Diffusivity enhancement of water vapor in poly(vinyl alcohol)–fumed silica nano-composite membranes: Correlation with polymer crystallinity and free-volume properties

The diffusion coefficients of water vapor in poly(vinyl alcohol)–fumed silica (PVA–FS) nano-composite membranes were determined using the gravimetric method. Water vapor was observed to diffuse more rapidly in membranes with increased FS content. The vapor diffusion coefficient was determined as 1.2 × 10⁻¹³ m²/s in pure PVA and was observed to increase to 3.0 × 10⁻¹³ m²/s in PVA composites containing 30% FS nano-particles. The free-volumes of PVA–FS membranes were characterized using positron annihilation lifetime (PAL) spectroscopy. PAL results showed that both the ortho-positronium (o-Ps) lifetime and intensity increased with the addition of FS. The intensity (I₃) was found to be higher than the estimated value determined from the linear combination of the data from pristine PVA and FS, and correlated excellently with the polymer amorphous content. The PAL results indicate that a higher FS content in PVA increases the free-volume hole size (a volume increase from 40 to 55 Å³) and free-volume hole density (an I₃ increase from 23 to 28%), resulting in a higher fractional free-volume in the nano-composites. The increase in the relative polymer free-volume with higher FS content was associated with a decrease in the PVA crystallinity, as determined from differential scanning calorimetry measurements. It is postulated that the incorporated FS nano-particles interrupt polymeric chain packing and retard crystallization during membrane formation. More crystalline segments were transformed into amorphous regimes in the nano-composites containing more FS. A correlation between water diffusivity and the fractional free-volume was obtained, and the water diffusivity was successfully expressed by the free-volume theory.     

Influence of the leprosy drug, dapsone on the model membrane dipalmitoyl phosphatidylethanolamine

The influence of the sulfone drug, diamino diphenyl sulfone (DDS or dapsone) on the phase transitions and dynamics of the model membrane, dipalmitoyl phosphatidylethanolamine (DPPE)-water/buffer has been studied using DSC and (¹H and ³¹P) NMR. These investigations were carried out with DPPE dispersion in both multilamellar vesicular (MLV) and unilamellar vesicular (ULV) forms for DDS/DPPE molar ratio, R, in the range 0-0.5. DSC results indicate that the mechanism by which DDS interacted with the DPPE membrane is independent of the morphological organization of the lipid bilayer and the solvent (water or buffer) used to form the dispersion. DDS affected both the thermotropic phase transitions and the molecular mobility of the DPPE membrane. Addition of increasing amounts of DDS to the DPPE dispersion, resulted in the lowering of the gel to liquid-crystalline phase transition temperature (T_m) hence increased membrane fluidity. At all concentrations, the DDS is located close to the interfacial region of the DPPE bilayer but not in the acyl chain region. The interesting finding with MLV is that the gel phase of DPPE-water/buffer both in presence and absence of DDS, on prolonged equilibration at 25 °C, transforms to a stable crystalline subgel phase(s). The DPPE-water system forms both crystalline subgel L_LC (with transition temperature T_LC < T_m) and L_HC (with transition temperature T_HC ≥ T_m) phases, while the DPPE-buffer system forms only subgel L_LC phase. The presence of the drug seems to (i) increase the strength of the subgel L_LC phase and (ii) decrease the strength of subgel L_HC (for R < 0.5) phase. However, the value of the transition temperatures T_LC and T_HC does not change significantly with increasing drug concentration.     

Study on solid–liquid phase equilibria in ionic liquid: 1. The solubility of alkali chloride (MCl) in ionic liquid EMISE

This paper reports the solubility of alkali chloride MCl (M is Li, Na and K) in IL EMISE was measured in the temperature range of 293.15 K to 343.15 K. The relationship between solubility, m and temperature T may be expressed in an empiric formula: ln(m/m⁰) = A₁ + A₂T₀/T + A₃T/T⁰, where m⁰ is 1 mol/kg, T⁰ is 1 K. The observed sequence of solubility is LiCl > NaCl > KCl. The fact implies that the less the radius of alkali ion, the greater is its solubility because little ion is easy to get into the interstices of IL EMISE.     

