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1.
Andrei Jitianu Kristin Lammers Georgia A. Arbuckle-Kiel Lisa C. Klein 《Journal of Thermal Analysis and Calorimetry》2012,107(3):1039-1045
Hybrid melting gels were prepared by a sol–gel process, starting with a mono-substituted siloxane and a di-substituted siloxane,
methyltrimethoxysilane (MTES) together with dimethyldimethoxysilane (DMDES). Five gel compositions were prepared with concentrations
between 50% MTES–50% DMDES and 75% MTES–25% DMDES (in mol.%). The consolidation temperature, the treatment temperature after
which the melting gel no longer softens, increased from 135 to 160 °C with a decrease in the amount of the mono-substituted
siloxane. The glass transition temperature, recorded with differential scanning calorimetry, decreased from −0.3 to −56.7 °C
with a decrease in the amount of the mono-substituted siloxane. When a sample was heat treated isothermally for 2 h at the
consolidation temperature, the glass transition temperature increased by about 15°, indicating further crosslinking of the
siloxane network. 相似文献
2.
Cellulose powder and cellulose pellets obtained by pressing the microcrystalline powder were studied using differential scanning
calorimetry (DSC), differential thermal analysis (DTA), and thermal gravimetry (TG). The TG method enabled the assessment
of water content in the investigated samples. The glass phase transition in cellulose was studied using the DSC method, both
in heating and cooling runs, in a wide temperature range from −100 to 180 °C. It is shown that the DSC cooling runs are more
suitable for the glass phase transition visualisation than the heating runs. The discrepancy between glass phase transition
temperature T
g found using DSC and predictions by Kaelbe’s approach are observed for “dry” (7 and 5.3% water content) cellulose. This could
be explained by strong interactions between cellulose chains appearing when the water concentration decreases. The T
g measurements vs. moisture content may be used for cellulose crystallinity index determination. 相似文献
3.
Joseph D. Menczel 《Journal of Thermal Analysis and Calorimetry》2011,106(1):7-24
The first experimental evidence of the existence of the rigid amorphous fraction (RAF) was reported by Menczel and Wunderlich
for several semicrystalline polymers. It was observed that the hysteresis peak at the glass transition was absent when these
polymers were heated much faster than they had previously been cooled. In the glass transition behavior of poly(ethylene terephthalate)
(PET), the hysteresis peak gradually disappeared as the crystallinity increased. At the same time, it was noted that the ΔC
p of higher crystallinity PET samples was much smaller than could be expected on the basis of the crystallinity calculated
from the heat of fusion. It was also observed that this behavior was not unique to PET only, but is characteristic of most
semicrystalline polymers: the sum of the crystallinity calculated from the heat of fusion and the amorphous content calculated
from the ΔC
p at the glass transition is much less than 100% (a typical difference is ~20–30%). This 20–30% difference was attributed to
the existence of the “RAF”. The presence of the RAF also affected the unfreezing behavior of the “mobile (or traditional)
amorphous fraction.” As a consequence, the phenomenon of the enthalpy relaxation diminished with increasing rigid amorphous
content. It was suggested that the disappearance of the enthalpy relaxation was caused by the disappearance or drastic decrease
of the time dependence of the glass transition. To check the validity of this suggestion, the glass transition had to be also
measured on cooling in order to overlay it on the DSC curves measured on heating. However, before this overlaying work could
be accomplished, the exact temperatures on cooling had to be determined since the temperature of the DSC instruments that
time could not be calibrated on cooling using the usual low molecular weight standards due to the common phenomenon of supercooling.
Therefore, a temperature calibration method needed to be developed for cooling DSC experiments utilizing high purity liquid
crystals using the isotropic → nematic, the isotropic → cholesteric, and other liquid crystal → liquid crystal transitions.
After the cooling calibration was accomplished, the cooling glass transition experiments indicated that the glass transition
in semicrystalline polymers is not completely time independent, because its width depends on the ramp rate. However, it was
shown that the time dependence is drastically reduced, and the midpoint of the glass transition seems to be constant which
can explain the absence of the enthalpy relaxation. The work presented here has led to a number of studies showing the universality
of the rigid amorphous phase for semicrystalline polymers as well as an ASTM standard for DSC cooling calibration. 相似文献
4.
