Sol–gel transition characterization of thermosensitive hydrogels based on water mobility variation provided by low field NMR

Sol–gel transition properties play a key role in various applications of thermosensitive hydrogels, but conventional methods for studying the sol–gel transition have some limitations. For extensive characterization of the water–polymer interaction and microstructure change during sol–gel transition, we propose a rapid and nondestructive method based on monitoring water mobility through low field NMR (LF-NMR), and this was applied to chitosan/β-glycerophosphate (CS/GP) hydrogels. The spin–spin relaxation time (T ₂) that depicted water mobility was measured by LF-NMR within 90 s. The T ₂ component corresponding to water protons trapped in polymer networks (T ₂₁ ) was very sensitive to sol–gel transition. A remarkable decrease of T ₂₁ value indicated obvious variations of water mobility when CS/GP was heated, and a turning point was observed on the T ₂₁–time curve. The gel point associated with this turning point could be easily determined by fitting the T ₂₁–time curves to a bilinear regression model, and the results showed good accuracy and repeatability owing to the nondestructive nature of LF-NMR. Variations in water components and microstructure of CS/GP caused by water migration after solidification were also analyzed by monitoring dynamic changes of T ₂. This rapid, nondestructive method provides a powerful tool for studying the sol–gel transition of hydrogels.     

Investigation of the fast relaxation in glass-forming selenium by low-frequency Raman spectroscopy

The low-frequency Raman spectra of liquid and vitreous selenium are investigated. It is demonstrated that the temperature dependence of the intensity of the fast relaxation at the glass transition temperature (T _g = 308 K) exhibits a specific feature. This feature manifests itself in a sharper increase in the intensity at temperatures T > T _g as compared to that observed at lower temperatures. The intensities of the fast relaxation at the critical temperature T _c are evaluated by the extrapolation of the linear dependence to the temperature range T > T _g in the framework of the mode-coupling theory. The new results obtained for selenium are compared with the available data for other glass-forming materials (boron oxide, toluene, arsenic sulfide). It is shown that, for all the glasses under investigation, the parameter describing the contribution of the fast relaxation to the Raman spectrum takes on the same value at the critical temperature T _c and is approximately equal to 0.3.     

Problems of compatibility of the values of glass transition temperatures published in the world literature

The influence of different factors (primarily, the temperature-time conditions for preparation and measurement of samples) on the glass transition temperatures determined from the temperature dependences of properties is analyzed using the calculations performed in terms of the relaxation theory of glass transition. The most optimum conditions for measurement of the glass transition temperatures T _g that ensure the compatibility of the values of T _g obtained by different researchers are recommended. The validity of the assertion that the glass transition temperature T _g is a temperature at which the viscosity of glasses is equal to 10¹³ P is considered.     

Ion conducting nano-composite polymer electrolytes: synthesis and ion transport characterization

Synthesis and ion transport characterization of a new K⁺-ion conducting nano-composite polymer electrolytes (NCPEs): (1?x) [70PEO:30KBr] + x SiO₂, where 0 < x < 20 wt%, are reported. The present NCPEs have been cast using a novel hot-press technique in place of the traditional solution cast method. The conventional solid polymer electrolyte (SPE) composition: (70PEO:30KBr), identified as the highest conducting composition at room temperature, has been used as first-phase host matrix and nano-size (~8 nm) particles of SiO₂ as second-phase dispersoid. As a consequence of dispersal of SiO₂ in SPE host, two orders of conductivity enhancement have been observed in NCPE composition: [95(70PEO:30KBr) + 5SiO₂] and this has been referred to as optimum conducting composition (OCC). The polymer-salt/nano-filler SiO₂ complexation and thermal properties characterization were done with the help of XRD, FTIR, SEM, DSC and TGA studies. The ion transport behavior in NCPEs have been discussed on the basis of experimental measurements on some basic ionic parameters, viz. conductivity (σ), ionic mobility (μ), mobile ion concentration (n), ionic transference number (t _ion), etc. The temperature-dependent conductivity studies of NCPE OCC have been done and activation energy (E _a) value was determined using log σ?1/T Arrhenius plot.     

