Chapter 10: The Melt Viscosity Properties of Fluoroplastics – Correlations to Molecular Structure and Tailoring Principles

The viscosity properties and the related structure–property relationships of fluoroplastics are highlighted with the example of statistical poly(TFE-co-HFP-co-VDF) samples (TFE, tetrafluoroethylene; HFP, hexafluoropropylene; VDF, vinylidene fluoride). As observed for many other fluoropolymers prepared by free radical polymerizations, they consist of linear chains showing a tight molar mass distribution with a dispersity of 1.6≤Đ≤2. This leads to linear low-density polyethylene (LLDPE)-like shear flow characteristics, such as flat viscosity curves with a marked Newtonian viscosity plateau, a simple thermo-rheological behavior and a limited shear thinning during melt processing. By tuning the amount of chain-transfer agent, the average molar mass can be effectively adjusted in the polymerization process. Hence the molar mass distribution can be discernibly broadened by multistage processes, resulting in an increased structural viscosity that facilitates melt processing. Moreover, the melt rheology properties can be drastically affected by modifying the polymer topology from linear to long-chain branched. The most striking feature of this tailoring concept is displayed by the long-chain branching poly(TFE-co-HFP-co-VDF) melts under extensional deformation. Even if the mass fraction of LCB remains below 5% m/m, their melts exhibit a pronounced strain-hardening behavior in elongational flow. This induces the unique ability of self-healing localized constrictions in the melt contour and results in final articles with a more uniform wall thickness distribution. Being equipped with these concepts for molecular tailoring, the melt-processing characteristics of fluoroplastics can be adjusted from LLDPE like to almost LDPE like.