Diffuse reflectance and photoacoustic (PAS) IR techniques are used for spectral measurements of poly(butylene terephthalate) (PBT) fibers. The spectral observations on the fibers are similar to those observed in films. The stress-induced phase transformation is reversible with annealing of the drawn fibers. The annealing process also increases the amount of crystallinity in the fiber. With the use of recently developed PAS methodology to study orientation of the surface species, the orientation of surface aromatic and carbonyl groups is analyzed. In the alpha phase, the surface aromatic rings are preferentially perpendicular on the surface whereas carbonyl groups are preferentially parallel. A similar orientation is observed in the drawn fibers. Annealing of the drawn fibers changes the orientation of the aromatic rings. Upon annealing, the phenyl rings are preferentially parallel to the surface. The depth of penetration at 3000 cm<sup>−1</sup> in PBT fibers is found to be approximately 4 μ.

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