Universal character of relaxation of the intensity of light scattering by boron oxide melt in the temperature range 300–340°C

It has been established that the process of stabilization of the boron oxide melt in the temperature range 300–340°С is accompanied by universal changes in the intensity of the polarized V _v component of light scattering characterized with the formation of a minimum. It has been demonstrated that the above feature does not depend on the direction of the intensity approaching to the stationary state for the selected temperature; i.e., it is observed in the process of intensity relaxation from both high and low temperatures. Upon attainment of the minimal values, the increase of the V _v intensity with the stabilization time was found to be satisfactorily described by the empirical dependence of the exponential type. The characteristic times of intensity changes calculated in accordance with the respective equation significantly exceed the times of structural relaxation and increase along with the temperature decrease in accordance with the exponential dependence. It has been established that the characteristic times of intensity changes as its values approach the stationary one from a higher temperature are larger than when approaching from a lower temperature. It has been shown that under these conditions, changes in the intensity of the depolarized H _v component are characterized with the formation of a maximum registered for both modes (sample cooling and heating). It has been revealed that the increase of the H _v intensity in the maximum does not exceed 10% of its regular value, which allows relating the formation of a minimum of the V _v component to the decrease of the isotropic light scattering intensity.     

Change of the thermal behavior of 55Bi<Subscript>2</Subscript>O<Subscript>3</Subscript> ⋅ 45B<Subscript>2</Subscript>O<Subscript>3</Subscript> glass at annealing close to the vitrification temperature

The characteristic properties of relaxation processes for 55Bi₂O₃ ? 45B₂O₃ glass were described. The pattern of changes of the vitrification temperature (T _g ) is found to be quasi-periodic; the value of the endothermic effect near T _g (ΔC’ _p ) and the difference between the softening point and the vitrification temperature (T _m –T _g ) were determined. The temperature-time parameters of the transition from quasi-periodic to the continuous change of the properties were determined and an explanation of such behavior of the system was offered.     

Effect of nitrogen to niobium atomic ratio on superconducting transition temperature of δ-NbN<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> powders

Powders of cubic niobium nitride δ-NbN _x with a particle size of below 20 μm were prepared by reactive diffusion at T = 1455?1475°C under nitrogen pressures of P ₁(N₂) = 0.1?3 MPa and P ₂(N₂) = 25 MPa. For these powders, the values of the stoichiometric coefficient x, lattice parameter a, and the superconducting transition temperature Tc were measured and the a(x), T _c(x) and T _c(a) functions were analyzed. The T _c values were found to linearly grow with increasing a (decreasing structure imperfection). A maximum value of T _c (15.8 K) corresponded to a maximum value of a (4.3934 Å). Maximain the a(x) and T _c(x) curves were found to correspond to a slightly substoichiometric nitride with x = 0.98. Having synthesized cubic niobium nitrides with 0.892 < x < 1.062, we managed to measure the dependences of a and T _c on x all over the almost entire homogeneity range for δ-NbN _x . Our a(x) and T _c(a) functions were found to reasonably agree with those previously reported for SHS-produced δ-NbN _x powders.     

Experimental Study on Pyrolysis of Black Non-Charring Polymers in the Reduced-Pressure Environment

A series of experiments on a black non-charring polymer in the low-pressure chamber is conducted under different external heat fluxes. The surface and bottom temperatures and the mass loss of the sample are measured. A parameter T _p is introduced to describe the impact of pressure on the surface temperature. There is a loose layer of the char residue left with significant pyrolysis bubbles under the low heat flux, and the bubble size decreases with pressure. The parameter T _p is found to exhibit a significant decline trend with increasing pressure, and the mass loss rate of the sample decreases apparently as the pressure increases. However, under a high heat flux, the char residue is denser, and the pyrolysis bubbles are not observed. The value of T _p and the mass loss rate of the sample have no obvious relationship with pressure. The average pyrolysis rate is linearly proportional to p ^a .     

A coarse-grained model for polylactide: glass transition temperature and conformational properties

In this article, we present a coarse-grained (CG) model of poly(lactic acid) (PLA) developed by the iterative Boltzmann inversion (IBI) method. The coarse-grained potential was derived by matching the structural probability distribution functions to those of reference atomistic simulation. The resulting coarse-grained potential was found to be temperature-dependent when trying to reproduce the thermal expansion behavior of PLA. To satisfactory reproduce this behavior, the potential needs to be modified by a temperature factor of (T/T ₀)^0.3; T ₀ = 327 K is the temperature at which the potential has been derived. The glass transition temperature (T _g) as predicted by the modified CG potential compared favorably with those from experiment and atomistic simulation. Chain conformational properties were also evaluated in terms of a chain length (N)-radius of gyration (R _g) relation and the persistence length. The model we develop was also noted to provide a considerable speed-up of computer time compared to its atomistic counterpart.     

