High-temperature extensional rheology of linear, branched, and hyper-branched polycarbonates

The high-temperature extensional viscosity of three commercially available linear, branched and hyper-branched polycarbonates (PCs) were measured using a high-temperature capillary breakup extensional rheometer (CaBER) in both air and nitrogen. The experiments were performed at temperatures ranging from T = 250 to 370 °C to a maximum Hencky strain of ten. At lower end of the temperature range, no significant degradation of the linear and branched PC was observed either in the shear or extensional measurements. Beyond, T > 300 °C degradation of the three different PCs was observed. Changes to the molecular structure of the PC were observed which resulted in a dramatic increase in the extensional viscosity. The rate of growth in the extensional viscosity was found to increase with temperature, time at temperature, and, in the case of the linear and branched PC, the presence of air. For the hyper-branched case, changes to the molecular structure of the PC were found to occur more quickly under nitrogen. At these high temperatures, the increase in extensional viscosity was found to grow large enough to stop the breakup of the fluid filament all together, essentially stopping all capillary drainage. This temperature-induced cross-linking and increase in the extensional viscosity of the PC can improve the anti-dripping properties of the polycarbonate by slowing and even restricting dripping from polymeric components near high heat surges. Through these experiments, we have demonstrated measurement of the extensional viscosity to be several orders of magnitude more sensitive to temperature-induced changes to the molecular structure than measurements of shear rheology.