Odda Ruiz de Ballesteros

On the form IV of syndiotactic polypropylene

The packing of the chains in (T6G2T2G2)n conformation of the form IV of s-PP is revisited on the basis of packing energy and structure factor calculations. According to this analysis, an alternative mode of packing has been suggested. A monoclinic structural model, with the unit cell centered on the C face, is obtained, after small changes of the atomic coordinates in the triclinic structural model as proposed by Chatani et al. The monoclinic model presents a lower packing energy than the triclinic model and a good agreement between the calculated and observed structure factors. The triclinic structural model implies that all the chains are rotated by the same amount around the chain axis with respect to the monoclinic structural model. Since clockwise and counter clockwise rotations are equivalent, the monoclinic structural model may be taken as descriptive of the order in the long range, for the form IV of s-PP, or in other terms, descriptive of an average structure (space group C2, unit cell constants equal to am = 14.17 A, bm = 5.72 A, cm = 11.6 A, and βm = 108.8°). The triclinic structural model for this polymorph, instead (space group P1, unit cell constants equal to at = 5.72 A, bt = 7.64 A, ct = 11.60 A, αt = 73.1°, βt = 88.8°, γt = 112.0°) is probably more properly descriptive of local situation of order (the symmetry, locally, is broken). Analogies between the monoclinic limit ordered structural model for the form IV and the orthorhombic limit ordered structural model for the form II (with chains in the more stable (TTGG)n conformation) of s-PP are also provided.

Influence of the stereoregularity on the crystallization of the trans planar mesomorphic form of syndiotactic polypropylene

The crystallization of the trans-planar mesomorphic form of syndiotactic polypropylene by quenching the melt at 0°C is investigated as a function of the stereoregularity of the samples. The formation of the trans-planar mesomorphic form at 0°C is followed as a function of the permanence times at 0°C by X-ray diffraction and FTIR spectroscopy. Samples kept at 0°C for short time rapidly crystallize into the helical form I at room temperature, whereas longer permanence times at 0°C increase the stability of the trans-planar mesomorphic phase which remains stable and inhibits the normal crystallization of the sample into the helical form at room temperature. The stereoregularity of the polymer sample strongly influences the rate of formation of the mesomorphic form at 0°C. Higher the syndiotacticity, easier the formation of trans-planar mesomorphic form. Very short permanence times (few hours) at 0°C are enough to form and stabilize the mesomorphic form for highly syndiotactic samples, whereas for stereo-irregular samples the amount of mesomorphic form observed when the sample is removed from the 0°C bath and heated to room temperature, remains always very low, even for very long permanence times (months) at 0°C and the crystallization into the normal helical form is not inhibited.

Mechanical Properties and Elastic Behavior of High-Molecular-Weight Poorly Syndiotactic Polypropylene

An analysis of the mechanical properties and elastic behavior of low-stereoregular and nearly amorphous syndiotactic polypropylene (sam-PP), prepared with heterocycle-fused indenyl silyl amido dimethyltitanium complexes, is presented. High-molecular-weight poorly syndiotactic sam-PP samples show good elastic behavior at room temperature in a large range of deformation. While highly stereoregular and crystalline syndiotactic polypropylene shows good elastic properties only for previously stretched oriented fibers, sam-PP samples present good elasticity even for unoriented compression-molded films during the first stretching. Because of the very low crystallinity, these samples experience a negligible irreversible plastic deformation and show a typical behavior of thermoplastic elastomers. The small crystalline domains in the amorphous matrix act as physical knots of the elastomeric lattice, preventing the viscous flow of the amorphous chains. The elastic behavior is associated with a reversible polymorphic...

Influence of the quenching temperature on the crystallization of the trans-planar mesomorphic form of syndiotactic polypropylene

Abstract The crystallization from the melt of the trans-planar mesomorphic form of syndiotactic polypropylene is investigated at different quenching temperatures. The formation of the trans-planar mesomorphic form at −5, 0 and 6 °C is followed as a function of the residence time at these temperatures by X-ray diffraction and FTIR spectroscopy. The quenching temperature influences the rate of formation of the mesomorphic form as well as the maximum amount of the obtained mesomorphic form. By increasing the quenching temperature, in the examined range between −5 and +6 °C, an increase in the rate of formation of the mesomorphic form is observed. The maximum amounts of mesomorphic form obtained at 6 and −5 °C are lower than the amount achieved at 0 °C, which corresponds to nearly 100% of the total crystalline phase.

Pseudohexagonal crystallinity and thermal and tensile properties of ethene-propene copolymers

Structural (X-ray diffraction), melting (differential scanning calorimetry), as well as mechanical (tensile tests) characterizations on uncrosslinked ethene-propene copolymer samples, obtained using a metallocene-based catalytic system and having an ethene content in the range 80-50% by mol, are reported. Samples with an ethene content in the range 80-60% by mol present a disordered pseudohexagonal crystalline phase, whose melting moves from 40°C down to -20°C as the ethene content is reduced. The dramatic influence of the crystalline phase on tensile properties of uncrosslinked ethene-propene copolymers is shown. In particular, highest elongation at break values are obtained for samples being essentially amorphous in the unstretched state and partially crystallizing under stretching. On the other hand, lowest tension set values (most elastic behavior) are observed for samples presenting, already in the unstretched state, microcrystalline domains acting as physical crosslinks in a prevailing amorphous phase.

Highly stereoregular polymerization of 1,3-cyclohexadiene in the presence of Cp2Ni-MAO catalyst

Polymerizations of 1,3-cyclohexadiene, in the presence of Cp 2 Ni (Cp = cyclopentadienyl) activated by methylaluminoxane (MAO), were carried out at different temperatures and Ni-MAO mole ratios. The polymers obtained are stereoregular with a 1-4 structure, and are highly crystalline with a melting point close to 315°C. Copolymerizations of 1,3-cyclohexadiene and butadiene with the title catalyst give polymers with a nearly random distribution of the comonomeric units.

Pseudo-Hexagonal Crystallinity in Ethene-Styrene Random Copolymers

A comparative X-ray diffraction study on unoriented and oriented samples of ethene based random copolymers with styrene and propene, obtained by single site metallocene catalysts is presented. For ethene-styrene copolymers, as usual for ethene copolymers with comonomers bulkier than propene, an orthorhombic crystal phase is generally observed. However, an oriented pseudo-hexagonal crystalline phase has been obtained in stretched samples, and compared with that one known for ethene-propene copolymers.

Structural features of the mesomorphic form of syndiotactic poly(p-methylstyrene)

A possible structural model for the mesomorphic form of syndiotactic poly(p-methylstyrene) is discussed. The structural analysis is performed through the comparison between the experimental X-ray diffraction intensities and the calculated intensities for model structures constituted by ordered and disordered aggregates of chains. This analysis provides support for the hypothesis that the mesomorphic form of sPPMS may be described in terms of small aggregates of trans-planar chains having a packing mode very close to that one of form III, in which disorder in the relative arrangement of neighbouring chains is present to some extent.

Influence of the guest molecules on the structural organization of the clathrates of syndiotactic poly(p-methylstyrene)

The influence of the shape and size of guest molecules on the structural organization of the a and β class clathrates of syndiotactic poly(p-methylstyrene) is described, through the analysis of the packing model proposed for the crystal structures of the clathrate forms containing o-dichlorobenzene and tetrahydrofuran. Preliminary data of the crystal structure of the s-PPMS clathrate containing benzene and a comparison with the crystal structure of the clathrate forms of syndiotactic polystyrene are also presented.