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Phase behavior and structure of semiflexible polymers in spherical confinement

Subject Area Experimental and Theoretical Physics of Polymers
Term from 2014 to 2023
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 261177998
 
Final Report Year 2025

Final Report Abstract

In this project, we used a combination of Density Functional Theory (DFT) and large scale Molecular Dynamics (MD) simulations to study the structure and dynamics of semiflexible polymers. In particular, we focused on mixtures of semiflexible polymers and confined systems, with the aim of understanding the interplay between the conformation of semiflexible macromolecules in solution and their phase behavior under geometric constraints imposed by planar, spherical or cylindrical walls. Altogether, our efforts have resulted in 19 publications in international peer-reviewed journals or book chapters. We began by studying the bulk behavior of semiflexible chains, identifying transitions between isotropic, nematic and smectic phases. Notably, we showed that while the nematic-smectic transition is continuous, the transition from the smectic phase to the crystalline structure with true 3D long-range order is clearly firstorder. For nematic blends of semiflexible polymers, we demonstrated with MD and DFT that macromolecules with the same degree of polymerization N but different stiffnesses ℓp separate into two different nematic phases, depending on the pressure. We also observed isotropic-nematic demixing when the stiffness disparity was sufficiently large. These transitions are completely entropy-driven and highlight the complex interplay between translational, rotational and mixing entropy contributions to the free energy. Moreover, we explored the impact of chain stiffness on the adsorption transition, demonstrating that the critical adsorption strength scales as ℓp^−1/3. Further, we found that partially adsorbed chains (with free non-adsorbed “tails”, surface-attached “trains”, and intermittent “loops”) are not well-described by the Kratky-Porod wormlike chain model. Our results regarding adsorption on cylindrical surfaces further elucidated the competing effects of pore surface curvature and chain stiffness. Another goal of our research was to understand the role of geometric constraints on the structure and phase behavior of semiflexible polymers. Among several results, we highlight the behavior of a binary blend of chains with identical length but different stiffnesses, confined within a sphere or cylinder with repulsive walls. Here, a subtle interplay was observed between wall repulsion, entropic mixing, and packing effects, leading to confinement-induced separation of the two species. In an extension of these investigations, we analyzed the behavior of a inwardly-grafted polymer brushes in spherical capsules to test earlier theoretical predictions on concave brushes, revealing new scaling relationships. Notably, we found that flexible concave brushes exhibit a negative surface tension γ, whereas γ changes sign in semiflexible concave brushes.

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