NMR spin–spin relaxation in solid and molten polyethylene structures

The NMR spin-spin relaxation (T₂) spectra of high-density polyethylene (PE) has been investigated over a wide range of temperatures, both in the solid and molten states. Previous work in these laboratories has shown that the T₂ relaxation spectrum of molten polyethylene differs from that of other polymers studied in that (a) it cannot be decomposed into two relaxation spectra (T_2S and T_2L) and (b) there is some evidence of a memory effect. This paper attempts to elucidate these observations, and compare them with the spin-spin relaxation of polyethylene at lower temperatures. In the solid state, the T₂ decay comprises both a Gaussian distribution for the crystalline region, and an exponential decay for the amorphous component. The effects of crystallization conditions and of temperature were determined. In the molten state the T₂ decay is more complex, but can be resolved into three exponentials. The longest (T_2L) component arises as expected from the most mobile, low molecular weight fraction. The T_2S component is due to an entangled but mobile network, as in other polymers. In addition, a short relaxation component T_2X is observed, which is influenced by previous crystallinity and the processing history of the material, and is ascribed to some vestigial degree of structure in the molten phase.