The Modern Student laboratory Analysis of Ethylene-Vinyl Acetate Copolymers A Combined TGAIFTIR Experiment Kathryn R. Williams University of Florida, Gainesville, FL32611
In a recent article in this Journal, Mathias et al. (1)describe the analysis of ethylene-vinyl acetate (EVA)copolymers. The experiment serves as an introduction to both the use of FTIR for quantitative analysis and the general concepts of copolymers. This paper reports further instructional enhancements made possible by equipping the FTIR with an Attenuated Total Reflectance accessory and by combining the infrared results with Thermogravimetric Analysis data. The method of Mathias and co-workers involves transmission mode measurements with quantitation based on the ratio of absorbances at 1020 m-' (C-0 stretch of the acetate group) and 720 cm-' (CH2 rock of the ethylene backbone). For the calibration the authors use nominal values of the wt% vinyl acetate (VA) provided by wmmercia1 suppliers. Although the plots of absorbance ratio versus wt% VAare linear, the authors observe a large variability i n the measurements for each of the individual samples. There is an inherent problem with the analysis of films in the transmission mode, because usually the absorbance~are too high for meaningful quantitative measurements. Mathias and co-workers attempted to avoid this problem by stretching the films and by limiting measurements to moderately weak bands. The actual absorbances are not given in the paper, but, as the authors mention, there is indication of out-of-range absorptions for the more intense peaks. The major advantage of the Fourier Transform design over a scanning infrared instrument a c m e s from the capability to obtain a spectrum in a few seconds. Repeated acquisitions can be signal-averaged in a short analysis time. and. in association with the light-gathering - - .proper. ties of the interferometer, this leads to increased signal-tonoist:ratios. This benefit has led t u the development ofconvenient sampling techniques, in particular the Attenuated Total Reflectance (ATR) method (also called Internal Reflection Spectroscopy). The ATR mode is suited ideally to the analysis of highly absorbing substances, because the effective path length is only a few micrometers. The sample is placed in close contact with a crystal of high refractive index, commonly zinc selenide (ZnSe, n = 2.4 at 1000 cm-'1. As shown in Figure 1, the infrared radiation is directed at an angle into the length of the crystal. The system is designed to produce an incidence angle greater than the critical angle at the crystal-sample interface. Accordingto Snell's Law, the light should be reflected back totally into the crystal. However, it can be shown (2)that a component of the electromagnetic radiation penetrates to a depth of a fraction of a wavelength into the sample. There is no net flow of energy into the sample, unless the wavelength of the radiation corresponds to an absorption band. If so, reflection is no longer total, and the beam intensity is
Crystal
Sample
Figure 1.Schematic of a flat ATR device, showing the angle, of the incoming beam with the crystal normal. decreased (i.e., attenuated). Thus, the initials ATR also could represent Almost Total Reflectance. The effect is multiplied, because the beam meets the interface several times as it zigzags to the opposite end of the crystal. Accessories for ATR work are designed with flat crystals, as shown in Figure 1, for analysis of films and viscous materials and with recesses for use with highly absorbing liquids (e.g., aqueous solutions). For a homogeneous material the sampling depth, and hence the measured absorbance, is constant at a given wavelength and incidence angle, provided that there is intimate contact with the crystal. Commercial ATR accessories are equipped with clamping devices, but for the analysis of films, changes in stiffness andior position inevitably result in variations in the effective path length. Thus, the
Figure 2.TGAoutput for a typical EVA sample (Continued on nezt page)
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the CH, rock at 726 cm-I or one of the C-H stretches at 2920 and 2841 cm '. During the initial design period it was noted that the ahsorbance ratio for a given pair of peaks is not the same for the two sides ofthe film. The variation is small but systematic (2%relative). At first it was considered that there was possible surface damage due to rapid cooling after heat pressing. Another set of samples was prepared, in which the films remained in the press until it returned to room temperature, but similar experimental results were obtained. This observation indicates that the absorbance ratios obtained with the ATR accessory may not correspond to those of the bulk material. This is bothersome, because the TGA gives a bulk analysis. However, as noted above, the effect is small, as is evident in the calibration plot shown in Figure - ( H ~ - c H ~ ~ ~ H c - c H ~ ) - --fH2C-CH2)fHC=C+ X I Y Y Y 3. Four absorbance ratios were 0 n measured for each film (two for each 4side of the film). Except for the + yCH3-C, 33.49% VA sample, the four points OH are almost coincidental. The "point" a t about 9.5% VA is an es~eciallv good indicator of the validit; of thk in Figure 2. At about 350 "C the copolymer loses acetic acid method, because eight absorbance ratios are actually plotand the wt% VA can be calculated from the mass loss. At ted. Four ratios were obtained for a film with 9.473% VA, the University of Florida students in instrumental analywhile the other four correspond to a film with 9.486% VA sis use TGAdata to evaluate the mmpositions of the EVA obtained from another supplier. All eight ratios fall on the "standards". They then