solving power and thermal hazards. The state of development indicates possible further improvement in separating thermally supersensitive liquids. ACKNOWLEDGMENT
The authors thank E. W. Deeg for discussions.
(1948). (4) Jaeckel, R., 2. Techn. Physik 23, 177 (1942). ( 5 ) Watt, p. R.,“&Iolecular Stills,” Chapman and Hall, London, 1963.
LITERATURE CITED
(1) Biehler, R. &I., Hickman, K. C. D.
Perry, E. s., i i N . 4 L . CHEM. 2 1 , 638 (1949). (2) Burrows, G., “Molecular Distillation,” Clarendon Press, Oxford, (1960). ( 3 ) Hickman, K. C. D., Chem. Rev. 34, 51 (1944); Ind. Eng. Chem. 39, 686
RECEIVEDfor review March 30, 1964. Accepted June 15, 1964.
Infrared Determination of Hydroxyl Content of Oxidized Polyethylene DAVID E. KRAMM, JOHN N. LOMONTE, and JOSEPH D. MOYER Washington Research Center, W. R. Grace &
b The hydroxyl content of oxidized polyethylene is determined by infrared analysis of the recovered polymer after quantitative acetylation with acetic anhydride. The analysis measures acetyl group content by use of the 1245-cm.-’ C-0 stretching vibration band typical of acetate esters, and equates ihis to the hydroxyl content. The method is standardized by the infrared spectrometric examination of polymers for which the hydroxyl content had been previously determined by acetylation with acetic anhydride-C14 and subsequent radiochemical analysis. A simple titration, as used for standard acetylation analyses, cannot be used. Although the polymer can be dissolved a t elevated temperatures for acetylation, when excess acetic anhydride is hydrolyzed and the temperature i s lowered, the polymer is precipitated; this makes titration data unreliable. Also, the hydroxyl content is often quite low, making sample and blank titrations approach each other, an undesirable situation.
D
content in oxidized polyethylene by standard infrared spectrometric techniques is difficult, if not impossible, because of the proximity of other absorption bands. The use of integrated absorption, which has been useful in this type of system (3, 4),is precluded not only because the band is not resolved but because suitable model compounds are not available. Figure 1 shows the spectrum of a typical oxidized polyethylene. From this spectrum it can be seen that the hydroxyl band which would be located a t about 3300 em.-’ is completely lobt in the shoulder of the C-H stretching band in the 2900-cm.-’ region. I t will be shown that hydroxyl groups are indeed present by the ability to acetylate them and ETERMINATION OF HYDROXYL
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ANALYTICAL CHEMISTRY
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Figure 1.
Infrared spectrum of a typical oxidized polyethylene
that this acetylation is free of side reactions. By acetylation of the polymer and examination of the infrared spectrum (Figure 2 ) , it has been found that hydroxyl content can be estimated in the acetylated product from the absorption a t 1245 em.-’ arising from the C-0 stretching vibration band of acetate esters (1). The procedure is standardized by an absorptivity constant, a, obtained from measurements with polymers of known hydroxyl content. The known hydroxyl content of these polymers is determined by reaction with acetic anhydride-U4 followed by radiochemical analysis ( 2 ) . EXPERIMENTAL
Apparatus. The radiochemical counting was done with a windowless, gas-flow counter made by Nuclear Measurements Corp. The infrared spectra were recorded on a Beckman IR-4 spectrophotometer using the following instrumental conditions: operating mode, double beam; slits, 1.5 x standard; filter period, 2 seconds; scan speed, 80 e m - ’ per minute.
Acetylation Procedure. T o an accurately weighed, 1-gram sample of oxidized polyethylene in a 500-ml. iodine flask is added 200 ml. of reagent grade xylene, 15 ml. of dry pyridine (less than 0.05% water), and 2 ml. of acetic anhydride. A Teflon-covered stirring bar is added, and the mixture is heated to reflux on a hot plate with magnetic stirring, and allowed to reflux for 3 hours. The sample is cooled and 50 ml. of methanol containing 10% water (by volume) is added. The sample is mixed vigorously and allowed to stand for 20 minutes to let the polymer coagulate. The precipitated polymer is collected by gravity filtration on N o . 42 Whatman paper and allowed to air-dry for an hour. The sample is then dried in a vacuum oven a t 50’ C. for 3 hours. Radiochemical Analysis. Several samples of oxidized polyethylene are acetylated with acetic anhydride-Ct4. The absolute specific activity of a purified derivative, 5-a~etyl-C’~-p-toluidine is determined by a liquid counting procedure (6). The reagent has a specific activity of 0.648 pc. per millimole. The recovered polymer is molded into plaques and counted, and the count is
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The factor of 17/1000 is necessary for converting microcuries per millimole into microcuries per gram. Infrared Analysis. An absorptivity constant, a, is calculated for t h e 1245-em. -l acetate ester C-0 stretching vibration band by:
a
=
A / ( b X c)
40
A b
30 20
c
= = =
io 0
Figure 2.
