Anal. Chem. 2006, 78, 3875
Thermal Analysis Sergey Vyazovkin
Department of Chemistry, University of Alabama at Birmingham, 901 South 14th Street, Birmingham, Alabama 35294 Review Contents Method Development and Calibrations Thermodynamics Kinetics Inorganics Polymers Energetics Pharmaceutical, Biochemical, and Biological Applications Literature Cited
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The present review covers the thermal analysis literature published in the years 2004 and 2005 and is limited to 200 references. Although an attempt has been made to select representative publications that present new and significant developments in major areas of thermal analysis, accomplishing this task poses a major problem because of the colossal growth of the field. Suffice it to say that according to ISI Web of Science, the application of only one technique, DSC, to the process of crystallization alone has been reported in over 1100 papers. In this situation, we can only regret not being able to cite all quality papers. We start by introducing new thermal analysis books published over the past two years. These include Thermal Analysis of Polymeric Materials by Wunderlich (1), Thermal Analysis of Plastics: Theory and Practice by Ehrenstein et al. (2), and Thermal Properties of Green Polymers and Biocomposites by Hatakeyama and Hatakeyama (3). The latter appeared as volume 4 in the series Hot Topics in Thermal Analysis and Calorimetry. The series has also added volume 5, which presents a collection of papers combined under the title The Nature of Biological Systems As Revealed by Thermal Methods (4). Another significant compilation (5) comes from the Japanese Society of Calorimetry and Thermal Analysis. A major event in the life of the international thermal analysis community was the 13th Congress of ICTAC held in Chia Laguna in 2004. There were almost 400 presentations, a representative selection of which has been published as a separate volume of Journal of Thermal Analysis and Calorimetry (6). METHOD DEVELOPMENT AND CALIBRATIONS Hansen and Hart (7) provide an introduction to the various types of calorimeters and discuss the requirements for precise temperature measurements, methods, and equations for accurate heat exchange corrections, the characteristics and design principles for constant-temperature baths, and the construction of devices for addition of reagents, of stirrers, and of calibration heaters. A new method of full deconvolution of the instrumental coefficients in heat flux DSC has been proposed by Toda and Hikosaka (8). Nogent et al. (9) explore the practical use of an earlier designed differential reaction calorimeter and present mathematical models and software for deconvoluting the collected 10.1021/ac0605546 CCC: $33.50 Published on Web 00/00/0000
© 2006 American Chemical Society
signal into two parts respectively related to the real heat flow and reagents accumulation. Zucca et al. (10) present further development of a theoretical model of a power-compensated twin calorimeter with the aim of improving the calibration techniques of the instrument. Drebushchak (11) describes an optimal procedure for heat flow calibration that is based on the relationship between the sensitivity of a thermocouple and calibration coefficient of a DSC sensor. For calibration of DSC on cooling, Malheiro et al. (12) explore the use of metal standards as an alternative to typically used liquid crystals. They conclude that any pair of the metal standards can be used for calibration with results similar to those obtained when using liquid crystals. Marti et al. (13) propose a new method of evaluating the temperature resolution for DSC instruments that makes use of two caloric events such as closely spaced λ transition and melting in n-hexatriacontane. Urakami and Beezer (14) report a technique that reduces the equilibration time prior to data capture in ampulebased heat conduction isothermal microcalorimeters. The technique makes use of a device that allows the sample and reference ampules to be lowered very slowly from the preequilibration position to the measuring position. Minakov et al. (15) has constructed a nonadiabatic thin-film nanocalorimeter based on a commercially available microchip. The instrument can be used for fast scanning calorimetry of submicrogram samples with sensitivity of 1 nJ/K and time resolution of 5 ms. Efremov et al. (16) describe an ultrasensitive, fast, thinfilm DSC that operates at high heating rates (15-200 K/ms) and is very sensitive (30 pJ/K). The sources of noise in the instrument are discussed, and simple models and methods for reducing noise are presented. Camirand (17) presents an improved technique for measuring