Thermal Analysis | Analytical Chemistry

Anal. Chem. 2004, 76, 3299-3312

Thermal Analysis Sergey Vyazovkin*

Department of Chemistry, University of Alabama at Birmingham, 901S 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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This review is an attempt to select 200 publications that present new significant developments and applications of the thermal analysis techniques that occurred in the years 2002 and 2003. According to ISI Web of Science, only two major methods, DSC and TGA, have been mentioned in well over 6000 articles during this period. In this situation, selecting 200 representative publications is obviously a daunting task, accomplishment of which regrettably leaves some of fine work not cited. During the past two years, the collection of thermal analysis books has seen new important additions. The four-volume series Handbook of Thermal Analysis and Calorimetry has been completed with the arrival of the last volumes that deal with the large variety of applications to polymeric (1) as well as inorganic and miscellaneous materials (2). Both volumes include a great number of articles written by renowned experts. Another compilation has appeared under the title Principles of Thermal Analysis and Calorimetry (3) and provides an efficient introduction to the thermal analysis techniques. A comprehensive introduction to the techniques of sample controlled thermal analysis (SCTA) has been given in a book edited by Sorensen and Rouquerol (4). The ESTAC-8 conference held in Barcelona in 2002 was a major event in the life of the international thermal analysis community. A total of 270 presentations were made at the conference, whose proceedings have been published as a separate volume of Journal of Thermal Analysis and Calorimetry (5). METHOD DEVELOPMENT AND CALIBRATIONS Gallagher et al. (6) have developed an improved protocol for calibrating TGA instruments by using magnetic transition temperatures. The observed values of transitions are corrected using the values for the melting temperatures of pure metals, whose melting is measured simultaneously with the magnetic transitions. Mercuri et al. (7) present an improved TGA technique for determination of the specific surface area of nanoporous materials. The method allows one to estimate accurate values of the specific surface area and the volume of primary mesopores, which are in a good agreement with those evaluated from low-temperature * E-mail: [email protected]. 10.1021/ac040054h CCC: $27.50 Published on Web 04/10/2004

© 2004 American Chemical Society

nitrogen adsorption isotherms. In SCTA, the heating program is adjusted continuously in order to accomplish the desired rates of transformation that are normally monitored as changes in the sample mass, sample dimensions, or the evolved gas. Charsley et al. (8) introduce a new technique of sample controlled thermomicroscopy where the intensity of the light reflected or transmitted by the sample is used to monitor the rate of transformation. Majling (9) gives a brief overview of the optical transmittance thermal analysis that monitors in situ changes in the optical transmittance of samples as a function of temperature. The utility of the method is illustrated by analysis of heat treatment of thin colloidally processed xerogel plates. Yasuda et al. (10) present an optical DTA apparatus capable of examining the solidification behavior of the Al2O3-based materials. A direct current through the two Mo crucibles effectively heats specimen and reference up to 2400 K. Two-color pyrometers are used for evaluating the temperature changes. Marcos et al. (11) have developed a high-sensitivity DSC that is capable of working under magnetic fields up to 5 T in the temperature range 10-300 K. The system is especially useful for investigating materials that exhibit the magnetocaloric effect due to magnetostructural phase transitions. Zimmermann and Keller (12) have designed a new calorimeter for simultaneous measurements of heats and isotherms of gas adsorption. It consists of a volumetric gas adsorption devise whose adsorption vessel is placed inside a second vessel filled with inert gas, which acts as a pressure-temperature sensor. The authors present the results of numerous calibration measurements and provide examples of measuring the heats of adsorption and adsorption isotherms for N2 on alumina oxide and CO2 on zeolite. Rouquerol et al. (13) analyze the use of microcalorimetry for assessing microporosity by immersing samples into liquid nitrogen or argon. They suggest that this method gives more realistic estimates of the surface area in the case of microporous samples. Pijpers et al. (14) provide a comprehensive report on the use of the new technique of high-speed calorimetry and demonstrate its advantages for the study of the kinetics of glass transition, crystallization, and melting of macromolecules. In addition to excluding slow phenomena, the technique provides high sensitivity that affords reducing samples sizes down to the microgram level. By using a thin-film Si3Nx membrane with a film thermopile and a film heater as a microcalorimeter cell, Adamovsky et al. (15) have accomplished controlled cooling and heating rates up to 5 × 103 K s-1. The instrument can be calibrated by applying a simple algorithm developed by the authors. The efficiency of the instrument is illustrated by analyzing a ∼120-ng sample of linear polyethylene in the melting-crystallization region. Olson et al. (16) have developed a sensor for use as a calorimetric cell in an ultrasensitive, thin-film, DSC technique. The sensor contains a Analytical Chemistry, Vol. 76, No. 12, June 15, 2004 3299

free-standing, thin silicon nitride membrane that, along with a thin metallization layer, forms a calorimetric cell. This very sensitive calorimetric cell allows one to make heat capacity measurements of nanometer-thick metal and polymer films. Wenz et al. (17) describe a system for the determination of the calorific value of gases that permits continuously monitoring the mass of the gas burnt in the calorimeter. This is accomplished via a special weighing procedure that accounts for an additional force exerted to the balance due to the capillary connection between the gas bottle and the calorimeter. Hohne et al. (18) use the linear response theory to discuss various approaches to the problems of calibration of temperaturemodulated DSC. They suggest the ways of determining the transfer functions of the sample and instrument as well as a method of correcting the complex heat capacity. By applying different calibration methods to the same set of experimental data, the authors (19) conclude that the more simple calibration procedures lead to practically the same uncertainties of the results as the more sophisticated “third-order” calibrations, which, however, should be preferred when the heat-transfer conditions change during the measurement. Kamasa et al. (20) have modified a DSC instrument by introducing high-power infrared lightemitting diodes that are used to produce a sinusoidal temperature modulation. The use of the separate modulation source enables increase of the range of modulation frequencies. Jiang et al. (21) have provided a very instructive overview of the principles of temperature-modulated DSC paying special attention to the problem of separating the signal into thermodynamic and kinetic components. They stress that the correct separation requires a knowledge of the molecular time scale that is not normally known “a priori”, which is an important limitation of the technique. Kanari and Ozawa (22) analyze sources of errors in measuring the complex heat capacity. On the basis of their theoretical analysis, they also suggest correction procedures for eliminating the errors. DSC measurements are frequently performed on biological substances such as proteins, nucleic acids, and lipid assemblies. Hinz and Schwarz (23) describe the procedures for measurement, calibration, and performance testing of DSC. They emphasize that thermodynamic transition models should be applied to the analysis of the heat capacity curves if the transition temperatures and enthalpies are independent of the scan rate. Application of thermodynamic transition models involving two states and dissociation has been tested on solutions of hen egg white lysozyme that were sent to six different laboratories. It is reported that an average unfolding transition temperature for lysozyme has been 331.2 K with the values ranging between 329.4 and 331.9 K, and an average transition enthalpy has been 405 kJ mol-1 with the values ranging from 377 to 439 kJ mol-1. It is recommended that the reporting of DSC data should necessarily include the composition of the solution, the operating conditions and calibrations, and determination of the baselines. Yao et al. (24) describe a precise ac nanocalorimeter for measuring heat capacity of biological macromolecules in solution. The instrument allows measurement of the heat capacity of 10 µL of liquid with an extremely high sensitivity of 0.001%, which corresponds to heat capacity changes of 300 nJ K-1. The performance is tested by measuring the heat capacity during thermal denaturation of lysozyme dissolved in buffered solution. O’Neill et al. (25) report the findings of a 3300

