Cooling and heating scans

In contrast, the enthalpy values illustrated in Table 6.2 for several nematic RF PLCs are counterintuitive at first glance e.g. supercooling is observed as expected (with Tj but is larger than AH j. The magnitude of supercooling and the difference between and ah, is relatively small for these low mass samples, but tend to increase with chain length [57]. 

If one assumes that a fully developed N phase (the degree of liquid crystallinity is unity) is formed on cooling, a process which is facilitated by the large drop in viscosity at the I-N transition, the smaller value of Ah, may be attributed to an incomplete isotropization on heating. Perhaps, as a direct result of molecular segregation across the biphase. 

Molecular masses range between 2500 and 6700. Enthalpies are given in kj per mol of repeating unit. Values recorded on cooling are in parentheses. 

Thermal history scanning rates are 20Kmin Samples are cooled to 20 K below and heated to Dafa are from second heating and cooling runs (5 min isothermal hold at 

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DSC cooling and heating scans obtained with a 2.0 M aqueous glycinebetaine solution. An inserted figure indicates glass-transition behavior. 

A series of heating, cooling and heating scans is the general approach to get an impression of the melting/recrystallisation behaviour of a semi-crystalline polymer. The Tml/Hfl-values are influenced by the thermal history of the sample. The Tc/Hc-values are characteristic for the recrystallisation of the polymer under standard (thermal) conditions. Finally, the Tm2/Hf2-values can be used to compare different samples recrystallised under identical conditions. 

In conclusion, for understanding metastability phenomena, one would like to be able to choose combinations of cooling and heating scans dedicated to the problem at hand. For most processes in practice, the present range of scan rates of Standard DSC is too limited to influence the metastability of the systems studied and hence to provide understanding. HPer DSC - see next section - is very useful to chart the problem and, in some cases, to cure it. 

Figure 5.3b shows cooling and heating scans where a sample is undergoing crystallization and melting. The values most... 

Figure 5.7 SSA thermal protocol schematic representation. Cooling and heating scans are performed at a constant scanning rate.

DSC cooling and heating scans of the nanostructured block copolymers presented here can be found elsewhere [207,208]. Figure 5.15 shows the peak melting and crystallization temperatures (T and T ) and the crystallinity degree (%XJ for the... 

Hie analysis of each of the most abundant FAMEs by DSC at 5 C/min has permitted us to identify the peaks obtained on the cooling and heating scans of rapeseed and palm biodiesels. Table 13.2 lists the FAME composition of rapeseed (ME2) and palm (MEl) biodiesels as well as the melting and crystallization onset and enthalpy. Methyl oleate and elaidate are the predominant FAMEs in MEl and ME2 at 53.2% and 57.1 %, respectively. For palm biodiesel, methyl palmitate was the next most abundant FAME (27.4 %), followed by methyl stearate (9.4 %) and methyl linoleate (6.4 %). Concerning the rapeseed biodiesel, methyl linoleate was the next most abundant FAME (24.5 %) after methyl oleate, followed by methyl linolenate (8.6%) and methyl palmitate (4.6%). Rapeseed biodiesel (ME2) has a higher amount of unsaturated fatty acids (UFA, 92 %) than palm biodiesel (UFA, 61 %). The total saturated fatty acid (SEA) content for palm ester was 38 %, and 7 % for rapeseed ester. 

Figure 12.3 DSC cooling and heating scans (10°C/min) for the neat PE homopolymer and for the PE-fe-PS diblock copolymers (a) PE , (b) 53847 (c) E26Sy4 and (d) EnS89 . Curves have been shifted vertically. The arrow indicates the PS block glass transition for the PE-fe-PS diblock copolymers.

Fig. 11 Cooling and heating DSC scans (10°Cmin 1) of original and purified E24EP57EO19. (Reprinted with permission from [29], Copyright 2002 American Chemical Society)...

The result for the thermal expansion coefficient, a, which is equal to (dV/dT)/V, is shown in Fig. 13.36 for the cooling and heating process. In the cooling process a decreases gradually from tq to ag. Hysteresis in the volume causes in the subsequent heating process an anomalous effect in the thermal expansion coefficient, depicted by undershoot and overshoot, as also shown in Fig. 13.36. A similar effect occurs in enthalpy H and accordingly in cp, the specific heat capacity, equal to dH/dT. This effect is frequently observed in DSC (Differential Scanning Calorimetry) experiments. 

Nishimoto, Y. Ichimura, Y. Kinoshita, R. Teramoto, Y. Yoshida, H. Heat capacity measurements by heat-flux type DSC on cooling and heating cycles at low scan rates. Thermochim. Acta 1991, 179, 117-124. 

Enthalpy relaxation is one of the most widely studied in the context of both non-linearity and non-exponentiality of the measured glass properties. A convenient technique for these studies is scanning calorimetry. In simple cooling and heating experiments, heat capacity curves exhibit normal increase with characteristic hump of Cp above the glass transition as represented in Figure 9.08(A). The fictive temperature,... 

Emulsion II differs from emulsion I only in the dispersed phase, the composition of which is 30% of urea dissolved in water. Equal amounts of emulsions I and II are mixed manually and the resulting emulsion is kept at ambient temperature. From time to time an emulsion sample is submitted to cooling and heating in a differential scanning calorimeter. 

The mesophase state of liquid crystals is normally opaque due to relatively large sizes of ordered domains. Its transition point to the isotropic melt state is called the clear point T,. The DSC scanning curves of liquid crystals can exhibit either enantiotropic or monotropic phenomena. For the thermodynamically stable mesophases of liquid crystals, they occur between the melt and the crystal states during both cooling and heating processes, as illustrated in Fig. 10.5. When both the cooling and heating curves show two symmetric consecutive phase transitions, it is known as the enantiotropic phenomenon. In contrast, for the metastable mesophase... 

Enthalpy and Heat Capacity Relaxation. The conventional techniques based on differential scanning calorimetry by which enthalpy relaxation is measured at different cooling and heating rates is discussed elsewhere in the encyclopedia by others. Extensive review of the subject can be foimd in Reference 160. Measurements of heat capacity and entropy of polymers as a function of temperature are well documented and the data can be found in Reference 161. Frequency-dependent heat capacity spectroscopy has been developed (162,163) and apphca-tion to study the local segmental d5mamics of polymers has been made (163). 

The full heating, cooling and reheating scan of a pyrene + 9,10-dibromoanthracene mixture... 

The catalyst for the in situ FTIR-transmission measurements was pressed into a self-supporting wafer (diameter 3 cm, weight 10 mg). The wafer was placed at the center of the quartz-made IR cell which was equipped with two NaCl windows. The NaCI window s were cooled with water flow, thus the catalyst could be heated to 1000 K in the cell. A thermocouple was set close to the sample wafer to detect the temperature of the catalyst. The cell was connected to a closed-gas-circulation system which was linked to a vacuum line. The gases used for adsorption and reaction experiments were O, (99.95%), 0, (isotope purity, 97.5%), H2 (99.999%), CH4 (99.99%) and CD4 (isotope purity, 99.9%). For the reaction, the gases were circulated by a circulation pump and the products w ere removed by using an appropriate cold trap (e.g. dry-ice ethanol trap). The IR measurements were carried out with a JASCO FT/IR-7000 sprectrometer. Most of the spectra were recorded w ith 4 cm resolution and 50 scans. 

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