Polymer extension flows and instabilities

The basics of the uniaxial stretching of polymer melts and solutions are considered, including analysis of the transient regimes of deformation, as well as the different types of instability in the final stage of stretching which result in breakup of the systems in question. A steady flow can be realized only at low deformation rates. Increase in the strain rate leads to large draw ratios that can be reached only by development of elastic (reversible) deformations of a polymer, while the flow (irreversible deformations) is practically suppressed. High rate deformations of polymer melts and solutions consisting of entangled chains are quite similar to the stretching of cured rubber. Quantitative conditions of breakup for polymer melts are also similar to the rupture of cured rubber. Thus, one can treat high rate large elastic deformations of linear polymers in stretching as strain-induced transition to the rubber-like state. This mechanism of extensional deformations is clearly seen in experiments with linear monodisperse polymers. However, similar physical processes also take place for polydisperse and branched polymers, though they are more complicated; and special phenomena such as strain hardening are observed. In the stretching of dilute or semi-dilute solutions, surface effects begin to play an essential role. However, in this case, elasticity also stabilizes a stream, creating a highly oriented core within a filament. In dilute solutions, a solvent forms a regular structure on this core ("bead-on-a-string" structure) while in entanglement solutions, it is squeezed out of a filament, forming separate drops. The latter is reminiscent of the stress-induced phase separation effect. Concentration redistribution along and across a filament then occurs, leading to modification of its temporal and spatial rheological properties. A special case of strong (high strain rate) stretching of polymer solutions is electrospinning, which is accompanied by loss of solvent. In all cases of high rate stretching, elastic deformations inherently related to macromolecules are dominant. (C) 2014 Elsevier Ltd. All rights reserved.