A Critical Evaluation of Internal Reflection Spectroscopic Methods for Determining Macromolecular Orientation in Polymer Surfaces

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Abstract
A critical evaluation was made of the available internal reflection spectroscopic (IRS) methods for the determination of macromolecular orientation in surfaces. A method involving the measurement of band ratios within a single spectrum, which were used to calculate the molecular orientation along the three principle orthogonal axes (MD, TD, and ND), was found to be in good agreement with independent orientation determinations on the same set of oriented polypropylene sheets by x-ray diffraction and polarized refractometry methods. Methods in which dichroic ratios are determined from two separate spectra were shown to lead to unsatisfactory results due to differing sample contact to the internal reflection element (IRE) for each spectrum. This was demonstrated for a method using a rotatable IRE and sample holder which permitted the measurement of spectra along orthogonal axes in the sample without demounting and remounting of the sample. It was found that this method yielded irreproducible results which were not in agreement with the aforementioned independent methods. The reason for the irreproducible results was clearly due to differing sample contact along the two paths through the rotatable IRE—a contact problem that was exacerbated for more highly oriented samples. The conclusion was drawn that only IRS methods involving band ratios within a single spectrum can yield quantitative and reproducible orientation functions, since the sample contact problem does not exist for these methods.