diethyl ether solution, the excess molybdate remaining in the aqueous phase. After a thorough washing of the extract with the hydrochloric acid wash solution, the absorbance of the molybdovanadophosphoric acid in the organic phase was measured at 308 mp. These absorbances were then plotted us. the corresponding mole ratios of molybdate to phosphate. As the spectrophotometric data listed in Table I and plotted in Figure 3 show the molybdate-phosphate ratio was 11:l. Therefore, if the vanadate to phosphate ratio was 1 :1 and the molybdate to phosphate ratio was 11 :1, it was concluded that the molybdate to vanadate ratio must also be 11 to 1. Diverse Ions. A study was made to determine the permissible amounts of various ions that may be present without interfering with the determination of parts per million of vanadium. N o attempt was made to determine the effects of ion concentration larger than 500 ppm, since this concentration is rather large compared to the concentration of vanadium that was being determined. Errors twice the relative standard deviation were considered negligible.
Table I1 summarizes the results of this study. As expected silicate and tungstate which also readily form various heteropoly complexes interfere. Iron(II1) gives a negative error presumably because of complexation with the phosphate while iron(I1) acts as a reductant with the mixed heteropoly complex. The observed interference with the Ti(1V) is attributed to the high concentration of sulfate resulting in preparation of the titanium(1V) solution. Reproducibility. An estimate of the precision of this method was ascertained from the results of eight samples each containing 0.48 ppm of vanadium. These samples gave a mean absorbance value of 0.422 at 228 mp. The standard deviation was 0.0045 absorbance unit, or a relative standard deviation of 1.06z. In a series of determinations, about 40 minutes is required for each determination, RECEIVED for review August 28, 1967. Accepted November 27, 1967. Presented at the 15th Anachem Conference, Detroit, Mich., October 1967.
DifferentiaI Therma I Ana Iysis CrystaII inities and Melting Points of Ethylene-Vinylpyrrolidone Copolymers Bert H. Clampitt and Richard H. Hughes Gulf Research and Deuelopment Co., Kansas City Division, Merriam, Kan.
THEMELTING POINTS and crystallinities of various high pressure polyethylenes (HPPE) have been the subject of several recent publications (1-4). It is generally inferred in these articles that all types of hydrocarbon branching (ethyl, butyl, etc.) behave identically in their effect on crystallinity of HPPE's. In fact, articles on hydrocarbon copolymers prepared with Ziegler catalysis systems indicate that in these systems this is the case. It is the purpose of this paper to show that vinylpyrrolidone (VP) groups in ethylene copolymers affect the crystallinity and melting point of them in a manner analogous to hydrocarbon branching. Further, it is the object of this paper to propose a method for estimating hydrocarbon branching in ethylene copolymers. A previous publication from this laboratory indicated a linear relationship between crystallinity determined by differential thermal analysis (DTA) and mole per cent comonomer in ethylene copolymers (5). The polymers reported in that paper were all prepared under similar reactor conditions where the amount of hydrocarbon branching would be expected to be similar. When DTA crystallinities of ethylene copolymers prepared under widely different reactor conditions, as in the present paper, are compared, no correlation exists between crystallinity and comonomer content. It is generally realized that ethylene copolymers produced by high pressure polymerization techniques contain both hydrocarbon and comonomer branching; however, a quantitative (1) K. Casey, C. T. Elston, and M. K. Phibbs, Polymer Letters, 2, 1053 (1964). (2) I. J. Bastien, R. W. Ford, and H. D. Mok, Polymer Letters, 4, 147 (1966). (3) D. Bodily and B. Wunderlich, J. Po/ymer Sci. A , 4, 25 (1966). (4) L. Mandelkern et a/.,Polymer Letters, 3,803 (1965). ( 5 ) K . J. Bombaugh and B. H. Clampitt, J. Polymer Sci. A , 3, 803 (1965).
66202
measure of the hydrocarbon branching is extremely difficult. This difficulty arises because most comonomers contain methyl groups which interfere with the determination of hydrocarbon branching by infrared methods. Vinylpyrrolidone is an unusual comonomer since it contains no methyl groups, and therefore both comonomer and hydrocarbon branching can be measured by infrared techniques in ethylene-vinylpyrrolidone copolymers. These infrared measurements then allow correlations to be made between total branching and DTA crystallinity and melting point measurements. EXPERIMENTAL
The DTA measurements were made using a Perkin-Elmer differential scanning calorimeter calibrated for temperature with indium metal. Sample weights of 10 f 0.5 mg were used with the samples being fabricated from approximately 10-mil thick film. The annealing procedure and the heat rate of 20" C/minute were identical to those described by Casey et al. (I). Areas of the thermograms were measured with a planimeter, and the melting points were taken as the peak of the endotherms. Methyl groups were measured on a Perkin-Elmer Model 221 spectrophotometer using the method described by Willbourn (6) with a polymethylene wedge in the reference beam. Infrared determination of vinylpyrrolidone branchings were made by measuring the relative intensities of the 5.9-p band to the 6.8-11 CH2 band on films of about 0.5 mil in thickness. A calibration curve relating this IR parameter to VP content was obtained previously using elemental carbon analysis of the copolymers as the absolute standard. The polymers discussed in this paper were prepared in semicommercial high pressure polyethylene reactors. Widely .