Cerium is exploited as a probe cation for elucidating the structure of an alkoxide-derived silica gel and its progressive
evolution to a glass network as a function of heat-treatment up to 1000°C. At intermediate temperatures, the host structure
exhibits inhomogeneity due to insufficient formation of siloxane bonds, which is reflected by at least two different sites
and co-ordination spheres (termed “high” and “low” water ligation) for cerium. This is proved by the response of the gels
heated up to 700°C to rehydration. Further formation of Si−O−Si network (900°C) leads to the destruction of the “high water”
sites of cerium and progression towards a glassy structure. It is, however, only after heat-treatment at 1000°C that a dense
silica glass network, not responding to rehydration, is finally obtained with cerium ions embedded in it. 相似文献
5.
L. M. Ramenskaya E. P. Grishina N. O. Kudryakova 《Russian Journal of Physical Chemistry A, Focus on Chemistry》2018,92(1):24-28
Thermochemical properties of the 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide ionic liquid [EMim]NTf2 containing moisture absorbed from the atmosphere (0.242 wt %) are investigated. The phase behavior and thermal stability relative to salt dried in vacuum are studied by means of thermogravimetry and differential scanning calorimetry at different heating and cooling rates. The glass transition, crystallization, and melting temperatures, the enthalpies of phase transitions, and the changes in heat capacity during the formation of glass are determined. It is established that the absorbed water crystallizes at a temperature of around ?40.6°C and has virtually no effect on the thermal stability and phase behavior of the salt. Rapid cooling results in the ionic liquid transitioning into the glass state at ?91.7 °C and the formation of three mesophases with different melting temperatures; one crystalline modification that melts at a temperature of ?19.3°C forms upon slow cooling. 相似文献
6.
Seven polymorphic modifications of doxazosin mesylate, designed as forms A, D, E, F, G, H, I, and the amorphous state were
studied by thermal methods (TG and DSC), temperature resolved X-ray powder diffractometry, hot stage and scanning electron
microscopy and by FT-IR spectroscopy. Amorphous form was obtained either by fast evaporation of the solvent or by fast cooling
of the melt in the DSC. Polymorphs A and F were found to be stable in the temperature range from room temperature to their
melting points at 277.9 and 276.5°C, respectively. Form G, which melts at 270.8°C, was found to be hygroscopic. Polymorph
D undergoes irreversible solid–liquid–solid phase transition at 235.5°C to polymorph I which melts at 274.9°C. Form H, which
melts at 258.0°C, was found to be unstable at high temperatures. DSC examinations revealed that form H is irreversibly transformed
to polymorph F during heating above the temperature of about 240°C. The amorphous state was found to be stable at room temperature
but when heating above the glass transition (T
g=144.1°C) it crystallizes at 221.6°C, what leads into a mixture of polymorphic forms. The new polymorphic form designed as
E was identified in the mixture. The polymorph E is converted by heating to the more stable form F. The solubilities at 25°C
for forms A, and F in methanol are 3.5 and 7.7 mg mL−1and in water they are 3.8 and 6.2 mg mL−1, respectively.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
7.
《Journal of Coordination Chemistry》2012,65(8):1361-1366
Ionic liquids comprising cationic cobalt(III) complexes [Co(acacen)L2][Tf2N] (L?=?3-butylpyridine (1), 1-butylimidazole (2); acacen?=?N,N′-bis(acetylacetone)ethylenediamine, Tf2N?=?bis(trifluoromethanesulfonyl)amide) were prepared. 1 is a liquid at room temperature and exhibits a glass transition at ?12?°C, whereas 2 is a solid at room temperature with a melting point of 74.6?°C and glass transition temperature of ?15?°C upon cooling from the melt. These salts are reddish brown diamagnetic materials that are stable against air and water; these properties differ from those of the corresponding iron(III) salt. Desorption of the axial ligands of 1 and 2 occurs at 180 and 207?°C, respectively. 相似文献
8.
Carbon nanotube (CNT) Buckypapers can be infused with resin and easily incorporated into conventional fiber reinforced composites. In this paper, we propose to use Buckypaper (BP) as a new measuring method to determine the glass transition temperature of polymeric composites. The CNT-only BP was fabricated by spray-vacuum filtration method with monodispersion of multi-wall carbon nanotubes, and then co-cured with polymeric composites. After manufacturing, the glass transition temperature of polymeric composites could be obtained from the relationship between resistance and temperature of BP during the dynamic heating process. Experimental results show that the glass transition temperature of composite samples A and B monitored by BP sensors were 127 °C and 180 °C, while such temperatures obtained from a dynamical mechanical analyzer (DMA) were 128 °C and 184 °C respectively. This paper not only reveals the ability of BP as a sensor for monitoring the glass transition temperature of composite but also provides a new way to understand the glass transition phenomenon of composite. 相似文献
9.
