The thermodynamic properties of polytetrafluoroethylene

Polytetrafluoroethylenes of different crystallinity were analyzed between 220 and 700 K by differential scanning calorimetry. A new computer coupling of the standard DSC is described. The measured heat capacity data were combined with all literature data into a recommended set of thermodynamic properties for the crystalline polymer and a preliminary set for the amorphous polymer (heat capacity, enthalpy, entropy, and Gibbs energy; range 0–700 K). The crystal heat capacities have been linked to the vibrational spectrum with a θ₃ of 54 K, and θ₁ of 250 K, and a full set of group vibrations. C_v to C_p conversion was possible with a Nernst–Lindemann constant of A = 1.6 × 10^?3 mol K/J. The glass transition was identified as a broad transition between 160 and 240 K with a ΔC_p of 9.4 J/K mol. The room-temperature transitions at 292 and 303 K have a combined heat of transition of 850 J/mol and an entropy of transition of 2.90 J/K mol. The equilibrium melting temperature is 605 K with transition enthalpy and entropy of 4.10 kj/mol and 6.78 J/K mol, respectively. The high-temperature crystal from is shown to be a condis crystal (conformationally disordered), and for the samples discussed, the crystallinity model holds.     

The heat capacity of solid poly(p-xylylene) and polystyrene

The constant-pressure heat capacity C_p of poly(p-xylylene) (PPX) has been measured from 220 to 625 K by differential scanning calorimetry. The constant-volume heat capacities C_v of both, PPX and its isomer polystyrene (PS) have been interpreted in the light of literature data on full normal-mode calculations for PS and estimates from low-molecular-weight analogs for PPX for the 39 group vibrations. Nine skeletal vibrations were used in this discussion with characteristic temperatures θ₁ and θ₃ of 534.5 and 43.1 K for PS. It was also possible to calculate a heat capacity contribution of a phenylene group within a polymer chain. Single 48-vibration θ₁ temperatures of 3230 K for PS and 2960 K for PPX are sufficient to describe C_v above 220 K. Below 140 K, PS heat capacity shows deviations from the Tarasov treatment.     

Quasi-isothermal measurement by TMDSC in isotactic polypropylene

We measure the frequency dependences of complex heat flows for isothermally crystallized isotactic polypropylene (iPP) by the quasi-isothermal TMDSC. Regarding the quasi-isothermal melting processes as a kind of the single relaxation process, we analyze them by the Debye model. The resultant heat capacity of iPP is larger (about 11%) than usual thermodynamic heat capacity. We also found that the excess of the heat capacity, C _p (excess), has non-monotonous temperature dependence. A simple model introducing some kinetic modes into amorphous producing after and during temperature modulation can reproduce the temperature dependence of C _p (excess) very well.     

The thermodynamic functions of fullerene derivative (<Emphasis Type="Italic">t</Emphasis>-Bu)<Subscript>12</Subscript>C<Subscript>60</Subscript> over the temperature range from <Emphasis Type="Italic">T</Emphasis> → 0 to 420 K

The temperature dependence of heat capacity C _p ^o = 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 C _p ^o (T), H ^o(T)−H ^o(0), S ^o(T) and G ^o(T) − H ^o(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.     

Thermodynamic properties of some polymeric forms of fullerite C<Subscript>60</Subscript> at temperatures ranging from 0 to 320 K

The temperature dependences of the heat capacityC ⁰ _p of fullerites C₆₀ were studied at temperatures ranging from 5 to 320 K in an adiabatic vacuum calorimeter with an accuracy of 0.4–0.2%. The fullerite C₆₀ samples were prepared by treating the starting fullerite C₆₀ under 8 GPa at 920 and 1270 K and “quenched” by a sharp decrease in pressure to −10⁵ Pa and in temperature to ∼300 K. Fullerite C₆₀(8 GPa, 920 K), a crystalline polymer with layered structure formed by polymerized fullerene C₆₀ molecules, was obtained at 920 K and 8 GPa. Fullerite C₆₀(8 GPa, 1270 K), a three-dimensional polymer with a graphite-like structure formed by fragments of decomposed C₆₀ molecules and containing many C(sp³)−C(sp³) bonds, was obtained at 1270 K and 8 GPa. Both polymers are metastable polymeric phases. The anomalous character of the temperature dependence of the heat capacity was revealed in the 49–66 K range for the polymer formed at 1270 K. The thermodynamic functions of the substances under study were calculated for the 0–320 K region along with entropies of their formation from graphite. The entropies of transformation of the starting fullerite C₆₀ into metastable phases and that of intertransformation of phases were estimated. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 277–281, February, 2000.     

