Determination of the Ratio of Methyl to Phenyl Groups in Silicone Polymers J. HAROLD LADY, GEORGE M. BOWER, ROBERT E. ADAMS, and F. P. BYRNE Westinghouse Research laboratories, Pittsburgh 35, Pa.
b The ratio of the methyl to phenyl groups in silicone polymers can b e determined by measuring the intensity of the methyl-silicon and phenyl-silicon bands at 7.92 and 6.97 microns, respectively. A suitable calibration curve may b e prepared covering a methyl to phenyl ratio of 0.5 to 5 and an R/Si range of 1 .O to 1.7.
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HE ratio of methyl to phenyl groups in silicone polymers is of interest, because physical properties such as curing, thermal stability, and flexibility are dependent on it. The ratio of absorption intensities of infrared bands arising from the methyl and phenyl groups is related to the amount of these groups present ( I >2 , 5 ) . However, the lack of reference standards has prevented the conversion of this information into the actual molar ratio of the two constituents. The 6.97-micron phenyl band and the 7.9micron methyl band were chosen because of their favorable intensity and proximity.
APPARATUS
This work was performed using a Model 21 Perkin-Elmer infrared spectrophotometer, and a slit resolution program setting of 927 which gives a 52-micron slit at 6.9 microns and a 60micron slit a t 7.9 microns. EXPERIMENTAL
The infrared spectra of silicone polymers containing all methyl and no phenyl groups, all phenyl and no methyl groups, and a polymer containing both groups are shown in Figure 1. Figure 1,A, shows the spectrum of a polymer containing methyl groups only. The band near 7.9 microns has been identified by a number of n-orkers as arising from vibration of the methyl group (3, 6-8). The other bands present are labeled as to the groups from which they arise in the cases in which the frequencies have been well established. Figure 1,B, shows a spectrum of a phenyl-silicone polymer containing phenyl groups only. It was prepared by hydrolysis and cocondensation of diphenyldichlorosilane and phenyltrichlorosilane. The band a t 6.97 microns is generally regarded as being due to the phenyl-silicon linkages (4,8). Other bands are labeled in the cases in which
1100
ANALYTICAL CHEMISTRY
the assignments are fairly well established. The small band near 7.9 microns no doubt contributes a small amount to intensity measurements made on the methyl group Then polymers have a very high phenyl to methyl ratio; however, this does not adversely affect the determination, because the band is of relative weak intensity. The only effect will be that of preventing the calibration curve from passing through the origin. Figure l,C, shows a typical polymer containing both phenyl and methyl groups (molar ratio, M e / + = 2 ) . As indicated in the figure. a baseline technique mas used to obtain the peak intensities of the two bands. The C-H band a t 7.1 microns is sufficiently separated from the phenyl-silicone band a t 6.97 microns so as not to make a significant contribution to the intensity of this band. The analytica1 measurements are made by placing the cast film of the polymer in the sample beam and a blank sodium chloride plate in the reference beam to compensate for reflection losses. The sample thickness should be such that peak intmsities of both bands fall within the absorbance region of 0.2 to 0.7, if possible. I n general, three of the four films n-ere cast and an average of the two or three which fell within the proper absorbance region vere taken. The standards used were 20 polymers prepared hy hydrolysis and cocondensation of thp appropriate ratios of phem-lmethyldichlorosilane, methyltrichlorosilane. phrnyltrichlorosilane, and diniethyldichlorosilane. The calibration curve prepared for this work is shon-n in Figure 2 . The composition of the standards varied considerably. .A variation of the methyl to phenyl ratio from 0 to 5 is shoim. For commercial thermosetting silicone polymers the primary rpgion of interest is a methyl to phenyl ratio of approximately 0.5 to 3.0. The most accurate region of the curve from the standpoint of a spectrophotometric measurement is from a ratio of 0.3 to 3.5. This represents the allovdde limits when both bands are confined to the ahsorhance region of 0.2 to 0.7 normally preferred for spectrophotometric measurements. I n this type of measurement where tn-o parameters are measured with but one degree of freedom, nothing can he done about the fact that the ratio does not fall nithin the prescribed area, hut to accept the fact that the relative error of a measurement becomes increasingly larger
