Polydimethylsiloxane - polyoxazoline based symmetrical block copolymers and their self-assembly

Abstract

The presented thesis reflects on methods of science and aims to discriminate between the occupation science vs. engineering. As both occupations are strongly linked and most often depend on each other, it is crucial to define the two precisely since the goal is always one or the other but the path a completely different one. Engineering is strictly goal orientated whereas the generation of new knowledge is the only valid outcome of scientific endeavour. This discrimination between science and engineering is further illustrated in the present thesis by giving examples for nano science and nano technology. Self-assembly and self-organisation are fundamental processes structuring the physical world we live in and are of key interest for nano science and nano engineering. Polymers, generally known as plastics, are used in a wide range of everyday life products and are of key interest for many technological advances. A special form of polymer is amphiphilic block copolymers here blocks with different physical and chemical properties are covalently linked together. Owing to their orthogonal properties, these blocks are not mixable and tend to segregate and thereby enable the self-assembly of three-dimensional nano-scale objects relevant for drug-design, nanoreactor development or molecular systems engineering. A library of amphiphilic block copolymers was synthesised by cationic ring opening polymerisation of 2-methyl-2-oxazoline (MOXA) on commercial poly(dimethylsiloxane) (PDMS) macroinitiators. This specific synthesis is affected by the formation of side products that negatively affect the self-assembly of the final PMOXA-b-PDMS-b-PMOXA triblock copolymers. In the course of this thesis a novel cosolvent fractionation method was developed that enables the separation of the desired triblocks from the side products. The obtained triblock copolymers were shown to self-assemble into polymersomes and enabled a fundamental study of membrane fluidity. The cosolvent fraction was further successful in obtaining narrowly dispersed macroinitiator fractions from broadly dispersed commercial PDMS products enabling the synthesis of narrowly dispersed triblock copolymers consecutively. Mechanical stability and molecular retention are key features of membranes self-assembled from PMOXA-b-PDMS-b-PMOXA. To improve these properties a pentablock copolymer was synthesised where the PDMS block is flanked by a few 2-phenyl-2-oxazoline (PhOXA) units, forming an additional aromatic barrier. The self-assembly of the pentablock did not yield the desired polymersomes but polymer-beads with internal nanoscale structuring. The self-assembly of polymersomes is achieved by many methods all with their unique disadvantages and advantages. The challenge of developing a more universal method was undertaken and resulted in a novel method for the self-assembly of PMOXA-b-PDMS-b-PMOXA-based polymersomes. The encapsulation of a model compound was shown to be reproducible, the method allows for large-scale polymersome assembly, yields complete rehydration of the polymer and the precursor is stable for at least a year under ambient conditions. The top fractions yielded by the cosolvent fractionation were found to be of a multiblock copolymer composition. Their bulk self-assembly was studied and compared with that of triblocks, revealing distinctly different nano-scale organisations. While the triblocks exhibited random organisation, the multiblocks were found to assemble in a ball-like manner with hexagonal symmetries. A model for the path of their self-assembly along with a reflection about the possible driving forces is provided. Electro-spray deposition gave access to films of ca. one cm2 which were populated with pillar-like structures. Light diffraction on these soft nano-scale structures was observed under their diffuse illumination.