Unraveling the Guest-Induced Switchability in the Metal-Organic Framework DUT-13(Zn)**

The switching mechanism of the flexible framework Zn₄O(benztb)_1.5 (benztb=N,N,N’,N’-benzidine tetrabenzoate), also known as DUT-13, was studied by advanced powder X-ray diffraction (PXRD) and gas physisorption techniques. In situ synchrotron PXRD experiments upon physisorption of nitrogen (77 K) and n-butane (273 K) shed light on the hitherto unnoticed guest-induced breathing in the MOF. The mechanism of contraction is based on the conformationally labile benztb ligand and accompanied by a reduction in specific pore volume from 2.03 cm³ g⁻¹ in the open-pore phase to 0.91 cm³ g⁻¹ in the contracted-pore phase. The high temperature limit for adsorption-induced contraction of 170 K, determined by systematic temperature variation of methane adsorption isotherms, indicates that the DUT-13 framework is softer than other mesoporous MOFs like DUT-49 and does not support the formation of overloaded metastable states required for negative gas-adsorption transitions.     

Comb‐like polymers pendanted with elastin‐like peptides showing sharp and tunable thermoresponsiveness through dynamic covalent chemistry

Comb‐like polymers carrying two elastin‐like polypeptide (ELP) pendants in each repeat unit were synthesized. The densely attached peptide chains afford these polymers with sharp thermally induced phase transitions, and their lower critical solution temperature (LCST) can be varied with molecular weights, solution pH and salt concentrations. Through amino terminals in ELP pendants, oligoethylene glycol (OEG)‐based dendrons cored with aldehyde were attached to the polymers through dynamic covalent imines. By virtue of dynamic characteristics of these novel dendronized polymers, their LCSTs can be tuned significantly by dendron coverage to shift from that dominated by ELPs to that dominated by OEG dendrons. Furthermore, dendron coverage can be enhanced obviously by the thermally induced phase transitions or greatly by freezing the polymer aqueous solutions. The work provides a convenient methodology to improve thermoresponsiveness of ELPs through polymer topology and to switch their properties through dynamic covalent chemistry. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 3379–3387     

Overall analysis of first‐order phase transitions in some alkyloxycyanobiphenyl liquid crystals

Measurements of pressure, molar volume and specific heat as functions of temperature in the isotropic (I) phase as well as in the smectic A (SmA) and nematic (N) mesophases of some alkyloxycyanobiphenyl compounds (nOCB, n = 6–10) were carried out using differential thermal analysis under pressure, densitometry, X‐ray powder diffraction and modulated differential scanning calorimetry. Thermodynamic properties, such as latent heats and volume jumps at the different phase transitions, were determined. The coherence of this whole set of data was tested using pressure–temperature data through the slopes associated to their phase transitions, extrapolated at normal pressure in the light of the Clausius–Clapeyron equation.     

Effect of pressure on the thermotropic phase behavior of surfactant assemblies in water

The temperature (T)—pressure (P) phase diagrams of aqueous solutions of a homologous series of cationic surfactants, tetradecyl- (C14TAB), hexadecyl- (C16TAB), and octadecyltrimethylammonium bromide (C18TAB), have been determined by observing the sudden change of the transmittance accompanying the phase transition under high pressure up to 160 MPa. Regarding three kinds of phase transitions which have been previously assigned by the differential scanning calorimetry (DSC) (S. Kaneshina and M. Yamanaka, J. Colloid Interface Sci.131, 493, 1989), all the transition temperatures were linearly elevated by applying pressure. The volume changes associated with the transitions were estimated from the Clapeyron—Clausius equation by using the values of the T—P slopes on the phase diagrams and of the transition entropies taken from the DSC study. A chemical potential vs pressure profile, of which slope reflects the partial molar volume, among the states of surfactant assemblies, i.e., micelle, gel, and coagel, was drawn schematically on the basis of the transition volumes. The phase boundary between the coagel phase and the micellar solution should be the critical solution line of the surfactant, representing the pressure dependence on the Krafft temperature. In the C18TAB-water system, the phase boundary line between the metastable gel and the supercooled micelle had a break point at 45 MPa, suggesting the existence of a new pressure-induced mesophase above 45 MPa. The metastable gel phase of C14TAB disappeared in the pressure range up to 160 MPa.     

