Analysis of Some Errors in the Calculation of Orientation Functions for Polymers from Infrared Dichroism Measurements

Infrared dichroism is an important means of characterizing polymer orientation (via the orientation function) which relies upon spectra obtained with polarized infrared radiation. A quantitative analysis is presented here for three different types of errors in the analysis of orientation via this method. First, the importance of accounting for the conformer distribution within a monomer unit is considered with respect to an example polymer, poly(ethylene oxide). Second, the effects of imperfect polarizer operation (“leakage”) are considered in detail for high-precision polarization optics. It is found that polarizer leakage results in at most ∼4% error in the orientation function at reasonable values for measured absorbances and relatively high frequencies (∼3000 cm⁻¹). Third, since it is common practice to make certain assumptions regarding transition moment angles in the course of calculating orientation functions, an assessment is made of the effects on the orientation function of erroneously choosing the angle value. The likelihood of adversely affecting the orientation function is found to be lowest near 0° and 90°, increasing steeply as one approaches the magic angle (54.7°) from either direction.