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1.
Three component percolative W/O microemulsions were studied by differential scanning calorimetry. Water-AOT-Decane, D 2O-AOT-Decane, Water-AOT-Isooctane and Water-Ca(AOT) 2-Decane systems were analyzed. Thus by changing, in the order, the dispersed phase, the dispersing medium, and by modifying
the interphase region. The thermal history of the samples was monitored by a suitable thermal program. Following the latter,
first order phase transitions associated with the freezing and/or melting of the two massive phases were obtained, as well
as the higher order phase transition associated with the percolation process. From the melting spectra an estimate of the
amount of water bound to the hydrophilic groups of the AOT as well as of that of oil bound to the hydrophobic surfactant tails
was obtained. The latter result shows a difference in the behaviour of the continuous oily phase at the O/W interphase. From
the freezing spectra, the percolative character of the microemulsion was evidenced by the exotherms associated with the freezing
of the water phase.
This work was supported by M.U.R.S. T. and I.N.F.M. 相似文献
2.
Composite materials (CM) based on poly(ethylene) (PE) and nanocrystalline nickel (Ni) have been produced. The effect of the
content of nanocrystalline Ni and processes of structure formation of its particles on a melting temperature ( T
m), interval of melting, true melting heat (Δ H
m), degree of crystallinity (χ) as well as characteristics of CM thermodestruction have been determined by DTA and thermogravimetry
techniques. It was found that these characteristics are changed non-linearly when the content of nanocrystalline Ni increases.
The most efficient influence of Ni on the above mentioned characteristics was observed for its low content (0.01 volume part
of Ni). It was shown that a formation of a branched multifractal cluster of nickel above a percolation threshold favored a
decrease in T
m, Δ H
m, χ of filled PE and a majority of thermal characteristics of CM thermodestruction as well.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
3.
The
melting and crystallisation behaviour of poly( m-xylene
adipamide) (MXD6) are investigated by using the conventional DSC, X-ray diffraction
and polarised light microscopy. Triple, double or single melting endotherms
are obtained in subsequent heating scan for the samples after isothermal crystallisation
from the melt state at different temperatures. The lowest melting peak can
be ascribed to the melting of secondary crystals. The melting of primary crystals
causes the medium melting peak and the highest melting peak is attributed
to the melting of recrystallised species formed during heating. Following
the Hoffman–Weeks theory, the equilibrium melting temperature is equal
to 250°C and the equilibrium melting enthalpy Δ H
m
0
to 175 J g –1. Then, using the Lauritzen–Hoffmann
theory of secondary crystallisation, the analyse of the spherulitic growth
shows that the temperature of transition between the growing regimes II and
III is equal to 176°C. Finally the Gibbs-Thomson relationship allows the
determination of the distribution function of crystalline lamellae. 相似文献
4.
The thermal properties of a perfluoropolyether (PFPE) W/O microemulsion were investigated by Differential Scanning Calorimetry
(DSC) both on freezing the liquid samples and upon their melting. PFPE systems as a function of increasing volume fraction
(Φ=water+surfactant/total), were studied along a dilution line with a water/surfactant molar ratio W/S=11. The percolative
nature of these systems emerged directly from the spreading of the exothermic peaks associated with the freezing of the dispersed
phase. This behaviour was found to depend on whether the starting temperature of the DSC measure was at, below or above the
percolative threshold temperature of the given sample. A low temperature 'needle-like' peak was found around 143 K, immediately
after the glass transition due to the oil continuous phase. The low temperature peak was also present in other percolative,
three-component microemulsions. The higher order phase transition at the percolation temperature was also evidenced.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
5.
Differential
scanning calorimetry (DSC) is a thermal analytical tool for preformulation
studies. Extrapolated melting temperature ( TP)
and heat of fusion (Δ Hf)
can be used as parameters for optimizing the DSC performance. Two model pharmaceuticals
acetaminophen and nicotinamide are used in this study. Using a factorial design
for the experimental model and matrix analysis the results, the effect of
sample mass, heating rate and the nitrogen flow rate were evaluated on the
Δ Hf values
and TP values. Two
levels for each of the procedural variables were used as a balanced experimental
design with two sample sizes, two heating rates and two nitrogen flow rates.