the thermal conductivity by DSC. The technique makes use of several samples having different heights and allows one to measure the thermal conductivity with precision better than 10%. Ray et al. (18) have developed a DTA-based method that allows for determining the critical cooling rate for glass formation of crystallizable melts. The method has been successfully tested on materials of known critical cooling rates. Binner et al. (19) present the design and construction of a calorimeter in which the specimen may be heated by microwave radiation as well as by hot air. The effect of the intensity of microwave radiation was examined by measuring the melting of benzyl and the solid-state phase transition in silver iodide. For the latter, transition temperature has been found to vary significantly with the intensity of microwave irradiation. Le Parlouer (20) has introduced a new high-pressure microcalorimeter for determining thermodynamic properties and kinetics of the gas hydrates formed from water and gases under low-temperature and high-pressure conditions. The utility of the technique is illustrated by its application to Analytical Chemistry, Vol. 78, No. 12, June 15, 2006 3875
drilling muds. Navarrete et al. (21) suggest that microcalorimetric measurement performed by immersion into liquid nitrogen or liquid argon allow one to better assess the internal surface area of a sample as illustrated for a set of microporous and mesoporous samples. Beurroies et al. (22) propose a methodology based on thermoporometry that reveals the hysteresis between melting and solidification of a confined fluid. The hysteresis loops are reminiscent of those observed for gas adsorption-desorption. Grolier et al. (23) demonstrate advantages of scanning transitiometry that permits scanning one of the independent variables (P, V, or T) while keeping the others constant and simultaneously measuring the resulting thermal effect. The instrument has a wide operating range (173 K< T < 673 K and 0.1 < P < 200 MPa) and can be used to study bulk properties and phase transitions, as well as chemical reactions. The evolution of the coefficient of thermal expansion of a thermosetting polymer is an important property, whose measurement provides a major challenge. Leroy et al. (24) approach the problem by suggesting an indirect method based on measuring ionic conductivity and correlating it to the coefficient of thermal expansion. A novel technique for measuring thermal pressure in fluids as well as isotropic stress development is presented by Merzlyakov et al. (25). Isotropic constraints are imposed by using a sealed stainless steel spherical pressure vessel with strain gauges and thermocouples that allows for measuring both isotropic stresses and reaction kinetics during cure at cure temperatures as high as 300 °C. Ogreten et al. (26) has designed and tested a thermostimulated creep instrument that is aimed at precisely measuring the thermomechanical properties of polymer samples having a thickness from 1 mm to 10 µm. The instrument utilizes magnetic field as a means of applying the stress to the samples. Zhang and Martins (27) discuss the problems of temperature calibration of a parallel plate rheometer when applied to polymer crystallization. They use an additional thermocouple to monitor the sample temperature and evaluate the errors of calibration. Mano and Cahon (28) advocate the use of hydrophilic polymericbased matrixes swelled in water for temperature calibration of TMA and DMA instruments. A drastic change in the mechanical response is detected at the temperature of melting ice that is compared with the reference melting temperature. Parkes and Williams (29) describe the development of a thermobalance based on a multimode microwave cavity utilizing pulse-width modulation control of the power and 0.01-mg resolution. Methods for calibrating TGA-FT-IR instruments are discussed by Slager and Prozonic (30), who use decomposition of solids and evaporation of liquids as standards. The value of the technique is demonstrated by quantification of overlapping components in the TGA effluent. Vyazovkin et al. (31) have combined DMA with mass spectrometry that provided a new technique for studying mechanochemical effects in degradation of polymeric materials. THERMODYNAMICS Willson et al. (32) present a pragmatic approach to calculation of equilibrium constants that have been determined from systems at equilibrium by measuring a component of the equilibrium at several different temperatures. A mathematical method is described that uses the linearity of the van’t Hoff plot as the basis to systematically test for an appropriate total amount of measured 3876
