IMPROVEMENTS IN THE USE OF ATTENUATED TOTAL-REFLECTION FOURIER-TRANSFORM INFRARED DICHROISM FOR MEASURING SURFACE ORIENTATION IN POLYMERS

Improvements in the use of attenuated total reflection (ATR) Fourier t ransform 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 intensitie s relative to a nondichroic band, prior to calculating dichroic ratios , eliminates the need to maintain identical contact areas/pressures wh en removing, rotating, and reclamping samples to the ATR element, whic h has been a major historical drawback to this technique. The normaliz ation is vital; it makes the calculated dichroic ratios largely insens itive to variations in sample/prism contact area, and less sensitive t o uncertainties in the refractive indices and birefringence of the pol ymer. 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 birefrin gent than PET can also be analyzed by using the formalism described in this paper, provided that the three independent indices are known. Th is paper is presented in two parts; first, equations are derived which allow the calculation of all second-order orientation parameters (P-2 00, P-220, P-202, and P-222), and the averaged squared direction cosin es, from the normalized ATR dichroic ratios. Second, we show how a sin gle-reflection diamond ATR unit is an ideal tool for this work, since it allows small, hard, or irregularly shaped samples to he examined wi thout fear of damaging the ATR element. We illustrate the technique us ing data obtained from a series of uniaxially drawn films, and one bia xially drawn film, using a commercially available accessory. From thes e data, orientation parameters were calculated as a function of draw r atio and compared with those obtained from specular-reflectance FT-IR and birefringence analysis of the same samples. The method should be a pplicable 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 dia mond). It must be realized that condition 1 is not trivial; careful in vestigation is required to identify truly nondichroic bands (if any ex ist for the polymer of interest).