It was found that the change in the heating rate caused significant changes
in the Δ Hf
values but not the Tp
values for acetaminophen. However, no significant effect was found for the Tp value but Δ Hf value was affected to a
certain extent for nicotinamide. 相似文献
6.
The relative errors ( e%) in the determination of the activation energy from the slope of the Kissinger straight line ln(β/β T
p2) vs. 1/ T
p (β is the heating rate) are in-depth discussion. Our work shows that the relative errors is a function containing the factors
of x
p and Δ x
p, not only x
p ( x
p = E/ RT
p, E is the activation energy, T
p is the temperature corresponding to maximum process rate, R is the gas constant). The relative error between E
k and E
p will be smaller with the increase of the value of x and/or with the decrease of the value of Δ x. For a set of different heating rates in thermal analysis experiments, the low and close heating rates are proposed from
the kinetic theory. 相似文献
7.
The sensitivity and precision of headspace solid-phase micro extraction (HS-SPME) at an analyte solution temperature ( T
as) of +35 °C and a fiber temperature ( T
fiber) of +5 °C were compared with those for HS-SPME at T
as and T
fiber of −20 °C for analysis of the volatile organic compounds benzene, 1,1,1-trichloroethane, trichloroethylene, toluene, o-xylene, ethylbenzene, m/ p-xylene, and tetrachloroethylene in water samples. The effect of simultaneous fiber cooling and analyte solution freezing
during extraction was studied. The compounds are of different hydrophobicity, with octanol/water partition coefficients ( Kow) ranging from 126 and 2511. During a first set of experiments the polydimethylsiloxane (PDMS) SPME fiber was cooled to
+5 °C with simultaneous heating of the aqueous analyte solution to +35 °C. During a second set of experiments, both SPME fiber
holder and samples were placed in a deep freezer maintained at −20 °C for a total extraction time of 30 min. After approximately
2 min the analyte solution in the vial began to freeze from the side inwards and from the bottom upwards. After approximately
30 min the solution was completely frozen. Analysis of VOC was performed by coupling HS-SPME to gas chromatography-mass spectrometry
(GC-MS). In general, i.e. except for tetrachloroethylene, the sensitivity of HS-SPME increased with increasing compound hydrophobicity
at both analyte solution and fiber temperatures. At T
as of +35 °C and T
fiber of +5 °C detection limits of HS-SPME were 0.5 μg L −1 for benzene, 1,1,1-trichloroethane, trichloroethylene, and tetrachloroethylene, 0.125 μg L −1 for toluene, and 0.025 μg L −1 for ethylbenzene, m/ p-xylene, and o-xylene. In the experiments with T
as and T
fiber of −20 °C, detection limits were reduced for compounds of low hydrophobicity ( Kow<501), for example benzene, toluene, 1,1,1-trichloroethane, and trichloroethylene. In the concentration range 0.5–62.5 μg
L −1, the sensitivity of HS-SPME was enhanced by a factor of approximately two for all compounds by performing the extraction
at −20 °C. A possible explanation is that freezing of the water sample results in higher concentration of the target compounds
in the residual liquid phase and gas phase (freezing-out), combined with enhanced adsorption of the compounds by the cooled
fiber. The precision of HS-SPME, expressed as the relative standard deviation and the linearity of the regression lines, is
increased for more hydrophobic compounds ( Kow>501) by simultaneous direct fiber cooling and freezing of analyte solution. Background contamination during analysis is
reduced significantly by avoiding the use of organic solvents. 相似文献
8.
Heat capacity at constant pressure C
p
( T) of a dysprosium boride DyB 62 single crystal obtained by zone melting was studied experimentally in the temperature range of 2 to 300 K. Abnormally high
values of dysprosium boride heat capacity were revealed in the range of 2–20 K, due to the magnetic contribution and the effect
of disorder in the boride lattice. Temperature changes in DyB 62 enthalpy, entropy, Gibbs energy, and standard values of these thermodynamic functions were calculated. 相似文献
9.
