Controlling Hydrophobicity of Colloidal Silica to Promote Miscibility in Polymer Nanocomposites

This is the authors' abstract. We don't add key points for this paper.

Controlling the dispersion of nanoparticles in polymer matrices is desired for nearly all applications ranging from consumer electronics to automotive tires. In polymer nanocomposites, the commonly accepted picture is that individual nanoparticles are separated from each other in the polymer matrix, but this well-dispersed morphology is only realized in a small subset of model systems. Such model systems often rely on hydrophobically modified silica particles available from commercial suppliers. In this work, we investigate how the surface chemistry of hydrophilic colloidal silica can be rationally engineered to promote miscibility in organic solvents and in melts of semicrystalline polymers. The hydrophobicity of colloidal silica (LUDOX AS-30, DNP = 18 nm) is controlled via the addition of trimethylsilyl, −Si(CH3)3, (TMS) groups. 1H/29Si CP-MAS solid state NMR shows that both a commercially available organosol (Nissan Chemical MEK-ST, DNP = 16 nm) and the modified AS-30 NPs have partially silylated surfaces with similar areal densities of TMS. Concentrated (≥300 mg/mL) organosols of the hydrophobically modified silica are prepared in methyl ethyl ketone (MEK) and analyzed using dynamic light scattering (DLS), X-ray scattering (SAXS), and transmission electron microscopy (TEM). Particle size distributions before and after surface modification show that nanoparticle agglomeration is mitigated during the surface modification process. Electrophoretic light scattering (ELS) and SAXS were used to characterize the surface potentials and electrostatic interactions between particles stabilized in MEK. The surface-modified AS-30 particles are then dispersed in high-Mw poly(ethylene oxide) (PEO) and characterized using SAXS with in situ heating and TEM with cryo-microtomed slices. In agreement with previous work, interparticle structure factors, S(q), develop with prolonged thermal annealing in the polymer melt. Quantitative analysis of SAXS data suggests that the AS-30 particles are coated with repulsive PEO bound layers of thickness δ ≈ 7 nm. The results of this study indicate that the surface chemistry of hydrophilic colloidal nanoparticles can be rationally engineered to promote dispersion in commonly used polymers. Our perspective is that similar control of colloidal properties will enable researchers to fabricate well-dispersed polymer nanocomposites from a wider variety of nanoparticles and polymer materials.