Computational investigation of chain dynamics in architecturally complex polymers

Abstract

We investigate the chain dynamics in the polymer melts with complex architecture by means of molecular dynamics simulations. Our study is focused on the following architecturally complex polymers: T- and Y-shaped asymmetric stars, symmetric stars, mixtures of Tshaped asymmetric stars and linear chains, H-polymers, combs and Cayley trees. Dynamics in these architectures is strongly influenced by the presence of one or more branchpoints. The overall chain dynamics in branched structures is slowed down comparing to the linear chain and the relaxation of these materials extend over several time decades. Extensive molecular dynamics simulations allow us to study the relaxation processes ocurring in the branched polymer melts at the molecular level. We pay particular attention to the role of the branchpoint in the dynamics of these systems. Our simulations reveal details about the branchpoint motion that can be further compared to the theoretical hypotheses, experimental data and finally introduced in the predictions of the viscoelastic properties of the industrially produced materials. The time evolutions of the mean squared displacements of the particular molecular segments confirm that the arm retraction is the main relaxation mechanism in symmetric systems, i.e. symmetric stars and Cayley trees. In these systems the branchpoint remains localized during the whole simulation time. We study the role of constraint release on the branchpoint dynamics and compare the simulation results with a theoretical model. The fluctuations of the branchpoint at time scales smaller than the Rouse time τR are affected by the early tube dilation process that leads to a weaker branchpoint localization than expected. After the incorporation of the early and late tube dilation processes quantified from the simulation data into the theoretical model, we were able to fully describe the branchpoint dynamics at the times smaller than τR. After the relaxation of the short side arms in the asymmetric structures the arms act as sources of friction and the whole molecule can be described as an effective linear chain. We studied the diffusion of the branchpoint after the arm relaxation. The calculation of the diffusion constant involves the knowledge about the arm relaxation, dilution of the tube and the friction related to the reptation of the molecule. We estimated these observables from the simulations and tested theoretical hypotheses used for the prediction of the branchpoint diffusive behaviour in experimental studies of branched polymer melts. We perform a detailed analysis of the branchpoint trajectories and present a robust method for finding regions of strong localization. We characterize the time and length scales for the branchpoint motion between traps of localization, and discuss the consequences for the interpretation of the long-time branchpoint motion dynamics proposed by hierarchical tube models.