13C NMR Studies of Photoinitiated Cross-Linking of Low-Density Polyethylene

Structures of cross-links produced in benzophenone-photoinitiated cross-linking of low-density polyethylene (LDPE) and a model compound n -C 24 H 50 have been studied for the first time by solution-state 13 C NMR spectroscopy. Well-resolved resonance lines, assigned to CH H, α-CH 2, and β-CH 2 groups related to H-cross-links, have been observed for the photo-cross-linked samples below the gel dose. At the same time, the resonance lines associated with Y-type long branches are also found to be significantly formed during the photoinitiated cross-linking of LDPE. The concentrations of H-links and Y-branches are estimated to be 5.7−16.9 and 7.6−21.6 per 10 000 carbon atoms, respectively, for the samples UV-irradiated for 5−10 s in the melt, indicating that Y-links also are effectively produced in the photoinitiated LDPE samples. These results are slightly but significantly different from those reported for high-energy radiation-induced cross-linking of high-density polyethylene.