A new treatment is proposed for quantitative analysis of two-component polymer systems by infrared spectroscopy. Like much previous work, it is based on a ratio involving two peaks in the same spectrum. The relationship between such a ratio and the concentration of a given polymer is inherently nonlinear. It is shown that this nonlinearity can be well described by a simple equation derived from the laws of optical transmission. This equation has the form X<sub>1</sub> = <i>m</i><sub>1</sub> + <i>m</i><sub>2</sub><i>R/</i>(1 + <i>m</i><sub>3</sub><i>R</i>), where X<sub>1</sub> is the weight fraction of polymer 1, the <i>m</i><sub>1</sub> are adjustable coefficients, and the ratio <i>R</i> is equal to <i>A</i><sub>a</sub>/(<i>A</i><sub><i>a</i></sub> + <i>A</i><sub><i>b</i></sub>). The quantities <i>A</i><sub><i>a</i></sub> and <i>A</i><sub><i>b</i></sub> are the absorbances (peak heights or areas) at two frequencies <i>a</i> and <i>b</i> of which the first is associated mainly with polymer 1 and the second with polymer 2. This equation has been applied to various peak combinations in spectra of miscible blends of poly(phenylene ether) with polystyrene (both mid-IR and near-IR data) and immiscible blends of polypropylene with polyethylene (mid-IR data). It is shown that the equation is valid in all cases, covering the full concentration range from 0 to 100% even when the peaks used for the analysis involve absorption by both polymers. It is therefore believed to be of broad general usefulness for the analysis of polymer blends and copolymers.

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