Polymorphous materials

Oxides. Owing to the importance as nuclear fuel material, actinide oxides have been intensively investigated. These are very compHcated compounds because of the formation of non stoichiometric or polymorphic materials. Actinide oxides are very heat-resistant and Th02 is the highest... 

Besides phthalocyanine pigments developed for office copiers, new polymorph materials for laser printer utilization have been prepared. These materials, whose absorption characteristics were extended to the near-infrared region, were prepared by vacuum sublimation techniques. Recently, for safety reasons and to reduce the cost of production, solvent-pigment interactions have received great attention [41]. 

A recent report presented Continuous Composition Spread (CCS) approaches (37) to identify thin film dielectrics with varying composition among the system Zr-Sn-Ti-O (78). A structural-driven approach was applied to the discovery of a high dielectric perovskite polymorphic material (79). 

A third compilation intended to be devoted to polymorphic materials is that of Kuhnert-Brandstatter (1971). The body of this book is an identification table for hot stage studies of pharmaceutical materials (see Sections 4.2 and 7.2), in which materials are arranged by increasing melting point, with eutectic data for mixtures with azobenzene and benzil. There is considerable descriptive detail on the melting behaviour and identification and description of polymorphic forms, albeit only microscopic determinations, for approximately 1000 pharmaceutically important... 

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). 

The topochemical principles have also been applied to the interpretation of mframolecular photochemical reactions of polymorphic materials depending on polymorphic form. 1,14-cyclohexacosanedione 6-XLII can be prepared as conformational dimorphs that undergo Norrish type II photochemistry which can be correlated with the molecular conformations in the two modifications, one leading to a cw-butanol, while the other leads to a trans product (Gudmunsdottir et al. 1996). Similarly, these principles can be invoked to understand the relative photochemical stability of two polymorphs of the active ingredient in a compound developed for the treatment of psoriasis (Lewis 2000). 

In spite of the wealth of information contained therein, as of this writing there are no citations to this 1995 compilation of Whitaker, which is a useful resource for reference powder diffraction patterns of many organic colorants, polymorphic and non-polymorphic. This may be due to the fact that it was published in a specialist monograph or that subsequent workers would use and cite the primary sources cited by Whitaker. Whatever the reason, it appeared to be of some benefit and convenience to the reader to compile here those primary references to polymorphic materials that are designated as colourants by the Colour Index, with the primary sources given by Whitaker. Whitaker also presents references for many other related dye materials that have not received such a designation. 

Other than its ability to demonstrate the anhydrous nature of genuine polymorphic materials, one extremely useful aspect of TG analysis is in the differentiation and characterization of solvatomorphs. The methodology is particularly useful in the determination of a solvate phase in the presence of its anhydrate phase, or vice versa. 

Polymorphic Materials Phase 1 Bands (cm ) Phase 2 Bands (cm )... 

Similar to polymorph materials, nonsolvate materials and solvate exhibit different physical properties and behaviors. Figure 2-17 shows solubility curves of typical nonsolvate and solvate solids as a function of solvent composition and temperature. As shown in the figure, the nonsolvate solid and the solvate solid cross over at composition Aj. Above composition Aj solvate solid has lower solubility (shown as a solid line) than nonsolvate (shown as a dotted line), and is more stable. Similarly, below composition Aj, a nonsolvate sohd has a lower... 

Materials that exhibit polymorphism present an interesting problem. First, it is necessary to control conditions to obtain the desired polymorph. Second, once the desired polymorph is obtained, it is necessary to prevent the transformation of the material to another polymorph. Materials that form polymorphs often will transform from one form to another. This is known as a polymorphic transition. Often a simple change of temperature will cause a material to change form. In many cases, a particular polymorph is metastable, meaning that after crystallizing the material will eventually transform into a more stable state. This transformation can be relatively rapid in some systems while in others it can be infinitely slow. At room temperature, a diamond is a metastable form of carbon. 

It is also possible to create top contact devices using printing processes. Transfer and contact printing has been explored by several groups. Deposition of electrodes from solution on pentacene and related polymorphic materials was once thought impossible because of the solvent phase conversion problem. Using an advanced femtoliter inkjet system in which the solution dries before hitting the substrate, however, Sekitani et al. have been able to directly print top contact devices [93]. 

Other than its ability to demonstrate the anhydrous nature of genuine polymorphic materials, the entire utility of TG analysis is in the differentiation and characterization of solvate species. The multitude of solvate species formed by the disodium salt of fl -4-[[[l-(4-fluorophenyl)-3-(l-methylethyl)-lH-indol-2-yl]-ethynyl]-hydroxyphosphinyl]-3]-hydroxybutanoic acid was most effectively characterized by TG analysis of materials exposed to various relative humidity values [58]. Representative thermograms of the different hydrates are shown in Fig. 5, where it is evident that the well-defined plateaus observed upon completion of the individual dehydration steps were ideally suited for the evaluation of the hydration state of an isolated material. 

The type and size of crystals in polymorphous materials depend on the crystallization process itself. In particular, the cooling conditions will affect the crystal size and shape and can produce substances with different properties. This topic is of particular interest for manufacturing processes of pharmaceutical and food products, as is discussed in more detail in [3, 8, 11, 15]. 

This concept of a size-dependent phase stability in polymorphic materials is addressed here by confronting this model and testing it against the well-characterised H-bonded compound L-glutamic acid (l-GA) which is a material with two mono-tropically related polymorphic modifications the metastable a-form that transforms into the stable p-form through a solvent-mediated transformation [66]. 

As previously stated in Chapter 3, isotaetie polypropylene is a polymorphic material with four basic crystalline forms, namely the monoelinie (a), trigonal (P), orthorhombic (y) and mesomorphic smectic (intermediate state between ordered and amorphous phase) forms. The a-form exhibits excellent modulus and tensile strength but poor fracture toughness. ... 

Morphological features, kinetics of growth, formation of structure and melting behavior of iPP spherulites were discovered [1, 2 and references therein]. During the crystallization of iPP, being a polymorphic material with several modifications [5], different types of spherulites may develop, which imply crystallites of the a-, p- and v-modification [1, 2, 5-7]. All these polymorphs consist of right-, and/or left-handed threefold helices with 0.65 nm chain axis repeat distance. The molecular... 

During the manufacture of polymer-polymer composites, the amorphous-crystalline structure of the constituents can also be problematic. It is well known that polymers are polymorphic materials therefore, they can undergo many different crystalline and amorphous modifications. It is characteristic of plastic that the physical and chemical properties of these modifications can differ significantly. The main difference among the polymorph modifications is that they are stable at different temperature and pressure ranges, and they are interconvertible. 

Aluminum oxide is a polymorphic material [132,133]. The thermodynamically stable phase is o -Al203 (corundum, S.G. R-3c, Z = 6) where all A1 ions are... 

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