Modulated Differential Scanning Calorimetry Observation of Physical Ageing in Polymers

Modulated-temperature differential scanning calorimetry (M-TDSC) is becoming a useful tool in the characterisation of thermal behaviour of polymers. In this paper, we discuss whether the non-reversing M-TDSC signal can be used, quantitatively and directly, to study the process of physical ageing in polymers. Difference exists between the values of relaxation enthalpy determined by using average heat capacity, <C _p>, signal as for conventional DSC and non-reversing heat capacity, C ⁿ _p, signal. When the signal of reversing heat capacity of unaged sample is considered as baseline for <C _p> and C ⁿ _p signals, the difference disappears. It is concluded that non-reversing M-TDSC signal can be used to observe the process of physical ageing semi-qualitatively and directly. With increasing annealing time, the peak of the imaginary part, C″_p, of the complex heat capacity becomes narrow, but peak area changes little. This indicated that C″_p is not correlated with relaxation enthalpy. It may be related to entropy change during the modulation. However, the entropy change is quite small. This revised version was published online in August 2006 with corrections to the Cover Date.     

Low-temperature heat capacities and derived thermodynamic functions of cyclohexane

The low-temperature heat capacities of cyclohexane were measured in the temperature range from 78 to 350 K by means of an automatic adiabatic calorimeter equipped with a new sample container adapted to measure heat capacities of liquids. The sample container was described in detail. The performance of this calorimetric apparatus was evaluated by heat capacity measurements on water. The deviations of experimental heat capacities from the corresponding smoothed values lie within ±0.3%, while the inaccuracy is within ±0.4%, compared with the reference data in the whole experimental temperature range. Two kinds of phase transitions were found at 186.065 and 279.684 K corresponding solid-solid and solid-liquid phase transitions, respectively. The entropy and enthalpy of the phase transition, as well as the thermodynamic functions {H_(T)-H _{298.15 K}} and {S _(T)-S_{298.15 K}}, were derived from the heat capacity data. The mass fraction purity of cyclohexane sample used in the present calorimetric study was determined to be 99.9965% by fraction melting approach. This revised version was published online in July 2006 with corrections to the Cover Date.     

Measurements of excess molar enthalpy and excess molar heat capacity of (1-heptanol or 1-octanol)+(diethylamine or <Emphasis Type="Italic">s</Emphasis>-butylamine) mixtures at 298.15 K and 0.1 MPa

Experimental data of excess molar enthalpy (H _m^E) and excess molar heat capacity (C _pm^E) of binary mixtures containing (1-heptanol or 1-octanol)+(diethylamine or s-butylamine) have been determined as a function of composition at 298.15 K and at 0.1 MPa using a modified 1455 Parr solution calorimeter. The excess molar enthalpy data are negative and show parabolic format over the whole composition range; however, the excess molar heat capacity values, whose curves show a S-shape, are positive in the 0.0 to 0.7 molar fraction range and negative between the molar fraction values 0.7 to 1.0. The applicability of the ERAS-model to correlate the excess molar enthalpy data was tested. The calculated data values are in good agreement with the experimental ones. The experimental behavior of H _m^E is interpreted in terms of specific interactions between 1-alkanol and amine molecules.     

Thermal and kinetic study of the ferroelectric phase transition in deuterated triglycine selenate

The specific heat and the enthalpy variation of a highly deuterated crystal of ferroelectric triglycine selenate have been measured around its first-order phase transition using the technique square modulated differential thermal analysis (SMDTA). The low temperature variation rate has allowed analyzing the kinetics of the phase transition. Due to an internal crack in the sample, the transition is carried out in two steps and an intermediate region where the transition is blocked and both phases coexist without transformation has been found. The latent heat on cooling (L _c=1.32±0.02 J g^–1) is higher than on heating (L _h=1.08±0.02 J g^–1) due to the thermal hysteresis and the great difference between the specific heat in both phases. Nevertheless, the enthalpy balance is fulfilled on heating and on cooling.     

