Polydiacetylene single crystals

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]. 

Cottle et al. [15] reported in 1978 that the imposition of hydrostatic pressure caused an increase in frequency of the four Raman-active vibrational modes in 

Values of the strain-indiioed Raman band shifts for the C=C stretching band of different substituted polydiacetylene single crystals 

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. 

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Weiser G (1992) Stark-effect of one-dimensional Wannier excitons in polydiacetylene single-crystals. Phys Rev B 45 14076... 

Fig. 33 Photocurrent (transient maxima) versus light intensity for PTS-polydiacetylene single crystals. (After Reimer and Bassler, 1975)...

Fig. 34 Photocurrent (steady state) versus light intensity for PTS-polydiacetylene single crystals. Data for F II b taken under illumination with 457 nm line of Ar laser F L b measured using a tungsten lamp (457 nm). Electric field, F = 4 x 103 V cm-1. (After Lochner et al., 1976a)...

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.,...

Fig. 37 Temperature dependence of the photocurrent for both field directions (F Lb, F II b) for PTS-polydiacetylene single crystal. (After Lochner et al, 1976b)...

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. 

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)]. 

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. 

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. 

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). 

More recently, Schmid et al. [12] have shown that a separation of the different contributions to the degenerate four-wave mixing signal can be obtained by a modulation technique two of the incoming laser beams are chopped at different frequencies /, and /2 and the detection occurs at special combination frequencies of /, and /2- This technique was applied to the measurements of the third- and fifth-order autocorrelation functions of a picosecond laser using a polydiacetylene single crystal. 

The initial study employing polydiacetylene single crystals proved extremely successful (177). This system is quite interesting for several reasons ... 

Reimer, B. and Bassler, H., Motion and recombination of charge carriers in a polydiacetylene single crystal. Chem.Phys. Letts. (1976), 43, 81. 

Reimer, B. and Baueisler, H., Trauisient photoconduction in a polydiacetylene single crystal. Determination of transit time, free lifetime amd recombination time of chaurge caurriers. Phys.stat.solidi. (b) (1978), 85(1), 145. 

In polydiacetylene single crystals electroreflectance revealed the weak interband transitions, hidden under the stronger vibronic excitons (2). The data yield the band gap, the exciton binding energy and derive in case of DCHD a surprisingly small effective mass m = 0.05 m (3). Charge transfer excitons too respond to an external... 

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. 

Although it Is the Young s modulus In the chain direction that Is of the greatest Importance for engineering applications. It must be remembered that a large number of elastic constants are needed to fully describe the elastic behaviour of a crystal [4]. Polydiacetylene single crystals usually possess monocllnic symmetry and therefore have 13 elastic constants. There have been several attempts to calculate the 9 elastic constants for orthorhombic polyethylene [42] but as yet there has been no similar calculation for a... 

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