Polydiacetylene single-crystal polymers

The polymer surface, as in the bulk itself, may vary from partially polycrystalline to amorphous. Except for the polydiacetylenes, single crystal polymer surfaces are essentially unknown. 

Fig. 35 Transient photocurrent action spectra for holes and electrons in PTS (p-toluenesulphonate)-polydiacetylene single crystals. The dashed curve in the spectra for holes corresponds to the absorption spectrum of the polymer dispersed in KBr. (After Chance et al.,...

FIGURE 13-30 The reciprocal tensile strength of polydiacetylene single crystals [redrawn from the data of C. Galiotis and R, J. Young, Polymer, 24, 1023 (1983)]. 

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. 

Polydiacetylenes. Single crystals of poly(TCDU) and poly— (DMDA) were deposited on a variety of alkali halide crystals. The resultant morphology of both the monomers and polymers were identical and were elongated platelets oriented in the substrates <110> directions (Figures 5 and 6). 

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. 

The first reported and one of the best examples of the use of Raman spectroscopy to follow drformation in polymers is the case of substituted polydiacetylene single crystals [8-12]. The macroscopic polymer crystals are produced by the solid-state polymerization of substituted diacetylene single crystal monomers. The reaction is a topochemical solid-state polymerization [13], and the oystals produced have a high degree of perfection [14]. 

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

The preparation of polydiacetylene single crystals will be outlined. Studies of oligomeric intermediates observed during polymerization will be described since they are of theoretical interest and provide information on the properties of short polymer chains which cannot be obtained in any other way. The electronic properties of these crystals, as revealed by studies of their spectra and electrical conductivity, will be discussed together with models used to interpret them. Finally studies of disordered systems will be described, in particular the recent observations of carrier recombination in damaged and disordered samples. The outstanding experimental and theoretical problems will be emphasised. 

The solid-state polymerization of diacetylenes is an example of a lattice-controlled solid-state reaction. Polydiacetylenes are synthesized via a 1,4-addition reaction of monomer crystals of the form R-C=C-CeC-R. The polymer backbone has a planar, fully conjugated structure. The electronic structure is essentially one dimensional with a lowest-energy optical transition of typically 16 000 cm-l. The polydiacetylenes are unique among organic polymers in that they may be obtained as large-dimension single crystals. 

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