Investigation of internal microstructure and thermo-responsive properties of composite PNIPAM/silica microcapsules

• Published in: Journal of colloid and interface science 2010-06, Vol.346 (2), p.352-360
• Main Authors: CEJKOVA, Jitka, HANUS, Jaroslav, STEPANEK, František
• Format: Article
• Language: English
• Subjects: Acrylic Resins - chemistry; Asymptotic properties; Atomic force microscopy; Capsules - chemistry; Chemistry; Colloidal gels. Colloidal sols; Colloidal state and disperse state; Crosslinking; Drying; Emulsions. Microemulsions. Foams; Exact sciences and technology; General and physical chemistry; Hydrogel; Hydrogel, Polyethylene Glycol Dimethacrylate - chemistry; Hydrogels; Microencapsulation; Nanoparticles; Particle Size; Physical and chemical studies. Granulometry. Electrokinetic phenomena; Pickering emulsion; Scanning electron microscopy; Shrinkage; Silicon dioxide; Silicon Dioxide - chemistry; Stimuli-responsive polymer; Temperature
• ISSN: 0021-9797; 1095-7103
• DOI: 10.1016/j.jcis.2010.02.060

The dynamic thermo-responsive behavior and drying/re-hydration properties of composite PNIPAM–silica microcapsules have been studied as function of the polymer cross-linking ratio. During shrinking, the particle retains a spherical shape but during swelling, buckling occurs as the rate of water ingress cannot keep up with the rate of shell relaxation. Composite microcapsules consisting of a thermo-responsive hydrogel poly- N-isopropylacrylamide (PNIPAM) and coated by silica (SiO 2) nanoparticles have been synthesized by the inverse Pickering emulsion polymerization method. The composite capsules, whose mean diameter is in the 25–86 μm range in the expanded state, were characterized by static light scattering, atomic force microscopy (AFM), scanning electron microscopy (SEM), and laser scanning confocal microscopy (LSCM). It is reported that the hydrogel surface is uniformly covered by a monolayer of silica nanoparticles and that depending on the capsule size and degree of polymer cross-linking, either hollow-core or partially-filled hydrogel-core microcapsules can be created. Equilibrium thermo-responsive behavior of the composite microcapsules is investigated and it is found that after heating the particles above the lower critical solution temperature (LCST) of PNIPAM, the shrinkage ratio V/ V max varies from 0.8 to 0.4 for a cross-linking ratio from 0.6% to 9% on a mass basis. Dynamic temperature cycling studies reveal no hysteresis in the shrinking and recovery phases, but a small measurable dependence of the asymptotic shrinkage ratio V/ V max on the rate of temperature change exists. The composite capsules are stable under long-term storage in both dried and hydrated states and easily re-dispersible in water.