Uniform and highly charged microgels via emulsion polymerization
- Einheitliche und hochgeladene Mikrogele mittels Emulsionspolymerisation
Tiwari, Rahul; Möller, Martin (Thesis advisor); Plamper, Felix Alois (Thesis advisor)
Dissertation / PhD Thesis
Dissertation, RWTH Aachen University, 2017
This thesis deals with the synthesis and solution properties of uniform and highly charged (cationic, anionic and polyampholyte) microgels via emulsion polymerization with tunable size and degree of crosslinking (swelling). The intermediately synthesized hydrophobic latex particles are later post-modified into hydrophilic charged microgels via post-modification or deprotection techniques. Apart from this, polyamphiphatic microgels form hydrophobic bridges or nanostructured domains in aqueous media visualized via cryogenic (cryo)-TEM.Microgels with cationic charged groups are synthesized by the post-functionalization of uniform latex particles via Cu-mediated Huisgen-type alkyne/azide click chemistry. The particle hardness and swelling can be tuned as a function of the used ratio of monomer/crosslinker. Post modifications in the interior of the particles are conducted in the swollen state in organic solvent. Combined Raman spectroscopy and elemental analysis reveal the kinetics and degree of modification. After modification, the resulting microgels display a pH-dependent ionization and swelling behavior in water. Additionally, it is shown that degree of functionalization depends on the crosslinking degree. In terms of anionically charged, strong polyelectrolyte microgels, the synthesis of highly uniform microgels is performed via emulsion polymerization with monomer (poly(styrene sulfonic ethyl ester) (PSSE)) carrying protecting groups, which are later transformed into highly charged strong polyelectrolyte (poly(styrene sulfonic acid) (PSSA)) microgels. The tuning of particle size, as well as particle softness is demonstrated with solution state characterization via dynamic light scattering (DLS) and cryo-TEM.Apart from that, microgels with internal and reconfigurable complex nanostructure are synthesized by incorporating permanent (poly(methyl methacrylate) (PMMA)) and switchable hydrophobic pockets (poly(N,N’-diethylaminoethyl methacrylate) (PDEAEMA)) in highly charged poly(methacrylic acid) (PMAA) microgels via emulsion polymerization. The tuning of the size, crosslinking density and tailored incorporation of functional comonomers into the polyacid microgels is shown. Analysis via cryo-TEM and pyrene probe measurements reveal switchable hydrophobic pockets inside the microgels as a function of pH. The particles show a rich diversity of internal phase-segregation that adapts to the surrounding conditions. Large amounts of hydrophobic pockets even lead to hydrophobic bridging between particles. The study shows ways towards tailored polyelectrolyte microgels with narrow dispersity, high charge density, as well as tailored and reconfigurable hydrophobic compartments and interactions.Furthermore, microgels having well-defined core-shell morphology with opposite charged groups in core and shell are synthesized via surfactant-free seeded emulsion polymerization. The swelling behavior of microgels are measured at different pH via DLS and cryo-TEM. Cryo-TEM reveals the core-shell morphology for PMAAcore-P2VPshell microgels due to ionization of PMAA shell and P2VP core at low and high pH. The homogenous shell around the core allows to understand the pH-dependent swelling behavior of core-shell microgels.In general, emulsion polymerization technique allows to synthesize highly uniform latex particles with different monomers carrying hidden functionalities, which are readily converted into charged units via post-functionalization by click-chemistry, deprotection of protected groups or modifications.