Studying the Glass Transition by DSC and TMDSC

First, the principal features of the glass transformation process in polymers are reviewed, and then it is shown how they are manifest in conventional DSC, and the quantitative analysis of typical DSC data is discussed in terms of the Tool-Narayanaswamy-Moynihan (TNM) model. Subsequently, the way in which the glass transition is manifest in Temperature Modulated DSC is presented, and the effects of both experimental and material parameters are discussed. In conclusion, the two techniques are compared in terms of the information they provide about the glass transformation process. This revised version was published online in July 2006 with corrections to the Cover Date.     

TOPEM,a new temperature modulated DSC technique

TOPEM is a new temperature modulated DSC technique, introduced by Mettler-Toledo in late 2005, in which stochastic temperature modulations are superimposed on the underlying rate of a conventional DSC scan. These modulations consist of temperature pulses, of fixed magnitude and alternating sign, with random durations within limits specified by the user. The resulting heat flow signal is analysed by a parameter estimation method which yields a so-called ‘quasi-static’ specific heat capacity and a ‘dynamic’ specific heat capacity over a range of frequencies. In a single scan it is thus possible to distinguish frequency-dependent phenomena from frequency-independent phenomena. Its application to the glass transition is examined here.     

Crystal Perfection of Poly(p-phenylene Sulfide) During Cooling Using Temperature-Modulated DSC

Temperature-modulated DSC (TMDSC) was used to enhance the perfection of crystals of different poly(p-phenylene sulfide) samples formed during slow cooling from the melt. The sample preparation was made with modulated cooling using a cool-heat mode. Re-heating the samples prepared by slow conventional and modulated coolings indicated that the melting point of the samples prepared by modulated cooling is considerably higher than the melting point of the samples crystallized with conventional cooling. Thus, the perfection of crystallites can be improved if the outer layers just deposited on their surface are re-melted and re-crystallized immediately.This revised version was published online in November 2005 with corrections to the Cover Date.     

Thermal hazards of cracker mixture using DSC

DSC studies of cracker mixtures of different compositions of potassium nitrate, sulphur and aluminum have shown some critical characteristics either with the increase or decrease in the composition of the components. Specifically, sulphur composition below 8% showed no exothermic activity. The studies revealed that a minimum of 11% to a maximum of 17% of sulphur is required for good cracking characteristics. The kinetics of decomposition of cracker mixture is carried out employing DSC multiple heating rate kinetic method. Increase in sulphur content decrease the energy of activation facilitating easy ignition of the cracker mixtures. Arrhenius parameters for cracker mixture decomposition are reported in this paper.     

Temperature Modulated DSC. Theoretical interpretation

The application of non-linear heating program to a heat-flux DSC apparatus has attracted much attention. From thermodynamics viewpoint, it is shown that the variation of enthalpy of a sample changing with temperature change is due, to both the true heat capacity of the sample and the enthalpy of some transformations occurring in the sample, characterized by its degree of advance. Using the simple assumption that the rate of the transformation is proportional to the distance from the thermodynamic equilibrium, an electrical model of the thermal event is given. Using the coupled cell model of the DSC apparatus, we show how to obtain the rate of transformation of the sample and heat capacity, which is directly related to the base line of the experiment. This revised version was published online in August 2006 with corrections to the Cover Date.     

DSC测量中校正对热分析结果的影响

介绍了灵敏度及温度校正对基线和实验结果的影响.仪器温度和灵敏度校正可以减小差示扫描量热法(DSC)曲线的漂移,曲线上得到的温度和焓变值更接近理论值,减小了实验误差.所以,定期对仪器进行重新校正是必要的.     

Temperature modulated DSC at intermediate-low temperatures

In this paper we present a new cooling system for temperature modulated DSC (TMDSC) working down to about 60 K. In order to demonstrate the features of this new system in combination with commercial TMDSC apparatus, we present measurements of the specific heat capacity (c_p) around the phase transitions of betaine borate and betaine phosphate. For SilGel 604 we report c_p and sound velocity data around the melt, as well as around the glass transition.     

Application of DSC as a tool for honey floral species characterization and adulteration detection

The thermal behaviour of authentic honeys and sugar syrups (industrial and homemade) was investigated by DSC. To confirm the first previous results concerning the effect of adulteration on the thermal behaviour of authentic honeys, 30 honey samples (Robinia, Lavender, Chestnut and Fir) were analyzed by DSC and their T _g were measured following a suited experimental protocol. The results indicated that this parameter was useful to characterize and to distinguish significantly these varieties between them. Applied to honey samples artificially adulterated with different industrial syrups, DSC showed a detection level of 5–10% depending on the type of syrup. An endothermic phenomenon occurring between 40–90°C during the heating was studied by TMDSC and a new thermal transition similar to a glass-transition was highlighted.This revised version was published online in November 2005 with corrections to the Cover Date.     

