Pulsed laser-induced oxygen deficiency at TiO2 surface: Anomalous structure and electrical transport properties

We have studied pulsed laser-induced oxygen deficiencies at rutile TiO₂ surfaces. The crystal surface was successfully reduced by excimer laser irradiation, and an oxygen-deficient TiO_2−δ layer with 160 nm thickness was formed by means of ArF laser irradiation at 140 mJ/cm² for 2000 pulses. The TiO_2−δ layer fundamentally maintained a rutile structure, though this structure was distorted by many stacking faults caused by the large oxygen deficiency. The electrical resistivity of the obtained TiO_2−δ layer exhibited unconventional metallic behavior with hysteresis. A metal–insulator transition occurred at 42 K, and the electrical resistivity exceeded 10⁴ Ω cm below 42 K. This metal–insulator transition could be caused by bipolaronic ordering derived from Ti–Ti pairings that formed along the stacking faults. The constant magnetization behavior observed below 42 K is consistent with the bipolaronic scenario that has been observed previously for Ti₄O₇. These peculiar electrical properties are strongly linked to the oxygen-deficient crystal structure, which contains many stacking faults formed by instantaneous heating during excimer laser irradiation.     

Correlation of chemical structure and swelling behavior in N-alkylacrylamide hydrogels

The thermoshrinking properties have been studied for the series of N-alkyl-acrylamide hydrogels (alkyl = methyl, ethyl, isopropyl, and n-propyl), which were prepared by free-radical copolymerization of the alkylacrylamide, sodium acrylate, and N,N′-methylenebis(acrylamide) (BIS) in aqueous solution. The reaction mixtures were prepared using the same nominal compositions in an effort to study the effect of the chemical structure of the alkyl substituent on the gel swelling behavior as a function of temperature. The alkyl group was found to have a pronounced effect on the features of gel swelling. Generally, larger alkyl chains produced dramatic decreases in gel transition temperature. In addition, a change in the nature of the swelling behavior from continuous to discontinuous was noted upon changing the alkyl group from ethyl to the two propyl derivatives. Discontinuous transitions were accompanied by hysteresis. The transition temperatures of the isomeric propyl derivatives were found to differ by 12°C, with n-propyl exhibiting the lower value. Additionally, a quantitative correlation was found between the gel transition temperatures and the water/octanol partition coefficients for appropriately chosen small molecule model compounds. The transition temperatures of other gels in the series, including the cyclopropyl derivative and the n-propyl/isopropyl copolymer gels (NIPA/NNPA), also fit this correlation. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 2095–2102, 1998     

Catalytic reaction accompanied by capillary condensation, 3. Influence on reaction kinetics and dynamics

The influence of capillary condensation of reagents on the catalytic reaction kinetics and dynamics was studied. The hydrogenation ofp-xylene over Pt/SiO₂ was used as a model reaction. Two types of SiO₂ were used (KCK-1 with large pores and KCM-5 with small pores). It was shown that capillary condensation could modify the kinetics and the transition regimes. The proposed mathematical model demonstrates good agreement with experimental results for both steady-state and dynamic regimes, including reaction rate—temperature hysteresis.     

Confined Crystallization of Spin-Crossover Nanoparticles in Block-Copolymer Micelles

Nanoparticles of the spin-crossover coordination polymer [FeL(bipy)]_n were synthesized by confined crystallization within the core of polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) diblock copolymer micelles. The 4VP units in the micellar core act as coordination sites for the Fe complex. In the bulk material, the spin-crossover nanoparticles in the core are well isolated from each other allowing thermal treatment without disintegration of their structure. During annealing above the glass transition temperature of the PS block, the transition temperature is shifted gradually to higher temperatures from the as-synthesized product (T_1/2↓=163 K and T_1/2↑=170 K) to the annealed product (T_1/2↓=203 K and T_1/2↑=217 K) along with an increase in hysteresis width from 6 K to 14 K. Thus, the spin-crossover properties can be shifted towards the properties of the related bulk material. The stability of the nanocomposite allows further processing, such as electrospinning from solution.     

