The phonon frequency spectrum g(ω) of a crystal, such as body centred cubic (bcc) Rb, is known to be characterized by the Van Hove singularities at ω?≠?0. However, for a liquid metal like Rb, g(ω) has a single, hydrodynamic-like singularity, namely a cusp ∝ ω (1/2), at ω?=?0. Here, we note first that computer simulation on liquid Rb near freezing has revealed a rather well-defined Debye frequency ωD. Therefore, we propose here a zeroth-order model g0 (ω ) of g(ω) for Rb, which combines the Debye model with the ‘hydrodynamic’ ω (1/2) cusp. The corresponding velocity autocorrelation function 〈 v (t)·v (0)〉 has correctly a long-time tail ∝ t-(3/2). The terms from g0 (ω ) involving ωD are then damped by weak exponential factors exp (-α it), and the resulting first-order approximation, g1 (ω ) say, to the frequency spectrum is found to have features in common with the molecular dynamics (MD) simulation form. Thus ωD is fixed, as well as transport coefficients for the known thermodynamic state. The article concludes with a more qualitative discussion on supercooled liquids, and on metallic glasses such as Fe, for which MD simulations exist. 相似文献
Summary: The sol–gel transition of a radical chain cross‐linking copolymerization system [N‐vinylcaprolactam/2‐hydroxylethyl methacrylate/allyl methacrylate] has been studied using in situ time‐resolved dynamic light scattering (DLS) and in situ rheology. A critical dynamic behavior was observed near the sol–gel transition, which was characterized by the presence of a power‐law spectra over three decades in the time–intensity correlation function g2(t) − 1 ∼ t−μ and over two decades in the oscillatory shear experiment G′(ω) ∼ G″(ω) ∼ ωn. A comparison of the obtained critical exponents μ ≈ 0.62 and n ≈ 0.75 was made. The theory predicts a relationship between these exponents, but up to now no experimental comparison has been done. The experimental results favor the percolation model, with a fractal dimension df of the gel clusters of 1.67.
Double‐logarithmic plot of time–intensity correlation functions g2(t) − 1 versus the delay time t. 相似文献
A series of epoxy resins derived from diglycidyl ethers of bisphenol A with differing initial linear molecular chain lengths have been studied during and after curing with the diamines MDA (4,4′-methylene dianiline) and DDS (4,4′-diamino diphenyl sulfone). The properties that were measured during curing were the volume, the fictive temperature Tf, the gel fraction, the viscosity, and the equilibrium compliance. Graphs of Tf as a function the time of curing tc obtained at four curing temperatures between 40 and 100°C have been reduced to a common curve. After curing, creep compliance curves J(t) were determined which characterize the viscoelastic response from the glassy compliance level to a rubbery equilibrium compliance level. The change in properties that occurs during the time-dependent spontaneous densification below the glass temperature Tg was monitored with repeated measurements of J(t). Time-scale shift factors as a function of volume contraction obtained during this physical aging below Tg were reduced to a common curve. 相似文献
Quasielastic light scattering measurements are reported for experiments performed on mixtures of gelatin and glutaraldehyde
(GA) in the aqueous phase, where the gelatin concentration was fixed at 5 (w/v) and the GA concentration was varied from 1×10−5 to 1×10−3 (w/v). The dynamic structure factor, S(q,t), was deduced from the measured intensity autocorrelation function, g2(τ), with appropriate allowance for heterodyning detection in the gel phase. The S(q,t) data could be fitted to S(q,t)=Aexp(−Dfq2t)+Bexp(−t/τc)β, both in the sol (50 and 60 ∘C) and gel states (25 and 40 ∘C). The fast-mode diffusion coefficient, Df showed almost negligible dependence on the concentration of the crosslinker GA; however, the resultant mesh size, ξ, of the
crosslinked network exhibited strong temperature dependence, ξ∼(0.5−χ)1/5exp(−A/RT) implying shrinkage of the network as the gel phase was approached. The slow-mode relaxation was characterized by the stretched
exponential factor exp(−t/τc)β. β was found to be independent of GA concentration but strongly dependent on the temperature as β=β0+β1T+β2T2. The slow-mode relaxation time, τc, exhibited a maximum GA concentration dependence in the gel phase and at a given temperature we found τc(c)=τ0+τ1c+τ2c2. Our results agree with the predictions of the Zimm model in the gel case but differ significantly for the sol state.
