C. De Rosa

On the crystal structure of the orthorhombic form of syndiotactic polystyrene

Abstract The crystallization conditions for the preparation of different modifications of the orthorhombic form (β form) of syndiotactic polystyrene (s-PS) are described. Crystallizing from the melt produces the β′ or α form depending on the cooling rate. Crystallization by casting a film from o-dichlorobenzene solutions produces clathrate structures (δ form) at low temperatures (T

Recrystallization kinetics of isotactic polypropylene (α-form)

The recrystallization kinetics of isotactic polypropylene (i-PP) (α-form) in the usual melting temperature region (155°–170°C) has been studied by calorimetric and dilametric techniques. The recrystallization is possible over a large range of temperatures also when the residual crystallinity is very low (⋍ 1%). Low values of the Avrami exponents independent of Tr (n ≤ 1.6), have been found, although the residual crystallinity shows a large variation (1–35%).

Influence of constitutional defects on polymorphic behaviour and properties of alternating ethylene-tetrafluoroethylene copolymer

The influence of constitutional defects (those resulting from variations of the comonomer molar ratio from 50/50 and those due to the introduction of termonomer units) and thermal history on the polymorphic behaviour of ethylene-tetrafluoroethylene (ETFE) copolymer is investigated. At low temperatures, while well developed orthorhombic structures are obtained for copolymers with a C2F4 content in the range 50–70 mol%, a content of the perfluoropropylperfluorovinylether termonomer of 1.8 mol% is already sufficient to favour formation of the so-called pseudo-hexagonal mesophase. At high temperatures, the degree of crystallinity as well as the correlation length of the hexagonal domains remain essentially unaltered by an increase of the C2F4 molar content from 50 to 80%, while both are strongly reduced by low concentrations (1–3 mol%) of the termonomer units (substantially excluded from the crystalline phase). This is expected to be a determinant factor for the improved ultimate properties typical of commercial terpolymers of ETFE.

Fourier-transform analysis of models for the orthorhombic crystal phase of the alternating ethylene-tetrafluoroethylene copolymer

Disorder in the relative positions of neighbouring chains in the orthorhombic form of ethylene-tetrafluoroethylene (ETFE), which has a polyethylene-like packing in the projection along the c axis, has been studied through a comparison of the observed diffraction profiles on the layer lines with the calculated Fourier transforms of models. The analysis suggests that, along the c axis, integral translational displacements of c2 are present in the crystals owing to the inclusion of defects in the alternation of the comonomers. On the basis of the analysis, an approximate evaluation of the amount of constitutional defects included in this crystal structure has been attempted. In the crystalline phase, associated with this kind of constitutional disorder, large nearly random translational displacements are present; correlations in relative positions do not go much beyond the first neighbours.

Structural and morphological aspects of some polymorphs of syndiotactic poly(p-methylstyrene)

Novel data about the crystal morphology and the electron diffraction of syndiotactic poly(p-methylstyrene) (s-PPMS) in form I and in the clathrate forms including o-dichlorobenzene and tetrahydrofuran are presented. Preparative routes of s-PPMS samples in the various polymorphs suitable for electron microscopy and diffraction have been devised. This investigation provides data, unavailable in the past literature, helpful to get direct structural information and to better understand the ways of obtaining and stabilizing the various polymorphs.