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Reversible Phase Transitions (reversible + phase_transition)

Distribution by Scientific Domains

Polymers and Materials Science	50%
Chemistry	50%

Selected Abstracts

Reversible phase transition of pyridinium-3-carboxylic acid perchlorate

ACTA CRYSTALLOGRAPHICA SECTION B, Issue 3 2010
Heng-Yun Ye
Pyridinium-3-carboxylic acid perchlorate was synthesized and separated as crystals. Differential scanning calorimetry (DSC) measurements show that this compound undergoes a reversible phase transition at ,,135,K with a wide hysteresis of 15,K. Dielectric measurements confirm the transition at ,,127,K. Measurement of the unit-cell parameters versus temperature shows that the values of the c axis and , angle change abruptly and remarkably at 129,(2),K, indicating that the system undergoes a first-order transition at Tc = 129,K. The crystal structures determined at 103 and 298,K are all monoclinic in P21/c, showing that the phase transition is isosymmetric. The crystal contains one-dimensional hydrogen-bonded chains of the pyridinium-3-carboxylic acid cations, which are further linked to perchlorate anions by hydrogen bonds to form well separated infinite planar layers. The most distinct differences between the structures of the higher-temperature phase and the lower-temperature phase are the change of the distance between the adjacent pyridinium ring planes within the hydrogen-bonded chains and the relative displacement between the hydrogen-bonded layers. Structural analysis shows that the driving force of the transition is the reorientation of the pyridinium-3-carboxylic acid cations. The degree of order of the perchlorate anions may be a secondary order parameter. [source]

Effect of the different rigidity of the chiral crosslinker on phase behaviors of side-chain chiral liquid crystalline elastomers

JOURNAL OF APPLIED POLYMER SCIENCE, Issue 3 2008
Bao-Yan Zhang
Abstract In the present work, the phase behaviors of two series of side-chain liquid crystalline elastomers (PI and PII series) derived from the same nematic liquid crystalline monomer and the different rigidity of chiral bisolefinic crosslinking units have been compared and studied extensively, and the effect of the different rigidity of crosslinker on the phase behavior of elastomers has been discussed. The chemical structure of the monomers and polymers obtained were confirmed by FTIR and 1H NMR spectroscopy. The phase behaviors were investigated by differential scanning calorimetry, polarizing optical microscopy measurement, thermogravimetric analyses, and X-ray diffraction measurement. The two series of elastomers showed smectic or cholesteric phases. When the amount of different crosslinking units was less than 15 mol %, both of the elastomers displayed elasticity, reversible phase transition with wide mesophase temperature ranges, and high thermal stability. It is shown that the isotropization temperature values of PII series are higher than those of PI series, and the glass transition temperature values of PII series varied smoothly and that of PI series changed smoothly first and then abruptly with increasing the contents of crosslinkers with different rigidity. In addition, PI series showed an interesting change in LC texture near clearing point, but PII series did not. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008 [source]

Reversible phase transition of pyridinium-3-carboxylic acid perchlorate

Polymorphism of CaTeO3 and solid solutions CaxSr1,,,xTeO3

ACTA CRYSTALLOGRAPHICA SECTION B, Issue 2 2009
Berthold Stöger
Single crystals and microcrystalline samples of the calcium tellurate(IV) phases ,-, ,-, ,,- and ,-CaTeO3 as well as of two solid solutions CaxSr1,,,xTeO3 (x = 0.55 and 0.77) have been synthesized and characterized by X-ray diffraction and thermal analysis. A comparative description of the structures and the relations between the polymorphs is given. The main building units of the hitherto unknown structures are isolated [TeIVO3]2, units and [(Ca,Sr)Ox] (x = 6,8) polyhedra. All structures exhibit channels in which the TeIV electron lone pairs protrude. The low-temperature phase ,-CaTeO3 is stable up to 1168,K. It exhibits nearly cylindrical channels (diameter ,,4,Å) and differs structurally from the other phases, whereas the metastable high-temperature phases are closely related to each other. They feature oval channels (shortest and longest diameter ,,2 and 8,Å). ,-CaTeO3 can be described as an order,disorder (OD) structure of two non-polar layers with layer groups p121/m1 and p1211. The ,-CaTeO3 crystal under investigation consists of two polytypes with a maximum degree of order. The two phases CaxSr1,,,xTeO3 (x = 0.55 and 0.77) are isostructural to the MDO1 polytype of ,-CaTeO3. ,-CaTeO3 shows a distinct reversible phase transition at ,,293,K. The low-temperature modification ,,-CaTeO3 as well as its high-temperature modification ,-CaTeO3 can be considered as threefold superstructures along [100] based on the MDO1 polytype of ,-CaTeO3. [source]

