May, 1956
RADIATION CHEMISTRY OF POLYETHYLENE, POLYMETHYLENE AND OCTACOSANE 599
THE RADIATION CHEMISTRY OF HYDROCARBON POLYMERS : POLYETHYLENE, POLYMETHYLENE AND OCTACOSANE’ BY A. A. MILLER,E. J. LAWTON AND J. S. BALWIT Contribution from the General Electric Research Laboratory, Schenectady, N . Y . Received October $1, 1066
Polymethylene, polyethylene and octacosane were irradiated with high-energy (800 kv.) electrons and crosslinking, changes in unsaturation, and gas evolution were measured. The only t e of unsaturation formed 1s trans-vinylene and thls is produced in approximately equal yields in all three hydrocarbons. g o u t 40% of the hydrogen evolved comes from this reaction, the remainder arising from crosslinking. Volatile hydrocarbons result from C-C scissions near the chain-ends and also at the short branches in polyethylene. The results indicate that in the unbranched hydrocarbons, yethylene the and octacosane, permanent C-C scissions do not occur at random throughout the hydrocarbon chain. In POpolymethylene evidence is not as clear but the tentative conclusion is that some main-chain cleavage leading t o methyl end-groups may occur. Radiation yields (Gvalues) for crosslinking, vinylene unsaturation, and gas evolution, and a general mechanism for crosslinking of these hydrocarbon structures are presented.
Introduction In the past few years there has been an increasing amount of research on the effects of high-energy, ionizing radiation in organic polymers. Most of the reported work, however, has been concerned with changes in physical properties due to crosslinking or d e g r a d a t i ~ n ~and - ~ detailed studies of the chemical changes have been limited. Dole, Keeling and Rose have measured gas evolution and changes in unsaturation in pile-irradiated polyethylene6 and Lawton, Zemany and Balwit have reported similar measurements for electron-irradiated polyethylene.6 Earlier studies of the radiation chemistry of pure, low molecular weight organic compounds, particularly hydrocarbons in the gas phase7J’ and in the liquid p h a ~ e , ~ have - l ~ provided the basis for a general interpretation of radiation effects in gaseous and liquid hydrocarbon^.^*^^^ However, although the primary processes may be similar, it is expected that the over-all chemical changes which are observed in solid, high molecuIar weight hydrocarbons should be greatly modified by such factors as the “cage-effect”la and molecular chain length. This paper discusses the radiation chemistry of polyethylene, polymethylene and octacosane with respect to changes in molecular weight, changes in unsaturation and gas evolution. These hydrocarbons all have a basic methylene structure, the chain length of which differs widely for the three materials. Also, the polyethylene molecule con(1) Presented before the Polymer Division (Symposium on Polymer Irradiation) at the Cincinnati Meeting of the American Chemical Society, April 5, 1955. (2) 0. Sisman and C. D. Bopp, ORNL-928 (1961); C. D. Bopp and 0. Sisman, ORNL-1373 (1953). (3) A. Charlesby, Nucleonica, la, 18 (1954). (4) E. J. Lawton, J. 8. Balwit and A. M. Bueche, Ind. Eng. Chem., 46, 1703 (1954). (5) M. Dole, C . D. Keeling and D. G. Rose, J . A m . Chem. SOC.,16, 4304 (1954). (6) E. J. Lawton, P. D. Zemany and J. 6. Balwit, ibid.. 1 6 , 3437 (1954). (7) S. C. Lind and D. C. Bardwell, ibid., 48, 1575, 2335 (1926). (8) R. E. Honig and C. W. Shepard, THISJOURNAL, BO, 119 (1946). (9) C. S. Schoepfle and C. H. Fellows, Ind. Eng. Chem., 23, 1396 (1931). (10) J. P. Manion and M. Burton, T H I NJOURNAL, 56, 600 (1962). (11) L. H. Gevantman and R. R. Williams, Jr., ibid., 56, 508 (1952). (12) P. F. Forsyth, E. N. Weber and R. H. Schuler. J. Chem. Phys., a i , 60 (1954). (13) See M. Burton, THISJOURNAL, 51, 611 (1947). (14) See J. L. Magee, Ann. Rev. Nue. Sci., 3, 171 (1953).
