Vittorio Petraccone

Polymorphism in isotactic polypropylene

Abstract The paper is intended to summarize the studies devoted to the elucidation of the different crystal structures and crystal morphologies and of the different conditions favouring one or the other of the polymorphs of isotactic polypropylene. This outline takes into account 103 contributions produced in the course of about thirty-five years and is completed with 22 figures.

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

Polymorphism of isotactic poly-α-butene: Conformational analysis of the chain and crystalline structure of form 2

Abstract The polymorphism of isotactic poly-α-butene is studied on the basis of a complete conformational analysis of the chain. The results show that chain polymorphism occurs between two different minima of the energy surface separated by a very low energy gap. The complete determination of the structure of form 2 is also an object of the present work. A possible slight distortion from the perfect s(3·65) helical symmetry of the chain in the form 2 is indicated.

Conditions for the α1-α2 transition in isotactic polypropylene samples

Abstract Transitions between structural modifications of isotactic polypropylene, characterized by increasing degrees of order in respect to the up and down positioning of the chains, have been studied for samples having different previous thermomechanical histories or containing small amounts of ethylenic units. The annealing temperature ranges, in which such transitions are observed, are strictly related to the positions of the melting endotherm positions for the various samples. For all the recrystallized samples, the same relation has been found between the increases in the melting peak positions and the increases in the parameter selected as a measure of the up-down degree of order.

The role of intermolecular interactions in determining the mode of packing of crystalline polymers. Energy calculations on isotactic polypropylene

Abstract Packing energy calculations for isotactic polypropylene chains have been performed with the purpose: (a) to predict the most stable kind of monoclinic lattice for the α-modification; (b) to contribute to the understanding of the structural disorder which is commonly found in the α-modification; (c) to evaluate the order of magnitude of the differences in energies among the three modifications (α, β, γ) experimentally observed. The results for the α-form show that the best packing of the chains is achievable for the ordered P2 1 /c space group, while more or less disordered structures may exist, which only slightly differ in energy from the most stable structure. In connection with point (c), our results are in agreement with the possibility of polymorphism in isotactic polypropylene.

Energetic calculations of the chain conformation of isotactic polymers of olefins in the crystalline state

Abstract Energetic calculations have been carried out on a correlated series of isotactic aliphatic polymers (polypropylene (PP), poly-α-butene (PB), poly-3-methylbutene (P3MB), poly-(S)-3-methylpentene-1 (P(S)3MP) and isotactic polystyrene (PS). The possible variation of the CCC bond angles and of all the internal rotation angles (no group of atoms being taken as a unit) was considered. The possibility to predict the experimental helical parameters without any previous assumption is discussed. The differences in the shape of the minima for polymers with aliphatic and aromatic branched chains have been critically evaluated. For the P(S)3MP the results have shown how the asymmetric configuration of the side group influences the chain conformation of the polymer.

Conformational and packing energy of the crystalline α modification of syndiotactic polystyrene

Abstract Conformational and packing energy calculations have been performed on the α modification of syndiotactic polystyrene. The conformational energy has been optimized as a function of the internal parameters of the chain. The packing energy has been calculated considering at first the best packing of chains in triplets and then the best packing of triplets in the space groups R 3 and P 3 . The comparison of the results of the energy calculations with the X-ray experimental data corroborates a rhombohedral mode of packing of the triplets, corresponding to the space group R 3 .

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%).

Analysis of chain folding in crystalline isotactic polypropylene. The implications of tacticity and crystallographic symmetry

Abstract Tacticity and crystallographic considerations are used to predict fold models for a perfect crystal of isotactic polypropylene (α-form). If we assume adjacent re-entry chain folding we conclude that the folding plane must be parallel to the (010) plane. This is in agreement with morphological experimental evidence. We also show that this result is likely to apply even to non-perfect crystals.

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.