Nonreversing heat flow

FIGURE 4.18 Nonreversing heat flow as a function of storage time at 90°C for amorphous lactose. (From Craig, D.Q.M., Barsnes, M., Royafl, P.G., and Kett, V.L., Pharm. Res., 17, 696, 2000. With permission.)... 

Figure 7. TMDSC reversing and nonreversing heat flow curves for the heating cycle of as-received Nitinol SE [37]. Reproduced with permission from Elsevier Science.

Broadj low-temperature exothermic peaks appear on the nonreversing heat flow curves in Figures 7 and 8, indicating transformation within martensite to another structure (M ). Low-temperature peaks had previously been reported from DSC [10] and electrical resistivity measurements [38] of engineering and NiTi orthodontic alloys, respectively. However, our group was unable to confirm these peaks on DSC plots for five representative orthodontic wires[25]. 

Similar results were obtained for TMDSC analyses of the in vivo shape-memory wire, Neo Sentalloy, shown in Figures 9 and 10 [37]. While our previous DSC study [25] suggested that direct transformation of martensite to austenite occurs during heating, the nonreversing heat flow curve in Figure 9 shows that a two-step transformation involving R-phase actually takes place. 

This polyether impression material undergoes a gleiss transition near -SO C and has melting peaks near -20 °C and 50 °C. While there are exothermic peaks indicative of crystallization on the nonreversing heat-flow curve for modulation condition (b) in Figure 18, only endothermic peaks that would not correspond to crystallization are found on the nonreversing heat-flow curve for modulation condition (a) in Figure 17. 

The TMDSC results are shown for the Coe-Flex polysulfide impression material and modulation condition (a) in Figure 19. There is a glass transition near -55 °C and an apparent weak crystalline polymer melting peak near 70 °C. No evidence of an exothermic crystallization peak can be seen on the bottom nonreversing heat-flow curve. The peak near 190 °C requires further study, but is not clinically relevant for the properties of the impression material. 

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. 

The experiment of Figure 2.2 will now be considered in more detail as a typical example of isothermal cure with vitrification. It shows the nonreversing heat flow (Figure 2.2a), the heat capacity (Figure 2.2b) and the heat flow phase (Figure 2.2c) as a function of reaction time for the quasi-isothermal cure of an epoxy-anhydride resin at 100°C for 200 min. The reaction exotherm obeys an auto-catalytic behaviour the heat flow increases at... 

For the epoxy resins studied, the mobility factor based on heat capacity coincides very well with the diffusion factor, calculated from the nonreversing heat flow via chemical kinetics modelling, and describing the effects of diffusion control on the rate of conversion of the cure reaction. Although the two resins behave quite differently, this coincidence between the mobility factor and diffusion factor is valid for both systems. Therefore, the mobility factor can be used for a quantitative description of then-altered rate of conversion in the (partially) vitrified state for the decrease in rate during vitrification, the increase in rate during devitrification and the diffusion-controlled rate in the (partially) vitrified region in between both processes. 

The nonreversing heat flow can be calculated from the difference between the total heat flow and the reversing heat flow ... 

Figure 2.89. The total heat flow, reversing heat flow, and the nonreversing heat flow from an MTDSC heating of quenched poly(ethylene terephthalate) (courtesy of TA Instruments).

Different from the other MTDSC techniques is the TOPEM technique, which is based on a quasistochastic temperature modulation superimposed on a conventional DSC temperature program as seen in Fig. 2.90 (Schubnell et al. 2005). This allows separate determination of the reversing and nonreversing heat flows and the quasistatic (zero-frequency) heat capacity, as well as a complex heat capacity (Schawe and Hutter 2005). Since the response at a desired frequency is determined after the run, multiple run parameters can be used to optimize the analysis of different transition regions during a run as noted in text below (see also Fig. 2.91). 

Nonreversing heat flow Kinetic heat flow... 

In general, TA Instruments recommends underlying heating rates of less than 5 °C/min for MDSC experiments with modulation amplitudes between 0.1 °C and 2°C, and modulation periods between 40 and 100 s (TA Instruments 2005). For quasiisothermal experiments, the nonreversing heat flow... 

Figure 2.97. The crystallization exotherm during an MDSC heating experiment seen in the nonreversing heat flow (top curve) is confirmed by superimposing it on the modulated heat flow signal for polyamide 12 that had been annealed at 160 °C for one hour. The exothermic direction of this figure is upward [from Judovits (1997) reprinted with permission from the North American Thermal Analysis Society].

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