The Modern Student laboratory: Fourier Transform Infrared Rnalysis Quantitative Analysis by FTIR Thin Films of Copolymers of Ethylene and Vinyl Acetate Lon J. Mathias, Marie G. ~ankins: Craig M. ~ e r t o i u c c i , ~ ~L. i n~rubb?and a Jeno Muthiah Department of Polymer Science, University of Southern Mississippi, Hattiesburg, MS 39406
Teaching infrared spectroscopy requires more than just introducing functional group analysis and "fingerprinting." Information on theory, operation and characteristics of Fourier transform (FT)instruments, and sampling methods is required. Experiments using real-world polymer samples show students the relevancy ofchemistry and polymer science to everyday life, and demonstrate drawbacks and limitations of spectroscopy. Polymer thin films are attractive for IR study in organic or ph$sical chemistry laboratories because they are readily available annmerciallv and are easilv samuled as obtained or after stretching or solvent casting; homopolymers, polymer blends, mixtures, and copolymers can be examined; both qualitative and quantitative experiments are possible; and students can analyze samples from a wide variety of everyday products. Excellent IR experiments using polymers have been described. Qualitative identification of commercial thin films was reported in 1977 (11, although reference spectra have been scarce. This makes ditlicult the identification of copolymers, blends and polymers containing plasticizers or additives. We have developed a manual for teaching FTIR and FTNMR emphasizing polymer analysis with substantial numbers of IR and NMR spectra of common polymers (2). Other recently published polymer IR experiments deal with using interference peaks to calculate film thickness (3-4); quantitation of acrylonitde-methyl methacrylate copolymers (51, and chain branching in polyethylene (6); use of dichroic IR to evaluate polyethylene properties versus methyl p u p content (7); and analysis of aqueous solutions using inexpensive plastic sample holders (8).Here we extend a published experiment (3) and literature reports (9-101, dealing with quantitation of copolymers of ethylene and vinyl acetate using FTIR.
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'Summer visifng professorsponsored by the donors of the Petroleum Research Fund, administered oy tne Amer can Chemical Socle 'Summer undergraduate research fellow sponsored by the industrial donors of the Department of Polymer Science, University of Southern Mississippi. 'Summer undergraduate researcher sponsored by the industrial donors of the Joint Polymer Education Comminee of the American Chemical Society Divisions of Polymer Chemistry and Polymeric Materials: Science and Engineering. 4~o(ysciences, lnc., 400 Valley Road, Warrington, PA 18976; 'scientific Polymer Products. Inc., 6265 Dean Parkway, Ontario, NY 14519; 'Aldrich Chemical Co.. P.O. Box 2060. Milwaukee, WI 53201.
Experimental We use a preliminary introduction to F"MR where students collect commercial films (usually 2 0 4 0 per student with five or so unique samples each) for qualitative analysis. Ethylene-vinyl acetate copolymers usually turn up that are evaluated quantitatively in this follow-on experiment. Alternatively, commercial unknowns can be handed out by the instructor. Sample mounting employs thin cardboard strips (old file holders) cut to fit the sample holder of the IR with a hole cut where the IR beam passes. Each sample is mounted permanently on a holder turned in .with the spectrum for future use. Thin film samples are simply taped to the sample holder. Samples too thick for analysis are stretched with careful warming or cast from a volatile solvent. Melt pressing may be done with a hot clothes iron and a preheated hot plate to "squeeze" samples between a folded 2 in. x 2 in. piece of household aluminum foil. The foil is placed on the hot plate and quickly pressed as flat as possible with the iron. Samples should not be heated for more than a minute and possible degradation requires a hood or good ventilation. After cwling for 30-45 s, the foil is carefully tom in two from the edge down through the sample, causing it to stretch and thin to films 2-5 times their original length; the thinner the sample, the better the results. The film is then mounted on the sample holder with tape. Establishing a Beer's law relationship for vinyl acetate content is most accurately done (10) using the ratio of absorbance of the CO peak at 1020 m-' to the polyethylene C-H peak at 720 cm-'. Peak heights are measured from a sloped baseline (3) drawn through adjacent "valleys." Data is collected for "knownncommercial samplesM of poly(ethy1ene-vinyl acetate) or "ethylenelvinyl acetate copolymer." Samples used here had nominal vinyl acetate contents of 14, 18, 25, 28, 33 and 40% (by weight). FTIR spectra were obtained on a Perldn-Elmer 1600 spectrometer with four scans with a resolution of 4.0 cm-'. Each sample was stretched to 3-5 times its melt-pressed length. Three to seven different samples were measured for each nominal composition. Results and Discussion Controlled temperature and pressure laboratory presses are available for preparing thin polymer films, although they are expensive and not really needed. The procedure described here is easy to learn, uses readily available hot plates and clothes irons, and is fast enough to allow empirical determination of the best conditions for each sample by each pair of students. FTIR acquisition and plotting times of a few minutes per sample allows each pair of stu-
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The Modern Student laboratory: Fourier Transform Infrared Rnalysis Comparison of 10201720 Absorbance Ratlos and Llnear Regression Analysis Results for Data from Several Sources Plus this Laboratory
mw #
1
#data points 3
range of
wt-% VAc
2-15
slope inter- correlation source ref cep coefficient 0.058
0.06
0.961
-0.12 -0.10 0.18
0.999 0.999 0.62
3
(1 bad)
2 3 4
24
3.6-11.1 0.094 0.093 2-11.1 14-40 0.10
5
29
14-40
4 6
6 (ave)l4-40
6
Figure 1. FTlR spectraof elhylene-vinyl acetate copolymers wntaining nominal vinyl acetate wntents (weight-%)of: A, 40%; B, 33%; C. 18%; D. 14%
dents to establish individual Beer's law plots for the standard samples, and then to quantitate commercial samples or unknowns given by the inst~ctor.