Hysteresis in the temperature dependence of the intensity of light scattering by boron oxide melt in the 300–450°C temperature range

It has been found that the temperature dependence of the intensity of the polarized V _v and depolarized H _v components of light scattering by a boron oxide melt in the range of 300–450°C is characterized by the existence of hysteresis: the specific feature of the change in the polarized V _v component is the formation of a minimum, while the specific feature in the behavior of the depolarized H _v component is associated with the appearance of a maximum in the sample heating mode; the cooling mode is accompanied by a monotonic change in the intensity of the light-scattering components. It has been found also that in the process of a low-temperature stabilization of the melt at 300°C, the polarized V _v component after reaching its minimum values increases up to the initial level, while the depolarized H _v component after passing its maximum decreases to the values smaller than the initial one. The characteristic time of the change in the intensity of the V _v component at 300°C is determined; its value was found to be ~100 min. It has been shown that an insignificant change in the melt synthesis conditions affects the detected features of the intensity change in the range of 300–450°C.     

Multilayer coatings on Ti substrate by SHS method

Explored was the deposition of multilayer TiC _x –Ti _x Si _y -based coatings onto Ti substrate by SHS method. Sandwich-type green multilayer structures were assembled from Ti foils and Ti + 0.5C, Ti + Si, 5Ti + 3Si, and Ti + 0.65C pellets and ignited under 1 atm of Ar and a load (400 g). Burned sandwiches were characterized by SEM, EDS, and XRD. In all cases, we observed good metal–ceramic joining. Prerequisites for such a joining are (i) the presence of the liquid phase in combustion products, (ii) good metal–ceramic wettability, and (iii) closeness of reaction temperature to the melting point of substrate.     

Effect of Sodium Carbonate on the Cloud Point in Alkyl Ether/Brine Systems: Apparent Relation with Dynamic Interfacial Tension Minimum

The effect of Na₂CO₃ on the cloud point in Na₂CO₃/surfactant/brine was investigated using two series of nonionic surfactants, C₁₃EO _x and C₁₇EO _x . The cloud point, T _cp, was found to decrease linearly with increasing Na₂CO₃ concentration. This was attributed to Na⁺ and particularly to CO₃ ^2?salting-out effect. The slope a = dTcp/d[Na₂CO₃] became more and more negative as the degree of ethoxylation is increased, suggesting that the higher the number of ethylene oxide (EO) groups the stronger is the cloud point depression for a given increment in Na⁺and CO₃ ^2?ions in solution. This was also illustrated by the linear variation of ΔT _cp = T _cp,0 ? T _cp,[Na2CO3] with the surfactant degree of ethoxylation.     

Crystal structure of Pb<Subscript>6</Subscript>O[(Si<Subscript>6</Subscript>Al<Subscript>2</Subscript>)O<Subscript>20</Subscript>]

The crystal structure of Pb₆O[(Si₆Al₂)O₂₀)] is investigated using X-ray diffraction. The compound has tetragonal symmetry, space group I4/mmm, a = 11.7162(10) Å, c = 8.0435(12) Å, and V = 1104.13(2) ų. The structure is refined to R ₁ = 0.036 for 562 unique reflections with [F ₀] ≥ 4σF. The structure contains two symmetrically independent positions of the Pb²⁺ cations coordinated by five O atoms (Pb²⁺-O^2? = 2.34–2.68 Å). The TO₄ tetrahedra (T = Si, Al) form tubular [(Si₆Al₂)O₂₀] chains extended along the c axis. The O₄ oxygen atom is not bonded to the Si and Al atoms and is octahedrally coordinated by six Pb atoms with the formation of an oxo-centered OPb₆ octahedron. The assumption is made that, in some of lead silicate and aluminosilicate glasses, a number of oxygen atoms are located outside the tetrahedral structure and represent segregation centers of the Pb²⁺ cations due to the formation of oxo-centered complexes.     

Stability of polymer structures based on fullerene C<Subscript>60</Subscript> under their oxidation with oxygen

A comparative study of the oxidative destruction of the monomer face-centered cubic (FCC) lattice and various polymer [dimeric (D), orthorhombic (O), tetragonal (T), and rhombohedral (R)] phases of C₆₀ in an atmosphere of oxygen has been performed in the temperature range of 100–500°C with the use of a flow microunit connected to a gas chromatograph. From direct measurements of the content of CO₂ in the gaseous products of destruction, the temperature dependences of oxidation rates are measured for the materials under study. It has been established that, with respect to stability against oxidative destruction, different forms of C₆₀ may be arranged as follows: the monomer FCC phase of C₆₀ > D > O > T > R. The reasons behind this tendency are discussed.     

