PhD defense : Modeling and simulation of the micromechanical behavior of semi-crystalline polyethylene including the effect of interphase layer

PhD Thesis: Akbar GHAZAVIZADEH

Team: MMB

Title: Modeling and simulation of the micromechanical behavior of semi-crystalline polyethylene including the effect of interphase layer

Abstract : Interphase layer in semi-crystalline polyethylene has been the least known constituent of this widely used polymer, in terms of the mechanical properties. Because of the metastable nature and nanometric size of the interphase and its intimate mechanical coupling to the neighboring crystal and amorphous domains, detailed characterization of its mechanical properties have eluded any experimental evaluation. Mechanical characterization of the interphase layer in polyethylene is the focus of two major technical parts of my dissertation. The characterization scenarios are deployed by applying micromechanics and continuum mechanics relationships to the relevant atomistic simulation data. The third technical section deals with the viscoplastic deformation simulation of an aggregate of polyethylene using a multiscale, homogenization analysis.

Elastic characterization of the interphase layer is implemented by applying the relationships of two distinct micromechanical homogenization techniques to the Monte Carlo molecular simulation data available for the interlamellar domain. The micromechanical approaches are “Extended Composite Inclusion Model” and “Double-Inclusion Method” and the atomistic data includes the variation of the interlamellar stiffness as well as the amorphous and interphase thicknesses with temperature for 350-450 K. To implement this characterization, the temperature dependence of the amorphous stiffness is also required, which is established using the relevant findings from the literature. The interphase stiffness is successfully dissociated form that of the interlamellar domain using the abovementioned micromechanical techniques whose results match perfectly. Interestingly and contrary to conventional materials, the interphase stiffness lacks the common feature of positive definiteness, which is indicative of its mechanical instability. An ad hoc sensitivity analysis is devised to assess the impact of the existing uncertainties on the dissociated results. The effective Young’s modulus of the interphase is evaluated using its dissociated stiffness, which compares well with the effective interlamellar Young’s modulus for highly crystalline polyethylene, reported in an experimental study. This satisfactory agreement along with the identical results produced by the two micromechanical approaches verifies the new information about the interphase elastic properties and endorses the proposed dissociation methodology to be applied to similar problems...

The defense will take place on Friday 13th December, at 14h00, in the seminar room, ICube, 4 rue Boussingault, Strasbourg.