Jeongihm Pak

Temperature-modulated differential scanning calorimetry of reversible and irreversible first-order transitions

Abstract Temperature-modulated differential scanning calorimetry of first-order transitions has led to many new observations. Some of these involve non-linear processes or deal with transformations of practically instantaneous response. The latter may cause serious lags within the calorimeter due to limited thermal conductivity of the sample and the instrument. In both cases the “reversing heat capacity” or a “complex heat capacity” is not a precise representation of the transition since both are computed from abbreviated Fourier transforms, limited to the evaluation of the first harmonic component. One has in these cases to work in the time-domain with the raw output. But even from these analyses in the time-domain many interesting new insights about the transition and the calorimeter performance can be generated.

CAN ONE MEASURE PRECISE HEAT CAPACITIES WITH DSC OR TMDSC? A study of the baseline and heat-flow rate correction

SummaryDuring 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.

Thermal analysis of paraffins as model compounds for polyethylene

The n-paraffins C50H102, C44H90, and C26H54 were analyzed with standard and temperature-modulated differential scanning calorimetry. Crystallization and ordering from the melt to the condis phase showed practically no supercooling. These observations were confirmed with hot-stage microscopy and a melting-point apparatus. Only the organization of the C26H54 condis crystals to fully ordered crystals showed a supercooling of 4.0 K. The measurement of the apparent reversing heat capacity with a 0.05-K modulation amplitude revealed that the melting of C50H102 was completed within 1.0 K and the isotropization of C26H54 was completed within less than 0.6 K, but 62–78% of the total transitions occurred over a much narrower temperature range of 0.1 K or less. The link to polyethylene was made with fractions of the masses 15,520, 2150, and 560 Da. The 560-Da sample corresponded to C40H82 and showed also almost no supercooling, whereas the others, with folded and extended-chain crystals, supercooled by about 10 K.

Multi-frequency heat capacity measured with different types of TMDSC

Abstract The heat capacities of sapphire (Al2O3) and sodium chloride (NaCl), have been measured to establish the accuracy and precision of two different temperature-modulated differential scanning calorimeters operated in diverse multi-frequency modes. The calorimeters have then been applied to find the apparent, reversing heat capacity of polystyrene as a function of frequency in the glass transition region. The first modulation mode consisted of a series of linear heating and cooling segments and produced four harmonics with practically equal temperature amplitudes (1st, 3rd, 5th, and 7th), one of lower amplitude (9th), and almost negligible higher harmonics. The second modulation mode is a rather sharp step ending in an isotherm or slow temperature-decrease and leads to a controlled spike in the heat-flow rate response which produces Fourier components of similar amplitudes for all harmonics of the rates of changes of temperature. The apparent, reversing heat capacity is evaluated from the amplitudes of the heat-flow rates and the corresponding sample temperatures or heating-rates. A time-constant or calibration constant which accounts for thermal conductivities and resistances within the calorimeters can be evaluated from the different harmonics of each run. Measurements in the glass transition region have a slow response of the sample. They are evaluated by separating the sample effect from the calorimeter response which can be extrapolated from data gained outside the transition. One measurement is thus sufficient for the evaluation of the frequency dependence of the heat capacity in the glass transition region.