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Materiali nanostrutturati e nanotecnologie
No full textGiornata di Studio e Approfondimento sulle bioplastiche e tecnologie ecocompatibili per imbottigliamento ed imballaggio delle acque mineral
Crystal structure of the trigonal modification (alpha form) of syndiotactic polystyrene
No full textA model of packing for the ordered modification of the alpha 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 Angstrom, c = 5.04 Angstrom, according to a statistical rhombohedral symmetry for the disordered alpha' modification and a trigonal, quasi-rhombohedral, symmetry for the ordered alpha '' modification. The glide plane symmetry of the isolated chain is not maintained in the lattice; we suggest that the space group is R (3) over bar for the statistically disordered alpha' modification; it is P3 for the ordered alpha '' modification (instead of R (3) over bar c and P3c1 previously proposed). A rotation of nearly 7 degrees of the triplets of chains with respect to the symmetric situation corresponding to the space group P3c1 for the alpha '' modification, can account for the observed differences between the intensities of the hkl and khl reflections in the electron diffraction patterns of syndiotactic polystyrene
CHAIN CONFORMATION, CRYSTAL STRUCTURES AND STRUCTURAL DISORDER IN STEREOREGULAR POLYMERS
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Crystal structure of form II of isotactic poly(4-methyl-1-pentene)
No full textThe crystal structure of form II of isotactic poly(4-methyl-1-pentene) is presented. Samples of isotactic poly(4-methyl-1-pentene) in the pure form II have been obtained by crystallization from tetramethyltin solutions. The crystal structure has been determined by using X-ray powder diffraction data and packing energy calculations. Chains in 41 helical conformation are packed in a monoclinic unit cell with axes a = 10.49 Angstrom, b = 18.89 Angstrom, c = 7.13 Angstrom, and gamma = 113.7degrees, according to the space group P2(1)/b. Four chains are included in the unit cell with a calculated density of 0.862 g/cm(3), according to the experimental density of 0.850 g/cm(3). Statistical disorder in the position of up and down chains in each site of the lattice is present in the structure. This structure provides an example of symmetry breaking in polymer crystals since the local 4(1) helical symmetry of the chains is not maintained in the lattice as crystallographic symmetry. As a consequence, the chains do not pack in a tetragonal lattice, as generally occurs for chains in 4(1) helical conformation but are packed in a monoclinic lattice having lower symmetry
Geometrical analysis and conformational and packing energy calculations on the isotactic alternating copolymer between carbon monoxide and styrene
No full textA geometrical and energy analysis on the isolated chain of the isotactic optically active alternating styrene-carbon monoxide copolymer is presented. Relations between the bond and torsion angles of the main chain, which give helical s(2/1) repetition and highly extended chains, have been found. The results of the conformational energy calculations indicate that the best conformation in the crystalline field corresponds to a slight deviation of the torsion angles from 180 degrees, according to the experimental structure, Packing energy calculations for the space group P2(1) and the experimental unit cell with axes a = 5.47 Angstrom, b = 8.367 Angstrom, c = 7.57 Angstrom, and gamma = 110 degrees are presented. The present. analysis indicates that the packing interaction influences the chain conformation. Precisely, the need to optimize intermolecular contacts between the atoms of the phenyl rings of adjacent chains produces a relevant deviation (nearly 30 degrees) of the torsion angle which characterizes the orientation of the lateral phenyl group from the value (approximate to 60 degrees) that keeps the phenyl ring plane bisecting the valence bond at the tertiary carbon. These results are in agreement with those obtained in the refinement of the crystal structure by X-ray diffraction
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