Analysis of Polysiloxanes by Alkali Fusion Reaction Gas Chromatography David D. Schlueter' and Sidney Siggia' Department of Chemistry, University of Massachusetts, Amherst, Massachusetts 0 1003
The technique of alkali fusion reaction gas chromatography has been applied to the determination of alkyl and aryl groups in polysiloxanes. The method involves the quantitative cleavage of ail organic substituents bonded to silicon, producing the corresponding hydrocarbons. Reactions are driven to completion, with no apparent decomposition, in less than 10 minutes by fusing the sample with potassium hydroxide in an Inert atmosphere. After concentration of the volatile products, they are separated and determined by gas chromatography. Sample losses are minimized by performing the total analysis in a single piece of apparatus. Fluids, gum rubbers, and resins are handled with equal ease. The percent relative standard deviation of the method is 1.0%; the average deviation between experimental and theoretical or check method results Is 0.5% absolute.
Polysiloxanes, also referred t o as silicones or polyorganosiloxanes, are commercially important because of unique properties resulting from their dual organic-inorganic composition. The analytical chemistry of this class of polymers has been reviewed in a recent monograph by Smith ( I ) . Spectrometric techniques, such as IR and NMR, are powerful tools for t h e qualitative and quantitative analysis of many functional groups in t h e intact polymer. Alternatively, these organic substituents can be cleaved from the polymer t o produce simple compounds that can b e determined by chemical or instrumental techniques. Alkyl and aryl substituents are readily detected by t h e above approaches, b u t quantitative analyses of these groups may be difficult with existing methodology. Instrumental techniques such as N M R ( 2 , 3 ) , IR (4-13), and UV (14-17) spectrometry are useful for analyzing methyl and/or phenyl groups in silicone polymers, b u t they are generally unable t o differentiate isomers or homologues. Chemical cleavage coupled with a separation technique is a more selective approach. Several reaction-based methods have been reported for t h e quantitative determination of alkyl and aryl groups bonded t o silicon and are included in Table I. T h e limited scope of these methods has led t o t h e present study. This paper describes the application of alkali fusion reaction gas chromatography to t h e quantitative analysis of a variety of alkyl a n d aryl groups in polysiloxane fluids, gums, and resins. Fusion of t h e polymer with a powdered potassium hydroxide reagent converts these substituents into t h e corresponding hydrocarbons, which are trapped and then analyzed by gas chromatography (GC). Application of this technique to other reactive functional groups bonded t o silicon was also investigated briefly.
EXPERIMENTAL Fusion Reagent. The alkali fusion reagent is a prefused mixture of potassium hydroxide (J. T. Baker, Analytical reagent grade) and 0.5% sodium acetate. The procedure for its prep'Present address, Biochemicals Department, E. I. du Pont de Nemours & Co., Inc., Wilmington, Del. 19898.
aration has been described in detail ekewhere (30). Typically, the mixture melts around 110 "C. and contains 13-14% water. When preparing this reagent, it is important to avoid excessive heating. If too much water is lost, the molten reaction mass may solidify before the hydrolysis reaction i:; complete. On the other hand, too much water will make the reagent sticky and difficult to handle. Additionally, it is possible to plug the cold trap if large amounts of water are released. To prevent adsorption of moisture from the atmosphere, the powdered reagent is stored in a small desiccator within a nitrogen-filled glove bag. Samples. The polysiloxane samples were obtained from Dow Corning Corporation (DC),General Electric Company (GE), Union Carbide Corporation (UC), and gas 