The role of water in structural changes of poly(N-isopropylacrylamide) and poly(N-isopropylmethacrylamide) studied by FTIR, Raman spectroscopy and quantum chemical calculations

Temperature-induced phase transition in water solutions of poly(N-isopropylacrylamide) (PNIPAM) and poly(N-isopropylmethacrylamide) (PNIPMAM) have been studied by ATR FTIR and Raman spectroscopy in combination with quantum chemical calculations. The presence or absence of the α-methyl group has a strong effect on the physical structure of water solutions. Although the hydrophobic interactions for PNIPMAM and PNIPAM are very similar, PNIPMAM with additional methyl group exhibits significantly weaker intermolecular interactions between the amide groups. That effect is the cause of the higher transition temperature T_t by about 8 °C for PNIPMAM compared to PNIPAM due to the formation of larger compact structures. The presence of the methyl group is significant for the reversibility of the temperature transition during the backward cooling as the dissolution of more stable compact PNIPMAM requires overcoming of a higher energy barrier and shows a strong hysteresis.     

Infrared Monitoring of Interlayer Water in Stacks of Purple Membranes†

The thermodynamic behavior of films of hydrated purple membranes from Halobacterium salinarum and the water confined in it was studied by Fourier transform infrared spectroscopy in the 180–280 K range. Unlike bulk water, water in the thin layers sandwiched between the biological membranes does not freeze at 273 K but will be supercooled to ～256 K. The melting point is unaffected, leading to hysteresis between 250 and 273 K. In its heating branch, a gradually increasing light‐scattering by ice is observed with rate‐limiting kinetics of tens of minutes. Infrared (IR) spectra decomposition provided extinction coefficients for the confined water vibrational bands and their changes upon freezing. Because of the hysteresis, at any given temperature in the 255–270 K range, the interbilayer water could be either liquid or frozen, depending on thermal history. We find that this difference affects the dynamics of the bacteriorhodopsin photocycle in the hysteresis range: the decay of the M and N states and the redistribution between them are different depending on whether or not the water was initially precooled to below the freezing point. However, freezing of interbilayer water does block the M to N transition. Unlike the water, the purple membrane lipids do not undergo any IR‐detectable phase transition in the 180–280 K range.     

Alkoxide bridged Copper(II) Hinokitiolato and Tropolonato Complexes: Polymorphism,Reconstructive Phase Transition,and Magnetic Properties

Polymorphism and an unexpected reconstructive phase transition in [Cu(trop)(μ‐OMe)]₂ (trop = tropolonate) were studied by single crystal and powder X‐ray diffraction; the phase transition is associated with a huge hysteresis of ca. 200 °C. In the readily reproducible crystal form, the methoxide‐bridged dinuclear subunits aggregate to infinite chains by longer bonds in the Jahn‐Teller distorted coordination sphere. Analogous alkoxide‐bridged derivatives with the substituted ligand hinokitiol (hino), [Cu(hino)(μ‐OR)]₂ (R = Me, Et, iPr) form pairs of dinuclear complexes and aggregate to discrete tetranuclear molecules. The inter‐cation distance patterns are reflected in the magnetic properties of these two structure types: Strong antiferromagnetic coupling within the dinuclear subunits is observed in either case, but susceptibility measurements confirm differences in exchange coupling between neighboring central Cu^II atoms.     

Studies of reactions with polymers. V. The reaction mechanism and resultant structure of PVA with PAN in DMSO without any catalyst

The reaction of polyacrylonitrile with poly(vinyl alcohol) in dimethyl sulfoxide without any catalyst was studied, and it showed that the adjacent nitrile groups on polyacrylonitrile could be linked up to form conjugated carbon-nitrogen sequence by the presence of poly(vinyl alcohol). However, no such reaction occurred when poly(vinyl alcohol) was replaced by i-propanol or poly(vinyl alcohol) graft copolymers. The structure of the resulting polymers were proposed by means of IR, UV, ¹H, and ¹³C-NMR spectroscopies. On the basis of the results, the effect of polymer feed and polymerization condition on this reaction were discussed. The compositions were determined by elemental analysis. The viscosity and thermal analysis of the products were also determined. At feed weight ratios of poly(vinyl alcohol) to polyacrylonitrile above one-half, gels were obtained.     

