Search

Dielectric spectroscopy is employed to study the relaxation phenomena in natural polyhydroxyalkanoates (PHAs) upon temperature and frequency variations. Effects of PHAs molecular structure on the relaxation, arising from the differences in monomeric composition, are investigated under identical conditions in a frequency range of 10−2–106 Hz, and at different temperatures. All PHA samples showed different dielectric response at different temperature. Primary α -relaxation signals are observed at temperature corresponding to the glass transition temperature. On the other hand, secondary β - and γ -relaxations are detected at low temperatures, and attributed to local motions of polar groups and small segments of the polymer chain. The dielectric properties of representative PHA samples are compared and discussed. [ABSTRACT FROM AUTHOR]

[Display omitted] • Thermotropic phase behaviours of α -D-mannosides containing nonsymmetrical Guerbet branched chains at sub-zero temperatures. • Occurrence of rippled structures implies the competition between hydrophobic volume and headgroup surface packing. • Temperature-dependent changes in the primary relaxation time were investigated through distortion-sensitive analyses. • Shear viscosity test indicates all liquid crystal phases exhibits shear thinning behaviour. Many aspects govern the nature of the resulting phase of a self-assembly of glycolipid, including its detailed stereochemical structure, solvent type, and state condition. Glycolipid has attracted considerable attention due to its extensive lyotropic applications in surfactant industry and material science. However, its application as thermotropic liquid crystal is unknown and rarely investigated. Herein, the thermotropic properties of a series of glycolipids, namely Guerbet branched chain α -D-mannosides (C 8 to C 24 total carbons) were studied by X-ray scattering, dielectric spectroscopy, and rheology. The shortest chain α ManC 6 C 2 exhibited lamellar phase over the entire temperature range whereas both α ManC 8 C 4 and α ManC 10 C 6 only at elevated temperatures since these have larger hydrophobic volumes. Interestingly, at the room temperature, both anhydrous α ManC 8 C 4 and α ManC 10 C 6 showed formation of rippled structures. Prior to transforming into the fluid lamellar phase, these complex structures possess greater viscosity than the former. The longer chain mannosides (α ManC 12 C 8 and α ManC 14 C 10) adopted an inverse bicontinuous Ia 3 d cubic and inverse hexagonal phases, respectively. The temperature-dependent evolution of dielectric relaxation times, τ (T) of primary relaxation within the lamellar, hexagonal, and isotropic phases is explored. Distortion-sensitive tests, enabled by derivative-based analysis, evaluate the suitability of τ (T) parametrisation using the Vogel-Fulcher-Tammann (VFT) and critical-like equations. According to the dielectric and rheological analyses, as the temperature increases, both ε ‖ a n d ε ⊥ increased, while the viscosity decreased. The findings suggest that higher temperatures are accountable for higher molecular mobility and fluidisation of the phase structure. These fundamental investigations are important to the bottom-up approach development of regulated and specially designed nanoscale material (e.g., a cryoprotective agent). [ABSTRACT FROM AUTHOR]

Medium-chain-length poly-3-hydroxyalkanoates (mcl-PHAs) with varied monomeric compositions were biosynthesized by producer bacteria fed with different fatty acids as carbon source. Octanoic-, lauric-, stearic-, and oleic acids were used to produce four types of mcl-PHAs viz. PHA-OC, PHA-LA, PHA-ST, and PHA-OL, respectively. The mcl-PHAs as film-casted preparations exhibit distinct traits e.g., PHA-OC and PHA-ST films are less flexible than PHA-LA while PHA-OL is a sticky, glue-like material; PHA-ST is opaque whereas PHA-OC, PHA-LA, and PHA-OL displayed transparent layers. The observation is attributed to polymer chain packing and side chain crystallization. A structure-property investigation of these biopolymers was carried out employing different spectroscopic and microscopic analyses in addition to thermal analyses. Comparative analyses of the results were applied in the interpretation and discussion of structure-property relationship. [ABSTRACT FROM AUTHOR]

• Exhaustive photochromic, dielectric and electrochemical analysis of an azobenzene-based organic compound, tAZOi. • tAZOi assembles into molecular dimers by hydrogen-bonding between thymine heads. • An ITO/tAZOi/Al device presents two switchable conductance states with preservation of memory performance. • Conductivity in the transition between ground and excited states is p-type, involving hydrogen-bonded thymine heads. • tAZOi increases dielectric permittivity driven by trans-to-cis isomerisation of the azobenzene groups upon UV irradiation. • Molecular simulations suggest that device performance can be enhanced by light exposure. In the search to obtain new and more efficient components of memory devices, we report the photochromic, dielectric and electrochemical response of a light-responsive organic compound, and its memory performance under electrical fields. The so-called N(1)-[12-(4-(4′-isobutyloxyphenyldiazo)phenoxy)dodecyloxy)]thymine, tAZOi, molecule contains one azobenzene group, which provides with photochromic character, and one terminal thymine group, capable to form hydrogen bonds and assemble supramolecular dimers, (tAZOi) 2. We have calculated the optical absorption coefficient, extinction coefficient and refractive index of tAZOi, which obeys the single oscillator Wemple–DiDomenico model. An ITO/tAZOi/Al device has been prepared and presents two switchable conductance states with preservation of memory performance. The mechanism linked to the resistive random-access memory (RRAM) has been evaluated by molecular modelling and is controlled by p-type conduction, possibly involving hydrogen-bonding. Upon UV irradiation at ∼ 365 nm, tAZOi displays an increase in the complex permittivity driven by trans -to- cis (E -to- Z) isomerisation of the azobenzene groups. Molecular simulations suggest that conductivity and device performance can be enhanced (and controlled) by light exposure through the formation of activated Z isomers that could transfer charge to other neighbouring molecules, resulting in photo-electric responsive devices. [ABSTRACT FROM AUTHOR]

