Study On Rheological Behavior Of Core@shell Nanoparticles Filled Polystyrene Composites

Nanoparticles are widely used in cosmetic, functional fiber, high-grade painting, rubber, medicine fields and their extensive application prospects in resistant-water, durability, anti-pollution coatings and multi-function membrane materials. Incorporation of a few particles into polymers can obviously improve weather and abrasive resistances, mechanical, antibacterial and photocatalytic activity properties. Structure and property relationships of nanoparticles filled polymer have been becoming research fronts and hotspot of nanomaterials science.In this dissertation, Polystyrene (PS) coated silica (SiO2) core@shell composite particles (SiO2@PS) were prepared via conventional emulsion polymerization and miniemulsion polymerization, respectively. Effect of two polymerization methods on composites particles structure and morphology were discussed in detail. Rheological behavior of untreated SiO2and SiO2@PS particles filled polymer solutions and melts were systematically investigated through steady rheological measurement, dynamic rheological measurements, thermal analysis, Transmission electron microscopy (TEM) and Scanning electron microscopy (SEM) observations in order to establish the correlation between particles dispersion stability and rheological behavior, and reveal the viscoelastic nature of core@shell particles filled polymer.Comparision with the two preparation methods, the composite particles of bigger particle size and wider distribution prepared by conventional emulsion polymerization have multi-core@shell structure, and morphology features of composite particles could not be controlled. On the contraty, morphology features of nanoscale core@shell composite particles with uniform particle size and narrow distribution prepared by miniemulsion polymerization could be effectively controlled through changing reaction conditions. Best design proposal:0.225wt%sodium dodecyl sulfate (SDS),5wt%Hexadecane (HD), reaction temperature80℃。Rheological behavior and dispersion stability of SiO2@PS (290nm) suspended in10wt%PS solution were compared with suspensions of untreated SiO2and silane modified SiO2nanoparticles. Suspensions of the untreated and the silane modified SiO2obviously exhibited shear thinning. The SiO2@PS suspension exhibited shear viscosity considerably smaller than suspensions of untreated and silane modified SiO2at low shear rates. TEM showed that the composite particles can uniformly and stably disperse in PS solution compared to other suspensions, implying that the PS shell can effectively enhance the particle compatibility with PS macromolecules in solution.Shell-crosslinked core@shell nanoparticles (SCCSN) of32-98nm and containing58.6wt%polystyrene (PS) shell were prepared using miniemulsion polymerization. Dynamic rheology of SCCSN suspended in PS/toluene solution was compared with suspensions of naked silica. The critical strain (γc) for onset of rheological nonlinearity was independent of SCCSN concentration above a concentration threshold, which differs from the silica suspensions. Linear dynamic rheological investigation revealed that the SCCSN suspensions with20vol%PS volume fraction were fluid-like at low particle concentrations while the suspension containing4.2vol%SCCSN formed a gel-like structure. On the contrary, the silica suspensions with20.0vol%PS underwent a fluid-to-solid like transition with increasing silica concentration.Steady and dynamic rheologies of SCCSN (63-104nm,79.05wt%PS) filled PS were compared with bare silica filled PS. Time sweeps were performed to investigate the rheological stability of the filled melts during thermal annealing. The results showed that the SCCSN filled PS composites exhibited exceedingly good rheological stability than silica filled ones during annealing. Both bare silica and SCCSN introduced a non-terminal dynamic rheology while they did not introduce additional mechanism responsible for origination of nonlinear steady flow except for macromolecular disentanglement of the PS matrix. The reinforcement of SCCSN to PS was related to the silica core even though the crosslinked shell could effectively eliminate filler aggregation as the case of silica filled PS.The structure characterization of SCCSN were examinated by thermogravimetry (TG), differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA). It was found the glass transition temperature of SCCSN obviously increased with increasing divinyl benzene amounts. Thermal analyses showed that both silica and SCCSN led reduction in glass transition temperature of PS. Dynamic rheology showed the reinforcement mechanism of SCCSN filled polymer is related to the formation of a jammed network is involved in the absorbed PS chains providing the mechanical bridges between SCCSN. The SiO2core in SCCSN exhibited a reinforcement effect slightly stronger than bare SiO2particles and the reinforcement was slight stronger with increasing the crosslinking degree of the PS shell. In addition to the a relaxation associates with the glass transition of the polymer matrix, a slower a’relaxation was observated in66nm SiO2and SiO2@PS filled PS nanocomposites using dielectric relaxation spectroscopy. No significant effects were observed on the a relaxation of polymer matrix.