Quantitative analysis of copolymers with the Cary 630 FTIR




Because of its unique combination of features and classleading performance, the new Agilent Cary 630 FTIR spectrometer makes quantitative analysis of polymers especially fast and easy.

cary-630-diamond-atrIn this application note, the amount of key components in two important copolymeric materials are measured — the styrene content in styrene butadiene rubber (SBR) and the ratio of polyethylene to vinyl acetate in polyethylene vinyl acetate (PEVA) polymer. The Cary 630 FTIR equipped with its single reflection Diamond ATR sampling accessory is used for these measurements.


cary-630-diamond-atr-spectrumThe measurement of a polymer sample by the Cary 630 FTIR equipped with an ATR accessory is extremely straightforward. The polymer material is placed on the diamond crystal and the sample pressure press is rotated downward until adequate pressure is placed on the sample to observe a spectrum in the Cary 630’s realtime analysis MicroLab FTIR software. The real-time analysis mode provides instantaneous spectral update and makes it easy for even novice users to get highly repeatable results. The sample press on the Cary 630 is designed so that it cannot be over-tightened, thus protecting the diamond crystal against over-pressure.



Styrene concentration in SBR polymer

A method is added to the MicroLab FTIR software that enables the polystyrene percentage in an unknown sample to be automatically displayed. The limit of detection for the quantitative analysis of PS in SBR is 0.09%, calculated as three times the standard deviation of the 0% replicate data (StDev= 0.03% PS).


The FTIR spectra of four SBR rubber standards with increasing polystyrene concentrations: 0% (purple), 5% (red), 23% (green), and 45% (blue)












FTIR calibration curve for polystyrene in SBR rubber using the 699 cm-1 peak height absorbance; R2 =0.999.



Ratio of polyethylene to vinyl acetate in PEVA

Since the ratio of polyethylene (PE) to vinyl acetate (VA) in PEVA can affect the physical properties of the final product, it is important for manufacturers to have a fast, easy measurement procedure for these components. As in the previous example, the Cary 630 FTIR spectrometer with single reflection diamond ATR is ideal for this measurement.

pe-pvaThe calibration samples were measured with one minute collection times, at a resolution of 4 cm-1. The FTIR spectra exhibit strong acetate ester carbonyl bands at 1737 cm-1 and an ester C-O stretch band at 1236 cm-1 (Figure 5) arising from polyvinyl acetate (VA). Both of these bands are ideal for quantitative analysis of the VA in the polyethylene (PE) matrix. The characteristic PE absorbance bands are located at 2921cm -1, 2852 cm-1, 1467 cm-1 and 720 cm-1. The best calibration is obtained by a peak area ratio of the 1236 cm-1 VA absorbance band ratioed to the PE absorbance at 1467 cm-1.


When a sample is run using this calibrated FTIR method, the results can also be displayed in color coded format (Figure 7), indicating that the sample is in-spec (green), marginal (yellow), or out of spec (red). This enables an operator to get a rapid, visual indicator of the quality of the material.



The result for the 1% VA validation standard — green color indicates an in-spec sample



The Agilent Cary 630 FTIR equipped with ATR sampling technology is an exceedingly effective spectrometer for analyzing copolymer blends. The combination of its compact size, sampling technology, performance, speed of analysis, and intuitive software enables quantitative methods for polymers to be rapidly developed and deployed in quality assurance and quality control applications. The measurement of both SBR and PEVA copolymers yields highly linear calibrations with excellent quantitative accuracy and reproducibility.


Excerpt from Agilent Technologies Application Note 5990-8676EN