A. Kyritsis

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.

Poly(methyl acrylate)/poly(hydroxyethyl acrylate) sequential interpenetrating polymer networks. Miscibility and water sorption behavior

Sequential poly(methyl acrylate)/poly(hydroxyethyl acrylate) interpenetrating polymer networks with different poly(hydroxyethyl acrylate) contents were prepared by free radical polymerization of hydroxyethyl acrylate inside the previously polymerized poly(methyl acrylate) network. Differential scanning calorimetry on dry samples shows that the interpenetrating polymer networks exhibit phase separation, and no differences are found between the glass transition temperatures of the two phases present in the interpenetrating polymer network and those of the pure components. Thermally stimulated depolarization current experiments were used to study the influence of water sorption on the mobility of the different molecular groups in the poly(hydroxyethyl acrylate) phase of the interpenetrating polymer network. Isothermal water sorption of the interpenetrating polymer networks and pure poly(methyl acrylate) and poly(hydroxyethyl acrylate) networks is analyzed with different theories to compare the behavior of the poly(hydroxyethyl acrylate) phase in the interpenetrating polymer networks with that of the pure poly(hydroxyethyl acrylate) network. Diffusion coefficients of water in the interpenetrating polymer networks are obtained by means of dynamic sorption experiments.

Protein and Water Dynamics in Bovine Serum Albumin–Water Mixtures over Wide Ranges of Composition

Dielectric dynamic behavior of bovine serum albumin (BSA)-water mixtures over wide ranges of water fractions, from dry protein until 40 wt % in water, was studied through dielectric relaxation spectroscopy (DRS). The α relaxation associated with the glass transition of the hydrated system was identified. The evolution of the low temperature dielectric relaxation of small polar groups of the protein surface with hydration level results in the enhancement of dielectric response and the decrease of relaxation times, until a critical water fraction, which corresponds to the percolation threshold for protonic conductivity. For water fractions higher than the critical one, the position of the secondary ν relaxation of water saturates in the Arrhenius diagram, while contributions originating from water molecules in excess (uncrystallized water or ice) follow separate relaxation modes slower than the ν relaxation.

Dielectric studies of dipolar relaxation processes in nylon 11

The dielectric properties of semicrystalline nylon 11 (mixture of γ and α phase, degree of crystallinity 62%) were investigated in wide ranges of frequency and temperature by broadband dielectric relaxation spectroscopy, thermally stimulated depolarization current techniques and triangular voltage measurements. The main interest was focused on characterizing the relaxation processes in nylon 11, so measurements were, in general, limited to temperatures lower than about 100 °C. The secondary γ and β relaxations, in order of increasing temperature in isochronal measurements, the primary α relaxation, associated with the glass transition of the amorphous polymer, and a process at about 95 °C, related to a structural phase transformation from triclinic α phase to pseudo-hexagonal γ phase, were observed and studied in detail. Absorption of water was found to have a significant effect on the secondary relaxations, giving, in particular, rise to splitting of the β relaxation into two relaxations, β1 and β2 in order of decreasing temperature in isochronal measurements. The γ relaxation is assigned to the motion of methylene sequences involving adjacent dipolar amide groups and the β relaxation to motion of water-polymer complexes of different molecular configurations.

Glass transition and dynamics in BSA–water mixtures over wide ranges of composition studied by thermal and dielectric techniques

Protein-water dynamics in mixtures of water and a globular protein, bovine serum albumin (BSA), was studied over wide ranges of composition, in the form of solutions or hydrated solid pellets, by differential scanning calorimetry (DSC), thermally stimulated depolarization current technique (TSDC) and dielectric relaxation spectroscopy (DRS). Additionally, water equilibrium sorption isotherm (ESI) measurements were performed at room temperature. The crystallization and melting events were studied by DSC and the amount of uncrystallized water was calculated by the enthalpy of melting during heating. The glass transition of the system was detected by DSC for water contents higher than the critical water content corresponding to the formation of the first sorption layer of water molecules directly bound to primary hydration sites, namely 0.073 (grams of water per grams of dry protein), estimated by ESI. A strong plasticization of the T(g) was observed by DSC for hydration levels lower than those necessary for crystallization of water during cooling, i.e. lower than about 0.3 (grams of water per grams of hydrated protein) followed by a stabilization of T(g) at about -80°C for higher water contents. The α relaxation associated with the glass transition was also observed in dielectric measurements. In TSDC a microphase separation could be detected resulting in double T(g) for some hydration levels. A dielectric relaxation of small polar groups of the protein plasticized by water, overlapped by relaxations of uncrystallized water molecules, and a separate relaxation of water in the crystallized water phase (bulk ice crystals) were also recorded.

