Study On Shear Induced Aggregation And Gelation Of Nanoparticles

Emulsion polymerization, as one of the most widely used processes for polymer manufacturing, produces million tons of polymers worldwide every year, in the form of latexes. Although latexes can be applied directly, such as surface coatings, paints, adhesives,etc, as the progress of polymeric powder industry, the polymer particles in the latexes have to be extracted from the disperse medium through a proper aggregation process by forming clusters or gels, often using electrolytes. In general, the added electrolytes cannot be completely eliminated from the polymers after the aggregation and lead to changes in electronic and mechanical properties of the final polymer products, as well as their color. In principle, when the colloidal stability of the polymer particles in a latex is solely governed by the DLVO (Derjaguin-Landau-Verwey-Overbeek) interactions (i.e., electrostatic repulsion plus Van der Waals attraction), coagulation is always possible if one imposes a collision (kinetic) energy that is high enough to overcome the energy barrier generated by the DLVO interactions. However, in industrial practice, the colloidal stability of polymer latexes is governed not only by the DLVO interactions but also non-DLVO ones. both of which depend on many physicochemical properties of the latexes such as size and morphology of the particles, particle surface physicochemical properties (fixed charge, surfactant surface coverage, type of the surfactant charge groups, surface roughness, tendency to hydration, etc.), ionic strength and ion types in the disperse medium, and so on.Rigid colloidal systems that are well stabilized by DLVO interaction can be activated by intense shear flow and transformed into solid-like gels, without adding any electrolytes. We have experimentally quantified the critical particle volume fraction for such a transition and found that it is a function of primary particle radius and shear rate. In particular, the values of the critical particle volume fraction obtained under different conditions can be represented through a single power-law function of the Breakage Number (Br), which is defined as the ratio between the shearing energy and the interparticle bonding energy. This finding indicates that, instead of shear rate or stress, the correct parameter quantifying the criticality for shear-induced gelation is Br. Moreover, the ratio between the radius of gyration of clusters constructing the gel network and the primary particle radius at criticality decreases as Br increases, following a power-law scaling with exponent of ?0.31, which is in good agreement with that for breakup of dense fractal clusters of the same fractal dimension in laminar flow. Moreover, shear-induced aggregation or gelation of rigid particles with different materials and particle size is investigated, and found that shear-induced aggregation in z-MC follows a certain universal behavious, e.g.the clusters formed in the shear aggregation process exhibit fractal scaling with fractal dimension(Df) equal to 2.4±0.04, independent of materials, particle size and shear rate. The colloidal system, after passing through the z-MC, no matter if it is liquid-like or solid-like, is composed of two distinct classes of clusters: Class 1, constituted mainly of primary particles, and at most some dimers and trimers, and Class 2, constituted of large clusters (or gels) with sizes at least two orders of magnitude larger than that of the primary particles.The effect of primary particle morphology on intense shear-induced gelation without adding electrolytes is studied. The primary particles are composed of a rubbery core grafted with a polystyrene shell. Depending on the shell-to-core mass ratio, the core can be partially covered by the shell, leading to strawberry-like morphology. It is found that the fractal dimension of the clusters constructing the gel decreases (i.e., more open cluster structure) as the shell coverage increases, and they have relationship as: D_f＞0.66Γ+2.97 The SEM pictures of the gels reveal that the structure variations are due to occurrence of partial coalescence among particles, which reduces as the shell coverage increases. In the region where the fractal dimension reaches a plateau, the coalescence is negligible. The conversion of the primary particles to the gels is incomplete and increases as the coalescence extent decreases.The effect of surfactant type on the aggregation/gelation of strawberry-like particles induced by intense shear without adding any electrolytes is studied. In the absence of any surfactant, after the latexes system passes through a microchannel, not only the shear-induced gelation but also partial coalescence among the particles occurs. Then, an ionic (sulfonate) surfactant and a nonionic (Tween 20) steric surfactant have been added to the system, respectively, and the same shear-induced aggregation/gelation has been carried out. It is found that for both the ionic and nonionic surfactants, although the shear-induced gelation does occur at very low surfactant surface density, the conversion of the primary particles to the clusters constituting the gel decreases as the surfactant surface density increases. When the surfactant surface density reaches a certain value, the intense shear-induced gelation or even aggregation becomes unachievable, due to the strong non-DLVO interaction ( e.g. short-range repulsive hydration force, steric repulsion ). Moreover, the nonionic steric surfactant can also protect the strawberry-like particles from coalescence, leading to the fractal dimension of the clusters constituting the gel decreasing from 2.76 to 2.45, while the ionic sulfonate surfactant fails in protecting the particles from coalescence.The polymeric surfactant poly(buty acrylate/ acrylate acid), P(BA/AA) is synthesized by Atom Transfer Radical Polymerization (ATRP). We study the adsorption of randon and block P(BA/AA) on surface of the strawberry-like particles and their effect on the gelation of strawberry-like particles induced by intense shear without adding any electrolytes. It is found that the randon and block P(BA/AA) has different adsorption behaviour on the surface of strawberry-like particles, the adsorption isotherm of randon P(BA/AA) is L shape, while that of block one is S shape. Both of them can improve the stability of the particles, the conversion of the primary particles to the clusters constituting the gel decreases as the surfactant surface density increases. Moreover, the block P(BA/AA) can also protect the strawberry-like particles from coalescence, while the random one fails in protecting the particles from coalescence.Polymeric surfactant P(BA/AA) is synthesized, with BA and AAas monomers. Acrylate latex is synthesized through a sequential polymerization technique, with the above surfactant P(BA/AA) as stabilizer, potassium persulfate(KPS) as initiator, methyl methacrylate(MMA) and BA as core-monomers, MMA,BA,MAA andβ-hydroxypropyl methacrylate(HPMA)as shell-monomers. Redispersible polymer powders are obtained in a free-flowing form by spray drying the above latex, with silica sol and abio-powders as anti-agglomeration. The obtained redispersible polymer powders have good redisperibility and film forming properties when the content of P(BA/AA) is 8% and the pH value of latex is 8. Interior wall coatings, with good performace, is prepared by using the obtained redispersible polymer powders and cement as the film forming materials.