How did we find the rigid amorphous phase?

The first experimental evidence of the existence of the rigid amorphous phase was reported by Menczel and Wunderlich [1]: when trying to clarify the glass transition characteristics of the first main chain liquid crystalline polymers [poly(ethylene terephthalate-co-p-oxybenzoate) with 60 and 80 mol% ethylene terephthalate units] [2], the absence of the hysteresis peak at the lower temperature glass transition became evident when the sample of this copolymer was heated much faster than it had previously been cooled. Since this glass transition involved the ethylene terephthalate-rich segments of the copolymer, we searched for the source of the absence of the hysteresis peak in PET. There, the gradual disappearance of the hysteresis peak with increasing crystallinity was confirmed [1]. At the same time it was noted that the higher crystallinity samples showed a much smaller ΔC _p than could be expected on the basis of the crystallinity calculated from the heat of fusion (provided that the crystallinity concept works). Later it was confirmed that the hysteresis peak is also missing at the glass transition of nematic glasses of polymers. When checking other semicrystalline polymers, the sum of the amorphous content calculated from the ΔC _p at the glass transition, and the crystallinity calculated from the heat of fusion was far from 100% for a number of semicrystalline polymers. For most of these polymers, the sum of the amorphous content and the crystalline fraction was 0.7, meaning that ca. 30% rigid amorphous fraction was present in these samples after a cooling at 0.5 K min⁻¹ rate. Thus, the presence of the rigid amorphous phase was confirmed in five semicrystalline polymers: PET, Nylon 6, PVF, Nylon 66 and polycaprolactone [1]. Somewhat later poly(butylene terephthalate) and bisphenol-A polycarbonate [3] were added to this list.     

Confined Crystallization of Spin‐Crossover Nanoparticles in Block‐Copolymer Micelles

Nanoparticles of the spin‐crossover coordination polymer [FeL(bipy)]_n were synthesized by confined crystallization within the core of polystyrene‐block‐poly(4‐vinylpyridine) (PS‐b‐P4VP) diblock copolymer micelles. The 4VP units in the micellar core act as coordination sites for the Fe complex. In the bulk material, the spin‐crossover nanoparticles in the core are well isolated from each other allowing thermal treatment without disintegration of their structure. During annealing above the glass transition temperature of the PS block, the transition temperature is shifted gradually to higher temperatures from the as‐synthesized product (T_1/2↓=163 K and T_1/2↑=170 K) to the annealed product (T_1/2↓=203 K and T_1/2↑=217 K) along with an increase in hysteresis width from 6 K to 14 K. Thus, the spin‐crossover properties can be shifted towards the properties of the related bulk material. The stability of the nanocomposite allows further processing, such as electrospinning from solution.     

Phase transitions in mesophase macromolecules. III. The transitions in poly(ethylene terephthalate-co-p-oxybenzoate)

The glass and melting transitions of poly(ethylene terephthalate-co-p-oxybenzoate)s have been studied by differential scanning calorimetry. Despite the higher glass transition expected for polyoxybenzoate, there is almost no change in the glass transition temperature up to 63 mol % oxybenzoate (353 ± 4 K). At high oxybenzoate concentration there seems to be a discontinuous jump to a glass transition of 450 K. This high glass transition has been observed in two-phase materials down to 30 mol % oxybenzoate. The melting transition shows signs of isodimorphism with a minimum in melting temperature at about 60–70 mol % oxybenzoate, 60 K below the melting temperature of poly(ethylene terephthalate). The thermal properties are little affected by the change of the noncrystalline parts of the molecules to a mesophase structure. The transition to the isotropic phase could not be analyzed because of prior decomposition.     