as the ratio becomes more removed from this region. It is thus apparent that the upper and lower points on the calibration curve are not strictly valid. I n the standards the ratio of the sum of the methyl and phenyl groups to silicone atoms-(R/Si)-varied from 1.0 to 1.7. The methyl to phenyl ratio was generally lower with ion-er R/Si values. This is also the practice follored in commercial polymers. Most of the standards n-ere prepared from phenylmethyldichlorosilane and methyltrichlorosilane in the appropriate proportions. T o check with regard to polymers of a different structure a polymer was prepared from 0.5 mole of dimethyldichlorosilnne and 0.5 mole of phenyltrichlorosilane. The ratio of the intensities of methyl and phenyl bands was 2.75, falling nicely on the calibration curve. The standard having this methyl to phenyl value of 2 n-as prepared from 0.5 mole of phenylmethyldichlorosilane and 0.5 mole of methyltrichlorosilane. The relative intensities of the methyl-phenyl bands for the standard was 2.86. I t is still possible that other variations in polymer structure may lead to significant deviations from this calibration curve. Because instrument differences may introduce errors, it seems advisable for anyone using this method to prepare a calibration curve. RESULTS
The results of a number of determinations are shown in Table I along with the estimated standard deviation (0.091) obtained by combining the data. Also included in the table are the methylphenyl ratios previously obtained for these materials as calculated from a carbon-hydrogen analysis. The agreement was not particularly good. To test the reliabiIity of both the infrared and carbon-hydrogen methods, three p o l p e r s were prepared and analyzed by both methods. I n the course of preparing the polymers caution was taken to minimize material losses to ensure the theoretical composition of the final product. The polymers were prepared in approximately 90% yield. The yields obtained q-ere considered sufficiently large to indicate that the theoretical methyl-phenyl ratios were essentially correct. The results of the two methods of analysis are summarized in Table 11. The infrared method agrees with the theoretical values within
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exaggerated sornen-hat in calculating the methyl-phenyl ratio. In commercial pol?-mers any unknown nonvolatile additives iyould contribute to the carbon-hydrogen content and hence lead to erroneous results.
DISCUSSION
There are a number of possible reasons why the carbon-hydrogen analysis might result in erroneous methyl-phenyl ratios. Some of these are: Occlusion of solvent in the polymer. Selective fractionation of portions of the polymer while being heated under vacuum to remove solvent. Occlusion of carbon in the silicon dioxide during the combustion process in the carbon-hydrogen analysis. Errors in the hydrogen content are
The use of additives in commercial polymers causes the last reason given to become a serious limitation to the hydrogen-carbon method of determination. It is evident that the infrared method is considerably more accurate and reliable than the carbon-hydrogen method. This method produces a determination of the ratio of methyl to phenyl
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Figure 2.
Calibration curve
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VOL. 31, NO. 6, JUNE 1959
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1101
Table I.
Comparison of Ratios Obtained by the Two Methods
Polymer
Methyl/Phenyl A 2.54 2 70 2 50 B 3.30 3 06 3 25 C 0.99 0 94 0 99 D i.is 1 25 1 20 E 2 78 2 55 2 36 F 1.96 1 87 1 95 G 0.90 0 99 0 89 IT 0 45 0 55 0 53 I 0.74 0 74 0 78 J 1.17 1 22 1 22 K 1 79 1 79 1 71 L 2 09 1 91 1 91 Standard deviat ion = 0.091.
Ratio 2.31
2.33
3.03
0 89 0:94 1.15 1.25 2.67 1 93 1:85 0.92 0.89
..
0.78 1.22 1.70
..
Value from C-H Analysis
Average 2.48 3.iS 0.95 1 20 2 59 1.91 0.90 0.51 0.76 1.21 1.75 1.97
2.5 3.47 0.68 1.50 2.09 0:82
..
..
smear of the polymer on a sodium chloride plate and allow about 5 minutes for the solvent to evaporate. The strong intensities of the absorption bands in silicone polymers require that the cast films be very thin; hence, solvent evaporation takes place very rapidly. The solvents generally used are toluene or xylene, which have no appreciable absorption at 8 microns and only slight absorption a t 7 microns due to a shoulder of a band near 6.8 microns; hence, traces of solvents do not interfere with the measurements. LITERATURE CITED
Table II.
