Computational modeling of tethered and entangled polymers [electronic resource]
Keywords: Mechanical Engineering

In the first part of the thesis, we study the dynamics of a dilute polymer in shear flow. We use Brownian dynamics simulations to examine how the shear dynamics of a polymer is affected as one changes the flexibility--moving from the long flexible polymers, consisting of multiple Kuhn-lengths, to small rod-like semiflexible polymers with sub-Kuhn lengths. Using our simulations, we reproduce the experimental data from both Guihua et al. (2011) and Harasim et al. (2012), spanning the flexible to the stiff polymer regime. We use these simulations to examine the stochastic nature of tumbling dynamics and relate it to the mean fractional extension in free shear flow. We show an interesting contrast between the extensional behavior of flexible and stiff polymers in free shear flow, wherein the former's extension increases with the increase in shear rate while the latter's decreases. In addition, we also study wall-tethered single molecules in shear flow and analyze the effect of stiffness on the scaling behavior of this different dynamical system. We ultimately compare the physical mechanisms of polymer extension between the free and wall-tethered polymer systems over the broad range of stiffness. In the second part of the thesis, we study the non-equilibrium dynamics of entangled polymers, which is still an open problem in polymer physics. Recent experimental data of the extensional viscosity of a monodisperse polystyrene melt (Bach et al., 2003) has revealed a failure in the available models for entangled polymers. The data shows an extensional viscosity thinning exponent of -0.5, in contrast to the value of -1 predicted by the standard models. Another failure of the standard theories lies in the prediction of the viscosity upturn. Unlike the predictions of the standard theories, Bach's experimental data shows no signs of an upturn in the extensional viscosity for extensional rates of the order of inverse Rouse time of a single chain. We introduce a mesoscopic model for simulating non-equilibrium dynamics of entangled polymers. This model is an extended version of a slip-link based model which was originally proposed by Masubuchi et al. (2001). Based on our extended slip-link simulations in planar extensional flow (Kushwaha et al. 2011), which predict a thinning exponent closer to -0.5 than -1, we propose an explanation for the thinning dynamics that relies on disentanglement caused by the flow. We also propose a dynamics for the upturn in the extensional viscosity curve based on the semiquantitative agreement of our simulations with the experiments of Bach et al. (2003), as neither our simulations nor Bach's experiments show any sign of an upturn at the extensional rates on the order of inverse Rouse time of a chain in entangled system. We also use our model to simulate the dynamics in shear flow, and show that the predictions compare well against both the experimental data of Jary et al. (1999) and the molecular dynamics simulations of Baig et al. (2010). Additionally, we show the possible extension of the model to mixed flows and report some simulations results.