Xi-Qiang Liu

Control of morphology and properties by the selective distribution of nano-silica particles with different surface characteristics in PA6/ABS blends

The roles of nano-silica particles in the morphology, rheological, crystallization, and melting properties of polyamide 6/acrylonitrile–butadiene–styrene (PA6/ABS) blends were investigated. With the addition of nano-silica particles possessing different surface characteristics (hydrophilic or hydrophobic), the blends showed a notable difference in the morphology of the dispersed ABS phase in the melt or solid state, which is mainly caused by different distribution states for the two kinds of nano-silica particles. Particularly, it was found that the hydrophilic nano-silica particles tended to distribute in PA6 matrix, whereas the hydrophobic nano-silica particles were almost located at the PA6/ABS interface. Besides, the shear thinning behavior of the composites was significantly changed with the incorporation of hydrophobic nano-silica particles and the contribution from the interface was regarded as the dominated one to influence the rheological properties. Finally, with different surface characteristics and their distribution states, it was found that nano-silica particles can play different roles in crystallization and melting properties.

Temperature induced gelation transition of a fumed silica/PEG shear thickening fluid

The effect of temperature on the rheological behaviors of a shear thickening fluid (STF) prepared by dispersing fumed silica (SiO2) particles into polyethylene glycol (PEG) under mechanical stirring and ultrasonication was investigated using a rotational rheometer. Under steady shear, the system showed an obvious shear thickening behavior due to the formation of “hydroclusters” of SiO2 particles driven by hydrodynamic lubrication forces. The value of the critical shear rate at which the shear thickening begins grows monotonically with temperature. Dynamic temperature sweeps show that elevating the temperature induces a gelation transition of the SiO2/PEG system when the concentration of SiO2 exceeds a critical value, which is found to be lower for the system consisting of higher average molecular weight PEG. The gelation process also becomes more remarkable at a higher concentration of SiO2 particles. It is found that the temperature induced gelation of SiO2/PEG sol is essentially related to the disappearance of the solvation layer on the surface of SiO2 particles as well as the change of hydrogen bonds.

Suppression of phase coarsening in immiscible, co-continuous polymer blends under high temperature quiescent annealing

The properties of polymer blends greatly depend on the morphologies formed during processing, and the thermodynamic non-equilibrium nature of most polymer blends makes it important to maintain the morphology stability to ensure the performance stability of structural materials. Herein, the phase coarsening of co-continuous, immiscible polyamide 6 (PA6)-acrylonitrile-butadiene-styrene (ABS) blends in the melt state was studied and the effect of introduction of nano-silica particles on the stability of the phase morphology was examined. It was found that the PA6-ABS (50/50 w) blend maintained the co-continuous morphology but coarsened severely upon annealing at 230 °C. The coarsening process could be divided into two stages: a fast coarsening process at the initial stage of annealing and a second coarsening process with a relatively slow coarsening rate later. The reduction of the coarsening rate can be explained from the reduction of the global curvature of the interface. With the introduction of nano-silica, the composites also showed two stages of coarsening. However, the coarsening rate was significantly decreased and the phase morphology was stabilized. Rheological measurements indicated that a particle network structure was formed when the concentration of nano-silica particles was beyond 2 wt%. The particle network inhibited the movement of molecular chains and thus suppressed the coarsening process.

Nanoparticle retarded shape relaxation of dispersed droplets in polymer blends: an understanding from the viewpoint of molecular movement

The effect of nano-silica particles on the shape relaxation of dispersed droplets in polypropylene/polystyrene (PP/PS) blend was investigated. The nano-silica particles were controlled to be distributed in PS phase, which presented dispersed droplets in the blends. By using an optical microscope equipped with a camera and a hot stage, the shape relaxation of the deformed droplets in the blends after application of a big strain was observed in melt state. The shape relaxation process of PS droplets from highly elongated fibers towards spheres was observed. The nano-silica particles were found to retard the relaxation process of the deformed PS droplets, especially when a particle network was formed. It was showed that the shape relaxation of the droplets was essentially related to the movement of the molecular chains and the relaxation of the oriented chains. These results provide a new understanding of the evolution of morphology for nanoparticles filled polymer blends, based on the slow movement of molecular chains.

