title: Surface-Compartmentalized worm-like Crystalline-Core Micelles: Preparation and Applications
Creators: name: Hils, Christian
Publisher: name: University of Bayreuth
Year: 2022
Subjects: subject: patchy micelles
subject: CDSA
subject: triblock terpolymers
subject: 540

description: Crystallization-driven self-assembly (CDSA) of block copolymers with one crystallizable block has emerged as an outstanding method for the preparation of worm-like crystalline-core micelles (wCCMs) with tailored properties. The use of ABC triblock terpolymers with a semi-crystalline core-forming polyethylene (PE) middle block and incompatible end blocks of polystyrene (PS) and poly(methyl methacrylate) (PMMA) or poly(N,N-dialkylaminoethyl methacrylamides) (PDxA, alkyl = methyl, iso-propyl) leads to a patch-like microphase-separated corona, resulting in so-called patchy wCCMs. These micelles are characterized by an excellent interfacial activity and can be used as efficient templates for the embedding of nanoparticles or as hybrids in heterogeneous catalysis due to their functional corona patches. Based on these highly promising properties, this doctoral thesis addressed the preparation as well as functionalization of surface-compartmentalized micelles with a semi-crystalline PE core, with the aim to exploit new applications for these interesting materials. First, the influence of patch size and chemistry on the catalytic activity of gold nanoparticle (AuNP) loaded patchy hybrid nonwovens in the alcoholysis of silanes was studied. The patchy hybrid nonwovens were produced by coaxial electrospinning, employing PS as core and patchy wCCMs with functional PDxA patches in the corona, followed by loading with preformed citrate-stabilized AuNPs. Here, the accessibility of the patches for the reactants, i.e., its swellability in the employed solvent (n-butanol), turned out as the most decisive factor, resulting in a pronounced induction period and longer reaction times in the first catalysis cycles and for the more polar PDMA patches, respectively. Besides, even after 10 consecutive cycles no loss of catalytical activity occurred, demonstrating the excellent reusability of this catalysis system. Moreover, solubility studies on PDxA homopolymers (x = methyl, ethyl or iso-propyl) in aqueous media of different pH revealed a double responsive behaviour (pH and temperature) that can be tuned by varying the alkyl substituents at the tertiary amino-groups. While PDMA is soluble over a broad pH and temperature range, PDEA and PDiPA show a pH dependent LCST (lower critical solution temperature)-type phase behaviour. Solubility can also be switched reversibly by CO2, and multivalent counter anions can induce a simultaneous LCST- and UCST (upper critical solution temperature)-type behaviour upon proper choice of pH. Notably, the investigated PDxA homopolymers were produced by post polymerization amidation of PMMA and can even be employed on bulk PMMA samples for surface functionalization, showing that this approach is widely applicable to PMMA and respective block copolymers to introduce stimuli-responsivity in a highly efficient and facile way. A new concept for the construction of hierarchical superstructures was developed, harnessing the well-defined patch functionality (tertiary amino-groups) of patchy fibres produced by coaxial electrospinning. The functional patches can initiate the molecular self-assembly of a benzene-1,3,5-tricarboxamide (BTA) from the surface of the fibres to yield a fir tree-like superstructure, where the patchy fibres form the core (branch) and the BTA assemblies the respective needles. A prerequisite for the formation of defined superstructures by this combination of CDSA with molecular self-assembly is the chemical match between the tertiary amino-groups in the surface patches of the fibres and the periphery of the BTA. Amphiphilic worm-like Janus micelles with a superior interfacial activity could be realised by CDSA using a PS-b-PE-b-PEO (PEO = poly(ethylene oxide)) triblock terpolymer. Here, the strong incompatibility of the two end blocks combined with the ability of the PEO corona to crystallize is the key for the formation of a Janus-type corona instead of the entropically more favoured patchy structure. The presence of a Janus-type corona was proven by transmission electron microscopy studies using staining agents of different selectivity with respect to the PS and PEO corona blocks. In addition, the Janus micelles showed a distinct different morphology compared to that of wCCMs with a patchy PS/PMMA corona after dialysis in a non-solvent for the PS blocks. Finally, CDSA of PS-b-PE-b-PMMA triblock terpolymers in the confinement of emulsion droplets (toluene in water) followed by solvent evaporation opens a new way toward internally structured microparticles. Controlling the interplay between CDSA and microphase separation allows to direct self-assembly to yield different well-defined morphologies. If CDSA to patchy wCCMs occurs simultaneously to solvent evaporation, lenticular microparticles with a compart¬mentalized hexagonal cylinder lattice were formed. If, however, solvent evaporation takes place first at elevated temperatures (CDSA is hindered), differently oriented lamellar morphologies (e.g., bud-like, double staircase, spherocone) can arise because of microphase separation taking place prior to crystallization. Hence, upon cooling, the PE block can only crystallize within the confinement of the lamellar morphology, being preformed upon solvent evaporation. This doctoral thesis shows the enormous potential of CDSA for the preparation of patchy and Janus micelles and further fields of application for these surface-compartmentalized micelles could be opened. Additionally, other exciting applications are conceivable, as will be explained in the chapter outlook.