Adjustable Interfacial Structure & Rheology Of Fumed Silica/Polar Oligomer Nanocomposites

Fumed silica (FS) has been an irreplaceable component especially performing as rheological agent in adhesives, coatings, lithium battery, and liquid armor, etc. However, it is of great challenge to control rheological behavior of polar liquids such as reactive polar oligomers using FS because diverse interactions between surface of FS and polar molecules bring difficulty for formula design and rheological performance adjustment of industrial products.This topic comes from formula and rheology design of a novel moisture-cured one-component polyurethane (1C-PUR), in which diverse rheological response generates just like other FS filled polar oligomers. The main idea of this thesis is trying to reveal the mechanism behind the formula-induced complicated rheology of FS filled polar oligomers. Firstly, similarity and difference of sol and gel behaviors in 1C-PUR are systematically investigated through rheological test and electron microscopic method. The relationships between formula, microstructure and rheology are found and the sol and gel behaviors are classified. Secondly, temperature-modulated differential scanning calorimetry (MDSC) and broadband dielectric spectroscopy (BDS) are used to investigate the interfacial dynamics of FS filled oligomers. Finally, essence of sol-and gel-like behavior are revealed by correlating bound layer structures with rheological behavior. Some main results are as the following.FS with different surface areas (200-400 m2/g) and surface characteristics (hydrophilic or hydrophobic) are dispersed into carefully designed and synthesized 1C-PUR. The rheology is investigated in relation to the formula including polyols, the characteristic and content of FS and the curing catalysts. The microstructures by transmission electron microscope are associated perfectly with three different rheological behaviors:(ⅰ) sols with well-dispersed silica aggregates, (ⅱ) weak gels with agglomerate-linked networks and (ⅲ) strong gels with concentrated networks of large agglomerates. Though sols and gels are well distinguished by shear thickening or sustained thinning response through steady shear flow test, it is interesting that the sols and weak gels exhibit a uniform modulus plateau-softening-hardening-softening response with increasing dynamic strain at frequency 10 rad s-1 while the strong gels show a sustained softening beyond the linear regime.To further reveal the generality of the rheological response among sol, weak gel and strong gels, the onset of softening and hardening are normalized. The two softening responses are isoenergetic while the hardening is initiated by a critical strain. An energetic-viscosity determined diagram regardless of formula is introduced distinguishing sol and gel behaviors and the microstructural evolution of sols, weak gels and strong gels are illustrated and compared under nonlinear dynamic shear.Immobilized polymer fractions have been claimed to be of vital importance for sol-gel transitions in nanoparticle dispersions. Here the immobilized layer structures of tri-functionality polypropylene glycol (PPG) near the surfaces of FS nanoparticles are investigated to reveal the role of surface chemistry on the molecular dynamics and sol-gel transitions of the dispersions. Using MDSC, molecular dynamics of FS-PPG system during glass transition and cold-crystallization are detected. Comparing with hydrophobic FS that forms an incomplete glassy layer, it is clear that hydrophilic FS immobilizes more PPG, forming a partially immobilized outer layer being unable to crystallize next to the inner glassy layer. It is directly evidenced that percolation of glassy layer along the nearest neighbor nanoparticles is responsible for the sol-gel transition.Amine induced gelation in 1C-PUR seems interesting and potential in FS involved industry. It is found that introducing a trace amount of secondary/tertiary amine also promote gelation of hydrophilic FS/PPG dispersions even at extremely low contents of FS (<2 vol%). By MDSC, BDS, and rheology investigations, strong evidence is found that the amine-promoted gelation is due to thickening and easy-percolation of the inner glassy layer converted from an outer uncrystallizable layer.In order to further reveal the reinforcement mechanism, molecular dynamics of FS-filled PPG with different weight-averaged molecular weight (Mw) are investigated.The MDSC and BDS results indicate that the major adsorptions of low-Mw PPGs are dominated by strong hydrogen bonding between terminal-OH of PPG and Si-OH on silica surface, which tends to form glassy layers. However, in high-Mw PPGs, adsorption conditions turn into weak hydrogen bonding between PPG ether bonds and silica silanols. Nevertheless, rheological result shows that percolation of glassy layers is still demonstrated to be the main reason of sol-gel transition despite of different interfacial layer structures.