Chain flips in polyethylene crystallites and fibers characterized by dipolar C-13 NMR

The occurrence and rate of 180° chain-flip motions in the crystalline regions of two polyethylenes were studied by 13C NMR. In high-density polyethylene (HDPE) and in ultradrawn ultrahigh molecular weight polyethylene (UHMWPE) fibers, the changes in the 13C−13C dipolar couplings brought about by the reorientations of the 13C−13C internuclear vectors in the crystallites were observed. In the HDPE sample, which was labeled with 4% of 13C−13C pairs, the rotational motion was observed directly via two-dimensional exchange spectroscopy, stimulated-echo decays, and 1D line shape changes monitoring the 13C−13C dipolar coupling. The data show that the jumps occur between two sites, with a rotation angle of 180° and with a jump rate of ∼10/s at ambient temperature. The correlation function of the motion was found to be slightly nonexponential, with a stretched-exponential β parameter of 0.8 ± 0.1. The data yield an activation energy of 93 ± 10 kJ/mol for the 180° chain flips. In the fibers, the narrowing of natura...