Symmetric Bent-core Liquid Crystals of Some Schiff Bases Containing Azo Linkage

The paper presents the synthesis and liquid crystalline properties of some new symmetric bent-core liquid crystals containing a 1,3-dihydroxybenzene central core disubstituted with mesogenic units consisting of three aromatic rings connected through esteric, azomethinic and azo groups. The wings are ended with terminal acyloxy or alkyloxy chains containing 6 to 10 carbon atoms. Compounds were ob¬tained by the condensation reactions between some 4-((4-alcanoyloxyphenyl)azo)benzaldehydes or 4-((4-alkyloxyphenyl)azo)benzaldehydes with the 1,3-phenylene bis(4-aminobenzoate) core. The new synthe-sized compounds were characterized from structural point of view (1H NMR, 13C NMR, MS spectrosco¬py) and liquid crystalline properties (polarizing optical microscopy and differential scanning calorimetry). All the banana shaped compounds presented enantiotropic liquid crystalline properties, with a wide-ranging stability of mesophases both on heating and cooling cycles. The synthesized compounds present¬ed a good thermal stability in the existence range of the mesophases as evidenced by thermogravimetric studies. (doi: 10.5562/cca2150

Bent-Core Liquid Crystals: Structures and Mesomorphic Properties

Bent-core (BC) molecules became an attractive liquid crystal class due to their potential use in smart displays and photonic devices. In contrast to calamitic mesogens, bent-shaped mesogens are self-organized superstructures with remarkable properties, given the presence of polar order in mesophases, although the molecules themselves are not chiral. A particular interest represents the biaxial nematic liquid crystal materials that are used in display technology and allow a faster switching response, compared to calamitic liquid crystals, with considerably reduced costs. This chapter briefly reviews the bent-core liquid crystals with three different core units in the structure: (1) 2,5-disubstituted oxadiazole, (2) 1,3-disubstituted benzene, and (3) 2,7-disubstituted naphthalene. To the central bent units (BUs) containing reactive functional groups of phenolic or aminic type, various mesogenic groups are symmetrically or asymmetrically connected, via esterification or condensation reactions. The obtained compounds showed biaxial nematic and/or smectic mesophases with high transition temperatures in the case of oxadiazole derivatives or cholesteric and banana-type mesophases with lower transition temperatures in the case of benzene and naphthalene derivatives

Effects of (complementary) polyelectrolytes characteristics on composite calcium carbonate microparticles properties

This study follows the possibility to tune the thermal stability of some CaCO3/polymer composites by crystal growth from supersaturated solutions controlled by polymer structure or by using nonstoichiometric polyelectrolyte complexes (NPECs). As the ratio between the organic and inorganic parts in the composites controls the Ca2+/polymer network crosslinking density, the CaCO3/polymer weight ratio was kept constant at 50/1, varying the initial concentration of the polyanions solutions (0.05 or 0.06 wt.%), the NPECs molar ratio , n+/n- (0.2 or 0.4), or the inorganic precursors concentration (0.25 or 0.3 M). Poly(2-acrylamido-2-methylpropanesulfonic acid-co-acrylic acid) (PSA) and chondroitin-4-sulfate (CSA) were used as polyanions. Some NPEC dispersions, prepared with the same polyanions and poly(allylamine hydrochloride) (PAH), were also used for calcium carbonate crystallization. The characteristics of the prepared composites were investigated by scanning electron microscopy (SEM), flow particle image analysis (FPIA), particles charge density (CD), zeta-potential (ZP). The thermal stability of the composite particles was investigated as compared to bare CaCO3 microparticles prepared at the same initial inorganic concentrations

Synthesis of poly(aspartimide)-based bio-glycoconjugates

The purpose of this programme was to synthesize and analyze new bioconjugates of interest for the potential inhibition of the influenza virus, using poly(aspartimide) as a polymer support. The macromolecular targets were obtained by attaching various sialic acid-linker-amine compounds to poly(aspartimide). 1H and 13C NMR studies were then performed to analyze the degree of incorporation of the sialic acid-linker-amine compounds within the poly(aspartimide). These studies illustrated that the incorporation was dependent on the nature of the spacer between the sugar and the amine functionality. Thus aliphatic spacers favoured the inclusion of sialic acid onto the polymer support whereas compounds having only an aromatic moiety between the sialic acid and the amine could not be easily incorporated

ATRP grafting of styrene from benzyl chloride functionalized polysiloxanes: An AFM and TGA study of the Cu(0)/bpy catalyst

