Chain Flips in Polyethylene Crystallites and Fibers Characterized by Dipolar¹³C NMR

The occurrence and rate of 180° chain-flip motions in the crystalline regions of two polyethylenes were studied by 13 C NMR. In high-density polyethylene (HDPE) and in ultradrawn ultrahigh molecular weight polyethylene (UHMWPE) fibers, the changes in the 13 C− 13 C dipolar couplings brought about by the reorientations of the 13 C− 13 C internuclear vectors in the crystallites were observed. In the HDPE sample, which was labeled with 4% of 13 C− 13 C pairs, the rotational motion was observed directly via two-dimensional exchange spectroscopy, stimulated-echo decays, and 1D line shape changes monitoring the 13 C− 13 C 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 natural-abundance 13 C− 13 C dipolar satellites is a clear NMR signature of the chain motion, indicating a jump rate of 150/s at 360 K, which is 20 times slower than in the unoriented HDPE. The correlation time dependence of the 1 H T 1 ρ relaxation time, which probes the modulation of H−H dipolar couplings in the crystallites, was determined directly. Relations between the chain flip motion, the dynamic-mechanical α-relaxation, creep, and drawability are discussed.