Thin Pd–23%Ag/stainless steel composite membranes: Long-term stability,life-time estimation and post-process characterisation

The long-term stability of Pd–23%Ag/stainless steel composite membranes has been examined in H₂/N₂ mixtures as a function of both temperature and feed pressure. During continuous operation, the membrane shows a good stability at 400 °C while the N₂ leakage increases very slowly at a temperature of 450 °C (P_feed = 10 bar). After 100 days of operation (P_feed = 5–20 bar, T = 350–450 °C), the N₂ permeance equals 7.0 × 10⁻⁹ mol m⁻² s⁻¹ Pa⁻¹, which indicates that the H₂/N₂ permselectivity still lies around 500, based on a H₂ permeance equal to 3.0 × 10⁻⁶ mol m⁻² s⁻¹ Pa⁻¹. Despite the generation of small pinholes, a membrane life-time of several (2–3) years (T ≤ 425 °C) is estimated for the experimental conditions employed based on long-term stability tests over 100 days. Post-process characterisation shows a considerable grain growth and micro-strain relaxation in the Pd–23%Ag membrane after the prolonged permeation experiment. Changes in surface area are relatively small. In addition, segregation of Ag to the membrane surfaces is observed. The formation of pinholes is identified as the main source for the increased N₂ leakage during testing at higher temperature.     

Moisture barrier,wetting and mechanical properties of shellac/agar or shellac/cassava starch bilayer bio-membrane for food applications

Edible bilayer membrane composed of agar (AG) or cassava starch (CAS) as a cohesive structural layer and ethanol-cast shellac layer as a moisture barrier are investigated for their potential use in food preservation as bio-packaging film, membrane or coating. Bilayer membranes containing non-plasticized shellac exhibit low water vapor permeability (WVP), from 0.89 to 1.03 × 10⁻¹¹ g m⁻¹ s⁻¹ Pa⁻¹. A high value of contact angle (≈92°) and a low liquid water adsorption rate (26 × 10⁻³ μL s⁻¹) indicate that these barrier layers have a quite hydrophobic surface. However, the rigid and brittle characteristics of shellac induce a lack of integrity for this layer. It tends to be cracked and scaled off. The incorporation of PEG 200 (plasticizer) into shellac improves the flexibility that prevents the defects in structure and reinforces the adhesion between the shellac and the cohesive-structural layer. The use of plasticizer weakly affects the WVP of bilayer membranes; however, the surface hydrophobicity as well as the liquid water adsorption rate is comparable to that of non-plasticized shellac layer. Furthermore, PEG increases the stretchability of bilayer membranes. Either being plasticized or not, shellac layer could improve significantly the functional properties of bilayer barriers and give a promising use as biopackaging.     

Measurements of the isochoric heat capacity,the critical point (TC, ρC) and vapor–liquid coexistence curve (TS, ρS) of high-purity toluene near the critical point

Isochoric heat capacities (C_V, V, T), phase boundary properties (T_S, ρ_S) and the critical (T_C, ρ_C) parameters for high-purity (0.9999+ mole fraction) toluene have been measured with a high temperature, high pressure, nearly constant volume adiabatic calorimeter and quasi-static thermogram technique. Measurements were made at three selected liquid and vapor isochores 777.8, 555.25, and 214.64 kg m⁻³ in the temperature range from 379 to 591 K. For five near-critical isochores 268.68, 281.68, 296.62, 301.52, and 318.28 kg m⁻³, the measurements were made in the immediate vicinity of the coexistence curve in order to accurately determine the phase transition temperatures (T_S, ρ_S) (shape of the coexistence curve near the critical point) and the critical parameters (T_C, ρ_C). The total combined uncertainty of heat capacity, density, and temperature measurements were estimated to be less than 2%, 0.06%, and 15 mK, respectively. The uncertainties reported in this paper are expanded uncertainties at the 95% confidence level with a coverage factor of k = 2. The uncertainty of the phase transition and the critical temperature value was 0.02 K. The Krichevskii parameter for some toluene-containing binary mixtures was calculated. The derived values of the Krichevskii parameter were used to estimate the effect of dilute impurities on the critical parameters of toluene. The measured values of saturated density near the critical point were interpreted in terms of the “complete scaling” theory in order to study singularity behavior of the coexistence curve diameter. The measured isochoric heat capacities and saturated densities were compared with the data reported by other authors and values calculated from an equation of state and other correlations.     

