Experimental investigation into the heat capacity measurement using an modulated DSC

Experiments using a commercial modulated DSC (MDSC) for the measurement of specific heat capacity of a sample have been carried out. It is found that because the amplitude of heat flow of MDSC is a complicated non-linear function of various experimental conditions such as the modulation frequency and the heat capacities of a sample and pan, the methodology of heat capacity determination using an MDSC in a single run has not been justified. The experimental results, on the other hand, agree with the theoretical equation of one of the authors. It is therefore concluded that the capabilities of MDSC should be further examined.     

Heat capacity measurement by modulated DSC at constant temperature

The mathematical equations for step-wise measurement of heat capacity (C _p ) by modulated differential scanning calorimetry (MDSC) are discussed for the conditions of negligible temperature gradients within sample and reference. Using a commercial MDSC, applications are evaluated and the limits explored. This new technique permits the determination ofC _p by keeping the sample continually close to equilibrium, a condition conventional DSC is unable to meet. Heat capacity is measured at ‘practically isothermal condition’ (often changing not more than ±1 K). The method provides data with good precision. The effects of sample mass, amplitude and frequency of temperature modulation were studied and methods for optimizing the instrument are proposed. The correction for the differences in sample and reference heating rates, needed for high-precision data by standard DSC, do not apply for this method. Presented in preliminary from at the 22nd NATAS Conference in Denver, CO 9/19-22/93 (Proceedings, pages 59–64, editor K. R. Williams).     

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.     

Modeling the heat flow and heat capacity of modulated differential scanning calorimetry

Modulated differential scanning calorimetry (MDSC) uses an abbreviated Fourier transformation ?r the data analysis and separation of the reversing component of the heat flow and temperature signals. In this paper a simple spread-sheet analysis will be presented that can be used to better understand and explore the effects observed in MDSC and their link to actual changes in the instrument and sample. The analysis assumes that instrument lags and other kinetic effects are either avoided or corrected for.     

Miscibility of polymer blends studied by a simplified method of data analysis using light heating modulated temperature DSC

A simple method of application of light heating modulated temperature DSC to a study of miscibility of polymer blends has been developed. In this method only the sample was measured and the standard materials were not used. The total heat flow, the complex heat capacity, the reversing and non-reversing heat flows were obtained as values measured from those quantities in hypothetical glassy state at T>T_g. The values of the hypothetical glassy state were calculated by extrapolation from T<T_g. The present method gives relative values but useful information can be obtained from the results. Some results from miscible and immiscible polymer blends are shown. This revised version was published online in July 2006 with corrections to the Cover Date.     

Non-debye nature in thermal relaxation and thermal properties of lithium borate glasses studied by modulated DSC

Complex heat capacity, C _p ^*=C _p ^'–iC _p ^', of lithium borate glasses xLi₂O·(1–x)B₂O₃ (molar fraction x=0.00–0.30) has been investigated by Modulated DSC. We have analyzed the shape of C _p ^* by the Cole-Cole plot, performed fitting by the Havriliak-Negami equation, and then determined the parameters related to the non-Debye nature of thermal relaxation. Moreover, the concentration dependence of the thermal properties has been investigated. Glass transition temperatures become higher with the increase of molar fraction of Li₂O and shows the board peak around x=0.26. Temperature ranges of glass transitions become narrower with the increase of Li₂O concentration.     

A Theoretical Approach to Temperature Modulated Power Compensation DSC

The steady state of temperature modulated power compensation DSC has been theoretically investigated for measurements of complex heat capacity, taking accounts of heat capacities of heat paths, heat loss to the environment, and mutual heat exchange between the sample and the reference material. Thermal contact between the sample cell and the cell holder is also taken into accounts. Rigorous and general solutions are obtained. From these solutions application of the technique to heat capacity measurements is discussed. This revised version was published online in August 2006 with corrections to the Cover Date.     

A study of PBT/PC blends by modulated DSC and conventional DSC

Two poly(butylene terephthalate)/polycarbonate (PBT/PC) blends with different formulations were analyzed by modulated DSC (MDSC) and conventional DSC to determine differences in crystallization behavior. A significant difference (30°C in cold crystallization temperature) between the two samples was detectable by MDSC while no significant difference was seen by conventional DSC. That indicatesthe total heat flow from MDSC is not always equivalent to the heat flow from conventional DSC as we have assumed or seen before. The reason has not been fully understood, but may be related to unusual nucleation and crystallization induced by modulation. Alternative conventional DSC methods were developed and compared to the MDSC results.Dedicated to Professor Bernhard Wunderlich on the occasion of his 65th birthdayThe authors like to thank Drs. Bernhard Wunderlich and Robert Gallucci for helpful discussion, David Shaker and Mary Parsonage for some DSC experiments. Technical support from TA Instruments is also greatly appreciated.     