Sample encapsulation on glass transition of methylmetacrylate copolymers: TMDSC and DSC studies

The use of modulated temperature differential scanning calorimetry (TMDSC) and differential scanning calorimetry (DSC) in the measurement of the glass transition temperature (T _g) in polymer-water systems presents several important problems. These include the presence of water evaporation endotherms, partial water evaporation during scanning, changes in pan integrity due to vapour pressure developing in the pan headspace during analysis, and possible interaction between water and polymer at high temperatures. As a result, in most of the cases, only apparent T_g values can be obtained. In this study, TMDSC and DSC were used to determinate the thermal behaviour of methylmethacrylate copolymer-water systems. The samples were previously equilibrated at different relative humidities (RH) from 0 to 97% RH. Three different pan arrangements were used. In addition, thermogravimetric analysis (TG) was carried out to determine the initial amount of water in the sample. None of the pan arrangements was entirely suitable for the study of these systems. When sealed pans were used, the plastification effect of water was observed. Some evidence of degradation was also observed in which water and methylmethacrylate appeared to play roles.This revised version was published online in November 2005 with corrections to the Cover Date.     

Thermal stability of bovine serum albumin DSC study

A calorimetric study of thermal denaturation of bovine serum albumin in aqueous solutions has shown essential differences in stability of fatty acid containing and defatted albumin. The first one shows a single endotherm peak in DSC curve near 69°C with enthalpy change about 1000 kJ mol^-1. Defated albumin melts in two different temperature ranges: near 56 and 69°C with enthalpy changes about 300 and 200 kJ mol^-1 respectively. Deconvolution analysis shows that the single endotherm is well approximated as the sum of three independent two-state transitions. Two transitions of bimodal DSC curve for defatted albumin are not of a two-state type. This molecule melts probably as two structurally independent parts. This revised version was published online in July 2006 with corrections to the Cover Date.     

Can one measure precise heat capacities with DSC or TMDSC?</o:p>

Summary During a prior study of gel-spun fibers of ultrahigh-molar-mass polyethylene, a substantial error was observed on calculating the heat capacity with a deformed pan, caused by the lateral expansion of the fibers on shrinking during fusion. In this paper, the causes of this and other effects that limit the precision of heat capacity measurements by DSC and TMDSC are explored. It is shown that the major cause of error in the DSC is not a change in thermal resistance due to the limited contact of the fibers with the pan or the deformed pan with the platform, but a change in the baseline. In TMDSC, the frequency-dependence is changed. Since irreversible changes in the baseline can occur also for other reasons, inspections of the pan after the measurement are necessary for precision measurements.     

Finite-element calculations on the behaviour of a DSC in temperature-modulated mode

The influence of changes of sample properties on the amplitude and phase shift of the differential-temperature signal as well as the influence of frequency changes has been calculated for a one-dimensional model of a temperature-modulated DSC (TMDSC) using a computer program for finite-element-method (FEM) calculations. Amplitude and phase shift of the measured signal ΔT (which is proportional to the differential heat-flow rate) is strongly influenced by the heat capacity of the sample. The connection is only linear for rather small heat capacities. The influence of the heat-transfer coefficient between sample and sample pan on amplitude and phase shift of the signal is not so large and linear (within the framework of our calculations). The influence of the heat-transfer coefficient between sample and sample pan on amplitude and phase shift of the signal is not so large and linear (within the framework of our calculations). For precise measurements, a very careful “calibration” is needed, which must take all the aforementioned influences into account.     

Study of temperature profile and specific heat capacity in temperature modulated DSC with a low sample heat diffusivity

One important application of temperature modulated DSC (TMDSC) is the measurement of specific heat of materials. When the sample has very good thermal conductivity as in the case of metals, the temperature gradient is not normally an important factor and can be ignored most of the time. However, in the case of materials with poor heat transfer properties, for example, polymers, the thermal conductivity is only in the order of 1/1000 or so of that of metals. This could have a major effect on the test results. In this paper, a round analytical solution is given and a numerical model is used to analyze the effects of thermal diffusivity on temperature distribution inside the test sample and specific heat measurement by TMDSC, PET sample test results are presented to demonstrate the effects of material thermal diffusivity.     

Debye Temperature Dependent Lattice Thermal Conductivity of Silicon

The temperature dependence of the Debye temperature θ_D(T) was applied to analyze the lattice thermal conductivity of Si between 2 and 300 K. The analysis of experimental data in terms of the Dubey model of the two modes of conduction has been carried out by combining the relaxation time for phonon-phonon scattering, point defect scattering and boundary scattering. The relative importance of the contribution of each mode was examined by estimating their percentage contribution to the phonon conductivity. Agreement between theory and experiment is achieved over the whole temperature range of study. This revised version was published online in August 2006 with corrections to the Cover Date.     

溶菌酶热变性的DSC研究

用差示扫描量热法研究了固体溶菌酶的热变性以及水溶液中不同变性剂与浓度对溶菌酶变性的影响. 结果表明, 溶剂水的存在及变性剂尿素和盐酸胍的加入使溶菌酶的变性温度降低, 变性焓减小; 同时, 在一定的浓度范围内, 溶菌酶的变性温度和变性焓随变性剂浓度的增大而降低. 盐酸胍的变性效果较尿素强, 这是由于盐酸胍与蛋白质分子间除了氢键作用外还存在着静电作用.     

