Supracolloidal co-assemblies of microgels based on orthogonal host-guest interactions

• Suprakolloidale Co-Assemblierungen von Mikrogelen basierend auf orthogonaler Wirt-Gast-Wechselwirkungen

Han, Kang; Möller, Martin (Thesis advisor); Plamper, Felix Alois (Thesis advisor)

Aachen (2017)
Dissertation / PhD Thesis

Dissertation, RWTH Aachen University, 2017

Abstract

Colloids are valuable model systems to understand structure and dynamics of matter, explore new self-assembly concepts, and realize advanced materials. A key to simulate complex colloidal systems is to furnish colloids with defined, orthogonal, and switchable interactions beyond classical electrostatic stabilization. Microgels as a special class of colloids are ideal models to investigate the relationship of the interparticle interactions because of their stable swollen states in good solvents. The major body in microgel self-assembly focuses on harnessing ionic interactions and self-assemblies based on single components. In fact, there are very few studies about hetero-aggregation or co-assembling microgel systems, and they mostly focus on simple cationic/anionic microgel partners. Such ionic interactions yet offer only limited selectivity in multicomponent systems, and also limited pathways of switchability. On the other hand, supramolecular chemistry is highly developed and supplies a range of hetero or self-complementary interactions up to the level of DNA programming. Few attempts have been made to incorporate supramolecular interactions into microgel systems, but the prospects are significant as it for instance allows selective structure formation and new and orthogonal triggers. To address this challenge I focus on building supracolloidal co-assemblies of microgels based on orthogonal host-guest interactions and investigating the relationships of interparticle interactions. The entire thesis contains three parts of work. In the first part, I explore a simple and versatile synthesis route for the preparation of highly uniform and densely functionalized aqueous microgels by modification of latex particles composed of an active ester monomer (pentafluorophenyl acrylate; PFPA). The hydrophobic nature of the PFPA allows synthesizing very uniform latex particles via emulsion polymerization, whose size can be controlled by the surfactant concentration, while the degree of swelling is a function of the added crosslinker. The high reactivity of the PFPA groups toward nucleophilic substitution delivers a platform method to synthesize functional microgels by reaction with functional amines. This study demonstrates this process for the dense functionalization of the entire particle with an amine carrying a pH-responsive unit. This study further describes the influence of the crosslinking degree on the ability for swelling of the resulting microgels in aqueous dispersion.In the second part I demonstrate social self-sorting of co-assembled families of colloids by orthogonal host/guest recognition using cyclodextrins. It builds on the expertise gained on functionalization in the first study, but uses advanced core-shell microgel architectures enabling adavanced imaging using confocal microscopy. Mixtures of up to four partners can self-sort into their respective families without mutual interference. Additionally, the self-assemblies and their interactions are switchable using orthogonal triggers. This study goes beyond previous possibilities of molecular self-sorting, and opens the design space for future self-sorting colloidal systems via rationally designed molecular recognition.In the third part, I investigate one of the co-assembling microgel families in greater detail, and show how electrostatic repulsion needs to be balanced for the supramolecular recognition to take place. A gradual change from repellent microgels to stable clusters and ordered flocculates upon decreasing electrostatic repulsion are observed. The adaptive nature of the multivalent interactions embedded in the soft microgel shell leads to kinetically trapped scenarios and fibril formation from spherical building blocks.