In this work, different systems of colloidally stable, ampholytic microgels (μGs) based on poly(N-vinylcaprolactam) and poly(N-isopropylacrylamide), wherein the anionic and cationic groups are randomly distributed, were investigated. Fourier transmission infrared spectroscopy and transmission electron microscopy confirmed the quantitative incorporation and random distribution of ionizable groups in μGs, respectively. The control of hydrodynamic radii and mechanical properties of polyampholyte μGs at different pH values was studied with dynamic light scattering and in situ atomic force microscopy. We have proposed a model of pH-dependent polyampholyte μG, which correctly describes the experimental data and explains physical reasons for the swelling and collapse of the μG at different pHs. In the case of a balanced μG (equal numbers of cationic and anionic groups), the size as a function of pH has a symmetric, V-like shape. Swelling of purely cationic μG at low pH or purely anionic μG at high pH is due to electrostatic repulsion of similarly charged groups, which appears as a result of partial escape of counterions. Also, osmotically active counterions (the counterions that are trapped within the μG) contribute to the swelling of the μG. In contrast, electrostatic interactions are responsible for the collapse of the μG at intermediate pH when the numbers of anionic and cationic groups are equal (stoichiometric ratio). The multipole attraction of the charged groups is caused by thermodynamic fluctuations, similar to the those observed in Debye−Huckel plasma. We have demonstrated that the higher the fraction of cationic and anionic groups, the more pronounced the swelling and collapse of the μG at different pHs. ■ INTRODUCTIONStimuli-responsive nano-and microgels (μGs) represent unique macromolecular objects, which are potentially useful for applications including biotechnology, 1−8 drug delivery, 9−15 semiconducting materials, 16,17 sensor technology, 18,19 and many others. It is believed that the internal structure of μGs resembles elements of macroscopic polymer networks: linear chains (subchains) are covalently linked with each other into a three-dimensional frame of size ranging between tens of nanometers and a few micrometers. As a result, μGs show some properties of macroscopic gels, like high elasticity as well as the ability to swell and collapse depending on the solvent quality and external stimuli 20−22 (temperature, pH, etc.). Swollen μGs (good solvent conditions) are usually characterized by welldefined shape due to the swelling of each individual subchain, by low polymer volume fraction, and by high stability toward aggregation. In contrast, collapsed μGs have smaller size as compared to swollen μGs and high polymer density because of effective attraction between monomer units. As a result, they have poor colloidal stability and can aggregate (precipitate). However, compared to macroscopic gels, μGs reveal much faster responses to stimuli, which are primarily controlled by their size.Water-soluble polye...
A striking discovery in our work is that the distribution of ionizable groups in polyampholyte microgels (random and core−shell) controls the interactions with the captured proteins. Polyampholyte microgels are capable to switch reversibly their charges from positive to negative depending on pH. In this work, we synthesized differently structured polyampholyte microgels with controlled amounts and different distribution of acidic and basic moieties as colloidal carriers to study the loading and release of the model protein cytochrome c (cyt-c). Polyampholyte microgels were first loaded with cyt-c using the electrostatic attraction under pH 8 when the microgels were oppositely charged with respect to the protein. Then the protein release was investigated under different pH (3, 6, and 8) both with experimental methods and molecular dynamics simulations. For microgels with a random distribution of ionizable groups complete and accelerated (compared to polyelectrolyte counterpart) release of cyt-c was observed due to electrostatic repulsive interactions. For core− shell structured microgels with defined ionizable groups, it was possible to entrap the protein inside the neutral core through the formation of a positively charged shell, which acts as an electrostatic potential barrier. We postulate that this discovery allows the design of functional colloidal carriers with programmed release kinetics for applications in drug delivery, catalysis, and biomaterials.
We describe a facile method for the synthesis of microgels with covalent and noncovalent ionic cross-links. Microgels were synthesized by copolymerization of various Nvinyllactams with zwitterionic monomer (methacryloyloxy)ethyl]dimethyl-(3-sulfopropyl)ammonium hydroxide in the presence of a cross-linking agent (bis(acrylamide)) in waterin-oil emulsion. Monodisperse colloidally stable microgels with a high amount (>30 mol %) of zwitterionic groups were synthesized. High contents of zwitterionic groups in microgels led to the formation of reversible ionic cross-links along with permanent covalent cross-links generated by bis(acrylamide). The obtained microgels exhibit interesting temperaturetriggered swelling/deswelling behavior in aqueous solution. With an increase of the temperature above 10 °C, the microgels swell due to the destruction of the zwitterionic cross-links. Above the lower critical solution temperature of poly(N-vinyllactam) chains at T > 32 °C, the microgels shrink due to the destruction of the hydrogen bonds and enhanced hydrophobic interactions. The variation of zwitterionic groups and cross-linker concentrations influenced the extent of swelling/deswelling at different temperatures. New doubly thermoresponsive microgels were synthesized using three homologues: N-vinylcaprolactam (VCL), N-vinylpiperidone (VPi), and N-vinylpyrrolidone (VPy). It was shown that the temperature-triggered deswelling of microgels is strongly dependent on the size of the lactam ring.
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