Multiscale simulation of polymers under shear

This PhD thesis deals with the investigation of polymer-melt viscosity from coarse-grained simulations and with the development of a backmapping method from coarse-grained nonequilibrium systems. These studies involve both atomistic and coarse-grained (CG) descriptions. Besides these theoretical studies, efforts are also pursued on programming a code, which is designed for molecular dynamics simulations of coarse-grained polymer systems. Chapter 1 gives a short overview of polymer properties which can be investigated by means of coarse-grained simulations as well as the algorithms for viscosity calculations via molecular dynamics. Chapter 2 focuses on the study of the viscosity and the structural alteration of a coarse-grained model of polystyrene under steady shear flow via the reverse nonequilibrium molecular dynamics (RNEMD) method. The applicability of the RNEMD algorithm in predicting the viscosity of polymers is investigated. The viscometric functions predicted by the RNEMD are compared to previous studies of similar models where conventional nonequilibrium molecular dynamics (NEMD) methods have been used. The performance of the dynamics of the CG model, which has been developed taking only structural information into account, is investigated. For the shortest polymer chain, the zero-shear viscosity is compared to recent experimental results. The material functions (namely the first and second normal stress difference) are discussed. Structural alteration (the average chain dimension, shear-induced alignment) under a steady shear flow is also quantitatively characterized. In Chapter 3, the problems in backmapping coarse-grained polymer models, on which a nonequilibrium shear flow has been imposed, are discussed. Backmapping is the procedure, by which the atomistic description is re-inserted into a coarse-grained configuration. Some strategies and a new backmapping protocol are proposed. In this method, the deformed conformations are maintained globally during backmapping by applying position restraints. The local optimization of the atomistic structure is performed in the presence of these restraints. The artefact of segment isolation introduced by position restraints is minimized by applying different restraint patterns iteratively. The procedure is demonstrated on the test case of atactic polystyrene under a steady shear flow. Chapter 4 reports in detail the implementation of the RNMED algorithm and the dissipative particle dynamics (DPD) methodology used as a thermostat into a numerical-potential molecular dynamics program (Ibisco). The program is partiallyredesigned in order to meet the requirements of these new algorithms. The developed code provides a reliable tool for investigating the rheological behaviour of CG models. Finally, Chapter 5 outlines some perspectives of future research