Polymorphism and polymorphs

Monoclinic sulphur (in addition to the more common orthorhombic form) had also been recognized and documented by a number of other people (see, e.g. Partington 1952 which also contains many early references to polymorphism and polymorphic materials). 

Polymorphic structures of molecular crystals are different phases of a particular molecular entity. To understand the formation of those phases and relationships between them we make use of the classic tools of the Phase Rule, and of thermodynamics and kinetics. In this chapter we will review the thermodynamics in the context of its relevance to polymorphism and explore a number of areas in which it has proved useful in understanding the relationship between polymorphs and polymorphic behaviour. This will be followed by a summary of the role of kinetic factors in detecting the growth of polymorphic forms. We will then provide some guidelines for presenting and comparing the structural aspects of different polymorphic structures, with particular emphasis on those that are dominated by hydrogen bonds. 

Figure Al.3.23. Phase diagram of silicon in various polymorphs from an ab initio pseudopotential calculation [34], The volume is nonnalized to the experimental volume. The binding energy is the total electronic energy of the valence electrons. The slope of the dashed curve gives the pressure to transfomi silicon in the diamond structure to the p-Sn structure. Otlier polymorphs listed include face-centred cubic (fee), body-centred cubic (bee), simple hexagonal (sh), simple cubic (sc) and hexagonal close-packed (licp) structures.

Tomanek D, Wilke S and Scheffler M 1997 Hydrogen-induced polymorphism of the Pd(110) surface Phys. Rev. Lett. 79 1329... 

Parrinello M and Rahman A 1981 Polymorphic transitions In single crystals a new molecular dynamics method J. Appl. Phys. 52 7182-90... 

Diele S, Brand P and Saokmann H 1972 X-ray diffraotion and polymorphism of smeotio liquid orystals. II. D and E modifioations Mol. Cryst. LIq. Cryst. 17 163-9... 

Flardouin F, Levelut A M, Aohard M F and Sigaud G 1983 Polymorphism in polar mesogens. I—Physioo-ohemistry and struotural aspeots J Chem.Phys. 80 53... 

Prost J and Barois P 1983 Polymorphism in polar mesogens. II—Theoretioal Aspeots J.Chem.Phys 80 65-81... 

Hardouin F and Levelut A M 1980 X-ray study of reentrant polymorphism N-S -N-S in a pure liquid orystal oompound J.Physlque4 41-56... 

Alexandridis P, Olsson U and Lindman B 1997 Structural polymorphism of amphiphilic copolymers Six lyotropic liquid crystalline and two solution phases in a poly(oxybutylene)-poly(oxyethylene)-water-xylene system Langmuir 23-34... 

It is a white, deliquescent solid, very powdery, which exhibits polymorphism on heating, several different crystalline forms appear over definite ranges of temperature -ultimately, the P4O10 unit in the crystal disappears and a polymerised glass is obtained, which melts to a clear liquid. 

Many molecules are obtained and used in a crystalline form, the nature of which can have e significant impact on their properties and behaviour. Moreover, it is sometimes possible foi a given material to exist in more than one crystalline form, depending upon the conditions under which it was prepared. This is the phenomenon of polymorphism. This can be important because the various polymorphs may themselves have different properties. It is Iberefore of interest to be able to predict the three-dimensional atomic structure(s) that a gi en molecule may adopt, for those cases where it is difficult to obtain experimental data and also where one might wish to prioritise molecules not yet synthesised. 

Many different approaches have been suggested as possible approaches to this problem, from the 1960s onwards [Verwer and Leusen 1998]. What is obvious from all of these ellorts is that this is an extremely difficult problem. Both thermodynamics and kinetics can be important in determining which crystalline form is obtained under a certain se1 of experimental conditions. Kinetic effects are particularly difficult to take into accouni and so are usually ignored. A proper treatment of the thermodynamic factors would lequire one to deal with the relative free energies of the different possible polymorphs... 

Chatfield C and A J CoHns 1980. Introduction to Multivariate Analysis. London, Chapman Hall. Desiraju G R 1997. Crystal Gazing Structure Prediction and Polymorphism. Sdence 278 404-405. Everitt B.S. 1993 Cluster Analysis. Chichester, John Wiley Sons. 

HMX, the highest density and highest energy soHd explosive produced on a large scale, primarily for military use, exists in four polymorphic forms. The beta form is the least sensitive, most stable, and the type requited for military use. The mole fraction products of detonation of HMX in a calorimetric bomb are 3.68 N2, 3.18 H2, 1.92 CO2, 1.06 CO, 0.97 C, 0.395 NH3, and 0.30 H2. 

Absorption of x-rays by a powdered sample of soHd fat has been a useful method for determination of polymorphic character as discussed eadier. The a, and P forms may be distinguished however, interpretation is made more difficult because subsets of the P and P forms have often been encountered. Also, a fat may contain mixtures of polymorphic forms and properties may therefore be difficult to relate to the spectra. 

Unlike other synthetic polymers, PVDF has a wealth of polymorphs at least four chain conformations are known and a fifth has been suggested (119). The four known distinct forms or phases are alpha (II), beta (I), gamma (III), and delta (IV). The most common a-phase is the trans-gauche (tgtg ) chain conformation placing hydrogen and fluorine atoms alternately on each side of the chain (120,121). It forms during polymerization and crystallizes from the melt at all temperatures (122,123). The other forms have also been well characterized (124—128). The density of the a polymorph crystals is 1.92 g/cm and that of the P polymorph crystals 1.97 g/cm (129) the density of amorphous PVDF is 1.68 g/cm (130). 

Some electrical properties are shown in Table 3. Values of other parameters have been pubflshed (146). Polymorphism of the PVDF chains and the orientation of the two distinct dipole groups, —CF2— and —CH2—, rather than trapped space charges (147) contribute to the exceptional dielectric properties and the extraordinarily large piezoelectric and pyroelectric activity of the polymer (146,148,149). 

DCPA inhibits the growth of grass species by dismpting the mitotic sequence, probably at entry (190). DCPA influences spindle formation and function (181) and causes root-tip swelling (182) and britde shoot tissue (191). It has been reported that DCPA, like colchicine and vinblastine, attests mitosis at prometaphase and is associated with formation of polymorphic nuclei after mitotic arrest (192). Pronamide also inhibits root growth by dismpting the mitotic sequence in a manner similar to the effect of colchicine and the dinitroanilines (193,194). Cinmethylin and bensuhde prevent mitotic entry by unknown mechanisms (194). 

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