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Crystallization of copolymers: formation of alternating periodic structures and concepts for their modification

Brahim Bessif. Inaugural-Dissertation der Fakultät für Mathematik und Physik, 2022

Soft materials have remarkable properties, for example, their unique ability to respond to external stimuli, such as changes in temperature or vapor pressure. Even weak stimuli can induce significant changes in their behavior due to the "softness" and mesoscopic structuring of the material. While softness results from weak interactions between elements of the polymer systems and their surroundings, mesoscopic and macroscopic structuring within soft materials is often the consequence of self-assembly, i.e. the spontaneous ordering of matter at a size much larger than that of the building blocks (monomers). I seek to exploit these features and design new functional crystalline structures based on multiple components, varied interfaces, combined with complex and efficient interactions of monomers. Understanding the synergy of these factors and their impact on material properties has been a major scientific challenge. Hence, the progress in this research field promises to create novel soft materials with a level of functionality similar to that found in nature which can be then used for multiple applications. Recently, food and pharmaceutical packaging materials have been developed via employing various bio-based polymers. These packaging materials partly consist of crystalline structures that can act as barriers against the diffusion of certain penetrated molecules or promote the sorption of other molecules and improve their diffusion. Therefore, governing the process of polymer crystallization through a control of interactions can be used for developing powerful strategies to arrange polymers into crystalline barriers or organize in predictable pathways for diffusion and sorption of penetrant molecules within packaging materials. Within that context, this thesis focuses on two related subjects. Using various copolymers, I explore how crystallization in solution and thin films can generate ordered arrangements of crystalline structures. For the investigations of the impact of ordered structures generated by crystallization (in part) on diffusion and sorption of small molecules penetrating polymer materials, I have chosen the well-investigated symmetrical poly-styrene-b-poly-ethylene-oxide (PS-b-PEO) diblock copolymer with equal volume fractions of PS and PEO. In solution, I show, by using a self-seeding approach and thereby controlling the nucleation density, that this diblock copolymer can form crystalline stacks of correlated lamellae. The resulting three dimensional single crystals consist of stacks of square-shaped sandwich-like PS-b-PEO lamellae, with crystalline layers of PEO and glassy layers of PS. Using stacks of many correlated lamellar crystals of PS-b-PEO diblock copolymers, I study sorption and diffusion of water in ordered thin crystalline structures confined between glassy polymer layers. By exposing these platelet-like polymeric structures to a humid atmosphere, sorption and diffusion of water, a good solvent for PEO, but a non-solvent for PS, could be investigated. Due to water-PEO interactions, a sharp front of swollen PEO advanced at a constant velocity from the edges of these stacks towards the center. The process of swelling could be reversed in a controlled way by reducing the humidity of the surrounding atmosphere. Moreover, by repeatedly changing humidity, multiple and highly reproducible cycles of swelling and deswelling can be achieved. Under swelling conditions, the platelets are approaching equilibrium shapes with squircle-like caps, which can be attributed to the balance of osmotic pressure, surface tension and bending of the glassy PS layers. An innovative photosynthesis route allows the preparation of novel polythioether materials which combine short oligo-ethylene-sulfide blocks with short oligo-ethylene-oxide blocks into copolymers. These polymers precisely link di-thiols and di-enes groups in an alternating manner, resulting in linear polymer chains with internal periodicity. For these highly regular polythioethers, I study the kinetics of their internal arrangements and interactions of their various building blocks. Strong interactions between the building blocks introduce novel possibilities for controlling nucleation and the controlled formation of different crystalline phases, characterized by different melting and crystallization temperatures, through designed thermal protocols. Intriguingly, the observed phase with a higher melting temperature cannot be formed directly from an isotropic melt state. It only can be achieved when another phase with a lower melting temperature was formed first and subsequently molten. However, these molten chains remember for some time their previous order and lateral alignment and thereby highly facilitate the nucleation of the phase with higher melting temperature. Thus, this phase can be only initiated by taking advantage of the melt memory of the phase with the lower melting temperature. In summary, within this thesis, I developed strategies to create functional crystalline structures, by exploring concepts of polymer crystallization, growth kinetics, and also the processes of molecular transport into confined polymer crystals. Functionalities of crystalline structures are arising from the building units of diblock copolymers and alternating copolymers. The functionalities of polymer systems are of very broad interest in the field of physics and chemistry of materials, well beyond the systems discussed here.

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