Dipolar-decoupled carbon-13 NMR study of highly oriented polyethylene films

Abstract

High-resolution solid-state 13C NMR spectra and 13C relaxation times T1 and T1ρ have been measured at 40–100° C for uniaxially oriented polyethylene films with the drawing direction parallel to the magnetic field; this sample has a unique morphological structure that the noncrystalline chains are nearly disordered irrespective of the high degree of drawing. A sharp resonance line(line A) appears at the position corresponding to the principal value σ33 of the chemical shift tensor for the CH2 carbons with the trans-trans conformation. Another sharp linedine B) is observed at almost the same chemical shift as for the CH2 carbons of polyethylene in solution. Although these observations are similar to those for a cold-drawn polyethylene sample reported previously, line B is much enhanced in intensity and the linewidth is narrower in the present sample, reflecting the disorientation of the noncrystalline component. It is found from T1 measurements that line A contains two components with different molecular mobility, both being assigned to the crystalline components. On the other hand, the line B is composed of a single component assignable to the noncrystalline component with liquid-like molecular mobility.