Temperature slow-crystallized sample

Figure 4, Comparison of spectra of slow-crystallized sample at temperature...

Figure 6. Normalized peak heights of 731 cm absorption vs. temperature for raw data and interpolated data of slow-crystallized sample. Maximum error of...

Additional information is available by subtracting spectra taken at different temperatures for the same sample. Since the position of the film was not altered as the temperatures varied, a 1 1 subtraction is a systematic method to illustrate the thermal eflFects. The spectra at the two temperature extremes for a slow-crystallized sample are subtracted in Figure 20. The 909 and 990 cm vinyl bands narrow in width, increase in intensity, and shift to slightly higher frequency as the temperature is decreased. The crystalline absorptions at 1050 and 1176 cm shift to lower frequency and sharpen considerably. The 1303 cm amorphous absorption shifts its maximum to 1300 cm and possibly increases in intensity. The 1353 cm" band remains fixed in position. The most intense amorphous band, 1369 cmmoves to 1371 cm at 78 K and has an intensity increase. These results are shown clearly in the difference spectrum. Similar results are obtained for the isopentane-quenched sample before and after annealing. 

The shifting of the rocking mode with decreasing temperature has been investigated thoroughly (48,49), These effects are observed here also. The 720 and 731 cm" peaks move to 722 and 734 cm S respectively, at 78 K. The subtraction of the spectra of a slow-crystallized sample at the two temperature extremes is shown in Figure 22. Any changes in intensities are obscured by the frequency shifts. Similar difference spectra are observed for the quenched and annealed samples. 

Figure 21, Difference spectra of slow-crystallized sample at different temperatures (1 1). (A), 78-295 K (B), 78-151 K (C), 151-259 K (D), 259-295 K. All subtractions are plotted on the same absorbance scale.

A plot of peak height intensity vs. temperature for the 720 cm" band is shown in Figure 23. In this figure and all similar ones, the data is normalized to the lowest temperature measurement and is offset for clarity. The slow-crystallized sample shows a continual rise of intensity starting at the yn relaxation, ca. 110 K. This increase ends at approximately Tg(U) or 240 K. The isopentane-quenched sample, however, remains essentially constant in intensity until rg(U), whereupon it also shows a decrease. After the quenched sample is annealed, the intensity has minor fluctuations at yn and in the upper glass transition region. 

The 909 cm band is fairly isolated in frequency from the other absorptions and would normally be integrated easily. However, as compensation by the reference beam becomes poorer from condensation on the cryostat, the absorption at 926 cm" fluctuates and imparts large errors to the integration. Therefore, the best monitor that is available is the peak height (Figure 29). Quenched and annealed samples show definite changes in their slopes at Tg(L). The yi and Tg(U) transitions correspond to the temperatures where changes of slope are noticed for the slow-crystallized sample s intensity. 

The amorphous methylene wagging bands show a temperature behavior which is more amenable to discussion. The 1352 cm" band has been calculated to result from the deformation of the methylene isolated by the GG conformation (55,56). The intensity of this band increased at elevated temperatures relative to the other amorphous wagging modes as a consequence of the higher energy of its conformation (55,56). This was also observed in the present work for the slow-crystallized sample. However, increases for the rapidly quenched systems only occurred after... 

Reaction Time. The rate of formation of to at reflux temperature is illustrated in Figure 2. The rate was estimated by taking samples from the reaction mixture at various times and examining them by x-ray diffraction. The crystallization is characterized by an unusually long induction period of about 13 hr, which is followed by a relatively slow crystal growth. No alteration of the zeolite was observed during an extra 50 hr reaction time after crystallization had been completed. 

The spontaneous but slow crystallization of monoclonic selenium (Se8) from red amorphous Se, prepared by quenching of selenium vapor (1000°C) in liquid nitrogen, depends on the storage temperature of the samples. Below 303 K, red amorphous Se(vap) is completely transformed into the monoclinic phase, whereas above 303 K transformation into the metallic phase takes place. (The relative Se8 content of the material can be determined by DSC.) In contrast, chemically prepared red amorphous Se(redn) is transformed only into the metallic phase, even below 303 K (21). 

In order to record the dynamic polymer crystal growth process in-situ, two factors are significantly important. One is the use of a very high resolution technique. Such a technique can repeatedly record the same area without damage to or significant interactions with the sample. AFM has been proved to be a successful tool to fulfill this task. The other key factor is that the polymers must have an appropriate crystallization rate. It generally takes an AFM several minutes to produce an image. This requires that the polymer has a very slow crystallization rate. The crystallization rates of most semicrystalline polymers at room temperature are too fast. 

The hysteresis that may appear depends on the direction or the scan rate of temperature change. This tendency is based on the phase transition or slow diffusion process of sample. Remarkably such hysteresis can even appear where a sample shows crystallization within the measurement temperature range. Therefore the ionic conductivity measurement is usually performed at each temperature after reaching the constant value. On the other hand, ionic conductivity is measured at scan rate of 1° to 2° C min. Therefore, when hysteresis appears during the heating or cooling process, the relationship between the phase transition and the ionic conductivity can be used in the analysis at this scan rate with the DSC measurement. It is better to use a small cell design to avoid the temperature distribution in samples. 

For temperatures lower than Tc = 95 °C and higher than Tc = 100 °C only one slope, with an Avrami exponent n = 3, is observed. This means that in these cases the nucleation corresponds to a slow crystallization mode. At low temperatures (Tc — 90 °C) the time required to crystallize is very large (tc = 300 min) because the nuclei appear within the sample very slowly. In this case, the fast crystallization process is not observed because the crystallization temperature is too low. On the other hand, for temperatures above 100 °C (Tc = 105 °C) there is also a single region of crystallization (Fig. 4.21) because the temperature of crystallization is higher, and crystallization proceeds so rapidly (mostly by crystal growth) that it seems reasonable to associate it with the fast mode. Finally, the Avrami constant G increases with Tc indicating that the number of nuclei increases with the crystallization temperature (Balta Calleja et al, 1993). 

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