Anisotropic microgels : from synthesis to features in 2 and 3 dimensions

Nickel, Anne Catherine; Richtering, Walter (Thesis advisor); Crassous, Jérôme Joseph Emile (Thesis advisor)

Aachen : RWTH Aachen University (2022)
Dissertation / PhD Thesis

Dissertation, RWTH Aachen University, 2022


Particles in the submicrometer size range are interesting for applications and as model systems especially for the investigation of ordering phenomena. Inspired by the variation of biological systems, the development of soft anisotropic building blocks becomes necessary. While hard anisotropically particles are well studied, the synthesis of soft anisotropic particles remains difficult. An appropriate class for soft model systems are microgels which are cross-linked polymer networks swollen by a good solvent. In the first part of this thesis, the synthesis of anisotropically shaped microgels is developed. An elliptically shaped hematite-silica particle is used as a template onto which a polymeric network is synthesized with a seed and feed precipitation polymerization. It is shown that the shape of the resulting anisotropic core-shell microgels depends on the thickness of the shell. With a two-step etching procedure, both parts of the core can be removed separately leading to hollow anisotropically shaped microgels. Such core-shell and hollow anisotropic microgels are excellent soft anisotropic building blocks with a unique phase behavior.In the second part, the first study dealing with the interfacial phase behavior of anisotropically shaped microgels is illustrated. It is revealed that the softness and thickness of the shell has a major influence on its properties at the interface. Anisotropic microgels with a soft and thick shell spread strong at the interface leading to a spherically shaped microgel shell surrounding the elliptical hard core. In contrast, the anisotropic shape of the microgels remains at the interface for an anisotropic microgel with a thin and harder shell. This specific shape favors shape-dependent capillary interaction causing clustering of the microgels at the interface. First of all, the characteristics of such clusters depend on the shell properties. Additionally, they are influenced by the number of microgels at the interface. Anisotropic microgels with a thin shell show clustering for all interfacial concentrations. In contrast, anisotropic microgels with larger shells reveal shape-dependent capillary interactions only at high interfacial concentrations where the shell is compressed and the elliptical cores affect each other. The dominating order for anisotropic core-shell microgels with larger shells is tip-to-tip ordering. In contrast, anisotropic microgels with a thin shell reveal dominant side-to-side ordering independent of the existence of the hard core. In the last part of this thesis, the phase behavior of anisotropically shaped hollow microgels in solution is revealed for the first time. It is shown that anisotropic hollow microgels display a self-healing behavior once embedded in a highly concentrated solution. A highly concentrated spherical microgel dispersion exerts an isotropic pressure by the crystalline structure. This pressure forces the anisotropic hollow microgels to a similar spherical shape integrating them into their crystalline structure. In contrast, the anisotropic pressure resulting from the different ordering of the anisotropic microgels, lead to the preservation of the anisotropic shape of the anisotropic microgels once embedded in a high concentration of similar anisotropically shaped microgels.