Received: 25 May 1999 /Accepted in revised form: 27 July 1999 相似文献
The kinetic model of the physical process of evaporation of plasticizer from plasticized PVC foils was developed from the results of isothermal thermogravimetric investigation of evaporation of benzyl-butyl phthalate in the temperature range 120-150 °C under nitrogen flow. The kinetic parameters were estimated by integral method of analysis. Mathematical modeling of the kinetic of plasticizers evaporation was performed on the basis of function c=f(T,t) and kinetic equation of evaporation −dc/dt=f(T,c0,c(t)). The developed mathematical model was described by the general kinetic equation . The differential quotients δ(−dc/dt)/δT=f(T,c0,c(t))=f(T,c0,t) and δ(−dc/dt)/δc0=f(T,c(t))=f(T,c0,t) were performed, and mathematical definition of the changes of the evaporation rate constant with the change of temperature and the change of the initial plasticized concentration were discussed. 相似文献
Preparation, Vibrational Spectra, and Normal Coordinate Analysis of Hexachlororhenate(V) and Crystal Structure of [P(C6H5)4][ReCl6] By oxidation of A2[ReCl6], A = [(n-C4H9)4N]+, [P(C6H5)4]+, with Cl2 in dichloromethane/trifluoracetic acid A[ReCl6] is formed. [P(C6H5)4][ReCl6] crystallizes with tetragonal symmetry, space group P4/n-C, a = 12.967(4), c = 7.6992(8) Å, Z = 2. The octahedral complexion [ReCl6]? is compressed (C4v) with the bond lengths, axial Re? Cl1 = 2.28 and Re? Cl3 = 2.24 Å, equatorial Re? Cl2 = 2.31 Å. The infrared active antisymmetric Re? Cl stretching vibration is split into v′3 = 346 an v″3 = 326 cm?1. The assignment of all IR and Raman modes is confirmed by a normal coordinate analysis. The different valence force constants, fd(ReCl1) = 2.09, fd(ReCl3) = 2.10, fd(ReCl2) = 1.88 mdyn/ Å result from the distortion of the octahedron. On excitation with the Ar laser line 514.5 nm a resonance Raman spectrum is observed, showing 8 overtones of v′(A1) = 382 cm?1, from which the harmonic frequency ω1 = 382.1 cm?1, the anharmonicity constant X11 = ?0.76 cm?1, and the maximum bond dissociation energy of the [ReCl6]? ion to be 138 kcal/mol, are calculated. The vibrational fine structure of the intraconfigurational transitions in the near infrared has been resolved by measuring the absorption spectrum of [(n-C4H9)4N][ReCl6] at low temperature (10 K), resulting in the assignment of the following electronic origins: Γ3(3T1g) → Γ4(3T1g): 7 512, Γ3(3T1g) → Γ1(3T1g): 7 624 and Γ3(3T1g) → Γ5(1T2g), Γ3(1Eg): 8 368 cm?1. 相似文献
The temperature dependence of heat capacity Cpo = f(T) of fullerene derivative (t-Bu)12C60 has been measured by a adiabatic vacuum calorimeter over the temperature range T = 6–350 K and by a differential scanning calorimeter over the temperature range T = 330–420 K for the first time. The low-temperature (T ≤ 50 K) dependence of the heat capacity was analyzed based on Debye’s the heat capacity theory of solids and its fractal
variant. As a consequence, the conclusion about structure heterodynamicity is given. The experimental results have been used
to calculate the standard thermodynamic functions Cpo (T), Ho(T)−Ho(0), So(T) and Go(T) − Ho(0) over the range from T → 0 to 420 K. The standard entropy of formation at 298.15 K of fullerene derivative under study was calculated. The temperature
of decomposition onset of derivative was determined by differential scanning calorimetery and thermogravimetric analysis.