Microcalorimetric study of the biodegradation of cellulolytic compounds

Microcalorimetry can be used as a tool for the analysis of microbial growth on soluble or insoluble substrates. Modern apparatus allows studies under aerobic or anaerobic growth culture conditions. The heat quantity evolved during any microbial growth is dependent on catabolic and anabolic reactions according to:△H _met=(1-α)△H _cat+α△H _an In this relationship, δH_met, δH_cat, and δH_an are the enthalpy variations associated with metabolic, catabolic, and anabolic reactions, respectively, and a represents the molar fraction of the energy source that is incorporated into cellular carbon. The value of α is strongly influenced by culture conditions. Under anaerobic conditions, the α coefficient is low and does not exceed 0.2. Under aerobic conditions, α can reach a value as high as 0.6. Some experimental results were presented and discussed. Modern microcalorimeters record the power evolved by microbial growth as a function of time. The curves obtained are called power-time curves. These curves can be used for the determination of growth parameters. However, power-time curves allow visualization of some catabolic phenomena that are not related to growth because they occur during the stationary growth phase. Some examples were shown. More recently, microcalorimetry has been used for studies of degradation of insoluble substrates, such as lignocellulosic compounds, by pure or mixed cultures of microbes. The results obtained with pure cultures of bacteria growing on cellulose were interpreted using the same relationships that have been used for studies on soluble sugars. It was possible to show the influence of the structure of the cellulose on the kinetics of its degradation. The results obtained with mixed cultures were difficult to interprete at the present time. The power-time curves are very complex, showing many heat peaks that cannot be related to any known physiological phenomena. However, the total heat evolved during lignocellulosic fermentation by mixed cultures is proportional to the quantity of sugar fermented; this was demonstrated and discussed. Thus it can be concluded that microcalorimetry can be used for the estimation of the biodegradability of any solid substrate. This technique can also be used for evaluation of the efficiency of the pretreatments of lignocellulosic compounds that are done to increase the biodegradability of these substrates. Recent data will be shown.     

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.     

Reversible and irreversible heat capacity of poly[carbonyl(ethylene‐co‐propylene)] by temperature‐modulated calorimetry

The heat capacity of poly[carbonyl(ethylene‐co‐propylene)] with 95 mol % C₂H₄? CO? (Carilon EP®) was measured with standard differential scanning calorimetry (DSC) and temperature‐modulated DSC (TMDSC). The integral functions of enthalpy, entropy, and free enthalpy were derived. With quasi‐isothermal TMDSC, the apparent reversing heat capacity was determined from 220 to 570 K, including the glass‐ and melting‐transition regions. The vibrational heat capacity of the solid and the heat capacity of the liquid served as baselines for the quantitative analysis. A small amount of apparent reversing latent heat was found in the melting range, just as for other polymers similarly analyzed. With an analysis of the heat‐flow rates in the time domain, information was collected about latent heat contributions due to annealing, melting, and crystallization. The latent heat decreased with time to an even smaller but truly reversible latent heat contribution. The main melting was fully irreversible. All contributions are discussed in the framework of a suggested scheme of six physical contributions to the apparent heat capacity. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 1565–1577, 2001     

Calorimetric and structural studies of organic compound of tris(pentafluorophenyl)-4-pyridylethylgermane

In the present research, the temperature dependence of heat capacity of tris(pentafluorophenyl)-4-pyridylethylgermane (C₆F₅)₃Ge–CH₂–CH₂–C₅H₄N was studied by precise adiabatic vacuum calorimetry and differential scanning calorimetry over the temperature range from 6 to 450 K. The temperature and enthalpy of fusion of tris(pentafluorophenyl)-4-pyridylethylgermane and the total mole fraction of impurities have been determined. The thermal stability of the sample was investigated by thermogravimetric analysis. The experimental data were used to calculate the standard thermodynamic functions: heat capacity, enthalpy, entropy, and the Gibbs energy over the range from T → 0 to 420 K for crystalline and liquid states. For the compound under study, the standard entropy of formation in the crystalline state was calculated at T = 298.15 K. In addition, the structure of the investigated compound was established, and corresponding structural parameters were determined.

