The curing reaction of a thermosetting system is investigated by DSC and temperature modulated DSC (TMDSC). When the material
vitrifies during curing, the reaction becomes diffusion controlled. The phase shift signal measured by TMDSC includes direct
information on the reaction kinetics. For long periods the phase shift is approximately proportional to the partial temperature
derivative of the reaction rate. This signal is very sensitive for changes in the reaction kinetics. In the present paper
an approach to determine the diffusion control influence on the reaction kinetics from the measured phase shift is developed.
The results are compared with experimental data. Further applications of this method for other reactions are proposed.
A characteristic index
for the oxidation stability this is the oxidation induction time (OIT) which
is defined by the time between the start of oxygen exposure and the onset
of oxidation. Pressure DSC is required to increase oxygen concentration in
order to achieve faster reactions at lower temperatures. OIT measurements
of reference engine oils have been used to study the influence of oxygen pressure
in the range from 0.1 to 10 MPa. A power law relationship was derived to describe
this correlation between OIT and oxygen pressure. From this a quantitation
factor is proposed to represent the influence of stabilizer. The exponent
describes the sensitivity of the oxidation reaction of the oil towards the
oxygen pressure and the term 'inherent stability' is proposed
for that.. This relationship characterizes in more details the oxidation behavior.
Extrapolation to higher pressures indicates, that the stabilization effects
of additives can be overcome by the inherent stability. This signifies, that
the ranking of the oils can be affected by the oxygen pressure.
The interaction among moisture content, solvent loss and glass transition temperature is relevant for processing of spray-dried
pharmaceuticals, since the glass transition temperature determines the application range of a compound. Conventional Differential
Scanning Calorimetry (DSC) does usually not allow to separate glass transitions from common kinetic effects like evaporation
or crystallization. Based on classical DSC methods, the IsoStepTM method allows the independent determination of heat capacities and kinetic effects, and thus, the separation of kinetic effects
from effects arising from heat capacity changes. This technique is used to separate glass transition and evaporation processes,
and to find the relation between moisture content and glass transition temperature for a pharmaceutical sample based on a
modified Gordon–Taylor equation.
The thermal behavior of a drawn PET fiber has been investigated by thermomechanical analysis, TMA, and by differential scanning calorimetry, DSC. Above the glass transition temperature of 79°C, the fiber shrinks to a maximum of 8% of the initial length. Temperature modulated TMA enabled the separation of the thermal expansion from the overlapping shrinkage during the first heating and to calculate the expansivity,
e and the shrinkage coefficient,
s, independent of each other. Young's modulus, E, was measured by TMA with modulation of the tensile stress. Hence, it was possible to record the behavior of
s and E during the structural changes by combining both modulations in a single measurement.A new technique was developed to calibrate the sample temperature. With this, accurate control of the modulated temperature of the specimen was achieved, independent of the changing heating rate.
The glass transitions of different materials (a silicate glass, a metallic glass, a polymer, a low molecular liquid crystal and a natural product) were investigated. By means of the temperature-modulated DSC (TM-DSC) mode, the frequency was varied. In the case of DSC, the cooling rate was changed. TM-DSC was shown to be a practicable tool for the acquisition of dynamic parameters of glass transitions for all kinds of materials.
The measured signal of the temperature-modulated differential scanning calorimetry (TMDSC) is discussed in the case of polymer
melting. The common data evaluation procedure of TMDSC-signals is the Fourier analysis. The resulting information is the amplitude
and the phase shift of the first harmonic of the periodic heat flow component. It is shown that this procedure is not sufficient
for quantitative discussions if deviations from the symmetric curve shape occur in the measured heat flow curves. For polymer
melting it is demonstrated that asymmetric curves will be measured if the experimental temperature amplitude is too large.
In this paper a data evaluation method is presented, which is based on the Fourier transform of the measured curves. The peaks
of the first and second harmonics in the resulting spectra are used for the analysis of the asymmetry of the measured curves.
In the case of polymer melting this analysis yields the maximum temperature amplitude which follows a correct linear data
evaluation. This maximum temperature amplitude depends on the material.
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.
Authors:A. Hensel, J. Dobbertin, J. E. K. Schawe, A. Boller, and C. Schick
The results from temperature modulated DSC in the glass transition region of amorphous and semicrystalline polymers are described with the linear response approach. The real and the imaginary part of the complex heat capacity are discussed. The findings are compared with those of dielectric spectroscopy. The frequency dependent glass transition temperature can be fitted with a VFT-equation. The transition frequencies are decreased by 0.5 to 1 orders of magnitude compared to dielectric measurements. Cooling rates from standard DSC are transformed into frequencies. The glass transition temperatures are also approximated by the VFT-fit from the temperature modulated measurements. The differences in the shape of the curves from amorphous and semicrystalline samples are discussed.