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Results for “Transition Temperature”

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At least 19 recordsLinked to original sources

PubMed2002

New approach to characterise physicochemical properties of solid substrates by inverse gas chromatography at infinite dilution. II. Study of the transition temperatures of poly(methyl methacrylate) at various tacticities and of poly(methyl methacrylate) adsorbed on alumina and silica.

The determination of the temperature transitions in polymers and more particularly when polymers are adsorbed on oxides is very important in many industrial processes. In this second part, we used inverse gas chromatography (IGC) at infinite dilution to determine the second order transition temperatures of poly(methyl methacrylate) (PMMA) adsorbed (or not) on alumina or on silica. Three types of PMMA were used: atactic (a), syndiotactic (syndio) and isotactic (iso). The IGC technique allowed to obtain the net retention volume Vn and the dispersive component of the surface energy gamma(s)d for various theoretical models of molecules, using the results of Part I. By plotting RT ln Vn as a function of (1/T) or gamma(s)d as a function of the temperature T, we proved the presence of three particular temperatures that correspond respectively to the transition temperature relative to beta-relaxation (Tbeta), the glass transition temperature (Tg), and the liquid-liquid transition temperature or order-disorder transition (T1.1). Results obtained in this part allowed us to show the effect of the tacticity of PMMA and the recovery fraction of polymer adsorbed on alumina or silica on the transition temperatures.

Adsorption↗

PubMed2000

Nickel-titanium alloys: stress-related temperature transitional range.

The inducement of mechanical stress within nickel-titanium wires can influence the transitional temperature range of the alloy and therefore the expression of the superelastic properties. An analogous variation of the transitional temperature range may be expected during orthodontic therapy, when the archwires are engaged into the brackets. To investigate this possibility, samples of currently used orthodontic nickel-titanium wires (Sentalloy, GAC; Copper Ni-Ti superelastic at 27 degrees C, 35 degrees C, 40 degrees C, Ormco; Nitinol Heat-Activated, 3M-Unitek) were subjected to temperature cycles ranging between 4 degrees C and 60 degrees C. The wires were mounted in a plexiglass loading device designed to simulate clinical situations of minimum and severe dental crowding. Electrical resistivity was used to monitor the phase transformations. The data were analyzed with paired t tests. The results confirmed the presence of displacements of the transitional temperature ranges toward higher temperatures when stress was induced. Because nickel-titanium wires are most commonly used during the aligning stage in cases of severe dental crowding, particular attention was given to the performance of the orthodontic wires under maximum loading. An alloy with a stress-related transitional temperature range corresponding to the fluctuations of the oral temperature should express superelastic properties more consistently than others. According to our results, Copper Ni-Ti 27 degrees C and Nitinol Heat-Activated wires may be considered suitable alloys for the alignment stage.

Alloys↗

PubMed1991

Effect of the curing cycle on acrylic denture base glass transition temperatures.

Glass transition temperature of a polymer is an important physical property which may have a major effect on the dimensional stability of denture base materials. Thermomechanical analysis has been used to determine the glass transition temperature of specimens of denture base materials which had been produced by various selected curing cycles. It was found that different curing cycles produced variations in glass transition temperature of up to 20 degrees C.

Acrylic Resins↗

PubMed1998

Temperature transitions of protein properties in human red blood cells.

Human red blood cells (RBC) undergo a sudden change from blocking to passing through 1.3 +/- 0.2-micrometer micropipettes at a transition temperature (Tc) of 36.4 degrees C. For resealed RBC ghosts this transition occurs at 28.3 degrees C (Tg). These findings are attributed to an elastomeric transition of hemoglobin from being gel-like to a fluid and to an elastomeric transition of membrane proteins such as spectrin. Spectrin shows a uniform distribution along the aspirated RBC tongue above Tg in contrast to the linear gradient below Tg.

Biophysical Phenomena↗

PubMed1980

Relation between growth temperature of E. coli and phase transition temperatures of its cytoplasmic and outer membranes.