Low-temperature heat capacity and thermodynamic properties of endo-Tricyclo[5.2.1.0]decane

Endo-Tricyclo[5.2.1.0^2,6]decane (CAS 6004-38-2) is an important intermediate compound for synthesizing diamantane. The lack of data on the thermodynamic properties of the compound limits its development and application. In this study, endo-Tricyclo[5.2.1.0^2,6]decane was synthesized and the low temperature heat capacities were measured with a high-precision adiabatic calorimeter in the temperature range from (80 to 360) K. Two phase transitions were observed: the solid-solid phase transition in the temperature range from (198.79 to 210.27) K, with peak temperature 204.33 K; the solid-liquid phase transition in the temperature range from 333.76 K to 350.97 K, with peak temperature 345.28 K. The molar enthalpy increments, ΔH_m, and entropy increments, ΔS_m, of these phase transitions are ΔH_m=2.57 kJ · mol⁻¹ and ΔS_m=12.57 J · K⁻¹ · mol⁻¹ for the solid-solid phase transition at 204.33 K, and, Δ_fusH_m=3.07 kJ · mol⁻¹ and Δ_fusS_m=8.89 J · K⁻¹ · mol⁻¹ for the solid-liquid phase transition at 345.28 K. The thermal stability of the compound was investigated by thermogravimetric analysis. TG result shows that endo-Tricyclo[5.2.1.0^2,6]decane starts to sublime at 300 K and completely changes into vapor when the temperature reaches 423 K, reaching the maximal rate of weight loss at 408 K.     

Dynamic mechanical and thermal behavior of liquid-crystalline polybutadiene-diols with mesogenic groups in side chains

Liquid-crystalline polybutadiene-diols (LCPBDs) with the comb-like architecture were synthesized by reaction of a LC thiol with the double bonds of telechelic HO-terminated polybutadiene (PBD). LCPBDs with various initial molar ratios of thiol to double bonds of PBD, R₀, in the range from 0.15 to 1, were prepared by the radical reaction at temperature 60 °C for 48 h. The experimentally obtained degree of modification, R_e, after the reaction and purification, was determined from elemental analysis - from the amount of sulphur bounded in LCPBDs, GPC and from ¹H NMR spectra. The physical properties were investigated by differential scanning calorimetry and dynamic mechanical spectroscopy. With increasing R_e ratio the glass transition temperature of LCPBDs, T_g, increases from ∼ − 45 °C (neat PBD) to ∼20 °C (R_e ∼ 0.5). LC transition starts at R_e ∼ 0.27 (the transition temperature T_m ∼ 27 °C). With increasing R_e temperature T_m increases and for R_e ∼ 0.5 reaches the value T_m ∼ 74 °C; at the same time also the change in enthalpy at LC transition increases. The LC transition could be detected also by the dynamic mechanical spectroscopy; especially shape and position of mechanical functions on frequency and free volume parameters strongly depend on degree of modification.     

The investigation of the (p,ρ,T) and (ps,ρs,Ts) properties of {(1−x)CH3OH + xLiBr} for the application in absorption refrigeration machines and heat pumps

The (p,ρ,T) and (p_s,ρ_s,T_s) properties of {(1−x)CH₃OH + xLiBr} over a wide range of state parameters are reported for the first time. The experiments were carried out in a constant volume piezometer over a temperature range from 298.15 K to 398.15 K, at 0.08421, 0.13617, 0.19692, 0.23133 and 0.26891 mole fractions and from atmospheric pressure up to 60 MPa. The experimental uncertainties are ΔT=±3 mK for temperature, Δp=±5·10⁻² MPa for high pressure and Δp=±5·10⁻⁴ MPa for atmospheric pressure, Δρ=±3·10⁻² kg · m⁻³ for density. An equation of state was derived for correlation of the experimental data of the solutions.     

