Mechanical properties of uniaxially oriented polymers fibres

MECHANICAL PROPERTIES OF UNIAXIALLY ORIENTED POLYMERS (FIBRES) 

If an isotropic polymer is subjected to an imposed external stress it undergoes a structural rearrangement called orientation. In amorphous polymers this is simply a rearrangement of the randomly coiled chain molecules (molecular orientation). In crystalline polymers the phenomenon is more complex. Crystallites may be reoriented or even completely rearranged and oriented recrystallisation may be induced by the stresses applied. The rearrangements in the crystalline material may be read from the X-ray diffraction patterns. 

Nearly all polymeric objects have some orientation during the forming (shaping) of the specimen the molecules are oriented by viscous flow and part of this orientation is frozen in as the object cools down. But this kind of orientation is negligible compared with the stress-imposed orientation applied in drawing or stretching processes. 

Orientation is generally accomplished by deforming a polymer at or above its glass transition point. Fixation of the orientation takes place if the stretched polymer is cooled to below its glass transition temperature before the molecules have had a chance to return to this random orientation. By heating above the Tg the oriented polymer will tend to retract in amorphous polymers the retractive force is even a direct measure of the degree of orientation obtained. 

Orientation has a pronounced effect on the physical properties of polymers. Oriented polymers have properties, which vary in different directions, i.e. they are anisotropic. 

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