Abstract
Gelation behaviors of hydrogels have provided an outlook for the development of stimuli-responsive functional materials. Of these materials, the thermogelling behavior of poly(N-isopropylacrylamide) (p(NiPAm))-based microgels exhibits a unique, reverse sol–gel transition by bulk aggregation of microgels at the lower critical solution temperature (LCST). Despite its unique phase transition behaviors, the application of this material has been largely limited to the biomedical field, and the bulk gelation behavior of microgels in the presence of colloidal additives is still open for scrutinization. Here, we provide an in-depth investigation of the unique thermogelling behaviors of p(NiPAm)-based microgels through poly(N-isopropylacrylamide-co-2-hydroxyethyl methacrylate) microgel (p(NiPAm-co-HEMA))–silica nanoparticle composite to expand the application possibilities of the microgel system. Thermogelling behaviors of p(NiPAm-co-HEMA) microgel with different molar ratios of N-isopropylacrylamide (NiPAm) and 2-hydroxyethyl methacrylate (HEMA), their colloidal stability under various microgel concentrations, and the ionic strength of these aqueous solutions were investigated. In addition, sol–gel transition behaviors of various p(NiPAm-co-HEMA) microgel systems were compared by analyzing their rheological properties. Finally, we incorporated silica nanoparticles to the microgel system and investigated the thermogelling behaviors of the microgel–nanoparticle composite system. The composite system exhibited consistent thermogelling behaviors in moderate conditions, which was confirmed by an optical microscope. The composite demonstrated enhanced mechanical strength at gel state, which was confirmed by analyzing rheological properties.
Highlights
Phase transition behaviors of colloidal dispersion have long been studied to understand the fundamentals of colloidal interactions and to apply the system to various fields such as cosmetics, pharmaceutics, food industries, paints, inks, slurries, etc
We consider thermogelling behaviors of microgels–nanoparticle composite systems consisted of poly(N-isopropylacrylamide-co-2-hydroxyethyl methacrylate) (p(NiPAm-co-HEMA)) microgels and silica nanoparticles
The p(NiPAm-co-HEMA) microgels were synthesized by radical precipitation polymerization as illustrated in Figure 1a [23]
Summary
Phase transition behaviors of colloidal dispersion have long been studied to understand the fundamentals of colloidal interactions and to apply the system to various fields such as cosmetics, pharmaceutics, food industries, paints, inks, slurries, etc. Poly(n-isopropylacrylamide) p(NiPAm) microgel dispersion is one of the widely known temperature-responsive colloid system It shows reversible swelling/shrinkage at a lower critical solution temperature (LCST), which is around 32 ◦ C. It forms bulk gels around LCST due to the weak electrostatic repulsion among microgels [19] Because this gelation does not require additional crosslinking reactions, they have been studied as an in situ gelation material for biomedical applications. Despite many studies on understanding gelation and controlling their mechanical properties, the thermogelling behavior of p(NiPAm)-based microgels in the presence of colloidal additives has not been fully understood. We consider thermogelling behaviors of microgels–nanoparticle composite systems consisted of poly(N-isopropylacrylamide-co-2-hydroxyethyl methacrylate) (p(NiPAm-co-HEMA)) microgels and silica nanoparticles. We added silica nanoparticles to the microgel dispersion to make a model composite system to investigate the thermogelling behaviors
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