Thermal-dependent polymorphic

Figure 2.14 Resolution results of a thermal-dependent polymorphic transformation monitored by Raman imaging, (a) Distribution maps, (b) pure spectra, and (c) thermal process profiles.

A literature survey reveals that the term thermotropism is mostly used to describe temperature-dependent ordering of LCs—usually referring to phase changes that occur between the temperature below which the LC crystallizes and the temperature above which it becomes an isotropic liquid. Since the study of crystals is a separate (but somewhat related) field, the term can easily be borrowed to describe thermally induced polymorphic transitions in crystals. 

Polymorphism. Many crystalline polyolefins, particularly polymers of a-olefins with linear alkyl groups, can exist in several polymorphic modifications. The type of polymorph depends on crystallisa tion conditions. Isotactic PB can exist in five crystal forms form I (twinned hexagonal), form II (tetragonal), form III (orthorhombic), form P (untwinned hexagonal), and form IP (37—39). The crystal stmctures and thermal parameters of the first three forms are given in Table 3. Form II is formed when a PB resin crystallises from the melt. Over time, it is spontaneously transformed into the thermodynamically stable form I at room temperature, the transition takes about one week to complete. Forms P, IP, and III of PB are rare they can be formed when the polymer crystallises from solution at low temperature or under pressure (38). Syndiotactic PB exists in two crystalline forms, I and II (35). Form I comes into shape during crystallisation from the melt (very slow process) and form II is produced by stretching form-1 crystalline specimens (35). 

For analysis of experimental results, the static model density must be used to eliminate noise, truncation effects, and thermal smearing. Some caution is called for, because the reciprocal space representation of the Laplacian is a function of F(H) H2, and thus has poor convergence properties.2 This difficulty is only partly circumvented by use of the model density, as high-resolution detail may be quite dependent on the nature of the model functions, as is evident in the experimental study of the quartz polymorph coesite discussed in chapter 11. 

The results discussed show that crystal structure transformations are considerably dependent on the thermal history of the samples to be more specific, the crystallite size, particle size and surface area have measurable effects on the transformation. It would, therefore, probably be difficult to reproduce strictly transformation data with different samples. The magnitudes of these effects are, however, not too great to result in the wide variability of temperatures of polymorphic transformations. The wide variations in transformation temperature can only be due to other factors... 

SPS takes two different conformations in its crystal, TT and TTGG conformations, depending on the crystallization conditions, and exhibits a very complex polymorphic structure [6-14]. One is the TT conformation which appears when SPS crystallizes from the melt. Two different transcrystals are reported when SPS takes the TT conformation in the crystal, a-form [15-19] and 3-form. The identity period is 5.06 A (Figure 18.3) in both crystal forms. The 3-form is more thermally stable than the a-form. Figure 18.4 shows the detailed structure of the 3-form [6,10]. 

The kinetics of thermal decomposition of three of the modifications were studied by thermogravimetry, IR spectroscopy and optical and electron microscopy (Nedelko et al. 2000), with the conclusion that the rate increases in the series a > y > s. However, it was found that the results for a particular polymorph also depend upon the morphological features of the crystals as well as their size distribution and mean size. 

Polymorphism and Milkfat High cooling rates (>l°C/min), or high levels of supercooling (>15°C), lead to the rapid formation of metastable a nuclei (34). The persistence of these unstable nuclei is dependent on thermal treatments that occur after crystallization. These nuclei may remain in the a form or convert... 

Timms (21) has heat of fusion to 17.7-22.3 kcal/kg for milkfat, 24-31 kcal/kg for fully hardened milkfat, 26-29 kcal/kg for cocoa butter in the p polymorph, 22.6 kcal/kg for refined, bleached, and deodorized (RBD) palm oil, 29.7 kcal/kg for RBD palm kernel oil, 26.0 kcal/kg for RBD coconut oil, 31.6 kcal/kg for fully hardened palm kernel oil, and 31.2 kcal/kg for fully hardened coconut oil. The heat of fusion is an empirical physical property dependent on the thermal history or tempering of the oil. 

The open frameworks of zeolites are slightly less stable than the corresponding condensed structures [15,16] into which they will transform during severe thermal treatment. Nevertheless, the difference in energy between a-quartz, the most stable polymorph of silica, and siliceous faujasite, one of the most open and least stable, is only about 15 kj mol k The extensive occurrence of aluminosilicate zeolites and their widespread utility in industry therefore depend heavily upon both the strengths of their T-O bonds (e.g. Si-O 466 kJ mol ), which render them stable with respect to framework rearrangement. The challenge with many of the newer materials is that their stability with respect to transformation into alternative condensed structures is considerably lower and they frequently collapse on dehydration or other means of activation. It is for this reason that only a small subset of the many open-framework families of materials can be rendered truly nanoporous,... 

The crystal structure of zirconia and the catalytic properties of SZ generally depend on the synthesis method and thermal treatment adopted. In particular zirconia crystallises in three different polymorphs characterised by monoclinic, tetragonal and cubic symmetry. Among them only the tetragonal SZ phase displays significant catalytic properties [5-7]. Unfortunately, the synthesis of the pure tetragonal polymorph is difiBcult and, in the absence of promoted oxides [8], it could be stabilised only through an accurate control of the synthesis parameters, with particular attention to the thermal treatments. 

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