Fenomeni di disordine nel polipropilene sindiotattico cristallino

Auriemma, Finizia; Rosa, Claudio De; Corradini, Paolo

doi:10.1007/bf03001190

Cited by 14 publications

(16 citation statements)

References 12 publications

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“…We recall that disorder implying b/4 shifts among consecutive bc layers piled along a 11,12 can account for experimental electron diffraction data on single crystals of s-PP grown at low temperatures, 9 which show diffuse scattering along the h11 layer line when diffuse scattering is present also along the h20 layer line (like, for instance, in the case of single crystals grown at 100 °C9 ); these spectra show strong and discrete reflections corresponding to indices (211) when strong and discrete reflections corresponding to indices (020) are present (like, for instance, in the case of single crystals grown at 140 °C9 ).…”

Section: Resultsmentioning

confidence: 99%

“…[10][11][12] The latter kind of disorder has been observed in single crystals of s-PP obtained at low temperatures 8,9 and in fiber and powder specimens of s-PP. [10][11][12] Limit ordered modification of form I, characterized by the monoclinic structure P2 1 /a, 15 is obtained only when highly stereoregular samples of s-PP are crystal-lized from the melt at high temperatures (e.g. 140 °C) 15 or crystallized in single crystals grown at high temperatures (>130 °C).…”

Section: Introductionmentioning

confidence: 82%

“…The polymorphism of s-PP is complicated by the fact that, depending on the degree of stereoregularity and the mechanical and thermal history of the samples, the various crystalline forms show different kinds and amounts of disorder in the crystalline packing. [6][7][8][9][10][11][12]17,20,21 For form I, broad peaks observed in the X-ray powder and fiber diffraction patterns 10 and streaks observed in the electron diffraction spectra [6][7][8][9] have been explained through disordered models characterized by a departure from the regular alternation of twofold helices of opposite chirality along the a and b axes of the unit cell, [10][11][12] as well as by a statistical mixture of crystals of form I and form II. [10][11][12] The latter kind of disorder has been observed in single crystals of s-PP obtained at low temperatures 8,9 and in fiber and powder specimens of s-PP.…”

Section: Introductionmentioning

confidence: 99%

“…[6][7][8][9][10][11][12]17,20,21 For form I, broad peaks observed in the X-ray powder and fiber diffraction patterns 10 and streaks observed in the electron diffraction spectra [6][7][8][9] have been explained through disordered models characterized by a departure from the regular alternation of twofold helices of opposite chirality along the a and b axes of the unit cell, [10][11][12] as well as by a statistical mixture of crystals of form I and form II. [10][11][12] The latter kind of disorder has been observed in single crystals of s-PP obtained at low temperatures 8,9 and in fiber and powder specimens of s-PP. [10][11][12] Limit ordered modification of form I, characterized by the monoclinic structure P2 1 /a, 15 is obtained only when highly stereoregular samples of s-PP are crystal-lized from the melt at high temperatures (e.g.…”

Section: Introductionmentioning

confidence: 99%

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Disordered Polymorphic Modifications of Form I of Syndiotactic Polypropylene

1997

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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 P21/a, is obtained only at high values of the crystallization temperature (Tc > 135 °C). At crystallization temperatures in the range 120-135 °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 Tc < 120 °C, disorder in the stacking of bc 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 P21/a. In these modifications, the chains are already rotated around their axes by nearly 5°and displaced along c according to the packing pattern of the limit ordered modification P21/a.

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Section: Resultsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 82%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Disordered Polymorphic Modifications of Form I of Syndiotactic Polypropylene

1997

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“…These defects produce, locally, the C-centered mode of packing of form II(Figure 2.37b). A model for this kind of disorder is shown inFigure 2.38; antichiral packing of first neighboring chains may still be preserved but is lost in the long range because of the disorder in the stacking of the layers 182,183. Disorder in the stacking of ordered layers of chains along one lattice direction is present in samples of iPP crystallized in the γ-form.…”

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confidence: 99%