Polyethylene and the n-alkanes

Polyethylene -(CH2-CH2) - is the largest toimage plastic in 1995 world-wide production was 3.8 x 10 tonnes. It has been produced commercially since 1939 for a wide range of applications from electrical insulation to packaging and pipes. Its usefulness derives from its low cost, easy processability, chemical resistance and the ability to tailor its properties by the method of manufacture and the inclusion of comonomers (usually -alk-l-enes, where n = 3-12). 

Polyethylene can adopt a number of crystal forms depending on the method of preparation. The most common form is a centrosymmetiic orthorhombic form, space group Pnma = (number 62), with two formula units (CH2-CH2) in the unit cell rotated at 90° with respect to each other as shown in Fig. 10.1. Each chain adopts an all-tram planar zigzag conformation. This can also be described as a 2 helix with one complete turn for each two CH2 units. 

The normal modes at the centre of the Brillouin zone, ( 4.3.1), (A = 0) can be deduced by a conventional factor group analysis [4,5]. Since there are two chains in the unit cell, equivalent modes on each chain couple to give an in-phase and out-of-phase pair of modes (Davydov splitting). Above 600 cm the modes can be described using standard group frequency correlations CH2 rock 720-1170 cm, C-C stretch 

Below 600 cm are the V5 in-plane C—C—C bending modes (longitudinal acoustic modes, LAMs), the V9 out-of-plane C—C—C bending modes (transverse acoustic modes, TAMs) and the 12 lattice modes. These comprise in-phase and out-of-phase pairs of librations about the three axes, giving six modes, three optic translation and three 

The variation of the energy of a vibration with wavevector k is the dispersion of the mode and arises from coupling between oscillators in different unit cells. Since for polyethylene adjacent unit cells contain the 

---

A large body of literature exists on the band assignments of polyethylene and the n-alkanes in the melt and in the solid state. In the melt spectra of the lower molecular weight alkanes, chains shorter than n-Cg, band assignments have been made for particular conformers of the molecules /5 i.e., bands associated with the... 

---