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analyte and derive the best values of equilibrium constants. Piekarski (33) discusses various theoretical models that allow solution calorimetry data to be interpreted on the molecular level. The discussion is focused on dissolution enthalpy and heat capacity measurements for two- and three-component liquid systems. Enthalpies of dilution of aqueous solutions of 1-butanol, butanediols, 1,2,4-butanetriol, and 1,2,3,4-butanetetrol have been measured by Romero et al. (34) by using a flow microcalorimeter. The data have been analyzed by the McMillan-Mayer theory to determine the enthalpic interaction coefficients that showed a clear dependence on the number and position of OH groups. Waliszewski et al. (35) have compared the measured heat capacities of several ionic liquids with the values estimated by an additive group contribution method that assumes that the heat capacity of a molecular compound equals the sum of individual atomic group contributions. The estimates suggest that heat capacities of ionic liquids do not differ considerably from those typical for molecular liquids. Harrison et al. (36) have conducted a study of selective guest encapsulation by metal-assembled resorcinarene-based cages. The selectivity depends on the size, shape, and polarity of the guest and can be characterized by thermodynamic parameters derived from calorimetric measurements. Leal et al. (37) have measured hydration of a DNA-hexadecyltrimethylammonium complex and of pure DNA at 25 °C by using a sorption microcalorimeter that affords simultaneous measurements of water activity (sorption isotherms) and the partial molar enthalpy as a function of water uptake. A kink in the isotherm is reported at 20.0 ( 0.3 water molecules per base pair that is consistent with A-B transition of the DNA, although there is no detectable heat effect associated with this transition. Manetto et al. (38) apply DSC for evaluating thermodynamic properties of irreversible thermal unfolding of proteins. It is suggested that accurate thermodynamic parameters can be obtained when using the Lumry-Eyring model for extrapolating DSC data to infinite scanning rate. By using the quartz crystal microbalance/heat conduction calorimeter, Smith et al. (39) have measured vapor sorption in thin (0.75-8.5 µm) films of Tecoflex and demonstrated that the sorption isotherms generally follow a linear Henry’s law dissolution relationship between the vapor pressure and the amount of vapor sorbed into the film. Tombari et al. (40) have employed temperature-modulated DSC to measure the heat capacity of controlled amounts of water during its transfer from the bulk state to 2-nm size pores. It is reported that the heat capacity per molecule of water increases asymptotically toward a limiting value of 1.4 times the heat capacity of bulk water for 1.8 wt % water in Vycor and of 1.04 times for 10.0 wt %. This indicates 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 that they decrease as more water enters the nanopores and water clusters form. Dore et al. (41) have examined crystallization cyclohexane in mesoporous silicas with a pore size in the range 2.5-50 nm. The results show a consistent depression of the nucleation temperature with decreasing pore size that ultimately results in complete suppression of nucleation. Nanocalorimetry has been used by Olson et al. (42) to investigate size-dependent melting of Bi nanoparticles. It has been discovered that for particles smaller than ∼7 nm the measured melting temperatures
are ∼50 K above the value predicted by the homogeneous melting model. The discrepancy is analyzed in terms of a possible sizedependent crystal structure change and the superheating of the solid phase. Wunderlich (43) discusses the effect of the surface on the thermodynamics of the melting and glass transition of films and fibers. Although the effect may seem negligible in systems of microscopic dimensions (>1 µm), macroscopic polymer samples tend to contain phases of significantly smaller dimensions that are frequently metastable and require nonequilibrium thermodynamics for their description. A review paper by Alcoutlabi and McKenna (44) focuses on the effects of size and confinement at the nanometer-size scale on the glass transition temperature. They stress that the glass transition temperature may decrease, increase, remain the same, or even disappear depending upon details of the experimental (or molecular simulation) conditions and that the existing theories of glass transition are unable to explain the range of behaviors seen at the nanometer-size scale, in part because the glass transition phenomenon itself is not fully understood. By using DSC, Roux et al. (45) have measured