Comparison of Experimental and Theoretical Values
Me/d Ratio by Infrared Method Individual Determination 0.99,0.91,0.93,0.93,0.94 Av. 0 94 1.43,1.43,1.40,1.33,1.36 Av. 1.39 2.69,2 80,3.04,2 95 Av. 2.87
Theoretical Me/$ Ratio
groups and not an absolute determination of the amount of either of these two groups. An absolute determination of each constituent individually n-odd in general be more dcsirable; however. practical limitations make such a determination difficult, if not impossible. The reason for this is as follows: Commercial silicone polymers are usually supplied in aromatic solvents such as xylenes, and an absolute de-
0 86 1.33 3.00 1.91
>le/+ from C-H Analysis Individual Determination 1 30,l 34 1 32 2 02;l 88 1.95 1.70,2.12
termination would necessitate the separation of the solvent from the solids. For many polymers, the solids become set up and insoluble when the solvent is removed; hence, redissolrinp the material in a solvent for infrared analysis becomes a problem. The method used herein does not require that the sample be weighed or the sample thickness be known. The technique used is simply to make a thin
(1) Fishl, Walter, Young, I. G., Appl. Spectroscopy 10, 213 (1956).
(2) h!urphy, C. M., Saunders, C. E., Smith. D. C.. Ind. Ena. Chem. 42, 2462 (1950j. (3)Rank, D. H.,Saksena, B. D., Shull, E. R., Discussions Faraday SOC.9, 187 (1950). (4) Richards, R. E., Thompson, H. W., J . Chem. Sac. 1949, 124. (5) Smith, D. C., French, J. >I., O’Neill, J. J., Naval Research Lab. Publ., 2746 (January 1946). (6) Wright, Norman, Hunter, M. J., J . Am. Chem. SOC.69, 803 (1947). (7) Toung, C. )I7.,Xoehler, J. S., McKinney, D. S., Ibid.. 69, 1410 (1947). (8) Young, C. W., Servals, P. C., Currie, C. C.. Hunter. RI. J.. Ibid.. 70, 3758 (1948j.
RECEIVEDfor review June 24, 1958. Accepted December 10, 1958. Division of Analytical Chemistry, 133rd Meeting, ACS, San Francisco, Calif., April 1958.
Colorimetric Determination of Boron with Victoria Violet CHARLES A. REYNOLDS Deparfmenf of Chemistry, Universify of Kansas, Lawrence, Kan.
b In the p H range of 7.7 to 10.0, the absorbance of the dye, Victoria Violet, is markedly lowered b y the presence of boric acid. A simple colorimetric method for boron in the range of 0.02 to 0.60 mg. of boron has been developed based on this decrease in absorbance. Measurements were made at 540 mp on solutions adjusted to pH 8.75.
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sulfuric acid is required as the solvent for the majority of the colorimetric methods available for boron (1, 5 ) . I n this solvent a number of polyhydroxy compounds, including quinalizarin, carminic acid, and two new reagents introduced by Grob and Yoe ($), 5-benzamido-6-chloro-l,l’-bis (anthraquinONCENTRATED
1 102
ANALYTICAL CHEMISTRY
ony1)amine and 5-p-toluidine-1, 1’(anthraquinonyl)amine, give color changes in the presence of boric acid. Another commonly used reagent is curcumin, which forms a colored product with boric acid when a solution containing curcumin and boric and oxalic acids is evaporated to dryness. This colored product is subsequently taken up with 95% ethyl alcohol for absorbance measurements. Only one method for boron is available in which the absorbance of a n aqueous solution is a function of the boron content of that solution. This method, developed by Kuemmel and Mellon (4, involves the use of chromotropic acid, which forms a complex with boric acid in aqueous solution. However, neither chromatographic acid nor the complex formed between this
reagent and boric acid absorbs in the visible region of the spectrum; hence, an ultraviolet spectrophotometer is needed to utilize this method. Victoria Violet ((3.1. 53) is made by diazotizing p-nitroaniline, coupling it with chromotropic acid, and reducing the nitro group. It still has the adjacent hydroxy groups necessary for complexing with boric acid. I n addition i t absorbs in the visible region of the spectrum. This investigation was concerned with the development of a convenient and rapid procedure for the colorimetric determination of boron in aqueous solutions utilizing the colored complex formed between boric acid and Victoria Violet. REAGENTS AND APPARATUS
STANDARD BORICACID SOLUTIONS.
