Crystal polymorphism studies

Applications The general applications of XRD comprise routine phase identification, quantitative analysis, compositional studies of crystalline solid compounds, texture and residual stress analysis, high-and low-temperature studies, low-angle analysis, films, etc. Single-crystal X-ray diffraction has been used for detailed structural analysis of many pure polymer additives (antioxidants, flame retardants, plasticisers, fillers, pigments and dyes, etc.) and for conformational analysis. A variety of analytical techniques are used to identify and classify different crystal polymorphs, notably XRD, microscopy, DSC, FTIR and NIRS. A comprehensive review of the analytical techniques employed for the analysis of polymorphs has been compiled [324]. The Rietveld method has been used to model a mineral-filled PPS compound [325]. 

Recently, by crystal structure studies the number of different polymorphs of Copper Phthalocyanine Blue has been extended to nine, various of which are differing mainly in herringbone-type interaction [17]. 

The TiC>2 samples, mounted in a helium cryostat (10-300 K), are lmm-thick (001) and (110)-cut single crystals to study the electronic response with the (THz) electric field perpendicular ( L) and parallel (//) to the c axis, respectively. Rutile is the most common and stable TiC>2 polymorph and has a tetragonal structure with a=4.6A and c=2.9A. The band gap of rutile is 2.9 eV at room temperature, increasing slightly at lower temperatures. 

Another approach is to compare polymorphic forms of the same compound. This allows the comparison of molecules with the same constitution and configuration in different crystal fields. Do these different potential environments lead to different conformations There are few comparative crystal structural studies of organic polymorphs. For d-mannitol the molecules had the same conformation in all three polymorphic forms studied with differences in torsion angles for the two that were studied in detail of only a few degrees (see Table I). 

In this chapter we focus on atomistic predictions of thermophysical and mechanical properties of HMX crystals and liquid important to the development of reliable mesoscale equations of state. The outline of the remainder of the chapter is as follows In section 2 we describe briefly the philosophy and overall approach we have taken to force field development, including the results of quantum chemistry calculations for HMX and smaller model compounds that were used in the force field parameterization. The focus of section 3 is on the properties of liquid HMX, for which experimental data are completely lacking. Structural, thermal, and mechanical properties of the three pure crystal polymorphs of HMX are presented in section 4, where the results are compared to the available experimental data. At the ends of sections 3 and 4 we discuss briefly the importance of the various properties with mesoscale models of high explosives, with an emphasis on conditions relevant to weak shock initiation. We conclude in section 5, and provide our opinions (and justifications, based on our interactions with mesoscale modelers) regarding which HMX properties and phenomena should comprise the next targets for study via atomistic simulation. 

In the context of milk fat, DSC is most widely used to measure temperatures and heats of transitions (phase changes). It is also used to measure specific heat, solid fat content, crystallization kinetics constants and fat purity, and in the study of fat crystal polymorphism. 

In other studies, solutions of high supersaturation were employed which yielded complex, poorly defined solid phases. The multi-picity of complex crystal polymorphs and hydrates in the calcium phosphate system is now well recognized and must be considered in the interpretation of experiments involving the removal of phosphate from natural waters (]A, 15). 

To this point this account of instances of concomitant polymorphs has been phenomenological. We have discussed the thermodynamic and kinetic crystallization of polymorphs. There is still the question if any insight concerning controlling the polymorph obtained can be gained from the study of the crystal structure of concomitantly crystallized polymorphs. A qualitative attempt was made to see if details of the... 

Fig. 6.23 Normal incidence polarized reflection spectra of the two forms molecule 6-XVII (R = Et, R = OH). For each crystal modiflcation there are two spectra measured with the light polarized along each of the two directions (the so-called principal directions), as indicated in the upper right hand corner, which also shows the projection of the molecule(s) onto the crystal face studied, (a) Triclinic polymorph, (100) face (b) monoclinic polymorph, (100) face. (From Tristani-Kendra and Eckhardt 1984, with permission.)...

