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]