Near- Inf rared Spe,ctrophotometric Analysis of Ethylene-Propylene Copolymers Tsugio Takeuchi, Shin Tsuge, and Yoshihiro Sugimura Institute of Analysis, Faculty of Engineering, Nagoya University, Japan
SINCENatta's work ( I ) for determining ethylene-propylene (E-P) copolymer composition by radiochemical and solution methods, a considerable volume of work has been reported. Natta's CCId solution infrared method ( I ) had some disadvantages. In some cases the dissolution of copolymers which have low propylene content or higher molecular weight was difficult. Avoiding the dissolution difficulty, Wei (2), Gossl (3), and Smith er al. ( 4 ) carried out the analysis with easily-prepared copolymer pressed films. Stoffer and Smith (5) reported a method involving liquid scintillation counting to determine the compositions of labeled E-P copolymers. Most of the infrared methods in this field were carried out in the fundamental infrared region, but a few methods have been reported (6-8) which use the near-infrared spectrum. Bucci and Simonazzi (6) proposed a method to determine the composition of E-P copolymers which uses the overtone region of the near-infrared spectrum. However, the overtone region is not suitable for composition determination because of its weak intensity and poor resolution of the spectrum. Drushel and Iddings (7)reported a method which used the combination tone region-2.275, 2.31, and 2.35 p . Recently, Bly, Kiener, and Fries (8) developed a method based on the combination tone region for various types of E-P copolymers. In the present work, the combination tone region was studied with E-P copolymers and physical blends of homopolymers with absorbance at 2.466 p as well as at 2.275, 2.31, and 2.35 p. The new band at 2.466 was assigned to be a combination tone of the fundamental bands at 2880 cm-1 and 1150 cm-' and was sensitive to propylene content. The repeatability of the determination was examined in comparison with absorbance ratios between the four characteristic bands mentioned above. Among them, the absorbance ratios which were associated with the new band at 2.466 I.C introduced less fluctuations than the other ones. Shifts in wavelength of the band near 2.31 p were observed and discussed. EXPERIMENTAL Sample. The following samples were used: four random E-P rubbers (EPR), an isotactic polypropylene (PP), a highand a low-density polyethylene (PE), and physical blends of the homopolymers. For purification, about 1 wt polymer solution was prepared by dissolving into warm xylene, and was then reprecipitated in methyl alcohol. The precipitate was filtered through a glass filter, washed with methyl alcohol, (1) G. Natta, G. Mazzanti, A. Valvassori, and G. Pajoro, Chim. Ind. (Milan),39, 733 (1957). (2) P. E. Wei, ANAL.CHEM., 33, 215 (1961). (3) T. Gossl, Makromol. Chem., 42, 1 (1960). (4) W. E. Smith, R. L. Stoffer, and R. B. Hannan, J. Polymer Sci., 61, 39 (1962). ( 5 ) R. L. Stoffer and W. E. Smith, ANAL.CHEM., 33,1112(1961). (6) G. Bucci and T. Simonazzi, Chim. Ind. (Milan), 44,262 (1962). (7) H. V. Drushel and F. A. Iddings, ANAL.CHEM., 35,28 (1963). (8) R. M. Bly, P. E. Kiener, and B. A. Fries, ibid., 38, 1798 (1961).
184
ANALYTICAL CHEMISTRY
u 1.7
1.B
1.9
2.2
2.3
2.4
Wavelength (microns)
25
Figure 1. Near-infrared spectra of an EPR, a PP, a PE, and a physical blend of homopolymers ----:base line from 2.1-2.2 fi, -: peak, A ; physical blend (70 wt C: high-density PE, D: PP
base line across the bases of a EPR (69.9 wt PP),
zPP), B;
z
and dried in a vacuum oven at 90 "C for 24 hours. The composition of the EPR was calibrated by the cast film method proposed by Corish, Small, and Wei (9). Physical blends were prepared by dissolving a mixture of a desired ratio of the purified homopolymers in warm xylene, and then the same procedure described above was followed. Preparation of Films. The films for the near-infrared analysis were molded on the apparatus which was designed by modification of the oven of a gas chromatograph. A sample weighing about 10 mg was placed in the oven between two glass plates (35- X 25- X 0.8-mm) between which a lead 100-p spacer was also sandwiched. Nitrogen gas was introduced with a flow rate of 180 ml/min, and the oven was heated from room temperature to 280 "C in 5 min. After placing the sample in the oven at 280 "C for 2 min, the hot sample was rapidly transferred to a flat plate, pressed with a glass rod, and then quenched by air-blowing. Films prepared in this manner ranged from 100 to 125 p in thickness. Examination of the infrared spectra of the films in the region 2 to 16 p showed that the films had been free from oxidation and residual xylene. Near-infrared spectra from 1.7 to 2.5 p were obtained with a Shimadzu Spectrophotometer Model RV-50, using the films with cover glasses attached. To check the wavelength calibration of the instrument, 1,2,Ctrichlorobenzene was used (10). RESULTS AND DISCUSSION
Typical near-infrared spectra of the copolymer and the homopolymers and the physical blend of the homopolymers are shown in Figure 1. These spectra are divided into two regions, the overtone region ranged from 1.7 to 1.9 p and (9) P. J. Corish, R. M. B. Small, and P. E. Wei, ANAL.CHEM., 33, 1798 (1961). (10) E. K. Plyler, L. R. Blaine, and M. Nowak, J. Res. Nutl. Bur. Std., 58, 195 (1957).