Heat capacity of crystalline GaN

The heat capacity of gallium nitride has been measured by DSC method using DuPont Thermal Analyst 2100, DSC 951 unit in the temperature range (300–850 K). The temperature dependence of the heat capacity can be presented in the following form: C _p=32.960+0.162·10⁻¹ T+2360170T ⁻²-775370000T ⁻³.     

戊唑醇的分子结构、理论计算及热力学性质研究

在甲苯溶剂中利用缓慢蒸发法得到1-(4-氯苯基)-4,4-二甲基-3-(1H-1,2,4-三唑基甲基)戊醇-3（戊唑醇）的单晶,通过 X射线单晶结构分析法测定其晶体结构,晶体属单斜晶系,空间群为P2(1)/c,晶胞参数为：a = 1.1645(1) nm,b = 1.6768(2) nm,c = 1.7478(2) nm,β= 92.055(2),Dcalc.= 1.199 g/cm3,Z = 4,F(000)= 264。运用密度泛函理论 (DFT) B3LYP得到其优化几何构型并得到其频率。计算得到的结构参数与相应的实验值十分接近。运用微热量仪对标题物进行比热容测定,在所测温度范围283~353 K内,比热容随温度呈稳定的线性变化,根据测定的比热容方程,计算出戊唑醇以298.15 K为基础在283~353 K温区的的热力学函数：焓、熵和吉布斯自由能。     

Heat capacity of poly(vinyl methyl ether)

The heat capacity of poly(vinyl methyl ether) (PVME) has been measured using adiabatic calorimetry and temperature‐modulated differential scanning calorimetry (TMDSC). The heat capacity of the solid and liquid states of amorphous PVME is reported from 5 to 360 K. The amorphous PVME has a glass transition at 248 K (?25 °C). Below the glass transition, the low‐temperature, experimental heat capacity of solid PVME is linked to the vibrational molecular motion. It can be approximated by a group vibration spectrum and a skeletal vibration spectrum. The skeletal vibrations were described by a general Tarasov equation with three Debye temperatures Θ₁ = 647 K, Θ₂ = Θ₃ = 70 K, and nine skeletal modes. The calculated and experimental heat capacities agree to better than ±1.8% in the temperature range from 5 to 200 K. The experimental heat capacity of the liquid rubbery state of PVME is represented by C_p(liquid) = 72.36 + 0.136 T in J K^?1 mol^?1 and compared to estimated results from contributions of the same constituent groups of other polymers using the Advanced Thermal AnalysiS (ATHAS) Data Bank. The calculated solid and liquid heat capacities serve as baselines for the quantitative thermal analysis of amorphous PVME with different thermal histories. Also, knowing C_p of the solid and liquid, the integral thermodynamic functions of enthalpy, entropy, and free enthalpy of glassy and amorphous PVME are calculated with help of estimated parameters for the crystal. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 2141–2153, 2005     

Thermal Properties of Polycrystalline ZnIn2Se4

A pulse method was used to measure the thermal conductivity, specific heat capacity C _p and thermal diffusivityξ of polycrystalline ZnIn₂Se₄ in the temperature range 300–600 K. The temperature dependence of λ, C _p and ξ demonstrated a light decrease for this material in the temperature range 300–600 K, indicating that there is not a significant change in the structure in this temperature range; this was confirmed by DTA measurements. The results showed that the mechanism of heat transfer is due mainly to phonons; the contributions of electrons and dipoles are very small. This revised version was published online in July 2006 with corrections to the Cover Date.     

Heat capacity of methacetin in a temperature range of 6 to 300 K

Heat capacity of methacetin (N-(4-methoxyphenyl)-acetamide) has been measured in the temperature range 5.8–300 K. No anomalies in the C _p(T) dependence were observed. Thermodynamic functions were calculated. At 298.15 K, the values of entropy and enthalpy are equal to 243.1 J K⁻¹ mol⁻¹ and 36360 J mol⁻¹, respectively. The heat capacity of methacetin in the temperature range 6–10 K is well fitted by Debye equation C _p = AT ³. The thermodynamic data obtained for methacetin are compared with those for the monoclinic and orthorhombic polymorphs of paracetamol.     

Thermal conductivity and heat capacity of liquid and glassy poly(vinyl acetate) under pressure

The dependence of thermal conductivity λ and heat capacity per unit volume pc_p on temperature and pressure for poly(vinyl acetate) has been measured by a transient hot-wire probe technique. The measurements were made under pressures up to 0.5 GPa over a temperature range of 270–470 K. The temperature coefficient of thermal conductivity (? lnλ/?T)_p was found to increase with pressure for both the liquid and the glassy state. The change in heat capacity per unit volume in the region of the glass-transition temperature was found to decrease with increasing pressure. The Ehrenfest relation does not explain the variation of the pressure coefficient of the glass-transition temperature.     

