapid Thermal Processing

The self-assembly of asymmetric polystyrene-b-poly(methyl methacrylate) (PS-b-PMMA) block copolymer based nanoporous thin films over a broad range of molar mass (Mn) between 39 kg·mol(-1) and 205 kg·mol(-1) is obtained by means of a simple thermal treatment. In the case of standard thermal treatments, the self-assembly process of block copolymers is hindered at small Mn by thermodynamic limitations and by a large kinetic barrier at high Mn. We demonstrate that a fine tuning of the annealing parameters, performed by a Rapid Thermal Processing (RTP) machine, permits us to overcome those limitations. Cylindrical features are obtained by varying Mn and properly changing the corresponding annealing temperature, while keeping constant the annealing time (900 s), the film thickness (∼30 nm), and the PS fraction (∼0.7). The morphology, the characteristic dimensions (i.e., the pore diameter d and the pore-to-pore distance L0), and the order parameter (i.e., the lattice correlation length ξ) of the samples are analyzed by scanning electron microscopy and grazing-incidence small-angle X-ray scattering, obtaining values of d ranging between 12 and 30 nm and L0 ranging between 24 and 73 nm. The dependence of L0 as a 0.67 power law of the number of segments places these systems inside the strong segregation limit regime. The experimental results evidence the capability to tailor the self-assembly processes of block copolymers over a wide range of molecular weights by a simple thermal process, fully compatible with the stringent constraints of lithographic applications and industrial manufacturing.

The biaxial nematic phase of oxadiazole biphenol mesogens

Herein we review the attributes of the cluster biaxial nematic exhibited by bent-core mesogens derived from the oxadiazole biphenol mesogenic core. We present an array of static 2H NMR spectra as well as 2D powder spectra generated by rotating the nematic phase of directly labelled mesogens. Analysis of these motionally averaged NMR observations requires the nematic phase to have monoclinic symmetry. X-ray diffraction data, in particular the effects of electric and magnetic field effects, are also consonant with the cluster picture of this biaxial nematic phase.

Superior-Performance Polymeric Composite Materials for High-Density Optical Data Storage†

Holographic data-storage (HDS) technologies have the potential to meet the ‘‘information age’’ requirements for fast data transfer and high storage capacity. There are a variety of different optical materials that can be used to fabricate these kinds of optical memories. In particular, photosensitive polymers and inorganic photorefractive crystals can exhibit significant changes in their optical properties (in the entire sample volume in which holograms are written) when exposed to laser irradiation. Such a property is necessary to achieve high storage capacities (up to one Terabyte) and to outperform the present optical supports: compact discs (CDs), digital versatile discs (DVDs), high-definition DVDs (HD-DVDs), blue rays (BR). At present, the implementation of HDS technology is limited by the availability of suitable photosensitive materials. The main HDS requirements, in the blue-violet wavelength range (typically 405 nm), are the following: high recording sensitivity (>10 cm J ), high spatial resolution (>7000 lines/mm), easy processability, high optical quality over large areas or volumes, low shrinkage (<1%), high transparency (>80%), and long temporal stability. Photorefractive crystals are interesting materials for rewritable devices, but are characterized by low sensitivity and low storage capacity. On the contrary, photopolymers have been extensively studied because of their capability of satisfying most of the fundamental requirements of HDS systems. Moreover, they are cheap and easily processable, e.g., they can be spin-coated to form a uniform thick disk layer. However, a polymeric material that fulfils all HDS prerequisites still does not exist. For instance, polymers undergoing cationic ring-opening polymerization are usually characterized by low shrinkage, but exhibit reduced sensitivity due to their characteristic low rate of photopolymerization. On the other hand, free-radical polymerization reactions are usually faster, but characterized by much higher shrinkage values. In this work, we propose a new approach to overcome these limitations, based on the use of a new composite polymeric mixture. It consists of a combination of a multifunctional acrylate monomer (di-pentaerythritol-hydroxypenta/hexa-acrylate, DPHP/HA) and an epoxy-aromatic resin (tri-phenyl-o-methane-tri-glycidil-ether, TPMTGE). The latter, solid at room temperature, is known to be a low molecular weight glass-forming liquid. The final mixture is a highly viscous fluid that can be spin-coated over a substrate or introduced into a glass cell by capillarity, as explained in the Experimental section below. The high viscosity of the mixture and the absence of any solvent are useful features for data-storage applications, since thick polymeric films, hence high storage capacities, can be easily obtained. The DPHP/HA-TPMTGE syrup is photosensitized to blueviolet radiation (405 nm), by adding bis(2,4,6-trimethylbenzoyl)phenyl-phosphine-oxide (Irgacure 819), a very efficient photoinitiator, yielding carbon-centered and phosphorous-centered free radicals and a very high acrylate photo-polymerization rate. On the contrary, Irgacure 819 is not able to activate TPMTGE polymerization in our experimental conditions. The chemical formulae of all used compounds are in Figure 1. In this work, we also report a full characterization of the above-mentioned mixture, and discuss the physical-chemical mechanism at the basis of its optical properties. We recorded high spatial resolution ( 7400 lines/mm) reflection gratings using a standard two-beam holographic set-up operating at l1⁄4 405 nm (I1⁄4 18mW cm 2 per beam). The grating growth was monitored in real time, by observing the appearance of a notch in the transmission spectra of the sample, in correspondence of the Bragg reflection wavelength of the periodic structure. This technique allows the measurement of the full set of HDS optical parameters. Figure 2a shows the transmission spectra detected during and after irradiation for a sample containing DPHP/HA and TPMTGE in a 70:30 weight ratio. It clearly evidences a first important feature: the mixture transparency exceeds 80% at 405 nm (over 90%, if glass reflection losses are taken into account). In fact, due to the low Irgacure 819 concentration (1%), our material exhibits negligible scattering and absorption losses in the visible range, before and after polymerization, as shown in the inset of Figure 1. The concentration used is sufficient to activate a very fast photo-polymerization reaction as a result of the high photoinitiator efficiency. As a consequence, we observe the formation of a reflection grating just after an irradiation of 0.1 s (Fig. 2a). For a longer exposure, the refractive index modulation Dn and the corresponding grating diffraction efficiency h, i.e., the notch depth, increase (Fig. 2b). In this way a diffraction efficiency as high as 60% was obtained, corresponding to a refractive-index modulation Dn1⁄4 1.4 10 . This value is very significant when considering the very high spatial frequency of our reflection gratings. It was calculated using the standard expression for