Antipin, M. Y, Timofeeva, T. V., Clark, R. D., Nesterov, V. N., Dolgushin, F. M., Wu, J. and Leyderman, A. (2001). Crystal structures and molecular mechanics calculation of nonlinear optical compounds 2-cyclooctylamino-5-nitropyridine (COANP) and 2-adamantylamino-5-nitropyridine (AANP). New Polymorphic modifcation of AANP and electrooptic effects. /. Mater. Chem., 11, 351-8. [213] Apperley, D. C., Fletton, R. A., Harris, R. K., Lancaster, R. W., Tavener, S. and Threlfall, T. L. (1999). Sulfathiazole polymorphism studied by magic-angle spinning NMR. / Pharm. ScL, 88, 1275-80. [135,139f]... 

Gavezzotti, A., Flippini, G., Kroon, J., van Eijck, B. P. and Klewinghaus, P. (1997). The crystal polymorphism of tetrolic acid (CHsC CCOOH) a molecular dynamics study of precursors in solution, and in crystal structure generation. Chem. Fur. J., 3, 893-9. [183]... 

Louis Pasteur was the first scientist to study the effect of molecular chirality on the crystal structure of organic compoimds [23], finding that the resolved enantiomers of sodium ammonium tartrate could be obtained in a crystalline form that featured nonsuperimposable hemihedral facets (see Fig. 9.1). Pasteur was quite surprised to learn that when he conducted the crystallization of racemic sodium ammonium tartrate at temperatures below 28 °C, he also obtained crystals of that contained nonsuperimposable hemihedral facets. He was able to manually separate the left-handed crystals from the right-handed ones, and foimd that these separated forms were optically active upon dissolution. More surprising was the discovery that when the crystallization was conducted at temperatures exceeding 28 °C, he obtained crystals having different morphologies that did not contain the hemihedral crystal facets (also illustrated in Fig. 9.1). Later workers established that this was a case of crystal polymorphism. 

Due to the difference of the polymer adsorption onto different faces of different crystal forms, polymer adsorption onto the crystal face has played an important role in crystal polymorphic transformationJ Garti and Zour have studied the effects of surfactants on the polymorphic transformation of glutamic acid. Glutamic acid has two crystal forms, a and p, with the p-form being more stable than the ot-form. Those surfactants that preferentially adsorb onto the surface of the ot-growing crystals retard the transformation of the ot-form to the p-form. A Langmuir analysis indicates that the kinetic coefficient of crystal polymorphic transformation is related to the volume of the surfactant adsorbed at the crystal surface. 

There is no doubt that single crystal X-ray diffraction is a powerful technique for the study of polymorphs and solvatomorphs, but it is equally apparent that this methodology is not well suited for the routine evaluation of the crystalline state of powdered solids. It is definitely not a technique that one would want to use in the determination of polymorphic phase impurities in a sample, since by definition only a single crystal is studied at one time. That crystal would certainly be phase-pure, and its quantitative analysis not... 

In preformulation, IR may be applied to the study of polymorphism of solid crystals. Polymorphs pose different IR characteristics, and they may be used as a tool for fingerprint identification. In certain cases, the absorbance at a selected wavelength is proportional to the amount of specific polymorph present. This relationship may be used for quantitative determination in solid states for a given polymorph. IR has the ability to differentiate isomer groups such as cis-trans double bond compounds as well as position isomers in an aromatic ring. 

A study example concerning the solid structure of a-amino acid, polypeptide and a protein is given to introduce the basis of the chemical shift of proton NMR precisely, and studies to do this have only recently been undertaken. Some interesting work has been done, including the discrimination of amino acid crystal polymorphism, conformational analysis of polypeptides and fibrous proteins, and the determination of the N—H bond length in polypeptides. 

It was shown already above that cis-1,4-poly butadiene melts in one step with the expected entropy of fusion. In contrast, cis-l,4-poly(2-methylbutadiene) (natural rubber) has a more complicated fusion and crystallization behavior . The reported entropy of fusion of the common monoclinic (P2ja) crystal polymorph is only 14.4 J/(K mol), less than half of the expected value. The crystal structure has been reported statistically disordered, but only relative to packing of chains that are mirror images of each other along the crystallographic a-axis. Such geometric disorder cannot account for a 50% decrease in entropy of fusion. A full study of the thermod5mamic functions as available for the polybutadienes would be of value. 

---