Quasi-isothermal MTDSC

Manduva R, Keti VL. Banks SR. et al. Calorimetric and spatial characterisation of polymorphic transitions in caffeine using quasi-isothermal MTDSC and localised thermo-mechanical analysis. J Wiarm Sci 2007 97(2008) 1285-1300,... 

FIGURE 4.12 Reversing heat capacity of ra-pentacontane by standard DSC and quasi-isothermal MTDSC. (From Pak, J., Boiler, A., Moon, I., Pyda, M., and Wunderlich, B., Thermochim. Acta, 357-358, 259, 2000. With permission.)... 

The total heat flow obtained in quasi-isothermal MTDSC experiments agrees very well with the heat flow evolution obtained in a conventional DSC experiment, performed under the same conditions without of the modulation (Figure 2.2a). Neither changing the modulation amplitude nor the period had an effect on the reaction exotherm seen in the non-reversing heat flow. This illustrates the negligible effect of the perturbation on the cure reaction. 

Figure 4.30. Modelling of quasi-isothermal MTDSC with a small transition of a peak height of...

Figure 4.32. Modelling of quasi-isothermal MTDSC with an additional linear increase of the heat flow rate, beginning at 250 s (upper curves), and an additional exponential increase of the heat flow rate starting at the beginning of modulation (lower curves).

Figure 4.40 illustrates the change of sample temperature (dashed line) and heat flow rate (heavy solid line) for a quasi-isothermal MTDSC experiment, as suggested by the bottom heating rate program of Figure 4.28 = 0). This analysis should be compared to Figure 4.34, which was... 

Figure 4.45. Apparent heat capaeity in the melting range of 0.936 mg of -pentaeontane by standard DSC (heating rate 10 K min ) and quasi-isothermal MTDSC A = 0.05 K, / = 60 s...

Figure 4.46. Lissajous figures for the quasi-isothermal MTDSC of Figure 4.45 [28].

Figure 4.49 shows the results of adiabatic calorimetry, standard DSC and quasi-isothermal MTDSC for poly-/ -dioxanone (—CH2—CH2— O—CH2—COO—)x, (PPDX). The ordinate is labelled as apparent heat capacity since in the transition region, latent heat contributions may increase the heat capacity. Up to 250 K, the heat capacity is practically fiilly vibrational as is typical for glassy and crystalline solids. The skeletal and group vibrational contributions are then extrapolated to higher temperature, as is discussed with Figure 4.1 for polyethylene. The sample analysed with... 

Figure 4.49. Apparent heat capacity of poly-p-dioxanone (PPDX) using adiabatic calorimetry by calculation of Cp = (AT/corrected for heat loss/A corrected for temperature drift)p,n Standard DSC using Eq. (10), and quasi-isothermal MTDSC evaluated with Eq. (11). The data were...

A comparison of quasi-isothermal MTDSC and standard DSC of the melting of PET crystallised from the melt is shown in Figure 4.50. This graph was the first proof that there is an apparent, reversing heat-capacity contribution to the melting [63]. The higher melting temperature of the... 

Figure 4.50. Apparent heat capacity of melt-crystallised PET as measured by standard DSC and quasi-isothermal MTDSC, compared to the baselines of the heat capacity of the melt, solid (vibrational contributions only) and semicrystalline pol)mier [63].

The effects due to very slow processes in the sample, however, may not be removed even in the quasi-isothermal MTDSC with data collection after 10 min. The analysis of the slow response of the sample in the glass transition region will be treated in Section 4.3. The decrease in the heat capacity due to cold crystallisation can easily be converted into a plot of the crystallisation kinetics. Additional points for the kinetics plot can be generated at shorter and longer analysis times of the quasi-isothermal runs. The time-scale can easily be adjusted to modulations from 1 min to many hours, limited only by the patience of the operator and the stability of the calorimeter. 

Figure 4.52. Apparent heat capacity of PET on cooling by quasi-isothermal MTDSC compared to data on semicrystalline, melt-crystallised PET measured on heating, as shown in...

Figure 4.54. Apparent heat capacity of low crystallinity PTT by quasi-isothermal MTDSC compared to data from standard DSC measurements [51,65]. See, for review. Figure 4.38.

Figure 4.54 is a quantitative quasi-isothermal MTDSC trace for quenched, poorly crystallised PTT. The corresponding semiquantitative MTDSC is depicted in Figure 4.38. The cold crystallisation at about 325 K, the recrystallisation, 450 K, and the small enthalpy relaxation at 320 K are seen to be fully irreversible, and as in PET, the kinetics of the glass transition and the cold crystallisation can be further analysed quantitatively making use of the reversing heat capacity. It is also clear that during the standard DSC measurement, the cold crystallisation never stops completely between the two peaks and considerable errors in the crystallinity may result from choosing a baseline without MTDSC data.

Figure 4.74. Extended quasi-isothermal MTDSC of PET in the melting range. Sample as in...

Figure 4.75. Quasi-isothermal MTDSC of PET as in Figure 4.50 with one set of long-term modulation experiments at 522 K.

Figure 4.76. Extended quasi-isothermal MTDSC of PET in the melting range, similar to Figure 4.74, but with a diiferent calorimeter. At 573 K, the sample is melted, and at 513 K, it is in the melting range. For the five experiments at 513 K, the modulations were started at times [A]-[E].

Figure 4.78. Quasi-isothermal MTDSC of PTT as in Figure 4.56 with three sets of long-term...

Poly(ethylene oxide) of high molar mass behaves similarly to the PET and PTT and other polymers analysed, although special effects are seen for many analysed polymers [78,82]. Figure 4.80 represents an example of PEO of a molar mass of 35,000 Da. As before, at low temperature, standard DSC and quasi-isothermal MTDSC give the same result. Most of the melting is irreversible and shows only in the total apparent heat capacity. A small amount, however, is reversing. The irreversible melting occurs at a temperature expected for 4 folds per molecule [52]. 

Figure 4.84. Apparent heat capacity measured by quasi-isothermal MTDSC for PEG1500 with successively higher modulation amplitudes, crystallised after quick cooling to the indicated first analysis temperatures. The indicated erystallinities were obtained by parallel experiments with standard DSC on identically treated samples measured at 10 K min . Successive data are displaced upwards by the listed amounts [52],...

Figure 4.91. Heat flow rate and contour lines (slightly displaced for clarity) of sample no. 2 of PEcoO of Figure 4.87 on quasi-isothermal MTDSC as a function of time at 299 K.

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