Influence of DSC Measurement Conditions on Kinetic Parameters of Thermal Decomposition of 2,4,6-trinitrotoluene

An autocatalytic model involving the limited solubility of volatile catalytic products was applied to the thermal decomposition of 2,4,6-trinitrotoluene. The critical supersaturation of the thermal decomposition products with the catalytic properties was higher at a low heating rate. Decrease of the sample mass led to an increased critical supersaturation of the decomposition products. This is probably a result of the greater contribution of products adsorption on the aluminium pan surface. It is presumed that the differences observed in the rate constant are connected with the uncontrolled critical supersaturation of the volatile thermal decomposition products. This revised version was published online in August 2006 with corrections to the Cover Date.     

Thermal stability evaluation of doping compounds before GC-MS analysis by DSC

The Medical Commission of the International Olympic Committee forbids the use of anabolic androgenic steroids, β-agonists, stimulant and narcotic compounds to improve athletic performance. In this work, we evaluated the thermal stability of 17 compounds by the use of the DSC for their potential GC-MS analysis either under free form or under TMS derivative form. In DSC, esterified and unesterified anabolic steroids were characterized by a true melting peak, followed by a large exothermic peak at about 251–316°C due to oxidative degradation. They could be analysed by GC-MS mainly under TMS derivatives. Hydroxylated and unhydroxylated stimulant compounds (xanthines) seemed to be more stable at high temperature. As unhydroxylated xanthines were not silylated with BSTFA - TMCS, their GC analysis would be done under their free forms. TMS derivatisation of albuterol hemisulfate and codeine phosphate is preferable. In our conditions, to analyse by GC-MS all 17 doping compounds in the same GC-MS run, the optimal silylation temperature and best column initial temperature were determined at both 60°C.     

Evaluation of effective thermal conductivity of fireworks compositions

Effective thermal conductivity of fireworks raw materials and their mixture have been measured by the temperature modulated DSC and the hot wire method, in order to predict spontaneous ignition properties precisely. As a result, an excellent linear correlation has been obtained between the density and the λ_e by the TMDSC method. Moreover, the low-density data by the hot wire method lie on the extrapolated point of the linear correlation. Thus, the λ_e within the ordinary limit of fireworks composition can be measured by the TMDSC method. Krupiczka’s estimation method shows a good agreement with the experimental values.     

Heat Capacity Measurements Using Temperature-modulated Heat Flux DSC with Close Control of the Heater Temperature

The determination of heat capacity data with sawtooth-type, temperature-modulated differential scanning calorimetry is analyzed using the Mettler-Toledo 820 ADSC™temperature-modulated differential scanning calorimeter (TMDSC). Heat capacities were calculated via the amplitudes of the first and higher harmonics of the Fourier series of the heat flow and heating rates. At modulation periods lower than about 150 s, the heat capacity deviates increasingly to smaller values and requires a calibration as function of frequency. An earlier derived correction function which was applied to the sample temperature-controlled power compensation calorimeter enables an empirical correction down to modulation periods of about 20 s. The correction function is determined by analysis of the higher harmonics of the Fourier transform from a single measurement of sufficient long modulation period. The correction function reveals that the time constant of the instrument is about 5 s rad⁻¹ when a standard aluminum pan is used. The influence of pan type and sample mass on the time constant is determined, the correction for the asymmetry of the system is described, and the effect of smoothing of the modulated heat flow rate data is discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

Thermal analysis of macromolecules

This review traces the development of thermal analysis over the last 50 years as it was experienced and contributed to by the author. The article touches upon the beginning of calorimetry and thermal analysis of polymers, the development of differential scanning calorimetry (DSC), single-run DSC, and other special instrumentations, up to the recent addition of modulation to calorimetry and superfast calorimetry. Many new words and phrases have been introduced to the field by the author and his students, leaving a trail of the varied interests over 50 years. It began with cold crystallization and more recently the terms oriented, intermediate phase, glass transitions of crystals, and decoupled chain segments were coined. In-between the following phenomena were named and studied: extended-chain crystals, irreversible thermodynamics of melting of polymer crystals, zero-entropy-production melting, dynamic differential thermal analysis (DDTA), the rule of constant increase of C _p per mobile bead within a molecule at the glass transition temperature, superheating of polymer crystals, melting kinetics, crystallization during polymerization, chin-folding principle, molecular nucleation, rigid amorphous phase, system of classifying molecules, macroconformations, amorphous defects, rules for the entropy of fusion based on molecular shape and flexibility, single-molecule single-crystals, systems for classifying phases and mesophases including condis phases, and the globally metastable semicrystalline polymers with reversible, local subsystems. This review is update of a publication written in 1995 and published under the same title in the J. Thermal Anal., 46 (1996) 643. Parts F and G are fully new, and Part G is the basis for my lecture: ‘The development of the idea of thermodynamic decoupling in macromolecules’.