Rheological properties of anisotropic poly(para-benzamide) solutions

A study has been made of the viscous properties of poly(para-benzamide) (PBA) solutions in dimethyl acetamide, which undergo a transition from an isotopic to an anisotropic (liquid-crystal) state at a definite concentration C^*. The polymer solutions behave in many respects (as regards the concentration and temperature dependence of viscosity, etc.) like solutions of low molecular weight compounds forming a liquid crystal phase, although the transitions are less pronounced in the polymer solutions owing to their polydispersity. It is shown that the viscometric method, being extremely sensitive to C^*, is convenient for determining phase diagrams of anisotropic polymer solutions. The values of C^* as related to the molecular weight of PBA have been determined, and a general criterion for transition from isotropic to anisotropic solutions established; the latter has the form (CM?)^* ≈ 1.3 × 10⁵ at 20°C. This criterion is in line with the condition for the formation of the liquid-crystal structure in a dispersion of rodlike particles as proposed by Flory. Generalized concentration dependences of viscosity have been plotted by reducing concentration to C^* and viscosity, to the maximum viscosity at the phase transition point. In investigating the flow properties of PBA solutions we revealed the existence of a yield point in the range of low shear stresses, and an intersection of the flow curves of solutions of different concentration at high shear stresses, which excludes a generalized representation of the flow curves in reduced ordinary-type coordinates.     

Critical fluctuations and static inhomogeneities in polymer gel volume phase transitions

Thermoresponsive polymer gels exhibit pronounced swelling and deswelling upon changes in temperature, accompanied by dynamic concentration fluctuations that have been interpreted as critical opalescence. These fluctuations span lengthscales similar to that of static structures in the gels, such as the gel polymer‐network meshsize (1–10 nm) and static polymer‐network crosslinking inhomogeneities (10–1000 nm). To systematically investigate this overlay, we use droplet‐based microfluidics and fabricate submillimeter‐sized gel particles with varying static heterogeneity, as revealed on a molecular scale by proton NMR. When these microgels are probed by small‐angle neutron scattering, the detection of dynamic fluctuations during the volume phase transitions is strongly perturbed by the co‐existing static inhomogeneity. Depending of the type of data analysis employed, the temperature‐dependent evolution of the correlation length associated to the dynamic fluctuations does or does not agree with predictions by the critical scaling theory. Only the most homogeneous sample of this study, prepared by controlled polymer crosslinking in droplet microfluidics, shows a diverging correlation length in agreement to the critical scaling theory independent of the specific approach of data analysis. These findings suggest that care must be taken about polymer‐network heterogeneity when gel volume phase transitions are evaluated as critical phenomena. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 1112–1122     

Interfacial Modification and Structural Transitions Induced by Guest Molecules Solubilized in U‐Type Nonionic Microemulsions

Abstract

Alcohols and polyols are essential components (in addition to the surfactant, water, and oil) in the formation of U‐type self‐assembled nano‐structures, (sometimes called L‐phases or U‐type microemulsions). These microemulsions are characterized by large isotropic regions ranging from the oil side of the phase diagram up to the aqueous corner. The isotropic oily solutions of reverse micelles (“the concentrates”) can be diluted along some dilution lines with aqueous phase to the “direct micelles” corner via a bicontinuous mesophases (i.e., two structural transitions). This dilution takes place with no phase separations or occurrence of liquid crystalline phases. The structural transitions were determined by viscosity, conductivity, and pulsed gradient spin echo NMR (PGSE NMR), and are not visible to the eye. Two guest nutraceutical molecules (lutein and phytosterols) were solubilized, at their maximum solubilization capacity, in the reversed micellar solutions (L₂ phase) and were further diluted with the aqueous phase to the aqueous micellar corner (L₁ phase). Structural transitions (for the two types of molecule) from water‐in‐oil to bicontinuous microstructures were induced by the guest molecules. The transitions occurred at an earlier stage of dilution, at a lower water content (20 wt.% aqueous phase), than in the empty (blank) microemulsions (transitions at 30 wt.% aqueous phase). The transitions from the bicontinuous microstructure to the oil‐in‐water microemulsions were retarded by the solubilizates and occurred at later dilution stage at higher aqueous phase contents (50 wt.% aqueous region for empty microemulsion and >60 wt.% for solubilized microemulsion). As a result, the bicontinuous isotropic region, in the presence of the guest molecules, becomes much broader. It seems that the main reason for such “guest‐induced structural transitions” is related to a significant flattening and enhanced rigidity of the interface. The guest molecules of the high molecular volume are occupying high volume fraction of the interface (when the solubilization is maximal).     

