IIADIATION CHEMISTRY OF HEXAMETHYLDISILOXASE, A POLYDIR4ETHYLSILOXASE MODEL' BY H. A. DEWHGRST A V D L. E. ST. PIERRE Gpneral Electric Research Laboratoi Rirpiipd
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Schpnerfarli~,A\
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has been made of the products from the 800 kvp. electron radiolysis of hesamethyldisilosane. Thp yields of volatile products were established as G(H2) = 0.7, G(CH,) = I .4 and G(CsH6) = 0.4. These yields were decreased by only 50', C in the presence of high iodine concentrations indicating a significant contribution hy molecular-like processes. The non-volatile liquid products were examined by gas chromatography, and classified into low molecular weight, intermediate molecular xeight and dimer wit,h the yields, 0.5, 1.8 and 1.8 molecules/lOO e.v., respectively. The results are disciissed in terms of free radical and molecular-like reactions and their pertinence to radiation efi'ects in polydimethy1Piloxanes indicated.
Introduction It has been shown by a number of workers t,hat, t'he organopolysiloxanes are crosslinked on exposure t'o high energy radiations. 2 - 4 Recent'ly a precise det'ermination of t,he crosslinking efficiency has been reported.5 The nature of trhe crossliiiks have been only partjially elucidated by studying the radiolysis of hexamethyldisiloxane and oct"net,hylcyclotmetmsiloxane.6 To obtain a more complet,e understanding of the radiation chemical processes occurring in polydimet'hylsiloxanes, it was necessary to determine the t30tal chemistry which occurred in a model system. We chose hexamethyldisiloxane as a model compound for the polydimethyl system and were able t o study the nature of the products formed by gas chroma,tography and mass spectrometry. The pertinence of these results to em are discussed.
from vacuum irradiated samples were hydrogen, methane and ethane with the following yields, G(H2) = 0.7, G(CH4) = 1.4 and G(C2H6) = 0.4 molecules'100 e v. The relatively high yield of methane i.: of interest since the ma- spectral fragmentatioii pattern of hexamethyldisiloxane shows that the most abundant positive ion results from loss of a methyl group. The ratio of hydrogen to methane is in fair agreement with Warrick'* results with octamethylcyclotetrasiloxane; however the ratio of ethane to methane is about four times that reported by M7arrick.3 It is conceivable that the discrepancy iii the ethane-methane ratio could be due partly to the fact that different starting materials mere used and partly to a dose rate effect since the dose rate in the electron beam experiments was approximately 2000 times that in Warrick's y-ray experiments. In the presence of iodine (-lo-? X ) the total gas Experimental yield mas decreased by approximately one-half : Degassed or nitrogen purged samples were irradiated with the yield of hydrogen and methane wab 0.4 and 0.70 800 kvp. clevtroris from the General Electric Research Laboratory resonant transformer unit. A specially designed molecules,/ 100 c.v., respectively. These results are high pressure (*ell7was used which enabled irradiations t o be qimilar to those observed with liquid alkaliesh made up t o 10 atmospheres pressure of nitrogen. T o estab- where it was concluded that a significant fraction lish the effect of product gases, experimenh were carried (0.60) of the hydrogen yield is fornied by molecular out sepa.rately under 10 atmospheres pressure of hydrogen and of methane. Dosimetry was hmed on ionization cham- processes. Liquid Products.-A gas chromatogram, obtained ber measuremvnts ant1 t*hecked independently by chemical methods. a t 100" on a 2-meter didecyl phthalate column, of The hesamath?-ldisilosnne used had been purified by distillation ant1 s1~on.nt)y gas chromatography t o be of high the lion-volatile products is shown in Yig. 1. The product assignments shown in Fig. 1 were in general purity. Product identification and analyses were based on a coni- based on a comparisoii of the retention tinies of the bination of gas chrom:ttography, infrared and mass spectrom- ob,ierved peak5 TI ith the retentioii times of etry. Identifirntion of products by gas chromatography known compounds. The assignments were further was hased on retention time studies of known compounds on two differe!It coliimns, namely, didccyl phthnlate and a checked by determinations of the retention tinies a t silicone colnmn. For quantitative determination of prod- different temperatures. Since a standard sample ucts a ralibrai t d two meter dideq-1 phthalate column was for the 23.7 min. product (a-trimethpl~ilosyhexaused. Gas a d y e s were performed as previously described.7 The formation of Si-H bonds vias determined methyldisiloxane) , was not available, it. identity \I a* ehtablished indirectly. The pobition of thiy by infrared m+asurernents at it vr-;tve length of 2150 ern.-' as dcscrihed t ~ )€3iiec-he.4 product peak on the chromatogram established it a i a 3-silicon atom compound of which only four Results and Discussion possibilities can be derived froin hesamethyldiGas Products.--The volatile products (at - 120") doxaiie. T v o of thew posbihilitiei arc pi e w i t niid identifiable a* the 7.6 (octamethyltrisilosane) and (1) Presented a t t h e 136th meeting of the American Chemical Society, Atlantic City, September, 195R. 21.5 [ (trimethylsilylmethy1)-pentamethyldisiloxanej ( 2 ) (a) E. J. Lxu-ton, A . .\I. Bueche a n d ,J. S. Enlait. S n l u r e , 172,76 minute product\. h third possibility n-odd require (1953): ib) A , Cliarlrsby. I ' m , H o g . Sor.. (London). A230, 120 (1955). a product coritaiiiiiig aii Si-Si bond. Thi5 possi( 3 ) E. L. JVar-ick, I n d . Eng. Chem., 47, 2388 (1955). bility was ruled out since treal ment with bromine (4) A . .\I. Bueche. .I. P o l y m e r Sei., 19, 292 (1956). ( 5 ) W. Barnes, 13. .i. Deuhurst. R. \T. Kilb and L. 1:. St. Pierre, did not affect the 23.7 minute product TT hereas the ibid.. 36, 525 (1959). 31.6 minute, which had been eitablished as the Si(a) 9. W. Kaittor, .ibatract So. 55-0. Am. Chem. Sot-. AIeeting, Si dimer [l,2-bis-(trimet hylsiloxy) -tetramet hpldiSeptember, 1956. (7) H. .A. Deatiur-t, THISJOKTRXAL, 63, 813 (19.59).
