Multiple crystal forms

As with ceramics and metals, polymer crystals can have multiple crystal forms. Polyethylene has a metastable monoclinic form and a orthohexagonal high pressure form. A list of some of the more common polymers and their corresponding crystal strnctnres is given in Table 1.24. Finally, X-ray diffraction can be used to determine the amorphous to crystalline ratio in semicrystalUne polymers in much the same way that Eq. (1.61) can be used. Figure 1.66 shows a schematic illustration of the X-ray diffraction patterns for semicrystalline and amorphous polyethylene. The estimation of crystalline content is based upon a ratio of the peak areas in the two samples. 

To illustrate the interrelationship of these techniques, a summary of the multidisciplinary approach for the characterization of materials is presented. DSC, TGA, and XRD analyses are initially employed to characterize some of the basic physical properties of the material. DSC analysis of several lots of material produces curves that exhibit endothermic maxima at different temperatures, suggesting the existence of multiple crystal forms. Whereas DSC is utilized to identify the thermal properties of these materials, XRD is the definitive technique utilized to determine the presence of crystal forms. If XRD analysis shows that different powder diffraction patterns exist, corroborating evidence has been generated confirming that the lots of material under investigation comprise different crystal forms. In most cases, DSC and XRD... 

Compound 3 is a material that exhibits multiple crystal form changes at elevated temperatures. Thermal data from single crystals of this material are presented in Figure 7.22 (overlay of the DSC and TGA curves) overlay of the DSC curve, which exhibits endothermic maxima at 48.8, 86.4, 170.1, and 190.4°C, exothermic maxima at 119.4 and 173.1°C, and the TGA curve, which exhibits a 6.9% weight loss at 100°C. 

In spite of the efforts of a great number of research groups worldwide, and of a familiarity with the experimental factors that can lead to multiple crystal forms, our ability to predict or control the occurrence of polymorphism is still embryonic. In many cases the crystallization of a new crystal form or of an amorphous phase of a given substance turns out to be the result of serendipity [7] rather than a process under complete human control. 

New Developments in Detecting and Characterizing Multiple Crystal Forms... 

Qualitative analysis of powder patterns can be used to determine if multiple samples are the same crystal form or if multiple crystal forms have been produced. Mixtures of samples can also be evaluated. When mixtures are obtained, XRPD can also be used in a quantitative mode to calculate the amount of each phase present. 

Fig. 5 Benefit of obtaining multiple crystal forms to aid with protein-ligand crystallographic work. Different crystal forms of mutant RadA arranged from the most open (left) to the most occluded (right) binding site (Marsh and Hyvonen, manuscript in preparation). Atoms involved in crystal contacts are coloured red and atoms that comprise the ligand binding site are shown in blue. The crystal symmetry mates closest to the binding sites are shown as cartoons. Space groups are indicated...

Surveys of the CSD " have indicated that the prevalence of polymorphism in single-component organic molecules (circa 1.4%) is about the same as exhibited by all organic multicomponent crystals sustained by hydrogen bonding (circa 1.9%), and that the tendency of each class to form multiple crystal forms is significantly less than exhibited by APIs. The working hypothesis is that cocrystal... 

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