Modulation conditions

Figure 3.21 Time effects in p(V) for 60 seconds holding at various potentials in the anodic-going sweep, with the corresponding p(V) against potential profile in a following cathodic-going sweep at the same sweep rate. Modulation conditions and scan rate as in Figure 3.20. From...

The frequency of modulation is adjusted so that there are many cycles over the duration of the transition, which in this case is the glass transition and is therefore broad. A typical modulation condition is 0.5 C in a 60-s period for studying cure of an epoxy resin (Van Assche et al, 1997). The MDSC instrument provides a modulated heating programme of... 

An additional calibration constant is required for accurate MTDSC experiments this is the heat capacity calibration. The heat capacity constant is calculated as the ratio of the theoretical heat capacity of a known standard to the measured heat capacity of the material. The heat capacity constant is sensitive to changes in the modulation conditions, especially the frequency of modulation. 

Finally, the temperature modulation conditions should be selected so that there are at least six cycles across the width of the transition. Otherwise artifacts will be introduced into the deconvolution of the modulated heat flow into the reversing and nonreversing signals. This invariably leads to the operator having to use slow heating rate (l-3°C/min) when analyzing pharmaceutical materials. A more detailed discussion on the selection of appropriate experimental conditions can be found elsewhere (1-3). 

Periodic variation of the gas concentration induces an oscillating sensor response the amplitude of the oscillations depends on gas. Resonance in the response oscillations is an indicator that the gas fits the preferences of the system that are determined by an intentional choice of the modulation conditions for the gas injection. In general, the method seems acceptable to any type of the sensors in which the response signal is proportional to surface coverage by the gas particles. 

Figure 1 shows the absorption spectrum of the nematic phase of PAA obtained under high resolution conditions, at several temperatures. These spectra were obtained under first side band modulation conditions discussed by Lawson and Flautt (17). These modulation conditions produce base line stability while minimizing the influence of the amplitude and frequency of the modulating field. 

Electron spin resonance spectrometers are available commercially, and typical operating conditions are a frequency/of 10 ° cps (x band), and a magnetic field H of 3600 gauss. Sometimes the measurements are made at other frequencies such as 2.4 X 10 cps (k band). The samples may be examined as a function of the microwave power, the microwave frequency, the modulation conditions, and the temperature. 

Eq. (4.60) shows that Q is factorized as a product of frequency-dependent part and the time-dependent part in the slow modulation regime. It is important to note that Eq. (4.60) could also have been derived from the exact result presented in Appendix C by considering the slow modulation conditions. Briefly, from the exact expressions of and in the... 

Figure 16. TMDSC plot for Examix-Light Body and modulation condition (b) [45]. The total heat flow is shown in the top curve. Reproduced with permission from Elsevier Science.

The TMDSC results for Examix silicone impression material and modulation condition (b) in Figure 16 are similar to those for the maxillofacial silicone in... 

Figure 15. The very weak glass-transition near -125 °C is barely evident in Figure 16. As expected, for modulation condition (a) the relative nonreversing character of the crystalline polymer melting peak near -50 °C was substantially less than its reversing character (relative values). Similar TMDSC results were found for the second silicone impression material Reprosil [45].

The TMDSC results are shown for the Impregum polyether impression material and the two modulation conditions in Figures 17 and 18, respectively. 

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. 

Additional TMDSC study of other vinyl polysiloxane, polyether and polysulfide impression materials is important to verify if the polymer transitions shown in Figures 16 to 19 generally exist in different products and to investigate the effects of other temperature modulation conditions. Complementary research on correlations with clinically relevant mechanical properties of the elastomeric impression materials is needed to verify if these thermal analyses have useful predictive power. Interestingly, when compared at apparently similar viscosities, the reported values of the elastic modulus [3] are highest for the vinyl polysiloxane silicone impression materials, intermediate for the polyether impression materials, and lowest for the polysulfide impression materials, in reverse order to the relative values of Tg fovind in our thermal analyses [45]. Our X-ray diffraction and scanning electron microscopic study [47] of these impression materials has shown that they contain substantial amounts of crystalline filler particles in the micron size range, which are incorporated by manufacturers to achieve the clinically desired viscosity levels. Tliese filler particles should have considerable influence on the mechanical properties of the impression materials. 

Scheme 2.2 Biocatalytic resolution of racemic glycidyl phenyl ether using BmEH under modulated conditions.

However, the sum of these signals is quantitative. Linearity is maintained in the baseline, permitting accurate measurement of the onset as well as the end of the endothermic and exothermic events [3]. Melting and crystallization neither start nor end at the same temperatures and therefore different integration limits are required. Finally, the total heat flow signal is quantitatively correct regardless of the modulation conditions. Thus the initial crystallinity of the sample can be estimated by TMDSC (see Table 5.1). The data presented in Table 5.1 reveal that the observed initial crystallinity is approximately constant under various experimental conditions. 

The peroxisomal processes are stable no major changes will occur during fasting, eating, exercise or other modulating conditions. An exception is phytanic acid, which is exclusively derived from dietary sources. As a consequence newborns do not show accumulation of this substance and dietary measures may result in lowering of its levels. 

The conditions for K (t) to coincidate with Cj (t) are generally correctly fulfilled in dense fluids, since they demand that close interactions be frequent (the "fast modulation" condition) and torques be stronger enough than the mean thermal energy following (22) that condition reads... 

Figure 2.95 Selection of the proper modulation conditions through simple inspection of the modulated heat flow [TA Instruments (n.d.) Compendium courtesy of TA Instruments].

In normal field-modulated spectra the modulation is kept smaller than the line width, and in careful work the effect of the modulation on the line shape is considered. However, the sidebands at 100 kHz or less are almost always within the envelope of the line shape, except for very narrow lines. The fast modulation that is required to obtain a relaxation time-dependent response in modulation spectroscopy inherently has more widely spaced sidebands. For example, at 200 MHz modulation the sidebands are approximately 71 G away fi om the center band. The modulation index and the d.c. offset can be varied to obtain a wide range of modulation conditions cf. Haworth Richards, 1966 Losee et al., 1997 Reference Data for Radio Engineers, 1968). It should be noted that the finite bandwidth caused by resonator Q can attenuate effects of sidebands and resultant signal amplitude as the modulation frequency is increased. This calculation is beyond the scope of this chapter but needs to be performed for a given experimental arrangement in order to be able to use the predictions of this chapter to actually measure relaxation times. 

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