tains some unsaturation and occasional short branches averaging four carbon atoms. l5 Experimental The octacosane (Eastman Kodak Co.) was treated with fuming sulfuric acid to remove traces of unsaturated and branched hydrocarbons. Following dilution the molten hydrocarbon was shaken with ten separate portions of hot, distilled water. The oil was vacuum-dried a t 80’ and recrystallized from petroleum ether using methanol as the precipitant. The dried, crystalline product showed no discoloration when tested with fuming sulfuric acid. Its melting point was 60’ (lit. value 60-62’16). A one-gram sample of polymethylene was obtained from L. A. Wal! of the National Bureau of Standards, who had used simllar material in a study of thermal degradation.‘? The polymer had an intrinsic viscosity of 20 deciliters g.-l when measured in xylene at 120’ corresponding to a molecular weight of over 108. Bakelite DYNH polyethylene, of viscosity average molecular weight 20,000, was used in this work. All irradiations were done with 800 kv. (peak) electrons from a G.E. resonant-transformer cathode ray unit.’* The thickness of the irradiated samples was never more than 40 mils (0.1 g. cm.-2), well below the maximum penetration of the electron beam, which is about 125 mils (0.32 g. cm.-2), so that the dose was constant throughout the thickness. The irradiations were always done in a nitrogen atmosphere. In cases where very high doses were given, the samples were irradiated on a water-cooled aluminum block or the dose given in small increments to prevent excessive temperature rise. The sample temperature during irradiation was usually in the range 25-50’. The dosimetry is based on a calibration of the beam current of the cathode ray unit against the ionization current in a specially constructed air ionization chamber a t a fixed distance from the window of the cathode ray tube. All samples were irradiated in this same position. The total dose was determined by the exposure time a t a constant beam current, The irradiation dose is expressed in terms of roentgens from the ionization chamber measurements according to the definition 1 R. = 84 ergs g.-l of air. For convenience, the unit 1 MR.(Mega-roentgen) = lo6 R. is used. All radiation yields (G values) that are derived in this paper are based on the assumption that the roentgen equivalent in these solid hydrocarbons is also 84 ergs g.-l. In the measurements of gas evolution the procedure described in a previous papere waa followed except that a special irradiation chamber with a thin stainless steel window was used. In all cases the samples were thoroughly outgassed a t 50-80’ before and after irradiation and before pressure measurements were made. The total gas evolved was analyzed with a mass spectrometer. (15) F. M. Rugg, J. J. Smith and L. H. Wartman, J. Poly. Sei., 11, l(1953). (16) G. Egloff, “Physical Constants of Hydrocarbons,” Vol. 5, Reinhold Publ. Corp., New York, N. Y., 1953, p. 258. (17) L. A. Wall, 5. L. Madorsky, D. W. Brown, 5. Straus and 13. Simha, J. Am. Chem. Soc., ‘76, 3430 (1954). (18) E. J. Lawton, W. D. ,Bellamy, R. E. Hungate, et al., T a p p i , 34, 113A (1951); and .J, A. Knowlton, G. R. Mahn and J. W. Ranftl, Nucleonics, 11, 04 (1953).
600
A. A. MILLER,E. J. LAWTON AND J. S. BALWIT
Unsaturation was measured by infrared absorption on a Model 21 Perkin-Elmer instrument (NaCl prism) on 3 mil films of polyethylene and polymet(hy1ene and as 16% solutions in CCla for octacosane. The following absorption bands were used: trans-vinylene (-CH=CH-) a t 10.35 p , vinyl (-CH=CH2) a t 11.0 p , and vinylidene (>C=CHz) a t 11-25 p . From the absorbance the concentration of unsaturated grou s was calculated using the relation: weight fraction = A/PKo.ldl) where A is the absorbance, d is the density of the sample, and I is the thickness in units of 0.1 mm. For trans-vinylenea t 10.35 p, Andersen and Seyfried’s specific absorption coefficient was used, K0.l = 35.6.lB In some cases the total unsaturation also was measured by bromine addition with a correction for substitution.* In this method 1-gram samples, made up of 10 mil sheets in the case of polyethylene, were dissolved or swollen in hot CClr under nitrogen. After cooling to room temperature, 10 ml. of 0.1 N BrrCClr reagent was added and the solutions stored in the dark for 0.5 to 4 hours. One ml. of saturated, aqueous K I solution and 25 ml. of water were added with shaking to absorb the HBr from the gas phase. The liberated iodine was titrated with standard 0.1 N sodium thiosulfate ( VA). Excess solid KBrOa was added and the iodine titrated with addition1 thiosulfate (VS). A blank determination on 10 ml. of the BrrCC14 reagent was made ( VB). The number of moles of double bonds in the sample is given by ( VB - VA - 2Vs)N/2000 where N is the normality of the thiosulfate.
