Viscosity of Perfluoropolyether Lubricants: Influence of Structure, Chain Dimensions and Molecular Interactions

Several copolymeric perfluoropolyether (PFPE) lubricants, produced by Ausimont via the photooxidative method, and having the following names and schematic structures:

have been studied with respect to the following molecular and physical characteristics:

• Chemical structure and composition
• Average molecular weight M
• Glass transition temperature T_g
• Viscosity and temperature coefficient of viscosity.

The extensive data collected on a number of PFPE oils of different structure, composition and molecular weight have been used to show the complex interrelationship existing between the various structural parameters that control the viscosity of these lubricants.

For one of the three family investigated, the Z series, in which CF₃ side groups are absent, and for related homopolymers, the viscosity appears to be dependent, through the glass transition temperature T_g, on the intermolecular interaction energy, or CED, which itself is controlled, at hight M, only by the chemical composition of the copolymer (ratio O/C).

The second variable influencing viscosity is molecular weight, although the M dependence is weaker than that typical of other high molecular weight polymers, due to the fact that PFPE oils have relatively short chain lengths.