Reactive Extrusion And Application Of Polymer-based Nanocomposite Microspheres

The applications of organic-inorganic composite microspheres are pervaded in every field, which include bulk products, such as coating, paper, cosmetic, and high value added devices with special function, for example, microwave absorber, electrophoretic display, protein separation, targeting drug delivery, enzyme immobilization and biochemical reaction etc. Up to now, most of the polymer-based nanocomposite microspheres were prepared by various emulsion polymerization. These approaches are limited by many disadvantages, for instance, long preparing period, difficulty in nanoparticle dispersion, low encapsulation efficiency and nanoparticle content, and limitation in mechanical strength, wear resistance and heat resistance. To tackle these problems, based on the selective location of nanoparticles in one domain of immiscible melted polymer blends, we proposed a novel method to low-costly prepare size-controlable composite microspheres by reactive extrusion. In this work, various nanoparticles with different surface treatment, such as silica (SiO2), titanium dioxide (TiO2) and tetroxide (Fe3O4) nanoparticles, were mixing with the polystyrene (PS) and polyamide 6 (PA6) model blends to prepare PA6-based nanocomposite microspheres. The basic principle, processing technology, micro-morpology tailoring as well as application of reactive blending for fabricating polymer-based nanocomposite mocrospheres were systemically investigated.Firstly, the selective location of various nanoparticles in the PS/PA6 blends or in the reactively compatibilized PS/PA6 blends was studied. The results showed that the hydrophilic SiC>2 with-OH groups was preferentially distributed in the PA6 doamin having nothing with the size of nanoparticle, while, the hydrophobic SiO2 with-CH3 groups was selectively located at the interface of PS/PA6. For TiO2 filled PS/PA6 blends, when PS and TiO2 were blended first and then mixed with PS in the second step, the TiO2 was transferred itself from PS to the preferential PA6 phase and accumulated at the blend interface and in PA6. For PS/PA6/Fe3O4 blends, both the as-produced Fe3O4 and Fe3O4 with surface treated by a NH2-end silane coupling agent were selectively located in PA6 domain, while, the Fe3O4 with stearic acid surface-treatment was distributed at the interface. The results implied that the heterogeneous distribution of nanoparticles in immiscible polymer blends depends on the interaction between the nanoparticle and polymer chains (i.e. enthalpic interactions). In order to regulate the morphology and size of PA6 phase, a terminal maleic anhydride functionalized polystyrenes (FPS) was introduced to PS/PA6/Fe3O4 blends for reactive blending. It was found that a large part of Fe3O4 particles was pulled out from the PA6 phase or the interface to PS domain. The results showed that becaused the reaction of FPS with the surface ligands of particle is more competitive than that of FPS with PA6 chain, only when Fe3O4 is added after a complete reaction of FPS with PA6 could the Fe3O4 be preferentially dispersed into the PA6 domains.Secondly, the effect of selective dispersion of TiO2 in PA6 domian on the phase inversion and morphology evolution of PS/PA6 blend was researched. By adding a small amount of TiO2, the morphology of the PS/PA6 blend abnormally transformed from a co-continuous into a matrix-droplet structure. With further increases in nanoparticle loading, the increase of the content of PA6/TiO2in PS/PA6/TiO2 composite indued the transformation of PA6 phase from the dispersed phase to the continuous one, however, the PA6 domain becomes larger. Morphology evolution of PS/PA6/TiO2 blend during the static annealing process was real-time traced by optical microscopy. These experimental results are different from those in carbon black-filled or silica-filled immiscible polymer blends as reported previously. It was found that by adding a small amount of TiO2, the coarsening of PA6 domain is in accord with the Lifshitz-Slyozov-Wagner (LSW) mechanism, which is the same as the immiscible polymer blend. However, at higher TiO2 loads, the coarsening rate is sharply decreased. Further experiments revealed that unlike carbon black self-agglomeration to form 3D network structure, TiO2 nanoparticles appears to self-coagulation to form separated crowding of clusters in the PA6 phase, which induces PA6 phase to transformate from co-continuity to a matrix-droplet structure by adding low content TiO2 or by annealing process, and thus turns out a larger PA6 domain size at a higher TiO2 loading.Furthermore, the influences of the content of PA6, the viscosity ratio of PA6 and PS, the content of compatibilizer and annealing process glomerating PA6 domain on the diameter and size distribution of PA6-based microspheres were investigated. The results showed that a lower PA6 viscosity is favourable to decrease the size of PA6-based microspheres. By adding a small amount of FPS for reactive extrusion, it not only can sharply decrease the diameter of microspheres as well as the size distribution of microspheres, but also increase the phase inversion for PS/PA6 blend to 50/50. By modifying the surface of nanoparticles and optimizing the processing conditions, two kinds of uniform microspheres with different structure were fabricated. One owns structure in which nanoparticles are distributed evenly throughout PA6 microspheres, the other has structure in which nanoparticles were selectively located at the interior interface of PA6 microspheres. It should be mentioned particularly that PA6/Fe3O4 microspheres were prepared with number-average diameter of 1.4μm and very high Fe3O4 content. It's saturation magnetization is about 42.3 emu/g.Finally, the protein immobilization utilizing the reactively extruded PA6/Fe3O4 composite magnetic microsphere was preliminarily investigated. Carboxyl functional group, bonded with PA6/Fe3O4 microsphere by copolymerization of acrylic acid with PA6 chain was used as a ligand for protein adsorption. The results showed that the surface concentration of carboxylic acid of the functionalized microsphere can be add up to 1.0 m mol/g, and the adsorption capacity of BSA reaches 215mg/g microspheres which is much higher than the reported value, showing its potential for application in bioseparation and biomedical fields.