Cross‐Linking Density and Temperature Effects on the Self‐Assembly of SiO₂—PNIPAAm Core–Shell Particles at Interfaces

Abstract

SiO2-PNIPAAm core-shell microgels (PNIPAAm=poly(N-isopropylacrylamide)) with various internal cross-linking densities and different degrees of polymerization were prepared in order to investigate the effects of stability, packing, and temperature responsiveness at polar-apolar interfaces. The effects were investigated using interfacial tensiometry, and the particles were visualized by cryo-scanning electron microscopy (SEM) and scanning force microscopy (SFM). The core-shell particles display different interfacial behaviors depending on the polymer shell thickness and degree of internal cross-linking. A thicker polymer shell and reduced internal cross-linking density are more favorable for the stabilization and packing of the particles at oil-water (o/w) interfaces. This was shown qualitatively by SFM of deposited, stabilized emulsion droplets and quantitatively by SFM of particles adsorbed onto a hydrophobic planar silicon dioxide surface, which acted as a model interface system. The temperature responsiveness, which also influences particle-interface interactions, was investigated by dynamic temperature protocols with varied heating rates. These measurements not only showed that the particles had an unusual but very regular and reversible interface stabilization behavior, but also made it possible to assess the nonlinear response of PNIPAAm microgels to external thermal stimuli.

Cargo shuttling by electrochemical switching of core–shell microgels obtained by a facile one-shot polymerization

“…Recent progress in use of “smart” multi-responsive microgels as central component of advanced, functional colloidal materials for controlled drug delivery14 systems and sensors27–31 illustrates the importance and the need of further research in this direction 9. In many cases, temperature-responsive poly( N -isopropylacrylamide) (PNIPAM) acts as a microgel scaffold, which exhibits a VPTT (volume phase transition temperature) in aqueous environment within a narrow temperature range around 32 °C, close to body temperature 26,32…”

Stimuli-Responsive Microgels and Microgel-Based Systems: Advances in the Exploitation of Microgel Colloidal Properties and Their Interfacial Activity

“…The most interesting feature that puts microgels in the category of “smart materials” is their unique capability to adjust their dimensions in response to external stimuli, such as temperature, pH, ionic strength, and solvent quality [2,5,6,7,8,9]. Their excellent colloidal stability, ease of synthesis, and post-modification, large surface area enabling encapsulation of the desired cargo makes them potential candidates for antifouling coatings, drug and gene-delivery, tissue engineering, catalysis, water purification, cosmetic applications, and responsive macroscopic materials [10,11,12]. Several review articles can be found in the literature dealing with different fundamental aspects of microgels and the readers are referred to them for detailed information [13,14,15,16,17,18,19,20].…”

Irreversible Adsorption-Driven Assembly of Nanoparticles at Fluid Interfaces Revealed by a Dynamic Surface Tension Probe

“…Equation is an attractive alternative to eq because measurement of the surface (interfacial) tension of a nanoparticle-laden interface is much more straightforward than direct measurement of the contact angle of adsorbed nanoparticles. A number of recent studies have reported the steady state interfacial tension of nanoparticle-laden fluid interfaces. − Using data from these studies, |Δ E | computed from eq is plotted in Figure . Evidently, nanoparticle adsorption at fluid interfaces is irreversible in the majority of cases, given that the magnitude of Δ E delineating reversible and irreversible adsorption lies in the range 20 to 50 k B T .…”