Polymorphic transformation

At the temperature limits of thek stabiHty ranges, the main forms of siHca interconvert. The transformations involve a change in the secondary (nonnearest-neighbor) coordination and requke the breaking and reformation of Si—O bonds. The transformation processes, known as reconstmctive polymorphic transformations (44), are slow, as shown by the fact that the high temperature polymorphs can persist outside thek normal stabiHty range. 

Fig. 2.12. If solids undergo a shock-induced polymorphic transformation, the volume change at the transformation causes significant changes in the wave profile produced by shock loading. In the figure, is the applied pressure, Pj is the pressure of the phase transition, and HEL is the Hugoniot elastic limit.

The space lattice does not undergo polymorphous transformation. As with other kinds of fibers, no transformation of the space lattice under the effect of any physical or chemical treatment of PET fibers has yet been found. 

Boggs, J.L. Prentice, K.J. Kraeutle J.E. Crump, The Role of the Scanning Electron Microscope in the Study of Solid Propellant Combustion , inavwepsceiiu ir h/zo yiyoy) do) u,u, Graber, F.C. Rauch A.J. Fanelli, Observation of Solid-Solid Polymorphic Transformation in 2,4,6-Trinitro Toluene , JPhChem 73, (10), 3514—15 (1969) 39) J.E. Crump, J.L. Prentice K.J. Kraeutle Role of Scanning Electron Microscopy in the Study of Solid Propellant Combustion. Part 11—Behavior of Metal Additives , NavWepsCentr TP-5142-PT-2 (1969) 40) J.A. Markham A.R. Cox, Applications... 

XRD can be used for studies of polymorphic transformations in bloom formation. In combination with DTA, XRD has been used to study polymorphic transformations of Ca stearate and Ca stearate monohydrate [338]. Full pattern refinement has been used for quantitative analysis of mixtures of crystalline... 

Polymorphs are different crystal forms of the same compound [55], They are usually prepared by crystallization of the drug from different solvents under diverse conditions. However, exposure to changes in temperature, pressure, relative humidity, and comminution which are encountered in processes such as drying, granulation, milling, and compression may also lead to polymorphic transformations. 

T Matsumoto, N Kaneniwa, S Hihuchi, M Otsuka. Effects of temperature and pressure during compression on polymorphic transformation and crushing strength of chlorpropamide tablets. J Pharm Pharmacol 43(2) 74-78, 1991. 

Preferred orientation of the particles must be minimized. One of the most effective ways to achieve this is to reduce the particle size by grinding the sample [1], As already discussed in Section III.A, however, grinding can disorder the crystal lattice. Grinding can also induce other undesirable transitions, such as polymorphic transformations [59]. In order to obtain reproducible intensities, there is an optimum crystallite size. The crystallites have to be sufficiently small so that the diffracted intensities are reproducible. Careful studies have been carried out to determine the desired crystallite size of quartz, but no such studies have been reported for pharmaceutical solids [60]. Care should be taken to ensure that the crystallites are not very small, since decreased particle size can cause a broadening of x-ray lines. This effect, discussed earlier (Eq. 9), usually becomes apparent when the particle size is below 0.1 jum. 

Microscopy is used to identify crystal morphology and size, and to assess physical form issues like agglomeration and solvent occlusion. It can be used to observe polymorphic transformations in real time with the addition of a hot stage fitting. 

Near and Mid infrared spectroscopy can be used to monitor crystal form and polymorphic transformations, as well as solution phase concentration. 

FBRM provides a real time chord length distribution within the crystallizer. The technique is excellent for determining the onset of nucleation and in following general growth rate and rate transitions associated with polymorphic transformation, reaching equilibrium and attrition effects. 

Their normal crystal structure, at ambient conditions, corresponds to the body-centred cubic cI2-W-type structure. At very low temperatures, the close-packed hexagonal hP2-Mg-type structure has been observed for Li and Na, while for Rb and Cs the face-centred cubic close-packed cF4-Cu-type structure is known at high pressure. No polymorphic transformation has been reported for potassium. 

For many years, it was thought, purely on an empirical basis, that if the enthalpy change for a given reaction were negative, that is, if heat were evolved at constant pressure, the transformation could occur spontaneously. This rule was verified for many reactions. Nevertheless, numerous exceptions, exist such as the polymorphic transformation of a quartz to (3 quartz at 848 K and atmospheric pressure ... 

LiBH undergoes first a polymorphic transformation around 110°C, then melts with an endothermic peak centered around 280°C, and evenmaUy at the atmospheric pressure it decomposes with an endothermic peak centered at 470°C through the following reaction path... 

X. Yang, J. Lu, X. Wang and C.B. Ching, In situ monitoring of the solution-mediated polymorphic transformation of glycine Characterization of the polymorphs and observation of the transformation rate using Raman spectroscopy and microscopy, J. Raman Spectrosc., 39, 1433-1439 (2008). 

T. Ono, J.H. ter Horst and P.J. Jansens, Quantitative measurement of the polymorphic transformation of L-glutamic acid using in-situ Raman spectroscopy, Cryst. Growth Des., 4, 465-469 (2004). 

S.-Y. Lin, W.-T. Cheng and S.-L. Wang, Thermal micro-Raman spectroscopic study of polymorphic transformation of famotidine under different compression pressures, J. Raman Spectrosc., 38, 39 3 (2007). 

The above-mentioned on-line spectroscopic techniques have involved granulation monitoring for particle size assessments and end-point control however there are other concerns involved with wet granulation which include polymorphic transformations or solvate formation that can affect end product quality. ... 

The presence of an unwanted polymorph, even in small proportions, in an API can significantly affect its response by inducing its polymorphic transformation. A number of API polymorphs have been determined using NIR " " or Raman spectroscopy. "" Blanco et al. applied this technique to the determination of the crystalline forms in amorphous antibiotics " " and in tablets. ""... 

Some authors have examined polymorphic transformations resulting from hydration of the product " or the passage of time. ""... 

M. Blanco, M. Alcala, J.M. Gonzalez and E. Torras, NIR Spectroscopy in the study of polymorphic transformations. Anal. Chim. Acta, 567, 262-268 (2006). 

In 2007, in a very exhaustive paper, Paradies and coworkers carried out a comprehensive structural characterization of the colorless and yellow forms of Af-hydroxyphthalimide (NHPI) by means of single-crystal X-ray diffraction, FTIR and Raman spectroscopies and scanning electron microscopy. In the yellow form, the Af-hydroxyl group is significantly out of the plane (1.19°), but the Af-hydroxyl group in the colorless form is only 0.06° out of the plane. The irreversible conversion of the colorless crystalhne form to the yellow crystalhne form is more like a dynamic isomerism than a polymorphic transformation. 

The oxidation of an API in an oral liquid formulation can be difficult to control due to the trace amounts of impurities, which may be present from the API or excipient vendor, and oxidation and photolysis have relatively low activation energies (2-12 Kcal/mol) compared to solvolysis, dehydration, and polymorphic transformations (10-56 Kcal/mol) (Table 11) (10). 

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