(6) A. H. Willbourn, J . Polymer Sci., 39, 569 (1959). VOL 40, NO. 2, FEBRUARY 1968
449
b Figure 1. Melting point us. total branching
z
Table 11. Branching, Crystallinity, and Melting Point Data for Various Vinylpyrrolidone-Ethylene Copolymers
System 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220
450
ANALYTICAL CHEMISTRY
Crystallinity,
z
63 54 43 62 47 56 0
54 67 65 64 65 60 11 55
66 62 66 67 58 0
Melting point, “C 113 107 101 109 102 105 ... 106 114 111 110 110 109 90 105 112 109 109 110 105
*..
20
M
\
\,
40
so
SO4
TOTAL BRANCHINO
Figure 2.
Table I. Branching, Crystallinity, and Melting Point Data for Various High-pressure Polyethylenes Methyl branches/ lo00 carbon Crystallinity, Melting point, System atoms “C 100 28.5 42 102 101 12.1 77 118 102 33.9 35 97 103 19.8 52 108 104 10.3 83 122 105 13.1 65 114 106 14.3 61 113 107 17.5 60 112 108 22.0 48 109 109 24.2 47 108 110 23.0 47 108 111 10.1 79 120 112 23.2 53 106 113 17.9 63 111 114 20.2 64 111 115 36.9 27 96
Branches/1000 Carbon Atoms VP Methyl Total 7.0 10.8 17.8 3.2 22.1 25.3 9.6 16.9 26.5 4.0 12.4 16.4 11.7 15.4 27.1 7.0 18.4 25.4 6.3 46.9 53.2 5.5 18.3 23.8 5.5 9.8 15.3 0.9 16.3 17.2 1.3 15.4 16.7 3.1 12.0 15.1 4.7 13.7 18.4 26.0 16.1 42.1 12.6 11.o 23.6 8.3 8.5 16.8 2.8 15.3 18.1 2.7 15.6 18.3 3.0 15.6 18.6 9.9 13.3 23.2 38.5 22.7 61.2
Ib
Crystallinity us. total branching
varying temperatures, pressures, and comonomer contents were used. Specifically, 16 HPPE’s and 21 VP copolymers were characterized in this study. RESULTS
Infrared and DTA results for HPPE are given in Table I and the results for ethylene VP copolymers are given in Table 11. Hydrocarbon and comonomer branching are reported in terms of branches per 1000 carbon atoms. DTA crystallinity values given in both tables were obtained by normalizing the thermogram areas relative to the area of a HPPE of known crystallinity. This is similar to the procedure outlined by Ke (3,and is very useful in comparing the crystallinities of a series of samples. DISCUSSION
Both Casey et a / . ( I ) and Bastien et ai. (2) have reported a linear melting point-hydrocarbon branching relationship for HPPE’s. The present study confirms a linear relationship as can be seen in Figure 1. The slope of the line is very close to the one reported by Casey et a / . ( I ) . All of the melting pointbranching data for HPPE’s are included in Figure 1, and it is seen that they all fall reasonably close to the indicated line. In ethylene-VP copolymers, if either hydrocarbon or comonomer branching alone is plotted against melting point, no correlations exist. However, if the total branching of the copolymers is plotted us. melting point, the correlation depicted in Figure 1 exists. It therefore appears that the total branching of the copolymer determines the melting point of the SYStern. This is clearly seen in samples 207 and 214 of Table I1 where large differences exist in copolymer and hydrocarbon branching between the two systems. However, the total branching of the two systems is nearly identical and they possess similar melting points. Figure 2 shows a plot of DTA crystallinity us. total branching in both HPPE and VP copolymer systems. Again a linear relationship exists. Such a relationship for HPPE’s was reported previously by Ke (8); however, he reported a different slope for the relationship than that found in the present study. It was reported previously (5) that for copolymers prepared under similar reactor conditions, the greater the amount of comonomer, the less the crystallinity. The present study (7) B. Ke, J. Polymer Sci., 42, 15 (1960). (8) Zbid., 61,47 (1962).
