Free-energy landscapes of semiflexible theta-polymer aggregation with and without external force

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The main objective of this project is to explore universal aspects of semiflexible polymer aggregation with and without external force. The focus will be on generic structural properties rooted in statistical physics and free-energy landscapes of grafted semiflexible polymers subject to an external force. The analysis will rely on numerical data obtained with carefully tailored multicanonical computer simulations.We aim to investigate finite-size effects of semiflexible polymer aggregation in periodic boundary conditions and apply finite-size scaling theory to draw universal conclusions. Emphasis will be laid on the free-energy landscape, especially characterizing the scaling of the free-energy barrier with chain number, in dependence on the stiffness of the polymers. This approach in the mesoscopic range should allow us to distinguish different aggregate morphologies and may be used as a protocol for phase transitions with mixed phases, in general. Since the aggregation transition separatesenergy-dominated compact structures from an entropy-dominated soluble regime, this demands systems at low density and sophisticated simulation methods that allow efficient sampling of both energy and entropy contributions as well as sophisticated analysis tools to disentangle them.Motivated by experimental realizations of polymer bundles under force, we plan to study the conformational properties of grafted semiflexible polymers with and without pulling force. Determining the free-energy landscape and the associated free-energy barrier will allow the characterization of the involved transitions from disordered to amorphous or bundle (tower) phases. With an external force, wewill focus on rather flexible polymers where we expect an intermediate force regime in which the competition between self-attraction, mutual attraction and pulling force should give rise to bundles with a helical twist. In this case, we want to test if the worm-like chain approximation (which is well-applicable to rather stiff polymer bundles) becomes valid for flexible polymer bundles underexternal force.This will be extended for a few grafted polymers to a setup with an external field where we aim to study the free-energy landscape as a function of both temperature and field. Considering also alternating fields allows a glimpse at steady-state properties, where we want to identify relevant length scalesthrough finite-size scaling. Attaching extended geometric objects to the non-grafted polymer ends introduces additional constraints and may lead to interesting and promising experimental setups.