Masoud Salavati-Niasari Masoud Farhadi-Khouzani Fatemeh Davar 《Journal of Sol-Gel Science and Technology》2009,50(3):321-327
Nanocrystalline films of magnetite have been prepared by a novel sol–gel route in which, a solution of iron (III) nitrate
dissolved in ethylene glycol was applied on glass substrates by spin coating. Coating solution showed Newtonian behaviour
and viscosity was found as 0.0215 Pa.s. Annealing temperature was selected between 291 and 350 °C by DTA analysis in order
to obtain magnetite films. In-plane grazing angle XRD and TEM studies showed that magnetite phase was present upon annealing
the films at 300 °C. The films had crack free surfaces and their thicknesses varied between ~10 and 200 nm. UV–Vis spectrum
results showed that transmittance of the films increases with decreasing annealing temperature and increasing spinning rate.
Up to 96% transmittance was observed between the wavelengths of 900–1,100 nm. Vibrating sample magnetometer measurements indicated
that magnetite thin films showed ferromagnetic behavior and the saturation magnetization value was found as ~35 emu/cm3. 相似文献
10.
B. R. Matos E. M. Aricó M. Linardi A. S. Ferlauto E. I. Santiago F. C. Fonseca 《Journal of Thermal Analysis and Calorimetry》2009,97(2):591-594
Thermal analysis has been used to evaluate the stability, glass transition, and water retention of Nafion based polymer–ceramic
electrolytes. These electrolytes are envisioned as promising replacement of Nafion in fuel cells operating above 100 °C. The
polymeric matrix prepared by casting exhibits lower crystallinity than the extruded Nafion, a feature that affects the water
absorption properties. The addition of titania-based nanotubes and nanoparticles to the polymer has enhanced the water retention
at high temperatures (~130 °C) and the glass transition temperature, respectively. Such results are important for the design
of composite electrolytes for the operation of fuel cells at high temperatures. 相似文献
11.
Swati Soman Yoshiki Iwai Junichi Kawamura Ajit Kulkarni 《Journal of Solid State Electrochemistry》2012,16(5):1761-1766
Li2O–Al2O3–TiO2–P2O5 (LATP) glass was fabricated by conventional melt quenching route. Glass transition temperature (T
g = 296 °C) and crystallization temperatures (T
C1,2) were obtained from thermal analysis. LATP glass was converted to glass–ceramic by heat treatment in the range 550–950 °C
for 6 h. X-ray diffraction analysis revealed LiTi2(PO4)3 as a major phase. Ionic conductivity increased monotonically with concentration, reaching a maximum of ~10−4 S/cm. AlPO4 phase was detected in samples heat-treated above 850 °C. Its presence decreased the conductivity, suggesting LiTi2(PO4)3 phase as main contributor to high ionic conductivity. NMR spectra confirmed the presence of mobile 7Li ions in the entire sample series and also gave some information on the structure and dynamics of conductivity. 相似文献
12.
Fatma Demir Niko Van den Brande Bruno Van Mele Sabine Bertho Dirk Vanderzande Jean Manca Guy Van Assche 《Journal of Thermal Analysis and Calorimetry》2011,105(3):845-849
Defining appropriate annealing temperatures and times is vitally important for increasing the efficiency of bulk heterojunction
solar cells by favoring the crystallinity of the polymer-fullerene blend components. In order to better understand the annealing
process, the isothermal crystallization of poly(3-hexyl thiophene) (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM) blend investigated by means of rapid heating cooling calorimetry (RHC). Isothermal crystallization
experiments at temperatures in between the glass transition and melting, within the temperature range of 70–150 °C, can successfully
be performed since RHC permits cooling at a sufficiently high rate in order to prevent crystallization during cooling. Crystallization
isotherms were determined from the subsequent melting behavior of the blend. They were measured for a wide set of annealing
temperatures and times, and the evolution of the crystallization rate with temperature is compared for annealing from the
glassy state and from the melt state. 相似文献
13.
