| It is generally acknowledged that introduction of inorganic nanoparticles into the polymermatrix uniformly by in-situ polymerization can endow the composite materials with variousexcellent properties. However, many papers and our previous studies have shown that thepresence of a small amount of inorganic nanoparticles (<2wt%) remarkably reduced theirmechanical and thermal properties, even though these nanoparticles were evenly distributed inthe prepared composite materials. These defects, to some extent, limit the application of organicnanoparticles. Therefore, it is significant to investigate the intrinsic reason of above-mentionedproblem, which will finally benefit the improvement of the properties of composites.In this study, nanosilica particles (SiO2) and poly (methyl methacrylate)(PMMA) have beenused as the typical functional nanoparticles and organic substrate, respectively. Firstly, the SiO2were modified with3-(trimethoxysilyl) propyl methacrylate (MPS) and octyltrimethoxysilane(OTMS), respectively. A series of SiO2nanoparticles containing various graft percentage ofsurface group (MPS or OTMS) were successfully designed and prepared. Meanwhile, these SiO2nanoparticles were characterized by dynamic light scattering (DLS), Fourier transformedinfrared (FTIR), element analysis and transmission electron microscope (TEM). Then thesemodified nanoparticles were re-dispersed into methyl methacrylate (MMA) monomer by solventreplacement technique, and a series of PMMA/SiO2composites were prepared via in-situ bulkpolymerization and in-situ solution polymerization. Accordingly, the molecular weight ofpolymer matrix and the mechanical properties of composite materials were examined. Based onabove-mentioned studies, the intrinsic relationship of the surface properties of SiO2, themolecular weight of polymer matrix and the mechanical properties of composite materials hasbeen thoroughly clarified. Moreover, the influence of the inorganic particles and its surfacegroup on the polymerization kinetics of free-radical polymerization of acrylate monomer wasalso studied in detail. An influence mechainism has been proposed.First of all, the controllable modifications on the surface of SiO2using MPS or OTMS werestudied. The modification was carried out through the dehydration condensation reactionbetween the methoxyl of siloxane and silicon hydroxyl of SiO2. It is well known that MPS has a double bond, but OTMS has no double bond. Besides this difference, the molecular structure andmolecular weight of MPS and OTMS are very similar. As a result, the methoxyl reactivities oftwo siloxanes are also similar. Therefore, the anchorage densities on the surface of the SiO2arevery close at the same dosage of MPS and OTMS. After the modification, the hydrophobicity ofSiO2and the compatibility between SiO2and monomer were significantly enhanced, resulting inthe improvement of dispersibility of SiO2in monomer. Thus, this kind of modified SiO2issuitable for the in-situ polymerization, and the anchorage density of silanes could beconveniently adjusted and controlled by varying the dosage of silane.Secondly, the bulk free-radical polymerization of MMA was carried out in the presence ofSiO2with different surface modifications to prepare PMMA/SiO2composites. The role of themodified SiO2in the in-situ polymerization was illuminated. Herein, the study was focused onthe influences of the type and density of anchored silanes, and the SiO2content on thepolymerization kinetics, molecular weight of prepared PMMA, and hardness of the SiO2/PMMAcomposites. It was found that the polymerization rate is obviously reduced by the addition ofSiO2nanoparticles. It can be attributed to the inhibition and retardation effect induced by thesilicon hydroxyl of SiO2,and the double bond of MPS when the SiO2was modified with MPS.As a result, both the molecular weight of PMMA and the hardness of the composites decreasedsignificantly. However, when the SiO2was modified with OTMS (no double bond), the siliconhydroxyl on SiO2surface decreased. Accordingly, the inhibition and retardation effect wasreduced. With increasing amount of OTMS on SiO2surface, the molecular weight of producedpolymers and the hardness of the composites increased gradually. When the anchorage density ofOTMS on SiO2surface reached the saturated value, the inhibition and retardation effect could beignored. Based on these results, an inhibition and retardation mechanism of the SiO2in the thein-situ bulk polymerization of MMA was proposed.Moreover, the in-situ solution polymerization of MMA was also performed in the presence ofSiO2with saturated surface modification to understand the role of SiO2in solutionpolymerization. On the one hand, the inhibition and retardation effect of active group on thesurface of SiO2is greatly reduced due to the dilution effect of solvent; on the other hand, thedilution effect can promote the outspread of anchored silane molecular chain, and hence a smallamount of residual hydroxyl would be exposed. The exposed hydroxyl would annihilate part of living radical in the reaction system，resulting in the reduction of polymerization rate in thein-situ solution polymerization.This research is a continued work of preparation and characterization of high dispersibilitySiO2/PMMA composite material. Our research work is focused on studying the influence ofnano-SiO2on the in-situ polymerization kinetics and the properties of final composite materials.On this basis, the influence mechanism of inorganic particles and its surface group on the chaingrowth and chain termination in radical polymerization has been clarified. This research resultcan take an important guiding role in the design and fabrication of composite materials with highperformance... |
PDF Full Text Request