Infrared spectrum of acetylated oxidized polyethylene
compared to that of a plaque of polye t h ~ 1 e n e - C ' ~having a known specific activity. From these data, the acetyl group content of the recovered polymers can be calculated and, hence, the hydroxyl content of the original polymer. Infrared Analysis. T h e standard samples from t h e radiochemical analysis mere molded into thin films (0.1 mm. or less) a n d the infrared spectra were recorded. As the waxy nature of these samples precludes accurate film-thickness measurements by micrometer, the thicknesses are calculated from the absorbance of the 4350em.-' C-H combination band by a method previously reported ( 4 ) . A base line is drawn across the shoulders a t 1140 and 1330 cm.-', and the absorbance measured a t 1245 cm.-'. This absorbance is corrected for film thickness and compared to the hydroxyl content values obtained from the radiochemical analysis, thus deriving a calibration.
where Cl = acetylated sample, counts per second Cz = po1yethylene-Cl4, counts per second B = background, counts per second SB= specific activity of polyethyleneC14 standard microcuries per gram S1 = specific activity of acetylated sample, microcuries per gram The acetyl group content, and, hence, the hydroxyl content, is obtained, from the calculated activity of the acetylated film, by comparison with the activity of the acetic anhydride-C14 derivative, AV-acety1-C14-p-toluidine,by : (81 X
where
S1 = activity of the sample as calcu-
CALCULATIONS
Radiochemical Analysis. T h e specific activity of t h e acetylated polymer is calculated by comparison with a polyethylene-C14 film having a specific activity of 2.59 /JC. per gram by: (Cl
- B)/(CZ- B ) x
Table I.
chemistry, w t . ?(-
0 0 0 0
21
27 27 29
SS
=
s 1
lated above, microcuries per gram S = specific activity of the CI4 derivative, microcuries per millimole 17 = equivalent weight of hydroxyl groups
The hydroxyl content can now be calculated for acetylated, oxidized polyethylene samples by: c = A/(a
x
b)
where
a
=
average value of the absorptivity constant as calculated above, and the other terms are as before. RESULTS A N D DISCUSSION
The data from this standardization are shown in Table I. The standard deviation as shown in this table is excellent, considering the number of operations involved in this analysis. This indicates that analyses can be done by this method with good precision and accuracy. The acetylation reaction is not difficult and is apparently free of side reactions, even though it takes place a t elevated temperatures. This is shoan by plotting the absorbance a t 1245 cm.-l per centimeter of film thickness against per cent of hydroxyl from radiochemical analysis. -2 straight line through the origin results. +2 straight line displaced from the origin mould show a reproducible amount of side reaction, whereas a curved line would indicate a nonreproducible amount. As neither occurs, the reaction is free from side reactions.
Calibration Data for Infrared Determination of Hydroxyl Content
yo OH by radio0.07 0.07 0 16 0 21
17 X lOO)/(s X 1000) = Wt. % ' OH
absorbance a t 1245 cm.-l film thickness in millimeters hydroxyl content in weight per cent, from radiochemical analysis
Film thickness, em. 0.0076 0.0081 0 0 0 0 0
0048 0035 0057 0035 0027
Absorbance at 1245 cm.-' 0.113 0.107 0 208 0 215 0 157
0.345 0.195 0 153
Thickness l/cm. 14.9 13.2 33.5 44.8 44.9 60.5 55.7 56.7
Absorptivity constant,
l / c n i . - R t . yc
213 189
LITERATURE CITED
(1) Bellamy, L. J., "The Ipfra-red Spectra of Complex Molecules, p. 179, Wileyj
Kew York, 1958. (2) Benson, R. H., Turner, R. B., AXAL. CHEM.32, 1464 (1960). (3) Lomonte, J. S . , Ibid., 34, 129 (1962). (4) Ibid., 3 6 , 192 (1964). (5) Schwebel, 4.,Isbell, H. S., hloyer, J. D., J . Res. S a t l . Bur. Std. 53, 221
221 (1954). .4v. 208
Std. dev. 5 . 3 5
RECEIVEDfor review June 25, 1964. Accepted August 5 , 1964.
VOL. 36, NO. 1 1 , OCTOBER 1964
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