Abstract The sugar anomeric effect on the thermotropic phase transition of stearyl glucoside was investigated. Standard analysis to study the phase transition through differential scanning calorimetry (DSC), and optical polarizing microscopy (OPM) was applied. In addition, small- and wide-angle X-ray scattering (SWAXS) and impedance spectroscopy (IS) were used to elucidate the phase structural information and the dielectric response of the glycolipid self-assembly system, respectively. A distinctive solid-to-liquid crystal phase transition of α - and β -stearyl glucoside was observed. At ambient temperature we observed semi-crystalline structure for the former and gel phase for the latter. α -Stearyl glucoside showed only one-step transition from semi-crystal to lamellar liquid crystal phase. In contrast, the gel phase of β -stearyl glucoside showed several distinct transition states prior to lamellar liquid crystal phase. Here the underlying reason for the observed β -stearyl glucoside polymorphism possibly involved strong intermolecular interaction originating from the combination of interlayer H-bonding and intralayer van der Waals forces. Graphical abstract Unlabelled Image Highlights • Sugar stereochemistry affects the lipid packing and hydrogen bond of the glycolipid. • Both glycolipid anomers form different solid phase structure at ambient temperature. • α -Anomer shows one step of semi-crystalline–to–liquid crystalline phase transition. • β -Anomer shows multiple steps of gel–to–liquid crystalline phase transition. [ABSTRACT FROM AUTHOR]

[Display omitted] • Azo-esters comprising of 2-methylbutoxy group attached to the terminal azo phenyl. • The lateral methyl and terminal substituents affect mesomorphism of the azo-esters. • Enantiotropic nematic mesophase was observed in the mesogenic compounds. • Optical investigations show all the mesogenic compounds exhibit fluorescent nature. • The laterally methyl-substituted derivatives exhibit antiferroelectric switching. The effect of introducing a lateral methyl substituent and the two different alkoxy terminal groups on the mesophase behavior of three-benzene-ring molecular core of azo-ester compounds bearing 2-methylbutoxy unit on the opposite terminal side (C1 – C6) were explored. Their structures were confirmed by several spectroscopic techniques (Fourier transform infrared spectroscopy, 1H and 13C nuclear magnetic resonance spectroscopy). The mesomorphic properties of these compounds were studied by differential scanning calorimetry, polarized optical microscopy, and small- and wide-angle X-ray scattering. Except for the un-substituted terminals (C1 and C4), all other derivatives are liquid crystalline, exhibiting enantiotropic nematic liquid crystal phase. The alteration of the alkoxy chain length from methoxy to butoxy at the phenyl ester terminal group (ring A) for each set of laterally-neat and laterally methyl-substituted mesogens decreases the nematic temperature range. A decrease in the mesophase temperature range is also observed when a lateral methyl substituent is incorporated into phenylazo ring of the mesogenic core (ring C). Optical study demonstrated that their absorption spectra are identical in shape due to the structural similarities of the mesogenic core. The corresponding fluorescence spectra exhibited blue-emission with the laterally methyl substituted compounds having a relatively higher fluorescence intensity compared to that of its laterally un-substituted analogues. Furthermore, the laterally methyl-substituted derivatives were subjected to the planar alignment and their dielectric and electro-optic properties were investigated. The nematic phase of the aligned samples shows antiferroelectric switching behavior with spontaneous polarization, P s ~ 5 μC/cm2. [ABSTRACT FROM AUTHOR]

• Synthesis of new side-chain liquid crystalline copolymers containing polarisable groups. • Azobenzene groups confer light-responsive character to the electrolytes. • Electro-donating and electro-withdrawing groups tailor molecular structure. • High conductivity values are reached (10–2.9 S·cm−1) under anhydrous conditions. • Ion conductivity occurs through nematic and smectic phases, decoupled from the polymer backbone. We have prepared and characterised a series of side-chain liquid crystalline homopolymers and terpolymers containing different azobenzene derivatives (R AzB), sulfonic groups (2-acrylamido-2-methyl-1-propanesulfonic acid, AMPS), and methyl(methacrylate) groups (MMA), as monomeric units. We have evaluated the effect of different para-substitutes of 6-(4-azobenzene -4′-oxy)hexyl methacrylate, at the side chains, OCH 3 , NO 2 and H, on the phase behaviour and conductivity of the new polymers. The copolymers can form smectic and nematic phases, depending on their composition, and have light responsiveness, conferred by the azobenzene chromophores. The terpolymer with methoxy terminations, MeOAzB/AMPS/MMA, exhibits the highest conductivity values of the series (10−2/10−3 S·cm−1 range) through liquid crystalline phases and with signs of decoupling from polymeric segmental motions. These materials are promising candidates to develop new light-responsive polymeric electrolytes for electrochemical conversion devices in which ionic conductivity under anhydrous conditions can be controlled by their nanostructure. [ABSTRACT FROM AUTHOR]