Extended-chain fibres

Polymers Elastomers (rubbers) Soft plastics (e.g. low density PE) Isotropic hard plastics ( Amorphous [ Semi-crystalline Conventional fibres (nylon, PETP) "Extended chain" fibres (Extended zig-zag chains (Extended helical chains ( Glassy. Cross-linked (Tg < 275 K lTg> 325 K 0.001 0.2 2.5- 3 2.5- 5 1-3 3 5-15 250fl ca 50fl... 

There is overwhelming evidence that the aramide fibres possess a radially oriented system of crystalline supramolecular structure (see Fig. 19.1). The background of the properties, the filament structure, has been studied by Northolt et al. (1974-2005), Baltussen et al. (1996-2001), Picken et al. (2001), Sikkema et al. (2001, 2003), Dobb (1977-1985) and others. The aramid fibres (and the "rigid" extended chain fibres in general) are exceptional insofar as they were - with the rubbers - the first polymer fibres whose experimental stress-strain curve can very well be described by a consistent theory. 

The most extended chains, fibres and networks can be formed in the narrow intercellular space, but free ring-like and random coil forms could be found in liquid connective tissues. Consequently, the structure of hyaluronan in vivo should be viewed as a product of conformational ensembles and a result of the impact of local environment [8]. 

Note Fibrous crystals may comprise essentially extended chains parallel to the fibre axis however, macroscopic polymer fibres containing chain-folded crystals are also known. 

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). 

Until the 1970s there was a substantial gap between the theoretical modulus of polymer chains and the practical stiffness achieved in the existing processes. Since then the fibres made by extended chain crystallisation have bridged this gap. Solution spun fibres of high molecular linear polyethylene have been prepared with a Young modulus (at low temperature) of 90% of the theoretical value. Tables 19.9 and 19.10 illustrate the whole extent of elastic moduli in the various materials. 

A completely different approach to polymer crystallisation in extended-chain conformation became possible with the coming of a new class of polymers the para-para type aromatic polymers. These polymers possess inherently rigid molecular chains in an extended conformation (Preston, 1975 Magat, 1980 Northolt, 1974, 1980, 1985 Dobb, 1985). Theoretically they should give rise to high orientation in fibre form without the necessity of subjecting the as spun filaments to the conventional drawing process. 

A remarkable development in polymer science was the observation by A. J. Pen-nings that, when dilute solutions of polyethylene were cooled under conditions of continuous stirring, very fine fibres were precipitated on the stirrer. These fibres possessed a remarkable morphology consisting of a fine central core of extended chain polyethylene with an outer sheath of folded chain polymer material, so that electron microscopy revealed a beautiful shish-kebab structure. The possible significance of this result was recognised by Frank who emphasised the importance of extensional... 

Studies of the crystalline, thermal and mechanical properties of PPL and PMPPL indicate that the a-P transformation involves a change in conformation from a helix to the fully extended chain. A fibre repeat distance of 4.75 A, corresponding to the planar zigzag conformation, is evident in the x-ray diffraction patterns of the p-forms of PPL , PMPPL and PMEPL ... 

Dyneema UD is a unidirectional laminate made of two layers of extended chain polyethylene filament tows, cross plied and sandwiched in a thermoplastic film. It is one of the strongesf " and most exclusive fibre laminates designed for ballistic protection. It is very thin and one of the lightest ballistic protection materials available, with a relative density lower than that of water. - It offers good protection from cuts and slashes and will also help to protect against stabs. It will withstand temperatures of up to 140 °C and maintains its protective properties down to -150 °C (Fig. 7.15). 

Figure 11.31 shows how the DNA chains are wound around the histones, which are then tightly packed. Electron micrographs of extended chromatin fibres show how these histone units, called nucleosomes, are fastened together like a string of beads as indicated diagrammatically in Figure 11.31a). These units are then assembled into a tightly packed array as indicated in Figure 11.31b.

FRACTURE OF HIGHLY ORIENTED, CHAIN-EXTENDED POLYMER FIBRES... 

The time-dependent behaviour is different in the two types of highly oriented, chain-extended polymer fibres. Table 1 gives the results of studies in FIBRE TETHERS 2000 (1995), which were made because creep rupture is a concern in deep-water mooring of oil-rigs. The low-load creep in aramid fibres is due to a straightening of the initial. structure. It reduces in rate, even on a logarithmic scale, with time and is not a source of creep rupture. In Vectran, the creep is less and is absent after 10 days under load. 

The dominant factor influencing fracture in highly oriented, chain-extended polymer fibres is the high axial molecular strength, which depends on covalent bonding, and the... 

The strength of highly oriented, chain-extended polymer fibres is lime- and temperature-dependent, and, in principle, responses can be predicted by statistical mechanics. Fig. 14 compares experimental results with the theoretical predictions of Termonia and Smith (1986). The agreement is good. The Monte Carlo methods used in this and later studies are de.scribed in the paper by Termonia (this volume). 

Fracture of Highly Oriented, Chain-Extended Polymer Fibres 265... 

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