Crystal structure and thermal expansion of ammonium pentaborate NH<Subscript>4</Subscript>B<Subscript>5</Subscript>O<Subscript>8</Subscript>

Anhydrous ammonium pentaborate NH₄B₅O₈ has been synthesized by thermal dehydration of larderellite NH₄[B₅O₇(OH)₂] · H₂O at a temperature of 290°C for 7 h. The crystal structure has been determined from the X-ray powder diffraction data: a = 7.58667(5) Å, b = 12.00354(8) Å, c = 14.71199(8) Å, R _p = 6.23, R _wp = 7.98, R _B = 12.7, R _F = 8.95, and β-KB₅O₈ structure type. The double interpenetrating framework is formed by pentaborate groups, each consisting of a boron-oxygen tetrahedron and four triangles, in which all oxygen atoms are bridging. The thermal behavior of the NH₄B₅O₈ compound has been investigated using thermal X-ray diffraction. As for other pentaborates of this type, the thermal expansion of the NH₄B₅O₈ compound is anisotropic and reaches a maximum along the a axis. The thermal expansion coefficients are as follows: α _a = 39 × 10^?6, α _b = 6 × 10^?6, α _c = 20 × 10^?6, and α _V = 65 × 10^?6 °C^?1.     

Partition of <Emphasis Type="Italic">n</Emphasis>-Butanol Among Phases and Solubilization Ability of Winsor type III Microemulsions

The partition of n-butanol in Winsor type III (W-III) microemulsions was investigated in this work. Three kinds of anionic surfactants (sodium dodecyl sulfate (SDS), sodium dodecyl sulfonate (DSS), and sodium dodecyl benzene sulfonate (SDBS)) and two kinds of anionic/cationic surfactant mixtures (SDS/octadecyl trimethyl ammonium chloride (OTAC) mixtures and DSS/OTAC mixtures) were studied. Internal standard gas chromatography was employed in n-butanol content analysis. The results showed that no water exists in the excess oil (EO) phase and no oil exists in the excess water (EW) phase. For the W-III microemulsions obtained by salinity scanning, relatively constant n-butanol content in the EO (11–12 v%) and EW (1–4 v%) was found under different salinities. Accurate measurement of n-butanol content in each phase is important for those systems having low solubilization ability. For the W-III microemulsions prepared using SDS/OTAC surfactant mixture, the percentage of n-butanol distributed into the interfacial layer decreased while the fraction of n-butanol in the interfacial layer first increased sharply and then tended to be stable with the addition of n-butanol. For the different optimum W-III microemulsion systems tested, most of the surfactant-to-alcohol molar ratio data are near 1:3, but obvious deviation could be observed for some data. On the basis of the accurate measurement of n-butanol content in the EO and EW phases, the standard free energy, ΔG _o→in ^* (T = 298.15 K) of n-butanol transferring from the EO phase to the interfacial region was calculated. The results show negative ΔG _o→in ^* values. For microemulsions with the same components, n-butanol content is an important factor influencing the ΔG _o→in ^* value, and a high absolute value of ΔG _o→in ^* leads to high solubilization ability.     

EPR and optical studies on binary mixed alkali-alkaline earth oxide borate glasses doped with Cu<Superscript>2+</Superscript> ions

Mixed alkali alkaline earth oxide borate glasses of the composition (25 – x)Li₂O–xK₂O–12.5BaO–12.5MgO–49B₂O₃–1CuO (x = 0, 5, 10, 15 and 20 mol %) were prepared by the melt quenching technique. The X-ray diffractograms of all the glass samples were recorded at room temperature. Peak free X-ray spectra revealed the amorphous nature of all the prepared glasses. Modulated differential scanning calorimetry (MDSC) was used to determine the glass-transition temperature (T _g ). The probable mixed alkali effect was investigated using experimental techniques like density, molar volume, MDSC, electron paramagnetic resonance (EPR), and optical absorption studies. From the EPR spectra the spin-Hamiltonian parameters were evaluated. The spin-Hamiltonian parameter values indicated that the ground state of \(C{u^{2 + }}is{\kern 1pt} {d_{{x^2} - {y^2}}}\) orbital (²B_1g) and the site symmetry around Cu² is tetragonally distorted octahedral. The variation of g _|| and A _|| as a function of Li₂O content was found to be nonlinear. A broad optical absorption band was observed in all the glasses containing Cu² ions corresponding to ²B_1g → ²B_2g transition. From the optical absorption studies the values of the optical band gap (E _opt) for indirect, direct transitions and Urbarch energy (ΔE) have been evaluated. By co-relating the EPR and optical absorption data, bonding parameters α², β² and β ₁ ² were evaluated.     

