Study On Interface Structure And Properties Of PA66/SiO₂ Nanocomposites

The PA66-based nanocomposites containing different surface-modified nano-SiO₂ were prepared in a twin-screw extruder by melt blending, and the mechanical properties of nanocomposites were investigated. The results of mechanical testing showed that the addition of reactable nano-SiO₂ (RNS) could reinforce the tensile strength and Yong's modulus of nanocomposites, but the change of notched impact strength was not obvious. The incorporation of dispersible nano-SiO₂ (DNS) could increase the notched impact strength of nanocomposites in the case of holding the Yong's modulus of material; but it would produce a negative influence on the tensile property of nanocomposites. It was obvious that the addition of different surface-modified nano-SiO₂ had a different effect on the mechanical properties of nanocomposites. In order to analyse more deeply the interaction between different surface-modified nano-SiO₂ and PA66 matrix during the preparation of nanocomposites, the extraction treatment of nanocomposites was used, and extraction products were investigated by thermogravimetric analysis (TGA), Fourier-transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM). The results showed that there was a strong interaction at the interface of RNS and PA66 matrix, which promoted the formation of interface structure based on hydrogen bonding and covalent bonding. It was evident that the formation of this interface structure would help to enhance the interfacial adhesion strength and improve stress transfer between nanofillers and matrix, accordingly increasing the tensile strength of nanocomposites. The increase of interfacial adhesion strength, in turn, could limit the movement of PA66 chains and facilitate the formation the high density region of PA66 chains around the nanoparticles, which could increase the rigidity of nanocomposites. For material toughness, however, the formation of high density region of PA66 chains was unbeneficial to the improvement of material toughness, but the existence of flexible modification layer in RNS surface was propitious to the increase of material toughness. So the impact strength of PA66/RNS nanocomposites depended on the competition between surface modification layer of RNS and the high density region of PA66 chains around silica nanoparticles. The interfacial interaction of DNS and PA66 matrix was not very strong, which was connected each other only by few hydrogen bonding and intertwist of chains segments. Obviously, the interface structure was not propitious to the increase of tensile strength and Yong's modulus of material, but due to well dispersible of DNS in PA66 matrix and the existence of surface flexible modification layer of DNS, its addiotn could help to the enhancement of notched impact strength of nanocomposites. It was seen from above analyses that the formation of different interface structure between RNS, DNS and PA66 was the main reason for the change of mechanical properties of their nanocomposites. In addition, the effect of different preparation processes on mechanical properties of nanocomposites was studied. Twice melt-blending method could farther reinforece the tensile strength and rigidity of material, but not improve the toughness of material; while masterbatching method could bring on the decrease of the whole mechanical properties of nanocomposites. The results showed that mechanical properties of nanocomposites were closely correlated with the choice of preparation process.In this paper, melt and crystallization behavior and dynamic mechanical properties of PA66/DNS and PA66/RNS nanocomposites were also investigated. The result showed that the addition of DNS and RNS decreased the melt and crystallization temperature of nanocomposites, and reduced the size of PA66 crystal. The discrepancy of both RNS and DNS was that RNS was a good nucleating agent, which could hasten the crystallization rate of PA66, shorten the half-time of crystallization, and markedly decreased the crystallization activation energy of nanocomposite system, accordingly leading to the increase of PA66 crystallinity, while DNS manifested a weak nucleating effect, which could lower the crystallization rate of PA66, lengthen the half-time of crystallization, and increase the crystallization activation energy of nanoccomposite system, making the crystallinity of nanocomposites decreased. In the study of dynamic mechanical properties of nanocomposites, it was found that the addition of RNS could increase the storage modulus of nanocomposites, whereas the case of DNS was opposite to that of RNS, which led to the slight decrease of storage modulus of nanocomposites. But both DNS and RNS could increase the mechanical loss of nanocomposites. For the glass transition temperature (Tg), the addition of RNS could increase the Tg of nanocomposites, whereas that of DNS could decrease it owing to different surface structure of RNS and DNS.