Segmented poly(ether‐ block‐amide) copolymers are typically known as polyamide‐based thermoplastic elastomers consisting of hard, crystallizable polyamide block and flexible, amorphous polyether block. The melting characteristics of a poly(ether‐ block‐amide) copolymer melt‐crystallized under various quiescent, isothermal conditions were calorimetrically investigated using differential scanning calorimetry (DSC). For such crystallized copolymer samples, their crystalline structures under ambient condition and the structural evolutions upon heating from ambient to complete melting were characterized using ambient and variable‐temperature wide‐angle X‐ray diffractometry (WAXD), respectively. It was observed that dependent of specific crystallization conditions, the copolymer samples exhibited one, two, or three melting endotherms. The ambient WAXD results indicated that all melt‐crystallized copolymer samples only exhibited γ‐form crystals associated with the hexagonal habits of the polyamide homopolymer, whereas variable‐temperature WAXD data suggested that upon heating from ambient, a melt‐crystallized copolymer might exhibit so‐called Brill transition before complete melting. Based on various DSC and variable‐temperature WAXD experimental results obtained in this study, the applicability of different melting mechanisms that might be responsible for multiple melting characteristics of various crystallized PEBA copolymer samples were discussed. It was postulated that the low ( T m1) endotherm was primarily because of the disruption of less thermally stable, short‐range ordered structure of amorphous polyamide segments of the copolymer, which was only formed after the completion of primary crystallization via so‐called annealing effects. The intermediate ( Tm2) and high ( Tm3) endotherms were attributed to the melting of primary crystals within polyamide crystalline microdomains of the copolymer. The appearance of these two melting endotherms might be somehow complicated by thermally induced Brill transition. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 2035–2046, 2008 相似文献
10.
Melt crystallization behaviors of poly(ethylene terephthalate) (PET) and poly(ethylene terephthalate‐ co‐isophthalate) (PETI) containing 2 and 12 mol % of noncrystallizable isophthalate components were investigated. Differential scanning calorimetry (DSC) isothermal results revealed that the introduction of 2 mol % isophthalate into PET caused a change of the crystal growth process from a two‐dimensional to a three‐dimensional spherulitic growth. The addition of more isophthalate up to 12 mol % into the PET structure induced a change in the crystal growth from a three‐dimensional to a two‐dimensional crystal growth. DSC heating scans after completion of isothermal crystallization at various Tc's showed three melting endotherms for PET and four melting endotherms for PETI‐2 and PETI‐12. The presence of an additional melting endotherm is attributed to the melting of copolyester crystallite composed of ethylene glycol, tere‐phthalate, and isophthalate (IPA) or the melting of molecular chains near IPA formed by melting the secondary crystallite Tm (I) and then recrystallizing during heating. Analyses of both Avrami and Lauritzen‐Hoffman equations revealed that PETI containing 2 mol % of isophthalate had the highest Avrami exponent n, growth rate constant Go, and product of lateral and end surface free energies σσ e. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 2515–2524, 2000 相似文献
11.
The peak temperature ( T
p) and different temperature (Δ T) are the basic information in the differential thermal analysis (DTA). Considering the kinetic relation and the heat equilibrium
in DTA, a correctional differential kinetic equation (containing T
p and Δ T parameter) is proposed. In the dehydration reaction of CaC 2O 4·H 2O, the activation energy calculated from the new equation showed some smaller than that from Kissinger equation, but some
bigger than that from Piloyan equation. 相似文献
12.
Two closely series of poly(ester imide)s had been synthesized by solution polycondensation of p‐phenylenebis(trimellitate) dianhydride with aliphatic diamines. The differential scanning calorimetry (DSC) traces of the most poly(ester imide)s exhibited two endotherms representing the solid state to anisotropic phase transition ( Tm1) and the anisotropic to isotropic melt transition ( Tm2), respectively. Observation under polarizing microscope and wide‐angle X‐ray diffraction (WAXD) measurements suggested that the anisotropic phase formed above the melting points ( Tm1) had a smectic character. The thermogravimetric analyses (TGA) revealed that the thermal stabilities of the poly(ester imide)s were up to 350°C. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 211–218, 1999 相似文献
13.
