Missions

The successful candidate must have a strong interest in both theory and numerical work. Numerical work involves code development (e.g., changing the C++ LAMMPS code, programming of data analysis tools, etc.), carrying out large-scale molecular dynamics simulations with LAMMPS, and data curation.

Activities

Scientific context:

Our current understanding of polymer viscoelasticity is founded on single-chain models [2]. Such models draw on the fact that polymers exhibit universal behavior for length scales larger than the local scale size of their monomer units. This has motivated the study of coarse-grained generic models, using Monte-Carlo and molecular dynamics simulations, to help advance theories describing the scaling behavior of polymers. This project will employ such generic, particle-based, models to simulate polymer dynamics in the search for theoretical descriptions of stress correlations in polymeric liquids. The aim of this project is to extend the analysis method of viscoelasticity, devised in [6] and based upon collective spatio-temporal stress correlation functions, from glassy liquids to polymer melts. This will require generalizing the computational methods [6] and theoretical concepts [1, 4], implemented for glassy systems, to polymer melts.

The project aims to answer the following hypotheses:

1.) Given polymer liquids are isotropic, achiral, equilibrium systems can we expect the same phenomenology found previously in glasses to apply to polymer melts? In entangled melts, it should then be possible to identify residual stresses that give rise to long-ranged, angular dependent shear stress correlations similarly to those found for simple liquids close to the glass transition [1, 6, 7].

2.) The space and time dependencies of stress correlations are determined by collective viscoelastic memory functions (VMFs). To what extent can the VMFs be expressed by mean-field like, single-chain theories describing the Rouse-to-reptation crossover [2]?

3.) The VMFs determine the linear stress response of the polymer fluid to longitudinal and transverse deformations. Does this allow for a unified framework encompassing polymer density fluctuations [3] and shear relaxation?

4.) Can we harness these insights to understand viscoelastic hydrodynamic interactions [5] when entanglements emerge with increasing chain length? Does this then allow for an accurate modeling of the anomalous diffusion of the chain's center of mass in entangled melts?

Bibliography:
[1] N. Grimm, J. Baschnagel, A. N. Semenov, A. Zippelius, and M. Fuchs. Stress correlations and stress memory kernels in viscoelastic fluids. Soft Matter, 21:4256, 2025.
[2] A. E. Likhtman and T. C. B. McLeish. Quantitative theory for linear dynamics of linear entangled polymers. Macromolecules, 35:6332, 2002.
[3] M. Müller. Memory in the relaxation of a polymer density modulation. J. Chem. Phys., 156:124902, 2022.
[4] A. N. Semenov and J. Baschnagel. General relations between stress fluctuations and viscoelasticity in amorphous polymer and glass-forming systems. Polymers, 16:2336, 2024.
[5] A. N. Semenov, J. Farago, and H. Meyer. Length-scale dependent relaxation shear modulus and viscoelastic hydrodynamic interactions in polymer liquids. J. Chem. Phys., 136:244905, 2012.
[6] J. P. Wittmer, A. N. Semenov, and J. Baschnagel. Correlations of tensor field components in isotropic systems with an appli- cation to stress correlations in elastic bodies. Phys. Rev. E, 108:015002, 2023.
[7] J. P. Wittmer, A. N. Semenov, and J. Baschnagel. Strain correlation functions in isotropic elastic bodies: large wavelength limit for two-dimensional systems. Soft Matter, 19:6140, 2023.

Skills

Candidate profile:

We invite applications from candidates that hold a PhD in Physics or a related area. The successful candidate is expected to have an excellent background in soft matter physics, experience in programming (e.g., python, C++, etc.), and familiarity with LINUX. The successful candidate must have a strong interest in both theory and numerical work. Numerical work involves code development (e.g., changing the C++ LAMMPS code, programming of data analysis tools, etc.), carrying out large-scale molecular dynamics simulations with LAMMPS, and data curation. The positions can be filled at any time.

Work Context

The position, funded by the French National Research Agency (ANR), is available in the Theory and Simulation of Polymers (TSP) group at the Institut Charles Sadron (ICS) in Strasbourg, France. The ICS is a research institute of the CNRS (Centre National de la Recherche Scientifique) and associated with the University of Strasbourg (Unistra). The research activities of the ICS are concerned with the science of macromolecules and self-assembled soft matter systems at the interface between chemistry, physics and engineering. The ICS hosts eight research groups, one of which is the TSP group. All permanent members of the TSP group are involved, at various levels, in the research associated with this post-doctoral project. The position is offered for 18 months with a monthly gross salary starting at 3,021.50 euros depending on experience. The position may be extended to 24 months under certain conditions.

The position is located in a sector under the protection of scientific and technical potential (PPST), and therefore requires, in accordance with the regulations, that your arrival is authorized by the competent authority of the MESR.

Constraints and risks

None