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  • Author or Editor: R. Androsch x
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Abstract  

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.

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Abstract  

Reversible and irreversible crystallization and melting of high-density polyethylene at low temperature has been re-evaluated and is discussed in terms of the concept of the specific reversibility of a crystal. The concept of the specific reversibility links reversible and irreversible melting of a specific crystal such that reversible melting occurs only at slightly lower temperature than irreversible melting. In this study evidence for irreversible crystallization at low temperature in high-density polyethylene is provided, non-avoidable by primary crystallization and extended annealing at high temperature. The simultaneously observed reversible crystallization and melting at low temperature can be attributed to lateral-crystal-surface activity in addition to the well-established reversible fold-surface melting, dominant at high temperature, and evidenced by small-angle X-ray data available in the literature.

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Abstract  

Non-isothermal crystallization and melting of metallocene-catalyzed polyethylene was analysed using the power-compensating calorimetry as function of both cooling rate and branch-degree ranging from 1 to 300 K min-1, and from 0 to 72 hexyl-branches per 1000 carbon atoms, respectively. Onset crystallization temperature decreases linearly with increasing logarithm of cooling rate. The slope of this dependence increases with increasing branch-degree, which is explained by a stronger inhibition of the crystallization by the branches and the decreasing diffusion rate of molecules. The melting endotherms reveal a clear reduction of the crystallinity in the entire temperature range with increasing cooling rate.

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Abstract  

A newly developed Micro-Thermal Analyzer affords images based on thermal properties such as thermal conductivity, thermal diffusivity, and permits localized thermal analyses on samples of a square micrometer area by combining the imaging ability of the atomic force microscope and the thermal characterization ability of temperature-modulated differential scanning calorimetry. Since thermal penetration depth depends on frequency, one can obtain depth profiles of thermal conductivity and thermal diffusivity by varying the modulation frequency. Also, the analyzer can be used to characterize phase-transition temperatures, such as glass and melting transitions, of small sample regions with a precision of about ±3 K. Heating rates can be varied between 1 and 1500 K min–1. Modulation frequencies can be applied in the range from 2 to 100 kHz. We applied this new type of instrument to characterize microscopic thermal and structural properties of various polymer systems. The operation principles of the instrument are described, application examples are presented, and the future of the technique is discussed.

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