Controlled Release of Farmazin from Thermosensitive Gels Based on Poly(N-vinylcaprolactam)

With the aim to prolong the effect of a wide-spectrum antimicrobial agent, Farmazin (tylosine tartrate), this drug was immobilized on a gel of N-vinylcaprolactam-sodium itaconate copolymer, which undergoes a phase transition on heating. The phase transition, accompanied by a change in volume, in aqueous solutions of the pure copolymer gel and that with the immobilized drug was studied.     

Densities and Apparent Molar Volumes of Aqueous H<Subscript><Emphasis Type="Bold">3</Emphasis></Subscript>BO<Subscript><Emphasis Type="Bold">3</Emphasis></Subscript> Solutions at Temperatures from 296 to 573 K and at Pressures up to 48 MPa

Densities of four aqueous H₃BO₃ solutions (0.062, 0.155, 0.315, and 0.529 mol-kg^–1) have been measured in the liquid phase with a constant volume piezometer immersed in a precisely controlled liquid thermostat. Measurements were made at temperatures between 296 and 573 K and pressures from 0.82 to 48 MPa. The total uncertainties of the density, pressure, temperature, and molality measurements were estimated to be less than 0.06%, 0.05%, 10 mK, and 0.0005 mol-kg^–1, respectively. The accuracy of the method was confirmed by PVT measurements on pure water for two isobars (30 and 39 MPa) at temperatures from 313 to 573 K. The experimental and calculated (IAPWS formulation) densities for pure water show excellent agreement which is within their experimental uncertainties (average absolute deviation, AAD=0.012%;). Apparent and partial molar volumes were derived using the measured densities for solutions and pure water, and these results were extrapolated to zero concentration to yield the partial molar volumes of the electrolyte (H₃BO₃) at infinite dilution. The temperature, pressure, and concentration dependencies of the apparent and partial molar volumes were studied. Small pressure and concentration effects on the apparent molar volumes were found at temperatures up to 500 K. The parameters of a polynomial type of equation of state for the specific volume V_sol(P, T, m) as a function of pressure, temperature, and molality were obtained with a least-squares method using the experimental data. The root-mean-square deviation between measured and calculated values from this polynomial equation of state is ±0.2 kg-m^–3 for density. Measured values of the solution densities and the apparent and partial molar volumes are compared with data reported in the literature.     

Apparent Specific Volume and Apparent Specific Refraction of Some Poly(oxyethylene) Glycols in 1,4-Dioxane and Benzene Solutions at 298.15 K

Summary. The density and refractive index of 1,4-dioxane and benzene solutions of poly(oxyethylene) glycols of the type HO–(CH₂CH₂O)_n–H (n varying from 4 to 36) were measured at 298.15K. From the experimental data the apparent specific volume and the apparent specific refraction at infinite dilution were calculated. The limiting apparent specific volume and the limiting apparent specific refraction were found to be inversely proportional to the number average molecular weight of solute. From the limiting apparent specific values at the infinite degree of polymerization, the partial molar volume and partial molar refraction of the monomeric unit were calculated. The partial molar volume as well as the partial molar refraction of the investigated compounds at infinite dilution are additive and depend linearly on the number of oxyethylene groups. The volumetric data were analyzed in terms of the intrinsic volume of solute molecules and by a void partial molar volume. The packing density of the investigated compounds approaches a uniform value as the size of the molecules increases and in both solvents limiting values are reached.     

Positron annihilation lifetime studies of the volume phase transition of polyacrylamides

Discontinuous and continuous volume phase transitions of organic polymer hydrogels, such as polyacrylamide (PAAm) and poly(N-isopropylamide) (PNIPA) gels, uponpH and temperature were studied by the positron annihilation lifetime measurement, which allows the estimation of size, intensity and size distribution of the free volume. Microscopic changes of physical and chemical interactions between gel network and solvent molecules and among conjugated solvent molecules at volume phase transitions of polyacrylamide gels were discussed.     

