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Solid Sulfur Allotropes 43 The elastomeric properties of sulfur chains of differing lengths have been studied by means of Monte-Carlo simulations [124]. The results demonstrate a remarkable flexibility of polymeric sulfur. The configurational energy sur- faces of polymeric sulfur were investigated with the use of a parameter-free density-functional method by Springborg and Jones [125]. It was found that the energy surface of the chain molecule is flat over large regions of the con- figurational space and contains a variety of local minima and saddle points corresponding to widely differing structures. In fact, the distinct flatness at the minimum energy is consistent with the enormous flexibility of the poly- mer. Finally, Ezzine et al. investigated the intermolecular interaction ener- gies of polymeric sulfur chains by semiempirical methods. These studies were performed for several cases of chain arrangements including the Sw al- lotropes [126]. In the following, we present a summary of the structures of the three solid allotropes of polymeric sulfur at STP conditions which have been studied so far. Doubtful forms, not verified up to now, will be omitted here. Although several authors are claiming crystalline structures in the case of certain poly- meric allotropes it seems likely that in these solids regions with well defined ordered structures and regions with amorphous structures co-exist. This has clearly been shown by more or less sharp diffraction patterns in X-ray studies of different samples depending on the method of preparation. 2.2.3.1 Fibrous Sulfur (Sy) Crystalline samples of fibrous sulfur can be prepared by stretching freshly quenched liquid sulfur or by drawing filaments directly from a hot sulfur melt. The quenched melt (plastic sulfur) is viscoelastic and transparent yel- low but slowly transforms into an opaque crystalline material at room tem- perature. After several days the material contains besides the polymer a frac- tion of S8 rings and traces of S7 while all other rings have converted to either polymer or S8 [22]. The ring fraction can be extracted by washing the mate- rial with CS2. The insoluble fraction of polymeric sulfur is then allowed to crystallize (termed Sm after Smith [109] or sometimes Sw after Das [112]). If plastic sulfur is stretched (by a factor of thousand of its original length!) crystallization will be induced [127]. The resulting filaments are mixtures of S8 rings and polymeric sulfur. After extraction of the soluble ring fraction a fibrous crystalline allotrope is obtained (called Sy after Prins et al. [31, 110]). Because of the complications introduced by the special treatments and the interconversion of the ring and polymer fractions during the preparation the elucidation of the structure was a long-standing puzzle. An important contribution to the determination of the molecular structure came from Prins et al. in the late 1950s [31, 110]. The authors showed that the diffrac- tion patterns of filaments of sulfur can be understood assuming a superposi- tion of the patterns of two constituents, namely catena-polysulfur and monoclinic g-S8. From then on the true fibrous sulfur was termed Sy. PrinsPDF Image | Topics in Current Chemistry
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