The physical properties of ethylene-styrene (ES) copolymers, like many other copolymers, are determined primarily by copolymer composition. ES copolymers may also contain a small amount of polystyrene. It is important to quantify the copolymerized styrene and polystyrene in ES copolymers. The difficulty associated with this analysis, however, arises from the fact that copolymerized styrene and polystyrene have a similar infrared spectrum. The quantitative analysis of ethylene-styrene copolymers by the Fourier transform infrared (FT-IR) technique, combined with the partial least squares (PLS) method, is described. The use of the PLS method facilitates the analysis. The PLS factors, which are calculated iteratively to maximize the covariance between the independent variable and the dependent variable in PLS, remove the collinearity problem present in the original spectral data, and make this analysis possible. The standard error of calibration (SEC) for 60 samples included in the calibration is 0.19% for polystyrene and 0.48% for copolymerized styrene. The standard error of prediction (SEP) for polystyrene and copolymerized styrene, calculated from 120 samples that were excluded from the calibration, is 0.25 and 0.51%, respectively.

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