We studied thermal transitions and physical stability
of oil-in-water emulsions containing different milk fat compositions, arising
from anhydrous milk fat alone (AMF) or in mixture (2:1 mass ratio) with a
high melting temperature (AMF–HMT) or a low melting temperature (AMF–LMT)
fraction. Changes in thermal transitions in bulk fat and emulsion samples
were monitored by differential scanning calorimetry (DSC) under controlled
cooling and reheating cycles performed between 50 and –45°C (5°C
min –1). Comparison between bulk fat samples
and emulsions indicated similar values of melting completion temperature,
whereas initial temperature of fat crystallization ( Tonset)
seemed to be differently affected by storage temperature depending on triacylglycerols
(TAG) composition. After storage at 4°C, Tonset
values were very similar for emulsified and non-emulsified AMF–HMT blend,
whereas they were lower (by approx. 6°C) for emulsions containing AMF
or mixture of AMF–LMT fraction. After storage at –30°C, Tonset values of re-crystallization
were higher in emulsion samples than in bulk fat blends, whatever the TAG
fat composition. Light scattering measurements and fluorescence microscopic
observations indicated differences in fat droplet aggregation-coalescence
under freeze-thaw procedure, depending on emulsion fat composition. It appeared
that under quiescent freezing, emulsion containing AMF–LMT fraction
was much less resistant to fat droplet aggregation-coalescence than emulsions
containing AMF or AMF–HMT fraction. Our results indicated the role of
fat droplet liquid-solid content on emulsion stability. 相似文献
15.
Thermodynamic stability of CdMoO 4 was determined
by measuring the vapor pressures of Cd and MoO 3 bearing
gaseous species. Th vaporization reaction could be described as CdMoO 4( s)+MoO 2( s)
=Cd( g)+2/ n(MoO 3) n
( n=3, 4 and 5). The vapor pressures of
the cadmium ( p
Cd)
and trimer ( p
(MoO3)3)
measured in the temperature range 987≤ T/K≤1111
could be expressed, respectively, as ln ( p
Cd/Pa)
= –32643.9/ T+29.46±0.08 and
ln( p
(MoO3)3/Pa) = –32289.6/ T+29.28±0.08. The standard molar Gibbs free
energy of formation of CdMoO 4( s),
derived from the vaporization results could be expressed by the equations:
° f
G
CdMoO4
(s)
0= –1002.0+0.267 T±14.5 kJ mol –1
(987≤ T/K≤1033) and ° f
G
CdMoO4 (s)
0
= –1101.9+0.363 T±14.4 kJ mol –1
(1044≤ T/K≤1111). The standard enthalpy
of formation of CdMoO 4( s)
was found to be –1015.4±14.5 kJ mol –1
. 相似文献
16.
In order to develop a new functional product from lignin, sodium lignosulfonate (LS)-based polyurethane (LSPU) hydrogels were prepared from LS and hexamethylene diisocyanate (HDI) derivatives in water. Isocyanate/hydroxyl group ratio (NCO/OH ratio) was varied from 0.05 to 0.8 mol mol−1, and water content (Wc = mass of water/mass of dry sample) of the obtained LSPU hydrogels was varied from 0 to 3.0 g g−1. Phase transition behavior of hydrogels with various Wc’s was investigated by differential scanning calorimetry (DSC) and thermogravimetry (TG). In DSC heating curve of LSPU hydrogels, glass transition, cold crystallization, melting and liquid crystallization were observed. Cold crystallization, two melting peaks and variation of melting enthalpy indicate that three kinds of water, i.e., non-freezing water, freezing bound water and free water, exist in LSPU hydrogel. Glass transition temperature (Tg) decreased from 230 to 190 K in a Wc range where non-freezing water was formed in the hydrogel. Tg increased when freezing bound water was formed in the system. Tg leveled off in a Wc range where normal ice was formed. The effect of NCO/OH ratio on molecular motion of LSPU hydrogel is examined based on Tg and heat capacity difference at Tg (ΔCp). Water vaporization curve measured by TG also indicates the presence of bound water which evaporates at a temperature higher than ca. 410 K. By atomic force microscopic observation, the size of molecular bundle of LSPU hydrogel is calculated and compared with that of LS-water system. By cross-linking, the height of molecular bundle decreased from ca. 3–1 nm and lignin molecules extend in a flat structure. 相似文献
17.