Temperature dependence of the thermodynamic parameters of cobalt(II) clathrochelate: X-ray diffraction and calorimetric characteristics of the low temperature structural phase transition

The temperature dependence of heat capacity of the polycrystalline sample of cobalt(II) clathrochelate in a range of 6–300 K is studied. Based on the smoothed dependence C _p(T), the entropy and enthalpy values in a temperature range of 8–300 K and their standard values at 298.15 K are calculated. In the C _p(T) curve in a range of 50–70 K, a process is recorded whose entropy and enthalpy are 1.2 J·(K·mol⁻¹) and 68 J·mol⁻¹ respectively. A comparison of the results with the data of a multitemperature X-ray diffraction study makes it possible to attribute this process to the structural phase transition.     

Thermodynamic properties of poly[<Emphasis Type="Italic">bis</Emphasis>(trifluoroethoxy) phosphazene] in the range from <Emphasis Type="Italic">T</Emphasis>→0 TO 620 K

By adiabatic vacuum and dynamic calorimetry, heat capacity for poly[bis(trifluoroethoxy)phosphazene] has been determined over the 6–620 K range. Physical transformations of the polymer on its heating and cooling have been detected and characterized. Smoothed heat capacity C _p⁰(T) and standard thermodynamic functions (H ⁰(T)-H ⁰(0), S ⁰(T) and G ⁰(T)-H ⁰(0)) of poly[bis(trifluoroethoxy)phosphazene] have been evaluated for the temperature range from T→0 to 560 K. The standard entropy of formation Δ_f S ⁰ at T=298.15 K has been also determined. Fractal dimensions D in the heat capacity function of the multifractal variant of Debye’s theory of heat capacity of solids characterizing the heterodynamics of the tested polymer have been determined.     

Behavior of [`(a)] 20 \bar \alpha _2^0 T of aqueous urea near the singularity point

Relations for the apparent molar heat capacity ϕ_c of urea in an aqueous solution depending on the molality m and temperature were obtained. A transition to the relations ϕ_c(m,T) for D₂O-(ND₂)₂CO and T₂O-(NT₂)₂CO systems was effected by temperature scaling. At low temperatures, the isotherms of the molar heat capacity C _p(m) of the protium and deuterium systems have minima shifted to more dilute solutions at elevated temperatures. At m = 1, C _p of a solution does not depend on temperature in both systems. The dependences C _p(T) also have minima at constant concentrations. The temperature of the minimum heat capacity is most effectively lowered by small additions of urea. For m = 0.25, T _min is 7.5 K lower than T _min of pure water, and its heat capacity is 0.08 J/(mol K) higher. A transition from m = 1.5 to m = 2 lowers the temperature of the minimum heat capacity by 3.6 K; thus, the heat capacity of solutions differs by 0.02 J/(mol K) only.     

Heat capacity and characteristic thermodynamic functions of dysprosium boride DyB62 in the temperature range of 2 to 300 K

Heat capacity at constant pressure C _p (T) of a dysprosium boride DyB₆₂ 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 DyB₆₂ enthalpy, entropy, Gibbs energy, and standard values of these thermodynamic functions were calculated.     

Heat capacity of amorphous polyhexene-1 between 20 and 300°K: Intramolecular vibrational contribution to Cv below the glass transition

The heat capacity of polyhexene-1 was measured between 20 and 300°K. The apparatus, an adiabatic calorimeter giving results with a random error of 0.2–0.4%, is briefly described. The characterization of the sample by x-ray diffraction patterns established that it was amorphous at all temperatures. Gold foil was incorporated with the sample to increase the apparent thermal diffusivity and so to decrease the time needed for the measurements. The glass transition temperature was found to be 215.5 ± 1°K. On the C_p curve, no subglass anomaly was detected, unlike the results of experiments described elsewhere. The calculation of C_v is discussed, and an explanation is given for the choice of the number of intramolecular vibrational modes per monomer which are assumed to contribute to C_v. A linear continuum model with characteristic temperature θ₁ = 736°K allows us to fit the experimental curve over a temperature range of 140°K.     

Calorimetric relaxation and glass transition in poly(propylene glycols) and its monomer

The glass transition temperature T_g of propylene glycol (PG) and poly(propylene glycols) (PPGs) of molecular weight up to 4000 has been measured by differential scanning calorimetry, and the activation energy and change in heat capacity ΔC_p have been determined in the glass transition range. The activation energy increases with an increase in the molecular weight of the polymer, and ΔC_p measured at a fixed heating rate decreases. The increase in T_g with molecular weight is remarkably more rapid for poly(propylene glycols) than for other polymers, and a limiting value of T_g is reached for a chain containing 20 monomer units. These results are discussed in terms of the Fox-Flory and the entropy theories. The calorimetric relaxation times are comparable with the extrapolated dielectric relaxation times. The initial increase of ΔC_p from PG to PPG 200 is attributed to the decrease of H-bonding sites from 12 in 3 monomers to 4 on polymerization to PPG 200 and further decrease with increase in molecular weight to an increasingly large amplitude of the β-process at T < T_g.     

