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The report that the distinction could be made by measuring Cp and the complex heat capacity, Cp, simultaneously in a temperature-modulated scanning mode is presented. The sigmoid-shaped Cp increase observed in normal scanning indicates an endothermic transformation in the beta-D-fructose liquid at T > Tg and not unfreezing of tautomeric transformation on the time scale of heating.

The effect of reflective multipass interferometers on the transverse modes of laser cavities equipped with these devices is investigated. We demonstrate, both theoretically and experimentally in a pulsed C[O.sub.2] laser, how information on the characteristics of these modes can be directly derived by simply describing these cavities in terms of suitable equivalent geometric parameters. We also discuss and explain the transverse-mode selective properties of these multimirror cavities.

The dynamic and the apparent specific heats of 5-methyl-2-hexanol were measured in its vitrification temperature range during its cooling and then heating at the same and exceptionally slow rates of 12 K/h and 60 K/h. The relaxation time determined from dynamic measurements is 48 s at 149.8 K. The relaxation time estimated from the onset of the apparent Cp-endotherm measured on heating is found to be inconsistent with that determined from dynamic Cp measurements. The fitting of a nonexponential nonlinear relaxation model to the Cp,app data shows that β varies slightly with the heating rate, and this is attributed to contributions to temperature-dependent energy from change in the hydrogen-bond population. The unrelaxed Cp of the ultraviscous liquid is closer to that of its glassy state, thus showing that the vibrational part of Cp does not increase in a sigmoid-shape manner when the glass structure kinetically unfreezes on heating. The results have implications for use of calorimetry in inferring the dielectric relaxation mechanism. [ABSTRACT FROM AUTHOR]

The real and imaginary components of the complex heat capacity, Cp′ and Cp″, and Cp,app have been measured in real time during the linear chain polymerization on 12 K/h heating of six different (partially) polymerized states of a stoichiometric mixture of cyclohexylamine and diglycidyl ether of bisphenol A. Their Cp,app shows a sigmoid shape rise with different onset temperatures Tonset, which is followed by a deep exotherm as the viscosity decreases and further polymerization occurs at different rates. The rates of their enthalpy decrease on polymerization determined by subtracting Cp′ from Cp,app differ but Cp′ and Cp,app of their final states are the same. The relaxation time increases with polymerization and decreases with an increase in T. Cp′ rises in a sigmoid shape manner, and Cp″ shows a peak when the relaxation time of the polymerized state is equal to the inverse of the temperature modulation frequency, whether polymerization occurs or not. The unrelaxed or vibrational heat capacity Cp,vib of the polymers at T>Tonset is close to Cp of their glassy state at T≤Tonset, showing that Cp difference between the equilibrium liquid and its glass is mostly configurational. This contradicts a calculation showing that Cp,vib change of a polymer at Tg is generally ∼20% of the total Cp change. [ABSTRACT FROM AUTHOR]

The complex heat capacity Cp* of poly(vinyl acetate) has been measured at 20.95 mrad/s modulation frequency during the cooling as well as on heating at 24, 8, and 2 K/h and during cooling at 0.5 K/h. The study is complemented with (the rate-dependent) Cp,app measured during cooling and heating at 60, 24, and 8 K/h. At low temperatures, the real component of Cp* yields the unrelaxed Cp or Cp,vib, the vibrational part of Cp. It is found to be indistinguishable from Cp,glass and lies on a line extrapolated to its equilibrium melt’s temperature. At T near Tg,ΔCp(=Cp,melt-Cp,glass) shows no detectable contribution from Cp,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 ∼27% of ΔCp of poly(vinyl acetate) at T near Tg is vibrational in origin and the remainder configurational. At T≤Tg, the real component of Cp* varies more slowly with T than Cp,app. [ABSTRACT FROM AUTHOR]

The heat capacity, Cp, and enthalpy and entropy change of α-cyclodextrin, H2O, and 4-methylpyridine solutions have been studied during their freezing on heating, isothermal freezing, and the solid’s melting on cooling. Freezing occurs in several endothermic steps on heating to 383 K and α-cyclodextrin rich solutions freeze in four steps. The melting rate becomes slower with decrease in temperature and its steps merge. Decreasing the amount of α-cyclodextrin decreases the Cp change on freezing. The endothermic freezing phenomenon differs from freezing of a pure liquid and is attributed to formation of a solid inclusion compound and its incongruent way of exothermic melting. [ABSTRACT FROM AUTHOR]

