PNiPAm microgels are surface active at oil-water and air-water interfaces. They can thus not only be used to stabilize emulsions but may also act as stabilizers in foams, similar to foams stabilized by hard particles or latexes.[1-2] The decrease in surface tension of PNiPAm microgels at the air-water interface is less strong than at the oil-water interface,[3] but it has been observed in preliminary experiments that zwitterionic microgels carrying positive and negative moieties produce foams with high stability.[4]
This behavior can be used to produce switchable foams with specific stability and foam volume. It would also be interesting to investigate the interfacial behavior of PNiPAm chains instead of microgels in foams and emulsions. They adsorb to liquid interfaces and lower the interfacial tension, as it has been shown by IFT measurements and compression isotherms.[5-8] The interfacial coverage is different when chains are used instead of microgels and it can thus be expected that emulsion stability is influenced by the morphology of the stabilizing polymer.
The location of microgels at the interface can be investigated in more detail by means of different methods. FreSCa cryo-SEM offers the possibility to determine the protrusion height of microgels at oil-water interfaces very precisely. The measurements presented in chapter 5 yielded an estimation of the protrusion height but an exact determination was not possible due limitations arising from the shadowing angle. This problem may be overcome by using large microgels that probably protrude more into the oil phase and produce a clear shadow behind the microgels. The absence of a shadow would indicate that the protrusion into the oil phase is small irrespective of the nature of the microgel.
The microgels used for the TXM measurements in chapter 8 (lab book code SAS02[9-10]) are worth investigating with FreSCa cryo-SEM and direct comparison with the protrusion resulting from TXM would then be possible.
Scattering of neutrons or x-rays at interfaces provides information about the particle monolayer by analyzing a beam scattered at the flat particle-covered interface. Different setups can be used to measure GI-SANS or GI-SAXS and neutron or x-ray reflectivity (NR, XRR) at large scale facilities. Both techniques are established for molecules or nanoparticles at liquid (mostly air-water) interfaces. Difficulties concerning reflectivity and GI experiments on microgel-covered oil-water interfaces arise from different aspects that may complicate data acquisition and analysis. For instance, the microgels adopt an inhomogeneous shape at the interface and the distribution of microgels at the interface may not be hexagonal over the whole sample. Furthermore, small microgels (Rh ≈50 nm) have to be used to ensure a thin interfacial layer. The loss in intensity after travelling through the oil has to be small, which is achieved by choosing an oil with suitable attenuation length and by reducing the thickness of the oil layer. In brief, if it is possible to overcome experimental difficulties and develop a suitable measurement procedure,
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reflectivity and GI experiments should give valuable information about the microgel layer at liquid interfaces.
Several measurements on the Langmuir trough can be performed to improve on the one hand the knowledge about the influence of deformability on the compression; on the other hand, the possible desorption of microgels during compression is worth investigating.
Microgels with different morphologies can be used to explore the first aspect. For instance, core-shell particles with hard core (silica or polystyrene) and soft microgel shell can be pitted, resulting in hollow spheres with similar size but increased deformability compared to the corresponding core-shell particles. The resulting compression isotherms show the influence of the missing core on the behavior of the monolayer under compression. The desorption of microgels during compression can be investigated by installing a second Wilhelmy balance to the Langmuir trough, following work performed on core-shell nanoparticles by Stefaniu et al..[11-12] The second balance is mounted behind the barriers and measures the change in surface pressure arising from the adsorption of previously desorbed microgels at the free interface behind the barriers (formation of a Gibbs monolayer). Preliminary experiments concerning the desorption of microgels from interfaces have already been performed with the Langmuir trough and the results are shown in chapter 11. The possibility of microgels desorbing from the interface under compression is worth considering in future experiments and can give information about the affinity of different microgels towards interfaces and about the origin and extent of the hysteresis between compression and expansion isotherms.
Considering the arrangement of hard particles in 3D colloidal crystals, the crystallinity is lost if the polydispersity exceeds a critical value. Furthermore, multimodal size distributions may affect the formation of colloidal crystals.[13-15] The Langmuir-Blodgett method can be used to test if a 2D microgel layer at oil-water interfaces retains its high hexagonality when the polydispersity is increased or when the monolayer is formed of microgels with different size. These findings will support the systematic formation of microgel monolayers that can be used for instance as masks for particle lithography.
Another important aspect is the correlation between microgel arrangement, deformation and the viscoelastic properties of the interface. This has already been investigated in a collaborative project with the groups of Lucio Isa (ETH Zürich) and Véronique Schmitt (Université de Bordeaux).[16] The surface pressure (measured in a Langmuir trough) and the surface elasticity (measured with the oscillating pendant drop method) were related to the packing density of microgels at the interface under compression and after spontaneous adsorption. The compression isotherm can be expressed as a function of the center-to-center distance of the microgels, showing the pronounced compression of the microgels at high interfacial microgel concentration. A comparison with the elastic modulus of the interface shows that the maximum surface elasticity is correlated to the fist increase in
FUTURE PROSPECTS
surface pressure in the compression isotherm, which corresponds to the flattened conformation of the microgels. These results show the similarity of microgel monolayers prepared in the Langmuir trough at low compression and after self-assembly, as it has also been shown in the scope of this thesis. Furthermore, they underline the necessity to measure interfacial shear and dilatational rheology because the viscoelastic properties of the interface are obviously connected to the microgel arrangement and deformation. The influence of microgel charges on the interfacial properties can then be investigated in more detail.
The kinetics of microgel adosorption to liquid interfaces also differs from that of hard particles, as it was investigated in collaboration with the group of To Ngai (Chinese University of Hong Kong).[17-19] The adsorption takes place in two steps which comprise first the diffusion to the interface and second the deformation and spreading of microgels.
The effect of microgel concentration and deformability was investigated in the study of Li et al.; it can thus be anticipated that the presence of charges influences the kinetics of adsorption due to different deformability of the respective microgels. In a very recent paper, emulsion droplets covered with charged and uncharged microgels were investigated in detail with cryo-SEM.[20] The microgel arrangement is independent from the presence of charges and their special distribution inside the microgels. These results emphasize again the unexpected influence of charges on microgel layers, similar to what has been shown in this thesis with Langmuir trough and AFM experiments.
References
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[12] C. Stefaniu, M. Chanana, D. Wang, D. V. Novikov, G. Brezesinski, H. Möhwald, Langmuir 2011, 27, 1192-1199.
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APPENDIX