vaporization enthalpies of the dicarboxylic acids from C4 to C16 and found that the values for C4-C10 vary linearly with the number of methylene groups. Above C10, the vaporization enthalpies begin to deviate from linearity that is discussed in terms of the formation of a cyclic intramolecular hydrogen-bonded network. Huang et al. (46) have used step-scan DSC to measure chain length dependence of the specific heat capacity for linear and cyclic alkanes and polymers. For the linear alkanes, the specific heat capacity in the equilibrium liquid state decreases as chain length increases, converging above C10 (decane) to a constant value. For the cyclic alkanes, the heat capacity of the equilibrium liquid state is lower than that of the corresponding linear molecules and increases with increasing chain length. At sufficiently high molecular weights, the heat capacities of cyclic and linear molecules become equal as found for the polyethylenes and polystyrenes studied. Hatakeyama et al. (47) have used DSC to study the thermal behavior of glassy guar gum in the presence of water. They report that on heating the system undergoes glass transition followed by cold crystallization partially overlapped with liquid crystallization. DSC in combination with polarized light microscopy has allowed Roussel et al. (48) to establish the equilibrium phase diagram of UV-cured (2-ethylhexylacrylate-1,6-hexanedioldiacrylate) and the multicomponent nematic fluid E7. Griesser et al. (49) use DSC and hot-stage microscopy in combination with other techniques to identify and characterize five crystal polymorphs of the herbicide metazachlor. It is stressed that crystallization of the compound from solvents can result in the formation of two metastable forms that may turn in a stable polymorph causing problems with the application of the material. Saiter et al. (50) explore the effects of very long physical aging (up to 13 years) on GexSe1-x glasses (x ) 4, 8, and 12 at. %) by means of calorimetric methods. They find that long aging leads to a phase separation process that could explain why the configurational entropy does not reach its equilibrium value as reported in earlier experiments. Illekova et al. (51) have obtained the heat capacity data for devitrification of the rapidly quenched Al90Fe7Nb3 alloy. The heat capacity of the ribbon has been related to that of its master alloy and of the Al, Al3Fe, and Al3Nb
components. Sharpataya et al. (52) have measured the heat capacity of cesium hexafluoroarsenate in the temperature range 300-850 K and detected a solid-solid transformation from the rhombohedral to cubic phase, occurring in the range 235-360 K. The transition has been interpreted as an order-disorder type, and its enthalpy and entropy has been determined. The magnetocaloric effect has been discussed by Kalva and Sestak (53), who propose to use a quasiparticle formalism to model it. KINETICS An extended discussion of various mathematical problems associated with analysis of nonisothermal kinetic data has been provided by Caballero and Conesa (54). Zhou and Grant (55) have combined mathematical analysis with simulated data to explain the dependence of the activation energy on the choice of a reaction model. The results highlight the severe limitations of fitting nonisothermal data to kinetic models. Vlad et al. (56) explore an opportunity of building multistep fractal kinetics models. A set of relationships is derived to relate the equilibrium constants of the reaction steps, the corresponding overall rate coefficients, and the stoichiometric numbers of the rate-determining steps. Korobov (57) discusses discrete kinetic models of reactions in crystals, which simultaneously play the roles of reactant and reaction medium. The thermal decomposition of ammonium hydrocarbonate is considered as an example. By employing an approach based on distribution of activation energies, Burnham et al. (58) have developed a kinetic model for the β-δ phase transition in HMX. It is suggested that the model may have a wide application to other thermodynamically inhibited processes. A Maxwell-Boltzmann distribution of activation energies has been used by Skrdla and Robertson (59) to derive kinetic models applicable to polymorphic transformation as well as to the thermal decomposition of solids. L’vov (60) gives an extensive overview of the developed third-law kinetic methodology as applied to kinetics of solid-state decompositions. Model-free isoconversional methods have seen novel applications in the areas of the glass transition (61) and polymer melt crystallization (62). The method has also been applied to the processes of exsolution (63) and to contraction data as measured by TMA on