Thermal Stability of Polytetrafluoroethylene

Abstract

POLYTETRAFLUOROETHYLENE is notable among addition polymers for its high thermal stability. It decomposes at temperatures above about 400° C. to give tetrafluoroethylene as primary product¹. It seems possible to account for this high stability in thermodynamic terms. A useful, though arbitrary, measure of the ultimate stability of a polymer, or more correctly of its free radical form, with respect to its monomeric precursor, is the temperature at which a particular pressure of monomer is supported over the polymer. This is given by²: where δH _gc and δS _gc are the heat and entropy of polymerization for the chosen monomer pressure at a suitable temperature, conveniently taken as 25° C. in most cases. Thus the ultimate stability is determined by the heat of polymerization of the monomer (which is in the range −16 to −25 kcal./mole for most monomers), and the entropy of polymerization (which increases with the molecular weight and complexity of the monomer). The heats of formation of tetrafluoroethylene and of its polymer (−152 kcal./mole³ and −199 kcal./tetrafluoroethylene mole unit⁴, respectively) give the heat of polymerization of tetrafluoroethylene as about −47 kcal./mole. The entropy of polymerization at 67° C. (which temperature is chosen to avoid anomalies resulting from the second-order phase transition of the polymer at room temperature), derived from the measured entropies of monomer⁵ and polymer⁶, is about −45 entropy units/mole for one atmosphere pressure of monomer. These compare with the values⁷ δH _gc = −24.7 kcal./mole and δS _gc = − 37 e.u./mole for ethylene at 25° C., and account for the strikingly greater thermal stability of tetrafluoroethylene.