The standard thermodynamic characteristics of (t-Bu)12C60 and C60 fullerite were compared. 相似文献
The temperature dependence of the heat capacity Cpo= f(T) 2 of 2-ethylhexyl acrylate was studied in an adiabatic vacuum calorimeter over the temperature range 6–350 K. Measurement
errors were mainly of 0.2%. Glass formation and vitreous state parameters were determined. An isothermic shell calorimeter
with a static bomb was used to measure the energy of combustion of 2-ethylhexyl acrylate. The experimental data were used
to calculate the standard thermodynamic functions Cpo(T), Ho(T)-Ho(0), So(T)-So(0), and Go(T)-Ho(0) of the compound in the vitreous and liquid states over the temperature range from T → 0 to 350 K, the standard enthalpies of combustion ΔcHo, and the thermodynamic characteristics of formation ΔfHo, ΔfSo, and ΔfGo at 298.15 K and p = 0.1 MPa. 相似文献
The extraction of cerium(III) from weakly acidic chloride solutions by HDEHP-nitrobenzene-loaded polyurethane foams could be analyzed quantitatively in terms of the equation: log(9.056 Dc)=log Kc+2.14 log (Cd?6Cc)+3 pH+log fc where Dc is the distribution ratio of cerium(III) between the foam and aqueous phases, Cd and Cc are the total HDEHP and Ce(III) concentrations on the foam, respectively, log fc=[Ce3+](sq)/[ΣCe(III)](aq), and Kc is the equilibrium constant of the equation: Ce(aq)3++2.14(HX)2.8(o) ? ? CeX6·H3(o)+3H(aq)+. Values of Kc under the different extraction conditions tested are given. 相似文献
Temperature dependences of the heat capacities of disordered graphite-like nanostructures prepared by the thermobaric treatment
of fullerite C60 (p = 2 and 8 GPa, T = 1373 K) were measured in the temperature ranges from 7 to 360 K in an adiabatic vacuum calorimeter and from 330 to 650
K in a differential scanning calorimeter. At T < 50 K, the dependences obtained were analyzed using the Debye theory of the heat capacity of solids and its multifractal
version. The fractal dimensions D were determined and some conclusions on the heterodynamic character of the structures studied were made. The thermodynamic
functions CpoT), Ho(T) − Ho(0), So(T) − So(0), and Go(T) − Ho(0) were calculated in the temperature range from T → 0 to 610 (650) K. The thermodynamic properties of the graphite-like nanostructures studied and some carbon allotropes were
compared. The standard entropies of formation ΔfSo of the graphite nanostructures studied and diamond were calculated along with the standard entropies of the reactions of
their synthesis from the face-centered cubic phase of fullerite C60 and their interconversions at T = 298.15 K.
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1940–1945, September, 2008. 相似文献
Several predictions for a recently proposed mesoscopic model for polymer melts and concentrated solutions is presented. It is a single Kramers chain model in which elementary motions of the Orwoll-Stockmayer type are allowed. However, for this model, the bead jumps are no longer given by a Markovian probability, but rather are described by a fractal “waiting-time” distribution function, with a single adjustable parameter β, which describes the long-time behavior of the distribution: ∼ 1/t1+β. We find that the model predicts D ∼ 1/N2 and η0 ∼ N3.4 for β ≈︁ 1.4, where N is the degree of polymerization. The generalized model predicts that the relaxation spectrum has a plateau regime whose height is independent of N, but whose width is strongly N dependent, in agreement with experiment. The model also predicts that rings will diffuse somewhat more slowly than linear chains of the same molecular weight (about 80% as fast), with the same scaling dependence on N as linear chains, also in agreement with preliminary data. 相似文献
The ground state potential curve of Cl2 has been computed near Rc by means of the SCF, MC SCF, CI(SD), and the recently proposed CPF methods. The convergence of the total energy, of Dc and Rc is studied with the aid of computations for various basis sets which include up to three d, two f and one g set. Higher polarization functions have a larger effect than for F2 for all methods, the g set still affects Dc by 0.15 eV and Rc by 0.02 a0 on the CPF level. The most elaborate calculation, on the CPF [7,4,3,2,1] level, yields Dc and Rc with an accuracy of 0.08 eV and 0.02 a0. The same accuracy is obtained for the MC-178 CAS SCF treatment employing a 2d1f polarization basis set. The present results allow us to order the polarization functions according to their relative importance as: d(1) > f(1) > d(2) > f(2) = d(3) > g(1) for the SCF and d(1) > f(1) > d(2) > g(1) > f(2) = d(3) for methods including external correlation. CI(SD) or CPF. A comparison of the results for N2, F2, P2, and Cl2 shows the higher polarization functions d(3), f(2), g(1) to contribute 0.1 (F2) to 0.3 eV (F2, Cl2) to Dc and affect Rc by 0.005 a0 (N2) to 0.01 7 a0 (P2).We have focused our attention mainly on the impact of the atomic basis set incompleteness on the accuracy of results obtained from various methods of computation. Cl2 turns out to be more demanding than the first-row counterpart F2 on all levels of theory, which is mainly due to the slower angular convergence rate for E, Dc and Rc (for Cl2), tables 2 and 3. The more pronounced influence of higher polarization functions is already noticed on the SCF and MC SCF level: f(1) increases Dc by 0.18 eV and reduces Rc by 0.052 a0 on the MC-178 level for Cl2, table 6, typical corresponding results for F2 are only 0.04 eV and 0.013 a0 [1]. CAS SCF calculations furthermore appear to require larger active spaces for Cl2, as discussed in section 3.3.On the CI(SD) or CPF level — which aim to account for the entire external correlation — one even finds a pronounced influence of the first g set which contributes ≈ 0.15 eV to Dc and reduces Rc by ≈ 0.02 a0 (on the CPF level, table 3), the corresponding effects for F2 were only ≈ 0.04 eV and 0.01 a0 [1]. The 2d1f polarization basis, which will remain the “standard” large basis for treatments of tri- and tetraatomic molecules, appears to underestimate Dc by still 0.5 eV and to overestimate Rc by ≈ 0.02 a0 for P2 and Cl2, table 7, and probably all molecules in-between. This conclusion emerges from the cumulative effect of adding d(3), f(2) and g(1) which amounts already to 0.3 eV and 0.015 a0, table 7. 相似文献
Summary A thermodynamic treatment of homo-polymer systems out of linear chains with folded chain crystals is developed outgoing from appropriate models for single component systems. An expansion of thermodynamics to multi-micro-phase systems the structure of which is partially or totaly frozen is indispensable. General properties of melt crystallized homopolymers with folded chain crystals can be recognized indeed when the thermodynamic formalisms developed are applied.
