Bottom-up Self-Assembly across Hierarchies: From Triblock Terpolymers to Patchy Particles to Colloidal (Co-)Polymers
André H. Gröschel (12/2012)
Support: Axel H. E. Müller
In this work the bottom-up self-assembly of compartmentalized particles on multiple hierarchies was investigated. ABC triblock terpolymers were directed as basic build-ing blocks into nano-scale corona-compartmentalized (patchy) particles via selection of kinetic self-assembly pathways. An extremely efficient and versatile step-wise self-assembly process was developed offering unique nano-engineering capabilities over addressable corona patches. Thereby, carefully chosen solvent sequences were of outmost importance. Depending on the volume ratio of the core forming blocks, VA/VB, two species with different geometrical distribution of the patches were identified: for VA/VB > 1, a Janus-like AB distribution, with patches A and C emanating from opposing sides of the B core and for VA/VB < 1 an ABA distribution, with two A patches on opposing sides of the B core protected by an equatorial C corona. The par-ticles were then used as colloidal building blocks (CBBs) that, upon addition of non-solvent for A, underwent next level hierarchical self-assembly. The AB CBBs self-assemble into spherical multicompartment micelles (MCMs) with precise control over aggregation number (VA/VB). In contrast, ABA CBBs grow into extended linear colloidal polymers of up to several micrometres in length via a step-growth polymerization process. The cluster size (AB)x and the worm length [ABA]m are both conveniently controlled by the solvent quality for the corona block (expansion/contraction). This dynamic tuning of the corona volume is a unique key feature of the bottom-up approach to soft patchy nanoparticles from triblock terpolymers. In a consecutive work, the AB and ABA CBBs were mixed in specific ratios prior to self-assembly by addition of non-solvent for A. With both CBBs present, aggregation by mutual interaction of A patches into mixed colloidal co-assemblies was accomplished. Colloidal co-assembly is a hierarchical structuring process crossing multiple hierarchies primarily driven by the minimization of interfacial energies. It critically depends on both the dynamic volumes change of the corona and of the aggregating patches with changing solvent polarity. The extraordinary quality of the superstructures is ascribed to the selection of kinetic pathways for co-assembly and similarly, to the dynamic tailoring of patch volume. Particles with a large C corona, but small attractive A patch are stable over broad a range of solvent compositions. On the contrary, particles with a small C corona, but large attractive A patch start to cluster even with at low contents of non-solvent for A. Hence, the mismatch of onset of self-assembly is a set screw to either form the colloidal “substrate” in the first step and decorate subsequently or vice versa. Both approaches lead to well-defined and predictable mixed colloidal co-assemblies comprising colloidal molecules, multiblock co-assemblies, telechelic oligomers, ternary co-assemblies and two-dimensional networks, all of which are exclusively accessible with the presented approach. The spherical MCM consist of AB CBBs with a B-core with a Janus-like distribution of the A and C blocks as a result of symmetry breaking during cluster formation in non-solvents for A. This phase separation within MCMs represents a novel and versatile route for the template-free synthesis of terpolymer-based, sub 100 nm Janus particles. The synthesis encompasses facile cross-linking of the patches of spherical MCMs to preserve the phase-separated state. This approach yields narrowly dispersed Janus micelles and offers unique options to nano-engineer core diameter, the Janus balance (volume ratio of A and C hemispheres) and the chemistry of the patches. Homogeneous populations of MCMs even at very high concentrations of 100 g/L enable high throughput synthesis of soft Janus micelles making this novel approach technologically relevant. Beyond that, the Janus balance proved decisive for cluster shape and size when the particles were subjected to a selective solvent for either of the hemispheres. The Janus particles (JPs) with tailored Janus balance were finally applied as disper-sants for multi-walled carbon nanotubes (MWNTs). Thereby, the JPs attach to the tube surface with a suitable hydrophobic patch (polystyrene), while facilitating stabilization in the solvent with the other. Depending on the Janus balance, i.e., the size ratio of adsorbing to stabilizing patch, dense multilayer coatings were obtained or helical arrangements with defined JP-JP interparticle spacing. In both cases, the quantity of attached JPs was substantial and unparalleled. Besides the known applications of JPs in emulsion polymerization and as compatibilizers in polymer blends, JPs proved also effective as non-covalent supracolloidal dispersants for MWNTs and may also find application as general dispersant for other insoluble particulate matter.