Polymer composite materials containing inorganic or organic particles have been used in a variety of industries for improving the functions and properties of such materials. These polymer composites are generally manufactured by the coating and drying of polymer solutions containing the desired particles. However, control of the dispersibility of the suspensions is difficult because the solutions contain multiple components such as solvent, monomers, and polymers, resulting in complicated chemical/physical properties. As the performance of polymer composite materials is significantly affected by the dispersion conditions, it is necessary to obtain a stable dispersibility for such suspensions. However, qualitative and quantitative analyses of dispersion/aggregation behavior is challenging, as these properties are determined based on the sum of several complex interactions. To address this problem, the surface modification of particles is commonly used to modulate the dispersion stability, but this requires a time-consuming trial-and-error procedure to optimize conditions such as molecular structure and the loading of modifying agents. A colloidal probe atomic force microscopy (AFM) method that enables direct measurement of the interactions between the surfaces would therefore be useful for the evaluation of dispersion/aggregation behavior of modified particles, including estimating the repulsive/attractive forces acting on particle surfaces in solution.We herein report the surface modification of silica particles using a silane coupling reaction. The effects of surface modification on the dispersion behavior of the silica particles were assessed from the viewpoint of surface interaction forces using colloidal probe AFM.
3-Glycidyloxypropyltrimethoxysilane (GPTMS) and vinyltrimethoxysilane (VTMS) were used as silane coupling agents. The coupling reactions of silica (mean diameter 2.0 μm, specific surface area 4.2 g/m2) were carried out in xylene for 24 h at a range of temperatures (40-100 °C) in the presence of additive amounts (6.5-26.4 μmol/m2) of each coupling agent. The surface-modified silica loadings were determined using simultaneous thermogravimetric and differential thermal analyses (TG/DTA) at 1000 °C. To confirm the different dispersion behaviors between the surface conditions, silica particles modified with different coupling agents were dispersed in either water or toluene (1.0 wt%) by sonication and agitation for 30 min at room temperature. In addition, surface interaction forces between the surface-modified silica and the mica plate (treated in the same manner as the silica particles) were measured using colloidal probe AFM to examine the effect of the surface modifications. Using silicon adhesive, a surface-modified single particle was attached to the tip of a cantilever (spring constant = 0.18 N/m) to form a colloidal probe, which was then used for further measurements. The surface distance between the colloidal probe and the substrate was varied by moving the piezo device at a rate of 250 nm/s.
The loadings of surface-modified silica were found to increase with the increased addition of the coupling agents for both GPTMS and VTMS. However, little difference was observed with regard to the temperature at which the coupling reaction was carried out. In addition, the coupling reaction of GPTMS was found to be more efficient than that of VTMS. When the dispersibility was observed in water, GPTMS-modified silica showed good dispersion even at small loadings, while VTMS-modified silica caused aggregation and sedimentation at any loadings. In toluene, the opposite tendency was observed, with GPTMS-modified silica showing aggregation and sedimentation, while VTMS-modified silica showed good dispersibility that improved with loading. Aggregation and sedimentation were observed for GPTMS-modified silica dispersed in toluene at any loadings. These results suggest that the dispersion behavior was tuned via these coupling reactions, and that it could be optimized by varying the coupling agents and/or loadings. In order to analyze the difference in the dispersion behavior between the two samples, surface interaction forces were measured using colloidal probe AFM in both water and toluene. When water was used as a solvent, a greater repulsive force was observed between the GPTMS-modified silica particle and the mica plate, with the forces increasing with increased loading. This repulsive force was not observed in the case for VTMS modification. In contrast, GPTMS-modified silica did not display repulsive forces in toluene, and attractive forces were observed at short separation distances, independent of loading. In the case of VTMS-modified silica, repulsive forces were observed between the silica and the mica plate, with an increase in repulsive forces upon increased loading. These results suggest that good dispersibility could be obtained from strong repulsive forces acting on the surface of particles, and that aggregation could be caused by weak repulsive forces and by attractive forces at shorter distances. It can be concluded that colloidal probe AFM is an effective tool to assess the dispersion/aggregation properties of surface-modified silica with regard to repulsive and/or attractive forces, and can therefore be useful for gaining a better understanding of dispersion behavior.
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