Specific heat capacities of pure triglycerides by heat-flux differential scanning calorimetry

The specific heat capacities of some triglycerides commonly found in palm oil were determined with a heat-flux differential scanning calorimeter. The specific heat capacity measurements were made under the optimum operating conditions determined earlier: scan rate 17 deg·min^?1, sample mass 21 mg and purge gas (nitrogen) flow rate 50 ml/min. Pure triglycerides (four simple and four mixed) were used in the experiments. The four simple triglycerides were trilaurin, trimyristin, tripalmitin and tristearin, and the mixed triglycerides were 1,2-dimyristoyl-3-oleoyl, 1,2-dimyristoyl-3-palmitoyl, 1,2-dipalmitoyl-3-oleoyl and 1,2-dioleoyl-3-palmitoyl. The results of this study are compared with literature values and also with values obtained by using estimation methods. The experimental specific heat capacities are within ±1% precision with a 95% confidence level.     

Improved conditions for measurement of the specific heat capacities of pure triglycerides by differential scanning calorimetry

Reproducible specific heat capacities (C _p) of triglycerides can be obtained by using heat-flux DSC under improved operating conditions. The improved operating parameters, such as the scanning rate, the sample mass and the atmosphere within the DSC chamber, were established via statistical analysis of the experimental data with trilaurin as a sample. The specific heat capacity results on trilaurin were compared with the values calculated by using estimation methods. The precision of the specific heat capacity measured for trilaurin under these conditions was within ±1%.     

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.     

Quasi-isothermal measurement of frequency dependent heat capacity of semicrystalline polyethylene at the melting temperature using light heating modulated temperature DSC

Frequency dependence of the heat capacity of semicrystalline polyethylene was measured using light heating modulated temperature DSC (LMDSC). The quasi-isothermal measurement was carried out in the melting temperature range of the crystal. Two decades of the frequency, from 0.01 to 1 Hz, was covered by the LMDSC instrument constructed in the author's laboratory. It was found that in the melting temperature range polyethylene exhibited Debye relaxation with the relaxation time of 14 s and had excess heat capacity independent of the kinetics. The excess heat capacity and the relaxation strength could be attributed to the disordered crystals generated during rapid cooling from the molten state and/or its surrounding amorphous region instead of the stable crystals reorganized during the quasi-isothermal measurement.     

A modulated DSC study on the stress oscillation phenomenon in a syndiotactic polypropylene

The static loading-induced stress oscillation (SO) in syndiotactic polypropylene (sPP) was studied by modulated differential scanning calorimetry (TMDSC). Samples were taken from the initial necked, premature and mature SO oscillation ranges, respectively, and the related calorimetric responses were compared to those of the bulk material. It was established that necking caused some decrease in the crystallinity. In addition, necking resulted in cold crystallization that was assigned to a polymorphic transition (from all-trans to helical conformation) based on literature results. The TMDSC response was practically the same for necked samples with and without SO. A model was proposed to explain SO. The model assumes the presence of a network (similar to that of semicrystalline thermoplastic elastomers), which is highly stretchable and fails by sudden voiding at the intersections of shear micro bands intermittently.This revised version was published online in November 2005 with corrections to the Cover Date.     

Greater insight into the MTDSC technique involving fundamental sinusoidal heat flow equations

Summary Modulated temperature DSC was investigated, comparing data found experimentally to that derived from theory. Deviation from theory was found with regard to the amplitude of the modulated heat flow signal when large modulation amplitudes were employed in the experiment. These deviations were determined to be dependent on the absolute temperature and it was concluded that further investigation of the heat flow signal obtained during MTDSC experiments is required.     

Comparative study of specific heat capacities of some vegetable oils obtained by DSC and microwave oven

Summary TG and DSC analyses were carried out in this work to evaluate the changes in the denaturation of human hair keratin submitted to different chemical effects. Hair bleaching and chlorinating treatments caused changes in the denaturation temperatures and denaturation enthalpies of hair keratin. Bleached hair and hair kept in a chlorinated solution presented a lower denaturation enthalpy and a higher denaturation temperature compared to the control hair sample. The TG and DSC analyses allowed to quantify the degradation level of hair fibers after the chemical treatments. AFM was also utilized to characterize the morphological alterations in the hair fiber surfaces caused by the chlorinating and bleaching treatments.     

