Polydiacetylenes fibres

Kelly (29) has estimated the strength of a polyethylene molecule as 6 nN but this is now thought to be rather high (30). Kausch (31) has shown that a covalently-bonded molecule should be broken by a force of the order of 3 nN-identical to the value determined experimentally for polyDCHD. Hence it can be seen that these high levels of strength measured for the polydiacetylene fibres are consistent with the predictions of Frank (27) in 1970. 

Micromechanisms of reinforcement. It was pointed out earlier that the vibrational frequencies of certain main-chain Raman active modes are found to change with the level of applied strain (26). The biggest change is found for the C=C triple bond stretchTng frequency which is of the order of 20 cm-i/%. This property can be used to determine the strain in any polydiacetylene fibre subjected to any... 

Recent investigations [61. 623 into the behaviour of polydiacetylene fibres In epoxy resin matrices have shown that not only such composites have good mechanical properties but that Important fundamental details of the mechanisms of fibre reinforcement can also be revealed from their study. 

The increasing use of optical fibre in the telecommunications network will, ultimately, require all-optical signal processing to exploit the full bandwidth available. This has led to a search for materials with fast, large third order optical nonlinearities. Most of the current materials either respond in the nanosecond regime or the nonlinearity is too small (1-3). Organic materials are attractive because of their ultra-fast, broadband responses and low absorption. However the main problem in the materials studied to date, e.g. polydiacetylenes (4) and aromatic main chain polymers (5), has been the small nonlinear coefficients. 

Folydlacetylenes offer a unique opportunity of studying structure/property relationships in polymers. This paper is concerned with structural factors which control mechanical properties. The effect of the size of side-groups upon the Young s moduli of different polydiacetylenes is discussed briefly. The effect of internal and surface defects upon the strengths of individual fibres is also described. Examples are given of how Raman spectroscopy can be used to follow the deformation of fibres and it is shown how this can be extended to fibres in composites. The general mechanical properties of the composites are also described. 

This paper reviews recent work upon structure/mechanical-property relationships in polydiacetylenes. It is shown how this has led to the development of high strength polydiacetylene single crystal fibres and their performance as reinforcing fibres in composites is described. 

Morphology. In general,polydiacetylene single crystals are found in one of two crystal morphologies, either as lozenges or as fibres. 

Figure 1 (a) Photograph of polydiacetylene single crystal fibres on mm graph paper, (b) Lattice planes in poly DCHD, spacing 1.2 nm. 

Stress/strain behaviour. In 1974, Baughman, Gleiter and SeAdfeld (.2 demonstrated that fibre-like crystals of the phenyl urethane substituted polydiacetylene, polyPUHD could be deformed elastically to strains of over 3%. The crystals were found to have high values of Young s modulus in the chain direction of the order of 45GPa. 

Creep. One of the most remarkable aspects of the deformation of polydiacetylenes is that it is not possible to measure any time-dependent deformation or creep when crystals are deformed in tension parallel to the chain direction (14,24). This behviour is demonstrated in Figure 3 for a polyDCHD crystal held at constant stress at room temperature and the indications are that creep does not take place at temperatures of up to at least 100 C (24). Creep and time-dependent deformation are normally a serious draw-back in the use of conventional high-modulus polymer fibres such as polyethylenes (28). Defects such as loops and chain-ends allow the translation of molecules parallel to the chain direction in polyethylene fibres. In contrast since polydiacetylene single crystal fibres contain perfectly-aligned long polymer molecules (cf Figure lb) there is no mechanism whereby creep can take place even at high temperatures. 

It has been demonstrated that polydiacetylene single crystal fibres are relatively perfect and have excellent molecular alignment. In consequence they display high values of stiffness and strength and are very resistant to creep. It has been shown that such fibres have considerable promise as reinforcing fibres in an epoxy resin matrix and the study of such composite systems has enabled considerable fundamental information to be obtained concerning the mechanisms of fibre reinforcement. 

Electrochromatic carbon nanotube/polydiacetylene nanocomposite fibres. Nat. NanotechnoL 4,738-741. 

In addition to giving a measure of the deformation within fibres, the strain-induced band shifts in Raman spectra have been used to follow the micromechanics of fibre reinforcement in model polydiacetylene/epoxy composites (13). The critical length has been measured directly (13) and the effect of resin shrinkage has been examined in detail (14,15). It has also been demonstrated that the technique can be employed to measure fibre strain optically in a high voliine fraction Kevlar 49/epoxy composite (16). In this present paper we demonstrate that Raman microscopy can also be used to measure fibre strain in carbon fibre reinforced PEEK composites (17) and to give a direct measure of residual thermal shrinkage stresses in PEEK matrix composites. 

Recent results on the fracture behaviour of advanced fibers and model composite materials (microcoiposites) are presented. The fiber data utilized are strength results for polydiacetylene vdiiskers and Kevlar 149 fibres. The probabilistic approach adopted for the effect of fibre... 

Polydiacetylenes allow a unique opportunity to study the relationship between structure and mechanical properties in polymer crystals. The technique of solid state polymerization 11] enables highly-perfect poiydiacetylene single crystals to be produced with macroscopic dimensions. For example single crystal fibres can be grown with lengths in excess of 50 mm 12.3]. Crystalline polymers produced by crystallization from both dilute solution and the molten state are invariably only semi-crystalline 14]. Melt-crystallized... 

Dependence of the Young s modulus of polydiacetylene single crystal fibres upon the reciprocal of the area supported by each polymer chain. The line S and the open circles are for values calculated using force-constants determined by Raman Spectroscopy. The line M and the closed circles are for the mechanically-measured ones. 

It is well-established that the composites produced by incorporating high-modulus fibres in a matrix of epoxy resin or metal can have outstanding mechanical properties. Polydiacetylenes offer considerable promise for reinforcement In polymeric matrices because they have the following properties which have been described in previous sections. 

The dependence upon strain of the wavenumbers for the Raman modes of several different substituted polydiacetylene single crystal fibres has been measured by various groups of workers [10,12,14]. Most attention has been paid to the behaviour of the mode which is essentially the symmetrical stretching mode of the C C triple l nd, as this is the most sensitive to applied strain. The dependence of the wavenumber of this band upon applied strain for a polyTSHD single crystal [11] is shown in Figure 8.2, and the dependemx of the position of this Raman band upon strain for four polydiacetylenes [14] with different... 

Over the IS years since the original Raman deformation studies upon polydiacetylene single crystals, the technique has been developed and refined to involve the study of a wide range of different high-performance polymers and other materials. These have included rigid-rod polymer fibres [19-21], carbon fibres [22-24] and ceramic fibres [2S-27]. This present chapter will concentrate upon recent research concerning the use of Raman spectroscopy to follow the deformation of aramid fibres and gel-spun polyethylene fibres and the possibility of the extension of the technique to isotropic polymers, and also the important and developing application of the method to the study of the deformation of fibres within composites. 

Fig. 4.12 Examples of polydiacetylene single crystals in the form of (a) lozenges and (h) fibres. The crystals are placed on graph paper with mm spacing (crystals supplied by Dr D. Bloor).

Fig. 5.34 stress-strain curve obtained for a polydiacetylene single crystal fibre. 

Fig. 5.36 Dependence of the modulus of polydiacetylene single crystal fibres upon conversion into polymer.

Fig. 5.63 Scanning electron micrograph of a fractured polydiacetylene single crystal fibre.

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