Abstract
In nanocomposites, the adhesion between nanofillers and the polymeric matrix is key to the mechanical properties. The strength and spatial distribution of the adhesive layer around the nanofillers are important, particularly the presence of chemical bonding between the nanofillers and matrix. In this work, we studied a styrene-butadiene rubber composite filled with silica nanoparticles to visualize the spatial distribution of the adhesive layer. A silane coupling agent (SCA) was added to the nanocomposite for strong adhesion. The reaction involving the SCA on the silica surface was investigated by scanning transmission electron microscopy combined with electron energy-loss spectroscopy. Si-L2,3 spectra of the silica-filled rubber nanocomposite without the SCA were the same around the nanofillers, whereas in the nanocomposite containing the SCA the spectra were position-dependent. The spectra were fitted with the intensity profiles of the Si-L2,3 spectra of silica and SCA by multiple linear least-squares fitting. The fitting coefficients of silica and SCA were used to map the spatial distribution of the chemical bonding between silica and rubber chains. Chemical bonding was observed around the silica nanoparticles but not in the SBR matrix region, providing direct evidence of the reinforcing mechanism in the silica-filled rubber nanocomposite.
Highlights
In nanocomposites, the adhesion between nanofillers and the polymeric matrix is key to the mechanical properties
The spectrum of silane coupling agent (SCA) showed three peaks at the similar energy positions at 106.0, 108.3, and 115.2 eV. These peaks were assigned as unoccupied electronic states in S iO44– tetrahedron clusters based on molecular orbital calculations[26,27]
The spatial distribution of SCA, which contributes to the adhesion between the silica nanoparticles and styrene-butadiene rubber (SBR) matrix, was visualized directly using Scanning transmission electron microscopy (STEM)-energy-loss spectroscopy (EELS)
Summary
The adhesion between nanofillers and the polymeric matrix is key to the mechanical properties. Silica nanoparticles are a typical filler used with cross-linked rubber These rubber nanocomposites have non-linear mechanical properties, including a strain-dependent dynamic modulus[6,7], high hysteresis[8], and stress softening[9,10]. The SCA forms covalent bonds between the silica filler and rubber, which affects the mechanical properties of the rubber nanocomposite substantially. The interfacial reaction between silica and rubber can be followed by spectroscopic techniques such as X-ray photoelectron spectroscopy[17,18], X-ray-induced Auger electron spectroscopy17, NMR19,20, and infrared spectroscopy[21,22] These studies showed that O–Si–C covalent bonds with the SCA were present in the rubber.
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