Search

The mechanical properties of copolymers of syndiotactic polypropylene (sPP) with 1-hexene and 1-octene have been studied and correlated with the structural transformations occurring during stretching and relaxation. These copolymers crystallize in disordered modifications of form I of sPP, with crystallinity, melting, and glass-transition temperatures that decrease with increasing comonomer content. All copolymers show remarkable mechanical properties and elastic behavior, with great improvement of flexibility and ductility with respect to the sPP homopolymer already at small comonomer concentrations. For low comonomer content up to 4-5 mol %, the elastic recovery is associated to a reversible conformational phase transition between the trans-planar form III or the trans-planar mesophase and the helical form II, which gives an enthalpic contribution to the elasticity. As the comonomer content increases, the trans-planar conformation is progressively destabilized and the observed elastic properties are not associated with any conformational transformation during stretching and relaxation. In these cases, copolymers behave as conventional thermoplastic elastomers where elasticity is purely entropic and crystals only act as topological constraints of the elastomeric network. These copolymers represent an exemplary case of the possibility to combine in the same materials desired and apparently incompatible physical properties of elasticity, crystallinity, and strength by controlling the crystallization behavior, achieved by tailoring the chemical structure in the synthetic process.

A study of the crystallization behavior and mechanical properties of random isotactic butene–ethylene copolymers prepared with a metallocene catalyst is presented. The use of the metallocene catalysis ensures a fine control over the molecular structure with low concentration of rr stereodefects (0.8%), negligible amount of regiodefects, and random and uniform distribution of ethylene constitutional defects. This molecular characteristic has allowed evidencing the only effect of the presence of ethylene units on the polymorphic behavior and mechanical properties of isotactic polybutene (iPB). The presence of ethylene accelerates the transition of form II into form I at room temperature and at concentration of nearly 6 mol % favors the direct crystallization from the melt of the stable form I. The presence of ethylene also affects the mechanical behavior of iPB and produces increase of flexibility and ductility with increasing ethylene content. A significant modification of the properties of iPB is observed...

The crystal structure of isotactic cis-1,4-poly(1,3-hexadiene) has been determined through a combination of X-ray diffraction analysis and molecular mechanics. Two reasonable values for unit cell parameters were obtained from the fiber diffraction pattern of a well-oriented sample. Energy maps performed on a molecular model of the polymer chain yielded an absolute minimum corresponding to a conformation having the internal parameters similar to those of an s(2/1) helical polymer chain. Packing energy minimizations performed for all the orthorhombic and monoclinic space groups having crystallographic s(2/1) chain symmetry indicate unequivocally that the best mode of packing is obtained for a crystal structure characterized by a unit cell with parameters a = 8.98 angstrom, b=7.82 angstrom, and c=8.10 angstrom in the P2(1)2(1)2(1) space group. The calculated powder and fiber diffraction patterns of this structural model are, in very good agreement with the experimental diffraction patterns. Similarities and differences with the crystal structures of isotactic cis-1,4-poly(1,3-pentadiene) and isotactic cis-1,4-poly(2-methyl-1,3-pentadiene) are discussed.

The crystal structure of syndiotactic 1,2-poly(E-3-methyl-1,3-pentadiene) has been determined by the combination of X-ray diffraction and molecular mechanics. Energy minimizations performed by various potential functions yielded models of crystal structure with calculated X-ray powder profiles similar to the experimental profiles. The best model has been refined in order to fit all the experimental data. The space group is Pcam, and the parameters of the unit cell are a = 15.4 angstrom, b = 7.45 angstrom, and c = 5.15 angstrom. Similarities and differences with the crystal structures of syndiotactic 1;2-poly(1,3-butadiene), syndiotactic 3,4-poly(2-methyl-1;3-butadiene), and syndiotactic 1,2-poly(E-1,3-pentadiene) are discussed