Thermal-dependent polymorphic transformation

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.

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... 

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). 

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,... 

Lead oxide (PbO) exists in two modifications (polymorphs) (1) red tetragonal lead oxide (tet-PbO) (also known as a-PbO or litharge) and (2) yellow orthorhombic lead oxide (orthorhomb-PbO) (also known as P-PbO or massicot). Tet-PbO is stable at low temperatures and low pressures. The transition temperature of tet-PbO to orthorhomb-PbO is 486—489 °C and the thermal effect of the transition is 1.35 kJ mol When orthorhomb-PbO is cooled rapidly, it may remain unchanged and continue to exist at low temperatures. Eventually, it is slowly transformed into tet-PbO under external physical action. Lead oxide exists also in amorphous form. The latter s amount depends on the method of manufacture of PbO. 

One of the key features of zirconia lies in its polymorphism. Zirconia exhibits three polymorphs. The monoclinic phase is stable up to 1170°C where it transforms to the tetragonal phase, which is Itself stable up to 2370 C. Above this temperature, zirconia exists as a cubic Cap2 type phase. The reversible m- to t-Zr02 transformation is key to the use of zirconia in ceramics. First, it is reversible but occurs with a thermal hysteresis. Second, it is rapid and takes place by a diffusionless shear process similar to that of a martensitic transformation. Finally, it is dependent on particle size and occurs with a volume change (3 to 5%) [82]. 

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