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preliminary inter/intralaboratory study of the base-catalyzed hydrolysis of methyl 4-hydroxybenzoate (methyl paraben) and its suitability as a test and reference reaction for flow microcalorimeters used for determination of both kinetic and thermodynamic parameters. Downes et al. (26) combine microcalorimetric and mass spectrometric methods that allow them to correlate changes in metabolic heat rate with compositional changes in the ampule headspace, including those resulting from the insect’s metabolism. These results aid in assessing the effectiveness of controlled atmospheres, such as low oxygen, high carbon dioxide, or both, as potential replacements of methyl bromide for fumigation of fresh produce. A new batch microcalorimeter has been developed for measuring dissolution of small amounts of easily or slightly soluble solids (27). The calorimeter has been calibrated by dissolution of potassium chloride and tested successfully by measurements of the enthalpies of dissolution of acetanilide and adenine. Wadso and Goldberg (28) publicize an IUPAC technical report on the standards in isothermal microcalorimetry. The report discusses different chemical calibration and test reactions with a focus on reactions initiated by mixing of liquids and dissolution of solid compounds and of slightly soluble gases. It also proposes a standardized terminology and presents guidelines on the use of standardized chemical test and calibration reactions in isothermal microcalorimetry. Hakvoort et al. (29) provide the vapor pressure of a number of calibration standards and emphasize the importance of knowing this value at a transition temperature as well as at room temperature. They stress that substances with a high vapor pressure should be hermetically encapsulated in order to accomplish reliable calibrations. Lerchner et al. (30) have studied the accuracy of integrated circuit calorimeters by using several Joule heaters and laser signals at different positions. They report that the sensitivity has positional dependence, which is mainly caused by the planar geometry of the heat power detector, and that it can be corrected by a shape factor. Merzlyakov (31) has proposed a new calorimetric method that uses an integrated circuit thermopile in an oscillating mode. The setup can measure total heat losses that permit one to monitor enthalpy changes in a sample. Preliminary results show that the method is capable of measuring dynamic heat capacity in a frequency range from 1 mHz to 100 Hz. Zogg et al. (32) have designed a new reaction calorimeter with an integrated infrared-attenuated total reflection probe that has a sample volume of 45 mL. The new calorimeter has been tested by studying the kinetics of the neutralization of NaOH with H2SO4 and the hydrolysis of acetic anhydride. The kinetic parameters obtained from the IR and calorimetric measurements agree well with each other and with literature values. THERMODYNAMICS Thermal methods are widely used for detecting phase transitions and measuring important thermodynamic properties such as the heat capacity, thermal expansion, and enthalpy. A great number of thermodynamic papers have been collected in proceedings of the Symposium on Thermochemistry, Calorimetry, and Molecular Energetics at the 17th International Conference on Chemical Thermodynamics of IUPAC (33). Frenkel et al. (34) present “ThermoML”, an XML-based approach to storage and

exchange of experimental and critically evaluated thermophysical and thermochemical property data. ThermoML covers essentially all experimentally determined thermodynamic and transport property data (more than 120 properties) for pure compounds, multicomponent mixtures, and chemical reactions (including change of state and equilibrium). The role of ThermoML in global data submission and dissemination is discussed with particular emphasis on the new cooperation in data processing between the Journal of Chemical and Engineering Data and the Thermodynamics Research Center at the National Institute of Standards and Technology. Hatta (35) discusses thermal properties of nanoscale materials focusing on the heat capacity and thermal conductivity at low temperatures and at the interfaces. The discussion is illustrated by experimental measurements. Ramakumar et al. (36) have performed experimental evaluation of different ways of measuring the heat capacity by DSC. They studied the effect of the calibrant choice as well as of heating rate, sample mass, and the sample form on reproducibility and repeatability of the measurements. Rudtsch (37) provides a comprehensive analysis of experimental and computational uncertainties involved in evaluation of the heat capacity. The conclusion of this analysis is that the value can be measured with a relative uncertainty of ∼1.5%. Wilding et al. (38) report a first-order transition between two liquids in yttrium-aluminate liquids that is interpreted as the nucleation and growth of a low-density phase in a matrix of a higher density liquid when Y2O3-Al2O3 liquids are cooled below the liquidus. DSC in combination with structural data is used to discuss the thermodynamic features of Y2O3-Al2O3 liquids that lead to liquid-liquid transitions. Stolen et al. (39) have measured thermodynamics characteristics of liquids in the GeSe2-Se system that suggest the existence of a fragile melt at high temperatures and much less fragile behavior of the system close to the glass transition temperature. The behavior is explained by a fragile-tostrong transition in supercooled GeSe2 Se. Taketomi et al. (40) have used DSC to confirm that the earlier reported strong magnetooptical effect of a magnetic fluid under a magnetic field is due to the second-order phase transition from colloidal particles’ monodispersed phase to the particles’ anisotropically agglomerated microcluster phase. No phase transition heat has been observed within an experimental error of 0.03 kJ kg-1 for all the samples whether or not the field was applied. Johari (41) discusses calorimetric features of high-enthalpy amorphous solids and glasssoftening temperature of water. It is stressed that, in the DSC runs of the annealed samples of vapor-deposited, hyperquenched, and crystal-amorphized solids, the glass-softening or Tg endotherm is interrupted by the crystallization exotherm, thus making the endotherm appear like a rounded peak. This apparent peak is occasionally misinterpreted as a sub-Tg peak, which appears in the 0.7-0.8 Tg range of glasses preannealed at a specific temperature. Criteria are provided for distinguishing the apparent peak of the Tg endotherm from a real sub-Tg peak. Diky et al. (42) analyze the available experimental values of the enthalpy of sublimation of monocyclic, bicyclic, and “cage” hydrocarbons. In the series of these structurally related hydrocarbons, the value of the sublimation enthalpy for cubane presents an anomaly, the potential cause of which is discussed with emphasis on the reliability of the value. High-resolution DSC has