Results Polyethylene Crosslinking Efficiency.-Estimates of crosslinking in polyethylene by swelling and tensile measurements already have been reported from this L a b ~ r a t o r y . ~For DYNH polyethylene the crosslinking efficiency was found to be 1.1 to 1.5 crosslinks per “ion-pair” (32.5 e.v.), giving G (c.1.) = 3.44.6. Gas Evolution.-Earlier worka on polyethylene of 19,000 molecular weight showed that a t a dose of 16 MR. about 85% of the gas evolved is hydrogen, the remainder being condensable (in liquid nitrogen) hydrocarbons, predominantly Cz-Ca. Also, the ratio of saturates to unsaturates in the condensable fraction was found to be about 0.4. I n the present work we confirmed the value for the hydrogen/condensable ratio, obtaining about 85% hydrogen from D Y ” polyethylene. Both Charlesby’s and Dole’s groups using pile irradiation found 96-98% Hz and only 2-470 hydrocarbons. This difference may be due to the nature of the ionizing radiation or, possibly, to incomplete outgassing of the irradiated polymer in the Charlesby and Dole measurements. The radiation yields, based on the results in the previous paper,C are G(H2) = 5.7 and G(condensables) = 1.0. Unsaturation.-Rugg and co-workers showed from infrared studies that the total unsaturation of DYNH polyethylene is comprised of 60% vinylidene, and about 20% each of vinyl and transvinylene.Ib We have found that upon irradiation the initial vinylidene and vinyl decrease and disappear a t about 15 and 50 MR., respectively, and only trans-vinylene unsaturation is produced by irradiation. This is in qualitative agreement with the results of Dole’s group. The change in total unsaturation in D Y ” polyethylene a t 50 MR. was measured by bromine addition with the results shown in Table I. (19) J. A. Andersen and W. D. Seyfried, AnoE. Chem., SO, 998 (1948). (20) See F. R. Mayo, J . Am. Chem. Soc., 7 6 , 6136 (1953).
Vol. 60
TABLE I BROMINATION OF DYNH POLYETHYLENE Bromination time, hr.
Bromine added, (moles 9.) X 104 irradiated (50 MR.)
Unirradiated
1 0.54 (0)” 1.06(1.35) 2 .51 (0) l.OS(1.52) 4 .54 (0) l.OS(1.83) Values in parentheses are for bromine substituted in (moles/g.) X 104.
Since this method measures total unsaturation, the formation of trans-vinylene must be estimated indirectly. The total initial unsaturation is 0.54 X mole/g. agreeing satisfactorily with Rugg’s mole/g. Since vinyl and value of 0.38 X vinylidene constituting SOY0 of the initial unsaturation disappear, 0.54 X (0.20) = 0.11 X mole/g., which is the initial trans-vinylene, remains. The total unsaturation a t 50 MR. is 1.08 X mole/g., all trans-vinylene. Therefore, the trans-vinylene produced by a 50 MR. dose is = 0.97 X mole/g. (1.08 - 0.11) X = 2.2 In This corresponds to G(-CH=CH-) relation to the results of Dole’s group the bromination measurement described above indicates that considerable substitution may occur in irradiated polyethylene so that the total bromine absorption without a correction for substitution may lead to erroneously high values for unsaturation. This may explain Dole’s high value for the fraction of hydrogen arising from unsaturation (70-8001,). We obtain only 40% since G(-CH=CH-)/ G(H2) 0.4. The quantitative measurement of trans-vinylene unsaturation by infrared absorption (10.35 p ) is complicated by the uncertainty in the state (crystallinity) factor. It was reported earliera that the increase in 10.35 p absorbance for a 3 mil film irradiated at 50 MR. is 0.059. Using Andersen and Seyfried’s specific absorption coefficient of &.I = 35.6 we obtain 1.0 X mole/g. (G = 2.4) for a state factor of unity. However, if a state factor of 0.5 is used (see Fig. 1) a value of only 0.5 X loL4mole/g. (G = 1.2) results. Because of the uncertainty in the state factor we believe that the value derived from bromination (G = 2.2) is the more reliable one. Polymethy1ene.-A sufficient quantity of polymethylene was not available for an extensive investigation of this polymer. However, a detailed study of the irradiation of a similar structure (an unbranched, highly crystalline ethylene polymer) will be reported in a separate paper by Lawton and co-workers. The irradiation of polymethylene results in crosslinking but, on the basis of minimum dose for insolubility in hot toluene and the estimated initial molecular weight, a t a lower efficiency than for polyethylene. The mass spectrometric analysis of the gas evolved after an irradiation dose of about 100 MR. gave 99.9% hydrogen and only about 0.1% ethane and butane. The quantity of hydrogen evolved corresponds to a yield of G(H2) = 5.4. Infrared analyses of 3 mil films of polymethylene showed no unsaturation (10.35, 11.0 and 11.25 p ) in the unirradiated polymer and the formation
