in end point volume determination. Serious interference occurs when C1- and NOa- are present in 100-fold excess over SO,- (See Figure 2). The adverse effect of bicarbonate can be eliminated by adjusting the sample pH to 4. Titration curves in the absence of carbonate are independent of pH in the range pH 3.0 to pH 6.5. At low pH, the solubility of the electrode membrane increases the electrode potential, thereby decreasing the end point potential break.
Precision. In the absence of the interferences described in the preceding section, end point volumes are stoichiometric and reproducible to 3~0.2%at concentrations above 5 X lO-'M sulfate. Precision decreases with sample dilutions, approaching *l.O% at 10+M initial sulfate concentrations. RECEIVED for review February 7, 1969. Accepted March 19, 1969.
Nuclear Magnetic Resonance Analysis of Rubber Vulcanizates David W. Carlson and Alan G . Altenau The Firestone Tire L? Rubber Co. Central Laboratories, Akron, Ohio 4431 7
THEQUANTITATIVE DETERMINATION of the polymer composition of vulcanized stocks has always been a very difficult problem. Present methods consist of dissolving the vulcanizates in boiling o-dichlorobenzene, removal of the carbon black by filtration, and then infrared determination of the polymer ( I ) . This procedure is time consuming and may suffer from selective adsorption of different elastomer types on the filtration medium. Pyrolysis of the stock followed by infrared analysis of the products has also been used (2). This technique had the disadvantages of requiring calibration of the infrared instrument and of having some uncertainty about changes in the microstructure of the polymer due to pyrolysis procedure. Other infrared spectrophotometric studies have been made but no quantitative measurements were reported (3-6). The use of NMR rather than infrared was investigated as a means of solving this problem. Various vulcanizates containing two or three of the following, natural rubber, polybutadiene, and butadiene styrene copolymer (SBR), were analyzed. EXPERIMENTAL
Apparatus. A Varian HA-60-IL nuclear magnetic resonance spectrometer was used to record the spectra while a Varian V3521A integrator/decoupler was used to integrate the area of the peaks. All integrals were run in internal lock field sweep mode using 30 db-rf attenuation and a setting of 0.5 on the manual oscillator. Procedure. Samples of the vulcanized rubber were cut into small pieces and extracted with acetone overnight in a Soxhlet apparatus to remove the oil, antioxidant, and any other extractable compounds. Four milliliters of hexachlorobutadiene were then added to 0.25 gram of the extracted sample in a 15-ml beaker. The uncovered beaker and contents were placed on a hot plate having a surface temperature of about 160 "C. After one to two hours the polymer was in solution. In several cases complete dissolution of the polymer was not obtained in this time. If several more hours of heating still resulted in no change, a higher temperature was used and usually helped. It was noticed, however, that (1) H. L. Dinsrnore and D. C. Smith, Rubber Chern. Technol., 22, 512 (1949). (2) D. A. MacKillop, ANAL.CHEM., 40,607 (1968). (3) D. L.Harms, ibid., 25, 1140 (1953). (4) D.Hurnmel, Rubber Chern. Technol., 32,854 (1959). ( 5 ) M. Lerner and R. C. Gilbert, ANAL.CHEM., 36, 1382 (1964). (6) M. Tryon, E. Horowitz, and J. J. Mandel, J . Res. Not. Bur. Std., 55,219 (1955).
