Rheological molecular weight distribution determinations of ethylene/tetrafluoroethylene copolymers: implications for long-chain branching

The molecular weight distributions of ethylene/tetrafluoroethylene copolymers (PETFE) were estimated by dynamic mechanical analysis of their melts. The validity of the method had been established on model polystyrenes. The high temperatures required for solubility prevented analysis by classical techniques. The results indicated much broader distributions than determined by Chu and co-workers on dilute PETFE solutions using a unique high-temperature laser light scattering procedure. Dynamic mechanical analysis was performed on one of the samples after dissolution, filtering, and drying to reveal no significant change vs. the originally analyzed melt. This indicated that the conditions required to treat the solutions during the light scattering tests were not appreciably altering the polymer nor filtering out an insoluble portion. A broader distribution would be predicted by the rheological technique if long-chain branching was present. Strong evidence for the presence of such branching was observed when plotting the zero shear viscosity (from the rheological analysis) vs. the weight-average molecular weight (from light scattering). The well-known 3.4 power law would have been obtained if these were linear chains. Instead, a much stronger effect of molecular weight on viscosity was observed, which has been attributed to the effects of long-chain branching. The theory of portions of the PETFE chain acting as its own chain-transfer agent is a potential explanation for branch formation.