Characterization of polymer molecular weight distribution by transient viscoelasticity: Polytetrafluoroethylenes

The relationship between the relaxation time spectrum H(τ) in the terminal zone and the volume‐fraction differential molecular‐weight‐distribution function P(M) is derived by considering binary chain contacts for stress transmission, equation image where β and λ are constants for a given chemical type. This is used to determine the molecular‐weight‐distribution curves from the stress relaxation modulus spectrum (above the crystal melting point) at 370°C for a number of commercial and experimental poly(tetrafluoroethylenes) (PTFEs). It is found that PTFEs typically have bimodal molecular‐weight distributions. The lower‐molecular‐weight peak conforms essentially to the “most‐probable” distribution, and the higher‐molecular‐weight peak to the binary coupling distribution. The entanglement molecular weight Me is 5490, and the number of main‐chain atoms between entanglement points is 110, consistent with a flexible chain. The zero‐shear melt viscosity at 370°C is η0 = 1.79 × 10−13 Mw2.94±0.13, where η0 is in Pa.s and Mw/Me = 2,000 to 12,000. The monomeric friction coefficient is also determined.