Tetrafluoroethylene-Based Fluoropolymers: Synthesis In Supercritical Carbon Dioxide And Properties Investigation

Supercritical carbon dioxide (sc-CO₂) is an inexpensive, low-toxic, non-flammable,easily recycled and green polymerization medium, which can be used as a polymerization solvent instead of conventional organic solvents. Studying on the polymerization of fluorine-containing monomers in sc-CO₂ will contribute great to lower contaminant to environment, upgrade the quality of polymer products, widen their applications, thus impacting great influence on the perspective of fluorinated polymers.In this dissertation, a series of tetrafluoroethylene-based fluoropolymers with perfluoroalkyl end groups initiated by bis(perfluoro-2-n-propoxypropionyl) peroxide (BPPP) in sc-CO₂ are prepared and characterized. The research mainly includes four parts as follows.(1) Green low molecular weight (Mn) poly(tetrafluoroethylene) (PTFE) homopolymers were successfully prepared initiated with BPPP in sc-CO₂. Solid-state 19F-NMR and FT-IR analysis show that perfluoroalkyl end groups are present in the resultant PTFEs,endowed all polymers enjoying outstanding thermal stabilities. DSC measurements indicate that both melting and crystallization transitions of PTFE shift to lower temperatures when its Mns decrease, because shorter polymer chain can move easily at lower temperatures. Expanded fibers appear in the resulting polymers and their fraction significantly increased with the decrease of Mn since higher Mn limits the mechanical stress effect on the formation of fibers. Polymerization kinetics investigation suggests that the rate law for the polymerization has kinetic orders of 0.5 and 1.0 with respect to initiator and monomer concentrations, respectively, indicating that termination occurs through coupling of propagating chains. Degree of crystallinity of as-polymerized PTFE can be as high as 86%, and polymerization rate seems to be not obviously affected by the total interphase area of the polymer phase, implying polymerization mainly occurs in the CO₂-rich fluid phase; meanwhile, low viscosity and high diffusivity of sc-CO₂ endows propagating chains more opportunities to meet, thus yielding low Mn PTFEs.(2) PTFEs were characterized by solid-state 19F-NMR and FT-IR spectroscopy. From rational assignments of the characteristic signals, an overall reaction mechanism explaining the homopolymerization processes is proposed. It is found that carboxyl radicals resulting from thermal decomposition of BPPP completely decarboxylated before reacting with TFE. Additionally, n-C3F7OCF(CF3)·has undergone rearrangement to n-C3F7·with a small fraction which is decreased from 0.11 to 0.04 when the reaction temperature was lowered from 35 to 5°C. Initiation rate constants (kd) were evaluated and found to be slightly increased with elevated pressure. The initiation activation energy derived from kd at different temperatures is 90.3 kJ/mol. Such mechanism and kinetics insights into the TFE homopolymerizations in sc-CO₂ will be instructive for the syntheses of fluoropolymers with desired properties in future.(3) To improve the processability of PTFE, copolymers of TFE and perfluoropropylvinyl ether (PPVE) with stable perfluoroalkyl end groups were prepared in sc-CO₂. Reactivity ratios of TFE and PPVE were first reported. The rTFE and rPPVE values are about 8 and 0.08, respectively. Such parameters are significant for the modification of PTFE through copolymerization of TFE and PPVE. It is found that through increasing the reaction pressure from 8.5 to 25 Mpa, while rTFE increases by 12.0%, rPPVE decreases by 9.0%, which should be ascribed to the enhancement of the polarity of CO₂ under high pressures. Because rTFE is by two orders of magnitude higher than that of PPVE, on one hand, the copolymerization rate falls rapidly with the decrease of TFE feed ratio; on the other hand, TFE content in the copolymer decreases with the reaction time. DSC and XRD results indicate that there exist two forms of crystals with highly regular molecular arrangement or less ordered chain packing which is disturbed by perfluoropropyl pendants.(4) A series copolymers of tetrafluoroethylene and perfluoro(4-methyl-3,6-dioxa-7-octen)sulfonyl fluoride (PSVE) initiated by BPPP in sc-CO₂ were prepared. The reactivity ratios for TFE and PSVE were calculated to be 7.85, 0.079 and 7.92, 0.087 according to Fineman–Ross and Kelen–Tudos methods, respectively. When PSVE fraction in copolymer was increased, the melting points of the copolymers were decreased, whereas the glass-transition temperatures were improved. It should be ascribed to the disruption of the main chain packing and the enhancement of intra- and intermolecular interactions, respectively, due to the introduction of more PSVE units. It was indicated that the initiation process is affected by monomer and the initiation rate is related to the monomer content, leading to a higher reaction order (more than 1.0) of monomer. Because of the intense plasticizing capability of CO₂ to polymeric materials, polymeric chain radicals can be easily diffused indicating that termination occurs through coupling of propagating chains. The rate equation of polymerization is given by Rp=k[BPPP]0.48[M]1.8, and the apparent active energy is 58.85 kJ/mol. Comparison studies on above copolymers and another fluorinated sample prepared in emulsion using FT-IR, TGA, 19F-NMR, SEM and Fenton test showed that the copolymers enjoy comparable properties to that of the fluorinated one, thus offering an economic way to improve the properties of TFE-PSVE copolymers for their applications in diverse fields.