Orientation in synthetic polymers the drawing of thermoplastics

The idealised case of cold drawing which has been discussed, is typical of the behaviour of several major thermoplastics such as polyethylene, polypropylene, nylon and polyethylene terephthalate, which readily cold draw at ambient temperatures. Other thermoplastics, which 

Not all polymers show cold drawing there are requirements such as a minimum molecular weight for strain hardening, and this has been extensively discussed elsewhere (see for example Ref. 5, pp. 271 and 322). It is clearly necessary for the polymer to be above its brittle-ductile transition, but this is a necessary rather than a necessary and sufficient condition for cold drawing. It should perhaps also be emphasised that there is no immediately obvious relationship between the glass transition temperature and the brittle-ductile transition temperature Tb (see for example Ref. 6). 

It is possible to produce orientation at normal temperatures in a brittle thermoplastic like polystyrene, by the application of a sufficiently high hydrostatic pressure during extension. The hydrostatic pressure prevents brittle failure at small strains, and can be regarded as reducing Ty, below ambient temperature. Similar results may be obtained by hydrostatic extrusion, where a billet of the polymer is forced through a reducing die under the pressure of a hydraulic fluid. An example of the properties achieved in this way has been given in Section 1.2 above. 

In the case of thermosets, deliberate and extensive orientation is virtually unknown. This appears to be the result of the practical difficulties involved, rather than from any theoretical obstacle. For example, it is possible that the fibre Kynol produced by the Carborundum Corporation is oriented to some extent. This is produced from a melt-spun Novolak phenol-formaldehyde resin, which is later further cross-linked with formaldehyde. It is, of course, legitimate to consider carbon fibres as extreme examples of thermosets. Formed by the cyclisation and subsequent graphitisation of polyacrylonitrile (or other suitable precursors), they are highly oriented. 

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