Glass transition and water dynamics in hyaluronic acid hydrogels

Glass transition and water dynamics in hydrated hyaluronic acid (HA) hydrogels crosslinked by divinyl sulfone (DVS) were studied by differential scanning calorimetry (DSC), dielectric relaxation spectroscopy (DRS) and water sorption-desorption (ESI) measurements. A critical water fraction of about h (w) = 0.17 (g of water per g of hydrated HA) for a change in the hydration properties of the material was estimated. Water crystallization was recorded by DSC during cooling and heating for water fraction values h (w) a parts per thousand yenaEuro parts per thousand 0.31. The glass transition of the hydrated system was recorded in the water fraction region 0.06 a parts per thousand currency signaEuro parts per thousand h (w) a parts per thousand currency signaEuro parts per thousand 0.59. The T (g) was found to decrease with increasing hydration level, starting from T (g) = -48 A degrees C down to about T (g) = -80 A degrees C and then to stabilize there, for the hydration levels where water crystallization occurs, suggesting that the origin of the glass transition is the combined motion of uncrystallized water molecules attached to primary hydration sites and segments of the HA chains. DRS studies revealed two relaxation peaks, associated with the main secondary relaxation process of uncrystallized water molecules (UCW) triggering the mobility of polar groups and the segmental mobility of HA chains (alpha relaxation). The alpha relaxation was in good agreement with the results by DSC. A qualitative change in the dynamics of the alpha relaxation was found for h (w) = 0.23 and was attributed to a reorganization of water in the material due to structural changes. 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Synthesis and Study of Fully Biodegradable Composites Based on Poly(butylene succinate) and Biochar

Biodegradable polymers offer a promising alternative to the global plastic problems and especially in the last decade, to the microplastics problems. For the first time, samples of poly(butylene succinate) (PBSu) biocomposites containing 1, 2.5, and 5 wt% biochar (BC) were prepared by in situ polymerization via the two-stage melt polycondensation procedure. BC was used as a filler for the PBSu to improve its mechanical properties, thermal transitions, and biodegradability. The structure of the synthesized polymers was examined by 1H and 13C nuclear magnetic resonance (NMR) and X-Ray diffraction (XRD) along with an estimation of the molecular weights, while differential scanning calorimetry (DSC) and light flash analysis (LFA) were also employed to record the thermal transitions and evaluate the thermal conductivity, respectively. It was found that the amount of BC does not affect the molecular weight of PBSu biocomposites. The fine dispersion of BC, as well as the increase in BC content in the polymeric matrix, significantly improves the tensile and impact strengths. The DSC analysis results showed that BC facilitates the crystallization of PBSu biocomposites. Due to the latter, a mild and systematic increase in thermal diffusivity and conductivity was recorded indicating that BC is a conductive material. The molecular mobility of PBSu, local and segmental, does not change significantly in the biocomposites, whereas the BC seems to cause an increase in the overall dielectric permittivity. Finally, it was found that the enzymatic hydrolysis degradation rate of biocomposites increased with the increasing BC content

Polyethylene Glycol-Modified Poly(Styrene-co-Ethylene/Butylene-co-Styrene)/Carbon Nanotubes Composite for Humidity Sensing

Polymeric composites of the linear triblock copolymer poly(styrene-co-ethylene/butylene-co-styrene) grafted with maleic anhydride units (SEBS-MA) or MA modified by hydrophilic polyethylene glycol (PEG) and containing various amounts of multiwall carbon nanotubes (MWCNTs) as conducting filler—were prepared by solvent casting. The MWCNT surface was modified by a non-covalent approach with a pyrene-based surfactant to achieve a homogeneous dispersion of the conducting filler within the polymeric matrix. The dispersion of the unmodified and surfactant-modified MWCNTs within the elastomeric SEBS-MA and SEBS-MA-PEG matrices was characterized by studying the morphology by TEM and SAXS. Dynamical mechanical analysis was used to evaluate the interaction between the MWCNTs and copolymer matrix. The electrical conductivity of the prepared composites was measured by dielectric relaxation spectroscopy, and the percolation threshold was calculated. The prepared elastomeric composites were characterized and studied as humidity sensor. Our results demonstrated that at MWCNTs concentration slightly above the percolation threshold could result in large signal changes. In our system, good results were obtained for MWCNT loading of 2 wt% and an ~0.1 mm thin composite film. The thickness of the tested elastomeric composites and the source current appear to be very important factors that influence the sensing performance

The Zero Lower Bound and Market Spillovers: Evidence from the G7 and Norway

This paper investigates mean and volatility spillovers between the crude oil market and three financial markets, namely the debt, stock, and foreign exchange markets, while providing international evidence from each of the seven major advanced economies (G7), and the small open oil-exporting economy of Norway. Using monthly data for the period from May 1987 to March 2016, and a four-variable VARMA-GARCH model with a BEKK variance specification, we find significant spillovers and interactions among the markets, but also absence of a hierarchy of influence from one specific market to the others. We further incorporate a structural break to examine the possible effects of the prolonged episode of zero lower bound in the aftermath of the global financial crisis, and provide evidence of strengthened linkages from all the eight international economies