allowed Tozaki et al. (43) to discover a new phase transition in n-C32H66 that is observed at 338.1 K in addition to the already known solid-solid transition at 339.1 K and the melting transition at 341.9 K. The origin of the new transition is discussed. The authors (44) have also studied phase transitions of this substance in a magnetic field of 5 T and found that the transitions shift to higher temperatures. By using DSC, Cerdeirina et al. (45) measure the heat capacity of the nitromethane-1-butanol mixture near its upper critical point. By fitting the Cp data in the one-phase region, the authors have determined the value of the critical exponent R ) 0.110 ( 0.014 that is consistent with theoretical calculations and other experimental data. Yamashita et al. (46, 47) have carried out a calorimetric study of phase transitions in CsZnPO4. They find a heat capacity anomaly due to the IV-III phase transition around 220 K that becomes larger when the sample is annealed for longer time below the transition temperature (46). At 584 K, the authors detect the I-II transition that has a complex nature showing the features of both order-disorder and displacive transitions (47). By using the simultaneous DSC-XRD method, Yoshida et al. (48) have observed a self-ordering process of phenanthrene polyesters derived from 2,7-phenanthrenedicarbonic acid diethyl ester and alkanediols with even methylene carbon number. On cooling from the molten state, they detect a transition from the isotropic state to the smectic A phase, whose entropy depends on the methylene carbon number. Jurkowski et al. (49) suggest that measurement of the thermal expansion coefficients (R) can be used to identify the amorphous and crystalline states in polymers as they have a markedly different ratio of R in the liquid and solid states. The critical values of this ratio are discussed. Takamizawa et al. (50) have synthesized a novel copper(II) benzoate pyrazine that undergoes a phase transition induced by CO2 adsorption. As demonstrated by heating-cooling cycles in DSC, the transition is reversible and its temperature depends on the concentration of CO2. Dick et al. (51) report on the size dependence of the melting temperature of silica-encapsulated gold nanoparticles (1.5-20 nm). According to their data, the silica shell has little effect on the melting temperature. Nikolic et al. (52) use DSC to study new materials for solar thermal storage that are based on solid/liquid transitions in fatty acid esters, some of which undergo transitions close to room temperature with high enthalpy and low hysteresis. These substances can be readily combined with the building materials and may store ∼60 kJ of the thermal energy/kg of the composite in the form of latent heat of melting. Zhang et al. (53) apply microthermal analysis to investigate martensitic to austenitic transformations of nearequiatomic NiTi shape memory alloy thin films deposited on silicon wafer. The individual films investigated have shown a spatial variation on the micrometer scale in the transition temperatures that is explained by a nonuniform distribution of Ti and Ni in the film structure. Hinks et al. (54) have measured vapor pressure of quinizarin and leucoquinizarin by using a transpiration method and by a method based on TGA. They report that TGA provided vapor pressure-temperature dependence data for each compound with expediency and in agreement with the transpiration method. Arlabosse et al. (55) compare the accuracy of TGA/DSC with two other methods of measuring sorption isotherms. They find this Analytical Chemistry, Vol. 76, No. 12, June 15, 2004

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method to be accurate as long as the apparent diffusion coefficient is above 10-9 m2 s-1. Goodrum and Geller (56) use TGA for measurement of boiling points and vapor pressure of mediumand long-chain triglycerides. The data obey the ClausiusClapeyron model and are consistent with the data obtained by using DSC as well as with data previously reported for reduced pressure. Mintz (57) describes a new method that affords measuring the permeability of vapor through various materials by using TGA and the solubility-diffusion model. Preliminary results for permeability of 2,3-dimethyl-2,3-dinitrobutane vapor through polystyrene and polyethylene are presented. KINETICS The kinetics of thermally stimulated reactions is almost universally parametrized as a function of the temperature, T, and the extent of the reactant conversion, R, in the following form

dR -E ) k(T)f(R) ) A exp f (R) dt RT

( )

(1)

where f (R) is the reaction model, k(T) is the Arrhenius rate constant, A and E are Arrhenius parameters (the preexponential factor and the activation energy, respectively), and R is the gas constant. Galwey and Brown (58) discuss the problem of applicability of the Arrhenius equation to solid-state kinetics. The main theoretical objection is that the Maxwell-Boltzmann distribution does not adequately represent the energy distribution among the immobilized constituents of a crystalline reactant. However, they argue that the band structure of a solid may include interface energy levels whose occupancy is determined by the Fermi-Dirac and Bose-Einstein distributions that approximate to an exponential energy term of the Arrhenius equation. Thermal analysis methods are not species specific so that the rates measured with their help are likely to represent complex (multistep) processes. For this reason, the reliable methods of kinetic analysis should be capable of detecting reaction complexity. It has been a conclusion of the ICTAC Kinetics Project that various kinetic methods that employ multiple temperature programs (isothermal, nonisothermal) are capable of handling complex kinetics. The complexity can be detected in various interrelated forms such as multiple steps with respective values of E, a variable value of E, or a distribution of the E values, E being the effective activation energy (59). Perhaps, the use of isoconversional methods is the simplest and most popular way of detecting the complexity in the form of a variation of the effective activation energy with the extent of conversion. In particular, Galwey and Brown (58) mention that the existence of the interface levels with a limited range of energies would give rise to the variation of apparent activation energy with extent of reaction and with temperature, reported for many complex solid-state reactions. By its meaning, the effective activation energy is merely an experimental parameter that represents the temperature dependence of the overall rate (59). Attempts (60) to interpret directly the effective activation energy in terms of the energy barrier height lead to obvious confusion (59). Starink (61) have compared the accuracy of isoconversional methods that he classified into two types. Type A includes differential methods that make no mathematical approximations. Type B methods make use of various approximations of the 3302

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temperature integral so that their accuracy is dependent on the accuracy of the approximation. Although type A methods tend to be more accurate than type B methods, it is stressed that in cases where some uncertainty over baselines exists or where accuracy of determination of transformation rates is limited, type B methods will often be more accurate than type A methods. It should be added that the integral methods may produce a systematic error when the effective activation energy varies strongly with the extent of conversion (62, 63). However, this error can be eliminated when using the advanced integral isoconversional method that performs integration over small segments of the extent of conversion (62, 63). Integration over segments has also been implemented by Simon et al. (64) in an incremental integral isoconversional method. The asymptotic convergence of the advanced isoconversional method with the differential method has been demonstrated by Budrugeac (63). He has also proposed a nonlinear differential isoconversional method (63) that uses the numerical algorithm similar to that used in our advanced isoconversional method. Although isoconversional methods require running several experiments at different temperature programs, an activation energy can in principle be estimated from a single oscillating temperature experiment. While less time-consuming, a single experiment evaluation is also less reliable. Also, because the temperature perturbations are not instantaneous, this approach is only approximately isoconversional and the resulting estimates may be very imprecise. The issue of precision and other aspects of this approach has been discussed by Mamleev and Bourbigot (65). Budrugeac et al. (66) discuss the application of an isoconversional method to isothermal data and stress the importance of correcting the data for the initial “warm-up” time and induction period. Isoconversional methods provide the basis for so-called “modelfree” kinetic analysis. Note that the term “model-free” may be confusing as it is interpreted somewhat differently by different workers (67, 68). In particular, Sewry and Brown (67) draw attention to the nonparametric kinetics (NPK) method (69) that also fits the “model-free” definition. Opfermann et al. (70) stress that the model-free analysis is a powerful kinetic tool and explore the limits of its application by using some experimental and simulated data. The most popular representative of the modelfree methods is the method of Kissinger that allows the effective activation energy to be determined from the dependence of the peak temperature on the heating rate. While approximately isoconversional, the method provides a single value of the activation energy related to the conversion at the peak temperature. This makes the method ineffective in detecting the reaction complexity. Another important limitation of the method is that it cannot be applied to the processes that occur on cooling such as the melt crystallization (71). However, some workers force the Kissinger method into this type of application by unjustifiably dropping the negative sign for the heating rate. It is demonstrated (71) that this is a mathematically invalid procedure and that the problem of negative heating rates can be resolved by using either the advanced isoconversional method or the Friedman method. The application of the method to crystallization of a polymer melt yields a variable activation energy that can be interpreted in terms of the accepted crystallization models (72). The advantages of the model-free kinetic analysis originate largely from eliminating the fundamental uncertainty in choosing

