Methacrylate-based amphiphilic block copolymers in solution and at surfaces : synthesis, characterization and self-assembly

Part 1. Introduction
General concept of polymer self-assembly, synthesis of amphiphilic block copolymers and their application in biotechnology are briefly presented. Special attention is given to the principles of atom transfer radical polymerization (ATRP) and preparation of solid-supported amphiphilic copolymer membranes. Scope of the thesis and the contribution to the current knowledge in the field are presented.
Part 2. Self-organization behavior of methacrylate-based amphiphilic di- and triblock copolymers
ATRP synthesis of amphiphilic di- and triblock copolymers having different hydrophilic-to-hydrophobic block length ratio is described. The investigation of self-assembly of these AB and ABA block copolymers consisting of poly n-butyl methacrylate (B) and poly 2,2-dimethylaminoethyl methacrylate (A) using combination of DLS, NS-TEM, cryo-EM, and AFM is presented and discussed.
Two populations of self-organized structures in aqueous solution, micelles and compound micelles, were detected for diblock copolymers. Triblock copolymers assembled into vesicular structures of uniform sizes. Furthermore it was found that these vesicles tended to compensate the high curvature by additional organization of the polymer chains outside of the membrane. The chain hydrophilicity of the polymers appeared to have a critical impact on the self-assembly response towards temperature change. The self-reorganization of the polymers at different temperatures and its mechanism are revealed.
Part 3. Solid supported block copolymer membranes through interfacial adsorption of charged block copolymer vesicles
The properties of amphiphilic block copolymer membranes make them promising candidates for the development of new (bio-) sensors based on solid-supported biomimetic structures. Here we investigated the interfacial adsorption of polyelectrolyte vesicles on three different model substrates to find the optimum conditions for the formation of planar membranes. The polymer vesicles were obtained and characterized as described in part 2. We observed reorganization of the amphiphilic copolymer chains from vesicular structures into a 1.5±0.04
nm thick layer on the hydrophobic HOPG surface. However, this film starts disrupting and ‘dewetting’ upon drying. In contrast, adsorption of the vesicles on the negatively charged SiO2 and mica substrates induced vesicle fusion and the formation of planar, supported block copolymer films. This process seems to be controlled by the surface charge density of the substrate and the concentration of the block copolymers in solution. The thickness of the copolymer membrane on mica was comparable to the thickness of phospholipids bilayers.
Part 4. Functionalization of gold and silicon surfaces by copolymer brushes using surface-initiated ATRP
To further develop the solid-supported polymer membranes with improved stability and control over the membrane formation, we applied surface-initiated ATRP to grow step-by-step the poly (n-butyl methacrylate)-co-poly(2-dimethylaminoethyl methacrylate) (PBMA-co-PDMAEMA) brushes from gold and silicon substrates. Two different approaches for the initiator immobilization on surfaces were tested to find optimal conditions for the reaction. The polymer brushes were characterized in situ by contact angle measurements, ellipsometry, and XPS. Detachment of the polymer brushes from both substrates allowed an exact determination of molecular weight and polydispersity indexes given by GPC. 1H NMR confirmed the chemical structure of the detached brushes. We used microcontact printing for the structuring of the surface by copolymer brushes.
Part 5. Grafting and characterization of the amphiphilic triblock copolymer membranes from gold supports
Based on the previous experience with the growth of diblock copolymer chains from surfaces and optimized conditions for initiator immobilization (part 4), we continued the developing of the solid-supported copolymer membranes maximally mimicking the structure of biological membrane. Hence, amphiphilic triblock copolymer brushes composed of hydrophilic poly(2-hydroxyethyl methacrylate) (PHEMA) blocks and a hydrophobic poly(n-butyl methacrylate) (PBMA) middle part were synthesized using a surface-initiated ATRP. ATR-FTIR, PM-IRRAS, ellipsometry, contact angle measurements and AFM were used for the characterization of PHEMA-co-PBMA-co-PHEMA brushes. Additionally, a detachment of the polymer membranes from the solid support and subsequent GPC analyses allowed us to establish their compositions. Treatment of the amphiphilic brushes with block selective solvents led to reversible changes in the polymer surface topography. The PM-IRRAS analysis revealed an increase of the chain tilt towards the gold surface during its growth. It was suggested that the orientation of the
amphiphilic polymer brushes is influenced mainly by the chain lengths and interchain interactions. The presented results could serve as a good starting point for the fabrication of functional solid-supported membranes for biosensing application.
Part 6. Conclusions and Outlook
In this section the achievements of the research work are discussed. Further improvements and applications are proposed.