A Method for Dispersing Compounds in Polystyrene Films for Spectroscopic Study VIVELENBTH IIO-'. We wish to draw attention t o a simple and inexpensive method 'O * for preparing polystyrene films doped with solid samples for use in NIR-VIS spcctrophotometry. This method consists of dissolving sufficient palystryene (common white polystryene packing material is usually of sufficient purity) in ethyl acetate to form a solution with the consistency of thin syrup. This solution is then agitated under vacuum (a water aspirator is sufficient) for several minutes in order :: to remove trapped gases and thereby prevent the formation of cloudy films. The solution is poured onto a clean glass plate and the film g which farms is covered with an inverted beaker. The beaker must have a lip to insure evaporation of ethyl acetate from the film. After several , hours the dry film may be easily removed with a razor blade. These E films mav be mounted in 35 mm slide mounts for ease of handline and storage. The clear polystyrene filmmay serve as a reference blank in = subsequent spectraphotometric experiments. The infrared spectrum of this film may he obtained and compared to that of a polystyrene calibration film. In order to prepare a doped polystyrene f h i t is only necessary t o suspend or dissolve the material under investigation in ~ ~ $ ~ ~ M the ethyl acetate-polystyrene solution prior to vacuum degasing. The doped film is then prepared as above for the referenoe film. Several 10 12 14 16 18 na rr rr 26 28 30 32 34 FREOUENCI s 10.3. om-1 films of varying effective concentration may he required in order to specIra of methyl violet in aqueous Comparison o f NIR-VIS ohtain spectra of suitable intensity. W and ~ ~fluaroiubemull. In certain cases is may be possible to obtain useful spectral in- s0iL"ion. P O I Y Sfilm. formation for samples dispersed in thin films which is not obtained by solution or other solid-state spectroscopic techniques. Thus by using a combination of solution, mull, and film spectra one should obtain more information about the structure of the material under study than by using any single method alone. An example of such a situation is illustrated in the accompanying figure which shows several spectra of methyl violet in the range 10,000-33,000 em-'. The polystyrene film spectrum of methyl violet is strikingly different from either its mull or solution spectrum. Two hands in the aqueous solution absorntion envelooe near 18.000 em-' are sliehtlv shifted and resolved in the film soectrum. This resolution in the film ia nlao s;lhstantiallvhetter than in the mull soe&&. In addition the verv strone emission band observed a t 24.500 cm-' in
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of methyl violet. We have used this polystyrene film technique to successfully ohtain the NIR-VIS spectra of air-sensitive materials. Sample films can be prepared under an inert atmosphere and, when dry, the polystyrene forms a protective film on the sample. Air-sensitive materials may then he conveniently manipulated in spectrophotometers which have no capabilities for inert atmosphere teehnqiues. These films are also stable to temperatures as low as 20K and are quite suited to variable temperature sneetroohotometric studies. There is one limitation t o the use of these films. In the near infrared region of the soectrum a t room temverature these films nrr wsrrptilde ~lislorrion.Suffic~cntheat is produced by the source tu wnrp the film and r w e rhr wrt'~ep.Thls 1s najt n prolrlcm if rht. sample is cooled belgju.HIIK. There :ino stabil~typrchlem ulth the film rhcu visible qprctrn nrs recorded ~
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University of Vermont Brulington, Vt 05405
660 1 Journal of Chemical Education
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J a m e s T. Wrobleski Chester T. Dziobkowski David B. Brown
