Crystal Structure of the Trigonal Modification (α Form) of Syndiotactic Polystyrene

Abstract

A model of packing for the ordered modification of the R form of syndiotactic polystyrene is suggested, through comparisons between calculated structure factors and experimental intensities observed in the X-ray fiber and electron diffraction patterns. The present analysis improves the preliminary model proposed in previous papers. Trans planar chains are arranged in triplets and packed in the unit cell with axes a ) b ) 26.26 Å, c ) 5.04 Å, according to a statistical rhombohedral symmetry for the disordered R′ modification and a trigonal, quasi-rhombohedral, symmetry for the ordered R′′ modification. The glide plane symmetry of the isolated chain is not maintained in the lattice; we suggest that the space group is R3 h for the statistically disordered R′ modification; it is P3 for the ordered R′′ modification (instead of R3 hc and P3c1 previously proposed). A rotation of nearly 7°of the triplets of chains with respect to the symmetric situation corresponding to the space group P3c1 for the R′′ modification, can account for the observed differences between the intensities of the hkl and khl reflections in the electron diffraction patterns of syndiotactic polystyrene. IntroductionSyndiotactic polystyrene (s-PS) presents a very complex polymorphic behavior. 1-7 Four different crystalline forms have been described so far. Following the nomenclature proposed in ref 3, the crystalline R and forms are characterized by chains in the trans planar conformation, whereas the crystalline γ and δ forms contain chains in the s(2/1)2 helical conformation. The term δ form has been used to indicate different clathrate structures, that is, compounds which include molecules of solvent. The general pattern is complicated by the fact that both R and forms can exist in different modifications having different degrees of structural order, so two limiting disordered modifications (R′ and ′) and two limiting ordered modifications (R′′ and ′′) have been described. [3][4][5] More recently, two mesomorphic modifications of s-PS, containing chains in the trans planar and in the s(2/1)2 helical conformations also have been described. [8][9][10][11] The crystal structures of R 4,12 and 5,13 forms, as well as those of molecular compounds containing molecules of solvent (clathrate structures) 14,15 have been described.As far as the R form is concerned, a detailed description of the morphology of thin films by transmission electron microscopy has been given. 12,16 Greis et al. 12 have proposed, by electron diffraction on single crystals, a hexagonal unit cell with a ) 26.26 Å, c ) 5.04 Å, in which clusters of three chains are packed according to the space group P6 h2c.We have pointed out 3,4 the absence, in the X-ray powder diffraction patterns of the R′ modification, of hkl reflections with -h + k + l * 3n, and their weakness in the powder diffraction patterns of the ordered R′′ modification. On the basis of this observation, we have suggested that alternative models for the molecular packing, characterized by a statistical rhombohedral symmet...

Thermally Induced Phase Transition of Crystalline Syndiotactic Polystyrene Studied by Molecular Dynamics Simulation

“…[1,2] The main chain conformation of the a and b forms is trans zigzag (TTTT), [3,4] while that in the c and d forms is s(2/1)2 helical (TTGG). [5] Various solvents, e. g., p-xylene, toluene, benzene, and iodine, are clathrated in the d form, [5] from which the emptied clathrate d e form can be obtained by solvent extraction.…”

Polymorphic crystal forms and morphology of syndiotactic polystyrene in miscible states

“…[1][2][3][4] Ishihara et al proposed a zig-zag planar structure for s-PS. 1 Other forms are also reported.…”

Crystallization Condition of Glassy Syndiotactic Polystyrene in Supercritical CO2

“…So far four crystal modifications and two mesomorphic phases have been reported, since Ishihara et al 1 succeeded the synthesis of highly syndiotactic polystyrene. [2][3][4][5][6][7][8] The molecular conformation of the α and β forms is planar zigzag but that of the γ and δ forms is t 2 g 2 . Kobayashi and his coworkers revealed the characteristic vibrational modes for these conformations using the infrared and Raman spectroscopies.…”