Cold-Drawing and the Natural Draw Ratio

The simplest explanation is that there is a rubber-like network present and that this has a maximum extensibility due to the degree of entanglement, which is constant for a given grade of polymer and depends on its molar mass and method of polymerisation. This limiting extensibility is not to be confused with the limit of applicability of the affine rubber model for predicting orientation distributions discussed in section 11.2.1 because the limiting extension can involve non-affine deformation. 

The natural draw ratio for amorphous polymers is very sensitive to the degree of preorientation, i.e. the molecular orientation in the polymer before cold-drawing. This was reported for polyethylene terephthalate by Marshall and Thompson [20] and for PMMA and polystyrene by Whitney and Andrews [26]. 

It has been proposed [78] that the sensitivity of natural draw ratio to pre-orientation arises as follows. The extension of an amorphous polymer to its natural draw ratio is regarded as equivalent to the extension of a network to a limiting extensibility. This limiting extensibility is then a function of the original geometry of the network and the nature of the links of which it is comprised. 

The dimensions of the unstrained network can be measured by shrinking the pre-oriented fibres back to the state of zero strain, that is isotropy [78]. These results can then be combined with measurements of the natural draw ratio to give the maximum extensibility for the network. 

Drawing to a length I2 gives a natural draw ratio 

Initial birefringence (x 10 ) Natural draw ratio, N Shrinkage, s 1 -s N/a -s) 

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