Oa9
t
1
1
20
0
40
60
80
Figure 2. Calibration curves using the absorbance ratio by the base line from 2.1-2.2 h(----) 0:
Physical blend (with a high-density PE) EPR
0: 0:
AZ.466/A2.85
Physical blend (with a high-density PE)
A: Physical blend (with a low-density PE) 0:
60 80 100 Weight % of Ethylene
Figure 3. Shifts in wavelength of the band near 2 . 3 1 ~as a function of composition of polymers
100
Weight % of Ethylene
40
20
0
Y
EPR
= 2.80
x
10-3
x + 5.35 x
10-1
z
the combination tone region ranged from 2.2 to 2.5 p. Both regions are applicable to the analysis of the composition, but from the standpoint of intensity and resolution of the spectra, the combination tone region is superior. Consequently, the subsequent discussion is restricted to the combination tone region. Hitherto, three combination bands at 2.275, 2.31, and 2.35 p have been used for the composition analysis of E-P copolymers (7, 8). Assignment of these bands was also reported (7, 8). A new band at 2.466 p (4059 cm-l), which is most likely a combination tone of the symmetric methyl v(CH3) mode near 2880 cm-' and the methyl w(CH,) mode near 1150 cm-I, is sensitive to the propylene content. For calculation of the absorbance of these four characteristic bands two different base line methods were used, as shown in Figure 1. The precision of the determination, expressed as a percentage of the standard deviation of the average, is presented in Table I. Six observations were made with two replicate samples of a physical blend containing 30 wt polyethylene and 70 wt polypropylene. From the data, it is apparent that the method by the base line from 2.1-2.2 I.( is superior to that by the one across the bases of a peak. As might be expected from the resolution of the spectra (Figure l), the absorbance at 2.466 p introduces less deviation than that at 2.275 p. The differences between the results obtained by two reference bands at 2.31 and 2.35 p are negligible. However, because shifts in wavelength at 2.31 p cause nonlinear calibration curves, the band at 2.35 p is preferable as a reference to calculate the absorbance ratio. The same trend mentioned above was observed with the coQolymers. Figure 2 shows the calibration curves obtained by the method which uses the base line from 2.1-2.2 p and the absorbance ratio A2.466/A2.35. Because the calibration curves of the physical blends of the homopolymers and EPR are fairly different from each other, for the composition analysis of EPR it is recommended that the calibration curves which were obtained with physical blends of homopolymers not be used. The solid line in Figure 2 is obtained by the method of least squares with observed values of EPR. The linear relationship can be represented as follows:
where Xis wt of propylene in EPR and Y is the absorbance ratio A ~ . ~ ~ 6 / A 2 . 3Accuracy 8. data with the calibration curve are listed in Table 11. Because the composition of the EPR or the physical blends changes, shifts in wavelength were observed with every
Table I. Precision of the Determination Percentages of the Absorbance ratio standard deviation (%) Base line (- - -) Base line (-) ( AilA j) 2.21512.31 2.27512.35 2,46612.31 2.46612.35
1.8 2.0 0.7
3.9 4.8 2.0 2.0
0.1
i,j = p
Table 11. Accuracy Data Propylene Absorbance Propylene wt %s ratio wt X b Sample Of EPR ( A ) Az.466/Az.35 O f EPR ( E )
z
51.4 60.6 62.0 69.9
1 2 3 4
0.680 0.705
0.710 0.730
Deviation (E
51.6 60.4 62.3 69.5
- A)
$0.2 -0.2 +O. 3 -0.4
a Determined by the method proposed by Corish, Small, and Wie (9). b Calculated from the equation given above.