( 8 ) H. A , Dewhurst, % b i d ,62, 15 (19%)
s
I
0 4-
3-
C H 3 C H 2 Si-0-Si
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shown in Fig. 1, gas chromatograms obt'ained a t 50" showed small amount8s of tet,ramethylsilane and pentamethyldisiloxane. The formation of the more abundant, products vvas studied as a function of t'he energy absorbed and the results are shown in Figs. 2 and 3. The
111111.
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1.0 2.0
Assignment
muleculee/lOO e . r .
Low molecular weight prodiirts Si-CH3 0' SiOSi-H 0 . :
alyield of low molecular weight product was unchanged whereas the total yield of intermediate and dimer product's was essentially decreased by one-half at, liquid nitrogen t,eniperaturc. These results are in st,riking cont.rast with r'he alkane system where it was found that the intermediate product was eliminated on irradiation at, liquid nitrogen temperature.* It is also of interest to note t,hat the two major intermediate products (4.9 and 7.6 minutes) were decreased tly only :3O7' even in the presence of ten atmosplieres of o ~ y g e i i . ~The 65 minute dimer product was suppressed by low temperature irra(9) I,. E. 3t. PiPrre and H. A . DPahurst. (1 9601.
Titic.
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64, 1068
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diation to about the same extent, observed i l l the presence of ~ x y g e n . The ~ 31.6 minut'e dimer product, is absent both at3low temperatures and in the presence of oxygen9 whereas t,he 79.5 niinutr dimer product is e1iminat)ed by oxygenYbut unaffected by low temperatures. Another important difference was the formation of a small yield of trimet(hylsilano1 by low temperature irradiat'ion, a product, which was not' observed a t room temperat,ure. These result,s suggest that both radical and molecular-like react,ioiis are important in t.he radiolysis of hexamethyldisiloxane. Conclusion The observed products ca,n be account'ed for on the basis of the following simplified reaction scheme. In this scheme it is assumed that' only one radiation event, can occur per molecule. Primary cleavage of the three different bonds in hexaniethvldisiloxarie lead t'o the processes (CH,),,SiOSi(CH.,)r
+ CH.,. (1) _ _ -+ (CH,)RSiOSi(CH,),.CH? + H. ( 2 ) (CH,),SiO. + .Si(CH,), (3) +---j
(CH,),,SiOSi(C X j ) , .
~
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hlthough subsequent recombination reactions of t'he various fragments can qualit'atively account' for the observed product,s, it' is evident from the result,s that' a consideration of only random recombination of radical fragments cannot, explain all the observations. The result>s indicat'e tshat the 4.9 and 7.6 minute products are to a large extent formed by molecular-like processes. These products cannot be formed by a simple bimolecular process involving excit,ed molecules because of the non-equivalence of the conjugate product, in each reaction: that is the 3.9 and 2.0 minute product's should be formed with the same yield, similarly the 7.6 and 1.0 minute product.s should be formed with the same yield. It is conceivable that ion-molecule reactions could explain some of these observat.ions. On the basis of the present results wit,h a model compound i t is concluded t'hat t.he cross-links formed in the radiolysis of polydimet,hylsiloxanes are of t,he type Si-CH,-Si, Si-Si and SiCH2CH2Si in the rat~ioof 2,'1!0.6. The ratio of SiCHSSi t,o Si-Si t,ype cross-links is in good agreement] x i t h the results of Kantor6 and of Bueche4; however, these authors did not observe t.he formalmionof SiCH2CHrSi type cross-links. The result's of t'he model compound study show that, silethylene cross-links make an important contribution t'o t8hetotal cross-linking efficiency. The results of the model study also indicat'e that an appreciable amount of main chain seissiori and branching can occur in siloxa,nes. The small amount of siloxane cleamge ohserved in the model study i u in accord with the receiit results of IGlb. Acknowledgment.-The authors are indebt,ed to Dr. S. W. Kantor for the purified hexamethyldisiloxane and the dimer calibration snmples, to ,J. S. Balwit for the irradiat,ions and to Drs. A. .\. Sliller and S. W. Karitor for valutihle discussionq. 110)
R. W. Rilb, ibid., 63, 1838
(1959)