J. Ninov I. Doykov L. Dimova B. Petrov L. Brakalov 《Journal of Thermal Analysis and Calorimetry》2011,105(1):245-250
The kinetics of pozzolanic reaction metakaolin–lime is studied in the present work. Metakaolin is prepared by calcination
of enriched kaolin (deposit “Senovo”, Bulgaria) at temperature of 830 ± 10 °C in a labscale muffle oven. The reaction is performed
in intensively stirred water suspension at different temperatures in the range 20–100 °C. The kinetics is analyzed by comparing
the experimental data with theoretical curves, derived according to appropriate kinetic and diffusion models taking into account
the grain size distribution of metakaolin. The macroscopic mechanism and activation energy of the reaction are determined.
It is found, that the activation energy decreases gradually from 71 to 45 kJ/mol[Ca(OH)2] with the increase of the reaction degree from 0.2 up to 0.6, respectively, which is a characteristic for transition regime
reactions. 相似文献
14.
Jan Cocquyt Ulf Olsson Gerd Olofsson Paul Van der Meeren 《Colloid and polymer science》2005,283(12):1376-1381
Two endothermic transitions, at 36°C and 44°C, were observed with differential scanning calorimetry (DSC) upon heating dioctadecyldimethylammonium
bromide vesicle dispersions that were equilibrated below 15°C while in samples kept at 25°C there was only the transition
at 44°C, which was shown to be the gel to liquid–crystalline transition by 1H-NMR measurements. The transition at 36°C was reversed in an exothermic transition around 13°C upon cooling. The slowness
of this transition at ambient temperatures suggests that the presence of the transition at 36°C in a DSC upscan depends strongly
on the sample history. 相似文献
15.
The influence of the order of polymer melt on the subsequent crystallization and melting has been carefully studied. The
experimental data show that the order of isotactic polypropylene (iPP) melt decreases with increases in the fusion temperature.
For an iPP sample isothermally crystallized at 130 °C for half an hour, the degree of order of melt is higher when the fusion
temperature is lower than about 170.5 °C, hence the lamellae formed in a rapid cooling process are perfect. If the fusion
temperature is not higher than 167 °C, some thicker lamellae can exist in the melt. The melting of these unmelted lamellae
and those lamellae recrystallized in the cooling process result in double endotherms. On the other hand, when the fusion temperature
is higher than 170.5 °C, the order of the iPP melt decreases greatly; thus, the lamellae formed in the following cooling process
are imperfect. At a lower heating rate, the recrystallization or reorganization of these imperfect lamellae also leads to
double melting endotherms.
Received: June 16, 2000 Accepted: October 16, 2000 相似文献
16.
Richard K. Johnson Audrey Zink-Sharp Scott H. Renneckar Wolfgang G. Glasser 《Cellulose (London, England)》2009,16(2):227-238
Utilization of TEMPO-oxidized celluloses in bio-based nanocomposites is reported for the first time. TEMPO-oxidized wood pulps
(net carboxylate content 1.1 mmol/g cellulose) were fibrillated to varying degrees using a high intensity ultrasonic processor.
The degree of fibrillation was controlled by varying sonication time from 1 to 20 min. The sonication products were then characterized
independently and as fillers (5 wt% loading) in hydroxypropyl cellulose nanocomposite films. Nanofibril yields ranging from
11 to 98 wt% (on fiber weight basis) were obtained over the range of sonication times used. Suspension viscosities increased
initially with sonication time, peaked with gel-like behavior at 10 min of sonication and then decreased with further sonication.
The thermal degradation temperature of unfibrillated oxidized pulps was only minimally affected (6 °C decrease) by the fibrillation
process. Dynamic mechanical analysis of the nanocomposites revealed strong fibril-matrix interactions as evidenced by remarkable
storage modulus retention at high temperatures and a suppression of matrix glass transition at “high” (~5 wt%) nanofibril
loadings. Creep properties likewise exhibited significant (order of magnitude) suppression of matrix flow at high temperatures.
It was also believed, based on morphologies of freeze-fracture surfaces that the nanocomposites may be characterized by high
fracture toughness. Direct fracture testing will however be necessary to verify this suspicion. 相似文献
17.
Mehta Sanjay Raghuvanshi R. S. Panda A. K. 《Journal of Thermal Analysis and Calorimetry》1998,52(2):559-564
Melt polymerization conditions for D,L-lactide initiated with tetraphenyltin were studied with regard to polymer molecular
weight. The present study was undertaken to investigate the progress of polymerization of D,L-lactide through differential
scanning calorimetry (DSC), and also to explore the correlation between melt polymerization conditions and molecular weight.
