Crystallization and melting of unentangled poly(ε-caprolactone) cycles containing pendants.

The Rouse model provides a basic framework to understand the chain dynamics of polymers, which is confirmed to be more suitable for exploring the linear dynamics of unentangled polymers. The crystalline morphology governed by chain dynamics and crystallization kinetics is expected to differ in linear and cyclic polymers. Cyclic poly(ε-caprolactone)s (c-PCLs) containing two bi-anthracenyl group pendants with molecular weights close to the critical molecular weight (Mc) were synthesized to investigate the chain dynamics based crystallization and melting behavior by DSC, POM, and in situ simultaneous small-angle X-ray scattering/wide-angle X-ray scattering (SAXS/WAXS) investigations during heating of the isothermally crystallized samples. Double endothermic peaks were observed in the DSC curves with a low heating rate of c-PCLs without entanglement after isothermal crystallization, especially for c-PCLs with Mc. The structure evolution of the crystalline structures observed from the in situ investigations during the heating and double endothermic peaks in DSC heating curves of the c-PCLs indicate the role of pendants in the chain dynamics, which leads to the reorganization of the metastable structures. Banded spherulites of c-PCL without entanglement were observed for the first time, and the uneven growth of spherulites along the radial direction may be caused by the mismatch between chain dynamics and crystallization kinetics.

Effects of end groups and entanglements on crystallization and melting behaviors of poly(ε-caprolactone).

The topology including end groups, entanglement loops, and tie molecules has a significant impact on the rheological and crystallization behavior and consequently on the functionality of a polymer. Unentangled, weakly entangled, and strongly entangled poly(ε-caprolactone)s (PCLs) with end groups and various molecular weights were synthesized. POM and DSC were used to observe spherulite growth and characterize thermal properties during crystallization and melting. The viscosity and structure of the samples were probed by rheology and X-ray analysis, respectively. The crossover of the scaling relationship of viscosity vs molecular weight demonstrates that the samples cover a wide range of entanglement density, and the bulky end groups cause deviations from the classical scaling laws. In situ simultaneous SAXS/WAXS investigations showed that the crystal structure of PCLs did not change with end groups and heating. The results of POM and DSC imply that the end groups and entanglements affect the crystallization rate and the spherulite morphology. The melting of PCLs containing end groups was found to be a multi-step process involving various nanoscale crystalline structures. The evolution of nanoscale crystalline structures of isothermally crystallized PCLs during heating was analyzed by fitting 1D SAXS profiles, and the continuous structural evolution was found to be a process influenced by end groups and entanglements. The results show that end groups and entanglements affect the chain dynamics and lead to constrained crystallization behavior and the formation of metastable structures, ultimately affecting the structure evolution during melting.

Anatase titania coated CNTs and sodium lignin sulfonate doped chitosan proton exchange membrane for DMFC application.

Anatase titania coated CNTs (TCNTs) and sodium lignin sulfonate (SLS) were introduced to chitosan membrane to improve the conductivity based on extra proton transfer channels built by TCNTs and sulfonate groups supplied by SLS. Water uptake, mechanical properties, oxidation stability and methanol-rejecting property of composite membranes were characterized. The results show that TCNTs and SLS doped membranes have enhanced conductivity and the sample with 5% TCNTs and 2% SLS doped (CS/TCNT-5/SLS-2) achieved a conductivity of 0.0367 S cm-1 at room temperature and 0.0647 S cm-1 at 60 °C, which is much higher than pure chitosan membrane. Moreover, with TCNTs incorporation, the mechanical properties, oxidation stability and methanol-rejecting property also improved. Overall, selectivity of CS/TCNT-5/SLS-2 sample achieved 28.2 × 104 S s cm-3 which is much higher than 3.8 × 104 S s cm-3 of pure chitosan membrane. Thus, with enhanced properties, chitosan composite membrane could be promising as proton exchange membrane (PEM) in the use of direct methanol fuel cell (DMFC).