Internal Reflection Spectroscopy Characterizes Polymer Films
During the experiment, a standard 25 multiple internal reflection accessory was aligned to a Paragon 1000 fourier transform infrared spectrometer, both from Perkin-Elmer Corp. Because the accessory allows samples to be placed on both sides of the crystal, it delivers higher contact, higher absorbances, and better sensitivity than other devices. The accessory can accommodate a variety of crystal materials, incident angles, and both large (10 ml) and small (1 ml) volume liquid samplers.
The multiple internal reflection accessory can analyze both solids and liquids with a high degree of efficiency. Samples are slide mounted so that they can be changed easily.
The crystal used in this experiment was a 45° KRS-5 crystal. A background was obtained with the accessory in place and a spectra of the samples were obtained by ratioing against this background. Sixty-four scans were used for the background; 32 scans were performed for the samples.
The polymer film was cut to fit the crystal and both sides of the crystal were used. Soft black rubber pads, slightly smaller than the sample film, were placed in back of the sample and pressure was applied uniformly to ensure optical contact between the sample and the crystal.
This spectrum shows the results of the infrared analysis of the polymer film.
The top spectrum is that of the polymer film, while the middle one is that of the residue left on the crystal surface after the film had been removed. This middle spectrum may be identified as a plasticizer belonging to the long-chain ester-type plasticizer class.
If the absorption bands of the plasticizer are subtracted from the top spectrum, then the resulting spectrum on the bottom of the graph can be identified as an ethylene. This ethylene is a methacrylic acid copolymer for which some of the methacrylic acid has been neutralized to form the carboxylate ion. The sharp band near 1694 cm-1 is due to the acid, while the broad band near 1550 cm-1 may be assigned to the carboxylate ion.
As a result of the pressure that was applied to ensure good optical contact in this experiment, some of the plasticizer was squeezed out of the polymer film and left on the internal reflection crystal surface. In this case, there is an obvious benefit to this residue, as the plasticizer can now be characterized. There is also a problem, however, since the residue may not have been expected and the potential for carryover to subsequent samples must be considered.
Because of the likelihood of a spill of this nature occurring, researchers should analyze a crystal blank that does not contain a sample after each experimental analysis. The resulting spectrum will show the presence of any residue that might be important in answering the analytical questions posed for the experimental sample. This will also indicate if the apparatus needs to be cleaned before the next sample is prepared and scanned. In this specific case, the residue was removed by dipping the crystal in chloroform, under a hood, and air drying the crystal under a hood.
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