IDENTIFYING CROSS-LINKS IN POLYETHYLENE FOLLOWING GAMMA-IRRADIATION IN ACETYLENE - A MODEL-COMPOUND STUDY

Long-chain linear alkanes have been used as model compounds for polyet hylene in an attempt to identify the chemical nature of crosslinks for med in polyethylene when it undergoes gamma-irradiation in the presenc e of acetylene. IR and UV spectral analysis of alkanes and polyethylen e following acetylene-sensitized irradiation shows the formation of vi nyl, trans-vinylene, and diene groups. A correlation of the conditions of formation suggests that in polyethylene the vinyl groups are restr icted to amorphous regions, diene groups are restricted to the crystal line regions, and trans-vinylene groups are formed in both regions. Th ere is no information on the nature of crosslinks. C-13-NMR analysis o f alkanes following irradiation of molten alkanes in the presence of C -13-enriched acetylene has shown that a range of saturated alphatic st ructures are formed by inclusion of acetylene molecules in the alkane structure. They include ethyl branches, gamma-branches, -CH(CH3)-, and -CH2-CH2- branches as the major species; the latter two are potential crosslink sites in the irradiation of polyethylene. In addition, the NMR analysis confirmed that the C atoms of the vinyl groups come from acetylene molecules and those of the trans-vinylene groups come from a lkane molecules. Data on irradiation of the alkanes in the crystalline state showed that acetylene inclusion in the alkane structure is mini mal under these conditions. The principal finding of this work is that acetylene can be incorporated as saturated aliphatic crosslinks in th e amorphous regions of polyethylene during high-energy irradiation. (C ) 1997 John Wiley & Sons, Inc.