Relations force-longueur a temperature ambiante determinees pour une serie de polyethylenes d'une large gamme de masse moleculaire et contribution moleculaire

Influence of structural and morphological factors on the mechanical properties of the polyethylenes

Relaxations α et β d'une variete de polyethylenes etudiees en detail par des methodes mecaniques dynamiques a basse frequence

Dynamic mechanical studies of α and β relaxations of polyethylenes

In the twenty years since this subject was last reviewed for this journal by Andrew Keller there has been an almost explosive growth of research in the area of crystallisation of polymers. This growth has been almost exclusively in the area of crystallisation of polymers from the molten state, whereas when Keller wrote his review most of the work was on crystallisation from solution. At that time, the research was conducted exclusively on flexible chain polymers, but during the past ten years or so rigid and semi-rigid crystallisable polymers have been synthesised and become available in large enough quantities for physical measurements to be carried out. Our level of understanding of such systems at this time does not warrant an extensive review, so the major concerns of this area are simply summarised. The major developments in our understanding of polymer crystals and the kinetics of their formation have been many. It is now established that adjacent re-entry folding of the molecule occurs on crystallisation from solution, but when crystallised from the melt the molecular trajectory approximates a random coil when viewed from a distance. From a closer viewpoint, it appears that adjacent re-entry folding is only one contributor and that any given molecule exits and re-enters several crystals in a manner that is determined by the crystallisation temperature. In addition, it is now accepted that the interface between the crystal and the random melt may be macroscopic in nature. Variously referred to as rigid amorphous phase or interzonal material, the extent of formation of such an intermediate state is strongly dependent on the rigidity of the given polymer molecule. Our understanding of the kinetics of crystallisation has taken a major step forward through the realisation that three regimes exist, dependent on the relative rates of secondary nucleation and surface spreading. At the time of writing, the third regime is believed to be unique to long chain molecules, whereas the first two regimes have been observed in atomic solids. Although we are now beginning to understand the influence of molecular length and microstructural impurity levels on regime behaviour, our definitive knowledge is still in its formative stages. A further important development is the recognition that the skeletal structure of common superstructural crystalline aggregates, such a spherulites, is determined by a small number of leading or dominant lamellae and that the bulk of the crystallinity develops behind the growth front. Such behaviour had been recognised for many years in low-crystallinity polymers, but is now a recognised feature of growth in common high-crystallinity polymers also. This review was received in June 1989. 0034-4885/90/050549 + 56$14.00 @ 1990 IOP Publishing Ltd 549

REVIEW: Polymer crystals

Solid-state reorganization, melting and melt-recrystallization of conformationally disordered crystals (α′-phase) of poly (l-lactic acid)

In this paper, we report differential scanning calorimetry studies of the temperature and molar mass dependences for the primary and secondary crystallization behavior of bisphenol A polycarbonate ...

Influence of structural and topological constraints on the crystallization and melting behavior of polymers: 3. Bisphenol a polycarbonate

Changes in the crystallite size distribution during the isothermal crystallization of linear polyethylene fractions were studied using low frequency Raman spectroscopy (LAM). Increases in the crystallite size and broadening distributions were observed as a function of time for all molecular weights studied. Large increases in size occurred even after most of the isothermal crystallinity had formed. The rate of increase was faster for higher crystallization temperatures, while the process was retarded for the higher molecular weights. Similar results were obtained for an unfractionated polymer except for complications that are likely due to molecular weight fractionation.

Changes in Crystallite Size Distribution During the Isothermal Crystallization of Linear Polyethylene

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Changes in Crystallite Size Distribution During the Isothermal Crystallization of Linear Polyethylene