TMDSC heat-flow rate

To compare the functioning of a TMDSC with that of a DSC, Fig. 4.103 gives data for three DSC runs and a matching TMDSC trace (TA Instruments, MDSC 2910). Plotted are the instantaneous (not averaged or smoothed) heating rates q(t) and heat-flow rates HF(t) (proportional to AT). The DSC was run at heating rates q = 3,5, and 7 K min, the TMDSC at < q > of 5 K min. These conditions result in a maximum q(t) of 7.0 and a minimum q(t) of 3.0 K min for - Ts (27t/p. Note that the time to reach steady state with TMDSC is longer than in a stan d DSC. The four curves match well above 340 K. Typically, one should allow 2-3 min to reach steady state for the chosen calorimeter. 

Another topic to establish a good TMDSC practice deals with Lissajous figures (or Bowditch curves), plots of the time-dependent heat-flow rate HF(t) or AT(t) versus... 

An analysis of the glass transition of polystyrene by TMDSC is illustrated in Fig. 4.125. These measurements can be compared to the DSC results in Sect. 4.3.7. The left half of the figure shows besides the modulated heat-flow rate, AT(t), the three... 

The study of elastic and viscoelastic materials under conditions of cyclic stress or strain is called dynamic mechanical analysis, DMA. The periodic changes in either stress or strain permits the analysis of the dynamic response of the sample in the other variable. The analysis has certain parallels to the temperature-modulated differential thermal analysis described in Sect 4.4, where the dynamic response of the heat-flow rate is caused by the cyclic temperature change. In fact, much of the description of TMDSC was initially modeled on the more fully developed DMA. The instruments which measure stress versus strain as a function of frequency and temperature are called dynamic mechanical analyzers. The DMA is easily recognized as a further development of TMA. Its importance lies in the direct link of the experiment to the mechanical behavior of the samples. The difficulty of the technique lies in understanding the macroscopic measurement in terms of the microscopic origin. The... 

A strain-modulated DMA parallels the temperature-modulated DSC discussed in Sect. 4.4. Figure 4.161 shows a comparison to the results in Figs. 4.90, written for a common phase lag 0. Note that the measured heat-flow rate HF(t) lags hehind the modulated temperature, while the measured shear stress advances ahead of the modulated strain. Besides modulation of strain, it is also easily possible to modulate the stress, and even temperature-modulation is possible and of interest for comparison of DMA to TMDSC, as was established recently [44]. 

Slow, primary crystallization of PEEK is analyzed in Fig. 6.98 using long-time, quasi-isothermal TMDSC at 606.5 K, and in Fig. 6.99, using TMDMA at 605 K. The analysis of the TMDSC data is further illustrated in Fig. 6.100 and reveals an increase in heat capacity with time and crystallinity, instead of the decrease expected from a lower heat capacity of a semicrystalline sample. The crystallinity is derived from the integral of the total heat-flow rate, with time in Fig. 6.98. From Fig. 6.99 it... 

The sample (or furnace) temperature is controlled to follow a set course with superimposed periodical changes, and the heat flow rate is measured via the differential temperature between sample and reference (temperature-modulated differential scanning calorimetry, TMDSC [38]). 

The TMDSC enables another elegant possibility of Cp (magnitude) determination from the amplitude of the modulated part of the measured heat flow rate function both in the isothermal and. scanning modes of operation. This method is especially advantageous in cases of noisy signals with low sample masses or low heating rates. Precise calibration of the heat flow rate amplitude is a prerequisite for obtaining reliable results [43]. 

A simple analysis of an irreversible first-order transition is the cold crystallization, defined in Sect 3.5.5. For polymers, crystallization on heating from the glassy state may be so far from equilibrium that the temperature modulation will have little effect on its rate, as seen in Fig. 4.122. The modeling of the measurement of heat capacity in the presence of large, irreversible heat flows in Fig. 4.102, and irreversible melting in Figs. 3.89 and 4.123, document this capability of TMDSC to separate irreversible and reversible effects. Little needs to be added to this important application. 

TMDSC data are calculated from three measured signals time, modulated heat flow and modulated heating rate (the derivative of modulated temperature). The raw data are visually complex and require deconvolution to obtain standard DSC curves. However, raw data are useful for revealing the sample behaviour during temperature modulation, as well as fine-tuning experimental conditions and detecting artefacts. 

Temperature modulated DSC requires the same baseline, temperature and enthalpy calibration as conventional DSC (see Sections 3.1 and 3.2). Measuring the heat capacity is a necessary First step in calculating the reversing and nonreversing heat flows, so a heat capacity calibration must be performed for TMDSC. Calibration parameters such as sample vessel type, purge gas, heating rate, modulation amplitude and period must be identical to those used in subsequent experiments. 

In Fig. 6.11, it is also found that there are two regions where the total heat flow shows anomalous behavior, one is around 70° C and the other is between 110°C and 130°C. The latter one corresponds to the anomalous change in total heat flow observed at T g in Fig. 6.10. It can be expected that the observed upper shift of total heai flow at Tg is mainly due to the heat of crystallization. The position of this anomaly in Fig. 6.11 is different from that in Fig. 6.10. This may be due to the difference in heating rate (20K/min for DSC measurements in Fig. 6.10 and IK/min for TMDSC measurements in Fig. 6.11.)... 

The non-isothermal crystallisation of PETP was examined by temp, modulated DSC(TMDSC). A new analytical model of TMDSC was applied to the process, taking account of the response of exothermic heat flow to temp, modulation in an apparent heat capacity of complex quantity. By examining the frequency dependence of the apparent heat capacity, the applicability was successfully examined for the non-isothermal process. The method was capable of determining the temp, dependence of crystal growth rate from TMDSC data analysis. The results were in good agreement with the dependence determined from literature values of spherulite growth rate measured by optical microscopy. 12 refs. 

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