     

Heat Transfer in a Disc-Type DSC Apparatus IV. Applicability of the coupled cells model to periodic temperature modulation

The application of non-linear heating programs to a heat-flux DSC apparatus has attracted much attention. On the basis of thermodynamics, the change in enthalpy of a sample during a temperature change ΔT is due, on the one hand to the true heat capacity of the sample ΔT C_pξ and on the other, to the enthalpy of some transformation occurring in the sample Δ_rH Δξ. These contributions can be separated on the basis of the kinetics of the transformation. The coupled cells model of a disc type, heat flux DSC apparatus has been tested, using true heat capacities and a sine modulation of the temperature of the furnace around a constant temperature. In the range from 2 to 60 mHz, the amplitude and phase shift of the calorimetric signal were measured at several frequencies. Theoretical equations, based on the model, and using the thermal Ohm's law explains the results with a reasonable accuracy. A non-linear DSC experiment affords two ways of determination of the heat capacity of a sample making possible a distinction between the enthalpic effect and heat capacity. This revised version was published online in July 2006 with corrections to the Cover Date.     

Differential scanning calorimetry (DSC) of semicrystalline polymers

Differential scanning calorimetry (DSC) is an effective analytical tool to characterize the physical properties of a polymer. DSC enables determination of melting, crystallization, and mesomorphic transition temperatures, and the corresponding enthalpy and entropy changes, and characterization of glass transition and other effects that show either changes in heat capacity or a latent heat. Calorimetry takes a special place among other methods. In addition to its simplicity and universality, the energy characteristics (heat capacity C _P and its integral over temperature T—enthalpy H), measured via calorimetry, have a clear physical meaning even though sometimes interpretation may be difficult. With introduction of differential scanning calorimeters (DSC) in the early 1960s calorimetry became a standard tool in polymer science. The advantage of DSC compared with other calorimetric techniques lies in the broad dynamic range regarding heating and cooling rates, including isothermal and temperature-modulated operation. Today 12 orders of magnitude in scanning rate can be covered by combining different types of DSCs. Rates as low as 1 μK s⁻¹ are possible and at the other extreme heating and cooling at 1 MK s⁻¹ and higher is possible. The broad dynamic range is especially of interest for semicrystalline polymers because they are commonly far from equilibrium and phase transitions are strongly time (rate) dependent. Nevertheless, there are still several unsolved problems regarding calorimetry of polymers. I try to address a few of these, for example determination of baseline heat capacity, which is related to the problem of crystallinity determination by DSC, or the occurrence of multiple melting peaks. Possible solutions by using advanced calorimetric techniques, for example fast scanning and high frequency AC (temperature-modulated) calorimetry are discussed.     

Contribution of Polymer‐Solvent Interactions to Dynamics of Linear Polymers in Dilute Solutions

In this work a theoretical approach to dynamics of linear vinyl polymers in dilute solutions of high viscosity solvents is presented. The calculations for the relaxation time spectra, polymer intrinsic viscosity [η (ω)], complex elastic modulus G*(ω), total intrinsic viscosity [η^T (ω)] and specific heat capacity C (ω) were carried out in the non‐free‐draining limits. The relaxation time spectrum calculated for dynamics of low frequency modes exhibits a Rouse‐like character. Its position and shape corresponds to the ultrasonic relaxation time spectrum observed in the system at 10⁶ Hz. On the other hand, the relaxation time spectrum associated with moderate frequency mode dynamics is narrower and typical for ultrasonic relaxation observed at 10⁷ Hz. The polymer intrinsic viscosity [η (ω)] and elastic modulus G*(ω) are shown to be represented by the model within a low‐frequency range. In turn, the specific heat capacity C (ω) is displayed as a representation of the model in the acoustic region mentioned above. In the high‐frequency range the dynamics is described by the total intrinsic viscosity [η^T (ω)] tending to a plateau where the value is equal to the sum of the single‐bead intrinsic viscosity [η_N] and effective solvent viscosity [η_eff].     

