Single crystal fibre

The graph shown in Figure 6.7 plots the strength versus the modulus of some typical fibres. Note the very special performance of the whiskers (single-crystal fibres). 

Whiskers are single-crystal fibres. They are very expensive and difficult to produce with only a few specific applications, for example, submicron gears or connectors. 

A continuous monocrystalline sapphire (A1203) fibre has been prepared as single-crystal fibres by LaBelle and Mlavsky using a modified czochralski puller and radio frequency heating. The technique adopted in this method is called edge-defined film-fed growth (EFG) [18-22], Figure 3.4 shows a schematic of the EFG method. 

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. 

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

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. 

O. Sudre, A. G. Razzell, L. Molliex, and M. Holmquist, Alumina Single-Crystal Fibre Reinforced Alumina Matrix for Combustor Tiles, Ceram. Eng. Sci. Proc., 19 [4] 273—280 (1998). 

The single fibre materials used were PDA single crystal fibres produced recently by Galiotis et al. [9-11], and aramid Kevlar 149 single fibres (du Pont de Nemours) tested in our laboratory using a now well established procedure [12-14]. In particular, the diameter of each Kevlar 149 fibre was measured (by optical microscopy) at three sites along the fibre length and the mean value taken as the diameter of the fibre. 

Studies of the optical reflection spectra of a POA-TS single crystal fibre subjected to uniaxial stress parallel to the direction hcwed that peak A In Fig. 1 shifted to hl er energy (28). The energy of the peak Increased linearly with strain by 37 meV/% up to the maximum 4% strain at which the fibre broke. 

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... 

Stress-strain curve for a polyDCHD single crystal fibre. The closed circles are for loading and the open ones for... 

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. 

This behaviour was thought to be due to the presence of surface defects which give rise to a stress concentration when the fibres are deformed. Since the size of the defects was found to scale with the fibre diameter the size dependence was predicted 1593. However, more detailed examination of the data in Figure 13 has shown that equation (1) Is not accurately obeyed for polydiacetyiene single crystal fibres, in addition. 

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... 

RJ. Young, Polymer single crystal fibres, in I.M. Ward (Ed.), Developments in Oriented Polymers—2, Applied Science, London, 1987. 

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

A model system which can be used is the diacetylene polymer for which the stress-strain curve was given in Fig. 5.34. By controlling the polymerization conditions it is possible to prepare single crystal fibres which contain both monomer and polymer molecules. The monomer has a modulus of only 9 GN m along the fibre axis compared with 61 GN m" for the polymer and the partly polymerized fibres which contain both monomer and polymer molecules are found to have values of modulus between these two extremes as shown in Fig. 5.36. The variation of the modulus with the proportion of polymer (approximately equal to the conversion) can be predicted by two simple models. The first one due to Reuss assumes that the elements in the structure (i.e. the monomer and polymer molecules) are lined up in series and experience the same stress. 

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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