Relationship between structure and mechanical properties for aramid fibres

The relationship between structure and mechanical properties for a series of twelve wellcharacterized aramid fibres has been determined. The fibres were produced under a variety of processing conditions and the fibre structure has been characterized using transmission electron microscopy. In particular, both the overall degree of molecular orientation in the fibres and the difference in structure between the fibre skin and core regions have been investigated in detail. The mechanical properties of the fibres have been evaluated using conventional mechanical testing and molecular deformation followed using Raman microscopy to monitor strain-induced band shifts. It has been shown that the mechanical properties of the fibres are controlled by the fibre structure. In particular, it is shown that the fibre modulus is governed by the overall degree of molecular orientation. It is also demonstrated that the fibre strength is controlled principally by the overall molecular orientation but may also be reduced by the presence of a highly-oriented skin region. It has been found that the rate of shift of the Raman bands per unit strain is proportional to the fibre modulus except for fibres with large differences in molecular orientation between fibre skin and core regions. For these fibres the rate of shift reflects the higher orientation of the skin.