curing (64). Budrugeac and Segal (65) develop new computational procedures for isoconversional calculations and provide their comparison. By comparing various isoconversional methods, Simon (66) concludes that the differential and incremental integral methods give accurate values of activation parameters whereas the isothermal and integral methods provide some averaged values. An integral isoconversional method that uses local heating rates has been proposed by Tang and Chen (67), who demonstrate that the method yields the values of activation energy similar to those obtained by a differential isoconversional technique. Vyazovkin et al. (61) have proposed an advanced isoconversional method to be used for detecting a variation in the effective activation energy throughout the glass transition. The activation energy has been found (61, 68, 69) to decrease with increasing temperature for a number of glasses. The rate of the decrease appears to correlate with the dynamic fragility of a system. Ramis et al. (64) applied an isoconversional method to the curing data measured by DSC, TMA, and FT-IR and found that the activation Analytical Chemistry, Vol. 78, No. 12, June 15, 2006
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energies derived from DSC and FT-IR agree well whereas TMA data give rise to markedly smaller values. Sbirrazzuoli et al. (70) have applied isoconversional kinetic analysis to the vaporization kinetics of alachlor and metolachlor and found that the obtained activation energies are consistent with the respective enthalpies of vaporization. Hazra et al. (71) have used DSC and TGA to study vaporization kinetics of some essential oils and their key components. The obtained thermodynamic and kinetic parameters of the pure compounds have been compared to that of the multicomponent systems to quantitatively and qualitatively measure the influence of different compounds on each other. Zogg et al. (72) review various reaction calorimetry techniques and provide their comparison from the standpoint of kinetic applications. Criado and Perez-Maqueda (73) review the use of a sample controlled thermal analysis method for kinetic applications. Advantages over conventional techniques are specifically stressed. Koga and Yamada (74) use controlled rate evolved gas analysis to examine the effect of product gases on the kinetics and mechanism of the thermal decomposition of synthetic malachite. It has been demonstrated that the reaction is decelerated by CO2 and accelerated by H2O, whose anomalous effects on the reactivity appears only at the early stage of the reaction. Oxidation induction time is frequently used to characterize oxidative stability of organic compounds. Schawe et al. (75) stress that the induction time depends strongly on the oxygen pressure and provide an analysis of the dependence. Araujo et al. (76) employ factorial design to reveal an effect of experimental conditions (sample mass, furnace environment, furnace atmosphere, sample container composition) on the values of kinetic parameters for the thermal decomposition of a pharmaceutical compound zidovudine. They showed that none of the analyzed effects has been significant for the process under study. INORGANICS TGA-MS and TGA-FT-IR in combination with a quantitative chemical analysis has been effectively used (77) to establish the chemical formula for an unknown bismuth oxalate compound Bi(NH4)(C2O4)2‚3.71(6)H2O that has been confirmed by solving the crystallographic structure on the basis of X-ray powder data. The formation of porous TiO2 thin films from commercially available sols of amorphous TiO2 and anatase nanoparticles has been studied by Madarasz et al. (78) by using simultaneous TGA/DTA coupled with mass spectrometry that allowed for detecting the release of CO2 and crystallization of amorphous TiO2 to anatase. Both increased crystallinity and improved solar efficacy have been accomplished in the resulting porous material. Thermal decomposition of dried TiO2 gel in air has been monitored by simultaneous TG/DTA/EGA-FT-IR measurements by Krunks et al. (79), who report that decomposition occurs in five steps. They have detected the release of H2O below 120 °C, followed by acetone, isopropyl acetate, and 1-propanol around 200-300 °C, and finally CO and CO2 up to 550 °C. Crystalline TiO2-anatase is formed around 500 °C overlapping with exothermic reaction at 410-550 °C caused by combustion of carbon residues. Guo et al. (80) demonstrate the usefulness of TGA-FT-IR and TGA-MS techniques to explore preparation of Cu/Zn nanocomposites by means of the thermal decomposition of Zn[Cu(CN)3]. 3878