Thermal properties and crystallization behaviors of polylactide/redwood flour or bamboo fiber composites

A series of polylactide/redwood flour (PLA/RWF) and polylactide/bamboo fiber (PLA/BF) composites were successfully prepared using a solution mixing procedure. Fourier transform infrared spectroscopy (FTIR) and wide-angle X-ray diffraction (XRD) were employed to characterize these composites. Thermal properties and crystallization behaviors of PLA composites were determined by their respective techniques of differential scanning calorimetry (DSC) and polarized optical microscopy (POM). With the increasing content of fibers, the glass transition temperature (T _g ), crystallization temperature (T _c ), and melting temperature (T _m ) of PLA/RWF composites decreased first and then increased, but T _g and T _m of PLA/BF composites increased first and decreased afterwards. It is suggested that fibers could improve the segmental mobility of PLA; meanwhile, the different morphologies, sizes, and densities of RWF and BF have different effects on thermal properties of composites. Under the increasing content of RWF, the crystallization rate of the composite increased first and decreased afterwards. When the content of RWF was 5%, the crystallization rate was at its maximum. It could be possible that the addition of fibers was able to nucleate PLA and increase the degree of crystallinity, but the excess content of fibers easily led to heterogeneous composites and subsequent poor crystallization behaviors. In a word, thermal properties and crystallization behaviors of PLA composites were regularly changing by increasing content of fibers.     

Esterification of acrylic acid with ethanol using pervaporation membrane reactor

Esterification of acrylic acid with ethanol was carried out using an in-situ reactor with an integrated pervaporation assembly (IPAE) made of polyvinyl alcohol (PVA) membrane and was compared with a non-integrated (NIE) system. Effect of reaction temperature (T _r ), catalyst loading (C _C ), molar ratios of reactants (MR) and ratio of effective membrane area to unit volume of reaction mixture (S/V _o ) on kinetics of esterification reaction were studied. Conversions achieved in IPAE were found to be distinctly higher than the NIE. The highest conversion of acrylic acid was obtained as 83.3% at T _r =60 °C, M _R =3 : 1, C _C =2% and S/V _o =14.1 m^?1. Equilibrium conversion of acrylic acid in NIE was obtained as 55.1% at 60 °C, 1 : 1 in 7 h, while using IPAE conversion enhances to 67.6%. Esterification of acrylic acid and ethanol with presently studied operating parameters provides a new approach to existing literature reported esterification-pervaporation system.     

Kinetics of growth on dual substrates,production of novel glutaminase-free L-asparaginase and substrates utilization by <Emphasis Type="Italic">Pectobacterium carotovorum</Emphasis> MTCC 1428 in a batch bioreactor

Bacterial L-asparaginase has been widely used as a potential therapeutic agent in the treatment of various lymphoblastic leukemia diseases. We studied product and dual substrates utilization kinetics by P. carotovorum MTCC 1428 in batch bioreactor. The kinetic study revealed that the maximum growth of P. carotovorum MTCC 1428 was achieved at 2 g l ^?1 and 5 g l ^?1 of glucose and L-asparagine, respectively. Different substrate inhibition models were fitted to the growth kinetic data and the additive form of double Luong model was found to best explain the growth kinetics of P. carotovorum MTCC 1428. The kinetic parameters of growth studies showed that the predicted maximum inhibition concentration of glucose (S _mg ) and L-asparagine (S _ma ) was close to the experimentally observed value 15.0 and 10 g l ^?1, respectively. Modified form of the Luedeking-Piret model was used to describe the kinetics of L-asparaginase production, and the system seems to be mixed growth associated. Kinetic models of dual substrate growth, L-asparaginase production and substrate(s) utilization by P. carotovorum MTCC 1428 well fitted with experimental data with regression coefficients (R²) value of 0.97, 0.96 and 0.93, respectively.