Reversible structural transition in MIL-53 with large temperature hysteresis

The metal-organic framework, MIL-53, can have a structural transition from an open-pored to a closed-pored structure by adsorbing different guest molecules. The aid of guest molecules is believed to be necessary to initiate this "breathing" effect. Using both neutron powder diffraction and inelastic neutron scattering techniques, we find that MIL-53 exhibits a reversible structural transition between an open-pored and a closed-pored structure as a function of temperature without the presence of any guest molecules. Surprisingly, this structural transition shows a significant temperature hysteresis: the transition from the open-pored to closed-pored structure occurs at approximately 125 to 150 K, while the transition from the closed-pored to open-pored structure occurs around 325 to 375 K. To our knowledge, this is first observation of such a large temperature hysteresis of a structural transition in metal-organic frameworks. We also note that the transition from the open to closed structure at low temperature shows very slow kinetics. An ab initio computer simulation is employed to investigate the possible mechanism of the transition.     

Sorption of n‐hexane in amorphous polystyrene

Sorption properties of pure n‐hexane vapor in amorphous polystyrene (PS) were studied at 298 K by thermogravimetry under controlled vapor pressure. Two sorption–desorption cycles were performed by varying the relative pressure between 0 and 0.91. Mixing of PS with n‐hexane resulted in a strong plasticization, which was evidenced by quite significant depression in the glass transition temperature of the polymer as shown by differential scanning calorimetry. Maximum quantity of n‐hexane sorbed in the PS at 298 K and at a pressure close to saturation was about 12.4 wt %. The thermogravimetry yielded an isotherm with a strong hysteresis loop, explanation of which was hypothesized with the help of (a) Flory–Huggins sorption model extended by Vrentas, (b) analysis in terms of modification in the glass transition temperature of the n‐hexane/PS system as a function of sorbed quantity, and (c) change in total volume of the system. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014 , 52, 1252–1258     

Solvophobic Acceleration of a Diels–Alder Reaction in True Solutions in Organic Solvents

The rate of Diels–Alder reaction of diene 9,10‐bis(hydroxymethyl)anthracene with dienophile N‐ethylmaleimide was studied in a series of solvents with different polarity and hydrogen‐bonding ability. Enthalpies and entropies of activation were determined from the temperature dependences of the rate constants. Rate acceleration in nonaqueous protic solvents such as glycerol, propylene, and ethylene glycols was observed. In addition, enthalpy versus entropy of activation plots show a compensation pattern different from the other considered solvents, which can be linked with the solvophobic effects observed in polyhydric alcohols. However, the solvophobic acceleration was not as strong as the hydrophobic acceleration in water. Hydrogen bonding of the reactants and transition state with solvent also influences the reaction rate. The studied reaction is slightly promoted in hydrocarbon solvents in comparison with aprotic polar solvents. This was explained by hydrogen bonding of the hydroxyl groups of diene with dienophile in transition state, which requires prior breaking of the hydrogen bonds of these groups with polar solvent molecules.     

A Two‐Dimensional Iron(II) Coordination Polymer with Synergetic Spin‐Crossover and Luminescent Properties

A composite material, {[Fe(L)(TPPE)_0.5]?3 CH₃OH}_n, has been constructed by integrating the spin‐crossover (SCO) subunit Fe^II{diethyl(E,E)‐2,2′‐[1,2‐phenyl‐bis(iminomethylidyne)]bis(3‐oxobutanoate)‐(2‐)‐N,N′,O³,O³′} and the highly luminescent connector 1,1,2,2‐tetrakis(4‐(pyridin‐4‐yl)phenyl)‐ethene. Its structure contains four staggered 4×4 layers and intercalated methanol. The packing is dominated by considerable H‐bonds either between adjacent layers and between layers and guests. A crystal‐structure transformation was detected upon removal of the guest molecules. The SCO transition of the solvated crystals is centered at ca. 215 K with a non‐symmetrical hysteresis of 25 K wide, and the desolvated [Fe(L)(TPPE)_0.5]_n exhibits gradual SCO without hysteresis. Intriguingly, the intensity of the fluorescence at 460 nm for the latter is maximized at the SCO transition. The energy transfer between luminescent and SCO entities is achievable as confirmed by theoretical calculations.     

