AbstractAn x‐ray diffraction method is described for the determination of per cent crystallinity in oriented films and fibers of polyethylene terephthalate by using an x‐ray diffractometer in association with a scaler counter. Results are presented to show that sample rotation approximately averages out the crystallite orientation effects. Results are presented on per cent crystallinity and specific volume of a number of films and fibers of polyethylene terephthalate possessing different degrees of crystallinity. A linear specific volume‐x‐ray crystallinity correlation is found. Extrapolation to zero and 100 crystallinity gives values of specific volume for amorphous and crystalline polyethylene terephthalate in good agreement with the measured specific volume of amorphous polymer and the specific volume calculated from the x‐ray crystal structure analysis made by Bunn2for crystalline polyethylene tere
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