Search results for " transition"
showing 10 items of 2751 documents
Glassy dynamics in thin polymer films: recent MD results
2002
The influence of a film geometry on the glass transition is investigated via molecular dynamics (MD) simulations of a (non-entangled) polymer melt. The confinement is realized by two identical potential barriers of the form U wall = z -9 , where z denotes the distance of a particle from the wall. Despite the geometric confinement, basic qualitative features of the system dynamics can be well described in the framework of the mode-coupling theory (MCT). Examples are the two-step relaxation of the incoherent intermediate scattering function, the time-temperature superposition property of the late time α-process and the space-time factorization of the scattering function on the intermediate ti…
The glass transition in (KI)0.5(ND4I)0.5 mixed crystals as studied by deuteron spin-lattice relaxation
1993
Abstract Nuclear spin resonance has been used to study the deuteron magnetization recovery in (KI)0.5(ND4I)0.5 mixed crystals. At high temperatures the spin-lattice-relaxation is exponential. For T ⪅ 45 K deviations from this simple behaviour occur, signalling the onset of spatial inhomogeneities due to the formation of an orientational glass. The results demonstrate that the transition of (KI)0.5(ND4I)0.5 into the glassy state is driven by the freezing of random bonds.
Pressure Effect Studies on the Spin Transition of Microporous 3D Polymer [Fe(pz)Pt(CN)4]
2018
Pressure effects on the spin transition of the three-dimensional (3D) porous coordination polymer {Fe(pz)[Pt(CN)4]} have been investigated in the interval 105 Pa–1.0 GPa through variable-temperature (10–320 K) magnetic susceptibility measurements and spectroscopic studies in the visible region at room temperature. These studies have disclosed a different behavior of the compound under pressure. In the magnetic experiments, a temperature independent paramagnetic behavior has been observed under 0.4 GPa. In contrast, at room temperature and at 0.8 GPa, a complete HS-to-LS transition has been evidenced. The differences in the magnetic behavior are strongly related with the porous structure of …
Structure and Dynamics of Liquid Crystalline Polymers from Deuteron NMR
1983
The molecular structures of thermotropic liquid crystalline polymers, which have been synthesized systematically in recent years, are described according to the model of decoupling the molecular motions of the mesogenic groups and the polymer chain by inserting a flexible spacer. The molecular structure is related to the properties of these systems, e.g., phase transition temperatures, mesophases formed, etc. The structure and mobility of a specific side chain, liquid crystalline polyacrylate with phenyl benzoate as a mesogenic group is investigated by deuteron NMR and compared with the behaviour of low molecular weight analogues selectively deuterated at equivalent positions, i.e., at the …
Pressure Effect Studies on the 3D Spin Crossover System: {Fe(3CN-py)2[M(CN)2]2}·nH2O (n < 2/3, M = Ag(I), Au(I))
2007
[EN] Pressure effect investigations on the magnetic behaviour of the 3D SCO polymers {Fe(3CN-py)2[Ag(CN)2]2} · 2/3H2O (1) and {Fe(3CN-py)2[Au(CN)2]2} · 2/3H2O (2) have been carried out in the range of 105 Pa to 0.7 GPa. Despite both compounds are isostructural their magnetic behaviour under applied hydrostatic pressures is very different. Strong nonlinearity in the Tc(P) vs. P plot has been observed for compound 1 a fact which contrasts with the almost linear dependence observed for each spin transition in 2. However, both compounds are extremely sensitive to the application of pressure as well as the Tc(P) vs. P plots denote.
Vitrification of polymer solutions as a function of solvent quality, analyzed via vapor pressures.
2006
Vapor pressures (headspace sampling in combination with gas chromatography) and glass transition temperatures [differential scanning calorimetry (DSC)] have been measured for solutions of polystyrene (PS) in either toluene (TL) (10-70 degrees C) or cyclohexane (CH) (32-60 degrees C) from moderately concentrated solutions up to the pure polymer. As long as the mixtures are liquid, the vapor pressure of TL (good solvent) is considerably lower than that of CH (theta solvent) under other identical conditions. These differences vanish upon the vitrification of the solutions. For TL the isothermal liquid-solid transition induced by an increase of polymer concentration takes place within a finite …
Protein Unfolding:1H-NMR Studies of Paramagnetic Ferricytochrome c-550 from Horse Heart
2005
Electronic transfer protein cytochrome c-550 from horse heart is studied in the unfolded state by means of paramagnetic 1H NMR. The protein contains 104 aminoacid residues and a heme group with low spin FeIII ion in the oxidized form of protein. The global secondary structure is of the α-helix type as occurs in the case of very other cytochromes c investigated such as cyt c-550 from Thiobacillus versutus or cyt c-551 from Pseudomonas aeruginosa. We have studied the coordination characteristic and electronic properties of heme iron horse heart ferricytochrome c-550 at increasing denaturing conditions (up to 3.1 M GuHCl and 288-323 K). The 1H T1 values of the signals were measured and some re…
1H NMR studies of paramagnetic ferricytochrome c-551 from Pseudomonas aeruginosa at high pH: The role of histidine 16 in the spin transition
2005
Abstract Cytochrome c-551 from the mesophile Pseudomonas aeruginosa is an electronic transfer protein that contains 82 amino-acid residues and a c-type heme as the prosthetic group with low spin Fe(II) in the reduced form and low spin Fe(III) in the oxidized form of cytochrome c-551. We have studied the electronic properties of ferricytochrome c-551 from P. aeruginosa at high pH (9–11.4) by means of paramagnetic 1H NMR spectra and the T1 and T2 values of isotropically shifted proton resonances. We have also analyzed the temperature dependence of the hyperfine-shifts. Resonance assignment of some signals was based on 2D saturation transfer experiments, EXSY. These results indicate the existe…
Bile acid–amino acid ester conjugates: gelation, structural properties, and thermoreversible solid to solid phase transition
2010
Design, synthesis, and gelation properties of three novel biocompatible bile acid–L-methionine methyl ester conjugates are presented. Two of the conjugates have been shown to undergo self-assembly leading to organogelation in certain aromatic solvents. The properties of these gels have been investigated by conventional methods typical for molecular gel studies along with 13C CPMAS NMR spectroscopic studies of the native gel. In addition, properties in solid and solution states for all three compounds have been investigated, and single crystal X-ray structures of all compounds determined. Furthermore, powder X-ray diffraction studies have revealed that compound 1 undergoes a dynamic and reve…
Thermal, glass-forming, nonlinear optical and holographic properties of "push-pull" type azochromophores with triphenyl moieties containing isophoren…
2013
Molecular organic compounds with electron donating fragment bounded through π-conjugated system with electron acceptor fragment, as well as with incorporated triphenyl groups in their molecules show potential for creating cheap and simple solution processable materials with nonlinear optical properties. Additional insertion of azobenzene fragment in their structures makes them also possible to form holographic volume and surface relief gratings (SRG) after exposure to laser radiation, which could be useful for holographic data storage. For these purposes polymers are generally used. However, their application is complicated and challenging task as in every attempt to obtain the same polymer…