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

Summary

High-resolution solid-state ¹³C NMR spectra and ¹³C relaxation times T₁ and T_1ρ 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.