II
POLYSTYRENE W v)
z P 0
fn W
Ha
K
2.0
3.0
4.0
5.0
6.0
7.0
8.0
9.0
10.0
T
Figure 1. NMR spectrum of vulcanizate containing a mixture of polybutadiene and butadiene-styrene copolymer too long a heating period could precipitate the polymer from solution. The black solution was cooled and filtered by pushing the solution through Reeve Angel 802 filter paper which was supported in a millipore Sweeney pressure filter and attached to the end of a 2-ml syringe. This filtration only removed pieces of undissolved stock and a small amount of carbon black. The filtrate and a trace of TMS were added to the NMR sample tube and the spectrum was obtained at room temperature. RESULTS AND DISCUSSION
Equations used to calculate polymer compositions by NMR have been published by Mochel (7). These same equations were used in this work. The quality of the spectra was comparable to that of uncompounded polymers. Good agreement was obtained between the calculated amounts of butadiene, natural rubber, and styrene and the found values; however, the method will not distinguish between butadiene from polybutadiene and butadiene styrene copolymer (Table I). The relative amounts of 1,Zbutadiene and lP-butadiene were found in the cases of polybutadiene and SBR blends. The individual amounts of cis and trans 1,4-butadiene could not be calculated because of complete (7) V. D. Mcchel, Rubber Chem. Technol., 40, 1200 (1967). VOL. 41, NO. 7, JUNE 1969
969
Styrene
Table I. Determination of Polymer Composition Calculated compounded formula Found compounded formula Natural Butadiene Natural Butadiene Rubber Total 1941,2Styrene Rubber Total 1,4-
15.5 12.0
75.2 50.5
20.0 17.0 16.7 15.6 13.2 11.0 8.8
25.0 40.0 20.0 30.0 35.0
75.0 60.0 60.0 53.0 51.7
40.0
44.4 46.8
40.0 50.0 60.0
9.3 37.5
13.7 11.3 27.1 42.0 31 .O 36.0 41.7
72.9 58.0 55.7 53.5 48.0 41.7
44.0
42.9
50.0 64.0
40.3 31 .O
24.3
20.0 15.5 16.0 16.6 11.8 9.7 5.0
39.0 31.2
1,2-
9.8
76.5 47.3
41.4
fir J.4-PoJyisopr0118
W v)
B
W
a
t
1
2.0
I
3.0
I
A0
c I
5.0
60
7.0
8.0
9.0
I
I
2.0
I
3.0
I
I
4.0
5.0
I
6.0
I
7.0
1
8.0
I
9.0
I1 0
T
T
Figure 2. NMR spectrum of dcanizate containing a mixture of natural rubber and butadiene-styrene copolymer
Figure 3. NMR spectrum of vulcanizate containing a mixture of natural rubber, polybutadiene, and butadiene-styrene copolymer
overlapping of the two signals as is also the case in uncompounded polymers (Figure 1). Determination of the different microstructures in a blend of polybutadiene and/or SBR with natural rubber is very difficult because the signals from the olefinic protons severely overlap one another. No attempt was made to calculate the percentages of the different microstructures in these blends of rubber. Natural rubber is essentially 100% cis 1,4-polyisoprene. The signal of the methylene hydrogens from the butadiene and natural rubber overlap the methyl group signal of natural rubber. The degree of overlap will vary with the effectiveness of sample dissolution. Therefore, the area of the methyl group signal was measured exactly from the initial rise to the end of the signal in order to reduce any contribution from the overlapping signals (as shown by the arrows in Figures 2 and 3). If the measurement was not done that way, the results were usually high. o- and p-Dichlorobenzene, 1,2,4-trichlorobenzene, 1,1,2,2tetrachloroethane, and naphthalene also dissolved the vulcanizates at temperatures around 160 "C. Tetrachloroethylene at its reflux temperature did not dissolve the polymers.
970
ANALYTICAL CHEMISTRY
The reason in the latter case was probably due to its lower reflux temperature. The disadvantage of using an aromatic solvent is the interference of the aromatic proton signals of styrene with those of the solvent. If styrene were absent, aromatic solvents could be used. Although we say the rubber is dissolving in hexachlorobutadiene, there is first a breakdown, possibly at the sulfur bonds which result from the vulcanization process. Some breakdown or reduction in the molecular weight of the polymer during the dissolution period probably also occurs. The carbon black had minimal detrimental effects in the NMR spectrum as evidenced by the sharpness of the signals (Figures 1, 2, and 3) and the excellent quantitative results. ACKNOWLEDGMENT
The authors thank The Firestone Tire & Rubber Company for permission to publish this work.
RECEIVED for review January 31, 1969. Accepted March 27,1969.