prepare a calibration plot of IR ableast squares line. The students are instructed to obtain sorbance ratio versus wt% VA and use it to evaluate the and analyze their data in the same manner. composition of an unknown EVA sample from its IR absorbance ratio. Safety Note All FT infrared spectrophotometers contain an internal Experimental Procedures HelNc) laser. which is used for calibration of the distance EVA copolymers with a range of compositions were protravelled by'the interferometer's movable mirror. In the vided in pellet form by the manufacturer (5).To test the transmission mode the laser light passes directly through validity of the method additional samples were obtained the sample compartment. However, with the ATR accesfrom a private source (6). Films of about 0.2 mm thickness sory in place, a fraction of the light escapes vertically,when should be prepared prior to the laboratory period using a the crystal is exposed. Although the total intensity is low Carver heat press. The same films can be used by many (1mW laser power in the Nicolet 5-PC), students are instudent groups. structed to block the light beam with a card or the sample It was found that one-half of a pellet is sufficientfor the compartment shutter, whenever they change samples. TGA work. The TA Instruments Model 2950 TGA in the University of Florida teaching laboratory is equipped with Results and Discussion the HiRes software, which automatically slows the heating As Figure 3 indicates, the calibration plot is linear for ramp, when a mass loss is sensed. This allows a maximum samples from two different sources with VA contents up to ramp rate of 20 'Clmin to be used. Otherwise, a heating at least 33%. Some upward curvature is expected, because rate of no more than 10 "Clmin should be specified. the H-4-H bend is not a true constant for all samples. For the IR analysis a Nicolet 5-PC FTIR is equipped with Pure poly(ethy1ene) has two CH2 groups per monomer a SpedraTech Flat ATR accessory with ZnSe crystal. For unit, but one CH2 is lost when a VA is substituted for an each sample eight interferograms are signal-averaged ethylene. Thus, the absorbance at 1462 cm-' should deprior to Fourier Transformation; a resolution of 16 em-' is crease as the 1740 cn-' absorbance increases. That curvaadequate. The author has used the Nicolet quadtation ture is not observed may be due to the overall low mole software to prepare a stored method for evaluating the fraction of VAin the samples (XVA = 0.14 for 33 wt% VA). peak absorbances. The bands chosen for this purpose are Assuming strictly linear dependence, the students calcuthe carbonyl stretch at 1740 cm-' and the CHz bend of the late the least squares slope and intercept and use it to calethylene backbone at 1464 cn-'. To fulfill minimum reculate the wt% VA of an unknown film directly from the quirements students must perform the analysis using the peak ratio. For the data shown in Figure 3, the determinaabsorbance ratio for these two wavenumbers. To encourage tion of (12.4M.26) % VAcompares well with the instrucstudents to learn more about the capabilities of the instrutor's value (obtained by TGAanalysis) of 12.6%VA. ment, extra credit is given to those who edit the standard method to include more spectral peaks and prepare caliConclusion bration plots with additional absorbance ratios. For examAlthough this experiment is somewhat more compliple, vinyl acetate has strong C-0 stretching bands at cated and expensive than the version described by Mathias 1026 and 1230 em-', which may be ratioed versus either ratioingmethod described by Mathias ( 1 1and Pattacini (31 still must be used. but the absorbances are in a range - that allows a choice of peaks for analysis. In any analytical determination it is essential to follow proper calibration procedures, and students should appreciate the difficulties in obtaining accurate standards. This is true especially for commercial copolymer samples that are labelled with approximate compositions. Proper calibration for the EVAexperiment should, therefore, include measurement of the composition by an independent procedure. A convenient method (4) for the determination of wt% VA is Thermogravimetric Analysis (TGA), in which the mass is monitored as the sample is heated at a controlled rate. The thermogram of a typical EVAsample is shown
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,
the modern student laboratory
accessory infrared spectral measurements may be obtained directly on thick films, so that the stretching procedure of Mathias et al. is not needed. Students also appreciate the importance of the calibration procedure, andthe utility of the TGA for direct mavimetric analysis. The experiment is easy to perform &d the results a& favorable. Acknowledgment The development of this experiment was made possible bv NSF-ILI Grant s#' 9051294 and 9151666 for the Ther;a1 Analysis system and the FTIR, respectively. The author thanks Julianne P. Harmon for her help in preparing the films. Wt%VA from TGA Figure 3.Typical calibration plot of IR absorbance ratio versus the wtSb vinyl acetate obtained by TGA. et al.. the use of both the TGA and the FTIR provides an excellent opportunity to show students the complementary nature of two statc-of-the-artinstruments. \V~ththt, ATR
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Literature Cited 1. Mathias, L. J.; H a n k h . M. G.:Be,td"cti. C. M : Glubb. T. L.; Muthiah, J. J . Chsm. Edue. 1992,69,A217. 2. Hsmek. J. Inlernol Reflmiedlon Sppcfroscopy; Interrienee: NeuYork, 1967. C.: Porm, T. J A m . Lob. 1991,161,38. 4. Wendlandt, W. W. Thermal Analyah, 3rd Why-lnterrience: N w Ymk, 1986; p
N. 3.Pattcini.S.
ed.:
IW
5. El"= manufachlrd by E. I. D"P0"t de Nemours & co, In
