intermediates of interest in our work. The concentrations given with the illustrations are for the final carbon disulfide solutions. Of the dyes investigated, FD&C Red 2 [l - (4- sulfo - 1 naphthylazo) - 2naphthol - 3,6 - disulfonic acid], FD&C Red 4[2 - (5 - sulfo - 2 , P xylylazo) - 1 - naphthol - 4 - sulfonic acid], and FD&C Yellow 6(l-p-sulfophenylazo - 2 - naphthol - 6 - sulfonic acid) were quantitatively extracted a t the concentrations indicated (Figure 1). FD&C Yellow 5(3 - carbosy - 5 - hydroxy - 1 - p - sulfophenyl - 4 - p - sulfophenylazopyrazole) was extracted too slowly for quantitative purposes. Figure 2 shows the spectra of carbon disulfide, a 5% solution of the LA-2 resin in carbon disulfide, and a liquid film of the LA-2 resin. Except for strong C-H and C-CH, absorption, the resin
-
is relatively transparent throughout most of the 2- to 1.5-micron region. A 5% solution of the resin in carbon disulfide appears to be the maximum generally useful concentration for quantitative analysis in the 7- to 15-micron region. However, considerably stronger solutions could be used in some regions of the spectrum. Figures 3 t o 5 show the spectra of several isomeric groups of sulfonated dye intermediates. These spectra afford a simple way of distinguishing among the individual isomers in each group. The various isomers of each group often occur in admixture with each other. The results obtained in the analysis of several synthetic mistures (Table I) indicate that useful quantitative results are obtainable. Calculations for Group a were based on absorbances a t 13.22, 9.06, and 8.96
microns, for Group b a t 12.33 and 7.73 microns, and for Group c a t 13.41, 12.88, and 12.15 microns. I n addition to sulfonated dyes and intermediates, preliminary work has indicated that the method may also be applicable to the analysis of other types of acidic compounds such as halogenated fluoresceins and anionic surface-active agents. A limited series of experiments indicates that the method is also applicable to the preparation of carbon tetrachloride-resin solutions of sulfonic acids. LITERATURE CITED
(1) Ard, J. S., Fontaine, T. D., ANAL.
CHEM.23, 133 (1951).
RECEIVEDfor review August 3, 1961. Accepted October 13, 1961.
Infrared Determination of Vinylidene Unsatura tion i n Polyethylene JOHN N. LOMONTE Texas Division, The Dow Chemical Co., Freeport, Tex.
b A method is presented by which alkyl group interference at the vinyldensity idene frequency in low branched polyethylenes is eliminated and a primary standardization for vinylidene content is obtained. Because the 888-cm.-l vinylidene absorption is not completely resolved from the 895-cm.-l absorption arising as a consequence of butyl branches, a method was devised for determining the extent of the interference. The standardization is then done through the use of suitable model compounds and integrated absorption intensity.
