1H spin‐lattice relaxation and observation of interfacial material in solid polyethylenes

Abstract

AbstractThe 1H NMR laboratory‐ and rotating‐frame (LF,RF) spin‐lattice relaxation (SLR) behaviour in solid polyethylenes is reviewed in light of the two region spin diffusion coupled model. Highly crystalline high‐density materials show single T1 values indicative of the fast diffusion limit whilst RFSLR is close to the slow diffusion limit, allowing direct determination of the dimensions of the crystalline and disordered regions. The LFSLR and RFSLR of the HDPE's suggest the existence of an interfacial material which the SLR behaviour includes as part of the crystalline region in contrast to XRD and DSC. 13C CPMAS measurements of 1H RFSLR in a LLDPE provide an additional insight into this problem. The 1H RFSLR is best represented by a superposition of two exponentials, measured by either 1H or 13C NMR. The 1H RFSLR, measured at different chemical shifts in the 13C NMR spectrum, provides direct evidence of the interfacial material. The possible use of spin‐locking at angles other than 90° in the rotating frame to scale the spin diffusion rates is reviewed and its possible utility for LLDPE materials explored.