Miqiu Kong

Morphology and rheology of poly(l-lactide)/polystyrene blends filled with silica nanoparticles

The successful development of co-continuous structure from poly(l-lactide) (PLLA) blends by melt mixing with lower PLLA content is highly desired in preparing macroporous biomaterials. However, the low viscosity of PLLA makes it difficult to prepare co-continuous PLLA blends at low PLLA concentration. In this study, hydrophilic silica nanoparticle is adopted to control the morphology of co-continuous polystyrene (PS)/PLLA blends. The influence of nanoparticle concentration on the co-continuity intervals and rheological properties of PS/PLLA blends are examined. The morphological stability of blends against melt annealing is also determined and discussed with a conceptual coarsening model for co-continuous structure. The results demonstrate that the incorporation of silica nanoparticles into PS/PLLA blends can be used to prepare macroporous PLLA structure with controllable pore size at lower PLLA content.

Preparation of alumina-coated graphite for thermally conductive and electrically insulating epoxy composites

Herein, highly thermally conductive and insulating epoxy composites were reported. Firstly uniform alumina-coated graphite flakes were successfully prepared by a two-step coating method of chemical precipitation with the aid of a sodium dodecyl sulfonate (SDS) surfactant using an inorganic precursor (aluminum nitrate) as the starting material. Then the alumina-coated graphite particles were incorporated into the epoxy resin. The thermal conductivity value of epoxy/alumina-coated graphite composite shows a significant increase from 0.22 W mK−1 (neat epoxy) to 0.64 W mK−1 by a factor of approximately 3 at the filler loading of 18.4%. Moreover, due to the presence of the alumina nanolayers coating on the graphite surface, epoxy/alumina-coated graphite composites could retain high electrical volume resistivity of >1010 Ω cm up to high filler contents, which was much higher than that of epoxy/graphite composites (<105 Ω cm) at the same filler loadings. And they still could be regarded as insulators.

Fractionated crystallization and morphology of PP/PS blends in the presence of silica nanoparticles with different surface chemistries

The fractionated crystallization behavior of polypropylene (PP) droplets in its 20/80 blends with polystyrene (PS) in the presence of hydrophilic or hydrophobic fumed silica nanoparticles was studied by using differential scanning calorimetry, scanning electron microscopy, and transmission electron microscopy. It was found that the fractionated crystallization of PP droplets in the PS matrix was promoted by adding a low content of hydrophobic or hydrophilic nanoparticles due to their morphological refinement effect. However, discrepancies in the fractionated crystallization behavior of PP droplets occurred as the nanoparticle content increased. The crystallization became dominated by the heterogeneous nucleation effect of high content of hydrophilic nanoparticles, which possibly migrated into PP droplets during mixing and significantly suppressed their fractionated crystallization. In contrast, the morphological refinement effect still played a dominated role in promoting the fractionated crystallization of PP droplets in PP/PS blends filled with higher content hydrophobic nanoparticles as a result of the efficiently morphological refinement effect.

Unusual hierarchical structures of micro-injection molded isotactic polypropylene in presence of an in situ microfibrillar network and a β-nucleating agent

The microstructural and mechanical properties of isotactic polypropylene (iPP), in situ PET microfibrils, and β-nucleating agent blends obtained from micro-injection molding were investigated via polarized light microscopy, differential scanning calorimetry, scanning electron microscopy, and two-dimensional wide-angle X-ray diffraction. The results indicate that addition of PET microfibrils markedly increases crystallization temperatures, and increases the thickness of the final oriented layer. Introduction of PET microfibrils to β-nucleation agent-nucleated iPP samples leads to formation of oriented β-crystals epiphytic on the surface of PET fibers in the inner region; this feature improves adhesion between the fiber and the matrix and simultaneously improves the strength and toughness of the final PP/0.5/15 microparts (e.g., the tensile strength increased by 12 MPa and the elongation at break increased by 1.2%) compared with those of iPP microparts. Taken together, the results of this study introduce an alternative approach to optimize the properties of MIM parts.

Morphological hysteresis in immiscible PIB/PDMS blends filled with fumed silica nanoparticles

The morphological hysteresis behavior of immiscible polymer blend reflects the dependence of their steady-state morphology on the shear protocol applied. In this work, the influences of hydrophobic and hydrophilic fumed silica nanoparticles on the morphology hysteresis behavior of immiscible polyisobutylene (PIB)/polydimethylsiloxane (PDMS) (10/90) blends under simple shear flow were investigated by using optical shear technique. Compared with particle-free blend, the morphology hysteresis zone of filled blends was found to be expanded by the addition of hydrophobic or hydrophilic fumed silica nanoparticles. It was found that the expansion of the morphology hysteresis zone in hydrophobic nanoparticle-filled blend stemmed from the suppression of droplet coalescence. However, the expansion in the morphological hysteresis zone for hydrophilic nanoparticle-filled blend, which was less noticeable, might originate from the more difficult breakup of PIB droplets upon the addition of nanoparticles.

Pickering emulsions stabilized by shape-controlled silica microrods

Silica microrods with aspect ratios (AR) varying from 1 to 16 but similar surface chemical characteristics are synthesized and their potential in preparing stable oil-in-water Pickering emulsions is explored. The stability of hexadecane/water emulsions is found to strongly depend on the AR and concentration of particles. Emulsions stabilized with these silica microrods are quiescently stable for quite a long period of time (over months), while emulsions with spherical particles of similar diameters destabilize after only dozens of hours. The superior stabilization efficiency of microrods with larger ARs is attributed to their higher steric hindrance, interface adsorption energy and capillary forces.

