Quantitative NMR for Solid Polymer Materials: Characterization of Low-density Polyethylene

In solid-state NMR, DDMAS spectra (single-pulse decoupling MAS spectra) measured with a sufficiently long recycle delay may provide quantitative information. The signal inteisities of the ¹³C DDMAS spectrum for low-density polyethylene (LDPE; density 0.915) shown below can draw some quantitative conclusions on its microstructures.

Crystallinity

The crystallinity of the sample is evaluated to be approximately 46% from the main CH₂ peak intensity for crystalline phase (32.84 ppm) and that for amorphous phase (30.77 ppm).

Molecular weight

The intensity ratio of the peak at 23.32 ppm, assigned to the α-CH₂ in the main chain, and the main peaks at 32.84 and 30.77 ppm proves that the number of CH₂ monomer units is about 300 and the molecular weight Mw is about 4,000.

Branching ratio

For example, the intensity ratio of the peaks at 23.75 and 38.24 ppm and the main peaks shows that the following one branching structure exists within 50-60 CH₂ units.

Referene

Assignments of the ¹³C signals follow “Polymer Analysis Handbook” (Asakura Publishing Co. Ltd.) p1102.