An explanation is presented for the broadening of intense spectral features observed in the infrared spectra of polyethylene terephthalate fibers prepared as KBr pellets. Since band broadening occurs in the spectra of stronger, undegraded fiber, it is proposed that, the stronger the fiber, the more the difficulty one has in pulverizing the sample. Larger particles result from undegraded polyester, which lead to broader bands for intense features. The local sample pathlength in the region of the large particles is greater than that for small particles, which can be more homogeneously mixed. Fiber samples can also be presented to an infrared microscope either "as is" or flattened. Striking differences exist between spectra for flattened and round fibers that were otherwise similar.
R. O. Carter, K. R. Carduner, M. C. Paputa Peck, and D. H. Motry, "The Infrared Analysis of Polyethylene Terephthalate Fibers and of Their Strength as Related to Sample Preparation and to Particle Size," Appl. Spectrosc. 43, 791-794 (1989)