Disordered polymorphic modifications of form I of syndiotactic polypropylene

The structural characterization of samples of syndiotactic polypropylene (s-PP) isothermally crystallized from the melt is presented. Form I of s-PP, with variable amount of disorder, is obtained at every crystallization temperature. The limit ordered modification, characterized by a fully antichiral packing along both axes of the unit cell and by a monoclinic symmetry P2(1)/a, is obtained only at high values of the crystallization temperature (T-c > 135 degrees C). At crystallization temperatures in the range 120-135 degrees C, modifications characterized by disorder in the regular alternation of left- and right-handed helices along both axes of the unit cell are formed. At T-c < 120 degrees C, disorder in the stacking of be layers of helices piled along a, implying a shift of b/4 among consecutive layers (b/4 shifts disorder), can also arise. The amount of both kinds of disorder depends on the crystallization temperature and increases in the as-prepared samples and in samples rapidly crystallized by quenching the melt. These samples are crystallized in a disordered modification which can be described by the statistical space group Bmcm. This model is characterized by a statistical disorder in the substitution of enantiomorphous helices in each site of the lattice, although local correlation about the chirality of neighboring chains may still be preserved. This correlation is lost in the long range because of the occurrence of the b/4 shifts disorder. A ''continuum'' of intermediate disordered modifications of form I exists among the limit statistical disordered form Bmcm and the limit ordered monoclinic form P2(1)/a. In these modifications, the chains are already rotated around their axes by nearly 5 degrees and displaced along c according to the packing pattern of the limit ordered modification P2(1)/a.