'chromatography supply houses. The source of each sample is indicated in the tables of results. In most cases, samples were analyzed as received. Resin solutions were dried under vacuum before being sampled. Standards. The hydrocarbon calibration standards were purchased from commercial suppliers. The purity of each was checked by programmed temperature C:C on matched 6-foot by 1/8-inch 0.d. stainless steel columns packed with 50-80 mesh Porapak Q (Waters Associates, Milford, Mass. 01757). If necessary, the compounds were purified by conventional practices. m-Carborane (Ventron Corporation, Alfa Products, Beverly, Mass. 01315) was particularly impure, but vacuum sublimation at room temperature produced a suitable standard. Apparatus. The total analysis (reaction, trapping, separation, and quantitation) is performed in a single piece of apparatus. Figure 1 is a diagram of the reaction and trapping sections. The reaction portion consists of a pyrolysis tube, combustion furnace assembly, and a mounting frame taken from a commercial unit (Perkin-Elmer Pyrolysis Accessory 154-0825, Norwalk, Conn. 06856). Modifications made to the tube include the addition of a septum inlet (E) directly behind the reaction furnace (F) and the elongation of storage areas (A) and (B) for greater sample capacity. Samples, standards, and reagent are contained in miniature platinum boats (10 mm x 4 mm x 4 mm, Fisher Scientific Co., Pittsburgh, Pa. 15219) and are magnetically manipulated within the confines of the glass tubing with metal cylinders (J) and a hoe-shaped retriever (K). The combustion furnace assembly (F)monitors and controls the temperature within the reaction zone (C). A loop-shaped piece of 316 grade stainless steel tubing (G) (0.125-inch o.d., 0.093-inch i.d.), connected between the outlet of the reaction furnace and a special low dead-volume GC injector assembly (H) (Perkin-Elmer Part No. 009-0276), serves as a trap for efficiently concentrating most volatile reaction products. All of the transfer tubing from the reaction zone to the injection port, except the lower section of the loop, is heated to 300 "C by a clam-shell type combustion furnace (I) clamped around the tubing. The exposed piece of tubing, which is loosely packed with quartz wool, is cooled in a liquid nitrogen-filled Dewar flask when concentrating the evolved products and heated with a 400 "C nichrome wire heater (30) when "injecting" the products into the gas chromatograph. Helium carrier gas flows (60 mL/min) through the reaction unit and cold trap before entering the chromatographic column. Additional details and a photograph of the assembled apparatus are given in Ref. 31. The products are separated on matched 6-foot by '/*-inch 0.d. stainless steel columns packed with 50-80 mesh Porapak Q. A Perkin-Elmer Model 990 Gas Chromatograph, equipped with dual thermal conductivity detectors and a linear column temperature programmer, was used. The column temperature was maintained a t 90 OC during the reaction period. Upon injection of the products, time zero in the chromatograms, the temperature was ANALYTICAL CHEMISTRY, VOL. 49, NO. 14, DECEMBER 1977
2343
Table I. Reaction Methods for the Quantitative Determination of Organic Substituents Bonded to Silicon Group determined Reagent Reaction conditions Product Analysis Phenyl 60% aqueous KOH in 2hat120"C Benzene GC DMSO Phenyl Bromine in glacial Boiling solution Bromobenzene Titration of acetic acid excess bromine Ethyl and Phosphorous pent30-580 "C over Ethane and GC-FID phenyl oxide and water 4 5 min benzene Methyl and Powdered potassium 2 h a t 250-270 "C Gas buret Methane and hydroxide ethyl ethane Sulfuric acid Methyl 20 min at 280-300 Gas buret Methane
Ref. 18 19
20 21 22, 23
"C
Phenyl Vinyl Vinyl Vinyl Vinyl Vinyl
Ethylbromide in the presence of aluminum chloride Phosphorous pentoxide and water Phosphorous pentoxide and water 90% Sulfuric acid Sodium hydroxide pellets Potassium hydroxide pellets
Y
"
...
