Preparation, Mechanical And Thermal Properties Of Polycarbonate/SiO₂ Nanocomposites

Polymer/silica nanocomposites have attracted increasing attentions due to the improvement in terms of thermal, mechanical, optical, electrical properties and so on. Theoretically, remarkable improvement in physical properties of polymers is expected when adding a small amount of nano-Si O2. Unfortunately, the agglomeration behavior and the poor interfacial interaction result in that the actual values of physical properties are far less than the theoretical values. However, the disadvantages of the conventional modified methods, such as complicated process and high-cost, make them unsuitable for mass production of polymer nanocomposites. For this reason, the main purposes of this paper are to explore the suitable modification methods for industrializing polymer/Si O2 nanocomposite and study the effects of modified Si O2 on the mechanical properties and thermostability of composites.Firstly, the conventional modifier, silane coupling agent, was used to modify nano-Si O2, then PC/Si O2 nanocomposites were melt mixed and injection molded with a low melt temperature. Furthermore, Fourier transform infrared spectrometer, thermogravimetric analyzer, X-ray diffraction, scanning electron microscope, tensile and impact tests were empolyed to analyze the modification effect of nanoparticles and the effects of nano-Si O2 on the interfacial and mechanical properties of composites. The primary results are as following:(1) The silane coupling agent(KH560) was successfully grafted on the surface of Si O2, which improves the dispersity of nano-Si O2 in matrix and interfacial interaction between particle and matrix.(2) The tensile strength and toughness of PC were simultaneously improved by introducing low content of nano-Si O2, expecially for modified Si O2. But the tensile properties of the composites with high particle content sharp drop due to the enhanced stress concentration effect. Besides, an unexpected ductile-brittle transition for impact toughness happens after adding silica nanoparticles.(3) The storage modulus of composites increased observably as a function of silica content, especially for the composites with modified silica due to the chain entanglement between KH560 and PC chains, but the Tg value changed little.Secondly, the thermal degradation mechanism and kinetics of the above 1 wt.% polycarbonate/silica nanocomposites were studied by thermogravimetric analysis coupled with differential scanning calorimetry, thermogravimetric analysis coupled with Fourier transform infrared spectrometry, scanning electron microscope, Fourier transform infrared spectrometry and X-ray photoelectron spectroscopy and Kissinger-Akahira-Sunose and Flynn-Wall-Ozawa methods, respectively. It is turned out that the thermostability of PC was significantly improved by incorporation of nano-Si O2, in particular for KH560-Si O2. The analysis for degradation products suggested that nano-Si O2 enhanced the thermostability of PC via the following three effects.(1) At the beginning of degradation, Si O2 nanoparticles could capture the radicals produced by the scission of isopropylidene, which leaded to the improvement of onset degradation temperature.(2) At the main degradation process, the silicon hydroxyl of nanoparticles induced the alcoholysis reaction of carbonate groups in PC and accelerated the degradation rate.(3) At the the middle and later periods of degradation, nano Si O2 were gradually linked by means of the reaction with carbonate groups so as to form a barrier network which gives rise to the increase of char residuals. Moreover, the degradation activation energies of both composites increased significantly relative to neat PC, especially for the composite with KH560-Si O2.Finally, based on the view of polymer process, degradative polycarbonate chains were successfully grafted on the surface of silica nanoparticles by reactive melt blending. The transesterification between carbonate groups and Si-OH and the condensation reaction between silicon hydroxyls and hydroxyl end groups of degradative chains were considered to be the main formation mechanisms of the Si-O-C bond. The application of functionalized Si O2 in PC nanocomposites was subsequently explored. The results indicated that incorporation of functionalized Si O2 could simultaneously enhance the tensile strength, toughness and storage modulus of PC due to the improvement of compatibility between PC matrix and PC-functionalized Si O2.