Double-Chain Cationic Surfactants: Swelling,Structure, Phase Transitions and Additive Effects

Double-chain amphiphilic compounds, including surfactants and lipids, have broad significance in applications like personal care and biology. A study on the phase structures and their transitions focusing on dioctadecyldimethylammonium chloride (DODAC), used inter alia in hair conditioners, is presented. The phase behaviour is dominated by two bilayer lamellar phases, L_β and L_α, with “solid” and “melted” alkyl chains, respectively. In particular, the study is focused on the effect of additives of different polarity on the phase transitions and structures. The main techniques used for investigation were differential scanning calorimetry (DSC) and small- and wide-angle X-ray scattering (SAXS and WAXS). From the WAXS reflections, the distance between the alkyl chains in the bilayers was obtained, and from SAXS, the thicknesses of the surfactant and water layers. The L_α phase was found to have a bilayer structure, generally found for most surfactants; a L_β phase made up of bilayers with considerable chain tilting and interdigitation was also identified. Depending mainly on the polarity of the additives, their effects on the phase stabilities and structure vary. Compounds like urea have no significant effect, while fatty acids and fatty alcohols have significant effects, but which are quite different depending on the nonpolar part. In most cases, L_β and L_α phases exist over wide composition ranges; certain additives induce transitions to other phases, which include cubic, reversed hexagonal liquid crystals and bicontinuous liquid phases. For a system containing additives, which induce a significant lowering of the L_β–L_α transition, we identified the possibility of a triggered phase transition via dilution with water.     

Spin transitions in non-classical systems

Peculiarities of synthesis of chain polymeric, copper(II) hexafluoroacetylacetonate based complexes with stable nitroxyl radicals and the results of studies on correlations between the magnetic properties and structure of these compounds are summarized. Temperature variation causes structural rearrangements in the solid phases of the compounds, accompanied by the magnetic effects similar to spin crossover phenomena. Magnetic anomalies induced by phase transitions originate from specific motions in the Jahn—Teller coordination units containing two types of exchange clusters, Cu²⁺—O^·—N< or >N—^·O—Cu²⁺—O^·—N<, and are accompanied by significant changes in the crystal volume after multiple cooling/heating cycles. Chemical methods of controlling the character and temperature of spin transitions by both the formation of solid solutions of mixed metal hexafluoroacetylacetonates with the same nitroxyl radical, {Cu_1–xM_x(hfac)₂L} (M = Mn, Ni, Co), and by the formation of solid solutions based on copper(II) hexafluoroacetylacetonate with different nitroxyl radicals, {Cu(hfac)₂L_xL_1–x}, are discussed. Specific influence of isotope substitution CH₃ CD₃ in the paramagnetic ligand on both the structure of the heterospin polymer chain and the temperature of the magnetic anomaly is discussed.__________Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 2304–2325, November, 2004.     

Synthesis and crystal structure of anilinium pertechnetate and perrhenate at low and room temperatures

Anilinium pertechnetate (I) and perrhenate (II) salts were synthesized for the first time and their crystal structure was studied. The crystals of the compounds are isostructural (monoclinic system, space group P2₁/c). On cooling from 293 to 100 K, both compounds undergo a phase transition with doubling of the parameter a, the isostructural character being retained and the space group remaining the same. The phase transitions are accompanied by strengthening and change in the H-bond system.     

Apparent Molar Volume and Apparent Molar Expansibility of Rubidium, Cesium, and Ammonium Cyclohexylsulfamate in Aqueous Solution

Summary. The apparent molar volume of rubidium, caesium, and ammonium cyclohexylsulfamate was determined from the density data of their aqueous solutions at 293.15, 298.15, 303.15, 313.15, 323.15, and 333.15 K. From the apparent molar volume, determined at various temperatures, the apparent molar expansibility was calculated. The limiting apparent molar volume and apparent molar expansibility were evaluated and apportioned into their ionic components. The limiting partial molar ionic volumes and expansibilities are discussed in terms of the various effects of the ion in solution on the structure of water. It was shown that the limiting partial molar ionic expansibilities of the alkali-metal cations increase with their ionic radii. The coefficients of thermal expansion of the investigated solutions at 298.15 K were calculated and are presented graphically together with some alkali-metal cyclohexylsulfamates and tetramethylammonium cyclohexylsulfamate. The densities of the investigated solutions can be adequately represented by an equation derived by Redlich.     

