Characterization of Precursor and Stabilized Fiber

High molecular orientation is generally considered to be an indispensable prerequisite for good carbon fibers. As mentioned in Section 3.3, there is evidence in the literature that molecular order in the precursor fiber may take effect on carbon fiber properties. 

The sound velocity in a fiber, and the sonic modulus calculated therefrom, are related to molecular orientation (De Vries ). As shown by Moseley ), the sonic modulus is independent of the crystallinity at temperatures well below the T (which means that the inter- and intramolecular force constants controlling fiber stiffness are not measurably different for crystalline and amorphous regions at these temperatures). An orientation parameter a, calculated from the sonic modulus, is therefore taken as a measure for the average orientation of all molecules in the sample, regardless of the degree of crystallinity. The parameter is called the total orientation , as contrasted to crystalline and amorphous orientation, from X-ray data. 

The relationship between sonic modulus and sound velocity C is given by  

If the modulus is given in units of force per unit cross-sectional area, k is equal to the density. If E is expressed in units of force per unit linear density, k is a universal constant, depending only on the units of and C. In particular, E = 1.25 C, if is given in tex, and C in km/sec. 

Anticipating experimental results reported in detail in Section 4, we present in Fig. 9a correlation of the sonic modulus of carbon fibers and that of their precursors. Included are a variety of copolymers and blends. (For identification of the samples see Table 11.) Although there is some scattering, the trend shows clearly that high molecular order in the precursor is certainly a desirable property. 

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