Hexaethylbenzene
Gravimetric
24
80-600 " C over 40 min Ambient t o 500 "C over 4 5 min 75-250 "C a t 1 0 "C/min and 1 h a t 250 " C 300 C for 15 min
Ethylene
GC-FID
25
Ethylene
GC-FID
26
Ethylene
GC-TC
27
Ethylene
Colorimetric
28
Heat with Meker burner
Ethylene
GC-FID
29
I
II /I
F
l i
I
-
G
3 '*
Figure 1. Diagram of the reaction andtrapping unit used for alkali fusion reaction GC. The mounting rack and electrical control units are not shown. A, storage area for unreacted samples; 6,storage area for reacted samples; C, quartz reaction zone: D, carrier gas inlet; E, side a r m with rubber septum; F, variable temperature furnace; G, trap loop; H, injector assembly into gas chromatograph; I, combustion furnace surrounding transfer tubing: J, metal cylinder behind platinum sample boat; K, magnetic retriever
programmed from 90 to 260 "C at 8 "C per minute. Peak areas were measured with a Vidar AutoLab 6300 Digital Integrator (Spectra-Physics, Santa Clara, Calif. 95051). The detector signal was displayed on a Varian G-2500 Recorder (Palo Alto, Calif. 94303). Procedure. One to five milligrams (HFg) of powdered sample or standard is weighed into the tared platinum micro-boats. The boats are then filled with the powdered caustic reagent in the nitrogen-purged glove bag and quickly loaded into the storage arm of the reaction tube (Figure 1, A) with a metal cylinder (J) behind each. When making quantitative analyses, the first sample reacted in a series of runs is used to condition the apparatus. Boats are usually loaded a t the end of a work day, thus permitting entrapped air to be purged from the system overnight. When samples are loaded in the morning, the tube is purged for 1 h before turning on the thermal conductivity detector current. Once a stable baseline is obtained, the liquid nitrogen-filled Dewar flask (-196 "C) is positioned around the unheated section of the trap. The first metal cylinder is magnetically moved forward, pushing the boat in front of it into the heated reaction zone (C). The cylinder is removed to storage area (B). The optimum reaction temperature profile for a fusion reaction will depend on the reactivity of the samples. In this study, the furnace temperature was programmed from 100 to 300 "C over a period of 10 min. As the reaction occurs, the volatile reaction products and water, liberated from the molten mixture, are carried by the 2344
ANALYTICAL CHEMISTRY, VOL. 49, NO. 14, DECEMBER 1977
flowing helium carrier gas and concentrated in the trap. At the end of the reaction period, Le., 10 min, the boat is withdrawn from the furnace with the magnetic retriever (K) and deposited with its cylinder in (B). After adjusting the furnace control to its initial temperature setting, the trapped compounds are revolatilized and directed into the gas chromatograph by replacing the Dewar flask with the heater (400 "C). When the separation arid integration are completed and the initial conditions reestablished, the procedure is repeated for each sample and standard. After completion of a series of reactions, the spent boats and metal cylinders are removed from the storage area. Boats are cleaned by soaking in dilute hydrochloric acid, washing with warm water, and heating in the flame of a Meker burner. The inner surfaces of the reaction tube are periodically cleaned with a moistened swab to remove any reagent that may have spilled or splattered from the boats. Calibration. Calibration curves are prepared for each compound produced. Products which are gases under normal conditions, are injected with a calibrated gas-tight syringe through the septum inlet of the reaction tube. Ambient temperature and pressure corrections are necessary to determine the actual amounts of gas injected. Standard solutions of liquid or solid reaction products are injected by syringe into an empty boat. These products are volatilized by pushing the boat into the heated reaction zone. Solid products having low vapor pressures at room temperature are weighed directly into platinum boats, covered
__
:
1 :p.,c--
5-7-
__
&
:
- - c
-
Flgure 2. Proton NMR spectrum of DC 710 polymethylphenylsiloxane in carbon tetrachloride. Methyl proton resonance occurs at 0 ppm while phenyl proton resonance occurs downfieM at -7 pprn. The arrow indicates the location of the dimethylsulfoxide internal standard singlet
