SEDL / STP / STP1402-EB / STP10696S

Temperature-Modulated Calorimetry of Polymers with Single and multiple Frequencies to Determine Heat Capacities as Well as Reversible and Irreversible Transition Parameters

Wunderlich, B
The University of Tennessee, Knoxville, TN
Chemistry and Analytical Sciences Division at Oak Ridge National Laboratory, Oak Ridge, TN

Pages: 14    Published: Jan 2001

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Abstract

Temperature-modulated differential scanning calorimetry (TMDSC) generated with a centrosymmetric saw-tooth oscillation can be considered to be a sinusoidal modulation with multiple frequencies. Different harmonics of the Fourier series of the heat-flow rate and heating rate of a single sawtooth-modulation can be deconvoluted to extract data pertaining to different frequencies. In order to give the higher harmonics similar amplitudes, a complex, but simple-to-program, sawtooth-modulation is generated for the harmonics 1,3,5,7 and 9. In this fashion a single experiment can produce a frequency-dependent analysis under identical thermal history. Application of this method to TMDSC includes the calibration for heat capacity determination of high precision, even if steady state and a negligible temperature gradient are not achieved. The measurement of the frequency (ω) dependence of the heat-flow rate (A^HF) and sample temperature (A^Ts) allows to evaluate the expression: CP=AHF/(ATsνω)[1+Caret(τνω)2]0.5 where the relaxation time τ is to be determined empirically from the multiple data generated by the single run. Typical values for the relaxation time for commercial calorimeters are between 3 and 9 s rad^-1. Frequency-dependent, apparent reversing heat capacities in the glass transition region and within first-order transition regions may also be analyzed to study local equilibria in globally metastable polymeric solids.