Effect of Rheological Strain Hardening on Extrusion Blown Film of Polyvinylidene Fluoride

Abstract

In this work the effect of strain hardening on poly-vinylidene fluoride (PVDF) extrusion blown film is investigated. Controlled long chain branching is introduced via a multi-functional initiator to produce two PVDF samples with different molecular weight and chain architecture. The branched samples are compared to two reference resins having identical molecular weight and no chain branching. All samples are characterized by size exclusion chromatography (SEC) coupled with triple detection system comprising a Differential Refractive Index (DRI) detector, Intrinsic Viscosity (IV) detector, and Multi-Angle Laser Light Scattering (MALLS) detector, to determine their molecular weights and their distribution as well as to detect chain branching via measuring the coil size in dilute solution. The rheological properties are determined using oscillatory measurement and melt strength at 230°C while extensional viscosity measurements are conducted at 180°C to determine strain hardening at different extension rates. The resins are evaluated using a small scale extrusion blown film set-up to determine the blow-up ratio and the minimum thickness achievable. The characterization results show that the control samples are different in molecular weight and almost identical in the polydispersity index (Mw/Mn). The branched samples, however, have higher molecular weight and a slightly broader molecular weight distribution. Light scattering data together with inherent viscosity data show that the branched samples have a lower radius of gyration (RG) and inherent viscosity (IV) over the entire molecular weight distribution confirming chain branching. The rheological properties in oscillatory measurements show that the branched samples exhibit almost identical viscosities as the control samples. However, a broader transition from the Newtonian to the non-Newtonian region for the branched sample is observed confirming the SEC—MALLS results. This is corroborated using extensional viscosity and melt strength measurements, which show a significant strain hardening and an increase in melt strength, respectively. Blown film experiments show that the samples containing chain branching could be processed under similar process conditions as the control samples, and with a higher blow up ratio thereby achieving 5 μm film thickness with high clarity.