无水烟酸钾的合成、表征及热化学

选择分析纯烟酸和无水醋酸钾为反应物, 利用室温固相合成方法, 合成了无水烟酸钾. 利用FTIR和X射线粉末衍射等方法表征了它的结构. 用精密自动绝热热量计测定了它在77～400 K温区的低温热容, 将该温区的摩尔热容实验值用最小二乘法拟合, 得到热容随温度变化的多项式方程. 用此方程进行数值积分, 得到此温区内每隔5 K的舒平热容值和相对于298.15 K时的各种热力学函数值. 在此基础上, 通过设计合理的热化学循环, 利用等温环境溶解-反应热量计分别测定固相反应的反应物和生成物在所选溶剂中的溶解焓, 从而得到该固相反应的反应焓为 (25.87±0.47) kJ•mol^－1. 最后, 依据Hess定律计算出烟酸钾的标准摩尔生成焓为 ＝－(560.57±1.09) kJ•mol^－1.     

Thermal requirements to obtain calcined and frits of ulexite

Ulexite has a substantial amount of hydration water, which allows using calcination methods to increase the B₂O₃ content from 42.97 to 66.69%, due to the loss of 35.54% of its weight, corresponding to its hydration water.In this study, determinations of changes in the heat content at each temperature were made using an isoperibol calorimeter. The mean specific heat was calculated from these values. The reaction enthalpy was also determined by DTA methods.The sample used in the experiments contains 39.04% B₂O₃. The temperature was between 80 and 1000 °C. Curves of weight loss and enthalpy changes are presented and analysed.The results have industrial application in order to determine the thermal requirements for calcining ulexite at different temperatures, as well as its complete dehydration.     

配合物Zn(Met)SO4•H2O(s)的低温热容和标准摩尔生成焓

利用精密自动绝热热量计直接测定了配合物Zn(Met)SO₄•H₂O(s) 在78～370 K温区的摩尔热容. 通过热容曲线的解析得到该配合物的起始脱水温度为T₀＝329.50 K. 将该温区的摩尔热容实验值用最小二乘法拟合得到摩尔热容 (C_p,m)对温度(T)的多项式方程, 并且在此基础上计算出了它的舒平热容值和各种热力学函数值. 依据Hess定律, 通过设计热化学循环, 选择体积为100 cm³、浓度为2 mol•L^－1的盐酸作为量热溶剂, 利用等温环境溶解-反应热量计, 测定和推算出该配合物的标准摩尔生成焓为Δ_fH_m⁰＝－(2069.30±0.74) kJ•mol^－1.     

Thermal properties of seed proteins

The sample of LiCoO₂ was synthesized, and the heat capacity was measured by adiabatic calorimetry between 13 and 300 K. The smoothed values of the heat capacity were calculated from the data. The thermodynamic functions, standard enthalpy, entropy and Gibbs energy, of LiCoO₂ were calculated from the heat capacity and the numerical values are tabulated at selected temperatures from 15 to 300 K. The heat capacity, enthalpy, entropy, and Gibbs energy at T=298.15 K are 71.57 J K^–1mol^–1, 9.853 kJ mol^–1, 52.45 J K^–1 mol^–1, –5.786 kJ mol^–1, respectively. This revised version was published online in August 2006 with corrections to the Cover Date.     

The existence of four dielectric modes in the planar oriented S*c phase of a fluorinated substance

