Suspensions and Gels Composed of Non-Spherical Nanoparticles: Rheology and Scattering (Suspensies en gels opgebouwd uit niet-sferische nanodeeltjes: Reologie en lichtverstrooiing)

Abstract

@font-face { font-family: "ＭＳ 明朝";}@font-face { font-family: "ＭＳ 明朝";}@font-face { font-family: "Cambria";}@font-face { font-family: "Lucida Sans Unicode";}p.MsoNormal, li.MsoNormal, div.MsoNormal { margin: 0cm 0cm 0.0001pt; font-size: 12pt; font-family: Cambria; }.MsoChpDefault { font-family: Cambria; }div.WordSection1 { page: WordSection1; }Colloidalmaterials play an important role in science and technology. A study by DuPontshowed that about 65% of their products are produced or sold as colloidal dispersions. Hence, colloidal suspensions are animportant part of chemical engineering research. Avast body of work has dealt with the relations between the properties of colloids and the structure and properties oftheir suspensions. A factor which so far has not received much attention isparticle shape, even though non-spherical colloidalparticles are ubiquitous in nature with 90% disklike and 9% rodlike particles.The reason why non-spherical particles are such efficient basic building blocks is, they can form networks with a smalleramount of particles compared to spheres, they impart high strength to thenetwork, have rich optical properties and can formcomplex 3D structures. In spite of this abundant use of non-spherical particlesby nature, research has concentrated mostly on spherical particles and only recently we have been looking at non-sphericalparticles. This is due to the complex characteristics of non-sphericalparticles and the lack of suitable model systems. The mathematical framework isalso more complex when it comes to describing properties of non-spherical particles,compared to spherical particles. Dealing with non-spherical particles is acircular problem because, to describe or model the behavior/properties ofnon-spherical particles in suspensions or gels, a good mathematical model isrequired, but to obtain a good mathematical model, a good set of data on the properties/behaviors is required. Thisthesis focuses on the latter part of the problemmentioned above, i.e., characterizing non-spherical particles in suspensionsand gels, to obtain good data sets that can be used further to model thebehavior of non-spherical particles. The firstpart of this thesis is dedicated to the characterization of non-sphericalnanoparticles in suspensions. The rheological properties of a suspension are dependent on many factors such as the effective shape, size,concentration, effective interaction between the nanoparticles. For sphericalparticles the effective volume fraction, as determinedby the effective hydrodynamic diameter is the dominant factor controlling the rheological properties. For simplenon-spherical particles like rods, the effectivehydrodynamic aspect ratio can be determined roughly by theknowledge of the charge or polymer layer on the surface combinedwith microscopy images. But for objects with complex shapes such as disks orpentagons, defining the aspect ratio itself is notclear. Scattering has been a great tool for getting information on thehydrodynamic sizes of nanoparticles. In the first part of the thesisnon-spherical particles are characterized using dynamic light scattering (DLS),depolarized dynamic light scattering (DDLS), static light scattering and rheo-optical methods. Historically, DLS has been used to obtainthe hydrodynamic size of spherical particles fromthe translational diffusion coefficient and the results are in good agreementwith theory. For non-spherical nanoparticles both translational and rotationaldiffusion coefficients have to be known to calculate the effective hydrodynamic aspect ratio. In this work DLS has beenused to study the translational diffusion of thinfd-virus rodlike particles (length/diameter ≫ 1), which are used as basic building block to make gels. For smallaspect ratio rodlike nanoparticles made of gold, the localized plasmon resonancesenabled the determination of translational and rotational diffusions usingDDLS. The hydrodynamic aspect ratio of these gold nanorods calculated usingtranslational and rotational diffusion showslarge deviations from the actual values. This is due to the exponential type equations relating the measured diffusionvalues to the calculated hydrodynamic aspectratio. For complex shaped objects both DLS and DDLSare not suitable since the theory of scattering is not fully developed for arbitrarily shaped objects. To determine the effectivehydrodynamic aspect ratios of arbitrarily shaped objects, a rheo-optical method(dichroism) is used, from which the effectivehydrodynamic aspect ratio and the polydispersity of any complex nanoparticlecan be obtained. The relationship between the transient dichroism signal andthe effective hydrodynamic aspect ratio and polydispersity is linear and therefore does not lead to complications in determining thesevalues. Finally, using dichroism the shape, size and flexibility of goldnanorods, gold nanodisks and graphene sheets are obtained. In the secondpart of the thesis, gels made of long monodisperse rodlike particles with tunable interactions are characterized using rheology andscattering. The rodlike particles are virus nanoparticles which are negativelycharged at a pH of 8.2 and have surface groups such as -COOH, -NH2 and -OH, on which a thermoreversible polymer Poly(N-isopropylacrylamide) (PNIPAM) isgrafted. The repulsive interaction is controlled by varying the ionic strengthof the buffer solution and the attractive interaction iscontrolled by changing the temperature. The rheologicalproperties of these gels are characterized in both linear and nonlinearviscoelastic regime. The rheological data are analyzed using the critical geltheory from which the gel strength and gel structure aredetermined as a function of concentration,temperature and ionic strength. Finally, theknowledge obtained from the characterization of non-spherical nanoparticles in suspensions and in gels is used for practicalapplications. Oriented 3D self-assembled gold nanorods areobtained by controlling the evaporation rate through the knowledge of diffusioncoefficients calculated from the DDLS experiments.The knowledge of flexibility of graphene sheets was used in preparingwater-in-oil emulsions stabilized by an extremely small amount of flexiblegraphene sheets that are stable for over 2 years. Rodlike virusparticles in suspension are used to produce 3D honeycomb structures that can beused as a high-strength low-weight material. Aligned gelswith varying strength and optical properties are produced by changing thethermo-mechanical history of the gel.nrpages: 177status: publishe