RELATIONSHIP BETWEEN STRUCTURE AND MECHANICAL-PROPERTIES IN HIGH-MODULUS POLY(2,5(6)-BENZOXAZOLE) (ABPBO) FIBERS

The relationship between structure and mechanical properties has been investigated for high-modulus fibres of poly(2,5(6)-benzoxazole) (ABPBO). It has been found that the modulus of the fibres increases from 91 to 133 GPa upon heat treatment at 525-degrees-C. The strain to failure decreases from about 4.3% to 2.0%, whereas the fibre strength remains virtually unchanged at about 2.1 GPa. The improvement in fibre modulus is accompanied by the development of a three-dimensional crystal order in the fibres. Molecular deformation in the fibres has been monitored using Raman microscopy and it has been shown that the position of the Raman bands in the fibres is sensitive to the application of mechanical stress. This has enabled a detailed study of molecular deformation in the fibres to be undertaken for both tensile and compressive deformation. It has been found that failure takes place at relatively low stresses and strains in compression through the formation of kink bands. It has also been shown that the fibres tend to have a lower modulus in compression than for tensile deformation. The molecular processes involved during fibre deformation have been discussed in detail.