shows this not to be the case for copolymers prepared under widely different reactor conditions. This is strikingly illustrated in copolymers 200 and 201 of Table I1 where sample 201 contains only half as much comonomer as sample 200 and yet it is less crystalline. If all of the crystallinity data in Table I1 are plotted as a function of VP branching alone, no correlation exists. This points to the need for an additional parameter in defining the relationship between copolymer branching and crystallinity. This additional parameter turns out to be the hydrocarbon branching of the system which, when added to the copolymer branching, gives the linear relationship depicted in Figure 2. It will be noted that the intercept of Figure 2 is near 50 branches per 1000 carbon atoms. Samples 206 and 220 are more highly branched than this, and therefore possess no crystallinity or melting point. Points for these two samples are shown on the abscissa axis of Figure 2, but cannot be included in Figure 1 since they possess no melting point. Having established the relationships shown in Figures 1 and 2, of what value are they in the general characterization of ethylene copolymers? Again, it should be emphasized that VP-ethylene copolymers are unique, in that they are the only readily available systems where both methyl and comonomer branching may be measured. In other ethylene copolymerse.g., vinyl acetate, methyl acrylate-only the comonomer content can be measured by infrared techniques. If one assumes
all comonomers behave identically in their effects on crystallinity and melting point, then Figures 1 and 2 may be used in conjunction with IR comonomer measurements on non-VP copolymers to obtain hydrocarbon branching in them. For example, a certain vinyl acetate copolymer was analyzed by I R and found to contain a comonomer content equivalent to 20 vinyl acetate branches per 1000 carbon atoms. DTA crystallinity measurements showed it to be 3 9 z crystalline. From these measurements and Figure 2 it can be seen that this copolymer contained 10 hydrocarbon branches per 1000 carbon atoms. A limitation to this method is that it is limited to total branchings less than 50 per 1000 carbon atoms. Fortunately most commercial copolymers are in the range of applicability, ACKNOWLEDGMEh'T
The authors wish to express their gratitude to Dr. H. D. Anspon for advice and encouragement, and to Dr. F. E. Brown for preparation of the many polymer samples. Appreciation is also expressed for Mr. Bob Bartholomew and Mr. Gene Rouse for much of the experimental work. RECEIVED for review August 16, 1967. Accepted November 17,1967.
Nondestructive Neutron Activation Analysis of Small Samples of Witwatersrand Ore for Gold P. W. de Lange, W. J. de Wet, J. Turkstra, J. H. Venter Atomic Energy Board, Pelindaba, Pretoria, South Africa
A NUMBER OF TOPICAL PAPERS have appeared on the nondestructive activation analysis of gold (Au) in ore samples. De Silva (1) experienced that the inhomogeneity of the mother sample, although crushed to better than 200 mesh, was such that the specific activity of 19*Au from neutron activation showed deviations of more than 1000 when he took 200-mg samples from the mother sample for activation. At the Tashkent Conference on Activation Analysis ( 2 , 3) satisfactory activation of gold in ore samples was obtained when finely ground 30-mg samples were irradiated for 6 hours in a neutron flux of 8 X 10l2n cm-2 second-' and then allowed 5 to 8 days for decay. An average deviation of + 12 is stated in the Nuclear Science Abstract (4) of the Tashkent Conference. All these authors used NaI(T1) scintillation gamma-spectrometry. The general approach in the South African Mining industry (5) is to use samples of 60 to 100 grams for fire-assay quantitative analysis because of the large grain size of the gold present
z
(1) J. G . de Silva Filho, A. Abrao, F. W. Lima, Publ. I.E.A. 98, Inst. Energia Atomica, Sao Paulo, Brazil (1965). (2) G . A. Perezhogin, I. P. Alimarin, First All-Union Co-ordinating Conference, Tashkent, October 24-28, 1962; E. M. Lobanov, Ed., Israel Program for Scientific Translations, 1966, p. 55. (3) E. M. Lobanov, I. A. Miranski, V. F. Pozychanuk, D. G. Saifietdinov, A. A. Khaidarov, Zbid., p. 60. (4) E. M. Lobanov, I. A. Miranski, M. M. Romanov, A. A. Khaidarov, Nucl. Sci. Absrr., 18, Abstr. 43432 (1964). (5) C. H. Coxon, Corner House Laboratories, Johannesburg, private correspondence, June 1967
in the crushed ore. Gold grains tend to flatten out under continuous ball mill crushing and do not break up. N o doubt exists that neutron activation analysis of gold in ore samples can be accomplished, but sample inhomogeneity and interference by other gamma peaks in the analysis have prevented the widespread application of such activation analysis. PRINCIPLE OF THE METHOD
When a 1- to 2-gram gold-bearing ore sample is placed in an ORR-type nuclear reactor such as SAFARI-1 with a thermal neutron flux of 1013ncm-2 second-', the following reaction is the most important: l9lAu (n, y) lgaAu l?&
(1)
lQ8Auhas a half life of 64.8 hours and decays with a single gamma of 412-keV after /3-emission. The activation cross section is 98.8 barns for thermal neutrons and it has a resonance integral of 1558 barns with a major resonance of 4.9 eV. Uranium (U) is present in most of the Witwatersrand ores and in a reactor neutron spectrum the following reaction can be obtained:
a
238U (n, y)239U 239Np l?& (a, = 2.7b) (23.5m) (2.35d)
239pu
(2)
Neptunium-239 emits a number of gamma rays of which the 277-keV transition is convenient for analytical exploitation. VOL 40, NO. 2, FEBRUARY 1968
451