the reaction model which is, however, considered by some workers as the crucial piece of information. Identification of the model is the centerpiece of kinetic analyses developed by Criado et al. (73) and Perez-Maqueda et al. (74) who, in particular, mention that the identification is much more reliable when using the method of constant rate thermal analysis. Koga and Tanaka (75) give a comprehensive review of the kinetic models and their relation to the actual processes of thermal dehydration and decomposition of inorganic solids. Obviously, the complexity of solid-state processes extends far beyond the oversimplified theoretical description implemented in the commonly used models. The review (75) examines possible extensions of the conventional kinetic theory by incorporating various physicogeometric and chemical features in the model descriptions. As an alternative to the traditional physicogeometrical models, Korobov (76) explores a new class of discrete models based on Dirichlet tessellations whose application is illustrated by the thermal decomposition of NH4HCO3. Dissatisfaction with the traditional models gives rise to developing new model-free approaches. In addition to the already cited NPK method (69), we should mention the use of neural networks (77) and an approach proposed by Schiraldi (78). L’vov (79) provides a review of his alternative approach to solidstate decompositions kinetic that is based on the Langmuir-Hertz vaporization equation. The basic assumption of this approach is that decompositions of a solid occur via congruent dissociative evaporation. The approach is applied to decomposition of Ag, Cd, Zn, Mg, CaMg, Ca, Sr and Ba carbonates. L’vov’s theoretical estimates of the activation energy appear to be in fair agreement with the experimental values obtained by other authors for decomposition in a vacuum. Sestak and Chvoj (80) discuss the possibility of applying the methods of irreversible thermodynamics for generalized description solid-state reaction kinetics. The approach, however, faces substantial experimental and mathematical challenges associated with measuring fluxes and incorporating them into the system of differential equations. Holland and Hay (81) and Howell (82) discuss the value and limitations of nonisothermal methods in kinetic studies. For decades, the correlation between isothermal and nonisothermal data has been a fundamental issue not only because of the difference in experimental conditions but also because of the difference in the methods data analysis. There appears to be a constructive tendency in developing universal methods (62, 63, 69, 73, 74) that can be applied to isothermal as well as to nonisothermal data. INORGANICS Flame-made ceria/zirconia nanocrystals have been prepared and studied with respect to catalytic applications (83, 84). The method of pulse thermal analysis has been used to measure oxygen-exchange capacity that reaches the highest value (450 mmol of O2 kg-1) in silica-doped systems. Grunwaldt et al. (85) have performed in situ absorption measurements during methane combustion over Pd/ZrO2 catalysts. Thermal analysis in combination with X-ray data has provided new insights into the oxidation state of the constituents and of the structure of a Pd/ZrO2 catalyst during activation and catalytic combustion of methane. Fesenko et al. (86) summarize some applications of SCTA methods for the

characterization and preparation of catalysts. It is stressed that SCTA methods can produce significant enhancements in the resolution of complex reactions, provide a detailed insight into the energetics of surface and bulk processes, and, when applied to catalyst preparation, give improvements in the pore structure and uniformity of the resulting materials. Kruk et al. (87) use gas adsorption and thermogravimetry for the determination of the phase composition of MCM-48/lamellar phase mixtures, which often are formed at different stages of the MCM-48 synthesis. Because of the overlap of the characteristic peaks, XRD is inconvenient for analytical purposes, whereas gas adsorption and TGA readily provide good estimates of the phase composition. Sanders and Gallagher (88) have studied the kinetics of the oxidation of magnetite (Fe3O4) to hematite (R-Fe2O3) in air using simultaneous TGA/DSC. The process is found to be complex and suggested to involve the metastable spinel, γ-Fe2O3, as an intermediate. The formation of the intermediate is supported by thermomagnetometric evidence (89). Labus et al. (90) have studied the phase transitions and thermal effects occurring during annealing CrOx (x g 2.4) in air. The mass changes observed on heating of the materials are accompanied by exothermic effects that change to endothermic ones when the sample mass decreases. This phenomenon is explained by the competition between reconstruction of the crystalline lattice (endothermic effect) and recombination of the evolved atomic oxygen (exothermic effect). Hardy et al. (91) use thermal and spectroscopic techniques to follow the formation of ferroelectric bismuth titanate (Bi4Ti3O12) from an aqueous metal-chelate gel. The gel appears to stay homogeneous throughout the heat treatment and forms single-phase Bi4Ti3O12 at 625 °C. Krunks et al. (92) study zinc thiocarbamide chloride as a single-source precursor for obtaining zinc sulfide thin films by spray pyrolysis. By heating this compound to 1200 °C, they demonstrate that cubic ZnS (sphalerite) forms below 300 °C and stays in this form until 760 °C, when it transforms to hexagonal ZnS (wurtzite). By applying microthermal analysis to silicon nitrides materials, Ye and Okada (93) have obtained local thermal conductance distribution that suggests that the thermal conductivity is higher in silicon nitride grains and lower in grain boundaries. The authors observe and discuss a discrepancy between the grain and bulk thermal conductivities. Logvinenko et al. (94) demonstrate that the thermal decomposition of salts of transition metals with carboxylic acids (maleic, o-phthalic, terephthalic) leads to polymer-metal composites that consist of spherical conglomerates of metal grains coated with polymer. Kinetic analysis suggests that decomposition occurs autocatalytically. Tsirlin et al. (95) describe the synthesis and characterization (TGA, DSC, DTA, etc.) of nano-metallopolycarbosilanes that can be pyrolyzed into materials suitable for the fabrication of non-oxide ceramic fibers, interphase coatings. and high-temperature ceramic matrix composite. Lopez et al. (96) synthesize carbon nanotubes by decomposing acetylene on silica- or alumina-supported iron as catalysts. The authors use TGA to monitor the growth of nanotubes and acetylene pulses to control their dimensional size. Strong et al. (97) have developed a simple purification process for single-wall carbon nanotubes that employs oxidative heat treatment followed by acid reflux. It is demonstrated that the purity of the resulting nanotubes can be reliably determined by a combination of TGA Analytical Chemistry, Vol. 76, No. 12, June 15, 2004

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and Raman spectroscopy. By using DSC, Zhang and Dahn (98) demonstrate that the reaction of the impregnated carbons with air is accelerated by the presence of moisture adsorbed by the impregnated carbon. According to kinetics analysis, this acceleration is caused by an increase in the frequency factor that is determined to result from a decrease in the particle size of the impregnant through a series of steps initiated by the adsorbed water. Maftuleac et al. (99) report that dehydration of montmorillonite samples produces endothermic effects in the region 80-250 °C that may appear as single or double peaks depending on the nature of exchange cations. By studying different exchange forms (Na, Ca, Al, Fe), the authors conclude that the shape of endo effects depends on the state of the active centers of the mineral. Balek et al. (100) have studied interaction of various organic compounds with TIXOTON (Ca-bentonite activated by sodium carbonate) by emanation thermal analysis (ETA) supplemented by TG and XRD. The ETA is shown to reveal in situ microstructure changes of montmorillonite, iron hydrous oxide, and amorphous silica, occurring during the heating. Rafferty et al. (101) demonstrate the convenience of using DMTA for monitoring amorphous-phase separation in fluorophosphoaluminosilicate and sodium borosilicate glasses, both of which exhibit two maximums in tan δ that correspond to the glass transitions of the individual phases. Barium aluminosilicate (BAS) glass ceramics have the potential to be used in the production of cast prostheses for biomedical applications. Griggs et al. (102) demonstrate that the addition of fluoride to BAS glass can reduce the necessary processing time and temperatures by obviating the need for a separate crystal nucleation treatment. DTA measurements suggest that the resulting glass has a lower energy barrier to crystallization and transformed to 76% crystallinity within 30 min at 975 °C. Perepezko et al. (103) study primary nanocrystallization reactions in amorphous aluminum alloys and report a strong sensitivity of the process onset and its enthalpy to thermal history and the as-quenched state. It is noted that crystallization can also be induced in response to external force such as irradiation or mechanical alloying that suggests that the shear process during rolling effects a local rearrangement of atoms in the amorphous matrix. Kavouras et al. (104) explore binding of toxic lead-rich solid industrial wastes by vitrifying them with the aid of SiO2 and Na2O. DTA is used to monitor phase separation of the resulting nontoxic vitreous products that contain 60 wt % of solid waste. POLYMERS Wunderlich (105) discusses the multiplicity of actual phases that may exist in semicrystalline polymers and may involve crystals, mesophases, liquids, and glasses. The glasses, in their turn, may have structures that correspond to liquids or mesophases and can exist even above the glass transition temperature of the mobile macrophase as rigid amorphous fractions. Pak et al. (106) report that poly(oxy-2,6-dimethyl-1,4-phenylene) is the first example of a polymer that has the glass transition of the rigid amorphous fraction above the melting temperature of the crystals. By using temperature-modulated DSC, Xu et al. (107) demonstrate that a significant amount of the rigid amorphous fraction coexists with the crystalline and mobile amorphous phases in cold3304