Heat capacity of poly(trimethylene terephthalate)

The heat capacity of poly(trimethylene terephthalate) (PTT) has been measured using adiabatic calorimetry, standard differential scanning calorimetry (DSC), and temperature-modulated differential scanning calorimetry (TMDSC). The heat capacities of the solid and liquid states of semicrystalline PTT are reported from 5 to 570 K. The semicrystalline PTT has a glass transition temperature of 331 K. Between 340 and 480 K, PTT can show exothermic ordering depending on the prior degree of crystallization. The melting endotherm of semicrystalline samples occurs between 480 and 505 K, with a typical onset temperature of 489 K (216°C). The heat of fusion of the semicrystalline samples is about 15 kJ mol⁻¹. For 100% crystalline PTT the heat of fusion is estimated to be 30 ± 2 kJ mol⁻¹. The heat capacity of solid PTT is linked to an approximate group vibrational spectrum and the Tarasov equation is used to estimate the heat capacity contribution due to skeletal vibrations (θ₁ = 550.5 K and θ₂ = θ₃ = 51 K, N_skeletal = 19). The calculated and experimental heat capacities agree to better than ±3% between 5 and 300 K. The experimental heat capacities of liquid PTT can be expressed by: $ C^L_p(exp) $ = 211.6 + 0.434 T J K⁻¹ mol⁻¹ and compare to ±0.5% with estimates from the ATHAS data bank using contributions of other polymers with the same constituent groups. The glass transition temperature of the completely amorphous polymer is estimated to be 310–315 K with a ΔC_p of about 94 J K⁻¹ mol⁻¹. Knowing C_p of the solid, liquid, and the transition parameters, the thermodynamic functions enthalpy, entropy, and Gibbs function were obtained. With these data one can compute for semicrystalline samples crystallinity changes with temperature, mobile amorphous fractions, and resolve the question of rigid-amorphous fractions.© 1998 John Wiley & Sons, Inc. J. Polym. Sci. B Polym. Phys. 36: 2499–2511, 1998     

Analysis of the residual entropy of amorphous polyethylene at 0 K*

The residual entropy of amorphous polyethylene (PE) at 0 K is discussed within the framework of the heat capacity (C_p). The measured C_p of the liquid was extended from the glass transition to low temperature by separately finding its three parts—the vibrational, conformational, and external contributions—and extrapolating each to low temperature. The vibrational C_p was calculated from the frequency distributions of the group vibrations on the basis of force constants obtained from experimental infrared and Raman spectra as well as the skeletal vibrations in the amorphous solid (glass) obtained from fitting of the appropriate experimental C_p to Debye functions in the form suggested by Tarasov. The conformational part of C_p was evaluated from a fit of the heat capacity of the liquid, decreased by the contributions of the vibrational and external parts, to a one‐dimensional Ising model that can be extrapolated to 0 K and requires two discrete states described by stiffness, cooperativity, and a degeneracy parameter. The external part was computed from the experimental data for expansivity and compressibility, fitted to an empirical equation of state, and modified at low temperatures in accordance with the Nernst–Lindemann approximation. The computed C_p of the liquid PE agreed with the experiment from 600 K to the beginning of the glass transition at about 260 K. Extending the heat capacity to 0 K, bypassing the freezing of the large‐amplitude conformational motion in the glass transition, led to a positive residual entropy and enthalpy and avoided the so‐called Kauzmann paradox. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 1245–1253, 2002     

Thermodynamic properties of α-platinum dichloride

The heat capacity of crystalline α-platinum dichloride was measured for the first time in the temperature intervals from 11 to 300 K (vacuum adiabatic microcalorimeter) and from 300 to 620 K (differential scanning calorimetry). In the 300–620 K temperature interval, the C° _p values for α-PtCl₂ (cr) coincide with the heat capacity of CrCl₂ (cr) within the limits of experimental error, which made it possible to estimate the heat capacity of α-PtCl₂ (cr) at higher temperatures. The approximating equation of the temperature dependence of the heat capacity in the interval from 298 to 900 K C° _p (±0.8) = 63.5 + 21.4·10⁻³ T + 0.883·10⁵/T ² (J mol⁻¹ K⁻¹) was derived using the experimental values, as well as the literature data on the heat capacity of CrCl₂ (cr). For the standard conditions, the C° _p,298.15 and S°_298.15 values are 70.92±0.08 and 100.9±0.33 J mol⁻¹ K, respectively; H°_298.15 − H°₀ = 14 120±42 J mol⁻¹. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1136–1138, June, 2008.