Molecular Dynamics Simulation of Entangled Melts at High Rates: Identifying Entanglement Lockup Mechanism Leading to True Strain Hardening.

In the present work, molecular dynamics simulations are carried out based on the bead-spring model to indicate how the entanglement lockup manifests in the late stage of fast Rouse-Weissnberg number (WiR >>1) uniaxial melt stretching of entangled polymer melts. At high strains, distinct features show up to reveal the emergence of an increasingly tightened entanglement network. Chain tension can build up, peaking at the middle of the chain, to a level for chain scission, through accumulated interchain interactions, as if there is a tug-of-war ongoing for each load-bearing chain. Thanks to the interchain uncrossability, network junctions form by the pairing of two or more hairpins. It is hypothesized that the interchain entanglement at junctions can lockup through prevailing twist-like interchain couplings as long as WiR > 9. In this limit, a significant fraction of chains act like cyclic chains to form a network held by interchain uncrossability, and appreciable chain tension emerges.

Nanoconfined Crystallization in Poly(lactic acid) (PLA) and Poly(ethylene terephthalate) (PET) Induced by Various Forms of Premelt-Deformation.

The processing-structure-property relationship using poly(lactic acid) (PLA) and poly(ethylene terephthalate) (PET) is explored. Specifically, both pre-extension and preshear of amorphous PLA and PET above their glass transition temperatures Tg , carried out in the affine deformation limit, can induce a specific type of cold crystallization during annealing, i.e., nanoconfined crystallization (NCC) where crystal sizes are limited to a nanoscopic scale in all dimensions so as to render the processed PLA and PET optically transparent. The new polymer structure after premelt deformation can show considerably enhanced mechanical properties. For example, premelt stretching produces geometric condensation of the chain network. This structural alternation can profoundly change the mechanical characteristics, e.g., turning brittle PLA ductile. In contrast, after preshear of amorphous PLA above Tg , the NCC containing PLA remains brittle, showing the importance to have geometric condensation from processing. Both AFM imaging and SAXS measurements are performed to verify that premelt deformation of PLA and PET indeed results in NCC from annealing that permits the strain-induced cold crystallization to take place on the length scale of the mesh size of the deformed chain network.

Nonlinear rheology of entangled polymers at turning point.

Thanks to extensive observations of strain localization upon startup or after stepwise shear, a conceptual framework for nonlinear rheology of entangled polymers appears to have emerged that has led to discovery of many new phenomena, which were not previously predicted by the standard tube model. On the other hand, the published theoretical and experimental attempts to test the limits of the tube model have largely demonstrated that the most experimental data appear consistent with the tube-model based theoretical calculations. Therefore, the field of nonlinear rheology of entangled polymers is at a turning point and is thus a rather crucial area in which further examinations are needed. In particular, more molecular dynamics simulations are needed to delineate the detailed molecular mechanisms for the various nonlinear rheological phenomena.

Shear banding in entangled polymers in the micron scale gap: a confocal-rheoscopic study.

Recent shear experiments in well-entangled polymer solutions demonstrated that interfacial wall slip is the only source of shear rate loss and there is no evidence of shear banding in the micron scale gap. In this work, we experimentally elucidate how molecular parameters such as slip length, b, influence shear inhomogeneity of entangled polybutadiene (PBD) solutions during shear in a small gap H ∼ 50 µm. Simultaneous rheometric and velocimetric measurements are performed on two PBD solutions with the same level of entanglements (Z = 54) in two PBD solvents with molecular weights of 1.5 kg mol(-1) and 10 kg mol(-1) that possess different levels of shear inhomogeneity (2bmax/H = 17 and 240). For the PBD solution made with a low molecular weight PBD solvent of 1.5 kg mol(-1), wall slip is the dominant response within the accessible range of the shear rate, i.e., up to the nominal Weissenberg number (Wi) as high as 290. On the other hand, wall slip is minimized using a high molecular-weight PBD solvent of 10 kg mol(-1) so that bulk shear banding is observed to take place in the steady state for Wi > 100. Finally, these findings and previous results are in good agreement with our recently proposed phase diagram in the parameter space of apparent Wi versus 2bmax/H suggesting that shear banding develops across the micron scale gap when the imposed Wi exceeds 2bmax/H [Wang et al., Macromolecules, 2011, 44, 183].