A lattice vacancy raises the energy of the neighboring (flexible) molecule in a crystal, which may be enough to isomerize it to a tautomer that does not fit the lattice site, thus creating a liquidlike local region embedding the vacancy. Similar regions may appear elsewhere in the lattice and the regions may ultimately merge. Thus a crystal may spontaneously liquefy over a period of hours to years at a temperature below its normal melting point. Simultaneous heat capacity and heat absorption measurements of several such molecular crystals show that they spontaneously liquefy at a temperature far below their reputed melting point, according to a non-exponential rate kinetics and a temperature dependent rate constant, and do not crystallize on cooling. [ABSTRACT FROM AUTHOR]

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, Cp, 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 Cp 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 Cp 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 Cp of THF and ice are not additive in the clathrate. Cp 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. [ABSTRACT FROM AUTHOR]

Heat capacity of controlled amounts of water in Vycor’s 2 nm radius pores has been determined in real time during the course of water’s isothermal nanoconfinement from bulk state at 358 K, by using temperature-modulated calorimetry. As water transfers from bulk to nanopores via the vapor phase, its heat capacity per molecule increases asymptotically toward a limiting value of 1.4 times the heat capacity of bulk water for 1.8 wt % water in Vycor and 1.04 times for 10.0 wt %. The observations indicate that vibrational and configurational contributions to the heat capacity are highest when the amount of water is insufficient to completely cover the pore wall, and they decrease as more water is present in the nanopores and water clusters form. The heat capacity of water in completely filled nanopores approaches the value for bulk water, thus indicating that the heat capacity varies with the water molecules’ position in the nanopores. [ABSTRACT FROM AUTHOR]

Generally, a liquid freezes exothermally on cooling and a crystal melts endothermally on heating. Here we report an opposite occurrence—a liquid’s endothermic freezing on heating and the resulting crystal’s exothermic melting on cooling at ambient pressures. Cp decreases on freezing and increases on melting, and the equilibrium temperature meets the thermodynamic requirement. Melting on cooling takes longer than freezing on heating. A rapidly cooled crystal state becomes kinetically frozen, evocative of a nonergodic state. Both Cp and enthalpy relax like those of glasses, though the viscosity is only a few centipoise. The crystal state belongs to energy minima higher than those of the melt, which has consequences for the use of potential-energy landscape, or inherent structures, for a thermodynamic description of a material. [ABSTRACT FROM AUTHOR]

The heat capacity Cp 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, Cp of water increases, and the broad minimum in the Cp against T plot shifts to higher temperature. The increase in the Cp 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 (∼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. [ABSTRACT FROM AUTHOR]

The real and imaginary components, C′[SUBp] and C″[SUBp], respectively, of the complex heat capacity, C*[SUBp] = C′[SUBp]-iC″[SUBp], 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. C′[SUBp] 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 C′[SUBp] 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. [ABSTRACT FROM AUTHOR]

The equilibrium heat capacity C[sub p] of a liquid whose molecules polymerized to form a linear chain structure at a fixed temperature has been measured at a fixed frequency of 3.33 mHz in real time simultaneously with the extent of polymerization α. The polymerization temperatures and this frequency were chosen such that the fully polymerized state remained a liquid and no heat capacity relaxation occurred. The plots of C[sub p] against a show a maximum. This confirms the recent theoretical conclusions based on the polymer chain statistics [J. Wang and G. P. Johari, J. Chem. Phys. 116, 2310 (2002)] that during the course of a melt's polymerization, the configurational heat capacity of the equilibrium liquid reaches a maximum value at a certain α, and then decreases to the finite value of the fully polymerized melt. The results are also discussed in terms of the potential energy landscape at a fixed temperature where each new covalent bond formed and/or a new molecular weight distribution causes the state of the sample to enter a new landscape with a new set of potential energy minima of different depths and widths. [ABSTRACT FROM AUTHOR]