Elpidio Tombari

Thermodynamic functions of water and ice confined to 2nm radius pores

The heat capacity C(p) of the liquid state of water confined to 2 nm radius pores in Vycor glass was measured by temperature modulation calorimetry in the temperature range of 253-360 K, with an accuracy of 0.5%. On nanoconfinement, C(p) of water increases, and the broad minimum in the C(p) against T plot shifts to higher temperature. The increase in the C(p) of water is attributed to an increase in the phonon and configurational contributions. The apparent heat capacity of the liquid and partially frozen state of confined water was measured by temperature scanning calorimetry in the range of 240-280 K with an accuracy of 2%, both on cooling or heating at 6 K h(-1) rate. The enthalpy, entropy, and free energy of nanoconfined liquid water have been determined. The apparent heat capacity remains higher than that of bulk ice at 240 K and it is concluded that freezing is incomplete at 240 K. This is attributed to the intergranular-water-ice equilibrium in the pores. The nanoconfined sample melts over a 240-268 K range. For 9.6 wt % nanoconfined water concentration ( approximately 50% of the maximum filling) at 280 K, the enthalpy of water is 81.6% of the bulk water value and the entropy is 88.5%. For 21.1 wt % (100% filling) the corresponding values are 90.7% and 95.0%. The enthalpy decrease on nanoconfinement is a reflection of the change in the H-bonded structure of water. The use of the Gibbs-Thomson equation for analyzing the data has been discussed and it is found that a distribution of pore size does not entirely explain our results.

The Low-Temperature Endotherm in Poly(ethylene terephthalate) : Partial Melting and Rigid Amorphous Fraction Mobilization

A detailed investigation of the low-temperature endotherm of poly(ethylene terephthalate) (PET) performed by temperature-modulated differential scanning calorimetry is presented. The origin of the small endotherm, generally observed a few degrees above the crystallization temperature in PET and in many other polymers, is a widely discussed matter. The most frequent interpretation considers it the result of partial fusion with superposition of a recrystallization process even if it has also been proposed that it can originate from enthalpic recovery connected to mobilization of the rigid amorphous fraction. In an attempt to resolve the question, a new method for the interpretation of the modulated heat-flow-rate curve resulting from a temperature modulation program is proposed. The procedure consists of the analysis of the initial points of the steady-state heat-flow-rate signals in the heating and cooling semiperiods with the temperature modulation being performed with a sawtooth profile. The study conducted in parallel on the reversing specific heat capacity and the heat-flow-rate curves, observed on heating after isothermal crystallization at various temperatures, showed that multiple processes, involving both the crystalline and the rigid amorphous fraction, overlap in the temperature range in which the low-temperature endotherm is observed. The origin of the endotherm under investigation is therefore connected with both partial fusion of the crystalline portions and enthalpy recovery subsequent to structural relaxation of the rigid amorphous fraction. An estimation of the relative percentages of the two different processes is presented and discussed.

The role of the crystallization temperature on the nanophase structure evolution of poly[(R)-3-hydroxybutyrate].

The nanophase structure of semicrystalline polymers, which determines the mechanical, thermal, and gas permeability behavior, can be quantified by thermal methods. A detailed investigation of the nanophase structure of poly[(R)-3-hydroxybutyrate] (PHB) was performed under conditions of isothermal, quasi-isothermal, and nonisothermal crystallizations. The experimental analyses revealed that the establishment of the nanophase rigid amorphous fraction (RAF) in PHB depends on the temperature at which crystallization occurs. The RAF grows in parallel with the crystal phase during quasi-isothermal crystallization at 30 °C, whereas during nonisothermal crystallization at higher temperatures, RAF starts to develop at 70 °C, in correspondence with the final stages of the crystallization process. The influence of crystallization temperature on the nanophase structure was rationalized taking into account the effect of the mobility of the entangled chain segments during the phase transition. The melting behavior was found to change after isothermal crystallization at 70 °C, revealing that complete RAF mobilization is achieved approximately at this temperature. The temperature of 70 °C could be the limit for the formation and the disappearance of rigid amorphous fraction in the PHB analyzed in the present study.

Calorimetric effects of intergranular water in ice

The heat capacity, Cp, of microcrystalline and large or single crystal ice was measured from 270 to 272.6 K and the effect of the heat input (required before measuring the increase in temperature) on the measured Cp investigated. The measured Cp of microcrystalline solids was much higher than that of large and single crystal solids and increased further with increase in the heat input. At 271.16 K, the measured Cp was independent of the heat input, but for higher temperatures, it increased nonlinearly with the heat input. These effects are caused by the increase in the volume fraction of intergranular water in impurity free ice with increase in both its temperature and the heat input. A significant fraction of the heat used for Cp measurement is consumed in melting ice at grain junctions which does not raise the temperature. Thus, the measured Cp appears to be much greater than the true value. Quantitative analysis of the data yielded results which agree reasonably well with the prediction of a theory giv...

Heat capacity of tetrahydrofuran clathrate hydrate and of its components, and the clathrate formation from supercooled melt

We report a thermodynamic study of the formation of tetrahydrofuran clathrate hydrate by explosive crystallization of water-deficient, near stoichiometric, and water-rich solutions, as well as of the heat capacity, C(p), of (i) supercooled tetrahydrofuran-H2O solutions and of the clathrate hydrate, (ii) tetrathydrofuran (THF) liquid, and (iii) supercooled water and the ice formed on its explosive crystallization. In explosive freezing of supercooled solutions at a temperature below 257 K, THF clathrate hydrate formed first. The nucleation temperature depends on the cooling rate, and excess water freezes on further cooling. The clathrate hydrate melts reversibly at 277 K and C(p) increases by 770 J/mol K on melting. The enthalpy of melting is 99.5 kJ/mol and entropy is 358 J/mol K. Molar C(p) of the empty host lattice is less than that of the ice, which is inconsistent with the known lower phonon frequency of H2O in the clathrate lattice. Analysis shows that C(p) of THF and ice are not additive in the clathrate. C(p) of the supercooled THF-H2O solutions is the same as that of water at 247 K, but less at lower temperatures and more at higher temperatures. The difference tends to become constant at 283 K. The results are discussed in terms of the hydrogen-bonding changes between THF and H2O.