with reagent, and stored with the samples. The preferred approach is to use secondary standards, Le., well-characterized compounds or polymers that quantitatively react with the reagent to give the desired product. For example, when only methane is determined, GE Viscasil 60000, a high molecular weight polydimethylsiloxane, is used. DC 704, a polymethylphenylsiloxane, is used as the secondary standard when both methane and benzene are determined. In all cases, the standards are trapped and chromatographed in exactly the same manner as the reaction products. The best straight line calibration curves are determined by a least-squares regression curve-fitting computer program. NMR Check Method. A proton nuclear magnetic resonance (NMR) method was developed to determine the methyl and phenyl content of polymethylphenylsiloxanes. Polymer 0.2-0.4 g is weighed, to the nearest 0.1 mg, into a tared sample bottle. Approximately 0.1 g of dimethylsulfoxide (Aldrich Chemical Co., Inc.) internal standard is added and the bottle reweighed. After addition of 1.0 mL of carbon tetrachloride (Aldrich), the vial is tightly capped and shaken. The proton spectrum from -1 to +9 ppm was obtained on a Perkin-Elmer Model R-12A, 60-MHz NMR spectrometer. Figure 2 is a typical spectrum of a silicone containing both methyl and phenyl substituents. The total peak area for each type of proton is obtained by scanning the appropriate resonance regions of the spectrum and summing the integrator output with a Heath DVM. Ten to twenty area determinations were made for each type of proton. The weight percent of each proton type is calculated from the equation:
where A is the average integrated area, W is the weight, N is the number of absorbing protons, and M is the formula weight of the internal standard (subscript i), the hydrocarbon group (subscript h ) ,and the sample (subscripts). In the case of methyl groups, this equation simplifies t o the following:
Weight Percent Methyl = 38.49 Similarly, the phenyl content is calculated from the equation:
Weight Percent Phenyl
=
118.4
RESULTS AND DISCUSSION A number of compounds have been suggested for cleaving monovalent organic substituents attached t o silicon (32-39). T h e use of potassium hydroxide was investigated. Initial recovery studies a t different isothermal reaction temperatures showed t h a t theoretical amounts of methane and benzene could be obtained from polydimethylsiloxanes and polydiphenylsiloxanes. However, reaction times were long a t the lower temperatures and splattering of the molten mixture
!I 0
6
12
18
24
TIME (minutes )
Figure 3. Gas chromatogram of the methane produced from the alkali fusion of GE Viscasil60 000 polydirnethylsiloxane. The water IS liberated from the reagent during the reaction
occurred if the reaction zone was too hot. By placing the boat into the oven, initially a t 100 "C, and setting the furnace controller for a final temperature of 300 "C, a reproducible temperature-time profile was generated within the reaction zone. Under these reaction conditions, all methyl and phenyl groups were cleaved in 5 min. As a general practice, the boat was left in the reaction zone for 10 min. This temperature programming procedure proved to be suitable for all the samples studied. Improvements were made in the trapping section. A 0.093-inch i.d. piece of '/8-inch tubing provided better heat transfer and was less prone t o plug than the more commonly available 0.085-inch i.d. tubing. Aerosol formation and possible blowthrough of the product is avoided by loosely packing the tubing with quartz wool. This was found to be a much more stable packing material than the previously used silanized glass wool, which degrades with time. Methane (bp -164 "C) is one of the few compounds not retained and concentrated in the cooled loop. This gas, when produced or injected through the pyrolysis unit septum, selectively passes through the trap a t a fixed rate resulting in a rectangular shaped "peak". This is illustrated in Figure 3, the gas chromatogram obtained from the fusion of GE Viscasil 60000. The recorder trace preceding time zero is typical of the detector response due to methane. Accurate methane peak area measurement was accomplished by adjusting the integrator sensitivity controls. After the integration was initiated, the noise and filtering dials were set a t their most insensitive levels. This does not interfere with the integrating process but does prevent the instrument from prematurely turning off. The inability to trap methane is an advantage. Quantitation of the product during the reaction results in a more rapid analysis, signals the end of the reaction, and simplifies any remaining chromatography. Calibration curves were prepared daily for each product being determined. Over a period of six months, the percent relative standard deviation of the slope of the methane curve was 2.2%. T h e slope of the benzene calibration curve varied by 1.9% relative over a two-month interval. Analysis of peak area data revealed that the initial sample or standard reacted on any given day gave recoveries that were a few percent lower than expected. For this reason, the first boat was used solely to condition the apparatus. ANALYTICAL CHEMISTRY, VOL. 49, NO. 14, DECEMBER 1977
2345
Table 11. Analysis of Polydimethylsiloxanes by Alkali Fusion Reaction Gas Chromatography CH3-Si~-Si*-Si-CH3 I I
Methyl content, wt %
Sample
xa
Alkali fusionb
%
Recovery
a w +
-L.