Thermally induced self-assembly of cylindrical nanodomains in low molecular weight PS-b-PMMA thin films.

The phase behaviour in thin films of an asymmetric polystyrene-b-polymethylmethacrylate (PS-b-PMMA) block copolymer with a molecular weight of 39 kg mol(-1) was assessed at a wide range of temperatures and times. Cylindrical PMMA structures featuring a diameter close to 10 nm and perpendicularly oriented with respect to the substrate were obtained at 180 °C in relatively short annealing times (t ≤ 30 min) by means of a simple thermal treatment performed in a standard rapid thermal processing machine.

Electric field effect on the phase diagram of a bent-core liquid crystal

We have investigated the effects of a low frequency electric field on the thermotropic behavior of a nematogenic bent-core liquid crystal by means of X-ray diffraction. We have determined for the first time the 2D phase diagram of the bent-core mesogen over an extended temperature-field (T, E) space. The results show that the electric field strongly affects both the mesophase sequence and the transition temperatures of the sample. An extraordinary field-induced shift of the nematic–isotropic phase transition is found, which has neither any experimental counterpart in conventional calamitic LCs, nor can be explained by the presently available theories. The phase diagram points out the existence of exotic phase transitions driven by either electric field or temperature along specific paths in the (T, E) space. In particular, the isothermal field-induced transition from the NCybC to the NCybA phase represents a breakthrough in the interpretation of previous XRD experimental results. These experimental findings provide further strong support of the now widely accepted cybotactic cluster picture of the nematic phase of bent-core mesogens.

Large-area photonic structures in freestanding films

The authors report the fabrication of freestanding two-dimensional photonic crystals in organic materials. Large-area patterned structures, with submicrometric spatial resolution, have been recorded in spin-coated polymer-dispersed liquid crystals via holographic photolithography. After the removal of the liquid crystal, the polymeric film is peeled off from the substrate. Scanning electron microscope images show the high homogeneity of the structure in any spatial direction. These films can be conveniently used in a wide range of applications, from templates for high-refractive index photonic crystals to flexible photonic elements.

Distributed feedback all-organic microlaser based on holographic polymer dispersed liquid crystals

An optically pumped vertically emitting all-organic laser, based on a holographic polymer dispersed liquid crystal reflection grating operating in a distributed feedback configuration, is presented. The used experimental geometry overcomes the main drawbacks characterizing similar systems, allowing virtually infinite operations and absence of damages associated with the high energy of the optical pumping.

Nanocomposite polymeric materials for high density optical storage

We report the results of an extended investigation performed on composite polymeric materials with the aim of obtaining compounds suitable for holographic recording. In order to investigate the material properties a characterization of holographic reflection gratings at different writing wavelength (514.5, 457 and 405 nm) has been performed. The volume grating presents high diffraction efficiency (>60%), high sensitivity (>103 cm J−1) and refractive index modulation Δn≈0.01 even for writing wavelength in the blue range. We show that following a strategy of two basic components leading to phase separation during the photopolymerization process, most of the requirements for holographic data storage are achieved. The one that needs further improvement concerns long term mechanical stability.