Illustrating Infrared Spectroscopy Using Commercially Available Plastic Films In a s e r w of tutorials for first-year students we have sought to developan understanding of molecular structure and dgnnmlrs through rhe study of several forms ot mdecular spectroscopy, namely microwave, inirnred, visible, ultrdv~olet and nuclrsr magnetic resunance sprcrmscapies ! I 31. One ofrrvrml demonsrrntionv of !hem terhnlquesand rheir s t t ~ n d n n r instrumentation stood out in particular for clarity, interest and teaching effectiveness. In this instance, the infrared (ir) spectrum was recorded for a variety of plastic films that are commercially available. Each student in a tutorial of about 10 brought a sample of a readily available film, for example, food wraps, freezer hags, oven bags. Spectra were recorded directly with the film in the samole beam (samole oreDeration time was effeetivelvnil) usines bench-tor, instrument in the fast-scan mode. Tmnsoarent films and some tr&alu>edt films ~~~.(e.e.. ... those used to w k some &en foodsjeave ,. soectra ~,~~ of mod oualitv. . Each spectrum rook about 2 min to record, allwing ronsidernhlc class time for d~scursionof the results. The spectra ohtamed were used in three ways: 1 I I rhey were compared with k n o m apectra for qualirntrvr indmrdcatiun purposes; (2) charts of ir group frequencies were used to identify chemical functional groups present; (3)in some favorable cases, knowing the functional groups present, the chemical identity of the polymer in the plastic could he inferred. Plastics submitted were usually polyethylene, polyvinylchloride, palyvinylidineehloride, or a polyester such as polyethyleneterephthalate. Spectra of these have been reported in detail elsewhere (4-6).Briefly, for polyethylene a very simple spectrum was obtained consisting essentially of the C-H stretch just below 3000, C-H deformation a t 1470 and, near 720 cm-', the skeletal CHn-mcking made for the group -(CH+., where n exceeds 4. Polyvinylchloride exhibits a more complex spectrum, and the 720 cm-' band is absent. Bandsassigned to the C-CI stretch are observed a t low energy (700-600 cm-I). The C=O and C-0 stretching frequencies are easily recognizable features of the polyester spectrum. Complete analyses are complicated by the presence of bands attributable to the copolymer andlor plasticizers present in many commercial plastic films. Polyvinylchloride typically contains phthalate ester plasticizers, sometimes up to 3096 by weight. Nevertheless qualitative identification of the samples and of several functional groups was 1,eadily achieved in a short period of time. This illustration of ir spectroscopy was received enthusiastically, and elicited considerable surprise among students when they would show, far example, that some plastic films marketed under different propietry names were chemically identical. In our view, an important feature of this response was that the students, who had little sophisticated background in spectroscopy, were able to apply their knowledge to derive significant information about the nature of their individual sample of plastic film. ~
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Literature Cited Ill Brown,R.D.,Chem. inBn't. 9,45011973). 12) Dauis,J.C..Jr., Chemistry. 47.6(19711 and48.15 (1975). 191 Morhson. R. T.. and Boyd, R. N.;'Organie Chemistry: 3rd Ed.. Allyn & Bamn. Ine..Boaton.1973. 141 Zbinden. R.."Infrared S p t m 8 m p y of High Polymers." Academic Press.New York. 1964. 151 Hadam. J., Willis, H. A,, and Sqvinell. D. C. M.. "The Identification and Analysis of Piastiea." 2nd Ed.. Butteworth, London, 1912. 161 Bellsrny. L.J.. '"TheInfra-red Spectreof Complex Molecules: 3rd Ed., Chapman & HdI. London. 1975.
Australian National University Canberra, Australia 2600
J o h n Webb Malcolm Rasmussen Ben Selinger
Volume 54, Number 5. May 1977 1 303