01
Theory
theoretical methane
86.7 GE SF96-20 4 39.74 i 0.30 45.86 98.7 GE SF96-50 4 5 40.74 t 0.16 41.26 99.6 GE SF96-100 8 5 40.75 t 0.06 40.93 100.5 DC 2 0 0 - 1 0 0 ~ ~ 8 5 41.15 i 0.35 40.93 99.6 GE SF96-1000 340 40.50 i 0.28 40.65 100.4 DC 200-12 500 cs 850 40.74 i 0.49 40.59 99.9 GE Viscasil 1240 40.55 * 0.24 40.58 60 000 99.8 GE SE-30 . . . 40.47 i 0.24 40.55 GE SE-33 , .. 39.93 i 0.42 c ... DC Silastic . . . 39.75 f 0.30 c ... 430 GE SE-31 . . . 38.14 f 0.33 c ... a Average number of dimethylsiloxane repeat units The standard deviabased upon manufacturers’ data. Sample tion is based upon five or more determinations. also contained a low percentage of vinyl groups. See Table VI. Methyl substituted polysiloxanes are probably the most important class of commercial silicon-containing polymers (40). Their physical properties can be modified by controlling the degree of polymerization, branching, and/or cross-linking. Under very basic conditions, one methane molecule is produced for each methyl group present (21). A series of polydimethylsiloxanes was analyzed by alkali fusion reaction GC. Table I1 summarizes the results of this study. Methane gas was used to prepare the calibration curve for GE Viscasil 60 000. This polymer was then used as a secondary standard for preparing subsequent curves. The precision of these determinations varied from 0.2 to 0.8% relative. Theoretical weight percent values were calculated using manufacturers’ average molecular weight data and the assumption that the polymers were linear and chain-terminated with trimethylsilyl groups. With the exception of the lowest molecular weight sample, the recovery of methane was within 0.5% absolute of theory. T h e low value obtained from the GE SF96-20
2
I 0 TIME
6 12 (minutes ;
18
24
30
Figure 4. Gas chromatogram of the methane and benzene produced from the alkali fusion of DC 704 polymethylphenylsiloxane sample may be due to vaporization of low molecular weight oligomers before reaction. Polymethylphenylsiloxanes are also of commercial importance (40). A gas chromatogram of the reaction products obtained from DC 704, a polymethylphenylsiloxane, is shown in Figure 4. Methane and benzene are the only volatile reaction products observed; the water is liberated from the reagent. Calibration curves were initially prepared using benzene and GE Viscasil60 000 as standards. A sample of DC 704 was reacted 33 times over a period of several days. T h e percent relative standard deviation of each product was 0.870, and the weight percent values correlated well with the NMR results. For these reasons, DC 704 was used as the secondary standard when both methyl and phenyl groups were determined. Table 111compares the weight percent values determined by both reaction GC and NMR. Agreement between these two different approaches was excellent, well within the expected accuracy of the NMR method. T h e gum and resin samples could not be dissolved in the carbon tetrachloride-dimethyl sulfoxide mixture and, thus, could not be determined by NMR. Solubility is no problem with the fusion approach.
Table 111. Analysis of Polymethylphenylsiloxanesby Alkali Fusion Reaction Gas Chromatography and NMR Phenyl content, wt % Methyl content, wt % Sample Alkali fusion’ NMR Alkali fusion4 NMR DC 510 35.9 t 0.1 35.2 i 2.2 9.96 i 0.07 9.37 5 1.16 34.5 :0.8 6.99 i 0.23 7.02 I0.18 GE SF-1153 35.8 3 0.3 GE SE-54b 34.5 2 0.3 ... 11.4 ?: 0.05 ... GE SE-52 33.4 i 0.4 ... 11.7 i 0.04 ... OV-3 32.2 i 0.4 31.0 i 0.8 17.2 i 0 . 2 17.3 i 1 . 5 DC 550 21.0 i 0.2 21.0 t 0.4 42.1 * 0.3 41.1 i 0.8 GE SF-1154 21.0 i 0.2 20.8 t 1.8 41.5 * 0.2 41.3 i 4.1 DC 556 27.4 i 0.5 27.5 i 0.3 28.6 i 0.1 27.9 t 0.9 15.4 i 0.4 53.2 i 0.4 54.3 i 1.0 DC 710 14.8 I0 . 1 OV-17 13.8 I0.2 14.1 t 0.7 55.4 i 0.3 55.4 t 2.4 66.0 i 0.9 DC 704 12.7 i 0.1 12.6 i 0.1 66.2 i 0.5 8.68 i 0.06 ... 47.4 t 0.3 ... DC 8 0 6 A DC 840 8.40 i 0.09 ... 47.4 t 0.3 ... 70.0 i 1.1 OV-25 5.83 i: 0.09 6.30 i 0.21 68.6 i 0.3 0 ... 80.7 0.5 C Polydiphenylsiloxane Sample also contains a low percentage of vinyl a The standard deviation is based upon five or more determinations. The sample could have between 77.8 and 86.6% by weight phenyl dependent upon the molecular weight distribugroups. 3
tion 2346
ANALYTICAL CHEMISTRY, VOL. 49, NO. 14, DECEMBER 1977
Table IV. Analysis of Alkyl and Aryl Substituted Polysiloxanes by Alkali Fusion Reaction Gas Chromatography Percent Volatile alkali Weight theoretical recoveryb Sample fusion products percentu GE SF-1080 Methane 10.1 i 0.1 100.8 Hexane 24.7 i 0 . 6 Cumene 37.7 f 0 . 2 GE SF-1091 Methane 9.96 i 0 . 1 1 103.4 Octane 66.0 i 0.1 GE SF-1147 Methane 11.0 i 0 . 2 95.9c Decane 5 9 . 4 f 0.9 DC 203 Methane 7.77 i 0 . 0 6 95.7d Cumene 1 3 . 3 i 0.1 Dodecane 5 3 . 8 i 1.8 UC L-42 Methane 2 4 . 2 i 0.1 99.8 Ethylbenzene 33.3 i 0.8 The standard deviation is based upon five or more determinations. Refer to t h e text for the assumptions made in calculating this number. Two small unknown seaks were present in the reaction gas chromatogram. An unidentified peak was present in the reaction gas chromatogram.
