Improvements in the Use of Attenuated Total Reflection Fourier Transform Infrared Dichroism for Measuring Surface Orientation in Polymers

Abstract

Improvements in the use of attenuated total reflection (ATR) Fourier transform infrared (FT-IR) dichroism for measuring surface orientation in polymer films are described, with poly(ethylene terephthalate) (PET) as an example material. It is shown that normalizing band intensities relative to a nondichroic band, prior to calculating dichroic ratios, eliminates the need to maintain identical contact areas/pressures when removing, rotating, and reclamping samples to the ATR element, which has been a major historical drawback to this technique. The normalization is vital; it makes the calculated dichroic ratios largely insensitive to variations in sample/prism contact area, and less sensitive to uncertainties in the refractive indices and birefringence of the polymer. For PET, it is shown that the birefringence can be neglected in the analysis, and a single approximate refractive index used. This is a significant benefit since the birefringence will vary as a function of orientation and crystallinity. Polymers that are much more birefringent than PET can also be analyzed by using the formalism described in this paper, provided that the three independent indices are known. This paper is presented in two parts; first, equations are derived which allow the calculation of all second-order orientation parameters (P₂₀₀, P₂₂₀, P₂₀₂, and P₂₂₂), and the averaged squared direction cosines, from the normalized ATR dichroic ratios. Second, we show how a single-reflection diamond ATR unit is an ideal tool for this work, since it allows small, hard, or irregularly shaped samples to be examined without fear of damaging the ATR element. We illustrate the technique using data obtained from a series of uniaxially drawn films, and one biaxially drawn film, using a commercially available accessory. From these data, orientation parameters were calculated as a function of draw ratio and compared with those obtained from specular-reflectance FT-IR and birefringence analysis of the same samples. The method should be applicable to any polymer provided that (1) a suitable nondichroic band is available for normalization and (2) the largest polymer refractive index lies well below that of the ATR element (2.4 in the case of diamond). It must be realized that condition 1 is not trivial; careful investigation is required to identify truly nondichroic bands (if any exist for the polymer of interest).