Functional Nanostructures from well-defined Block Copolymers: Stimuli-Responsive Membranes, Multicompartment Micelles, and Interpolyelectrolyte Complexes
Felix H. Schacher (07/2006-02/2009)
Support: Axel H. E. Müller
This work describes the synthesis of different stimuli-responsive block copolymers of the AB- and ABC-type, their characterization, and their self-assembly in the bulk and in solution. Sequential anionic polymerization was used to obtain well-defined polymeric materials, which were utilized for the generation of functional nanostructures, e.g. for membrane applications.
In a first approach, five series of polybutadiene-block-poly(2-vinylpyridine)-block-poly(tert-butyl methacrylate) (BVT) block terpolymers were synthesized and their behavior in thin-films on substrates with different wettabilities was investigated. The aim was to generate microphase-separated structures which could serve as precursors for the fabrication of stimuli-responsive composite membranes. Thin-films were prepared via spin-casting and the self-assembly processes were facilitated through subsequent solvent annealing. Although no smart membrane could be prepared and tested, model systems were intensively studied and in-depth knowledge could be gained concerning the morphological behavior of BVT terpolymers under such conditions. The results in thin-films were always compared with the bulk studies.
Using a different strategy, smart ultrafiltration membranes could be fabricated via non-solvent induced phase separation (NIPS) processes. Amphiphilic polystyrene-block-poly(N,N-dimethylaminoethyl methacrylate) (PS-b-PDMAEMA) diblock copolymers were synthesized and cast onto planar glass substrates with a doctor blade. After final film formation in the coagulation bath, asymmetric membranes with tunable water flux and pore sizes were obtained. PS formed the matrix of these materials, while PDMAEMA was covering the pore walls. The pH- and temperature-responsive properties of those systems were attributed to the hydrophilic PDMAEMA segments. After initial characterization, the influence of several important parameters during the casting process onto the membrane morphology and permeability was thoroughly investigated: the solvent composition, the casted film height, the open-time, and the PDMAEMA content of the block copolymers.
Besides PS-b-PDMAEMA, several diblock copolymers with PDMAEMA as second block were also synthesized: PB-b-PDMAEMA, poly(tert-butoxystyrene)-block-poly(N,N-dimethylaminoethyl methacrylate) (PtBS-b-PDMAEMA), and PEO-b-PDMAEMA. For the latter, a novel one-pot strategy could be successfully employed, providing a facile changeover from an oxyanion to a carbanion. The kinetics of the DMAEMA polymerizations were monitored with a NIR probe and it could be shown that the reactions proceed considerably slower in presence of the tBU4-phosphazene base compared to polymerizations performed with an excess of alkoxides.
In a third approach, the self-assembly of BVT terpolymers in solution was explored. Narrowly dispersed micelles with a patchy core were formed in acetone, a selective solvent for polybutadiene. The micelles exhibited a PB core, a non-continuous P2VP shell, and a PtBMA corona. The micellar core was then crosslinked via different methods, enabling the transfer of such polymeric colloids into non-selective solvents, like dioxane, while still preserving their structure and shape.
Finally, polymer analogous reactions were performed with the BVT terpolymers. After hydrolysis of the PtBMA block to PMAA and, eventually, quaternization of the middle block, P2VP, amphiphilic block terpolymers with either one or two pH-responsive segments were obtained. Their aggregation behavior in aqueous systems, depending on salinity and pH, was studied. Micelles with a soft PB core, a P2VP shell and a PMAA corona were formed. It could be shown, that under certain conditions intra-micellar interpolyelectrolyte complexes (IPECs) were formed, generating multicompartment micelles with a patchy shell. Furthermore, the IPEC formation of those systems with oppositely charged double hydrophilic poly(N-methyl-2-vinylpyridinium)-block-poly(ethylene oxide) (P2VPq-b-PEO) diblock copolymers was investigated. In that way, a second IPEC shell could be formed through the electrostatically driven co-assembly of PMAA and P2VPq. PEO served as the new corona of the resulting colloidal structures. The time-dependent evolution of such systems was studied and intermediate, star-like, structures could be identified. Furthermore, the selective incorporation of in-situ generated Au-Nanoparticles inside the IPECs could be demonstrated.