Zusammenfassung Das Schmelzen in polymeren Einteilchensystemen mit Faltungskristallen einheitlicher Dicke kann thermodynamisch als Umwandlung 1. Ordnung in einer Richtung behandelt werden, wenn die Faltungslänge bis zur Umwandlungstemperatur konstant bleibt (Faltungslänge als innerer Zusatzparameter). Eine wesentliche begriffliche Erweiterung ist für eine phänomenologische Beschreibung mit den Mitteln der Thermodynamik unumgänglich, wenn eine Faltungskristallit-Dickenverteilung existiert, weil dann prinzipiell nur noch partielle Koexistenz bestimmter Fraktionen metastabiler autonomer Mikrophasen mit der Schmelze möglich ist. Partielles Aufschmelzen und Rektistallisation können so dann auch in Betracht genommen werden. Die entwickelten Konzeptionen bewähren sich in der Anwendung auf bekannte Experimente.
Notation
gc(y);gm(Y)
molar Gibbs-free energy of a chain of a lengthy within an extended chain crystal and the melt rsp
-
goc
;gom
molar free enthalpy of the unit in the crystal lattice and the melt rsp
-
g(y,y, f)
molar Gibbs-function of an ideally folded chain crystal with the fold heightyf
-
gco(y, yef,yf)
molar free enthalpy of the crystal coreyco
-
g0ex
((yef)
excess free enthalpy of the longitudinal layers of folded chain crystals
-
gf(yef,goex
)
molar free enthalpy of the longitudinal layers of the folded chain crystals
-
gtot
molar free enthalpy of a chain of the lengthy within a folded chain crystal with longitudinal layers
-
ho1c
,hom
molar enthalpy of the chain unit within the crystal lattice and the melt rsp
-
h =hom
-hoc
molar heat of fusion of the unit
-
Cp=Cpm
-Cpc
difference of the molar specific heat of a unit within the melt and within the chain crystal
-
hD
molar defect enthalpy of local defects within the crystal lattice
-
hD
molar defect enthalpy of the unit
-
soc
,som
molar entropy of the chain unit within the crystal lattice and the melt rsp
-
scm
conformational entropy of a chain in the melt
-
sgk
conformational entropy of a chain of lengthy within a super-lattice as indicated in figure 5,
- s
molar entropy of fusion of the melt
-
snc
nematic configurational entropy
-
T
absolute temperature
-
TM
melting temperature of extended chain crystals of infinite size
-
TM(y)
melting temperature of extended chain crystals containing only chains of the lengthy
-
TM (y, yf)
melting temperatureof folded chain crystals of the thicknessyf composed of chains of the lengthy
-
TM(yf)
melting temperature of folded chain crystals of the thicknessyfy
-
eh
excess free enthalpy of the chain ends occupying crystallographic places
-
ef
excess free enthalpy of a single fold loop
-
z
coordination number of the lattice
-
7
Euler's constant
-
R
Boltzmann's constant
-
y
number of chain units
-
yf
height of lamelliform folded chain crystals
-
f=(y/yf - 1)
number of fold loops of a chain of a lengthy when being built into a folded chain crystal of the thicknessyf
-
yco
thickness of the crystal core of the simplified twophase model
-
yet
average thickness of the surface layers of folded chain crystals
-
Nc
number of crystallized units of a chain of the lengthy
-
xc
molar number of crystallized units of a chain of the lengthy
-
xnc
molar number of noncrystallized units
-
excess free enthalpy parameter
-
(yf)
thickness distribution of the fold heightsyf
With 15 figures and 2 tables 相似文献
The temperature dependence of heat capacity of C70 fullerene was studied by calorimetry in the range between 6 and 390 K. Phase transitions were established and their thermodynamic
characteristics were determined. From the experimental data obtained, the thermodynamic functionsHo(T)-Ho(0),So(T),Go(T)-Ho(0) for temperatures between 0 and 390 K were calculated. The results were used to calculate the standard values of ΔfSo, ΔfGo, and logKfo
for the formation of C70 from graphite.
Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 647–650, April, 1998. 相似文献