Mixed polymorphism

The isolation of crystalline products having mixed polymorphic compositions (often referred to as concomitant polymorphism) remains a topic of interest, even though the phase rule predicts that a system at equilibrium consisting two components (solvent + solute) and three phases (solution + Form I + Form II) is uni variant. Hence, for crystallizations performed at a fixed pressure (typically atmospheric) the system becomes nonvariant and genuine equilibrium can exist at only one temperature. Therefore, concomitant products must be obtained under nonequilibrium conditions. Flexibility in molecular conformation was attributed to the concomitant polymorphs of a spirobicyclic dione [34] and of 3-acetylcoumarin [35],... 

Some properties of p-iPP differ significantly from those of a-iPP. In comparison with a-iPP, fi-iPP possesses lower crystal density, melting temperature and fusion enthalpy, but a similar glass transition temperature (Table 3). The chemical resistance of p-iPP seems to be lower than that of a-iPP. The former could be selectively extracted with hot toluene from samples of mixed polymorphic composition [8]. 

The 7-modification of iPP (y-iPP) may form in degraded, low molecular weight iPP or in samples crystallized under high pressure [5, 6]. Certain propylene copolymers with low comonomer content (4-10 wt.%) crystallize preferentially in y-form, as well. y-iPP has a face-centred ortho-rombic unit cell with parameters a = 0.85 run, b = 0.993 nm and c = 4.241 nm containing isochiral helices. The cell structure proposed by Bruckner et al. [5] is unique in polymer crystallography the chain axes in adjacent crystal layers are not parallel. The angle between the chain stems is about 80°. y-iPP is not usually observed as an independent phase, but crystallizes with and within the a-spherulites. According to Lotz et al. [7], the positive spherulites observed in samples with mixed polymorphic composition of a- and y-iPP are probably made of a... 

We have subsequently shown (see section 4.3.) that injection molded specimens, which were chilled in air, have mixed polymorphic composition where the skin coasists of a-ciystals while the core comprises /3-spherulites. This fact supports the existence of a lower critical temperature limit for the formation of /3 PP. 

At low mold temperatures (20 to 60° C) the thicker (3 mm or more) plates have mixed polymorphic composition where the skin consists of a-ciystals while the core comprises /S-spheruiites. The chaipy and Izod impact strength of thicker specimens are summarized in Tabs. 4, 5. 

Control of the polymorphic forms in cocoa butter is further compHcated by the presence of other fats such as milk fat. The fat in a chocolate can be likened to the mortar between the bricks in a mason s wall. The soHd particles in a weU-conched chocolate bed down better than the soHds in a coarsely refined and poorly mixed one (30). 

Methylsull anyT I //-tetrazole was found to crystallize in a monoclinic form, and could be sublimed into an orthorhombic form, with the structures differing in the relative polarity of the molecular layers in the two forms [47]. /) -1 o d o a ce t o p h e n one was found to crystallize in two polymorphs that both contained C—H-re points of contact, but where the contacts were shorter in one form than in the other [48]. A second monoclinic modification of the mixed salt benzimidazolium 3-carboxyphe-noxyacetate 3-carboxyphenoxyacetic acid was reported, where the acid hydrogen atom and the two monoanions comprised a carboxylate monoanion/neutral molecule in which the acid proton was disordered between the two anionic units [49]. 

Also mercury(II) selenite is polymorphic and two new modifications, p-HgSe03 and y-HgSe03, have been recently described.134 They show the mercury atoms in sevenfold coordination of oxygen atoms, and the [I Ig07] polyhedra are linked by the pyramidal selenite ions. A remarkable and unusual mercury selenite is the mixed valent compound (HgSe03)3E[gSe.134 As a characteristic feature, the crystal structure contains [Hg3Se] pyramids with distances Hg Se of 2.489 A. 

Using zinc oxide as the internal standard, the relative amounts of two polymorphic forms of fenretinide were quantified [55], Mixtures containing 25, 50, and 75% w/w form I were prepared by mixing authentic standards of form I and form II fenretinide. After the addition of the internal standard, the maximum intensities (peak heights) of the 4.6 A line (peak at 19.1° 20) of fenretinide form I (/19. i) and the 2.8 Aline (peak at 31.8° 20) of zinc oxide (/318) were determined. A plot of the intensity ratio (/19.//31.8) as a function of the weight percent form I was linear. The method was reported to be precise and accurate to within 6%. 

I hypothesised (Baker and Beilis 1995 Baker 1996) that the within-population frequency distribution of testes size reflects a balanced polymorphism. In this hypothesis, men with larger testes were programmed to spend less time with partners, ejaculate often, and expose their sperm more often to sperm competition. Men with smaller testes were programmed to invest more in mate guarding and only infrequently to expose their sperm to competition. Between the two extremes, the intermediate men were programmed to pursue a more mixed strategy. 

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