The extensional viscosity properties of long-chain branched fluorothermoplastics and correlations to molecular structure

The uniaxial extensional rheology behaviour of tailor-made long-chain branched TFE₃₉/HFP₁₁/ VDF₅₀ terpolymers, LCB-THVs, is investigated at 265°C using the extensional viscosity fixture (TA Instruments). Within the window of applied extension rates (0.03 s^− 1 ≤ ε^∙ ≤ 30 s^− 1) the model polymer fluids show weak to marked strain-hardening behaviour depending on their level of branching. A straightforward data filtering and evaluation procedure capable to extrapolate for quasi steady-state conditions from the similarity of the data sets is presented. Flexible approximation functions for the nominal transient elongational viscosity curves \(\eta _{\text{E}}^{+} (t,\varepsilon^{\bullet})\) and for quasi steady-state elongational viscosity curves \(\eta_E^{\text{\ddag}} (\varepsilon^{\bullet})\) are proposed. In the latter case, there are three adjustable parameters employed which have the physical meaning of: the elongational stress response time τ controlling the location of the maximum in the \(\eta_E^{\ddag} (\varepsilon^{\bullet})\) function, the polydispersity ξ of a unimodal LCB population, and its effective amount. An approximation function is also proposed for bimodal LCB distributions, which applies very well for the LDPE IUPAC A standard resin. Empirically, τ is found to depend on the size dimension of branching λ, derived from molecular characterization, by the relationship τ∼λ^3.3.