May, 1956
RADIATION CHEMISTRY OF POLYETHYLENE,
POLYMETHYLENE AND
OCTACOSANE
601
2.6
;1
2.4 \
0 0
0 Fig. 1.-trans-Vinylene absorbance (10.35 p ) in irradiated polyethylene ( Q O ) and polymethylene (a,.).
only of trans-vinylene (10.35 p ) with irradiation up to a dose of 200 MR. A comparison of the absorbance a t 10.35 p for 3 mil films of polyethylene and polymethylene measured a t room temperature and at 140°, above the crystal melting temperature, is shown in Fig. 1. The absorbances a t each dose are almost identical for the two polymers indicating that the formation of trans-vinylene is independent of the branching in polyethylene. The 50% decrease in absorbance measured at 140" suggests that the solid -+ melt state factor is about 0.5. However, the effect of temperature alone on the 10.35 p absorbance is not known. Octacosane : Molecular Weight Change.-The results of duplicate cryoscopic molecular weight measurements (cyclohexane solvent) for octacosme are shown in Fig. 2, in which the reciprocal number average molecular weight is plotted as a function of irradiation dose: The theoretical value for pure octacosane (M.W. = 394) is 1000/M, = 2.54, somewhat lower than the measured values. However, to be consistent, the experimental value was used for the unirradiated material. The decrease in number of molecules/g./R. is given by the product of the slope in Fig. 2 (2.7 X 1O-l2/R.) and 6 X lo2* or 1.6 X 10l2 molecules/g./R. and this must equal the number of crosslinks since each crosslink decreases by one the number of molecules. (It should be noted that the samples on which the cryoscopic measurements were made were not outgassed to remove the low molecular weight hydrocarbons formed in the irradiation; see next section. It was estimated, however, that the M , values of these samples would be changed by only 2y0 depending on whether the volatile products remain in the samples or are completely removed by outgassing prior to the cryoscopic measurement.) The value for crosslinks obtained above leads to 33 e.v. per crosslink or G(c.1.) = 3.1. Charlesby has reported a value of 32 e.v. per crosslink for saturated, unbranched hydrocarbons, including octacosane, based on infusibility measurements and with pile radiations.21.22 Gas Evolution.-A 0.112-g. sample of octacosane irradiated to a dose of 64.6 MR. gave an average rate of hydrogen evolution of 45.3 p/MR. and a rate of 41.2 p/MR. a t the end of the irradiation. Of (21) A. Charlesby, Proc. Roy. 8 0 0 . (London), AMs, 60 (1964). (22) A. Charleaby, Radiation Research, 4, 97 (1956).
2.2
2.0 DOSE, MR. Fig. 2.-Change in reciprocal molecular weight of octacosane by irradiation (0= calculated value for pure octacosane).