Glasses with compositions 60B 2O 3–40PbO, 60B 2O 3–40Bi 2O 3, and 60B 2O 3–30Bi 2O 3–10PbO have been prepared and studied by differential thermal analysis. The crystallization kinetics of the glasses was investigated
under non-isothermal conditions. From dependence of the glass transition temperature ( T
g) on the heating rate, the activation energy for the glass transition was derived. Similarly the activation energy of the
crystallization process was determined. Thermal stability of these glasses were achieved in terms of the characteristic temperatures,
such as the glass transition temperature, T
g, the onset temperature of crystallization, T
in
, the temperature corresponding to the maximum crystallization rate, T
p, beside the kinetic parameters, K( T
g) and K( T
p). The results revealed that the 60B 2O 3–40PbO is more stable than the others. The crystallization mechanism is characterized for glasses. The phases at which the
glass crystallizes after the thermal process have been identified by X-ray diffraction. 相似文献
18.
In the present research for the first time, the heat capacity
C\textp ° C_{\text{p}}^{ \circ } of crystalline tetraphenylantimony acetophenoneoxymate Ph 4SbONCPhMe has been measured using the methods of precision adiabatic vacuum calorimetry over the range from 6 to 350 K, the
standard thermodynamic functions: heat capacity
C\textp ° ( T ) C_{\text{p}}^{ \circ } (T ) , enthalpy H°( T) − H°(0), entropy S°( T), and Gibbs function G°( T) − H°(0) have been calculated over the range from T → 0 K to 350 K. Low-temperature (20 K ≤ T ≤ 50 K) heat capacity data have shown a chain-layered structure topology of the compound under study. The energy of combustion
of the compound has been determined in the isothermal combustion calorimeter with a stationary bomb. The standard thermodynamic
functions of formation of crystalline Ph 4SbONCPhMe at 298.15 K have been calculated. The differential scanning calorimetry and thermogravimetric analysis studies have
shown the compound melts with decomposition and its melting temperature has been estimated. Thermodynamic properties of Ph 4SbONCPhMe, Ph 5Sb and Ph 4SbONCPh 2 have been compared. 相似文献
19.
Heat capacity C
p( T) of the orthorhombic polymorph of L-cysteine was measured in the temperature range 6–300 K by adiabatic calorimetry; thermodynamic functions were calculated
based on these measurements. At 298.15 K the values of heat capacity, C
p; entropy, S
m0( T)- S
m0(0); difference in the enthalpy, H
m0( T)- H
m0(0), are equal, respectively, to 144.6±0.3 J K −1 mol −1, 169.0±0.4 J K −1 mol −1 and 24960±50 J mol −1. An anomaly of heat capacity near 70 K was registered as a small, 3–5% height, diffuse ‘jump’ accompanied by the substantial
increase in the thermal relaxation time. The shape of the anomaly is sensitive to thermal pre-history of the sample. 相似文献
20.
The
thermal effect accompanying the transition of Cu 2–xSe
into a superionic conduction state was studied by non-isothermal measurements,
at different heating and cooling rates (β=1, 2.5, 5, 10 and 20°C
min –1). During heating the peak temperature
( Tp) remains almost
stable for all values of β, (136.8±0.4°C for Cu 2Se
and 133.0±0.3°C for Cu 1.99Se). A gradual
shift of the initiation of the transformation towards lower temperatures is
observed, as the heating rate increases. During cooling there is a significant
shift in the position of the peak maximum ( Tp)
towards lower temperatures with the increase of the cooling rate. A small
hysteresis is observed, which increases with the increase of the cooling rate, β.
The mean value of transformation enthalpy was found to be 30.3±0.8
J g –1 for Cu 2Se and
28.9±0.9 J g –1 for Cu 1.99Se.
The transformation can be described kinetically by the model f(ǯ)=(1–ǯ) n(1+kcatX), with activation energy E=175 kJ mol –1,
exponent value n equal to 0.2, log A=20 and log( kcat)=
0.5. 相似文献
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