Cells of wild-type E. coli B were grown at 17, 27 and 38 degrees C, and their cell membranes were fractionated into the cytoplasmic and the outer membranes. Chemical assay proved that the molar ratio of saturated to unsaturated fatty acids increases in phospholipids extracted from each membrane as the growth temperature increases. The transition temperature at which the solid phase disappears was determined by X-ray diffraction in these biomembranes and also membranes of extracted phospholipids and of extracted lipopolysaccharide. The transition temperatures of the cytoplasmic membrane and of the membranes of phospholipids extracted from the cytoplasmic and the outer membranes increased with the growth temperature in good parallelism to the molar ratio of saturated to unsaturated fatty acids. The transition temperature of the outer membrane was less sensitive to the growth temperature, presumably due to the presence of lipopolysaccharide. The transition temperature of the membranes of lipopolysaccharide extracted from the outer membrane was 25 degrees C, for the cells grown at 27 and 37 degrees C. For the cells grown at 17 degrees C, the extracted lipopolysaccharide gave a broad diffraction peak and did not exhibit a solid-fluid phase transition between --5 and 40 degrees C.

Cell Membrane↗

PubMed2005

Smectic tilt susceptibility: anharmonic behavior in surface-induced smectic layers above the nematic--smectic-a transition temperature.

Fréedericksz transition measurements were performed on the Merck liquid crystal SCE12R. The results were used to determine the quartic contribution to the free energy associated with molecular tilt relative to the layer normal in the surface-induced smectic layers above the nematic-smectic-A transition temperature T(NA) . Both the quadratic and quartic coefficients are consistent with the scaling relation (T- T(NA))(-3nu) , where nu is the correlation length critical exponent, and their ratio was approximately constant with T . The dielectric constants, the refractive indices, and the bend elastic constant for SCE12R also are reported.

Journal Article↗

PubMed2001

Usefulness of the Kohlrausch-Williams-Watts stretched exponential function to describe protein aggregation in lyophilized formulations and the temperature dependence near the glass transition temperature.

PURPOSE: We studied the feasibility of using the Kohlrausch-Williams-Watts stretched exponential function (KWW equation) to describe protein aggregation in lyophilized formulations during storage. Parameters representing "mean aggregation time" (taua) and stretched exponential constant (betaa) were calculated according to the KWW equation by assuming that the time required for protein molecules to aggregate (tau) varies because of the fact that protein aggregation occurs at a rate that depends on the degree of protein deformation resulting from stresses created during freeze-drying. The temperature dependence of the parameters near the glass transition temperature was examined to discuss the possibility of predicting protein aggregation by accelerated testing. METHODS: Protein aggregation in lyophilized bovine serum gamma-globulin (BGG) formulations containing dextran or methylcellulose, at temperatures ranging from 10 to 80 degrees C, was followed by size-exclusion chromatography. RESULTS: Non-exponential BGG aggregation in lyophilized formulations could be described by the KWW equation. The taua and betaa, parameters changed abruptly around the NMR relaxation-based critical mobility temperature for formulations containing dextran and methylcellulose. In the glassy state, in contrast, the taua parameter of these formulations exhibited continuous temperature dependence. The parameter taur, as calculated from taua, and betaa, reflected differences in tau values between the two excipients. CONCLUSIONS: The results indicate that the parameter betaa is reflective of physical changes wihtin lyophilized formulations. Within the temperature range, during which no abrupt changes in betaa were observed, knowledge regarding the taua and betaa parameters allows the rate of protein aggregation to be predicted. The parameter taur was found to be useful in comparing the protein aggregation behavior of formulations having different taua and betaa values.

Algorithms↗

PubMed2001

The molecular basis for the inverse temperature transition of elastin.