Polystannanes: Polymers of a Molecular, Jacketed Metal,Wire Structure,

ADVANCED MATERIALS, Issue 11 2008
Fabien Choffat
Abstract Organometallic polymers comprising a backbone of covalently connected metal atoms can be regarded as molecular metal wires surrounded by a jacket of organic matter. Such polymers are rare and their materials properties are largely unexplored. Here, we report on polystannanes, (SnR2)n, that is, polymers with a backbone of tin atoms, which are synthesized by dehydropolymerization of dialkylstannanes (H2SnR2) with the catalyst [RhCl(PPh3)3]. The polystannanes feature reversible phase transitions into liquid-crystalline states, remarkably, even below room temperature, and, interestingly, oriented either parallel or perpendicular to external driving forces, depending on the length of the alkyl substituents. [source]

In situ high-temperature powder diffraction study of reversible phase transitions in decagonal Al71.2Co12.8Ni16

JOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 4 2003
Christian Soltmann
This experimental work is aimed at confirming and broadening findings of former investigations into the type I,S1 phase transition in Al,Co,Ni quasicrystals. In situ high-temperature synchrotron radiation measurements were carried out on Al71.2Co12.8Ni16 powder samples. The cell parameters were precisely determined at various temperatures in the range 290,1110,K. The data provided a new extended insight into the thermal expansion of Al71.2Co12.8Ni16 and into fundamental properties of the phase transition. [source]

Acid-labile, thermoresponsive (meth)acrylamide polymers with pendant cyclic acetal moieties

JOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 13 2008
Xiao-Nan Huang
Abstract Acid-labile, thermoresponsive polymers with pendant six-membered cyclic acetal groups were prepared by radical polymerization of two monomers, N -(2,2-dimethyl-1,3-dioxan-5-yl) methacrylamide (NDMM) and N -(2,2-dimethyl-1,3-dioxan-5-yl) acrylamide (NDMA). The aqueous solution properties of the polymers, PNDMM and PNDMA, were studied by turbidimetry, 1H NMR, fluorescence, and DSC measurements. It is found that both polymers show sensitive and reversible phase transitions with distinct lower critical solution temperatures (LCST). Below their LCSTs, there are still some polymer aggregates as evidenced by measurements of pyrene excitation spectra and urea effects on the cloud points (CP) of polymers. The salting effect of six inorganic sodium salts on the phase transition behavior of PNDMM was investigated by turbidimetric approach. The salting-out to salting-in effect is in the order of SO42, > F, > Cl, > Br, > I, > SCN,, following the Hofmeister's series. pH-dependent hydrolysis of PNDMM and PNDMA was studied by turbidimetric and 1H NMR methods. They are both pH-sensitive and their hydrolysis rates significantly increase with decreasing pH value. The CP of PNDMM gradually increases with the acid-triggered hydrolysis of the acetal groups and the hydrolyzed polymer with , 30% hydrolysis degree does not show thermally induced phase transition. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 4332,4343, 2008 [source]

Stimuli-responsive properties of aminophenylboronic acid-carrying thermosensitive copolymers

POLYMER INTERNATIONAL, Issue 5 2003

Abstract Thermosensitive copolymers of N -isopropylacrylamide (NIPA) and N -acryloxysuccinimide (NASI) were obtained by solution polymerization using azobisisobutyronitrile as the initiator in a tetrahydrofuran,toluene mixture at 65,°C. A boronic acid-carrying ligand, m -aminophenylboronic acid (APBA) was covalently attached to the thermosensitive copolymer via the reaction between amino and succinimide groups. APBA-coupled thermosensitive copolymer exhibited both temperature and pH sensitivity. Thermally reversible phase transitions were observed both in the acidic and alkaline pH region for the APBA-modified copolymers obtained with different NASI feed concentrations. In our study, ribonucleic acid (RNA) was selected as a biomolecule having reactive groups which could potentially interact with the boronic acid functionality. The response of boronic acid-carrying thermosensitive copolymer against RNA was investigated in aqueous media in the pH range 4,9. In the acidic pH region, an increase was observed in the lower critical solution temperature (LCST) of the APBA-coupled thermosensitive copolymer with increasing RNA concentration. However, LCST decreased with increasing RNA concentration at both neutral and alkaline pH values. The LCST of the APBA-attached copolymer varied linearly with the RNA concentration at pH of 3, 4 and 7. © 2003 Society of Chemical Industry [source]