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In-situ Spectroscopic Investigations of High Energy Li-S Batteries Based on New Carbon Cathodes

Subject Area Physical Chemistry of Solids and Surfaces, Material Characterisation
Polymer Materials
Preparatory and Physical Chemistry of Polymers
Term from 2015 to 2019
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 273723695
 
Final Report Year 2019

Final Report Abstract

The ultimate goal of this project was the preparation and investigation of nanoparticle/block copolymer (NP/BCP) composites consisting of NP hierarchical superstructures confined inside BCP domains. In particular, the project focused on investigation of the process superstructure formation under soft BCP confinement and understanding the effect of various experimental parameters on the characteristic of these superstructures. The project was subdivided into experimental and theoretical parts. Synergetic experimental and theoretical studies allowed gaining information about the effects of intrinsic properties and experimental conditions on NP/BCP co-assembly process, and elucidating possible mechanism of NP superstructure formation. Series of Au and Ag NP of different core size were synthesized using common wet chemistry methods and functionalized with polystyrene (PS) ligands to make them compatible with the same block of poly(styrene)-block-poly(4-vinylpyridine) (PS-b-P4VP) BCPs. Block copolymer of varied block ratio and molecular weight were used. The cooperative selfassembly process of PS-coated NPs and cylinder-forming PS-b-P4VP BCP comprising PS as minority block was investigated in details. Both experimental and modelling results demonstrate that NP/BCP co-assembly process, which takes place during solvent evaporation step, leads to densely packed NP-filaments embedded in cylinder-forming PS block. The resulting NP packing morphology is determined by the synergy of confinement strength, solvent selectivity and NP surface characteristic. The process of NP/BCP co-assembly involves initial entrapment of NP in micellar aggregates formed by the BCP. These micellar aggregates are formed due to the higher selectivity of the solvent towards P4VP block. Subsequently, these micellar aggregates coalesce to result in 3D ordered structure of block copolymers where the closely packed NP are localized in the PS cylindrical domains. The morphology of closely packed NP superstructure depend on the ratio between BCP cylindrical domain size and NP size (DC/DNP). As the DC/DNP ratio increases, the number of NP layers normal to the cylinder axis also increased. However, the packing density and the regularity of the NP structures decreased at higher DC/DNP. The experimental results were further supported by the results obtained from MD simulation. The simulation results reveal an increase of DC/DNP ratio with decreasing size of the NP. The simulation also revealed that for smaller NP block chains could interpenetrate in the space between the particles, plausibly to gain conformational entropy. Both experimental and simulation results indicate that a high fraction of NP inside cylindrical copolymer structures can be obtained during solvent evaporation. Two other types of NP/BCP systems studied were CdSe/PS-b-P4VP composites comprising CdSe with distinct affinity towards P4VP phase. In case of PS-b-P4VP with minority P4VP block, cylindrical morphology of pure BCP was preserved also for CdSe/PS-b-P4VP mixture, and QDs were localized at PS/P4VP interface. In case of PS-b-P4VP with minority PS block, transition from cylindrical to lamellar morphology took place upon addition of QDs being attribute to the displacement and migration of TOPO ligands from the surface of QDs to the PS phase. The general goal of the theoretical part of the project was to recognize parameters and conditions which allow to obtain nonspherical nanoparticle assemblies. The major focus was to investigate the role of interaction of nanoparticles with: 1) asymmetric block copolymer matrix with short block being attractive with respect to nanoparticles (soft confinement); 2) physical confinement in a form of cylindrical nanochannel (strong confinement). For this purpose we have used extensive molecular dynamics simulation of the coarse-grained model for nanoparticles and polymer monomers. We showed that the breaking of the spherical symmetry of the assemblies can be achieved by introducing the block copolymers. We identified that transition from spherical to cylindrical assemblies can be tuned by the size of nanoparticles, nanoparticle volume fraction in the system and the strength of interaction between short blocks and nanoparticles. We have also demonstrated that strong confinement effect provided by cylindrical pore induces helical and spiral arrangement of nanoparticles. In the latter case the key physical parameter that controls the degree of helical packing is the ration between the pore size to the nanoparticle size.

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