Elastin undergoes an "inverse temperature transition" such that it becomes more ordered as the temperature increases. To investigate the molecular basis for this behavior, molecular dynamics simulations were conducted above and below the transition temperature. Simulations of a 90-residue elastin peptide, (VPGVG)(18), with explicit water molecules were performed at seven different temperatures between 7 and 42 degrees C, for a total of 80 ns. Beginning from an idealized beta-spiral structure, hydrophobic collapse was observed over a narrow temperature range in the simulations. Moreover, simulations above and below elastin's transition temperature indicate that elastin has more turns and distorted beta-structure at higher temperatures. Water was critical to the inverse temperature transition and elastin-associated water molecules can be divided into three categories: those closely associated with beta II turns; those that form hydrogen bonds with the main-chain groups; and those hydrating the hydrophobic side-chains. Water-swollen, monomeric elastin above the transition temperature is best described as a compact amorphous structure with distorted beta-strands, fluctuating turns, buried hydrophobic residues, and main-chain polar atoms that participate in hydrogen bonds with water. Below the transition temperature, elastin is expanded with approximately 40 % local beta-spiral structure. Overall the simulations are in agreement with experiment and therefore appear to provide an atomic-level description of the conformational properties of elastin monomers and the basis for their elastomeric properties.

Amino Acid Sequence↗

PubMed1999

Tyrosine-PEG-derived poly(ether carbonate)s as new biomaterials. Part II: study of inverse temperature transitions.

Tyrosine-poly(alkylene oxide)-derived poly(ether carbonate)s represent a new group of degradable biomaterials that exhibit inverse temperature transitions. Poly(DTE co 70%PEG,1000 carbonate) was chosen as an example to study this special phase transition behavior of the polymers. The observed transition temperature varied slightly depending on the technique used, e.g. CD always gave a lower temperature than UV/Vis. CD and UV/Vis studies indicated that the transition temperature was both heating rate and concentration dependent. Thermodynamic parameters of the transition (enthalpy, entropy, and free energy) were determined by DSC. The molecularity of the transition was 2.6, as calculated from UV and DSC data. The transition temperature could be varied from 18 to 58 degrees C by changing the polymer structure. The new poly(ether carbonate)s may be used in medical applications such as injectable drug delivery formulations and bioresorbable barriers for the prevention of surgical adhesions.

Biocompatible Materials↗

PubMed1985

Diacylglycerols, lysolecithin, or hydrocarbons markedly alter the bilayer to hexagonal phase transition temperature of phosphatidylethanolamines.

The bilayer to hexagonal phase transition temperatures of dielaidoylphosphatidylethanolamine and 1-palmitoyl-2-oleoylphosphatidylethanolamine are 65.6 and 71.4 degrees C, respectively. Using high-sensitivity differential scanning calorimetry, I have shown that these transition temperatures are extremely sensitive to the presence of small amounts of other lipid components. For example, at a mole fraction of only 0.01, dilinolenin lowers the bilayer to hexagonal phase transition temperature of 1-palmitoyl-2-oleoyl-phosphatidylethanolamine by 8.5 degrees C. Other diacylglycerols have similar effects on this transition temperature, although the degree of unsaturation of the acyl chains has some effect, with distearin being less potent. In comparison, the 20-carbon alkane eicosane lowers this transition temperature by 5 degrees C, while palmitoyl-lysolecithin raises it by 2.5 degrees C. Similar effects of these additives on the bilayer to to hexagonal phase transition temperature are observed with dielaidoylphosphatidylethanolamine. At these concentrations of additive, there is no effect on the gel-state to liquid-crystalline-state transition temperature. The observed shifts in the temperature of the bilayer to the hexagonal phase transition can be qualitatively interpreted in terms of the effects of these additives on the hydrophilic surface area and on the hydrophobic volume. Substances expanding the hydrophobic domain promote hexagonal phase formation and lower the bilayer to hexagonal phase transition temperature. The sensitivity of the bilayer to hexagonal phase transition temperature to the presence of additives is at least as great as that which has been observed for any other lipid phase transition.

Calorimetry, Differential Scanning↗

PubMed2003

Monte Carlo studies of three-dimensional O1 and O4 phi4 theory related to Bose-Einstein condensation phase transition temperatures.