Various combinations of Cu(0), CuCl, 2,2'-bipyridine (bpy) and 1,10-phenanthroline (phen) were used as catalysts for the grafting polymerizations of styrene from polysiloxane macroinitiators functionalized with benzyl chloride. While Cu(0)/bpy alone promotes the grafting, narrower polydispersities were obtained in the presence of CuCl. Analysis of the Cu(0) surface before and after polymerization by a combination of AFM, TGA and FTIR investigations reveals the formation of bpy or phen films on Cu(0). In the presence of CuCl, the ligand film appears decorated with CuCl particles which increase in size with increasing the CuCl concentration. The initial layer occurs most likely as a result of complexation between the ligands and the Cu(0) surface and acts as a support for the rest of the film. These observations are consistent with the film formation on Cu(0) from related nitrogen donors and indicate that the reactivity of the Cu surface may depend not only on its prior treatment but also on the deposition of ligands from the reaction Mixture. (c) 2005 Elsevier Ltd. All rights reserved

Unsymmetrical banana-shaped liquid crystalline compounds 1 derived from 2,7-dihydroxynaphtalene

The synthesis and characterization of new bent-core asymmetric compounds derived from 2,7-dihydroxynaphtalene with various connecting groups between the aromatic rings and alkyloxy terminal substituents at the end of the long arm are presented. As calamitic promesogenic units some 1,4 - disubstituted phenylene rings with azo or ester linkage between them have been used. The synthetic strategies to obtain the final esteric derivatives involved the esterification of 7-(benzyloxy)naphthalen-2-ol with 4-(4-alkyloxyphenylazo)benzoyl chlorides or with 4-((4- (alkyloxy)benzoyl)oxy)benzoic acids in the presence of DCCI and DMAP. The mesomorphic properties have been assigned by optical polarizing microscopy and differential scanning calorimetry. All the compounds showed mesomorphic properties of enantiotropic or monotropic type, the liquid crystalline behavior depending on the linking group between the phenylene rings. Thermogravimetric studies evidenced that all compounds were stable in the range of the existence of mesophases

Direct observation of athermal photofluidisation in azo-polymer films

The surface relief gratings (SRGs) can be generated when azo-polymer films are exposed to laser beam interference as a result of mass migration. Despite considerable research effort over the past two decades this complex phenomenon remains incompletely understood. Here we show, for the first time, the athermal photofluidisation of azo-polysiloxane films exposed to 488 nm light, directly monitored by optical microscopy. A process of surface relief erasure occurring in parallel with its inscription was also observed during laser irradiation. We therefore propose a new mechanism of SRG formation, based on three different processes: (1) the polymer photo-fluidization in illuminated regions, (2) the mass displacement from illuminated to dark regions and (3) the inverse mass displacement, from dark to illuminated regions. The mechanical properties of the films during UV light irradiation were investigated by classical rheology and, for the first time, by using amplitude modulation-frequency modulation atomic force microscopy (AM-FM AFM)

TECNICHE ANALITICHE COMBINATE PER LO STUDIO DELLA STABILIT\uc0 DEI VINI BIANCHI

I vini bianchi sono estremamente sensibili ad alterazioni del colore, spesso accompagnate dalla comparsa di difetti sensoriali. La ricerca si \ue8 concentrata dapprima sulla valutazione delle caratteristiche sensoriali ed eventuali percezioni di off-flavour in grado da individuare il trattamento luminoso pi\uf9 invasivo, dopodich\ue9 sono stati acquisiti gli spettri (UV-Vis) con studio della derivata seconda e analizzate diverse combinazioni di stress chimici e luminosi su diverse tipologie di vino bianco. Sono stati testati vini con diversi livelli di stabilizzazione tecnologica allo scopo di individuare il test in grado di stimare al meglio la potenziale instabilit\ue0 del vino in funzione della sua shelf-life. Sono stati valutati singolarmente e in combinazione: stress chimici con perossido di idrogeno e acido ascorbico, stress luminosi nel visibile e tempistica di esposizione. Dalle esperienze di laboratorio l\u2019esposizione alle luci bianca e azzurra si \ue8 dimostrata la pi\uf9 efficace nei confronti dell\u2019instabilit\ue0 fenolica. In ogni caso le risposte delle singole luci o loro combinazioni sono correlate alla tecnica di vinificazione/stabilizzazione. Prove successive hanno mostrato che la combinazione di stress luminosi e chimici evidenzia rischi di instabilit\ue0 non rilevabili con le metodiche tradizionali. Informazioni complementari e interessanti sono state ottenute con l\u2019analisi della derivata seconda degli spettri grazie all\u2019interdipendenza d2A/d\u3bb2. \uc8 stata evidenziata l\u2019utilit\ue0 dei diversi approcci analitici per la valutazione dell\u2019instabilit\ue0 dei vini bianchi che dovr\ue0 essere studiata in funzione delle tecniche di vinificazione adottate e della futura shelf-life. La finalit\ue0 della presente ricerca \ue8 approfondire gli studi per cercare di elaborare un ulteriore test in grado di stimare con pi\uf9 accuratezza il reale rischio di instabilit\ue0 del vino bianco