Apparent Molar Volume and Apparent Molar Expansibility of Rubidium, Cesium, and Ammonium Cyclohexylsulfamate in Aqueous Solution

The apparent molar volume of rubidium, caesium, and ammonium cyclohexylsulfamate was determined from the density data of their aqueous solutions at 293.15, 298.15, 303.15, 313.15, 323.15, and 333.15 K. From the apparent molar volume, determined at various temperatures, the apparent molar expansibility was calculated. The limiting apparent molar volume and apparent molar expansibility were evaluated and apportioned into their ionic components. The limiting partial molar ionic volumes and expansibilities are discussed in terms of the various effects of the ion in solution on the structure of water. It was shown that the limiting partial molar ionic expansibilities of the alkali-metal cations increase with their ionic radii. The coefficients of thermal expansion of the investigated solutions at 298.15 K were calculated and are presented graphically together with some alkali-metal cyclohexylsulfamates and tetramethylammonium cyclohexylsulfamate. The densities of the investigated solutions can be adequately represented by an equation derived by Redlich.     

Effect of alkyl substituents structures and added ions on the phase transition of polymers and gels prepared from methyl 2‐alkylamidoacrylates

The effects of structure alteration of alkyl groups and addition of ions to solutions of new thermosensitive polymer series of poly(methyl 2‐alkylamidoacrylate)s were investigated. Potential advantages of the thermosensitive polymers are their simplicity of functionalization and transition temperature control that result from their unique α,α‐disubstituited structures. Poly(methyl 2‐propionamidoacrylate) (PMPA) and poly(methyl 2‐isobutyracrylate) (PMIBA) were thermosensitive polymers, and poly(methyl 2‐acetamidoacrylate) and poly(methyl 2‐n‐butyramidoacrylate) were completely water‐soluble and ‐insoluble, respectively. The PMIBA solution showed endotherm during the phase transition, while endotherm was not detected for PMPA. The difference between the two polymers resulted from the size of the hydrophobic groups. MPA gel was prepared by copolymerization with N,N′‐methylenebis(acrylamide) and temperature‐induced volume change of the gel was continuous. The salting‐out effect of inorganic ions on PMPA solution and MPA gel followed the Hofmeister series. The inorganic (I^? and SCN^?) and organic ions (ⁿPr₄N⁺ and ⁿBu₄N⁺), which showed the salting‐in effect, were indicated to directly interact with PMPA chains. These ions widened the temperature range of the phase transition of the PMPA solutions. This reduced cooperativity of the phase transition was caused by size decrease of cooperative domains, which resulted from the interaction of the ions with the polymer. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 4942–4952, 2005     

Volumetric Study of Binary Solvent Mixtures Constituted by Amphiphilic Ionic Liquids at Room Temperature (1-Alkyl-3-Methylimidazolium Bromide) and Water

At room temperature, the 1-decyl-3-methylimidazolium bromide (DMImBr) is a long alkyl chain imidazolium ionic liquid miscible with water and forming a gel zone between 5 and 40% w/w H₂O. We measured the density of the liquid mixtures of water and DMImBr. We determined the apparent molar volume of the molten salt for dilute solutions. For the concentrated solutions the partial molar volume of each component was evaluated by a perturbation method. These results are shown to be substantially different from those obtained with a short chain bromide ionic liquid, 1-butyl-3-methylimidazolium bromide (BMImBr). The amphiphilic ionic liquid (DMImBr) has been shown to form micelles and its critical micelle concentration (cmc) has been determined. Below the cmc, the Debye–Hückel limiting law for 1:1 electrolytes describes very accurately the behavior of low concentrations of the DMImBr salt in water. Above the cmc, the partial molar volume of the micellized monomer was approximately equal to the molar volume of the pure fused salt. The partial molar volume of water in these mixtures was similar to that of pure water. The concentrated solutions behave like mixtures of interpenetrated phases.     

Apparent Molar Volume and Apparent Molar Expansibility of Lithium,Sodium, Potassium,and Tetramethylammonium Cyclohexylsulfamate in Aqueous Solution

Summary. The apparent molar volume of lithium, sodium, potassium, and tetramethylammonium cyclohexylsulfamate was determined from the density data of their aqueous solutions at 293.15, 298.15, 303.15, 313.15, and 323.15 K. The apparent molar expansibility was calculated from the apparent molar volume at various temperatures. The limiting apparent molar volume and apparent molar expansibility were evaluated and divided into their ionic components. The partial molar ionic expansibilities were discussed in terms of the hydration of the ion in solution, as well as in terms of the hydration effects on the solute as a whole. From the partial molar expansibility of the solute at infinite dilution the partial molar expansibility of the hydration shell was deduced. The coefficients of thermal expansion of the investigated solutions at 298.15 K were calculated and are presented graphically. The density of the investigated solutions can be adequately represented by an equation derived by Root.