The phase transition temperature for the Bose-Einstein condensation (BEC) of weakly interacting Bose gases in three dimensions is known to be related to certain nonuniversal properties of the phase transition of three-dimensional O(2) symmetric phi(4) theory. These properties have been measured previously in Monte Carlo lattice simulations. They have also been approximated analytically, with moderate success, by large N approximations to O(N) symmetric phi(4) theory. To begin investigating the region of validity of the large N approximation in this application, the same Monte Carlo technique developed for the O(2) model [P. Arnold and G. Moore, Phys. Rev. E 64, 066113 (2001)] to O(1) and O(4) theories has been applied. The results indicate that there might exist some theoretically unanticipated systematic errors in the extrapolation of the continuum value from lattice Monte Carlo results. The final results show that the difference between simulations and next-to-leading order large N calculations does not improve significantly from N=2 to N=4. This suggests that one would need to simulate yet larger N's to see true large N scaling of the difference. Quite unexpectedly (and presumably accidentally), the Monte Carlo result for N=1 seems to give the best agreement with the large N approximation among the three cases.

Journal Article↗

PubMed2003

Understanding the physical stability of freeze dried dosage forms from the glass transition temperature of the amorphous components.

Modulated differential scanning calorimetry has been applied to understanding the long-term physical stability of freeze-dried units. It is known that these units are liable to contract on exposure to elevated temperature or humidity. The contraction occurs when the storage temperature is above the glass transition temperature of the amorphous components in the system. The effect of moisture content on the glass transition temperature of the amorphous components in the system has been studied. By combining this information with the moisture sorption isotherm it has been demonstrated that it is possible to predict the temperature and humidity conditions that will induce contraction of the unit. The magnitude of the glass transition temperature is composed of the contribution of each of the amorphous components in the system. It is proposed that it should be possible to develop a more robust system by the rational selection of excipients that increase the glass transition temperature or by modification of the processing conditions to promote crystallization of components that would otherwise depress the glass transition temperature.

Dosage Forms↗

PubMed2006

Characteristic behavior of short-term dynamics in reorientation for Gay-Berne particles near the nematic-isotropic phase transition temperature.

A specific transition behavior was found in the tumbling motion near the nematic-isotropic phase boundary using molecular dynamics simulations of the Gay-Berne mesogenic model under isobaric conditions at a reduced pressure P* of 2.0. The relaxation time for the motion obtained from the second-rank orientational time correlation function and the rotational diffusion coefficient showed a clear jump at the nematic-isotropic phase transition temperature. Regardless of the temperature dependence of the relaxation time, the change in the rotational diffusion coefficient evaluated from the orientational order parameters and the relaxation time agreed qualitatively with that of real mesogens. The rotational viscosity coefficients gamma(1) and gamma(2) were obtained from the simulation data for the relaxation time for the short-term dynamics and for the rotational diffusion coefficients. gamma(1) was proportional to (2), where is the second-rank orientational parameter. Furthermore, the rotational behavior of the model was compared with that of the Debye approximation in the isotropic phase.

Journal Article↗

PubMedDate not supplied

Effect of bovine serum on the phase transition temperature of cholesterol-containing liposomes.

The phase transition temperature of liposomes composed of dipalmitoylphosphatidylcholine (DPPC)/hydrogenated soy phosphatidylcholine (HSPC) at a 2:1 molar ratio was estimated in buffer, 30% and 50% bovine serum by monitoring the leakage of encapsulated self quenched doxorubicin (Dox) from the vesicles when exposed to a temperature increasing from 30-52 degrees C. The results showed that bovine serum caused a slight decrease in the phase transition temperature from 44 to 41 degrees C in 50% serum. Addition of 50% cholesterol to this liposomal composition resulted in the disappearance of transition temperature in buffer and 30% serum, whereas 50% bovine serum resulted in the reappearance of the transition temperature at 46 degrees C. The data suggest that bovine serum affects the transition temperature of liposomes in a concentration-dependent manner, and this effect is more pronounced in cholesterol-rich liposomes. The time course for the release of Dox from both kinds of liposomes (cholesterol rich and cholesterol free) during incubation in 50% bovine serum, at temperatures close to the transition temperature (42 degrees, 45 degrees C), was followed. The results showed an increased leakage of Dox from both kinds of liposomes, at both temperatures. However, liposomes with high cholesterol content released more drug at 42 degrees than at 45 degrees C. The size of these liposomes was monitored in 10% bovine serum at 25 degrees C for a period of 1 h using photon correlation spectroscopy. The data showed no variation in the size of both cholesterol-rich and cholesterol-free liposomes for this period, which indicates that bovine serum does not affect the size of either cholesterol-rich or cholesterol-free liposomes.