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crystallized isotactic PS. The amount of this fraction increases with the crystallization time, and a greater amount is formed at a lower crystallization temperature. Garrett et al. (108) have investigated the phase behavior of two series of MDI-poly(tetramethylene oxide) soft segment copolymers, chain-extended with ethylenediamine or a diamine mixture. Both DSC and DMA experiments indicate that the soft-phase glass transition temperature decreases with increasing hard-segment content that can be rationalized by taking into consideration soft-segment crystallinity and the concentration of “lone” MDI units in the soft phase. The determination of the equilibrium melting temperatures of polymers that show recrystallization during heating presents a serious challenge. Al-Hussein and Strobl (109) investigate the problem for isotactic polystyrene by using SAXS and DSC measurements that have shown a good agreement in estimating the Gibbs-Thomson melting line. Androsch and Wunderlich (110) demonstrate that temperature-modulated DSC allows one to identify a reversible melting in polymers that were earlier thought to exhibit only fully irreversible crystallization and melting. It is suggested that reversible melting mainly occurs only between the temperatures of specific crystal fractions formation and their zero-entropy-production melting temperature, at which they change to a melt of the same degree of metastability. Toda et al. (111) suggest that the superheating effect in melting of polymer crystals may be due to a nucleation mechanism. They demonstrate that for some polymers the melting rate is linearly proportional to the degree of superheating, which indicates the kinetics controlled by heat diffusion. However for other polymers (isotactic polypropylene, poly(ethylene terephthalate), poly(caprolactone)), the dependence is close to exponential, which indicates nucleation-controlled kinetics of melting. Yamada et al. (112) propose a new method to determine the Gibbs-Thomson plot and equilibrium melting temperature of polymers. The method is illustrated by estimating the melting point of isotactic PP. In their DSC study of melting behavior of nylon 6 fiber, Kubokawa et al. (113) analyzed the behavior of several characteristic temperatures, including the temperature where the DSC curve starts to depart from its baseline. They demonstrated that at heating rates above 60 °C min-1 this temperature increases linearly with the heating rate, which allowed them to extrapolate the value to zero rate and obtain 190 °C, which is interpreted as the temperature of the zero-entropy-production melting of the most imperfect crystallites of the nylon 6 fabric. Cheng and Lotz (114) discuss some fundamental problems of building molecular kinetic models of polymer crystallization. They stress that the models tend to use a “mean-field” approach whereas crystallization involves a number of molecular pathways occurring on different length and time scales. It has been demonstrated (71) that the Kissinger method is not applicable to evaluating activation energies of polymer melts. On the contrary, the Friedman method and advanced isoconversional method can be applied (71) to estimate a variable activation energy that can be interrelated in terms of accepted crystallization theories (71). Following these recommendations, Supaphol et al. (115) estimate activation energies for the nonisothermal crystallization of the melts of three polyterephthalates. Waddon and Petrovic (116) report retardation spherulite growth rates for crystallization of poly(ethylene oxide) (PEO) in the presence of the silica nanoparticles. A similar effect

is reported by Strawhecker and Manias (117), who examined the crystallization behavior of PEO in the presence of sodium montmorillonite. However, they report that the overall crystallization rate increases with silicate loading as a result of extra nucleation sites. Sur et al. (118) have prepared composites by penetrating PEO chains into mesoporous silica. As a result of confinement, the melting temperature of the polymer decreased and ultimately disappeared. Taden and Landfester (119) have studied crystallization of PEO in the form of narrowly distributed nanodroplets (∼100 nm) and report that nucleation in these droplets occurs only at large supercooling. Fornes and Paul (120) have investigated the crystallization behavior of nylon-6 nanocomposites formed by melt processing and found that very low levels of clay result in dramatic acceleration of crystallization kinetics, whereas increasing the concentration of clay beyond a certain levels retards the rate of crystallization. The effect of ZnO nanoparticles on the crystallization kinetics of nylon-6 have been analyzed by Zheng et al. (121), who report enhanced nucleation as well as retarded mobility of polymer chains. Wei et al. (122) have produced an inclusion complex between nylon-6 and R-cyclodextrin. After removal of R-cyclodextrin, the authors obtained coalesced nylon-6 that exists predominantly in the R crystalline phase and has increased crystallinity and elevated melting and crystallization temperatures. The glass transition temperature of organic liquids confined to nanoporous matrixes can decrease dramatically from the bulk value. Simon et al. (123) suggest that the effect can be explained by the development of hydrostatic tension during vitrification under confinement that results in a concomitant increase in the free volume. The phenomenon is explored in experimental and modeling studies of relaxation of o-terphenyl confined in 11.6-nm pores. By analyzing thin films of polystyrene with different entanglement concentrations, Bernazzani et al. (124) have demonstrated that the reduction of the glass temperature observed in thin films cannot be due to the reduced entanglement concentration. The ultrasensitive DSC is used by Efremov et al. (125) to observe the glass transition in thin (1-400 nm) spincast films of polystyrene, poly(2-vinylpyridine), and poly(methyl methacrylate). The authors do not observe appreciable dependence of the glass transition temperature over the thickness range from hundreds of nanometers down to 3-nm-thick films, although transition becomes broader. By using atomization spraying and drying of a dilute polymer solution, Mi et al. (126) have prepared nanosized polymer globules. The resulting nanosized particles exhibit a significantly higher glass transition temperature than does the corresponding bulk polymer. Bershtein et al. (127) report that silica plays a dual role in polyimide-silica nanocomposites; they observe an enhancement of small-scale motion at temperatures below the β-relaxation region and suppression of segmental motion above that region. Schonhals et al. (128) have employed dielectric spectroscopy and temperature-modulated DSC to study the glassy dynamics of poly(propylene glycol) (PPG) and poly(dimethyl siloxane) (PDMS) confined to a nanoporous host system. They observe that ∆Cp step vanishes in PPG when it is confined to pores of e1.8 nm and in PDMS in pores of e5 nm. Below these sizes, the temperature dependence changes to an Arrhenius-like behavior with a low activation energy.

