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

Abstract

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.

Modeling elongational viscosity and brittle fracture of polystyrene solutions

“…Fracture occurs only at or to the right of this line for all solutions considered. The onset of fracture of the polymer solutions of PS-820k is shifted to higher Wi/φ 2 with decreasing polymer fraction in agreement with earlier results of other polystyrene solutions (Huang 2019;Wagner et al 2021). The magnitude of ε c depends on the polymer fraction φ, and the strain at fracture increases with decreasing polymer concentration.…”

“…3b and c. One may speculate that the high concentration of solvent OS8.8k in these solutions prevents immediate brittle fracture of the filament after breakage of polymer chains (Huang 2019). We also note that the number of entanglements in these solutions is reduced to only two or three (Table 1), and therefore, modeling based on tube segments may come to its limit.…”

“…Progress in understanding failure of polymer samples in elongational flow was hampered by the fact that failure of stretched filaments includes the phenomena of ductile failure ("necking") and cohesive failure ("rupture" or "brittle fracture"), and until recently, the experimental separation of these two fundamentally different failure modes has been difficult or even impossible. A further achievement of the filament stretching rheometer as shown by Huang et al (2016b), Huang and Hassager (2017) and Huang (2019) was that when the true Hencky strain rate is controlled rather than nominal Hencky rate, the four failure zones of the so-called Malkin plot (Malkin and Petrie 1997) (purely viscous zone, viscoelastic zone with failure by necking, rubbery zone, glassy zone) are reduced to just two possible states: liquid or solid, and a clear distinction exists between liquid-like behavior (unlimited steady-state elongation) and solid-like behavior (brittle fracture). A quantitative criterion for brittle fracture of entangled polymer liquids (Wagner et al 2018) was recently extended by taking finite chain extensibility and polymer fraction of the solutions into account (Wagner et al 2021).…”

Elongational viscosity and brittle fracture of bidisperse blends of a high and several low molar mass polystyrenes

“…By measuring the local diameter of the polymer sample during elongation, the true Hencky strain and strain rate can be determined and controlled, while by elongational rheometers prescribing the global deformation of the filament only nominal values of strain and strain rate can be obtained. A further achievement of the filament stretching rheometer as shown by Huang et al (2016b, Huang andHassager 2017) and Huang (2019) was that when the true Hencky strain rate is controlled rather than the nominal Hencky rate, the four failure zones of the so-called Malkin plot (Malkin and Petrie 1997) (purely viscous zone, viscoelastic zone with failure by necking, rubbery zone, and glassy zone) are reduced to just two possible states: liquid or solid, and a clear distinction exists between liquid-like behavior (unlimited steady-state elongation) and solid-like behavior (brittle fracture). A quantitative criterion for brittle fracture of entangled polymer liquids (Wagner et al 2018) was recently extended by taking finite chain extensibility and polymer fraction of the solutions into account (Wagner et al 2021a(Wagner et al , 2021b.…”

“…

Elongational viscosity data of four well-characterized blends consisting of 10% mass fraction of monodisperse polystyrene PS-820k (molar mass of 820 kg/mol) and 90% matrix polystyrenes with a molar mass of 8.8, 23, 34, and 73 kg/mol, respectively, as reported by Shahid et al Macromolecules 52: 2521-2530, 2019 are analyzed by the extended interchain pressure (EIP) model including the effects of finite chain extensibility and filament rupture. Except for the linear-viscoelastic contribution of the matrix, the elongational viscosity of the blends is mainly determined by the high molar mass component PS-820k at elongation rates when no stretching of the lower molar mass matrix chains is expected.

…”