Beniamino Pirozzi

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.

Geometrical and energetical analysis of the chain conformations of syndiotactic polystyrene in the crystalline state

Abstract A geometrical and energetical analysis of the chain conformation of syndiotactic polystyrene has been performed. The geometrical analysis shows the internal parameters in the line repetition groups which are compatible with the constitution and configuration of the polymer. Conformational energy calculations performed by maps and minimizations have given the best conformations in the crystalline field having chain axes in accordance with experimental data. The results provide corroborating evidences in favour of the existence of conformational polymorphism in syndiotactic polystyrene.

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 .

Conformational and packing energy calculations on the two crystalline modifications of poly(trans-1,4-butadiene)

Conformational and packing energy calculations have been performed on the two crystalline modifications of poly(trans-1,4-butadiene), and the results have been compared with experimental data. The conformational energy calculations predict the chain axis and conformation of the modification that is stable below 76°C, without any a priori assumption. Packing energy calculations on this modification show that the best space group is P21a and the position of the chains is in good agreement with X-ray data. Conformational energy calculations on the modification that is stable above 76°C predict that the lowest energy conformation is a statistical sequence with a random distribution of the three minimum torsion angles around the single bonds adjacent to the double bond, with a trans conformation around the other single bond.

The role of intra- and intermolecular interactions in determining the conformation and mode of packing of crystalline polymers—I. Poly(cis-1,4-butadiene)

Abstract Conformational energy calculations on an isolated chain of poly(cis-1,4-butadiene) have been performed, allowing for the variation of all bond angles and torsion angles. Various minimum energy and high symmetry conformations are accessible to the chain. Packing energy calculations have been performed for chains having tci symmetry, allowing for the variation of symmetry, unit cell constants and internal rotation angles. Unlike the behaviour observed for other polymers, the chain conformation which minimizes the total (conformational plus packing) energy is sensibly different from that of minimum energy for the isolated chain. The agreement between experimental and calculated data of the crystal structure analysis may be considered as very good.

Thermodynamic stabilities of the three crystalline forms of isotactic poly-1-butene as a function of temperature

Abstract Thermodynamic stabilities of the 3 crystalline forms of isotactic poly-1-butene as a function of the temperature has been evaluated by packing energy calculations using the values of the axes of the unit cells obtained by X-ray measurements. The lengths of the axes of the unit cells have been determined from X-ray powder spectra at different temperatures in a range between 170 K and temperatures near the melting point for each form. The packing energy has been calculated by minimizations performed using the experimental values of the axes of the unit cells at the various temperatures. The results show that form 1 is the stablest form in all the range of temperature and that form 3 is lower in energy with respect to form 2, but the 2 forms become almost isoenergetic at high temperatures.

Geometrical and energetical feasibility of a highly extended chain conformation for isotactic polystyrene

Abstract Highly extended, four-atom helices with h = 5.1 A and t = 60° are geometrically feasible for a vinyl polymer. The energetical feasibility for the case of isotactic polystyrene (in crystalline gels) is discussed on the basis of appropriate conformational maps.