C-13 NMR INVESTIGATION OF MOLECULAR CHAIN DIFFUSION IN SEMICRYSTALLINE POLYETHYLENE .1. EFFECT OF CROSS-LINKING

Solid-state molecular chain diffusion in linear high-density polyethyl ene (HDPE) is established as the dominant mechanism for the crystallin e C-13 longitudinal relaxation at 60 degrees C, confirming previous wo rk of Schmidt-Rohr and Spiess. A progressive saturation NMR experiment was undertaken on several samples of HDPE, where different degrees of cross-linking were introduced by electron irradiation. A decrease in the rate of recovery of the,crystalline signal was observed with an in crease in cross-link density, as measured by gel fraction. As irradiat ion forms cross-links on the fold surfaces of the lamellae, this behav ior cannot be explained through a conventional dipolar spin-lattice me chanism or C-13 spin diffusion. Transport of magnetization via chain d iffusion to the amorphous region, where it experiences an efficient re laxation process, is consistent with the relaxation data and the Overh auser enhancement. As the gel fraction increased from zero to 87%, the effective diffusion coefficients almost halved from the nonirradiated sample (0.033 nm(2).s(-1)) to 0.018 nm(2).s-1.