Responsive properties of crystalline colloidal arrays formed by core-shell microspheres with pH and temperature sensitivities

Abstract

Polymeric microspheres responsive to pH and temperature changes have been prepared by a two-step surfactantfree emulsion polymerization with a polystyrene core and a hydrogel shell. The double sensitivity is achieved by the copolymerization in the shell layer of acrylic acid and N-isopropylacrylamide, which are pH-and thermo-responsive, respectively. Above a certain critical concentration, these microspheres are able to self-assemble into crystalline colloidal arrays, causing intense visible light diffraction. The behavior of these structures has been studied and correlated to the properties of the microspheres. The packing of these microspheres results in stimuli-sensitive colloidal crystals, with a behavior directly linked to the interactions between the particles and the surrounding media.Résumé : Des microsphères doublement sensibles et composées d'un coeur de polystyrène entouré d'une couche d'hydrogel ont été synthétisées par polymérisation en émulsion sans tensioactif. La double sensibilité a été obtenue grâce à la copolymérisation de l'acide acrylique et du N-isopropylacrylamide, tous deux connus pour être sensibles respectivement au pH et à la température. Au dessus d'une concentration critique, ces microsphères sont capables de s'auto-assembler pour former des cristaux colloïdaux et affichent des couleurs intenses dues à la diffraction de la lumière visible. Le comportement de ces structures a été étudié en rapport avec les propriétés des particules. L'empilement de ces microsphères donne des cristaux colloïdaux sensibles à leur environnement, propriété qui semble être liée à la nature des interactions entre les particules.Mots-clés : microsphères coeur-coquille, polymérisation en émulsion sans tensioactif, sensibilité au pH et à la température, cristaux colloïdaux.

Lubrication with Soft and Hard Two-Dimensional Colloidal Arrays

“…For both dispersions, the hydrodynamic diameter increased with increasing pH as a result of the ionization-induced swelling of the PAA shell, as supported by the increase in the zeta potential (Figure ). Such swelling behavior has also been reported for similar PS-PAA core–shell NPs. , The protonation of the PAA shell at lower pH values facilitates the physical cross-linking of the PAA shell through hydrogen bonding between the carboxylic groups. However, the size increase between pH 3 and 10 was larger for PAA-80% (190%) than for PAA-25% (130%) even though they exhibited a smaller change in zeta potential (Figure ).…”

“…Such swelling behavior has also been reported for similar PS-PAA core−shell NPs. 46,51 The protonation of the PAA shell at lower pH values facilitates the physical crosslinking of the PAA shell through hydrogen bonding between the carboxylic groups. However, the size increase between pH 3 and 10 was larger for PAA-80% (190%) than for PAA-25% (130%) even though they exhibited a smaller change in zeta potential (Figure 1).…”