Modulated temperature DSC measurements relating to the cold crystallization process of poly(ethylene terephtalate)

The modulated temperature differential scanning calorimetric method (MT-DSC) yields three temperature dependent signals, an underlying heat capacity curve from the underlying heat flow rate (corresponding to the conventional DSC signal), and a complex heat capacity curve with a real part (storage heat capacity) and an imaginary part (loss heat capacity). These curves have been measured in the cold crystallization region for poly(ethylene terephtalate) with a modified Perkin-Elmer DSC-7. The underlying curve shows the well known large exothermic crystallization peak. The storage heat capacity shows a step change which reproduces the change in heat capacity during crystallization. This curve may be used as baseline, to separate the crystallization heat flow rate from the underlying heat flow rate curve. The loss heat capacity curve exhibits a small exothermic peak at the temperature of the step change of the storage curve. It could be caused by changes of the molecular mobility during crystallization.Dedicated to Professor Wunderlich on the occasion of his 65th birthday     

The heat capacities of selected inorganic alkali metal compounds

The heat capacities of selected inorganic binary and ternary alkali metal compounds are determined using differential scanning calorimetry (DSC). As part of an ongoing research program at Battelle Memorial Institute since 1983, the heat capacities of cesium and rubidium chalcogenides, aluminates, silicates and uranates in the temperature range 310 to 800 K have been added to the series of compounds. The measured data is to be combined with the standard enthalpies of formation and low temperature heat capacities to obtain reliable thermodynamic data on the alkali metal compounds to high temperatures.     

A Numerical Model of a Two-pan Heat Flux DSC

A numerical program has been written to treat a heat-flux DSC. The model operates in two modes. In the first,experimental data is used as input and the enthalpy is calculated as a function of the sample temperature rather than the sample thermocouple temperature. This allows accurate enthalpies and transition temperatures to be obtained without smearing. In the second mode, enthalpy is used as an input and the responses of the calorimeter are calculated. Using this mode it is possible to investigate the effects of sample size, heating rate and alloy composition. Non-equilibrium effects and difficulty in nucleation can also be included. This revised version was published online in July 2006 with corrections to the Cover Date.     

Temperature Modulated DSC for the Investigation of Polymer Materials: A brief account of recent studies

Temperature modulated differential scanning calorimetry (TMDSC), the most recent development that adds periodic modulation to the conventional DSC, has recently seen a fast growth due to availability of commercial instrumentation. The use of the technique necessitates a total control of all of the experimental parameters. The paper focuses on recent applications to investigate polymers [1].This revised version was published online in November 2005 with corrections to the Cover Date.     

Low-temperature heat capacities and thermodynamic properties of 2,2-dimethyl-1,3-propanediol

The molar heat capacities C _p,m of 2,2-dimethyl-1,3-propanediol were measured in the temperature range from 78 to 410 K by means of a small sample automated adiabatic calorimeter. A solid-solid and a solid-liquid phase transitions were found at T-314.304 and 402.402 K, respectively, from the experimental C _p-T curve. The molar enthalpies and entropies of these transitions were determined to be 14.78 kJ mol⁻¹, 47.01 J K⁻¹ mol⁻ for the solid-solid transition and 7.518 kJ mol⁻¹, 18.68 J K⁻¹ mol⁻¹ for the solid-liquid transition, respectively. The dependence of heat capacity on the temperature was fitted to the following polynomial equations with least square method. In the temperature range of 80 to 310 K, C _p,m/(J K⁻¹ mol⁻¹)=117.72+58.8022x+3.0964x ²+6.87363x ³−13.922x ⁴+9.8889x ⁵+16.195x ⁶; x=[(T/K)−195]/115. In the temperature range of 325 to 395 K, C _p,m/(J K⁻¹ mol⁻¹)=290.74+22.767x−0.6247x ²−0.8716x ³−4.0159x ⁴−0.2878x ⁵+1.7244x ⁶; x=[(T/K)−360]/35. The thermodynamic functions H _T−H _298.15 and S _T−S _298.15, were derived from the heat capacity data in the temperature range of 80 to 410 K with an interval of 5 K. The thermostability of the compound was further tested by DSC and TG measurements. The results were in agreement with those obtained by adiabatic calorimetry.