Study of poly(bisphenol A carbonate) relaxation kinetics at the glass transition temperature

In this work, the variations of the relaxation times are investigated above and below the glass transition temperature of a model amorphous polymer, the polycarbonate. Three different techniques (calorimetric, dielectric and thermostimulated currents) are used to achieve this goal. The relaxation time at the glass transition temperature was determined at the temperature dependence convergence of the relaxation times calculated with dynamic dielectric spectroscopy (DDS) for the liquid state and thermostimulated depolarisation currents (TSDC) for the vitreous state. We find a value of τ(T_g) = 110 s for PC samples. The knowledge of the temperature dependence, τ(T), and the value τ(T_g) enables to determine the glass-forming liquid fragility index, m. We find m = 178 ± 5.     

Heat capacity, enthalpy and entropy of bismuth niobate and bismuth tantalate

The heat capacity and the heat content of bismuth niobate BiNbO₄ and bismuth tantalate BiTaO₄ were measured by the relaxation method and Calvet-type heat flux calorimetry. The temperature dependencies of the heat capacities in the form C_pm=128.628+0.03340 T−1991055/T²+136273131/T³ (J K^-1 mol^-1) and 133.594+0.02539 T−2734386/T²+235597393/T³ (J K^-1 mol^-1) were derived for BiNbO₄ and BiTaO₄, respectively, by the least-squares method from the experimental data. Furthermore, the standard molar entropies at 298.15 K S_m(BiNbO₄)=147.86 J K^-1 mol^-1 and S_m(BiTaO₄)=149.11 J K^-1 mol^-1 were assessed from the low temperature heat capacity measurements. To complete a set of thermodynamic data of these mixed oxides an attempt was made to estimate the values of the heat of formation from the constituent binary oxides.     

A methodology to define the Cubic Equation of State of a simple fluid

A method to define the Cubic Equation of State (CES) of a simple substance is presented in this work. CES is constructed with only three parameters of the fluid, namely, the critical compressibility Z_c≡P_cv_c/RT_c, the acentric factor ω ≡ − log  (P^(sat)/P_c) − 1 (where P^(sat) is the saturated vapor pressure), and the saturated vapor volume v^(sat) at the temperature T^(sat)/T_c = 0.7 (where T_c is the critical temperature, v_c is the critical volume, and P_c is the critical pressure). The resulting CES is unique for each substance and, in general, it is different from other known CES in the literature.     

Pervaporation of water from aqueous sulfuric acid at elevated temperatures using Nafion membranes

The concentration of sulfuric acid by pervaporation has been studied using Nafion-112^® and Nafion-117^® membranes, which have been characterized in terms of flux, permeability, and separation factor at 100 and 120 °C. Feed acid concentrations investigated ranged from 40 to over 80 wt%. In general, water fluxes ranged from 100 to 8000 g/m² h, depending on feed acid concentration and separation factors as high as 10⁴ were observed. Membrane stability was probed using dynamic mechanical analysis that revealed an increase in the temperature at which the α transition is observed, which corresponds to the glass transition (T_g) of the hydrophilic domain, upon use, suggesting embrittlement of the polymer structure. Further studies showed that the embrittlement was due to an interaction with the acid and was not induced by the operating temperature.     

Unusual oxidation states give reversible room temperature magnetocaloric effect on perovskite-related oxides SrFe0.5Co0.5O3

The magnetic properties and the magnetocaloric effect are presented for the perovskite-related oxide SrFe_0.5Co_0.5O₃ prepared using electrochemical oxidation. SrFe_0.5Co_0.5O₃ exhibits a second order paramagnetic-ferromagnetic transition close to room temperature (T_C=330 K). The maximal magnetic entropy change ΔS_M^Max , the maximal adiabatic temperature change ΔT_ad and the refrigerant capacity are found to be equal to respectively 4.0 J/kgK, 1.8 K and 258 J/kg while raising the B-field change from 0 to 5 T.     

