Anna Spanoudaki

Relaxation processes of water in the liquid to glassy states of water mixtures studied by broadband dielectric spectroscopy

The relaxation processes of water mixtures of glycerol, ethylene glycol, ethylene glycol oligomers with two to six repeat units, poly(ethylene glycol) 400 and 600, fructose, and propanol have been studied by broadband dielectric spectroscopy at different water contents in the frequency range 10 µHz–20 GHz and in the temperature range 300–80 K without water crystallization. The results show that, in the vicinity of the glass transition temperature of the mixtures, two kinds of water exist. Part of the water behaves as excess water retaining its inherent mobility and appearing as a separate relaxation process (named here the ν-process) at frequencies higher than the structural α-process at subzero temperatures. Another part of the water moves cooperatively with solute molecules and contributes to the α-process.

Relaxation processes at the glass transition in polyamide 11: From rigidity to viscoelasticity

Relaxation processes associated with the glass transition in nonferroelectric and ferroelectric polyamide (PA) 11 are investigated by means of differential scanning calorimetry, dynamic mechanical analysis, and dielectric relaxation spectroscopy (DRS) in order to obtain information about the molecular mobility within the amorphous phase. In particular, the effects of melt quenching, cold drawing, and annealing just below the melting region are studied with respect to potential possibilities and limitations for improving the piezoelectric and pyroelectric properties of PA 11. A relaxation map is obtained from DRS that shows especially the crossover region where the cooperative alpha relaxation and the local beta relaxation merge into a single high-temperature process. No fundamental difference between quenched, cold-drawn, and annealed films is found, though in the cold-drawn (ferroelectric) film the alpha relaxation is suppressed and slowed down, but it is at least partly recovered by subsequent annealing. It is concluded that there exists an amorphous phase in all structures, even in the cold-drawn film. The amorphous phase can be more rigid or more viscoelastic depending on preparation. Cold drawing not only leads to crystallization in a ferroelectric form but also to higher rigidity of the remaining amorphous phase. Annealing just below the melting region after cold drawing causes a stronger phase separation between the crystalline phase and a more viscoelastic amorphous phase.

Nanostructured and nanocomposite hydrogels for biomedical applications

Polymer hydrogels are interesting for biomedical applications due to their biocompatibility and good water sorption/diffusion properties. In previous work their mechanical stability could be improved by combination with a hydrophobic polymer. Instead, here we prepare poly(hydroxyethyl acrylate), PHEA/silica nanocomposites by sol-gel techniques as potential scaffold materials for tissue engineering. Their morphology and properties were investigated by scanning electron microscopy, thermogravimetry, dynamic mechanical analysis, water sorption/desorption, dielectric spectroscopy, and thermally stimulated depolarization currents techniques at various levels of relative humidity/water content. The results show that the nanoparticles are homogeneously distributed, form a continuous phase, and significantly reinforce the matrix, without affecting its hydrophilicity and, thus, biocompatibility. Water is more homogeneously distributed, as compared to pure PHEA matrix, forming smaller clusters. The glass transition temperature is controlled by water content, and less affected by the concentration of silica.

Dielectric Spectroscopy on Propylene Glycol/Poly(Vinyl Pyrrolidone) Solutions: Polymer and Solvent Dynamics in Hydrogen‐Bonding Systems

The dielectric behavior of solvent‐rich solutions of poly(vinyl pyrrolidone) in propylene glycol has been studied through broadband dielectric spectroscopy. Measurements covered a broad temperature range both in the liquid and the glassy state of the samples and concentrations from 0% to 60% wt. of poly(vinyl pyrrolidone). Two distinct glass transitions are observed dielectrically, evidencing decoupling of the segmental motion of PVP and the rotation of the propylene glycol molecules. The decoupling appears not to be full, since the dielectric strength data hint to a participation of propylene glycol molecules in the relaxation connected with the glass transition attributed to poly(vinyl pyrrolidone). Interactions between the two components are expressed through the concentration dependency of the relaxation times, as well as through the marked broadening of the propylene glycol relaxation.