| The different surface modified nanosilica/PA6 hybrid materials with excellent performance were synthesized by in-situ polymerization, and the mechanical properties, thermal properties and melt crystallization behaviors of hybrid materials were evaluated. The effects of different kinds and composition of modifier on the surface of nanosilica and the way of adding nanosilica on the properties of hybrid materials were analyzed, and found that the different interface structure between PA6 matrix and nanosilica resulted in distinctly different properties of hybrid materials.1. When nanosilica was introduced into the reaction system at the early stages of polymerization, the results showed that both surface-modified nanosilicas containing saturated carbon chains (DNS-2 and DNS-3) and amino group (RNS-A) were well dispersed in PA6 matrix and were all able to effectively improve the comprehensive properties of PA6 even at a low dosage (≤1wt%). Thermogravimetric analysis (TGA) and Fourier transform infrared spectrometric (FT-IR) analysis etc. indicated that DNS-2 and DNS-3 were linked with PA6 matrix via hydrogen bond or physical adsorption, while RNS-A could act as a monomer to participate in the polymerization of PA6 and form local three-dimensional network structure via covalent bonding.2. DNS-2, DNS-3 and RNS-A could all strengthen and toughen PA6 matrix, but they had different strengthening and toughening effects. Of all the three kinds of nanosilica, RNS-A was the best one to improve the notched impact strength and tensile strength of PA6, because it was able to form three-dimensional network structure in the hybrid system. As RNS-A was doped in PA6 matrix at a dosage of 0.5wt%, resultant hybrid material had the maximum impact strength and tensile strength which were higher than that of PA6 matrix by 45.7% and 18.2%, respectively. DNS-3 could greatly increase the tensile strength of PA6. The PA6 hybrid doped with 0.2wt% and 1.0wt% DNS-3 possessed the maximum impact strength and tensile strength which were higher than that of PA6 matrix by 12.5% and 34.5%, respectively. Besides, DNS-2 could moderately increase the tensile strength but had little effect on the notched impact strength of PA6. The PA6 hybrid material doped with 0.4wt% DNS-2 possessed the maximum impact strength and tensile strength which were higher than that of PA6 matrix by 9.8% and 20.7%, respectively. 3. Both DNS-2 and DNS-3 had little effect on the thermal stability of PA6, while RNS-A significantly increased the thermal decomposition temperature of PA6. This is because RNS-A could participate in the polymerization of PA6 and form local three-dimensional network structure via covalent bonding, greatly limiting the movement of PA6 molecular chains. Besides, the destruction of a large number of interface chemical bonds require more energy. The thermal decomposition temperature of PA6 could be increased by 10℃when it was doped with 1.5wt% RNS-A.4. DNS-2, DNS-3 and RNS-A all had little effect on the melt temperature of PA6 but resulted in endothermic peaks at lowered temperature. In the meantime, DNS-2, DNS-3 and RNS-A could act as heterogeneous nucleation agents to increase the crystallization rate, crystallization temperature and crystallinity of PA6. However, the strong interfacial interaction between RNS-A and PA6 matrix greatly limited the movement of PA6 molecular chain; and the crystallization temperature and crystallinity of resultant PA6 hybrid material were shifted to higher temperature and then decreased with increasing content of RNS-A.5. When nanosilica was introduced in the early stage of polymerization reaction, we found, by adjusting the ratio of saturated carbon-chain compound and amino groups on the surface of nanosilica, that the amount of nylon chains grafted onto the surface of nanosilica was gradually reduced with decreasing amount of amino groups. This might be because the amount of amino groups participating in the polymerization of PA6 was reduced therewith. Besides, the mechanical properties of hybrid materials decreased gradually with reducing amount of amino groups, and the melt temperature, crystallization temperature and crystallization rate of the hybrid materials also tended to decline.6. When nanosilica was introduced into the reaction system at different periods of polymerization, by comparing the interfacial interaction between RNS-A and PA6 matrix and its effect on the mechanical properties and melt crystallization behavior of hybrid materials, we found that, when RNS-A was added at a postponed time, the amount of nylon molecular chains grafted onto the surface of nanosilica increased. This might be because nylon molecular chain had grown fully at an extended polymerization duration, allowing grafting of longer nylon molecular chains onto the surface of RNS-A. In the meantime, longer nylon molecular chains were beneficial to enlarging the grid of three-dimensional network structure, causing differences in the properties of the hybrid materials. When 0.5wt% RNS-A was introduced in the reaction system after polymerization reaction was conducted for four hours, resultant PA6 hybrid material possessed the best notched impact strength and tensile strength which were higher than that of PA6 by 113.4% and 48.2%, respectively. When RNS-A was added at postponed polymerization duration, resultant hybrid material had almost unchanged melt temperature, decreased crystallization temperature and crystallization rate, while its low temperature endothermic peaks disappeared, and its crystallinity was equal to or higher than that of pure PA6.
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