The physical characteristics, such as glass transition temperature (Tg) of the polymer and melting transition (Tm) of D,L-lactide
are correlated with GPC data. DSC data shows that the Tm of D,L-lactide is 122.8 at 150°C polymerization time. ΔHf is 83.2
J g-1, and Tg of polymer is untraceable. At 180°C the Tm is 101.4°C, ΔHf is 34 J g-1, and Tg is around 29.5°C. The drop in
Tm and ΔHf clearly shows the conversion of D,L-lactide to polymer. The maximum increment to molecular weight of polymer is
achieved at 160°C and 8 h. After a short induction period, the slow polymerization of D,L-lactide resulted in maximal molecular
weight followed by an almost constant value of molecular weight.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
18.
Marcelo Kobelnik Douglas Lopes Cassimiro Clóvis. A. Ribeiro Jorge M. V. Capela Diogines S. Dias Marisa S. Crespi 《Journal of Thermal Analysis and Calorimetry》2012,108(1):213-218
The thermal behaviour of Mg-diclofenac compound was evaluated by simultaneous TG-DTA and DSC. The profile of the DSC curves
showed that this compound possesses two transition phases: endothermic and exothermic between 170–180 °C and 185–195 °C, respectively.
The endothermic reaction is reversible (enantiotropic). Thus, different experimental conditions, i.e. masses sample, open
and crimped lids crucible, static and dynamic atmospheres were utilized for DSC analysis for evaluation of this transition
phase. In a static atmosphere the enantiotropic reaction was not observed. The obtained data were utilized to obtain the kinetic
parameters, which were calculated by the Capela and Ribeiro method. The results show that the activation energy for the transition
phase depends on the different experimental conditions. 相似文献
19.
A “dust-free” sol-gel microsphere pelletisation (SGMP) process has been developed for fabrication of (U,Pu)O2, (U,Pu)C and (U,Pu)N fuel pellets containing around 15% plutonium for the forthcoming prototype fast breeder reactor (PFBR)
in India. The objective was to produce homogeneous sintered pellets of ∼85% T.D. with a predominantly open-pore structure.
Hydrated gel-microspheres of UO3+PuO2 and UO3+PuO2+C have been prepared from nitrate solutions of uranium and plutonium by the “ammonia internal gelation” process, using hexamethylene
tetramine (HMTA) as an ammonia generator and silicone oil at 90±1°C as gelation bath. For oxide fuel pellets, the hydrated
UO3+PuO2 gel-microspheres were calcined at around 700°C in Ar+8% H2 atmosphere to produce “non-porous”, “free-flowing” and coarse (around 400 micron) microspheres which could be directly pelletised
at 550 MPa to green pellets. The mixed oxide pellets were subjected either to low temperature (∼1100°C) oxidative sintering
(LTS) in N2+air containing ∼1500 ppm O2 or to high temperature (1650°C) sintering, (HTS) in Ar+8% H2. For monocarbide and mononitride pellets, hydrated gel-microspheres of UO3+PuO2+C were subjected to carbothermic synthesis in vacuum (∼1 Pa) and flowing nitrogen (flow rate: 1.2 m3/h) in the temperature range of 1450–1550°C respectively. The monocarbide and mononitride microspheres thus produced were
relatively hard and required higher compaction pressure (∼1200 MPa) for making reen pellets which could be sintered to 85%
T.D. in Ar+8% H2 at 1700°C. The sintered oxide, monocarbide and mononitride pellets had a “blackberry” “open” pore microstructure with fine
grain size. The microspheres retained their individual identity in the sintered pellets because during sintering densification
took place mainly within and not between the microspheres. 相似文献
20.
Micro–mesoporous materials combining the structural and sorption characteristics of a mesoporous molecular sieve (MMS) and
zeolite BEA were obtained by the “dry gel conversion” method – partial zeolitization of silica MMS SBA-15 in the presence
of tetraethylammonium hydroxide. The volume of the mesopores reaches 0.65 cm3/g, while that of the micropores is in the region of 0.1 cm3/g. The acidity of the obtained zeolitized materials differs from that of BEA; the total concentration of medium-strength
acid centers (maximum thermal desorption of ammonia at ~315 °C) amounts to 0.15 mmol/g. 相似文献