Date: Wednesday 15th of March 2017, 14:00.
Location: JBL0C05 (Joseph Banks Laboratories).

‘Coarse-grained molecular dynamics simulations of structure and mechanics of filled elastomers’

by Alexey V. Lyulin,
Theory of Polymers and Soft Matter, Department of Applied Physics, Eindhoven University of Technology, Eindhoven, The Netherlands.

Abstract:
Polymer nanocomposites are materials with an abundant of industrial applications. The mechanical properties of elastomer-based nanocomposites, with inorganic nanoparticles dispersed in the polymer matrix, depend drastically on the interactions between the polymer matrix and the nanofillers. The fundamental problem is the experimentally observed significant loss of the composite’s rigidity appears upon shear, or the so-called Payne effect [1].

Using LAMMPS molecular-dynamics (MD) software package we performed [2-3] constant temperature–constant pressure (NPT) simulations of coarse-grained, amorphous polymer melt consisting of non-entangled bead-spring polymer chains, both cross-linked and non-crosslinked, and confined between two crystalline or amorphous walls, mimicking the inorganic filler surfaces, see Fig. 1a. The simulated glass-transition temperature displayed a steep increase once the crosslinking mesh size became smaller than the radius of gyration of the bulk chains, otherwise it remained invariant to mesh-size variations. The rise in the glass-transition temperature with decreasing mesh size and film thickness was accompanied by a monotonic slowing-down of the relaxation of the incoherent scattering function on all simulated length scales. Higher dynamic fragility was observed when smaller values of film thickness and mesh size were used. The high polymer density that was observed close to the crystalline walls vanished when the crystalline walls have been replaced by the amorphous walls. This surface roughness could also lead to slower segmental relaxation, though additional simulations are required to affirm this.

We have finished recently the larger-scale MD simulations of the polymer matrix filled with colloidal particles, Fig. 1b. We discuss the first results to elucidate the connection between the Payne effect and the size and volume fraction of the filler particles, and the underlying structural and dynamical properties of the polymer and filler phases of the systems.

Fig. 1a) The simulated films of random copolymer chains confined between two crystalline walls. Three different values of the film thickness were used in the simulations. For thinner films, larger lateral dimensions were used so as to maintain a constant density. Distinct colors denote different bead types.

Fig. 1b) On a conceptually larger scale we simulate polymer matrix with non-spherical filler particles, with the prospect of elucidating the microscopic mechanisms that cause the reinforcement of the mechanical properties and the Payne effect. Periodic boundary conditions were implemented in all three dimensions.

[1] A.R. Payne, J. Appl. Polym. Sci. 6 (19), 57 (1962)
[2] T. Davris, A.V. Lyulin, Polym Composite, 36 (6), 1012 (2015)
[3] T. Davris, A.V. Lyulin, J. Chem. Phys., 143, 074906 (2015)