No abstract

Core-shell structured PEO-chitosan nanofibers have been produced using a coaxial electrospinning setup. PEO and chitosan solutions, both in an aqueous acetic acid solvent, were used as the inner (core) and outer (shell) layer, respectively. Uniform-sized defect-free nanofibers of 150-190 nm diameter were produced. In addition, hollow nanofibers could be obtained subsequent to PEO washing of the membranes. The core-shell nanostructure and existence of chitosan on the shell layer were confirmed by TEM images obtained before and after washing the PEO content with water. The presence of chitosan on the surface of the composite nanofibers was further supported by XPS studies. The chitosan and PEO compositions in the nanofibrous mats were determined by TGA analysis, which were similar to their ratio in the feed solutions. The local compositional homogeneity of the membranes and the efficiency of the washing step to remove PEO were also verified by FTIR. In addition, DSC and XRD were used to characterize the crystalline structure and morphology of the co-electrospun nonwoven mats. The prepared coaxial nanofibers (hollow and solid) have several potential applications due to the presence of chitosan on their outer surfaces.

No abstract

Aqueous solutions of PEO exhibit a lower critical solution temperature (LCST) phase diagram. In this work, phase separation behavior of PEO/water solutions was investigated using small-amplitude oscillatory shear and steady shear rheological measurements. Binodal decomposition temperatures were determined from the sudden changes in the slope of the dynamic temperature sweep of storage modulus and loss tangent. The spinodal decomposition points were also estimated by a mean-field theoretical approach. Comparing the obtained critical points with other conventional methods revealed that rheological measurements are powerful and sensitive to detect even the early stage of phase separation of PEO solutions. This successful method was employed to investigate phase separation and miscibility of chitosan/PEO solutions at different compositions in aqueous acetic acid solutions that have already showed anomalous behavior in a forming process. Lower critical solution temperature (LCST) phase behavior was observed for chitosan/PEO solution blends. Phase separation temperature, miscibility range, and correlation length of the solutions were determined from isochronal dynamic temperature sweep experiments. The effect of chitosan/PEO ratio on the binodal and spinodal decomposition temperatures was studied. Finding phase separation information on polymer solutions through rheological measurement is very promising. Isothermal steady shear rheological measurements were also carried out on chitosan/PEO solutions over a temperature range in which phase separation occurs. Viscosity increase at low shear rates above but in the vicinity of phase separation temperature was observed, which confirms the validity of the theoretical approach employed to determine the critical temperatures through dynamic rheological measurements. Finally, the Flory–Huggins interaction parameters were estimated from critical solution temperature and concentration results.