Deformation Of Driven Polymers Studied By Computer Simulations

The deformation of polymer is the fundamental problem in polymer rheology. Different from small molecules, polymer diffusion is the integration of Brownian motions of monomers. In the flow of polymer solution or melt, Brownian forces maintain the polymer chain as a random coil. But when the internal tension generated by the external field exceeds Brownian forces, the coil begins to deform. The large deformation of polymers under flow is the source of nonlinear viscoelasticity. So far, the mechanism of deformation is not yet clear, but it apparently relates with the unique chain-like polymer structure.Because the the complexity of the reality, polymer chains which perform Brownian motions may be in the biased diffusion, e.g., interfacial migration upon phase separation, centrifugal/gravitation precipitation, electrophoresis, and driven shear flow upon polymer processing. Under uniform driving forces, the ideal integration of parallel transport of monomers brings no deformation to the polymer coil. Based on lattice Monte Carlo (MC) simulations, we observed the deformation of a single chain under a uniform driven flow. We studied the mechanism of the deformation by Brownian dynamics simulations under continuous space. Finally, using Monte Carlo simulations, we study the deformation of polymer shear flow.The main achievements and novel contributions of this thesis can be summarized as follows.(1) We investigated the biased Brownian motions of single-chain macromolecules, and observed the force-induced coil-deformation due to an intrinsic asymmetry between chain ends and chain middles. The deformation appears as steady and reversible under variable driving forces. The onset strengths of driving forces decay with the increase of chain lengths, at which the thinning followed with thickening behaviors of polymer shear flows can be reproduced.(2) We studied the molecular mechanism of chain deformation under uniform driven flow by means of standard Brownian Dynamics simulations. We design three different sampling methods and the random activation method corresponding to the Monte Carlo simulations. Our results will demonstrate that coil deformation can be induced by non-synchronous activations of monomers under strong enough driving forces. We attribute the mechanism of coil-deformation to an accumulation of local acceleration along the polymer chain. Our observations reveal the source of polymer deformation in the Monte Carlo simulations is the dynamic heterogeneity along a polymer chain.(3) Using dynamic Monte Carlo simulations, we studied the deformation of polymer chain brought by the shear force and shear rate in the polymer melt. Usually, we thought that only the velocity gradient brings deformation to the chain. The results demonstrate that both the shear stress and the shear rate can yield polymer deformation. Such an observation is apparently beyond the common sense that the velocity gradient (shear rate) is mainly responsible for polymer deformation in a shear field. Long chains seem to be easier in deformation than short chains, and their stress induced deformation causes thinning followed with thickening behaviors featured for non-Newtonian fluid behaviors of polymers in the fast flows. In addition, the velocity gradient appears as mainly responsible for polymer deformation at low shear stresses, while the shear stress is mainly responsible for polymer deformation at high shear stresses (or shear rates).As the local density fluctuations of the monomer, the situation for asynchronous movement of monomers always occurs in the bulk flow of polymers. From this point of view, the deformation of the chain may contain the contribution of the shear stress. The existing models of the polymer bulk could not explain the deformation phenomenon observed in experiments. The deformation of the chain under uniform driving flow can helps us to understand non-linear viscoelasticiry of the polymer, and allow us to study further polymer phase transitions under driven flow by means of molecular simulations.