-
I
NTEGRATED absorption has not been used extensively for standardization for infrared functional group analysis. It holds promise of assuring the validity of model compound comparisons, and in eliminating the effect of physical state of the sample. The subject of integrated intensity has been reported in a review article by Brown (3). Most prior functional group standardizations have been based on molar absorptivity, although this may not be suitable for the comparison of solid or polymeric material with liquid model compounds. Standardization based on integrated intensities should be more
reliable because it is more nearly a constant for a given series of compounds containing the same functional group than is the molar absorptivity. Integrated intensity also takes the band width into account. Work done in this laboratory shows that there is no abrupt change in the integrated intensity of the unsaturation bands in the 900-cm.-l region a t the crystalline melting point of the polymer. A plot of the integrated intensity for these bands us. temperature is a smooth curve identical to that of a liquid sample which does not undergo a transition in the temperature range. Other work shows that a solid noncrystalline material will have a larger band width than the same material run as a solution. This larger band width is accompanied by a smaller peak absorbance, so that the integrated absorbance will remain constant regardless of the physical state of the sample. Obviously, since molar absorptivity does not take band width into account, it will not give the same value for a solid and its solution. From the above, it is concluded that low-density polyethylenes can be compared t o suitable model compounds a t room temperature, provided the comparison is done through integrated absorbance. One exception to
the constancy of integrated absorbance is highly crystalline solids, where the effect of the crystallinity on the solid spectrum due to the polymorphism and random crystalline orientation gives unreliable results. However, in partially crystalline polyethylene, the crystallinity should have no effect on the vinylidene absorption, as the vinylidene group would be considered as a noncrystallizing copolymer unit according to Flory's theory (6),and as such would not lie in crystalline volumes of the polymer. I n their excellent paper, Richardson and Sacher (8) give a good explanation of the applicability of integrated absorption to the standardization for functional groups in a polymeric system by the use of selected model compounds. The calculations used are those of Ramsey (7), which assume that the band shape for suitably isolated absorption bands can be represented by a Lorentzian function. This function has the form In(lo/l)r =
a (v
- YO)*
+ b2
where ln(Zo/l)u = absorption intensity a t any frequency, v YO = frequency of band center VOL. 34, NO. 1, JANUARY 1962
129
a / b 2 = absorption intensity at v0 2b = width of band a t half absorption intensity, cm.-'
4
The integrated absorption intensity, A , is calculated from d = 1/CL X ~ / x 2 In(Zo/Z)vo x
3
AU,
where ln(lo/Z)va
=
A v l i ~=
C
=
L
=
peak absorption intensity at band center width of band at half absorption intensity, cm.-' concentration of absorbing group, moles per liter sample thickness, centimeters
The molar absorptivity,
e,
.2 . a l
is given by I
E
= 1/CL
x log ( Z O / l ) P O
where these terms have the same definitions as above. Boyd, Bryant, and Voter (2) in the investigation of the infrared spectrum of polyethylene found that the absorption at 895 em.-' which interferes with the vinylidene absorption is due to butyl branches on the polymer. Bryant and Voter (4) in their investigation of short-chain branching in polyethylene found, after the removal of vinylidene unsaturation by hydrogenation, that a constant ratio of this butyl group absorption t o the methyl group absorption a t 1378 cm.-' was obtained. They were interested in removing the vinylidene absorption t o investigate the butyl group band. As the purpose of this paper is the investigation of the vinylidene band, the correlation of the butyl interference to the methyl group absorption is desired for the calculation of the extent of this interference. By finding a more suitable method of removing the vinylidene unsaturation. not by hydrogenation which would convert the unsaturation present to more methyl groups, the estent of this butyl interference could be obtained from the above correlation. Such a method of removal of unsaturation should be don? so as not to degrade the sample and. if possible, should be a simple and convenient procedure.
Table 1.
0
Friquensy.
Figure 1. region
A. 6. C.
Original band Butyl band by bromination Vinylidene band by A- B
EXPERIMENTAL
A number of pure 2-methyl-1-alkenes
were obtained from the Carnegie Institute of Technology and scanned in carbon disulfide solution in a 0.0098-cm. cell which had been calibrated by interference fringes. The scans were made on a Becknian IR-7 spectrophotometer with sodium chloride optics and a prismgrating double monochromator with the following instrumental conditions : Operating mode Slits Dispersion Scanning speed Filter period Chart scale Scan length
Single beam Fixed at 1.0 mm. 1.5 cm.-l at 900 crn.-l 40 cm.-l per minute 2 seconds 5 cm.-l per inch 850 t o 935 cm.-l
The peak absorption intensities and the band widths a t half intensity were measured and from the calculated value
ANALYTICAL CHEMISTRY
Band Width, Cm.-' 7.70 7.31 7.52 7.64 8.98
7.76
cm-'
Absorption bands in the 865- to 925-cm.-'
Integrated Absorption Data for Model Hydrocarbons
Absorbance Compound a t 888 Cm.-' 0.571 2,3-Dimethyl-l-butene 2,3-Dimethyl-1,3-butadiene 0.642 0.498 2,3,3-Trimethyl-l-butene 2,3-Dimethyl-l-pentene 0.510 2,PDimethyl-1-pentene 0.418 2,4,4-Trimethyl-l-pentene 0.426
130
C
Vinylidene Concn., Mole/Liter 0,323 0.354 0.288 0.284 0.283 0.255
Integrated Absorbance, Liters/ (MoleSq. Cm.)
