PhD Thesis
Self-Organized Nanostructures from Stimuli-Responsive Triblock Terpolymers
Eva Betthausen (02/2010-07/2014)
Support: Axel H. E. Müller
This work describes the self-assembly of novel stimuli-responsive ABC triblock terpolymers into functional nanostructures. Sequential living anionic polymerization was used to synthesize well-defined triblock terpolymers that offer various possibilities for further modifications. This allows precise tailoring of the properties of each block and generates a multi-responsive system. Different assembly strategies in solution and in the bulk were applied to tune the architecture and functionality of the resulting particles or nanostructured films. This enabled the preparation of a plethora of complex and, in part, compartmentalized micellar architectures that demonstrated potential for various applications, such as switchable surface coatings and non-viral gene delivery.
The triblock terpolymers employed in all studies are polybutadiene-block-poly(tert-butyl methacrylate)-block-poly(2-(dimethylamino)ethyl methacrylate) (PB-b-PtBMA-b-PDMAEMA, BTD), which feature pH- and temperature-responsive properties. Using polymer-analogous modification reactions, such as ester hydrolysis of PtBMA to poly(methacrylic acid) (PMAA) or quaternization of PDMAEMA to PDMAEMAq, terpolymers with weak or strong polyelectrolyte blocks were generated. Consequently, choosing the appropriate preparation pathway allowed control over the size, architecture, and charge of the micellar structures formed in aqueous solution. The presence of two oppositely charged polyelectrolyte blocks in the modified terpolymer enabled the formation of intramicellar interpolyelectrolyte complexes (im-IPECs) in the shell of the micelles, yielding multicompartment or core-shell-corona micelles. In addition, the micelles can undergo rearrangements in both the shell and the corona in response to external stimuli, such as pH, temperature, or salinity.
The charged PB-b-PMAA-b-PDMAEMAq core-shell-corona micelles were used as colloidal templates for the build-up of multi-layered micellar particles. In alkaline media, complexation with oppositely charged homopolymers or diblock copolymers led to the formation of a second IPEC shell within the micelles. By complexation with bis-hydrophilic poly(acrylic acid)-block-poly(N-isopropylacrylamide) (PAA-b-PNIPAAm) diblock copolymers, the micellar IPECs were equipped with a PNIPAAm corona, yielding thermoresponsive particles. Upon IPEC formation the precursor micelles showed dynamic rearrangements due to the soft PB core. The dynamic processes were suppressed by crosslinking of the core, allowing the formation of uniform IPEC particles.
Aiming at a potential application in switchable coatings, the pH-responsive behavior of the core-shell-corona micelles was studied after immobilization on surfaces. The micelles, deposited as a monolayer on silica surfaces, showed two types of response to changes in pH. For pH cycling on a short time scale, reversible changes in the micellar morphology and composition comparable to their behavior in solution were observed. Long-term exposure to acidic solutions caused irreversible morphological changes, resulting in an exposure of the PB core. The stability of the micelles was retained via core crosslinking, yielding robust building blocks for the design of switchable surfaces.
Further, a novel strategy for the solution assembly of BTD triblock terpolymers mediated by additives was established. Co-assembly of the terpolymers with organic multifunctional acids in mixtures of THF and water enabled the preparation of rarely found non-spherical structures, such as ribbons and undulated ribbons. Tunable electrostatic interactions between the multiacids and the corona-forming PDMAEMA chains provided control over the interfacial curvature of the aggregates and thus the micellar morphology. The solvent quality, the PDMAEMA block length, as well as the chain architecture, amount and functionality of the added multiacid were identified as major set screws. In a systematic investigation of all parameters involved, a general concept for morphology evolution within this particular terpolymer/multiacid system was established.
For the preparation of anisotropic solution structures via bulk templating, the self-assembly of the BTD terpolymers in the bulk was investigated. By blending a BTD terpolymer with different amounts and mixtures of PtBMA and PDMAEMA homopolymers, a variety of bulk morphologies was prepared based on one single terpolymer. Rarely found structures, such as tetragonally perforated lamellae and double gyroid morphologies, were generated. Selective crosslinking of the PB domains of the bulk structures followed by sonication-assisted dispersion in non-selective solvents allowed the preparation of well-defined nanostructures. Crosslinked nanoporous sheets featuring highly regular pores were obtained in aqueous solution. These sheets are promising materials for the construction of nanoporous membranes with pH- and temperature-tunable permeability.