Figure 2. Plot of absorbance rams ot peam at 1020 and 720 cm ' determined by five individuals for thin films of wmmerdal samples containing 1440% by weight vinyl acetate. A218
Journal of Chemical Education
12
240
0.087 0.086 0.091
10 3+10
microscope 0.080 0.750 men-press
0.092 0.915 -0.07 0.978
rows 3+5
Figure 1gives typical IR spectra obtained on known samples.5 Trace A illustrates on-scale absorption across the entire spectrum: all peaks are sharp and n m w , and no peaks show ragged tops indicating complete absorption as seen for the 3100-2800 cn-' region in traces C and D. However, as long as the peaks at 1020 and 720 cn-I are sham and on-scale (traces B-Dl, the spectra may be used. to students should experiment with sample obtain the thinnest films wssible to eive reliable data. Figure 2 is a plot of the-1020/720 ratios obtained by five individuals for the same set of standards indicating unexpected sample inhomogeneity. Analysis by FTIR microscope of 20-pm regions at different locations of the same thin film samples showed even greater variations (the table, row 4). However, averaging data for each composition can give reasonably reliable results as shown in Figure 3 which gives a plot of our data (from l b 4 0 % ) plus that reported in the literature (2-11.1%). Clearly, averag-
rlgure 3. rlor or iuzur~zuaosomance ranos rrom rererences o an0 13 together with averaged values (foreach nominal composition) from our measurements.
variability comparable to that depicted in Figure 2. This is illustrated in Figure 4 which compares spectra of homopolymers of ethylene and vinyl acetate (bottom two traces, respectively) with that of a commercial packaging film that was estimated to have a vinyl acetate content of 1% based on a 10201720 ratio of 0.16. Conclusions
Previous work dealing with IR determinations of vinyl acetate in copolymers with ethylene suffered fmm too-few data points (3) or too-narrow a range of compositions (3, 10)to be useful for the type of samples available commercially as packaging material and hot-melt adhesives. In addition,-at least aome commercial samples containing higher vinyl acetate contents (>lo%) apparently suffer from inhomogeneity that can make individual determination of Beer's law relationships unreliable. However, when class averages are analyzed, correlations can be obtained that are usable for quantitating unknowns. This experiment eives students valuable emerience with samnle variabili6 multiple sample analysisby FTIR spectros~opy, linear remession determination of a Beer's law relationship, -. and use of this relationship for unknown evaluation. Acknowledgment
Figure 4. FTlR spectra of thm flms of polyethylene (oonom trace); poly(viny acetate) (middle trace);an0 a commerctal pacKaging fi m (toptrace). ing values for each sample and then determining a best-fit relationship gives a good correlation for unknown evaluations. This introduces students to experimental variations beyond their control and the use of multiple sample determinations plus averaging and linear regression to obtain useful correlations. The table comDares various literature results alone and in combination &th our data. Row 6combines data in rows 3 and 5. While the correlation coefficient is only 0.978, the slope and intercept agree well with those of row 3 and row 5 individually. We believe the combined data accurately correlates the actual relationship between the 1020/720 absorbance ratios with the weight-% vinyl acetate content in copolymers ranging from 2-40%. This relationship should be useful in determining the vinyl acetate contents of thin films obtained from commercial packaging samples and hot-melt adhes~vesalthough the latter may display
Grateful acknowledgment is made to the donors of the Petroleum Research Fund for nartial s u ~ ~ oofr this t work through a summer visiting p~ofessorshi~to MGH, to the industrial snonsors of the Polvmer Education Committee i f the ~ m e i i c a nChemical society Divisions of Polymer Chemistry and Polymeric Materials: Science and Engineering for a summer undergraduate research fellowship to TLG, and to the industrial donors of the University of Southern Mississippi for a summer undergraduate fellows h i to ~ CMB. The FTIR s~ectrometersused here were purc g s e d with funds from the USDA. Literature Cited 1. Webb, J.; Rasmuasen. M.; Selinge., B.J. Chem Edue.18??,54,303. 2. Mathia8.I. J.;Moore,R. B.;Hankins,M. G . k h i n g I T S @ m p y : B m k g m u d ,
Mefhads o d Eprimnts; MRC Polymer Reas, H a t t i e s M , MS 391064376, 1991.
8.
K N.;Cowell,8. J.: Herd,k C.J.Chrm Edue 19111.58.742.
4. Plumar, B.PJ. Chem Educ. 1984,61,439. 6 . EkpenyoK 1.;O h d u o , R.0.' C h m . Educ. lWS.60.429. 6. Powers,D. E.;Harris,W. C.;Kalaainab,V. F J.C h m . Edvc 1919,58,128. 7. Hoada, S.;EUrata.M.MokmmoL Chem Rapid Commun. 1881,2,671. 8 . Phillips, J. PJ. C h m . Edm. 1183,60,740. 9 . ~ o o ~ m a nR. s ,~.;vanderunden,~;v-ant, E . F . P O Z YEng ~ . sei iwa,az,8m. 10. pattaclru,S. C.;Porm, T J.; Paulik,J.Am.Lab, lm1, (6),3E41.
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