Swier and van Mele (129) demonstrate how the measurements of the nonreversing heat flow and heat capacity obtained from temperature-modulated DSC can be used for mechanistic modeling of the epoxy-amine reaction. The model is verified by simulating the reaction kinetics of DGEBA with aniline and methylenedianiline as well as by the effect of the polymeric modifiers. Ramis et al. (130) provide a comprehensive comparison of TMA, DMTA, and DSC as applied to the curing of a thermosetting powder coating made up of carboxyl-terminated polyester and triglycidyl isocyanurate. They note that below the gel point the degree of curing determined from the mechanical experiments is always greater than that found in DSC runs; they also discuss differences in the respective kinetic parameters. Montserrat et al. (131) applied conventional and modulated DSC in combination with dielectric relaxation spectroscopy to study the kinetics of an epoxy-amine reaction. The permittivity and loss factor has been correlated with the mobility factor estimated from the complex heat capacity that allowed the authors to probe the molecular dynamics of cross-linking. The application of isoconversional methods to epoxy curing reactions tends to yield a dependence of the effective activation energy on the extent of cure. Sbirrazzuoli and Vyazovkin (132) demonstrate that analysis of this dependence allows for untangling complex cure processes that may include different chemical reactions or a chemical reaction complicated by mass-transfer processes such as viscous relaxation and vitrification. Kessler and White (133) have used several kinetic methods to examine the effect of different concentrations of Grubbs' catalyst on cure kinetics of the ring-opening metathesis polymerization of dicyclopentadiene. As opposed to model-fitting procedures, the model-free isoconversional method has revealed a significant variation in the activation energy and demonstrated a clear advantage in modeling low-temperature experiments. Sbirrazzuoli et al. (134) combine isoconversional analysis with temperature-modulated DSC and dynamic rheometry in order to study curing of DGEBA with stoichiometric and with excessive amounts of m-phenylenediamine. For the nonstoichiometric system, the activation energy is practically constant (∼55 kJ mol-1), whereas the stoichiometric system demonstrates a decrease from 55 to 20 kJ mol-1 that is explained by shifting the rate-determining step from a kinetic to a diffusion regime, which appears to be associated with gelation rather than vitrification. He et al. (135) report a similar decrease (∼65 to 35 kJ mol-1) in the activation energy for curing of phenol formaldehyde resins that is explained by diffusion control. The diffusion-controlled kinetics is described by a diffusion rate constant that is both temperature and conversion dependent. To describe the effect of diffusion control on curing kinetics, Schawe (136) introduces a new phenomenological model that is independent of the reaction temperature and requires only one parameter, which has a meaning of the width of the glass transition. Van Assche et al. (137) propose another model based on the free volume theory and the Rabinowitch activated complex theory that relates the apparent rate constant to the difference between the reaction temperature and the glass transition temperature. Vinnik et al. (138) focus their attention of the later stages of the epoxy-amine curing process when its kinetics is largely determined by rearrangement of the cross-linked chains. They suggest that epoxy ring-opening alone is not responsible for the residual curing and Analytical Chemistry, Vol. 76, No. 12, June 15, 2004

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discuss the role of side reactions. Zvetkov (139) mentions that the actual kinetics of epoxy-amine curing may be inconsistent with the frequently assumed lower reactivity of secondary amine. He suggests that this inconstancy can be resolved in a kinetic model that accounts for the solubility of the reaction components. Brostow and Glass (140) have developed an analytical formula for the cure progress of epoxy systems as a function of both time and temperature. The cure progress is monitored by measuring complex viscosity or the storage modulus as a function of time under isothermal conditions. Ganguli et al. (141) have applied rheological, thermal, and spectroscopic techniques to study the effect of organically layered silicate nanoparticles on the flow and curing behavior of a phenolic triazine cyanate ester resin. They find that gelation and vitrification times and activation energies for the nanocomposite systems are lower than that of the neat resins, indicating a catalytic effect of the clays on the curing reaction. Chen et al. (142) study the interlayer expansion mechanism and thermal-mechanical properties of surface-initiated epoxy nanocomposites. It is concluded that differences in the activation energies of interlayer expansion and of curing influence the final nanostructures of the materials. Timmerman et al. (143) have studied the effect of nanoclay reinforcement on the cryogenic microcracking of carbon fiber/ epoxy composites. They report that in the proper concentration nanoclay reinforcement gives rise to laminates with diminished microcrack densities without significantly altering the mechanical properties of the materials. Zheng and McKenna (144) have studied an effect of isothermal jumps in relative humidity on a glassy epoxy material and found it to be similar to the effect of temperature jumps. They have extended the TNM-KAHR model to fit the experimental results of relative humidity-jump experiments. Holland and Hay (145) attempt to link the observed discrepancy in kinetic parameters of PMMA degradation in terms of the differences in polymerization processes that yield polymers having different structure and, in particular, different weak links. To test whether head-to-head units are the weak links responsible for the thermal instability of PS, Howell et al. (146) synthesized fully head-to-head polymer and compared it against the head-to-tail PS. By analyzing the products of degradation, they determine that degradation of the head-to-head polymer occurs via scission at head-to-head linkages while decomposition of the head-to-tail polymer occurs via unzipping. Yao et al. (147) have obtained crown ether-modified clays and polymerized styrene in their presence. By applying TGA and cone calorimetry to the resulting composites, they have discovered that as compared to virgin PS these materials demonstrate a markedly greater degradation temperature and smaller peak heat release rate. However, for PS graphite nanocomposites, Uhl and Wilkie (148) report only a slight increase in thermal stability that is offset by deterioration of mechanical properties. Organically modified layered silicates with high thermal stability are critical for synthesis and processing of polymer-layered silicate nanocomposites. Xie et al. (149) combine TGA with pyrolysis-GC/MS to study the nonoxidative thermal degradation of alkyl and aryl quaternary phosphonium-modified montmorillonites. It is concluded that the overall thermal stability of these systems is higher than that of usual ammonium-modified montmorillonite. Fernandes 3306

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et al. (150) have demonstrated the utility of kinetic analysis in estimating potential fire-retarding efficiency of additives to unsaturated polyester. Balabanovich et al. (151) report that addition cyclic diphosphonate ester and melamine gives rise to excellent fire retardant performance in poly(butylene terephthalate) that is explained by synergetic interaction, which is demonstrated by thermal analysis and IR spectroscopy. Tsukruk et al. (152) compare the technique of microthermal analysis with conventional methods of DSC and TMA as applied to a wide selection of materials ranging from poorly thermal conducting polymers to highly conductive metals.