Chitosan-silica hybrid porous membranes

Chitosan-silica porous hybrids were prepared by a novel strategy in order to improve the mechanical properties of chitosan (CHT) in the hydrogel state. The inorganic silica phase was introduced by sol-gel reactions in acidic medium inside the pores of already prepared porous scaffolds. In order to make the scaffolds insoluble in acidic media chitosan was cross-linked by genipin (GEN) with an optimum GEN concentration of 3.2 wt.%. Sol-gel reactions took place with Tetraethylorthosilicate (TEOS) and 3-glycidoxypropyltrimethoxysilane (GPTMS) acting as silica precursors. GPTMS served also as a coupling agent between the free amino groups of chitosan and the silica network. The morphology study of the composite revealed that the silica phase appears as a layer covering the chitosan membrane pore walls. The mechanical properties of the hybrids were characterized by means of compressive stress-strain measurements. By immersion in water the hybrids exhibit an increase in elastic modulus up to two orders of magnitude.

Dielectric studies of molecular mobility and microphase separation in segmented polyurethanes

Abstract This work deals with molecular mobility and microphase separation studies on segmented polyurethanes with different fragments, including crown ethers, as chain extenders and/or chain end groups. The techniques used include thermally stimulated depolarization current (TSDC) in the temperature range −185°–30°C and broadband dielectric relaxation spectroscopy (DRS) in the frequency and temperature ranges of 10 −2 –10 6 Hz and −55°–80°C. The combination of these techniques allowed the secondary γ and β mechanisms, the primary α mechanism, the Maxwell–Wagner–Sillars (MWS) mechanism associated with interfacial polarization and dc conductivity σ dc to be recorded. The results suggest that addition of crown ethers promotes microphase separation. Specific characteristics of the primary α and the MWS relaxations and σ dc were found to systematically change with the degree of microphase separation. They should be further quantified and tested on selected systems.

Glass transition in 2- and 3-dimensionally confined liquids

Abstract Broadband dielectric relaxation spectroscopy and thermally stimulated depolarization currents (TSDC) measurements were employed to investigate effects of confinement on the glass transition of the hydrogen bonded liquid propylene glycol (PG) and the non-associating liquid N-methyl-e-caprolactam (NMEC). The liquids were confined 2-dimensionally in the pores of porous glasses with mean pore diameter 2.5⩽d⩽20.0 nm and 3-dimensionally confined in butyl rubber with mean droplet diameter 7⩽d⩽11 nm. The data provide evidence for both the cooperativity concept and the existence of two states (interfacial layer and liquid). With decreasing d the α relaxation associated with the glass transition of the liquid becomes faster and broader and the glass transition temperature decreases. These effects are larger for 3- than for 2-dimensional confinement. The cooperativity length, ξ, at Tg is determined to ξ⩽6 nm for PG and ξ⩽12 nm for NMEC.

Water sorption and water-induced molecular mobility in dental composite resins

Water sorption in two resin composites, Kulzers Solitaire (S) and SDIs Wave (W), and in a polyacid-modified composite resin, 3Ms F2000 (compomer F), was investigated by means of equilibrium sorption isotherms (ESI) and of dynamic sorption (DS) measurements. Molecular mobility in these materials was studied by means of dielectric relaxation spectroscopy (DRS) and of thermally stimulated depolarization currents (TSDC) measurements. The results of ESI measurements show that at equilibrium, water is molecularly distributed in the materials and the effects of hydrophilic sites and clustering are negligible. Hysteresis effects in sorption–desorption cycles are larger in the resin composites than in the compomer. Equilibrium water uptakes in both ESI and DS conditions are rather low, in the range 1–2%. Diffusion coefficients of water are about 1×10−8 cm2/s in the resin composites and by a factor of about 2 smaller in the compomer. Molecular mobility increases with hydration, as suggested by preliminary DRS and TSDC measurements. Detailed dielectric measurements may give important information for understanding, at the molecular level, water-induced degradation in dental materials.

Characterization of thermoplastic interpenetrating polymer networks by various thermal analysis techniques

Thermoplastic interpenetrating polymer networks (t-IPNs), prepared by melting and pressing of crystallizable polyurethane (CPU) and styrene/acrylic acid random copolymer (S/AA) in wide ranges of composition, were investigated by the combination of various thermal analysis techniques: differential scanning calorimetry (DSC), thermomechanical analysis (TMA), thermally stimulated depolarization currents (TSDC) and thermally stimulated conductivity (TSC) measurements, as well as broadband dielectric relaxation spectroscopy (DRS). The results show that the t-IPNs under investigation are microheterogeneous systems with contributions to microheterogeneity from both the heterogeneity of the individual polymers and the thermodynamic incompatibility of the components.