Metastable monoclinic crystals

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. 

These crystal modifications differ in their molecular and crystal structures as well as in their physical properties. Many types of crystalline modifications are reported, including a stable orthorhombic phase and metastable monoclinic phase for PE a, and y forms for isotactic polypropylene (/-PP) trigonal and orthorhombic phases for polyoxymethylene a and y forms for Nylon 6 and others. Poly(vinylidene fluoride) (PVF), for example, appears in at least four types of crystalline modification (Lovinger, 1985 Dunn Carr, 1989). 

Hydroquinone, p-C6H4(OH)2, also known as quinol in the older literature) exists in three polymorphic forms (or crystalline modifications). a-Hydroquinone is the stable form at room temperature, whereas the metastable monoclinic y-form can be prepared by sublimation or rapid evaporation of a solution in ether. Crystallization of hydroquinone from a common solvent such as methanol generally yields a clathrate with guest solvent inolecules trapped inside cavities of the (3-hydroquinone host lattice.". The existence of the (3 polymorph of hydroquinone (empty P-hydroquinone clathrate), which can be obtained by crystallization from n-octane, was reported in 1981. Described in this article are the structural features of these compounds, and some recent developments are also covered. 

Primrose chrome This is a pale, greenish-yellow pigment with a metastable orthorhombic crystal structure containing a solid solution of lead chromate and lead sulfate. The crystals are normally stabilized by additives. Due to a less-stable crystal form, they have poorer fastness properties compared to other monoclinic chrome pigments. 

Crystals of polyethylene, PE, exist in three different modifications. The well known orthorhombic modification, a metastable monoclinic phase, and a hexagonal phase that appears at high pressure. 

Isotactic polypropylene (iPP) exhibits a stable crystalline a form consisting of chains in the 3i helical conformation (tgtgtg), packed in a monoclinic unit cell. Left- and right-handed helices face each other in the a form. The metastable -form crystals contain hexagonally packed 3i helical chains arranged in groups of the same helical handedness (left or right). There is also a smectic, or partially ordered, form... 

The monoclinic crystal form of polyethylene (also referred to as the triclinic form) is a metastable phase formed under conditions of elongation [18,19]. It may be present to a small extent in commercial samples that have undergone cold working after initial molding. Temperatures in excess of 60-70°C cause it to revert to the orthorhombic form [20]. The monoclinic phase is sometimes present in nascent granules of ultrahigh molecular weight polyethylene due to... 

Anhydrous Oxalic Acid. The anhydrous form of oxaUc acid is odorless and colorless. It exists in two crystal forms, ie, the rhombic or a-form and the monoclinic or P-form (3). The rhombic crystal is thermodynamically stable at room temperature, but the monoclinic form is metastable or slightly stable. The main difference between the rhombic and monoclinic forms exists in the melting points which are 189.5 and 182°C, respectively (Table 1)-... 

Pressure-induced phase transitions in the titanium dioxide system provide an understanding of crystal structure and mineral stability in planets interior and thus are of major geophysical interest. Moderate pressures transform either of the three stable polymorphs into the a-Pb02 (columbite)-type structure, while further pressure increase creates the monoclinic baddeleyite-type structure. Recent high-pressure studies indicate that columbite can be formed only within a limited range of pressures/temperatures, although it is a metastable phase that can be preserved unchanged for years after pressure release Combined Raman spectroscopy and X-ray diffraction studies 6-8,10 ave established that rutile transforms to columbite structure at 10 GPa, while anatase and brookite transform to columbite at approximately 4-5 GPa. 

For example, the difference between the monoclinic CaC2 phases is the presence of only one type of acetylide ion in phase II and two distinct C2 species in phase III, as concluded from their crystal structures and from C-NMR studies (Fig. 8.3) [8]. The transformation between phases II and III is induced by heating phase II above 150°C until the metastable phase. III, is formed. Phase III remains stable even when being cooled down to room temperature. However, when the metastable phase III is ground in a mortar at room temperature, it transforms back into phase II. 

There are two basic crystallisation forms for sulphur - monoclinic and rhombic. Rhombic is the most stable form, at least up to 96 °C the other types revert to this stable form at a rate dependant upon temperature. When sulphur solidifies from the molten state (melting point 114 °C) the crystalline form which occurs is monoclinic (needle-like crystal structure). Below 96 °C, the monoclinic form becomes metastable and changes into the rhombic form. 

In contrast, Van den Elzen and Rieck (79) reported a monoclinic structure for Bi2Mo209 with space group P21/c and lattice parameters a = 11.946 A, b = 10.795 A, c = 11.876 A, and 0 = 90.15°. The inability of several researchers to prepare single crystals and the existence of metastable modifications of the /3 phase of bismuth molybdate are apparently the main determents to additional clarity concerning its structure. 

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