ENERGETICS Thermochimica Acta has published two special issues that cover thermal properties and processes of energetic materials (153, 154). A great deal of attention is focused on the thermal decomposition of new energetic materials. Geetha et al. (155) use dynamic DSC and isothermal TG to determine kinetic parameters of hexanitro hexazaisowurtzitane (CL-20) decomposition. The obtained activation energies were close to the values reported for N-NO2 bond scission, suggesting it as a rate-limiting process. The hypothesis was supported by mass spectrometry. Thermal aging of rocket propellant formulations containing CL-20 and the GAP binder has been investigated by Bohn, (156) who measured kinetics of heat generation and mass loss as a function of time at the temperatures of 70, 80 and 90 °C. Although one out of seven formulations showed a noticeably lower activation energy; all formulations demonstrate a satisfactory aging behavior giving rise to the estimated useful life of ∼20 years. Tappan and Brill (157) employed the sol-gel to cryogel method to synthesize an energetic composite, in which spherical nanoparticles (20-200 nm) of CL-20 are coated with HDI-cross-linked nitrocellulose. The use of DSC and T-jump/FT-IR spectroscopy demonstrated that the decomposition properties of the composite were controlled mostly by nitrocellulose until the percentage of CL-20 was well above 50%. The drop weight impact sensitivity of the cryogels was essentially independent of the composition. Mishra and Russell (158) explore the problem of suppressing the low-temperature thermal stability of ammonium dinitramide (ADN) by investigating the decomposition rates below 90 °C. ADN has been studied in mixtures with 1-2% of potential stabilizers such as potassium fluoride, potassium dinitramide, a six-member ring or polymeric phosphorus compound [P(C6H5)], and perhydro-1,3,5-triazine-2,4,6trione. The latter is found to be a most effective stabilizer. Jones et al. (159) employed DSC and TGA-DTA-FT-IR-MS and accelerating rate calorimetry to determine the effect of aluminum nanopowder on decomposition of its mixtures with ADN and GAP. No chemical interactions were observed between ADN/Al, GAP/Al, and ADN/GAP. The nanopowders had a minor effect on the thermal stability of ADN; however, they appear to sensitize ADN to the electrostatic discharge, impact, and friction. de Klerk et al. (160) have studied the decomposition kinetics of 1,1-diamino-2,2dinitroethylene (FOX-7) and hydrazinium nitroformate (HNF). FOX-7 is considered as a lower sensitivity alternative to RDX, and HNF as a chlorine-free oxidizer. The effective activation energies determined by an isoconversional method are about 335 and 225 kJ mol-1 for FOX-7 and HNF, respectively. The obtained kinetic

parameters have been used to evaluate the lifetime of the materials at lower temperatures. Ammonium nitrate (AN) is the most common commercial fertilizer and explosive. It is typically processed and used in one of several forms that include neat AN, its mixture with fuel oil, and aqueous solutions, which form the basis of ammonium nitrate emulsion explosives. Turcotte et al. (161) perform hazard assessment of these AN systems by using laboratory-scale calorimeters. They study effects of sample mass, atmosphere, and formulation on the resulting onset temperatures and propose a method to extrapolate these results to large-scale inventories. Desensitizing of AN is an important practical problem. Oxley et al. (162) apply thermal analysis to screen various AN formulations in search of a possible deterrents. The sodium, potassium, ammonium, and calcium salts of sulfate, phosphate, or carbonate as well as certain high-nitrogen organics (urea, oxalate, formate, guanidinum salts) are considered as promising candidates because they enhance AN thermal stability and because they can be used with agricultural products. Singh and Felix (163) demonstrate that transition metal salts of 5-nitro-2,4-dihydro-3H-1,2,4-triaole-3-one increase the steady burning rate of an AN-based propellant. As found in their TGA and DSC experiments, these compounds also lower the decomposition temperature of AN and alter its phase transition temperatures. Drake et al. (164) have synthesized three new families of triazole-based salts with nitric, perchloric, and dinitramidic acids. Many of these salts have melting points well below 100 °C, yet high decomposition onsets that define them as new, highly energetic members of the well-known class of materials identified as ionic liquids. The kinetics of the β to δ solid-solid-phase transition of HMX has been measured by Weese et al., (165) who found that the process occurs with an activation energy of ∼500 kJ mol-1, markedly greater than previously reported by other workers. Singh et al. (166) have applied TGA and DSC to analyze decomposition of HMX and its plastic bonded explosives (PBX) containing Estane and found that increasing its content in PBX may lower decomposition temperature. The decomposition and explosion delay data have been used for determining the kinetic parameters. Pinheiro et al. (167) use DSC to examine the effect of the heating rate on the thermal decomposition of HMX. They demonstrate that the activation energies estimated from the slope of the logarithm of the heating rate against the reciprocal peak temperature depend significantly on the interval of the heating rates. It is, in particular, explained by self-heating as well as by shifting of the process from the solid to liquid phase that occurs on increasing the heating rate. On the other hand, Zeman et al. (168) demonstrate that for a number of explosives the reactivities, estimated as the activation energies from the Kissinger plot, correlate with the squares of the detonation rates in accord with the modified Evans-Polanyi-Semenov equation. Lee et al. (169) have examined the thermal behavior of HMX, RDX, pentaerythritol tetranitrate, and hexanitrostilbene. DSC was used to evaluate the decomposition kinetics as well as the compatibility of the explosives with silicone rubber. Long et al. (170) have studied the kinetics of decomposition of TNT by using high-pressure crucibles in DSC and found that the process follows an autocatalytic model, although the activation energy remains practically constant.

PHARMACEUTICAL, BIOCHEMICAL, AND BIOLOGICAL APPLICATIONS The Journal of Thermal Analysis and Calorimetry has devoted two special issues (171, 172) to pharmaceutical application of thermal analysis. Giron (173) reviews the current use of thermal analysis and combined techniques in research and development of pharmaceuticals. It is emphasized that the changes of temperature and humidity that occur at processing and storage may have a dramatic effect on activity, toxicity, and stability of solid compounds. Craig et al. (174) have reviewed pharmaceutical applications of microthermal analysis that allows samples to be spatially scanned in terms of both topography and thermal conductivity, by placing the probe on a specific region of a sample and heating. Current pharmaceutical applications of the technique include the identification of components in compressed tablets, the characterization of drug-loaded poly(lactic acid) microspheres, the analysis of tablet coats, and the identification of amorphous and crystalline regions in semicrystalline samples. To provide local gentamicin delivery for 1 week based on a biodegradable system, Friess and Schlapp (175) have developed special microparticles that are based on a blend of two Resomer polymers. The liberation mechanism has been investigated by thermal analysis methods to monitor the glass transition temperature and mechanical properties. DSC is one of the important thermoanalytical techniques used in preformulation studies of pharmaceutical substances. The attainment of proper results in a DSC experiment is dependent on a number of factors such as the sample size, heating rates, atmosphere, crucible type, and relative humidity. Roy et al. (176) suggest using experimental design to establish a general protocol for DSC performance. By applying DSC and hot-stage microscopy, Keymolen et al. (177) report that glassy nifedipine reveals several heating rate-dependent events occurring prior to the main melting endotherm (172 °C). The authors suggest that nifedipine is likely to have four polymorphic forms. Thermal analysis in combination with XRD has been used by Zhang et al. (178) to characterize racemic and homochiral sodium ibuprofen that is found to form a racemic conglomerate (gamma-form), as well as two polymorphic racemic compounds, R and β. Forms R and β are “enantiotropically related” with a transition temperature between 75 and 113 °C but can be considered as metastable monotropes of the stable γ form. The structural relaxation time is a measure of the molecular mobility of amorphous pharmaceutical solids that determines reactivity of drugs in amorphous formulations. Liu et al. (179) describe a novel method for estimating the structural relaxation times that depend on a number of variables, including nature of material, temperature, moisture content, thermal history, etc. They demonstrate that the resulting structural relaxation parameters provide data useful for rational development of stable peptide and protein formulations and for the control of their processing. Zhou et al. (180) use DSC to evaluate the molecular mobility and the entropic barrier to crystallization of five amorphous pharmaceuticals. The importance of both factors for estimating the physical stability of amorphous pharmaceuticals is supported by experimental crystallization studies under nonisothermal conditions according to which compounds with the highest entropic barriers and lowest mobilities are most difficult to crystallize. Analytical Chemistry, Vol. 76, No. 12, June 15, 2004