Animals↗

PubMed1982

Comparative study of transition temperature in adult and newborn human red blood cells.

Hypotonic haemolysis of newborn and adult human red blood cells (RBC) is more intensive at 4 than at 37 degrees C. Newborn RBC are more resistant to the decrease of temperature than adult RBC. Transition temperature of the RBC membrane can well be determined by illustrating osmotic haemolysis as a function of temperature. This parameter characterizes well the microviscosity of the membrane. The inclination point of Arrhenius curves representing transition temperature occurred always at 25 degrees C both in the case of adult and newborn RBC. The similarity of the transition temperature indicates the similarity of membrane microviscosity of newborn and adult human RBC.

Adult↗

PubMed1997

Effect of compaction temperature on consolidation of amorphous copolymers with different glass transition temperatures.

PURPOSE: The purpose of this study was to relate the combination of glass transition temperature (Tg) and temperature of measurement with the mechanical and compaction properties of some test materials. METHODS: Copolymers with different Tg'S were synthesised by free radical copolymerisation of methyl methacrylate with lauryl methacrylate. Elastic moduli were measured by dynamic mechanical analysis at different strain rates and temperatures. Compaction experiments were performed at different compaction speeds and temperatures. RESULTS: The difference between temperature of measurement and Tg appears to determine both elastic modulus and yield strength completely. They both decrease with decreasing difference between temperature of measurement and Tg and increase with strain rate. At temperatures of measurement higher than the Tg the elastic modulus is extremely low because the materials behave as rubbers. Consequently, the amount of energy stored during compaction decreases when the compaction temperature approaches the Tg and increases with strain rate. When the compaction temperature is higher than the Tg, the amount of stored energy is extremely large. The compaction experiments show that the final tablet porosity is completely determined by stress relaxation phenomena. Consequently, the final tablet porosity follows exactly the same relation as that of stored energy. CONCLUSIONS: The final tablet porosity is unequivocally determined by the amount of stored energy. This implies that tablet production at a temperature of about 20 K under the glass transition temperature of the material yields tablets with minimum porosity.

Glass↗

PubMed2006

The glass transition temperatures of sugar mixtures.

We measured the glass transition temperatures of mono-, di-, and trisaccharide mixtures using differential scanning calorimeter (DSC) and analyzed these temperatures using the Gordon-Taylor equation. We found that the glass transition temperatures of monosaccharide-monosaccharide and disaccharide-disaccharide mixtures could be described by the conventional Gordon-Taylor equation. However, the glass transition temperatures of monosaccharide-disaccharide and monosaccharide-trisaccharide mixtures deviated from the conventional Gordon-Taylor equation and the amount of deviation in the monosaccharide-trisaccharide mixtures was larger than those in the monosaccharide-disaccharide mixtures. From these results, we conclude that the size and shape of the sugars play an important role in the glass transition temperature of the mixtures.

Disaccharides↗

PubMed2006

Gauging the effect of impurities on lipid bilayer phase transition temperature.

We report on the gel-to-fluid phase transition behavior of unilamellar vesicles formed with 1,2-dimyristoyl-sn-phosphatidylcholine (14:0 DMPC). We have interrogated the gel-to-fluid transition temperature of these bilayer structures using the chromophore perylene incorporated in their nonpolar region. We observe a discontinuous change in the reorientation time of perylene sequestered within the bilayer at the known melting transition temperature of 14:0 DMPC, 24 degrees C. The perylene reorientation data reveal a local viscosity of 14.5 +/- 2.5 cP in the gel phase, and 8.5 +/- 1.5 cP in the fluid phase. We have also incorporated small amounts of 1,2-dimyristoleoyl-sn-glycero-3-phosphocholine (14:1 DMPC) into these unilamellar vesicles and find that the melting transition temperature for these bilayers varies in a regular manner with the amount of 14:1 DMPC present. These data demonstrate that very little "contaminant" is required to cause a substantial change in the gel-to-fluid transition temperature, even though these contaminants do not alter the viscosity of the bilayer sensed by perylene, either above or below the melting transition.

Chromogenic Compounds↗