5050 4920 4820
5090 4900 4810 Av. 4950
of vinylidene content of these compounds, a n average value of 4950 was calculated for the integrated intensity of 888-cm.-' vinylidene band. The data are shown in Table I. These bands as graphed by the spectrometer are well represented by the Lorentz equation. Four samples of polyethylene were taken as typical vinylidene-containing polymers. Before any measurements could be made on these samples, the butyl group interference had to be determined. This was necessary because the butyl group and vinylidene group band centers are separated by only 7 cm.-' and they are incompletely resolved. To determine this butyl interference it is necessary t o remove the vinylidene unsaturation which obscures it. This was done by bromination by placing a sample 1 mm. thick in a 5% by volume solution of bromine in carbon tetrachloride for 3 hours, washing the sample in fresh carbon tetrachloride for 2 hours, and drying in a vacuum oven a t 60' C. overnight. A study of the bromination rate showed the bromination to be complete a t the end of 2.5 hours, as indicated by no further change in the butyl band after that time. Using the same instrumental conditions as before, a brominated sample and a blank are run. A point by point subtraction of the brominated run from that of the sample blank scan is made, the result of which is the true absorption band of the vinylidene groups
present. These bands are shown in Figure 1. A fit of this vinylidene band to the Lorentz equation is very good. The vinylidene content of the polyethylene sample is calculated from the average value of integrated absorbance from the model compounds and the peak absorbance and band width measured on the corrected band. By this procedure, a molar absorptivity of 131 was calculated from the vinylidene content of the four representative samples. The data are compiled in Table IT.
Table II.
beam; the spectrum recorded gave the spectroscopic difference. If the butyl absorption were fully additive a t the vinylidene frequency, the arithmetic and spectroscopic differences would be equal. However, the arithmetic difference is only S5% of the spectroscopic, showing that the butyl band is 85% effective a t the vinylidene frequency, as shown in Table IV. From the above correlations, the butyl interference can be determined from the absorbance measurt.d a t the 13iS-cm.-' xnpt hyl band.
Calculation of Vinylidene Content and Molar Absorptivity for Polyethylene
Sample NO.
E 6€-' PE-7 PE-7.4 PE-10
iibsorbance at 888 Cni.-'
Band Width, Cm.-1
0,384 0.374 0.335 0.153
10.41
10.13 10.46
10.80
Molar Absorptivity, Liters/ (Mole-Cm.)
Vinylidene Content, Mole/Liter 0.0292 0,0277 0.0256 0.0121
131.5 135 0
130 8 127.0
Av.
