Solid-State 1H NMR Relaxation Analysis of Ultrahigh Molecular Weight Polyethylene Reactor Powder

Abstract

Solid-echo 1H NMR relaxation was analyzed for ultrahigh molecular weight polyethylene reactor powders. Their high crystallinities reveal the characteristics of crystalline free induction decay (FID). We propose here a very simple but appropriate fitting function for the crystalline component: combination of Weibullian and sine functions. The applicability of this function was tested for the series of samples having much higher crystallinity of over 90%, which was achieved by fuming nitric acid etching at room temperature over a period of 1 year. The typical beat at 20−30 μs on the observed FID was further emphasized by prolongation of etching time, which corresponds to “crystalline” relaxation. An introduction of the simple Weibullian/sine function for FID fitting reproduces well this beat profile. On the other hand, the FID relaxation of the amorphous chains could be represented by the single Weibullian function. However, they were classified into two categories: intermediate and mobile, depending on their relaxation characteristics. Among these three components (crystalline, intermediate, and mobile ones) exhibiting different chain motions, the intermediate amorphous relaxation was an effective guide for characterization of phase arrangements in our powder samples. The component percentage of this phase decreased with prolongation of the etching time, which was coincident with the crystallinity development detected by thermal analysis. Also, the spin−spin relaxation time of intermediate amorphous chains gradually decreased with the progress of the etching treatment. These results indicate that the molecular motion of the surviving intermediate amorphous phase is restricted. A comparison of morphological changes and these FID characteristic transformations obtained during etching suggested that such an intermediate amorphous phase is located in the region between crystalline domains within the powder globules.