Crystallization isodimorphic

Formula weight 109.815. Colorless salt crystallizes in the anhydrous form as clear, orthc onal, stubby prisms. Anhydrous NaBF does not etch glass. Readily soluble in water. Rhombic crystals, isodimorphous with NaClQ... 

The calcium ion is of such a size that it may enter 6-fold coordination to produce the rhombohedral carbonate, calcite, or it may enter 9-fold coordination to form the orthorhombic carbonate, aragonite. Cations larger than Ca2+, e.g., Sr2+, Ba2+, Pb2+, and Ra2 only form orthorhombic carbonates (at earth surface conditions) which are not, of course, isomorphous with calcite. Therefore these cations are incapable of isomorphous substitution in calcite, but may participate in isodimorphous or "forced isomorphous" substitution (21). Isodimorphous substitution occurs when an ion "adapts" to a crystal structure different from its own by occupying the lattice site of the appropriate major ion in that structure. For example, Sr2+ may substitute for Ca2 in the rhombohedral lattice of calcite even though SrC03, strontianite, forms an orthorhombic lattice. Note that the coordination of Sr2 to the carbonate groups in each of these structures is quite different. Very limited miscibility normally characterizes such substitution. 

Note Isomorphism is a general term in the strict sense, the crystal structure is essentially the same throughout the range of compositions in isodimorphism or isopolymorphism, there are two or more crystal structures, respectively, depending on composition. 

The crystals of potassium bromate were at first thought to be cubic, but C. F. Rammelsberg 30 showed that they are pseudo-cubic and belong to the trigonal system, where a c=l 1 3572, and a=85° 57. They are isodimorphous with potassium chlorate in that the two salts form trigonal mixed crystals if the bromate is in... 

The vinyl fluoride/vinylidene fluoride and the vinyl fluoride/tetra-fluoro ethylene copolymer systems were also studied (21). In the first case isomorphism is observed in the whole range of compositions, while the distribution of the two types of units is random. The crystal structure is that of polyvinyl fluoride, which is virtually identical with one of the three known crystalline forms of polyvinylidene fluoride, and characterized by a planar zig-zag chain conformation. High degrees of crystallinities in the whole range of compositions are also observed in the second case. However, the crystal structure of the two pure homopolymers is not the same hence we are in the presence of isodimorphism. In any case, for vinyl fluoride contents ranging between 0 and 75 mole-% the structure observed is essentially that of polytetrafluoro ethylene in the crystalline... 

The system consisting of copolymers of styrene and />-fluoro styrene (case b), instead, is characterized by isodimorphism. For the whole range of copolymer compositions the system exhibits crystallinity. The crystal structure is that of the homopolymer deriving from the predominant comonomer. Up to >-fluoro styrene, contents of about 50% by moles the copolymers crystallize in the threefold helix structure of iso-... 

P-ll - Isodimorphism of templates in zeolites. New crystal chemistry of analcime and its analogues... 

Random poly(ethylene-c< -butylene 2,6-naphthalate) copolymers are crystallizing over the entire range of composition and they exhibit a eutectic point. The copolymers form either exclusively PEN-type crystals or poly(butylene naphthalate) (PBN)-t) e crystals. A transition from the PEN-type crystal to the PBN-type crystal occurs approximately at a ratio of 1 1 of the alcohol components in the copolymer. This phenomenon is addressed as isodimorphic co-crystallization. 