T o use this value of molar absorptivity for routinely determining the vinylidene content of polyethylene, a more convenient method of measuring the butyl interference is desired, so that it is not necessary to brominate each sample. A series of polyethylene samples was brominated and the intensity of the 1378-cm.-' methyl group absorption was measured before and after bromination. As there was no change in this methyl group intensity, it was assumed that this bromination had no effect on the sample beyond removal of the unsaturation. Table I11 shows that a constant ratio of 28.9 exists between this methyl group absorption and that of the butyl band uncovered by bromination. It should be possible to calculate the butyl interference from the intensity of the 1378-cm.-l methyl band except for the cas? of samples containing pendant methyl groups, as found in ethylenepropylene copolymers, for which there are no corresponding butyl groups. Although the butyl and vinylidene bands lie so close together that they cannot be resolved, they are not sufficiently close to allow a strictly additive treatment. To determine the degree of overlap, samples were run before and after bromination and the peak absorbances subtracted. Then a compensated run was made with the brominated sample in the reference beam and the sample blank in the sample
131
Table 111. Ratio of Methyl to Butyl Group Absorption in Polyethylene
Polyethylene Sample PE-4~~
PE-5 PE-5A PE-6 PE-7 PE-7A PE-9 PE-9-4 PE-QB PE-10
Absorbance At 1378] 895 cm.cm.-1 3.87 0.129 3.80 0.129 2.82 0.099 4.24 0.145 4.49 0.156 0.138 3.91 4 28 0.144 2 97 0.103 0.103 2.91 2.92 0,104 Av. At
Ratio 1378/ 895 30.0 29.5 28.5 29.2 28.8 28.3 29.7 28.8 28.2 28.1 28.9
Table IV. Per Cent Interference of Butyl Absorption at Vinylidene Frequency
Poly-
ethylene Sample PE-6 PE-7 PE-iA PE-10
Arithmetic Difference in Absorbance SpectraArith- Spectfo- scopic metic scopic x 100 0.295 0.319 84.9 0.294 0.323 82.9 0.271 0.289 87.3 0.111 0.127 86.0 Av. 85
DISCUSSION
This method presented for the correction of butyl interference a t the vinylidene frequency is based on the assumption that because the absorbance ratio of the 1378-cm.-' methyl band t o the 895-cm.-l butyl band is constant, the number of butyl groups is proportional to the number of methyl groups. However, this is not the case in ethylene-propylene copolymers, where there are pendant methyl groups for which there are no corresponding butyl groups. To determine the butyl interference for this type of sample, bromination is necessary. ,4 comparison of the value of 131 for the molar absorptivity of vinylidene groups t o the literature values is not conclusive. Richardson and Sacher (8) report a value of 159 based on similar measurements for polyisoprenes. The band widths presented in their paper are larger than those obtained in this work, which by this scheme would lead to a high value of molar absorptivity. Whether these large band Tvidths are due to the larger spectral slit m-idth of their spectrometer or to difficulty in measuring band widths due to insufficient abscissa expansion, the effect is to calculate a value which is too large. Cross, Richards, and Willis ( 5 ) calculated a value of 103 from the work of Anderson and Seyfried ( 1 ) who did extensive work on liquid hydrocarbons. However, this value of 103 is calculated
from the hydrocarbons themselves rather than from representative polyethylene samples. Bryant and Voter (4) found the methyl t o butyl absorption ratio to be 29.4 and that the butyl band is 75% effective a t the vinylidene frequency. The values of 28.9 and 85% compare very well with these values. ACKNOWLEDGMENT
The author is indebted to D. E. Cofer of this laboratory for his development of the bromination technique and the subsequent brominations performed. LITERATURE CITED
(1) Anderson, J. A., Seyfried, W. D.,
ANAL.CHEM.20,998 (1948). (2) Boyd, D. R. J., Bryant, W. M. D., Voter, R. C., private communication. (3) Brown, T. L., Chem. Revs. 58, 581 (1958). (4) Bryant, W. M. D., Voter, R. C., J. Am. Chem. SOC.75, 6113 (1953). (5) Cross, L. H., Richards, R. B., Willis, H. A., Discussions Faraday SOC.9, 235 (1950). (6) Flory, P. J., J. Chem. Phys. 17, 223 (1949). ( 7 ) Ramsey, D. A,, J. Am. Chem. SOC. 74, 7 2 (1952). (8) Richardson, W. S., Sacher, A., J. Polymer Sci. 10, 353 (1953).
RECEIVED for review July 7 , 1961. Accepted October 19, 1961. VOL. 34, NO. 1, JANUARY 1962
rn
131
