Investigation On Construction, Structure And Properties Of Polypropylene-Based Nanocomposites Containing Ag

This paper is based on size-controlled and large-scale synthesis of organic-soluble Ag nanocrystals (NCs) with highly hydrophobic surface characteristcs, and structure design and construction of isotactic polypropylene (iPP) nanocomposites containing Ag. Based on synthetic organic-soluble Ag NCs, PP/Ag and PP/MMT/Ag nanocomposites have been constructed by simple mix-blending and novel melt intercalation assembly methods, respectively. The microstructure, properties and novel characteristics of the as-prepared iPP nanocomposites were comprehensively investigated and characterized; the interface interaction mechanism and structure-property-performance relations of the nanocomposites were discussed in depth.Organic-soluble Ag NCs with tunable sizes have been successfully synthesized by a facile, reproducible and easily scaled up route using water as solvent in the presence of oleic acid and an alkylamine. The synthetic Ag NCs in the form of powders exhibited excellent hydrophobicity and could be well re-dispersed in various non-or weak-polar media. The tunable sizes of the Ag NCs, ranging from5.4to22.1nm, were achieved by simply adjusting the initial precursor AgNO3concentration. The smaller and more narrowly distributed Ag NCs, the higher precursor AgNO3concentration. Large-scale synthesis of the Ag NCs has been realized at a higher AgNO3concentration (0.4M). As much as16.2g of highly uniform organic-soluble Ag NCs with5nm average diameter can be readily synthesized at low cost in a single reaction only using400ml of water as solvent, which is adaptable to industrial scale production. The structure of the as-obtained Ag NCs was detailedly investigated by X-ray diffraction, transmission electron microscopy, Fourier transform infrared and thermal gravimetric analysis measurements. The’results revealed that the surface of organic-soluble Ag NCs was coated with a monolayer of the surfactants consisting of oleic acid and the alkylamine. In addition, on the basis of the sufficient evidences, a proposed mechanism based on the "bilayer surfactant-controlled reaction" successfully demonstrated the formation of the high quality Ag NCs in our synthesis.Functional iPP nanocomposites based on designedly synthetic Ag NCs coated with high surfactant content were prepared by simple melt-processing route. The synthetic Ag NCs with highly hydrophobic surface were finely and homogeneously dispersed in the iPP matrix due to their good interface compatibility and adhesion with the polymer. The well-dispersed Ag NCs was shown to be very efficient in improving and tailoring the rheological behavior, thermal stability, mechanical properties and antibaceterial efficiency of iPP. As compared to the pure iPP, a remarkable enhancement in thermal stability by more than80℃was observed with the addition of2.0wt%Ag NCs. The Ag NCs induced the formation of large amounts of B-form crystals, resulting in the signifcant improvement of toughness of iPP. The elongation at break and impact strength for the nanocomposites containing1.0wt%Ag NCs were approximately9and2.7times that of pure iPP, respectively. The incorporation of the Ag NCs is an effective strategy to balance the toughness and stiffness of iPP due to the β-nucleating and stiffening dual-effect. The antibacterial efficacy of the PP/Ag nanocomposites was evaluate using Escherichia coli and Staphylococcus aureus as the target bacteria. The results revealed that the addition of a extremely small amount (0.1wt%for plastics and100ppm for fibers) of the Ag NCs could obtain good antibacterial efficiency due to the fine particle dispersion.The crystallization and melting behavior of the iPP nanocomposites based on synthetic oganic-soluble Ag NCs under dynamic and quiescent conditions were investigated. The results showed that the synthetic Ag NCs, as a highly effective B-crystal nucleating agent, could induce the formation of large amounts of B-crystals during both dynamic and quiescent crystallization. The relative content of β-crystals strongly depended on Ag loading and cooling rate. The β-crystal content increased with increasing Ag loading, and interestingly, the β-crystal content in the nanocomposites nearly increased linearly with increasing Ag loading under dynamic conditions. With increasing cooling rate, the relative content of B-crystals increased. Under qiescent conditons, a slightly increased B-crystal content with decreasing crystallization temperature (Tc) suggested that Tc had little effect on the relative B-crystal content. However, the increased Tc was beneficial for the formation of β1-crystals. By comparison, the synthetic Ag NCs had much higher β-nucleating activity for iPP than a commercial Ag NPs. The dynamic crystallization kinetics was also analyzed. The results showed the Ag NCs, acting as nucleating sites, promoted the crystallization rate of iPP, and the acceleration of the crystallization rate came from the increased crystallizing growth points. The crystallization activation energy was calculated based on Kissinger’s method. The significantly decreased crystallization activation energy further confirmed that the presence of the Ag NCs made iPP molecular chains easier to crystallize, increased the crystallization rate and was favorable for the formation of high content of β-crystals.The conception and mechanism of melt intercalation assembly were put forward for the first time. Based on synthetic organic-soluble Ag NCs, novel PP/MMT/Ag nanocomposites have been constructed by the novel melt-intercalation assembly method. The results showed that the Ag NCs with small sizes (less than dooi of MMT) intercalated into the interlayers of MMT under shear, which increased the interlayer space of MMT and was beneficial for the further intercalation of iPP chains or large Ag NCs into the interlayers, and then facilitated the fine dispersion of MMT in iPP matrix, forming the combined structure composed of intercalated and exfoliated galleries. The Ag NCs were concentratedly dispersed in the regions where MMT existed and simultaneously adsorbed on the surface of MMT layers, forming the novel assembled structure of MMT-Ag. The Ag NCs adsorbed on MMT, acting as bridging points, increased the interaction and adhesion between MMT and iPP matrix. The increased interaction exibited a significant effect on the rheological behavior, thermal stability and mechanical properties of the composites. The synergistic effect of added Ag NCs and MMT not only resulted in a sharp enhancement of thermal stabilty of iPP by more than130℃but also improved the toughness and tensile strength. In addition, the PP/MMT/Ag nanocomposites with a small quality of Ag NCs have outstanding antebacterial efficacy.The researches in this paper offer the new method for size-controlled and large-scale synthesis of organic-soluble Ag NCs, which provide the basis for their applications. Based on synthetic organic-soluble Ag NCs, novel iPP containing Ag nanocompostes have been constructed, which opens up the new method for preparation of high-performace iPP functional nanocomposites and broadens the application areas of iPP-based materials.