PhD Thesis
Exploring the Structural Complexity of Compartmentalized Nanostructures of Triblock Terpolymers
Tina Löbling (01/2012-07/2015)
Support: Axel H. E. Müller
Within this work, complex compartmentalized nanostructures were created via the self-assembly of linear ABC triblock terpolymers in selective solvents. Thoroughly chosen block sequences and lengths combined with precisely tuned self-assembly pathways enables the controlled formation of novel compartmentalized nanoparticles with so far unprecedented complexity. This thesis is divided in three main topics. Two of them emphasize on fabrication of complex nanostructures and comprehensive characterization to elucidate underlying formation mechanisms, while the third subject deals with synthesis and application of nanoparticles in industrial relevant amounts.
The first topic within this thesis is the fabrication of complex patchy nanoparticles via a combination of solution-based self-assembly of a charged amphiphilic ABC triblock terpolymer followed by interpolyelectrolyte complexation with oppositely charged polymers. The ABC triblock terpolymer with a hydrophobic A, a polycationic B and a polyanionic C block self-assembles in aqueous solution into multicompartment micelles (MCMs) with a B core, compartments of an intra-micellar interpolyelectrolyte complex (IPEC) between B and C while excess C forms the stabilizing corona. These polyanionic MCMs were used as precursor micelles for complexation reaction with polycationic homopolymers as well as bishydrophilic diblock copolymers. Instead of forming a continuous IPEC shell, addition of a polycationic homopolymer forms distinct IPEC-patches on top of the micellar core when complexed with the negative C chains. This behavior was ascribed to the fact that long corona chains allow the IPEC to form thick patches lowering the total interfacial area towards the micellar core compared to a continuous shell. An unfavorable interface with the aqueous medium is thereby prevented through the long corona of the precursor micelles. In case of polycationic bishydrophilic diblock copolymers, carefully chosen block length mismatch between the negatively charged corona chains and the polycationic block results in a dense packing of the ionically grafted diblock copolymer chains within the corona of the precursor MCMs. The collapse of newly formed hydrophobic IPEC from solution is hindered due to steric stabilization through the water-swollen but non-ionic part of the diblock copolymer. Cryogenic transmission electron tomography and computational 3D reconstruction reveal novel “sea-urchin” and “paddle-wheel” like IPEC structures.
The formation of core-compartmentalized nanoparticles through hierarchical self-assembly of linear ABC triblock terpolymers was the second main aspect within this thesis. The structuring concept is based on consecutive precipitation of the two hydrophobic blocks. This method, developed in our group to form spherical MCMs, was expanded in this work towards other particle geometries as well as patch morphologies. On the basis of diblock copolymer solution morphologies, ranging from spherical micelles to cylinders, sheets and vesicles, a synthetic library of terpolymers was established with proper choice of corona block length. The investigated terpolymer system was polystyrene-block-polybutadiene-block-poly(tert-butyl methacrylate) (SBT) and the final solvent mixture for the self-assembly was acetone/isopropanol. This solvent mixture is a good solvent for the T corona over all solvent compositions, while it is a non-solvent for B and S. It was proven that S is plasticized to a certain extend by acetone allowing sufficient chain rearrangement preventing kinetically trapped structures. Spherical MCMs, compartmentalized cylinders, sheets and vesicles were achieved with decreasing the T corona block length and/or swelling of the S block with acetone. Following the theory of block copolymer self-assembly in bulk, we hypothesized that the S and B blocks phase-separate analogously within the confined geometry of the particle core. The library of SBT triblock terpolymers was systematically expanded through synthetic variation of the S/B volume ratio, while adjusting the T corona to gain different particle geometries. A myriad of complex compartmentalized superstructures was studied with so far unprecedented detail applying transmission electron tomography to reveal the internal phase separation of the hydrophobic blocks in the final aggregate. Besides rather simple sphere-on-sphere and core-shell structures, complex helix-on-cylinder and unprecedented cylinder-on-sheet, cylinder-on-vesicle as well as sheets and vesicles with a bicontinuous membrane were realized. A consecutive work deals with parameters influencing the resultant compartmentalized nanostructures apart from synthetic encoding block volume fractions. Tuning the solvent conditions allows swelling or contraction of core and corona blocks and induces morphological transitions from spherical MCMs towards vesicles including cylinders and sheets as intermediate states. These transitions were also obtained through controlled blending with solvophobic homopolymers or post-modification of the B block. To complete the systematic investigation of as synthesized SBT triblock terpolymers, the phase behavior in bulk was investigated and a phase diagram established. The bulk morphologies comprise core-shell cylinder, lamella-lamella, core-shell gyroid and a rarely observed cylinder-in-lamella morphology.
Within the third section, SBM Janus particles were obtained through crosslinking of the B compartments and subsequent dissolution of spherical MCMs. The Janus particle synthesis was scaled up to fabricate 100 g of Janus particles per batch. They were used as efficient compatibilizers for immiscible polymer blends under technologically relevant conditions using industry scale blending equipment. The blend morphology was changed from a co-continuous to small droplet morphology upon compatibilization with Janus particles. The Pickering effect contributes to the particle adsorption at the interface between the two blend components and sufficient droplet stabilization was achieved between a content of 2-5 wt.-% of Janus particles within the blend mixture.