Low-moIecular-weight polytetrafluoroethylene bearing thermally stable perfluoroalkyl end-groups prepared in supercritical carbon dioxide

摘要

Low-molecular-weight (M_n) polytetrafluoroethylene (PTFE) homopolymers were successfully prepared using a perfluorodiacyl initiator, bis(perfluoro-2-n-propoxypropionyl) peroxide, in supercritical carbon dioxide. Solid-state ~(19)F NMR and Fourier transform infrared spectral analyses show that perfluoroalkyl end-groups are present in the resultant PTFEs.Thermogravimetric analysis suggests all polymers with various M_n have outstanding thermal stability. Differential scanning calorimetry measurements indicate that both melting and crystallization transitions of PTFE shift to lower temperatures when M_n decreases, because shorter polymer chains can move more easily at lower temperatures. Investigation of polymerization kinetics suggests that the rate law for the polymerization has kinetic orders of 0.5 and 1.0 with respect to initiator and monomer concentrations, indicating that termination occurs through coupling of propagating chains. Melt fusion crystallinity of as-polymerized PTFE can be as high as 86%, and the polymerization rate does not seem to be obviously affected by the total interphase area of the polymer phase, implying polymerization mainly occurs in the carbon dioxide-rich fluid phase; meanwhile, the low viscosity and high diffusivity of supercritical carbon dioxide mean that propagating chains have more opportunities to meet, thus yielding low-M_n PTFEs.