Rheological Study On Suspension System Of Nanoparticles With Different Dimensions

Suspension systems widely exist in daily lives, industrial manufacture and nature. Their rheological properties have become more closely related with industrial production in recent years and have therefore attracted more and more attention. Among the numerous suspension systems, colloidal one has the closest relations with the daily lives, so scientists have paid more and more attention on it. Size and dimensionality of particles, the interaction between particles and suspension medium make suspensions present different characteristics in their flow and deformation. Therefore, the rheological behavior of suspensions strongly depends on their internal hierarchical structures. The related research on the rheological properties of colloidal suspensions hence has very important scientific significance. In this work, therefore, the particles with different sizes and dimensionalities, such as silica (SiO2), polystyrene (PS) microsphere and nanocellulose crystal (NCC) were used as the dispersed phaseto prepare colloidal suspensions with polyvinyl alcohol (PVA) solution or carboxymethyl cellulose (CMC) one for the rheological studies. The linear and nonlinear, as well as the steady and transient rheology of as-prepared suspensions were then investigated in detail in terms of sizes and dimensionalities of dispersed particles, as well as interactions between particles and suspension media. The aim of this work is to study the aspects influencing the rheology of suspensions. The obtained results are included below.(1) For the SiO2/PVA and PS microsphere/PVA suspensions:the two suspensions showed well dispersion stability. The presence of nanoparticles made the viscosity of suspensions increased. Taking SiO2 suspension as an example, when mass concentration of SiO2 (50nm) achieved up to 10%, the suspensions showed shear thinning behavior. At the same concentration level, system viscosity increased firstly, and then "decreased with the increase of SiO2 particle size. In dynamic rheological measurement, the addition of SiO2 caused the increase of storage modulus (G"), loss modulus (G") and complex viscosity (|η*|) in the whole frequency range. But the suspensions were viscous-dominated within the experimental SiO2 concentrations. But for the SiO2 particle with the size of 350nm, its suspension system appeared solid-like response as the mass concentration reached 20%. For PS microsphere suspensions with the same particle size (150nm) and mass concentration, the viscosity and the degree of shear thinning were significantly higher than those of the SiO2 suspensions. The linear viscoelastic modulus growth of the former was higher than the latter as well.(2) For the NCC/PVA suspensions:due to hydrogen bonding, NCC dispersed well in PVA solutions to form stable suspensions. NCC/PVA suspensions showed monotonous increase of modulus with increase of NCC loadings, and NCC particles were percolated in PVA solution with a threshold value ranged in 0.4-0.7 wt%. Strain thinning and a weak strain overshoot could be observed under large amplitude oscillatory shear (LAOS). However, in transient rheological tests, this system showed evident stress overshoot behavior, and during stress development, the overshoot levels increased with increasing flow rates. The maxim of all overshoots approximately appeared at the strain of 4-5, showing typical strain-scaling behavior with nonzero residual stress in long-time scale. Besides, the presence of NCC particles improved the morphology of electrospun PVA fibers within moderate loading ranges.(3) For the NCC/CMC suspensions:NCC can be dispersed well in the CMC solutions, forming stable suspensions because of hydrogen bonding and similar molecular structure between NCC and CMC. NCC/CMC suspensions showed monotonous increase of modulus with increase of NCC loadings too, and NCC particles were percolated in CMC solution with a threshold value ranged in 1.0-1.5 wt%, higher than that in the PVA one. The threshold value decreased with increase of temperatures. Similar with NCC/PVA suspensions, this system also showed evident overshoot behavior in transient rheological tests, the maxim of all overshoots approximately also appeared at the strain of 4-5, revealing similar strain-scaling behavior. Whereas, NCC/CMC suspensions had far stronger stress overshoot response relative to the NCC/PVA ones. Besides, the presence of NCC could improve the spinnability of CMC solution, and the addition of PVA solution could broaden the electrospinning window of CMC solution in the presence of NCC.