I
0 TIME
n
i=
44.09
n
Pi
2
i=l
Fj
30
represents the total weight percent contribution of all the pendant organic groups in the polymer as determined by the alkali fusion technique. T h e second term represents the corresponding siloxane content; 44.09 is the formula weight of the siloxane repeat unit and the factor 2 is based on the fact that, in fluid-type silicones, there is, on the average, 1 mol of siloxane for every 2 mol of substituent. If quantitative cleavage and recovery of each organic group is achieved, the sum of these weight percent values should approach 100%. Lower values will result if any step in the analysis is not quantitative, the substituents d o not produce a volatile product, and/or the polymer is branched or cross-linked. Recovery numbers calculated from the equation are included in Table IV. G E SF-1080 and G E SF-1147 recoveries were expected to be a little low because of small amounts (el%) of an antioxidant (Ethyl Corporation's Antioxidant 702) that is chemically bonded to the polymer. This particular antioxidant is a phenolic compound and produces the nonvolatile phenolate salt upon alkali fusion. T h e presence of impurity peaks in the chromatograms obtained from DC 203 and G E SF-1147 also account for the low values. The fact that these
-2 ; -
1
24
18
Figure 5. Gas chromatogram of the methane, n-hexane, and cumene produced from t h e alkali fusion of GE SF-1080
Alkyl and aryl groups other t h a n methyl and phenyl may be present in silicones. Several such samples were obtained, but the identity of these substituents was not known. Thus, before a quantitative analysis could be performed, the volatile reaction products were trapped from the gas chromatographic column and their mass spectra obtained. The combined mass spectral and GC retention d a t a permitted identification of these compounds. Figure 5 is the gas chromatogram of the three volatile fusion products produced from G E SF-1080. T h e quantitative results obtained from these samples are compiled in Table IV. The appropriate aliphatic and aromatic hydrocarbons were used as standards. In the absence of a suitable check method, i t was impossible to determine the accuracy of these values. T h e consistency of the results, however, can be determined by the simple mass balance recovery equation given below:
Recovery = Z Pi+
(
6 12 mlnutes )
where P is the weight percent and F is the formula weight of each organic substituent, i. T h e first term in this equation
Table V. Analysis of Polysiloxane Copolymers and Formulations Copolymer DC 4 7 0 A DC 4 7 2 DC 4 7 3 Dexsil 3 0 0 GC
Weight percent Alkali fusion Check method
Volatile alkali fusion products Methane Methane
5.73 11.0
f
Methane Methane m-Carborane
6.99 23.8 38.1
i
i
i
t
0 . 1 2 (5)U 0 . 3 (11)
0.08 ( 5 ) 0.4 ( 6 ) 0.7 ( 6 )
4.87c 11.0 i 0 . 3 b 15.4c
...