Table 111. Fundamental Wavelengths of v(CH2)( 1 1 , 12) Sample Wavelength ( p ) Wavenumber (cm-1) PE PP v.:
1
.v 3.419 vd 3.505
i
v, 3.431 v. 3.546
2925 2853 2915 2820
Asymmetric stretching.
vd: Symmetric stretching.
(11) S. Krimm, C. Y.Liang, and G . B. B. M. Sutherland, J . Chern. Phys., 25, 549 (1956). (12) M. C. Tobin, J. Phys. Chem., 61, 216 (1960). VOL. 41, NO. 1. JANUARY 1969
185
characteristic band. In particular, the shift at 2.31 p was prominent. Figure 3 shows the relationship between the composition of the polymers and the wavelength of the band near 2.31 p. The band shifts as a monotonic function of the Composition of the physical blends (the high- and the lowdensity PE introduces no appreciable differences), while that of the EPR shifts only below 2.313 p. The band near 2.31 p is a complex, overlapping band between the symmetric or asymmetric methylene mode: v(CH2), 6(CH2), and o(CH2). Among these, the contribution of v(CH2) may be largest because of its strongest intensity. The wavelengths of the fundamental methylene C-H stretching vibration v(CH2) mode are listed in Table 111. The PP has a longer wave-
length both in the symmetric and asymmetric methylene v(CH2) than does PE. It is most likely that this causes the shift of the band near 2.31 p with the physical blends. On the other hand, it may be inferred that the fundamental methylene bands of random copolymers have shorter wavelengths (larger wavenumbers) than those of the homopolymers, and perhaps E-P block copolymers. By these wavelength shifts, the discrimination of random copolymers from block copolymers and physical blends of homopolymers may also be possible to some extent. RECEIVED for review December 1, 1967. Accepted June 17, 1968.
Oxidation of Coordinated Thiourea in Copper(1)-Thioulrea Complexes by Copper( 11)-Perchlorate in Acetonitrile H. C. Mruthyunjaya and A. R. Vasudeva Murthy Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore I 2
THIOUREA is known to be one of the versatile complexing agents. The copper(1) state is rendered stable in aqueous medium by thiourea by its coordinating capacity to form a variety of cuprous-thiourea complexes, depending upon the experimental conditions. Addition of thiourea to a cupric salt in aqueous medium results in the formation of such cuprous-thiourea complexes which are easily isolated and characterized ( J - 5 ) . When a suspension of such a complex in acetonitrile is titrated with copper(I1)-perchlorate in the same solvent, thiourea is oxidized to formamidine disulfide. Copper(I1) is reduced to copper(1) and remains in solution as the copper(1)acetonitrile complex. This reaction has been studied potentiometrically. EXPERIMENTAL
The following cuprous-thiourea complexes were prepared by standard methods and checked for purity by analysis for nitrogen and copper (6). 1. [Cu(NH2 CS NHJ3C1] (Nitrogen: calculated 25.68%, found 25.70%; copper: calculated 19.42%, found 19.31%). 2. [Cu(NHz. CS NH2)&S04 2Hz0 (Nitrogen : calculated 23.61%, found 23.36%; copper: calculated 17.752, found 18.00%). 3. [Cu2(NH2 CS NH&] (NO3)* 3Hz0. (Nitrogen: calculated 24.71%, found 24.35%; copper: calculated 18.54%, found 18.62%. Solution of about 0.02M in copper(I1)-perchlorate in pure and dry acetonitrile (7) was prepared by dissolving an appropriate quantity of C U ( C ~ O6H20 ~ ) ~ in a definite volume of
the solvent. The exact strength of the solution was determined by titration with EDTA (8-10). The oxidation reaction was followed potentiometrically with a Cambridge Research Model pH meter (No. 0-213497) using platinum as the indicating electrode and a glass electrode as the reference electrode (ZJ). About 10 mg of the cuprous complex was taken in 10 ml of acetonitrile in a potentiometric cell and titrated against copper(I1)-perchlorate solution. RESULTS AND DISCUSSION
A gradual increase in potential is observed with the progressive addition of oxidant. A sharp jump in potential (200250 mV) is registered at the end point of the titration (Figure 1). The results of a few representative experiments are given in Table I. It can be seen from the titration results that three moles of the oxidant are required for every mole of copper(1)chloride complex, while five and six moles are taken up by copper(1)-nitrate and copper(1)-sulfate complexes, respectively. The stoichiometry of these reactions can be formulated in the following way:
]
2lu(S=C