Phase transition properties of vanadium oxide films deposited by polymer-assisted deposition

Vanadium dioxide (VO₂) films were synthesized on mica substrates by a polymer-assisted deposition method, followed by rapid annealing with different annealing temperatures. The crystalline structure and morphology of the films were investigated by XRD and FE-SEM, and their phase transition properties were studied by in situ FT-IR. The results indicated that the annealing temperature affected the crystalline structure and morphology of the films remarkably, which then resulted in varied phase transition properties. In particular, the films annealed at higher temperature showed more polycrystalline structure, increased particle size and reduced phase transition intensity. But the films exhibited the similar hysteresis temperature width with increasing annealing temperature.     

Amphiphilic networks. VII. Synthesis and characterization of pH-sensitive poly(sulfoethyl methacrylate)-1-polyisobutylene networks

A series of environmentally sensitive amphiphilic networks consisting of 2-sulfoethyl methacrylate (SEMA) chains linked by methacrylate-ditelechelic polyisobutylene (MA–PIB–MA) chains have been prepared and characterized. Network composition was determined after sequential solvent extraction by elemental analysis. These networks are two-phase microheterogeneous systems containing hydrophobic rubbery PIB domains (T_g ～ ?60°C) and hydrophilic poly(2-sulfoethyl methacrylate) domains (T_g ～ ?15°C). They exhibit large contact-angle hysteresis in water which is due to surface segmental mobility and microheterogeneity. By increasing the SEMA content of the networks the contact-angle hysteresis increases. This phenomenon is due to an increase in the advancing contact angle most likely caused by the migration of the nonpolar PIB domains toward the surface and concomitant decrease of the receding contact angle. These amphiphilic networks exhibit non-Fickian swelling in n-heptane, as well as in water, and show pH-sensitive swelling in aqueous media. They rapidly and reversibly swell and deswell in response to increasing or decreasing the pH of the media (cycling between pH = 2 and 12). © 1994 John Wiley & Sons, Inc.     

An Order–Disorder Ferroelectric Host–Guest Inclusion Compound

The host–guest complex [(DIPA)([18]crown‐6)](ClO₄) ( 1 ; DIPA=2,6‐diisopropylanilinium) was constructed and found to undergo a sequence of phase transitions (Ibam–Pbcn–Pna2₁) at T₁=278 K and T₂=132 K, respectively. Systematic characterizations, such as differential scanning calorimetry, heat capacity, temperature‐dependent dielectric constant, and P–E hysteresis loop, reveal that the centrosymmetric‐to‐polar phase transition at T₂ is a paraelectric‐to‐ferroelectric transition. The symmetry breaking was also confirmed by temperature‐dependent second‐harmonic generation effect and X‐ray powder diffraction. The ferroelectric mechanism is attributable to the linear motion of the perchlorate counterions accompanied by the order–disorder transition of the [18]crown‐6 molecules and the anions.     

Radiation-induced polymerization of glass-forming systems. VII. Polymerization in supercooled state under high pressure

Radiation-induced polymerization of glass-forming monomers such as 2-hydroxyethyl methacrylate and glycidyl methacrylate under high pressure was studied. The glass transition temperature of these monomers was heightened by increased pressure. The temperature dependence of polymerizability showed a characteristic relation; similar to those in supercooled-phase polymerization under normal pressure, that had a maximum at T_v which shifted to higher levels of temperature as well as to T_g under high pressure. Polymerizability in the supercooled state also increased under increased pressure.     

Thermochemical studies of group iiib borates and mixed borates

The changes in energy, mass and structure of Sc, Y, La and In borate were studied at temperatures up to 1400°C with and without the solid diluents, alumina and magnesia. These compounds did not decompose, but based on the energetics of solid state transitions, the stability of the crystal lattice was found to be: ScBO₃ ～- LaBO₃ ～- InBO₃ > YBO₃. YBO₃ was found to be dimorphic. The high temperature form being the pseudohexagonal vaterite structure. LaBO₃ formed a solid solution with this structure. The reverse transition took place with considerable hysteresis.