Associative polymers. Part III: Shear rheology from molecular dynamics

作者：Castillo Tejas J^*; Castrejon Gonzalez O; Carro S; Gonzalez Coronel V; Alvarado J F J; Manero O

来源：Colloids and Surfaces A: Physicochemical and Engineering Aspects , 2016, 491: 37-49.

DOI：10.1016/j.colsurfa.2015.11.052

• Non equilibrium molecular dynamics
• Associative polymers
• Viscoelasticity
• Rheology

In this work, non-equilibrium molecular dynamics (NEMD) is used to simulate simple shear flow of associative polymer solutions of the telechelic and multi-sticker types. The objective of the work is the analysis of the effect of the hydrophobic groups of these polymers on the rheological behavior of the system. To generate the flow, the equations of motion are coupled to boundary conditions of Lees and Edward [A.W. Lees, S.F. Edward, J. Phys. C 5 (1972) 1921], where simulations are performed at constant temperature under a coarse-grain simulation scale. Three concentration regimes are identified: dilute, semi-dilute unentangled and semi-dilute entangled. In the dilute regime, the multi-sticker polymers exhibit a Newtonian constant-viscosity region, while in telechelic polymers a shear-thickening region is observed. In this regime, the molecules are isolated, so that the majority of hydrophobic interactions occur between sites of the same chain. Coordination number reveals the presence of a transient first-layer of hydrophobic neighbors, associated with the viscosity of the polymer. In the semi-dilute unentangled regime, both types exhibit a Newtonian region at low shear-rates followed by shear thinning. For telechelic polymers, the Newtonian viscosity increases with concentration with a slope of 4.4, attributed to the formation of non-isotropic aggregates with viscoelasticity levels of two orders of magnitude larger than those of the non-modified polymers. Static structure-factor suggests an increasing alignment of the multi-sticker polymer along the x-direction, revealing larger molecular mobility and lower viscosity than in the telechelic polymers. Finally, these results reveal that when the hydrophobic groups are located at the extremes of the polymer chain (telechelic type), the rheological behavior is strongly affected in the dilute regime by the formation of inter-connected aggregates. On the other hand, when the hydrophobic groups are located in the interior of the chain (multi-sticker type) hydrophobic associations are inhibited by topological restrictions, and in this case the viscoelastic response is only observed in the semi-dilute regime.

• 出版日期2016-2-20