the total gas evolved 2938 p, 2660 p (91%) was hydrogen (non-condensable in liquid nitrogen) and 278 p (9%) was condensable. The amount of evolved gas, calculated from the calibrated volume of the gas-measuring system and averaged up to a dose of 64.6 MR. was 2.52 X 10l2molecules/g./R. These data lead to G(Ha) = 4.3 and G(condensab1es) = 0.5. A mass spectrometric analysis of the evolved gas gave 91.0 mole % H2 and 7.2 mole % CnHan + 2 hydrocarbons distributed as follows: 0.5% CH4, 2.1% CaHa, 1.3% C4Hio, 0.9% CsHiz, 1.1% C6H14, 0.7%C ~ H and M 0.6T0 C8Hl8. The number average molecular weight (ATn = ZnLMi/Zni) of the volatile hydrocarbon fraction was Mn = 62 which is close to the value for butane (58). Unsaturation.-Samples of octacosane irradiated up to a dose of 200 MR. were examined as 16% solutions in carbon tetrachloride by infrared absorption. The only change that could be detected was the appearance of absorption a t 10.35 p (transvinylene). For a sample irradiated a t 50 MR. the 10.35 p absorbance was 0.058 for a 0.358 mm. thickness of a 16% solution in CCL, which with Andersen and Seyfried's value of K0.l = 35.6,20 gives a concentration of 0.81 X mole/g. trans-vinylene formed. The bromination method on two 2-g. samples, also irradiated a t 50 MR., gave 0.89 X and 0.80 X mole/g. for bromination times of 30 and 60 minutes, respectively. The average value, 0.85 X mole/g., is in satisfactory agreement with the infrared estimate and gives G(-CH=CH-) = 1.9. It is significant that the infrared measurement, made in CC14 solution where the uncertainty of the state factor is absent, is not appreciably lower than the total unsaturation measured by bromination. This must mean that cis-vinylene, which is not
A. A. MILLER,E. J. LAWTON AND J. S. BALWIT
602
readily observed by infrared absorption, is not formed in the irradiation of these hydrocarbon structures. Material Balance.-In the irradiation of octacosane, unsaturation (vinylene) is produced only in the solid product and butane was shown to be the average volatile hydrocarbon fragment. The results on octacosane may be associated with the following over-all changes
+ + +
C28H68 .-+ C ~ B H I I Hz ~ (crosslinking) w+ C52Hl06 “C4H10”(cleavage W+C2&6 Hz (vinylene)
+ crosslinking)
Since each crosslink and each vinylene group which is formed gives a molecule of H2 and each molecule of “butane” must result in the crosslinking of the residues, the sum of the crosslinks and vinylene double bonds formed should equal the number of molecules of H2 and “C4H10.” The material balance actually obtained, based on the individually measured G values at 50 MR. listed C=C) = 5.0 against ZG(H2 earlier is ZG(c.1. “C4H10”) = 4.8. This satisfactory material balance serves to verify the quantitative estimates of the individual G values in the irradiation of octacosane. Discussion Scission of C-C Bonds.-One of the basic questions in the crosslinking of polymers by ionizing radiation is the degree of scission occurring simultaneously in the main-chain. By comparing theoretical and experimental sol-dose relationships Charlesby concluded that in long chain paraffis, including polyethylene, the ratio of main-chain scissions to crosslinks is about 0.3 and he suggested, further, that this ratio is relatively independent of physical state and of molecular chain length over the extreme range from gaseous methane to solid polyethylene.21 Baskett and Miller2* by further irradiation of the extracted gel fraction of lightly crosslinked polyethylene derived a scission/crosslink ratio of about 0.2. Dole, Keeling and Rose6 concluded that in polyethylene C-C scissions at the branches occurred much more frequently than in the main chain but they did not make a quantitative estimate of the amount of main-chain C-C scission. On the basis of the results in the present paper we suggest that in the irradiation of long-chain paraffins of the unbranched, polymethylene-type, permanent random scission of the main chain does not occur to the extent indicated by Charlesby. We base our conclusion on (1) the results of gas evolution and (2) the absence of end-groups which might be expected by main-chain C-C scission. A comparison of gas evolution in the irradiation of the three hydrocarbon structures studied in the present work is shown in Table 11. Polymethylene with an extremely low chain-end/ CH2 ratio gives almost pure H2 in the evolved gas. Octacosane, which is also of unbranched structure but with many more chain-ends (ie., lower molecular weight), gives a much higher hydrocarbon/Hz ratio in the evolved gas. This must mean that permanent C-C scissions giving volatile hydrocarbons occurs only at the chain-ends. This
+
(23) A. C. Baskett and C. W. Miller, Nalure, 174, 364 (1954).
+
Vol. 60
TABLEI1 GASEVOLUTION IN CROSSLINKING HYDROCARBON POLYMERS Polymethylene
Oota-
aosane
Polyethylene
394 >loa 9,1005 Mol. wt. Chain-ends/-CHz0.07 99 87 H2 evolved, mole yo Hydrocarbons, mole yo 9
