Polyethylene forces, between chains

Nonpolar polymers such as polyethylene are attracted to each other by weak London or dispersion forces that result from induced dipole-dipole interaction. This arises from the instantaneous fluctuations in the density of the electron clouds along the polymer chains. The energy range of these forces is about 2 kcal/mol repeat unit and is independent of temperature. London forces are the dominant forces between chains in largely nonpolar polymers present in elastomers and soft plastics. 

The reason for the differences in surface tensions between PTFE-based materials, FAs, and analogous hydrocarbons will now be examined. From Eq. (1), the intermolecular dispersion force is proportional to ryf, which means that the forces between chains and molecules will be sensitive functions of the separation distance between them. The separation distance and packing density are related. An examination of the arrangement of these chemicals on a molecular scale is now necessary. It is known that chains of PTFE lie parallel to the air/material interface. This is shown in Figure 4.8B. The same is true of polyethylene,... 

Crystallinity is also favored by strong forces between polymer chains. In a nonpolar polymer such as polyethylene, the only forces holding the chains in place are van der Waals forces (see Section 2.5). Although these attractions are relatively weak, there are many of them, so their total force can be quite large. However, polymers with polar groups have stronger intermolecular forces and are more crystalline than nonpolar polymers, other factors being equal. 

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