Poly( 1,4-cyclohexylenedimethylene terephthalate-c< -1,4-cyclohexyl-enedimethylene 2,6-naphthalate) (P(CT-c< -CN)) copolymers behave similarly with respect to crystallization behavior than poly(ethylene-co-butyl-ene 2,6-naphthalate) copolymers. Poly(ethylene 2,6-naphthalate-co-l,4-cyclohexylenedimethylene 2,6-naphthalate) (P(EN-co-CN)) copolymers do not crystallize in the middle of copolymer composition, whereas poly-(hutylene 2,6-naphthalate-c< -1,4-cyclohexylenedimethylene 2,6-naphthalate) (P(BN-co-CN)) and poly(hexamethylene 2,6-naphthalate-co-l,4-cy-clohexylenedimethylene 2,6-naphthalate) (P(HN-c< -CN)) copolymers exhibit clear melting and crystallization peaks. This behavior indicates that both P(BN-co-CN) and P(HN-co-CN) copolymers exhibit a co-crystallization behavior. P(BN-co-CN) copolymers exhibit a eutectic melting and isodimorphic co-crystallization. 

Mitomo H, Morishita N, Doi Y (1993) Composition range of crystal phase transition of isodimorphism in poly(3-hydroxybutyrate-co-3-hydroxyvalerate). Macromolecules 26 5809-5811 Mitomo H, Morishita N, Doi Y (1995) Structural changes of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) fractionated with acetone-water solution. Polymer 36 2573-2578 Mitomo H, Takahashi T, Ito H, Saito T (1999) Biosynthesis and characterization of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) produced by Burkholderia cepacia Dl. Int 1 Biol Macromol 24 311-318... 

When polymorphs are not interconvertible, the crystal forms are said to be monotropic graphite and diamond are monotropic forms of carbon. The term isopolymorphism is used when each of the polymorphous forms of one substance is isomorphous with the respective polymorphous form of another substance. For instance, the regular and orthorhombic polymorphs of arsenious oxide, AS2O3, are respectively isomorphous with the regular and orthorhombic polymorphs of antimony trioxide, Sb203. These two oxides are thus said to be isodimorphous. 

The crystal structure of PHB is a orthorhombic form with unit cell parameters fl = 0.576 nm, = 1.320 nm, and c(fiber axis)=0.596 nm, and space group P2,2,2, (Alper et al. 1963 Okamura and Marchessault 1967). The conformational analysis by intermolecular energy calculation has indicated that the PHB molecule has a left-handed 2j helix conformation (Comibert and Marchessault 1972 Yokouchi et al. 1973 Bruckner et al. 1988). The crystal structure of random copolymers of 3HB and 3HV has been investigated extensively (Bloembergen et al. 1986 Kamiya et al. 1991 VanderHart et al. 1995). A structural characteristic of poly(3HB-co-3HV) is isodimorphism, i.e., cocrystallization, of the two monomer units. In contrast, the cocrystallization of 3HB with 3HH or (7 )-6-hydroxyhexanoate (6HH) does not occur. 

The crystal in Fig. 5.34 illustrates the concept of isomorphism, the possibility that two different motifs fit into the same crystal. For polymers isomorphism of repeating units is most important. Isomorphism of complete chains is seldom possible. Polyfvinyl fluoride) that fits into a crystal of poly(vinylidene fluoride) provides a rare exception. The repeating-unit isomorphism can be separated into three types Type 1 occurs when both homopolymers have die same crystal structure, and a smooth change of the lattice parameters occurs on changing the concentration. An example of type 1 repeating-unit isomorphism is poly(vinylidene fluoride-co-vinyl fluoride). Type 2 is also called isodimorphism and occurs if the homopolymers have different crystal structures. A change in structure occurs at an intermediate concentration. Type 3 occurs if one homopolymer does not crystallize by itself, but participates in the crystal of the other. 

More complicated is the description of the melting of copolymers with Ml or partial isomorphism or isodimorphism (Sect. 5.1.10). The changes in concentration in the mixed crystals as well as the melt must be considered and the change of the lattice energy with cocrystalhzation must be known. The simplest description assumes that there is no difference in concentration between melt and crystal, and there is a linear change of the heat of fusion, AH, with concentration. This leads to T = [ 1 - XB(AHj/AHu)]Tm°, where AH is the heat of fusion per repeating unit of the homopolymer. 

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