MEASUREMENT OF SURFACE ORIENTATION IN UNIAXIAL POLY(ETHYLENE-TEREPHTHALATE) FILMS USING POLARIZED SPECULAR REFLECTANCE FOURIER-TRANSFORM INFRARED MICROSCOPY

Polarised Fourier transform infrared (FT-IR) external-reflection spect roscopy has been used to measure molecular orientation at the surface of uniaxially drawn poly(ethylene terephthalate) (PET) films as a func tion of draw ratio. The spectra, which were obtained using an FT-IR mi croscope operating in the reflectance mode, were of high signal-to-noi se (S/N) ratio, and contained only the specular component of the refle cted radiation. The dichroic ratio of the 1019 cm(-1) ring stretching band was used to measure the PET orientation function, P-200. Prior to quantifying band intensities, the reflectance spectra were either tra nsformed using the Kramers-Kronig algorithm, or simply differentiated; it was found that the dichroic ratios obtained using either pretreatm ent were similar. It was shown that the P-200 values obtained using th e FT-IR method compared well with those obtained using surface refract ive index measurements, provided that the intensity of the 1019 cm(-1) band was normalised relative to a non-dichroic band in the spectrum p rior to computing dichroic ratios. The use of a reflecting microscope to perform the analysis of surface orientation not only simplifies sam ple alignment compared with ''macro'' reflectance accessories, but als o allows awkward-shaped or small samples to be examined with relative ease. Unfortunately, a significant current limitation is that samples must be sufficiently ''optically thick'' to prevent double-pass (''tra nsflectance'') radiation reaching the detector and yielding non-specul ar features in the reflectance spectrum. For polyaromatics, this proba bly means thicknesses in excess of 50 mu m to obtain good spectra in t he fingerprint region. Polarised attenuated total reflectance microsco py may present an alternate approach for examining thinner films.