Crystallinity polyethylene fibres

The results of such calculations for semi-crystalline polyethylene have been reviewed elsewhere [37]. A rather wide range of predicted values is obtained, due to the choice of force constants and also to sensitivity to detailed assumptions on the unit cell structure. In spite of these limitations the principal predictions for the elastic anisotropy are clear. These include the anticipated high values for C33 and the very low values for the shear stiffnesses C44, C55 and cee, which reflect the major differences between bond stretching and bond bending forces that control C33 and the intermolecular dispersion forces that determine the shear stiffnesses. It is therefore of value to compare such theoretical results with those obtained experimentally. Table 7.3 shows results for polyethylene where data for the orthorhombic unit cell at 300 K are used to calculate these constants for an equivalent fibre (Voigt averaging procedure see Section 7.5.2 below) compared with ultrasonic data for a solid sheet made by hot compaction. It can be seen that... 

The structure of high modulus polyethylene fibres obtained by optimized drawing of linear polyethylene is viewed as crystalline lamellae linked by intercrystalline bridges.Accordingly, the component B is then viewed as crystalline, and its content (1 — >1) corresponds to the volume fraction of the material incorporated in the crystalline bridges. A more complex model consisting of four components has been proposed for these fibres by Grubb. 

In fibres of some polymers, made under certain conditions, the crystalline regions are found to be tilted with respect to the fibre axis in a well-defined crystallographic direction. This is a very valuable feature, because the diffraction patterns of specimens in which this type of orientation occurs are of precisely the same form as tilted crystal diffraction patterns of single crystals rotated round a direction inclined to a principal axis. The unit cell cannot be obtained directly, for 90° oscillation tilted crystal photographs are required for direct interpretation, but unit cells obtained by trial can be checked by the displacements of diffraction spots from the layer lines this is a severe check, and consistent displacements would leave no doubt of the correctness of a unit cell. This procedure played an effective part in the determination of the unit cell of polyethylene terephthalate (Daubeny, Bunn, and Brown, 1954). 

A clear example for the absence of Xe in crystalline polymers is shown in Figure 12.2 for highly stretched polyethylene (PE) fibres. 

FIG. 19.1 Morphological models of some polymeric crystalline structures. (A) Model of a single crystal structure with macromolecules within the crystal (Keller, 1957). (B) Model of part of a spherulite (Van Antwerpen, 1971) A, Amorphous regions C, Crystalline regions lamellae of folded chains. (C). Model of high pressure crystallised polyethylene (Ward, 1985). (E) Model of a shish kebab structure (Pennings et al., 1970). (E) Model of paracrystalline structure of extended chains (aramid fibre). (El) lengthwise section (Northolt, 1984). (E2) cross section (Dobb, 1985). 

Regioselective dialkylation of naphthalene is another reaction of considerable interest as 2,6-dialkylnaphthalenes can be oxidised to naphthalene-2,6-dicarboxylic acid, which is used in the synthesis of the commercially valuable polymer, poly(ethylene naphthalenedicarboxylate) (PEN).22 PEN has properties that are generally superior to those of polyethylene terephthalate) (PET) and has become the polymer of choice for a variety of applications such as in films, industrial fibres, packaging, liquid crystalline polymers, coatings, inks and adhesives. However, the high cost of naphthalenedicarboxylic acid has been a major hindrance to widespread application. 

The significance of the amorphous regions in relation to the second order transition is shown clearly in the curves in Fig. 7.11. Orion, the most crystalline of the three fibres shows the least reduction of stiffness with increase of temperature. Dynel, a copolymer, with the least crystallinity exhibits the greatest reduction, and lying between these two is an experimental acrylic fibre specially prepared with an intermediate degree of orientation. Second order transition temperatures are polyethylene tere-phthalate (partly crystalline) 81 "C, nylon 66 (partly crystalline) 47°C, and polyacrylonitrile 81 "C. 

The properties of elastomeric materials are greatly influenced by the strong inter-chain, i.e., intermolecular forces which can result in the formation of crystalline domain. Thus the elastomeric properties are those of an amorphous material having weak inter-chain interaction and hence no crystallisation. At the other extreme of polymer properties are fibre-forming polymers, such as Nylon, which when properly oriented lead to the formation of permanent crystalline fibres. In between these two extremes is a whole range of polymers, from purely amorphous elastomers to partially crystalline plastics, such as polyethylene, polypropylene, polycarbonate, etc. 

Finally, there is the very interesting result that in oriented linear polyethylene and polypropylene fibres, a three component NMR spectra has been observed, first by Hyndman and Origlio. " In addition to the broad crystalline component and the narrow amorphous component, there was a third component of intermediate line width, which was attributed to strained amorphous material. This intermediate component was much reduced in intensity on annealing and consequent shrinkage of the fibre, which is consistent with the proposed interpretation. A three-... 

The tubular-film process is unsuitable for polymers with a very low melt strength such as polyethylene terephthalate. It is also not suitable for polypropylene films for packaging because the films are too crystalline, opaque and brittle due to too slow cooling. However, these films are often used as a precursor for fibrillated film fibres. 

The use of a fibre-coupled confocal Raman microscope and an infrared microscope for both point mapping and global imaging in the study of spatial variations in polymer chemistry and morphology is illustrated by studies of the curing of the UV-cured acrylate coatings, crystallinity in drawn polyethylene terephthalate (PET) film, molecular orientation in PET bottles, and the analysis of a PES/PEES copolymer blended with epoxy resin and cured at elevated temperature. 8 refs. 

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