Nanotribological interactions at the interface between polydimethylsiloxane and silane-modified silica

Abstract

This research was conducted to show that long alkyl chains of silane, grafted on the surface of silica, can interact with liquid polydimethysiloxane (PDMS) matrix through a so called nanotribological interaction at the interface. Some nonlinear behavior was reported as the result of this interaction. For this purpose, composites of PDMS and silica modified by silanes with two different alkyl chain lengths of three (short) and sixteen (long) carbons were prepared. Also, small molecule silicone oil was utilized as the matrix to be compared with the rubber matrix in this type of interaction. Initially, the surface energy of modified silicas as an enthalpic driving force for polymer-filler and filler-filler interactions were made almost equal in order to reveal features of nanotribological interactions resulting from long alkyl chains of silane. Microscopic results indicated that long alkyl chains of silane are effective in better dispersion of silica particles in the rubber matrix. Also, the strain-sweep rheological behavior of the composites showed that at concentrations below the percolation threshold of silica, the long alkyl chains of silane can tribologically engage with the rubber matrix, resulting in an instability in storage modulus, even with better dispersion of particles. This observation was attributed to some engagement-disengagement of polymer at the interface rather than networking of particles, known as the Payne effect. However, silica grafted by the short chain silane did not show this behavior. This type of interaction was showed to be also effective in filler-filler interactions and networking for silica grafted with long chain silane, high above the percolation threshold of silica in PDMS. It was shown that this type of interaction is absent in silicone oil, revealing the essential role of molecular weight of matrix in nanotribological interactions with the long alkyl chains of silane.