title: Block copolymers : Nanoreactors for the synthesis of coordination polymers
Creators: name: Göbel, Christoph
Publisher: name: University of Bayreuth
Year: 2021
Subjects: subject: spin crossover
subject: nanoparticles
subject: nanocomposites
subject: composites materials
subject: functional materials
subject: coordination polymer
subject: block copolymer
subject: 540

description: This thesis aims for the synthesis and characterisation of coordination polymer (CP)/block copolymer (BCP) nanocomposites. The used coordination polymers are either based on Schiff base-like ligands with an iron(II) centre connected by bis(monodentate) bridging ligands. Alternatively, zinc(II) complexes are also reacted with bis(monodentate) bridging ligands. The diblock copolymer consists of one block polystyrene and another block poly(4-vinylpyridine) resulting in polystyrene block poly(4 vinylpyridine) (PS b P4VP). The PS-b-P4VP polymer self-assembles in suitable solvents like tetrahydrofuran or toluene to micelles where PS is building up the shell of the micelles and P4VP is forming the core. These block copolymer micelles are used as a size template for the formation of nanoparticles of the respective coordination polymer. The size of the micelle cores can be enlarged by raising the percentage of P4VP of the block copolymer. Above a certain ratio also the shape of the micelles can be altered to rods and worm-like structures. The synthesis method for the formation of nanocomposites is as follows: The block copolymer is dissolved together with the respective complex in the suitable solvent and the reaction mixture is heated to reflux. After the addition of the bridging ligand and the subsequent heating the solvent can either be removed by cold distillation or the complex and the bridging ligand can be added simultaneously up to four times. Thus, several different nanocomposites were obtained. The sizes in the solid state and in solution, the crystallinity, the composition and, additionally for iron(II)-based CPs, the spin crossover (SCO) properties were analysed. Nanocomposites of three different one-dimensional iron(II) CPs [FeL(bpea)]n@BCP, [FeL(bpee)]n@BCP, and [FeL(bpey)]n@BCP with varying bridging ligands were synthesised. Their size, magnetic, and SCO properties were investigated. Transmission electron microscopy (TEM) images and dynamic light scattering (DLS) revealed that the sizes of the nanoparticles were equal in size independent from the formed CP (TEM: ~50 nm, DLS: ~150 nm). Microcrystals were observed for some samples in TEM images. The appearance of microcrystals was explained by the stability of the CPs regarding their ligand field splitting, their electronic configuration, and the rigidity of the bridging ligands. The magnetic measurements showed that samples with microcrystals exhibit a bulk-like behaviour, whereas the nanocomposites without microcrystals undergo a gradual spin transition. In the case of the nanocomposite [FeL(bpey)]n@BCP a gradual, two-step spin transition was found whereas the bulk [FeL(bpey)]n features an abrupt, half complete spin transition with a hysteresis width of 10 K. Powder X-ray diffraction explained the variation in the spin transitions of the nanocomposite which showed a different polymorph than the bulk material. By utilising another BCP as template, the particle core size of the BCP and the nanocomposite with the CP [FeL(bipy)]n could be reduced to 15 nm and 16 nm, respectively. The magnetic properties of these smaller particles were investigated and the influence of a stepwise increase of the annealing temperature on the SCO properties was evaluated. The SCO properties of the annealed nanocomposite improved compared to the as-synthesised product. The hysteresis width was broadened from 7 K to 14 K, while shifting the spin transition from 163 K to 203 K and lowering the residual high-spin fraction at 50 K from 52 % to 32 %. The change of the magnetic properties was supported by temperature-dependent Mössbauer spectroscopy, which also detected a decrease of the residual high-spin fraction. Furthermore, temperature-dependent powder X-ray diffraction revealed that the pattern of the nanocomposite resembled the bulk pattern after annealing. The integrity of the particles after the annealing was proven by subsequent TEM, DLS, and scanning electron microscopy measurements. The processability of the nanocomposite was demonstrated by electrospinning of fibres and non-woven. The templated synthesis using BCPs is not only limited to one-dimensional iron-based CPs. This was demonstrated by the synthesis of nanocomposites with the one-dimensional [Zn(OAc)2(bipy)]n CP and the two-dimensional [Zn(TFA)2(bppa)2]n coordination network. Two different PS b P4VP BCPs were used for this approach. Nanocomposites particle core sizes of 47 nm for the [Zn(OAc)2(bipy)]n CP in only one BCP and sizes of 46 nm and 15 nm for the [Zn(TFA)2(bppa)2]n coordination network in two BCPs were achieved. TEM images revealed chain-like structures for the particles of the nanocomposites of [Zn(TFA)2(bppa)2]n in the smaller particles and a tendency to worm-like structures in the larger particles. This is supported by DLS measurements showing an increase of the hydrodynamic diameter and a broadening of the size distribution in solution. The successful formation of the CP and the coordination network was confirmed by powder X-ray diffraction, by infrared measurements supported by computational calculations, and by scanning electron microscopy images. Since BCPs are also known for their possibility to form all kinds of structures five different BCPs and their resulting nanocomposites with [FeL(bipy)]n were tested for the size and shape control. The BCPs were varied in the ratio between polystyrene and poly(4-vinylpyridine). Raising the poly(4-vinylpyridine) fraction to 61% resulted in an increase of the spherical particle core size verified by TEM, DLS, and cryo-TEM measurements. Introducing the CP into the BCPs also resulted in spherical particles when using the BCPs with poly(4-vinylpyridine) fractions up to 42 % and in worm-like structures with a fraction of 61 %. The magnetic properties of the nanocomposites were investigated regarding the particle size and shape. It was found that the abruptness of the spin transition increased in the larger particles and in the worm-like structures and that the residual high-spin fraction can be reduced to 14 % in the worm-like micelles. The spatial distribution of the iron inside the nanocomposite with worm-like structures was detected by transmission electron microscopy – energy dispersive X-ray scattering showing that iron was only incorporated into the polymeric structure.