Suppressing phase coarsening in immiscible polymer blends using nano-silica particles located at the interface

The morphologies of polymer blends generated during processing are usually unstable and phase coarsening often occurs in the melt state, so suppressing the morphology coarsening is crucial to obtain polymer blends with tailored and stable structure and properties. The effect of nano-silica particles located at the interface on the phase coarsening of a polypropylene (PP)/polystyrene (PS) blend was studied in this work. In co-continuous 50/50 PP/PS blend, the particles at the interface can effectively suppress the coarsening process even at a very low particle loading. Real-time observation conducted by using an optical microscope equipped with a camera and a hot stage showed that a small loading of particles has little effect on the retraction process but can suppress the coalescence and at high loading of particles, both the retraction and coalescence process can be significantly suppressed. The suppressing effect towards the coalescence was confirmed in 70/30 blend with PS phase as the dispersed phase. The stabilization mechanism used in particle stabilized emulsions was adopted to explain the suppressing effect of nano-silica particles located at the interface towards the phase coarsening of PP/PS blends.

Suppressing phase retraction and coalescence of co-continuous polymer blends: effect of nanoparticles and particle network

The morphologies of polymer blends generated during processing are usually unstable and morphology coarsening often occurs in the melt state, so suppressing the morphology coarsening is crucial to obtain polymer blends with tailored and stable structure and properties. Here, we report the morphology coarsening behavior of a co-continuous polypropylene/polystyrene (PP/PS) blend, with and without nano-silica particles, subjected to quiescent annealing in the melt state. The filled nano-silica particles were controlled to selectively distribute in the PS phase and formed a rheological particle network at a concentration of 8 wt% relative to the mass of the PS phase. A significant coarsening, which can be divided into two stages with different coarsening rates, was observed for the pure blend. Real-time monitoring of the coarsening process showed that the morphology coarsening process proceeded via retraction of elongated domains first and then coalescence of the retracted domains. The filled nano-silica particles were found to be able to suppress the coarsening process, which was demonstrated to be achieved by slowing down the retraction process of elongated domains. It was also found that the suppression effect of nano-silica particles heavily depended on the particle concentration and when a particle network formed, the suppression effect was more prominent. A stabilization mechanism, based on the phase deformation being closely related to the movement of polymer molecular chains, was proposed to expound the role of the introduced particles. When a particle network structure was formed, the movement of polymer molecular chains was significantly retarded and the corresponding phase deformation became difficult, leading to suppressed retraction process of the elongated domains and the whole phase coarsening phenomenon.

Insight into the nucleating and reinforcing efficiencies of carbon nanofillers in poly(vinylidene fluoride): a comparison between carbon nanotubes and carbon black

Poly(vinylidene fluoride) (PVDF) nanocomposites containing homogeneously dispersed multi-walled carbon nanotubes (MWCNTs) and carbon black (CB) were fabricated by a small melt mixer. The uniform dispersion of the nanofillers in PVDF was confirmed by both scanning electron microscopy and transmission electron microscopy. Both the heterogeneous nucleation efficiency and crystallization half-time show that MWCNTs exhibit higher nucleation efficiency than CB for the crystallization of PVDF. Meanwhile, MWCNTs show greater contribution to the reinforcement of the storage modulus of PVDF as revealed by dynamic mechanical analysis, especially at low temperatures. However, the enhancement of the storage modulus in the melt state is reversed due to the network formed by serious agglomeration of CB. This study provides some insights into the nucleating and reinforcing efficiency of MWCNTs and CB in polymers.

Studies on the Blends of Polyamide66 and Thermoplastic Polyimide

The influence of the content of thermoplastic polyimide (TPI) on the structure and properties of polyamide66 (PA66)/TPI blends was studied. The results indicated that the addition of TPI showed little influence on the mechanical properties of the PA66/TPI blends, and the melting and crystallization behavior of the TPI/PA66 blends was not changed obviously. However, the addition of a small quantity of TPI significantly improved the heat resistance, and lowered the friction coefficient and the wear rate of the blends in comparison with pure PA66.