Retarded stress and morphology relaxation of deformed polymer blends in the presence of a triblock copolymer

The effect of linear triblock copolymer compatibilizer (styrene–ethylene/butylene–styrene, SEBS) with strong viscoelasticity on the stress relaxation behavior of polypropylene (PP)/polystyrene (PS) (20/80) blends under various shear deformations has been investigated. With the addition of SEBS, the initial deformation of dispersed droplets under step shear strains was suppressed, and the following stress relaxation was found to be continuously retarded. The strain sensitivity of the stress relaxation modulus became weaker with the addition of SEBS possibly due to the improved viscoelasticity and interfacial adhesion. But the increase of strain led to more pronounced retardation in the stress relaxation of compatibilized blends. These phenomena were discussed in terms of the competitive effect of morphology refinement and the changes in interfacial and viscoelastic properties brought by compatibilization. The dominant factors determining the relaxation behavior were suggested to rely on the SEBS loading.

Stress relaxation behavior of co-continuous PS/PMMA blends after step shear strain

Stress relaxation probing on the immiscible blends is an attractive route to reveal the time-dependent morphology–viscoelasticity correlations under/after flow. However, a comprehensive understanding on the stress relaxation of co-continuous blends, especially after subjected to a shear strain, is still lacking. In this work, the stress relaxation behavior of co-continuous polystyrene/poly(methyl methacrylate) (50/50) blends with different annealing times, strain levels, and temperatures was examined under step shear strain and was correlated with the development of their morphologies. It was found that co-continuous blends display a fast relaxation process which corresponded to the relaxation of bulk polymer and a second slower relaxation process due to the recovery of co-continuous morphology. The stress relaxation rates of co-continuous blends tend to decrease due to the coarsening of instable co-continuous structure during annealing. Furthermore, the stress relaxation of the co-continuous blends is strongly affected by the change of viscosity and interfacial tension caused by the temperature. The contribution of morphological coarsening, viscosity, and interfacial tension variation on the stress relaxation behavior of co-continuous blends was discussed based on the Lee–Park model and time–temperature superposition principle, respectively.

New understanding of the hierarchical distribution of isotactic polypropylene blends formed by microinjection-molded poly(ethylene terephthalate) and β-nucleating agent

Blends of isotactic polypropylene (iPP) and poly(ethylene terephthalate) (PET) were prepared by using a special injection molding process named microinjection molding (MIM). Interestingly, a strong continuous shear flow imposed on the melt of iPP/PET directly promotes the formation of the in situ PET microfibrils under microinjection molding. The hierarchical structures, including the shish-kebab-like structure, β-cylindrite, β-spherulite and α-spherulite are simultaneously formed in the iPP/PET microparts, which are closely related to formation of row-nuclei induced by the strong shear flow that is further amplified by incorporating in situ PET microfibrils. A surprising synergetic effect is observed between PET microfibrils and β-NA, resulting in the coexistence of shish-kebab, shish-kebab-like β-cylindrite, β-cylindrite and oriented β-crystal epiphytic on the surface of PET fibers in PP/0.1/15 microparts for the first time. Mechanical properties (e.g., tensile strength increased by 10.4 MPa) of the specimen are significantly improved compared with that of the iPP microparts because of the abundant hierarchical structures. A schematic model of the formation of hierarchical distribution of β-crystals via PET and β-NA addition is thus proposed.

Elongation thinning and morphology deformation of nanoparticle-filled polypropylene/polystyrene blends in elongational flow

Elongational thinning is observed in model polypropylene/polystyrene (PS) blends filled with nanoparticles at nanoparticle loadings ≥ 3 vol. % and high strain rates, wherein the nanoparticle network that forms at rest is destroyed by the deformation. The elongational thinning is stronger in blends with hydrophilic silica surfaces than with hydrophobic silica, apparently due to stronger interaction within the hydrophilic silica network and between PS and hydrophilic silica. Moreover, the elongational deformation of droplets is not significantly altered by the addition of hydrophilic silica at lower silica loadings but nearly completely inhibited at higher loadings. In contrast, hydrophobic silica significantly boosts the deformation of droplets in elongation possibly because of the reduced interfacial tension due to the preferential distribution of hydrophobic silica at the droplet-matrix interface, relative to the hydrophilic silica which resides mostly in the PS matrix.Elongational thinning is observed in model polypropylene/polystyrene (PS) blends filled with nanoparticles at nanoparticle loadings ≥ 3 vol. % and high strain rates, wherein the nanoparticle network that forms at rest is destroyed by the deformation. The elongational thinning is stronger in blends with hydrophilic silica surfaces than with hydrophobic silica, apparently due to stronger interaction within the hydrophilic silica network and between PS and hydrophilic silica. Moreover, the elongational deformation of droplets is not significantly altered by the addition of hydrophilic silica at lower silica loadings but nearly completely inhibited at higher loadings. In contrast, hydrophobic silica significantly boosts the deformation of droplets in elongation possibly because of the reduced interfacial tension due to the preferential distribution of hydrophobic silica at the droplet-matrix interface, relative to the hydrophilic silica which resides mostly in the PS matrix.