RADIATION-INDUCED REACTIONS IN n-HEXADECANE
2345
Radiation-Induced Reactions in n-Hexadecane'
by R. Salovey and W. E. Falconer Bell Telephone Laboratories, Inc., Murray Hill, New Jersey
(Received January 22, 1966)
The effects of irradiation of n-hexadecane are apparently independent of source and dose rate, the major effect being C-H rupture resulting in dimerization by cross linking to form isomers of dotriacontane and hydrogen. Chromatographic analyses of products indicate that scission of the paraffin chain is suppressed in the irradiation of solid n-hexadecane relative to irradiation in the liquid state. Mass spectrometric studies of gases evolved following irradiation and molecular weight measurements by vapor pressure depression are consistent with this observation. Increased chain scission in irradiated polyethylene melts may explain the reduction in gel measured under such conditions. From the distribution of isomeric dimers on irradiation of pure n-hexadecane we conclude that the main sites of cross linking are nonterminal. That is, cross linking occurs at random and there is no need to postulate radical-center mobility. However, irradiated solid n-hexadecane containing 5% n-octadecane end links to form linear dotriacontane to the extent of about 2% of total dimer.
Recent studies on the irradiation of hydrocarbon polymers have led to diverse views concerning the effect of state on the relative importance of the cross-linking reaction. Less crystalline polyethylenes cross link more readily than more ordered materials.2 On the other hand, gel formation is greater for highly crystalline polyethylene as compared to a supercooled melt of the same material several degrees below the melting point.3 The authors suggest4 a marked increase of chain mobility and cross-linking efficiency in the crystalline state a t temperatures exceeding 100". Further, preferential cross linking a t folds and radical center mobility were inferred from studies on irradiated polyethylene single crystal aggregates5 It was suggested that cross linking between chains in the crystal lattice was improbable. An extrapolation from n-hexane radiolysis at 77°K. to polyethylene at room temperature6 does not distinguish between state and temperature effect^.^ Moreover, a more justifiable comparison with polyethylene should involve n-paraffins having a lower endgroup concentration. The present study of the product distribution in irradiated n-hexadecane, both in the solid and liquid states over a narrow temperature interval near lo", concerns experiments which bear on the effect of physical state on radiation-induced react ions.
Experimental n-Hexadecane (0.5-ml. samples, Humphrey Chemical Co. petroleum-derived product, olefin-free grade, minimum purity 99%) was placed in thin-walled (0.5 mm.) Pyrex glass tubes, outgassed a t torr for several hours, and then sealed. Solutions of n-hexadecane containing about 5% n-octadecane (Humphrey Chemical Co., petroleum derived, 99% purity) were similarly prepared. Materials were irradiated with 1MeV. electrons a t 2.5 Mrads min.-' to total doses between 1.1and 20 Mrads,' or with Co60y-rays at 0.5 Alrad hr.-I to a total dose of 2.5 RIrads. For a 2.5-Mrad dose, conversion was