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Randzio et al. (181) have reexamined phase transformations in the starch-water system by using completely dried starch as a reference in DSC measurements. They observe five types of well-reproducible transformations that include the following: (1) sharp endothermic transition occurring over the temperature range 319-333 K associated with melting of the crystalline part of the starch granule followed by a helix-coil transformation in amylopectin; (2) water content-dependent slow exothermic transition associated with reassociation of the unwound helixes of amylopectin with parts of amylopectin molecules other than their original helix duplex partner; (3) water content-dependent lowtemperature exothermic transition associated with recrystallization of partly dehydrated starch; (4) high-temperature endothermic transformation associated with nematic-isotropic transition and small distortions on the main endothermic transition associated with smectic-nematic transition; (5) continuous slow exothermic transformation associated with the softening and sucking in water of the amorphous growth rings of the starch granule. Hug-Iten et al. (182) use thermal analysis in combinations with other techniques to understand the mechanism of bread firming during staling. Starch-degrading enzymes decrease the structural strength of the starch phase, and on the other hand, they also promote the formation of a partly crystalline amylose network that stabilizes the starch network. Sun et al. (183) use thermomechanical analysis to study transitions of rice kernels on heating from 0 to 200 °C. They identified three transitions with onsets at 45, 80, and 180 °C that are respectively associated with the glass transition, moisture evaporation, and melting of the crystalline phase of starch. The glass transition is a crucial process that determines the properties of many food products. Borde et al. (184) have performed an extensive study of the glass transition in partially hydrated amorphous polysaccharides and discuss their results in terms of structure-property relationships. Cordella et al. (185) apply DSC to study the thermal behavior of authentic honeys and industrial sugar syrups that are found to demonstrate significant differences in thermal phenomena. They report a linear relationship between the percentage of added syrup and the glass transition temperature as well as the enthalpy of fusion. Reguera et al. (186) apply DSC to water-induced chain dynamics protein-based polymers, poly(Val-Pro-Gly-Val-Gly) and poly(Val-Pro-Ala-Val-Gly). By applying an isoconversional kinetic analysis, the authors find that the kinetics of folding and unfolding is a multistep process. The polymers show a markedly different pattern of kinetic behavior that has been related to the hindrance factor associated with the methyl group of alanine (Ala). Salvetti et al. (187) use temperature-modulated DSC to measure the complex heat capacity of the lysozyme-water system in the 293368 K range. The observed endothermic peak results from heat absorption when the equilibrium constant between the native lysozyme state and a conformationally different intermediate state increases with temperature. This conclusion is consistent with the general view that the first step of denaturation of small one-domain globular proteins is a reversible conformational (unfolding) transition, and the second step is irreversible denaturation. Rosgen and Hinz (188) apply statistical thermodynamics models to interpret the heat capacity of ligand-binding proteins as measured by DSC. They stress that the microscopic statistical thermody3308

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namic heat capacity must be carefully distinguished from the macroscopically observable thermodynamic heat capacity in those cases where proteins unfold in the presence of high-affinity ligands. Cueto et al. (189) propose a new method of assessing the stability of protein solutions that utilizes DSC to measure the calorimetric enthalpy, the temperature at maximum heat flux, the ratio of maximum heat flux over peak area, and the ratio of calorimetric enthalpies from the second heating cycle to the first enthalpy. These parameters are interpreted using the three-step equilibrium model for protein degradation. Grinberg et al. (190) apply DSC to explore the order-disorder conformational transition in the tetramethylammonium salt of gellan that they define as a second-order phase transition, which is in agreement with predictions of a matching model of the double-helix-coil transition. The cooperativity parameter in the matching model (σ ) 0.62 ( 0.01) indicates a small contribution of the stacking effect into the cooperativity of the transition, while a larger contribution apparently originates from the loop factor. Tanaka et al. (191) looked at phase transition of liquid silk from domestic and wild silkworms. They report that DSC curves for the liquid silk of the domestic silkworm demonstrate weak endothermic peaks corresponding to the breaking of hydrogen bonds or to the untangling of physical network. However, the wild silk worm silks show clear exothermic peaks corresponding to a phase transition from the R-helix conformation to the β-form. Gorbunov et al. (192) explore the microscopic properties of infrared receptors of snakes with “infrared vision” by using scanning thermal microscopy and micromechanical analysis. The IR receptors are found to have lower surface thermal conductivity and greater mechanical compliance as compared to the nonspecific skin areas. By applying TMA to bovine chrome leather, Manich et al. (193) have been able to detect the collagen thermal transformations associated with the glass transition and denaturation. O’Neill et al. (194) demonstrate the utility of microcalorimetry to investigate antimicrobial efficacy of silver-containing wound dressings. Yeo et al. (195) describe preparation of polypropylene/ silver nanocomposite fibers for use as an antibacterial textile. Nanosilver in fibers has shown excellent antibacterial activity as evaluated by percent reduction of two kinds of bacteria; Staphylococus aureus and Klebsiela pneumoniae. Prado and Airoldi (196) apply isothermal microcalorimetry to compare the toxic effects of free and immobilized picloram on the microbial activity of agricultural soil. A decrease in the thermal effect is used as a measure of the pesticide activity that has found to be much lower for immobilized picloram. Jaffe et al. (197) discuss the role of thermal methods in biorelevant characterization of biopolymers. Because in biorelevant testing the temperature of interest is limited to 37 ( 3 °C, they stress the importance of the development of accelerated aging evaluation procedures. Lamprecht (198) presents the applications of calorimetry and classical and irreversible thermodynamics to six examples of a living system that include the following: (i) glycolytic oscillations far off the thermodynamic equilibrium; (ii) growth and energy balances in fermenting and respiring yeast cultures; (iii) direct and indirect calorimetric monitoring of electrically stimulated reptile metabolism; (iv) biologic and climatic factors influencing the temperature constancy, and distribution in the mound of a

wood ant colony; (v) energetic considerations of the clustering of honeybees in winter; and (vi) energetic and evolutionary aspects of the mass specific entropy production rate. Hansen et al. (199) suggest that kinetics of plant growth can be defined in terms of the specific rate of CO2 evolution and the metabolic heat rate as functions of environmental variables. They find that growth season temperature and temperature variability are major determinants of growth rates and distributions of plants. Ellingson et al. (200) compare four known methods for determination of the enthalpy change for anabolism that is a crucial value for modeling the growth/respiration relation in plants. The application of the methods to oat and corn seedlings has resulted in reasonable agreement of the values despite the different assumptions involved. Sergey Vyazovkin received his Ph.D. from the Byelorussian State University in 1989. He then joined the Institute for Physical Chemistry (Minsk) where he worked until 1993. Since 1993 he had held visiting positions at the Technical University of Vienna, the University of Toledo, and the Univerisity of Nice Sophia-Antipolis. Before joining the University of Alabama at Birmingham, he worked at the University of Utah as a research faculty member and the deputy director of the Center for Thermal Analysis. His research interests are concerned with the application of thermal analysis methods to study thermally stimulated reactions in polymeric, energetic, and pharmaceutical materials. The results of his work have been published in over 90 peer-reviewed articles, including several invited review papers. Prof. Vyazovkin is editor of Thermochimica Acta and a member of the editorial board of Journal of Thermal Analysis and Calorimetry. He is a member of the American Chemical Society Analytical Division, the North American Thermal Analysis Society, and the International Confederation for Thermal Analysis and Calorimetry.

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