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High-resolution TGA has been used (81) to study the adsorption of mercury(II) ions by modified MCM-41 material and regeneration of the loaded adsorbent with mercury ions by using different eluents. It has been demonstrated that the adsorption data for mercury ions measured by UV spectrophotometry correlate well with TGA measurements that makes the latter a viable technique for studying adsorption systems of environmental significance. Zelenak et al. (82) have synthesized mesoporous silica and used it as a host system for two bactericidal zinc(II) complexes. TGA has been used to confirm the removal of the surfactant and to determine the load of the complexes. TGA has been employed by Gervasini et al. (83) to collect thermodesorption curves of 2-phenylethylamine from acidic surfaces (alumina, silica, silica-alumina, silica-zirconia, silica-titania) and to determine the amount and distribution of the acid sites. The results compare well with those obtained from the differential heat curves of ammonia adsorption. Balek et al. (84) have utilized emanation thermal analysis (ETA) in combination with other thermal techniques to study the preparation of ruthenia-based catalytic materials. ETA has afforded obtaining in situ information about surface area and microstructure development of the intermediate oxide products during heating in air. TGA-MS pulsed thermal analysis plays a pivotal role in studies of activity and regeneration of catalysts. Stark et al. (85) demonstrate that, compared to conventionally prepared material, flamemade Pt/ceria/zirconia has significantly improved thermal stability and dynamic oxygen exchange capacity. Piacentini et al. (86) studied a series of Pt-Ba/Al2O3 catalysts with Ba loadings in the range 4.5-28 wt % and established the existence of three Ba-containing phases that differ in their crystallinity and thermal stability. Storage tests performed by exposing the catalysts to pulses of NO in oxygen-containing carrier gas at 300 °C indicated that the low-temperature BaCO3 phase has the highest reactivity for NO converting rapidly to Ba(NO3)2. The activity depends on the Ba loading and reaches its maximum at ∼17 wt % (88). Gallagher et al. (88) combined simultaneous TG/DSC with high-temperature X-ray diffraction to investigate reactions occurring in the system SrCO3-Fe2O3 on its heating to 1300 °C in an atmosphere of CO2. While elucidating the reaction mechanisms, the authors do not find any evidence of a Hedvall effect associated with the first-order phase transformation in SrCO3 at 927 °C. Nelis et al. (89) apply TGA-MS, TGA-FT-IR, high-temperature XRD, and diffuse reflectance FT-IR to follow the formation of the ferroelectric material strontium bismuth niobate (SrBi2Nb2O9) from an aqueous acetate-citrate precursor gel. On heating the amorphous precursor to ∼425 °C, they have detected the formation of an intermediate fluorite phase that converts into the desired ferroelectric perovskite phase at 625 °C. By using TGA-DTA and DSC, Koga et al. (90) have determined that thermal dehydration of dipotassium tetraborate tetrahydrate occurs in three steps giving rise to an anhydrous glass that shows the glass transition at ∼700 K and subsequently crystallizes in two consecutive steps at 770 and 900 K. The final crystallization product, triclinic K2B4O7, melts at 1072 K. Materazzi et al. (91) examine the thermal behavior of biomimetic complexes of 4(5)-hydroxymethyl-5(4)-methylimidazole with divalent cobalt, nickel, and copper by using TGA-FT-IR. The spectroscopic information has allowed the authors to correlate
decomposition mechanisms with kinetic data for individual steps. Ukraintseva et al. (92) have studied the thermal dissociation processes for clathrates [CuPy4(NO3)2]‚2G (G ) THF and CHCl3) by measuring the vapor pressure and mass loss as a function of temperature. The enthalpies of dissociation are ∼43 and ∼51 kJ mol-1 for the THF and CHCl3 clathrates, respectively. The activation energy of dissociation of the THF clathrate is ∼75 kJ mol-1 by the ASTM E 698 method. Yariv and Lapides (93) demonstrate the utility of thermo-XRD analysis in the study of organo-smectite complexes such as montmorillonite complexes with anilines, fatty acids, alizarinate, and protonated Congo red and of complexes of other smectites with acridine orange. The technique is very efficient in determining whether the adsorbed organic species penetrates into the interlayer space of the smectites mineral. Xi et al. (94) have used high-resolution TGA and XRD to examine the thermal stability of montmorillonite modified with octadecyltrimethylammonium bromide and found that the surfactant remains stable up to ∼180 °C. Starink (95) provides a comprehensive review of the application of calorimetry