Electricity Prices, Large-Scale Renewable Integration, and Policy Implications

This paper investigates the effects of intermittent solar and wind power generation on electricity price formation in Germany. We use daily data from 2010 to 2015, a period with profound modifications in the German electricity market, the most notable being the rapid integration of photovoltaic and wind power sources, as well as the phasing out of nuclear energy. In the context of a GARCH-in-Mean model, we show that both solar and wind power Granger cause electricity prices, that solar power generation reduces the volatility of electricity prices by scaling down the use of peak-load power plants, and that wind power generation increases the volatility of electricity prices by challenging electricity market exibility

Optimal portfolio from securities of the index FTSE/XAA20 of the Athens stock-exchange in the years 1997-1999

Περίληψη Σε αυτή την εργασία υπολογίζεται ένα χαρτοφυλάκιο οκτώ μετοχών από τις είκοσι που μετέχουν στον δείκτη FTSE/ΧΑΑ20 του Χρηματιστηρίου Αθηνών για τα έτη 1997-1999. Το Χαρτοφυλάκιο είναι βέλτιστο με το κριτήριο του Roy ,της μέγιστης πιθανότητας να έχει απόδοση μεγαλύτερη από την βασική επένδυση χωρίς κίνδυνο (ρίσκο) δηλ την απλή κατάθεση σε τράπεζα ή την επένδυση σε γραμμάτια δημοσίου

Effects of Hydration/Dehydration on Interfacial Polymer Fraction and Dynamics in Nanocomposites Based on Metal–Oxides and Physically Adsorbed Polymer

Effects of hydration on the interfacial dynamics of polydimethylsiloxane (PDMS) adsorbed on silica and titania nanoparticles in the form of aggregates of different specific surface area (<i>S</i>_BET, 25–342 m²/g) were studied employing equilibrium water sorption/desorption isotherms (ESI/EDI) and broad-band dielectric spectroscopy (BDS). ESI revealed a systematic increase of the water content, <i>h</i>, of initial oxides (varying up to 60 wt %) with increasing <i>S</i>_BET, whereas changes in <i>h</i> in nanocomposites (PNCs) are very weak (0.1–0.3 wt %) and practically independent from <i>S</i>_BET. BDS allowed for the detection of an increase in the interfacial polymer fraction with hydration, accompanied by a significant enhancement of dynamics and cooperativity of the corresponding segmental relaxation in the interfacial layer (α_int relaxation), changes being reversible as recorded by hydration and subsequent dehydration of the samples. At the same time, bulk PDMS dynamics in PNCs (α relaxation) was not significantly affected. Effects of hydration on the characteristics of α_int in PNCs were found to be qualitatively similar here with changes imposed by thermal annealing (resulting in increasing crystallinity) and surface modifications (resulting in changes in <i>S</i>_BET) recorded recently in similar PNCs. Hydration effects were additionally followed via monitoring the dielectric relaxation process attributed to hydrated hydroxyls at the surfaces of the oxides (<i>S</i> relaxation) in the PNCs. The results may suggest that with increasing <i>h</i> the concentration of contact points between the solid surface (oxides) and the polymer chains increases, while the polymer chain conformations at the interface change toward an increase in looplike interfacial polymer segments

Fabrication of poly(ethylene furanoate)/silver and titanium dioxide nanocomposites with improved thermal and antimicrobial properties

Poly(ethylene furanoate) (PEF)-based nanocomposites were fabricated with silver (Ag) and titanium dioxide (TiO2) nanoparticles by the in-situ polymerization method. The importance of this research work is to extend the usage of PEF-based nanocomposites with improved material properties. The PEF-Ag and PEF-TiO2 nanocomposites showed a significant improvement in color concentration, as determined by the color colorimeter. Scanning electron microscopy (SEM) photographs revealed the appearance of small aggregates on the surface of nanocomposites. According to crystallinity investigations, neat PEF and nanocomposites exhibit crystalline fraction between 0–6%, whereas annealed samples showed a degree of crystallinity value above 25%. Combining the structural and molecular dynamics observations from broadband dielectric spectroscopy (BDS) measurements found strong interactions between polymer chains and nanoparticles. Contact angle results exhibited a decrease in the wetting angle of nanocomposites compared to neat PEF. Finally, antimicrobial studies have been conducted, reporting a significant rise in inhibition of over 15% for both nanocomposite films against gram-positive and gram-negative bacteria. From the overall results, the synthesized PEF-based nanocomposites with enhanced thermal and antimicrobial properties may be optimized and utilized for the secondary packaging (unintended food-contact) materials