Supercritical fractionation of a perfluorinated copolymer

A perfluorinated copolymer, poly(tetrafluoroethylene-co-19.3 mol% hexafluoropropylene), is fractionated using supercritical SF6. The fractionations are performed isothermally at 163 degrees C using an increasing pressure profile to obtain gram-sized fractions that have molecular weight distributions that are narrower than that of the parent copolymer. The fractionation pressure is increased in increments of 34 bar from 276 to 683 bar. The parent material and the fractions are characterized using SEC and FTIR spectroscopy. The first seven samples, eluting between 276 and 552 bar, show a trend of increased molecular weight of the fraction with increasing fractionation pressure. Above 552 bar, the molecular weight of the remaining fractions decreases with increasing fractionation pressure. Although these last fractions do not differ in hexafluoropropylene content, they do differ in the type and concentration of their end groups. In particular, the fractions that eluted at pressures greater than 552 bar contain increasing amounts of carboxylic acid end groups. The strong hydrogen-bonding interactions of these end groups prevent oligomers with acid ends from eluting from the column at lower pressures, which was expected due to their lower molecular weight.