24.7c 39.OC
Formulation DC Stopcock grease Methane 3 5 . 2 i 0.2 ( 6 ) ... DC High vacuum grease Methane 35.5 i 0.2 ( 6 ) ... Ascolube (silicone) Methane 35.5 i 0.1 ( 6 ) ... stopcock grease and joint seal Analabs high temperature Methane 2 1 . 7 f 0.1 ( 2 ) ... red septum Benzene 1.88 t 0.01 ( 2 ) Analabs RS series high Methane 25.7 i 0.3 ( 2 ) ... temperature white septum Benzene 1 . 7 6 t 0.04 ( 2 ) ... Applied Science high Methane 25.8 i 0.2 ( 2 ) ... temperature blue septum Benzene 8.13 i 0.03 ( 2 ) . . , NMR value using benzene as the internal standard. The number in parentheses is the number of determinations. Approximate values calculated from suppliers' information bulletin. ANALYTICAL CHEMISTRY, VOL. 49, NO. 14, DECEMBER 1977
2347
Table VI. Analysis of Other Organofunctional Groups in Polysiloxanes by Alkali Fusion Reaction Gas Chromatography Volatile alkali fusion Weight Sample product determined percenta Ethylene 0.117 i 0.005 DC Silastic 430 Ethylene 0.146 i 0.010 GE SE-54 Ethylene 0.171 i 0.010 GE SE-33 Ethylene 0.179 f 0.009 GE SE-31 Methane 34.6 I0.7 Versilube F-50b a The standard deviation is based upon five or more determinations. b N o chlorobenzene peak was noted in the gas chromatogram.
recovery values were, in all cases, within 5% of the theoretical model, strongly suggests that a wide variety of alkyl and aryl groups can be accurately determined. Polydimethylsiloxanes copolymerized with polyoxyethylene, polyethylene, and/or polyoxypropylene were analyzed for methyl content (Table V). Although polyoxyalkalenes react with the reagent, all of their volatile reaction products were retained by the trap and, thus, did not interfere with the methyl determination. T h e Dexsil GC phases investigated are copolymers in which the siloxane blocks are joined through m-carborane linkages. Quite unexpectedly, the m-carborane produced in the fusion reaction was sufficiently volatile to be chromatographed and determined (Table V). Several formulations containing silicones were subjected to alkali fusion. T h e fact that no extraneous peaks were observed in the chromatograms indicates that the fillers were nonvolatile and did not react or decompose to form volatile products. T h e qualitative and quantitative results are included in Table V. T h e three grease samples produced essentially the same weight percent methane, whereas the gas chromatographic septa contained different ratios of methyl and phenyl groups. The other ingredients of the formulations do not appear to hinder the fusion reaction; thus, a preliminary separation of t h e polysiloxane is unnecessary. Vinyl groups (0.1-2.0%) are incorporated into polysiloxanes to facilitate cross-linking (40). Reaction gas chromatography is the most sensitive and selective method presently available for their determination (25-29). Published procedures are included in Table I. Although satisfactory results were reported, our reagent and apparatus offer some additional advantages. T h e fusion reaction with potassium hydroxide proceeds more rapidly under less drastic conditions than the methods employing acidic reagents (24-26). The continuous flow design eliminates secondary reactions, such as the conversion of ethylene to ethane, which are a problem with static systems (27). In addition, since all of the volatile product, rather t h a n a small fraction, is chromatographed, greater sensitivity is possible. Table VI lists the vinyl contents we found using our apparatus. The standard deviation at the 0.1 to 0.2 wt % level was between 4 and 7% relative. Ethylene gas was used to prepare the calibration curves. Chlorination of phenyl groups in polymethylphenylsiloxanes increases their lubricating action (40). Alkali fusion of Versilube F-50, a polymer containing chlorophenyl substituents, gave a reaction gas chromatogram similar to that of Figure 3; no chlorinated benzene products were observed. Apparently the reaction conditions were drastic enough to cause replacement of the chloro group; the aromatic material remains in the boat as the nonvolatile potassium phenolate salt (41, 42).
CONCLUSIONS Although this study was directed a t the determination of alkyl and aryl groups in polysiloxanes, it should also be applicable to alkoxy groups. Hanson and Smith (29) have 2348
ANALYTICAL CHEMISTRY, VOL. 49,NO.
14,DECEMBER 1977
demonstrated quantitative cleavage and recovery of the corresponding alcohols using a similar potassium hydroxide reagent. T h e poor precision (*2070 relative) and accuracy (-10% relative) appear to be due to their inefficient water trap. The proven efficiency of our liquid nitrogen bath for retaining alcohols (30) should improve the accuracy and precision significantly.
ACKNOWLEDGMENT T h e authors thank the following persons for supplying silicone samples: A. M. Quinn of the Dow Corning Corporation, N. G. Holdstock of the General Electric Company, and J. E. Pikula of the Union Carbide Corporation.
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RECEIVED for review February 11, 1977. Accepted September 19, 1977. This work was supported by the National Science Foundation, Grant Number GP-37493~.Preliminary results of this study were presented a t the 168th American Chemical Society Meeting, Atlantic City, N.J., September 13, 1974.
