CHAIN DIFFUSION BETWEEN CRYSTALLINE AND AMORPHOUS REGIONS IN POLYETHYLENE DETECTED BY 2D EXCHANGE C-13 NMR

Solid-state chain diffusion between crystalline and amorphous regions in linear polyethylene (PE), at temperatures down to 100 K below T(m), is detected by means of two-dimensional (2D) and one-dimensional (1D) exchange C-13 NMR experiments under high-resolution magic-angle-spinning (MAS) conditions. The jump rates and the activation energy of 105 kJ/mol extracted from the data agree well with previously reported values for the underlying chain jump motion, i.e., the dynamic alpha-process. The root-mean-square displacement amounts to more than 10 nm within 100 s near 360 K and is affected by entanglements. The measured effective diffusivities range between 10(-17) and 10(-21) m2/s. Chain diffusion also explains the nuclear Overhauser enhancement factor and the nonexponential C-13 longitudinal (T1) relaxation in the crystallites of PE, as well as the dependence of T1 on the crystallite thickness reported by other authors.