in analysis of a large variety of processes in aluminum based alloys. Toda et al. (96) apply temperaturemodulated DSC to measure the kinetics of the martensitic transformation in Ti-Ni alloy. By analyzing the temperature dependence of the relaxation time, they find that the process has two characteristic relaxation times apparently related to different processes. Ziewiec et al. (97) report preparation, thermal stability, and glass-forming ability of copper-nickel-phosphorus alloys. They find that depending on the composition melt spinning may result in either amorphous or partially crystalline systems whose thermal behavior has been characterized by DSC, DTA, and XRD. By combining calorimetry with three-dimensional atom probe analysis, Starink et al. (98) have examined room-temperature aging of Al-Cu-Mg-Mn alloys and found that the process is accompanied by a substantial exothermic heat evolution, whereas the only microstructural change involves the formation of CuMg co-clusters. Kost’al et al. (99) have applied the penetration method to measure the temperature dependence of viscosity for Cux(As2Se3)100-x melts (x ) 1, 5, 10, and 20) and found it to follow the Arrhenius law. Both kinetic and thermodynamic fragility exhibit a similar compositional dependence, indicating that increasing Cu content causes topological changes in the structure of As2Se3 supercooled melts. Budrugeac et al. (100) examine the kinetics of the nonisothermal crystallization of (GeS2)0.3(Sb2S3)0.7 by employing the methods of Friedman and of invariant kinetic parameters and demonstrate that process can be treated as a single step. A more complex kinetic situation has been encountered by Thomas and Simon (101) in recrystallization of nickel sulfide from the R- to β-form. Their analysis yielded evidence of at least two steps involved in the overall process. The complexity of crystallization of behavior of a metallic glass has been analyzed by Chrissafis et al. (102), who combined DSC with electron microscopy and revealed at least three different microscopic processes that are represented by a single DSC peak. POLYMERS A review by Cebe (103) focuses on new insights into the melting and glass transition of polymers that have been gained
through the development of temperature-modulated DSC and nanocalorimetry. The techniques allow thermal properties to be respectively studied under quasi-isothermal conditions (i.e., zero heating rate) and under rapid heating conditions (heating rates of thousands of degrees per second). Wunderlich (104) suggests that the ability of a macromolecule to crystallize depends strongly on it being in either an extended-chain or chain-folded macroconformation. Since both conformations may coexist within the same macromolecule, it may demonstrate decoupled processes of irreversible and reversible melting respectively related to the extended-chain and chain-folded macroconformations. Quasiisothermal temperature-modulated DSC is considered to be the tool for probing this phenomenon. In particular, a temperaturemodulated DSC study of poly(oxyethylene) by Qiu et al. (105) has shown that perfected extended-chain and once- or twice-folded crystals of the oligomers with a molar mass above 1100 Da melt practically fully irreversibly, whereas the folded-chain crystals of high molar mass show some locally reversible melting. The origin of the multiple melting DSC peaks observed for poly(ethylene terephthalate) has been discussed by Minakov et al. (106), who used in their study a chip calorimeter capable of very fast heating rates. The results suggest that the meting-recrystallizationremelting process typically observed at slow heating rates does not occur when applying the heating rate 2700 K s-1 at which only the melting process takes place. Van Durme et al. (107) demonstrate that modulated-temperature DSC can be applied for detecting the onset of phase separation in aqueous poly(N-isopropylacrylamide). Quasi-isothermal measurements through the phase transition show large excess contributions in the apparent heat capacity, whose time-dependent evolution represents the kinetics of phase separation. Fukao et al. (108) apply temperature-modulated DSC and dielectric analysis to aging poly(methyl methacrylate) in the glassy state and report that both dielectric constant and heat capacity decrease with increasing aging time. Cooling and reheating after aging shows a minimum in both dielectric constant and heat capacity. Temperature-modulated DSC and dielectric spectroscopy have been employed by Schonhals et al. (109) to probe the glass transition dynamics of poly(propylene glycol) and poly(methylphenylsiloxane) confined to nanoporous glasses of different pore sizes (
