Polydimethylsiloxane at the interfaces of fumed silica and zirconia/fumed silica

Polydimethylsiloxane (PDMS)/fumed silica A-300 and PDMS/ZrO₂/A-300 were studied using adsorption, thermogravimetry, temperature-programmed desorption (TPD) mass-spectrometry, infrared spectroscopy, XRD, and broadband dielectric relaxation spectroscopy. ZrO₂ was synthesized on fumed silica with zirconium acetylacetonate in CCl₄ at 350 K for 1 h and calcinated at 773 K for 1 h (1-4 reaction cycles). PDMS (5-40 wt.%) was adsorbed onto silica and zirconia/silica from hexane solution and then dried. Grafted zirconia changes the chemistry of the surface (because of its catalytic capability) and the topology of secondary particles (because of occupation of voids in aggregates of primary silica particles by zirconia nanoparticles) responsible for the textural porosity of the powders. Therefore, many properties (such as structural characteristics of the composites, reactions on heating in air and vacuum, interfacial relaxation phenomena, hydrophobicity as a function of treatment temperature, etc.) of PDMS/zirconia/silica strongly differ from those of PDMS/A-300. Broadening of the α-relaxation of PDMS at the interfaces of disperse oxides suggests both weakening of the PDMS-PDMS interaction and strengthening of the PDMS-oxide interaction.