Spontaneous decrease in the heat capacity of a glass

The real and imaginary components, Cp′ and Cp″, respectively, of the complex heat capacity, Cp*=Cp′−iCp″, of a molecular liquid have been measured in the temperature range of its vitrification and in the glassy state, and the effect of spontaneous structural relaxation has been determined in real time. Cp′ of the glassy state is found to decrease with time. Analysis shows that this is mainly due to the decrease of configurational entropy as the characteristic time of the Cp′ spectra increases and consequently the contribution from the unfrozen, faster modes of the α-relaxation process decreases. There may also be a significant decrease in the vibrational and anharmonic force contributions as the glass densifies. Interpretations in terms of the potential energy landscape model suggest that for each state of lower energy attained with time, the number of minima in the potential energy surface decreases, and the minima become deeper.

A modulated adiabatic scanning calorimeter (MASC)

Abstract The modulated adiabatic scanning calorimeter (MASC) described here is designed to work in both the time-domain (adiabatic-like step-scanning) and in the frequency domain (modulated temperature/power-scanning) operational modes. The cylindrical form of the cell and the controlled thermal environment in which the sample is located make it possible: (i) to write detailed and reliable equations to describe the heat flow; (ii) to derive analytical relations to calculate the energy release from the sample and the real and imaginary parts of the complex heat capacity at the modulation frequency; (iii) to study narrow phase transitions using the power modulated scanning mode; and (iv) to generate adiabatic-like conditions. The thermal environment is controlled by means of an active thermal shield which is kept at a temperature close to that of the sample cell in order to minimise the thermal gradients to which the sample is exposed. The temperature difference between the shield and the cell is set at the value required to control or cancel heat leakage from the cell. MASC can be used to carry out measurements on a single sample in different operational modes that can be made operative under the control of its built-in software. The parameters of interest can be automatically calculated by means of a program based on a model of the cell+sample system that will be described in a second paper. To test the performance of the calorimeter, samples of materials which undergo a first-order phase transition or a glass transition (indium, ice, and polystyrene) were studied. The specific features of the calorimetric cell, the calibration procedure, and the experimental results are reported and discussed.

The temperature and polymerization effects on the relaxation time and conductivity, and the evolution of the localized motions

To examine the manner in which molecular dynamics of a polymerizing liquid (stoichiometric amounts of 4,4′-diaminodicyclohexylamine and diglycidyl ether of bisphenol-A) evolves during thermal cycling from its (molecular) vitreous state to its fully polymerized vitreous state, calorimetry, and dielectric spectrometry were performed simultaneously in real time. The half-width of the relaxation spectrum of the liquid was relatively narrow and became narrower on heating. This was followed by an increase in the characteristic relaxation time and the spectrum became broader as polymerization occurred and reached completion. The dc conductivity initially increased and then decreased. The faster dynamics of the Johari–Goldstein relaxation in the fully polymerized state evolved as polymerization reached completion and the temperature increased. The dielectric polarization associated with this relaxation had a broad spectrum, whose half-width increased with decrease in the temperature. Its relaxation rate followed ...

Excess energy of polymorphic states or glass over the crystal state by heat of solution measurement

The excess internal energy of one polymorph of a material over another may be determined from their heat of solution in a given solvent. (This energy is not to be seen as reversible heat of transformation of the polymorphs.) Thus the difference between the internal energy of amorphous and crystalline forms of sucrose, glucose and glucose monohydrate have been determined from measurement of their heat of solution in water. These differences are 21.2, 15.5 and 28.1 kJ mol−1, respectively. This difference in energy is caused by differences in van der Waals interaction energy, the extent of and the total energy associated with H-bonding in the two solids, and their vibrational frequencies. The implication of these studies and further use of the procedure are discussed in general terms, and it is proposed that this method is more accurate than the usual method of integrating heat capacity-temperature data. The method can be used for determining the excess energy at 0 K of materials which decompose or melt incongruently, and when neither the heat capacity of the high temperature phase nor the heat of phase transformation can be measured.

Vibrational and configurational heat capacity of poly(vinyl acetate) from dynamic measurements

The complex heat capacity C(p) (*) of poly(vinyl acetate) has been measured at 20.95 mrads modulation frequency during the cooling as well as on heating at 24, 8, and 2 Kh and during cooling at 0.5 Kh. The study is complemented with (the rate-dependent) C(p,app) measured during cooling and heating at 60, 24, and 8 Kh. At low temperatures, the real component of C(p) (*) yields the unrelaxed C(p) or C(p,vib), the vibrational part of C(p). It is found to be indistinguishable from C(p,glass) and lies on a line extrapolated to its equilibrium melts temperature. At T near T(g),DeltaC(p)(=C(p,melt)-C(p,glass)) shows no detectable contribution from C(p,vib). The finding conflicts with a modified entropy theory calculation [E. A. DiMarzio and F. Dowell, J. Appl. Phys. 50, 6061 (1979)], which had predicted that approximately 27% of DeltaC(p) of poly(vinyl acetate) at T near T(g) is vibrational in origin and the remainder configurational. At T