Phase behavior of biodegradable amphiphilic poly(l,l-lactide)-b-poly(ethylene glycol)-b-poly(l,l-lactide)

This study describes the miscibility phase behavior in two series of biodegradable triblock copolymers, poly(l-lactide)-block-poly(ethylene glycol)-block-poly(l-lactide) (PLLA-PEG-PLLA), prepared from two di-hydroxy-terminated PEG prepolymers (M_n = 4000 or 600 g mol⁻¹) with different lengths of poly(l-lactide) segments (polymerization degree, DP = 1.2-145.6). The prepared block copolymers presented wide range of molecular weights (800-25,000 g mol⁻¹) and compositions (16-80 wt.% of PEG). The copolymer multiphases coexistance and interaction were evaluated by DSC and TGA. The copolymers presented a dual stage thermal degradation and decreased thermal stability compared to PEG homopolymers. In addition, DSC analyses allowed the observation of multiphase separation; the melting temperature, T_m, of PLLA and PEG phases depended on the relative segment lengths and the only observed glass transition temperature (T_g) in copolymers indicated miscibility in the amorphous phase.     

Equilibrium melting temperature and spherulitic growth rate of poly(phenylene sulfide)

The equilibrium melting temperature (T_m), through linear extrapolation method (HW) and nonlinear extrapolation method (MX) respectively, and the supercooling dependence of the spherulitic growth rate, in the context of the Lauritzen-Hoffman secondary nucleation theory, of poly(phenylene sulfide) (PPS) have been investigated by differential scanning calorimeter (DSC) and a polarized optical microscope (POM) equipped with a CSS450 shear hot stage. The results show that the T_m value obtained by MX method is more suitable for analyzing the regime transition behavior of PPS than that by HW method. And the regime I/II and II/III transitions can be really observed at T_X = 526 and T_X = 516 K, respectively, while analyzing the secondary nucleation theory of PPS by use of the T_m value obtained by MX method.     

Accurate thermodynamic and dielectric equations of state for high-temperature simulated water

The thermodynamic and dielectric properties of the simple point charge extended (SPC/E) water model are examined over wide temperature and density range by means of molecular dynamic simulations. Accurate analytical thermodynamic and dielectric equations of state for the SPC/E pair-potential are presented. Parameterizations cover a broad range of high temperature states including the critical region. The critical point parameters of SPC/E water were determined to be ρ_c = 0.276 g/cm³, T_c = 640.25 K and p_c = 164.37 bar. The value of the static dielectric constant of SPC/E water at its critical point was calculated to be 5.35, which compares remarkably well with the corresponding experimental value of 5.36. Analytical thermodynamic and dielectric equations for the saturated liquid and vapor densities are also given.     

Apparent molar volumes and apparent molar heat capacities of aqueous N-acetyl-d-glucosamine at temperatures from 278.15 K to 368.15 K and of aqueous N-methylacetamide at temperatures from 278.15 K to 393.15 K at the pressure 0.35 MPa

We determined apparent molar volumes V_? from densities measured with a vibrating-tube densimeter at 278.15 ? (T/K) ? 368.15 and apparent molar heat capacities C_p,? with a twin fixed-cell, differential, temperature-scanning calorimeter at 278.15 ? (T/K) ? 363.15 for aqueous solutions of N-acetyl-d-glucosamine at m from (0.01 to 1.0) mol · kg⁻¹ and at p = 0.35 MPa. We also determined V_? at 278.15 ? (T/K) ? 368.15 and C_p,? at 278.15 ? (T/K) ? 393.15 for aqueous solutions of N-methylacetamide at m from (0.015 to 1.0) mol · kg⁻¹ and at p = 0.35 MPa. Empirical functions of m and T for each compound were fitted to our results, which are then compared to those for N,N-dimethylacetamide. Estimated values of Δ_rV_m(m, T) and Δ_rC_p,m(m, T) for formation of aqueous N-acetyl-d-glucosamine from aqueous d-glucose and aqueous acetamide are calculated and discussed.