Abstract

Soft mode and Goldstone mode properties have been studied for a fluorinated substance. The dielectric spectra have been measured on the planar oriented samples, in the frequency range from 10 Hz to 10 MHz. The thickness of the sample was 10 μm and two kinds of capacitors were used: (i) a low resistance EHC cell and (ii) gold coated electrodes. A bias field up to 10kV cm^?1 has been applied to show the existence of both the soft mode and domain mode in the S*_c phase below T _c. In the S*_c phase a strong Goldstone mode has been observed with a low critical frequency (v ^G _C = 15 Hz). The high frequency side at the Goldstone mode spectrum is accompanied by a shoulder which consists of the soft mode and domain mode as well. In the vicinity of the S*_A-S*_C transition the dielectric parameters of the soft mode obey a Curie-Weiss law. The slope ratio is equal to ?1·62 for the inverse of dielectric increments (Δ∈^?1) and ?1·73 for the critical frequencies (v _c) obtained by using gold electrodes. The respective values received for the EHC cell are ?4·14 and ?2·1. The dielectric parameters of the domain mode have been obtained versus temperature and bias field. We can qualitatively show that a high frequency molecular relaxation is present in the S*_A and S*_C phases as the high frequency limit of dielectric permittivity is distinctly higher than the refractive index squared.     

Low‐Temperature Heat Capacities and Standard Molar Enthalpy of Formation of Gramine (C11H14N2)

Low‐temperature heat capacities of gramine (C₁₁H₁₄N₂) were measured by a precision automated adiabatic calorimeter over the temperature range from 78 to 401 K. A polynomial equation of heat capacities as a function of temperature was fitted by least squares method. Based on the fitted polynomial, the smoothed heat capacities and thermodynamic functions of the compound relative to the standard reference temperature 298.15 K were calculated and tabulated at 5 K intervals. The constant‐volume energy of combustion of the compound at T=298.15 K was measured by a precision oxygen‐bomb combustion calorimeter as Δ_cU=−(35336.7±13.9) J·g⁻¹. The standard molar enthalpy of combustion of the compound was determined to be Δ_cH_m⁰=−(6163.2±2.4) kJ·mol⁻¹, according to the definition of combustion enthalpy. Finally, the standard molar enthalpy of formation of the compound was calculated to be Δ;_cH_m⁰=−(166.2±2.8) kJ·mol⁻¹ in accordance with Hess law.     

Low-Temperature Heat Capacities and Thermodynamic Properties of Hydrated Nickel L-Threonate Ni（C4H7O5）2·2H2O（S）

Low-temperature heat capacities of the compound Ni（C4H7O5）2·2H2O（S） have been measured with an auto- mated adiabatic calorimeter. A thermal decomposition or dehydration occurred in 350--369 K. The temperature, the enthalpy and entropy of the dehydration were determined to be （368.141 ±0.095） K, （18.809±0.088） kJ·mol ^-1 and （51.093±0.239） J·K^-1·mol^-1 respertively. The experimental values of the molar heat capacities in the temperature regions of 78-350 and 368-390 K were fitted to two polynomial equations of heat capacities （Cp,m） with the reduced temperatures （X）, [X=f（T）], by a least squares method, respectively. The smoothed molar heat capacities and thermodynamic functions of the compound were calculated on the basis of the fitted polynomials. The smoothed values of the molar heat capacities and fundamental thermodynamic functions of the sample relative to the standard reference temperature 298.15 K were tabulated with an interval of 5 K.     

A new accurate calorimetric method for the enthalpy of fusion measurements up to 1000 °C

A new metrological approach is proposed by the Laboratoire National de Metrologie et d’Essais to lower the uncertainty of measurements of enthalpy of fusion from 23 to 1000 °C. It consists, as in our previous works, in putting the studied sample inside a calibration crucible and in performing the heat calibration of the calorimeter by electrical substitution at two temperatures before and after the temperature of fusion of the sample during the same low-rate heating run. The novelty lies in the “electrical compensation” of the endothermic reaction of fusion. The quantification of the electrical energy needed to melt the sample, which can be measured with high accuracy, and the integrated remaining heat flow rate divided by the calibration factor (sensitivity) of the calorimeter at the temperature of fusion, leads to the determination of the enthalpy of fusion of the sample with low uncertainty. This methodology reduces the influence of the uncertainty component associated with the area of the peak of fusion by comparison with the former procedure when no electrical compensation was applied. The relative expanded uncertainty (k = 2) associated with the enthalpy of fusion measurements, performed using this facility and this new procedure, has been assessed to be < 0.2% for tin and indium and to be about 0.3% in the case of silver.

     

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.