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Lamellar thicknesses and cross-hatching frequencies in α-isotactic polypropylene have been measured for two series of fractions using linear nucleation to provide large arrays of oriented lamellae in row structures for sampling. One series is of high tacticity polymers differing in molecular mass from ∼6×10 4 to ∼8×10 5 , the other has low and high tacticity materials for ∼9×10 ...
There are two key features brought to crystallization by molecular length. First transverse lamellae which, for longer molecules, involves chainfolding and thence spherulitic growth. Second the basal lamellar surfaces; their formation is secondary to the crystallization of stems so that they are only well-defined for slow growth. For faster growth they are increasingly rough, unstable and prone to...
New experiments underpin the interpretation of the basic division in crystallization behaviour of polyethylene in terms of whether or not there is time for the fold surface to order before the next molecular layer is added at the growth front. For typical growth rates, in Régime II, polyethylene lamellae form with disordered {001} fold surfaces then transform, with lamellar thickening and twisting,...
The use of high-melting fibres as linear nuclei for quiescent polymeric melts is instrumental in providing the superior mechanical properties of polymeric self-composites. It also has inherent advantages in the elucidation of fundamental aspects of polymeric crystallization and self-organization, not least in allowing systematic microscopic studies of polymeric crystallization from nucleation through...
The wide-ranging survey of twisted growth in polymers by Lotz and Cheng cites extensive evidence consistent with the relief of surface stress being the underlying cause. This complementary note contributes to the discussion by making three main points. First, it is necessary to go further and explain the key issue of how a consistent twist is maintained when, as commonly, this habit has a lower symmetry...
The poorly characterized and little understood phenomenon of isothermal lamellar thickening, central to melt crystallization, has been studied morphologically in polyethylene rows, grown around high-melting fibres as linear nuclei revealing that thickening is a function of position within the morphology as well as of elapsed time. In contrast to polyethylene spherulites whose central lamellae are...
Linear polyethylene crystallized isothermally on linear nuclei from blends with low density, branched polyethylene, cellulates at low concentrations with individual lead lamellae, typically ~1 μm or so apart, advancing at a constant, faster rate than intervening lamellae whose growth is slowed by accumulated segregants. In these circumstances it is found that while initially all lamellae form with...
Observations of individual lamellae within spherulites of linear polyethylene, drawn under affine conditions between room temperature and ∼100 °C, show lamellae surviving to sample failure, thereby providing a strong memory of the initial morphology in the final product. Lamellae rotate and deform according to the angle their plane makes with the draw direction. Those parallel to the draw direction...
A number of polyurethanes are synthesized, based on the novel diisocyanate 4,4'-dibenzyl diisocyanate (DBDI), the traditional 4,4'- methylene bis(phenyl isocyanate) (MDI), or combinations of these. Two equivalent series are prepared, one with hydroxyterminated poly(ethylene adipate) and one with poly(tetrahydrofuran) macrodiol. Mechanical measurements and X-ray crystallinity data are shown for the...
The morphology of woven oriented polypropylene tapes and fibres has been studied both before and after processing by hot compaction. In this technique bundles of oriented tapes or fibres are subjected to suitable conditions of temperature and pressure so that just sufficient of each tape or fibre is selectively melted; on cooling, this material recrystallizes to bind the whole structure together....
Linear-low-density polyethylene shows novel features, in addition to those found for the linear polymer, when crystallized as row structures from linear nuclei. There is the same division between disordered and ordered (inclined) fold surfaces for faster and slower crystallization, respectively, with the changeover occurring at a lower temperature and for a longer time than the linear polymer. The...
The development of morphology in tapes and woven cloths of oriented melt-spun Tensylon polyethylene has been studied both before and after hot compaction over a range of temperatures below and above the optimum. For both the unidirectional fibres and the woven cloths, the optimum temperature was found to be where approximately 30% of the original structure was lost which, for Tensylon tapes, was ~2K...
A study of the habits of individual melt-grown polyethylene crystals has shown how banded spherulites form in this polymer and the essential cause to be the relief of fold surface stress. An initially planar lamella develops a characteristic S-profile over time extending out from central regions. This is a consequence of reducing surface stresses by improving fold packing whose spatial requirements...
Stable hexagonal phases have been identified in the three monodisperse long n-alkanes C 390 H 782 , C 294 H 590 and C 246 H 494 at pressures ~0.5GPa and above. Their isobaric temperature range of stability widens with increasing pressure but decreases for shorter molecular lengths. Crystallization...
The isothermal crystallization kinetics and morphologies of two centrally branched monodisperse alkanes have been studied and compared with two long n-alkanes, one of equal and one of half, the molecular length of the branched molecules. The two branched alkanes were C 96 H 193 .CHR.C 94 H 189 with R being methyl or butyl, respectively,...
At high temperatures, linear polyethylene crystallizes from the quiescent melt as lamellae with inclined, ∼{201}, fold surfaces. Accordingly, when grown on linear nuclei to form row structures, the first lamellae to form do so epitaxially in two orientations symmetrically inclined to the axis of the row. With increasing radial distance, dominant lamellae widen, separate and become normal to the row...
Specimens of the polyethylene Sclair 2907, full of large banded spherulites, have been drawn to high extensions, and parts of these annealed close to their melting points under constraint. Etching with permanganic reagents and examination under the scanning electron microscope revealed that in transverse sections of the unannealed specimens, the legacy of the banded spherulitic morphology is seen...
Morphological investigations on a range of polyethylenes, which differ with respect to their molecular weight characteristics and which possess quite distinctly different superstructures, non-spherulitic and banded spherulitic, have been performed to follow structural developments during drawing. Of particular significance is the detailed examination of transverse sections, in addition to the more...
The crystallization kinetics of the monodisperse n-alkane C98H198, growing as extended-chain lamellae from the melt, have been measured as part of a wider programme on these novel model systems for polymeric materials. Crystal growth rates decrease linearly, by a factor of 30, over the temperature range of 113.5–114.9°C, an interval which encompasses a significant change in habit from individual through...
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