Exploring the mechanism of fracture for entangled polymer liquids in extensional flow

The critical strain and stress at fracture are systematically investigated for two groups of nearly monodisperse linear polystyrene liquids in an extensional flow. The samples in group I have similar number of Kuhn segments per entangled strand (Ne) but different number of entanglements per chain (Z), while the samples in group II have similar Z but different Ne. We found that the critical conditions, especially the critical stress, are independent of Z but influenced by Ne. The observations indicate that the fracture in entangled polystyrene liquids occurs at a length scale smaller than an entangled strand. Therefore, the fracture originates more likely from scission of primary bonds in polymer chains, rather than rapid entanglement slipping. The level of the critical stress also suggests that at fracture, the polymer chains approach their theoretical maximum stretch ratio, which is equal to Ne.