Evaluation of the orientation coefficient for the c axis in poly(ethylene terephthalate) fibers

The literature methods for the determination of the mean of the crystallite orientation distribution for the c axis, that is of the orientation coefficient fc, for poly(ethylene terephthalate) (PET), based on the azimuthal scan of the (105) reflection, are reviewed. These methods appear unsuitable for samples presenting the “tilted orientation”; that is, the molecular chain axis inclined by some degrees with respect to the fiber axis, as frequently occurs for PET fibers. A new method for the determination of fc for PET, also based on the azimuthal scan of the (105) reflection (which can be applied also to samples with “tilted orientation”), is proposed. This method implies as a first step the determination of the tilt angle, for which the complete fiber pattern is required. A possible simplifying assumption, which allows use of the sole azimuthal (105) profile and makes the method also applicable to poorly oriented samples (for which the determination of the tilt angle is not easy), is also discussed.