G. Guerra

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

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.

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.

Crystalline order and melting behaviour of isotactic polypropylene (α-form)

The dependence of the order in the orientation of the chains in the crystalline structure on the crystallization temperature is described for the α-form of isotactic polypropylene.From the present and previous results it is concluded that, for a fixed isotactic polypropylene sample, a biunivocal relation holds between a parameter selected as a measure to the degree of order and the melting endotherm position.ZusammenfassungDie Abhängigkeit der Orientierungsordnung von Ketten in kristallinen Strukturen von der Kristallisationstemperatur wird für die α-Form von isotaktischem Polypropylen beschrieben. Aus den jetzt und früher erhaltenen Ergebnissen wird geschlossen, daß für eine fixierte isotaktische Polypropylenprobe eine Beziehung zwischen einem als Maß für den Ordnungsgrad gewählten Parameter und der Lage der endothermen Schmelzpeaks besteht.РезюмеОписана зависимость степени ориентации ц епей в кристаллической ст руктуре α-формы изотактического пол ипропилена на темпер атуру его кристаллизации. На ос новании результатов настоящ ей работы и ранее полу ченных, установлен двухзнач ный характер связи между параметром, взя тым в качестве меры упорядоченности тве рдого образца изотак тического полипропилена, и поло жением эндотермы его плавления.

Possible Models for the Steric Control in the Heterogeneous High-Yield and Homogeneous Ziegler-Natta Polymerizations of 1-alkenes

In our research group, we have been investigating the mechanisms presi ding over the steric control in the Ziegler-Natta polymerizations of 1-alkenes to polymers of high or very high stereoregularity.

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.

Melting and solid phase transitions of isothermally crystallized copolymers of tetrafluoroethylene and hexafluoropropylene

Abstract The melting and the solid-solid transition behaviours of isothermally crystallized samples of FEP copolymers of various compositions are described. The results suggest that a significant contribution to the increases of the melting and transition temperatures, observed upon isothermal crystallization, is due to a reduction of the concentration of the perfluoromethyl groups in the crystallites, with partial segregation of them in the melt. Nevertheless a suitable extrapolation of the experimental melting temperatures leads to a reasonable value for the equilibrium melting temperature of poly(tetrafluoroethylene).

Unusual recrystallization and melting behaviour in DSC scans of isotactic polypropylene samples

The unusual dependence of the melting peak temperature of the species recrystallized during DSC scans on the isothermal crystallization temperature (Tc) is discussed for isotactic polypropylene samples.It is pointed out that such a phenomenon is not due to superheating effects; it is believed to be accounted for by assuming that the rates of the recrystallization phenomena at the same temperature are higher for samples obtained at higherTc values.ZusammenfassungDie ungewöhnliche Abhängigkeit der Temperatur des Schmelzpeaks der während des DSC-Versuchs rekristallisierten Species von der isothermen Kristallisationstemperatur (Tc) wird diskutiert. Es wird gezeigt, daß solch ein Phänomen nicht Überhitzungseffekten zuzuschreiben ist; unser Meinung nach kann es durch die Annahme erklärt werden, daß die Geschwindigkeiten der Rekristallisationsphänomene für bei höherenTc-Werten erhaltene Proben bei gleicher Temperatur größer sind.РезюмеДля образцов изотакт ического полипропил ена обсуждена необычная зависимость температурного пика плавления частиц, рекристаллизующихс я в течении ДСК измере ний на температуру изотерм ической кристаллиза цииTc. Отмечено, что это явле ние не может быть обусловлено эффекта ми перегрева и объясн яется тем, что при той же самой те мпературе скорость рекристалл изации выше для образ цов, полученных при более высоких значенияхTc.

Surface addition of flame retardants in acrylic cast sheets

A technique for the addition of flame retardants into outer layers of polymeric articles rather than in the whole mass is described. It is pointed out that this technique can be usefully applied in incorporating some flame retardant ad ditives in acrylic cast sheets.