Chapter 9 Polyethylene and paraffins

Abstract

This chapter discusses the structure and dynamics of polyethylene and paraffins. Paraffins and polyethylene have been the target of many high resolution solid-state nuclear magnetic resonance (NMR) studies on the chemical shift values and motions because only one kind of methylene exists except for the terminal carbons in the molecule. The chemical shift value is a good index of the structure for molecules of interest. For paraffins and polyethylene trans-zigzag conformation is stable independent of chain length in the solid state. Chemical shift values of paraffins and polyethylene in the solid state are in the range 33-35 ppm, which are typical chemical shift for methylene carbons taking the trans-zigzag conformation. The chemical shift of n-paraffins in the liquid state and solution appears to be of lower frequency than those in the solid state. In the liquid state and solution, paraffins and polyethylene adopt some rotational isomers containing gauche conformation. The low frequency shifts in the liquid state and solution are caused by γ- gauche effect like an amorphous phase. The conformational state in the liquid state and solution can be estimated by using chemical shift in the solid state as a reference for trans-zigzag conformation and γ- gauche effect. Chemical shifts in the solid state vary within about 2 ppm, which is much larger than the variation in the liquid state and in solution. This large scattering is closely associated with a crystal structure. Several models have been proposed to depict the conformation in the folded structure region of polyethylene single crystal— namely, sharp fold model, switchboard model, and loose loop model.