Yadong Lv

The intrinsic thermal-oxidative stabilization effect of chemically reduced graphene oxide on polypropylene

The antioxidative effect of chemically reduced graphene oxide (rGO) on the thermal-oxidative stability of polypropylene (PP) was evaluated by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). rGO was prepared by reduction of graphene oxide (GO) and characterized by atomic force microscopy, X-ray photoelectron spectroscopy and X-ray diffraction. PP/rGO nanocomposites were then prepared without using a compatibilizer by melt blending. It was found that the thermal-oxidative degradation of PP was retarded noticeably by the rGO. The stabilization mechanism of rGO was discussed in terms of the changes in carbonyl bands and oxygen diffusion. It was proposed that the improved thermal-oxidation stability of PP/rGO nanocomposites can be attributed to the decline in both the concentration of peroxy radicals and oxygen permeability. The acceptor-like electronic property afforded by the long conjugated CC bonds and the barrier effect of rGO were suggested to be responsible for the improved thermal-oxidation stability of PP.

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.

Dispersion and rheology of polypropylene/organoclay nanocomposites: effect of cation exchange capacity and number of alkyl tails

Nanocomposites of montmorillonite organoclays and polypropylene (PP) were prepared via direct melt intercalation using maleic anhydride functionalized polypropylene (PP-g-MA) as a compatibilizer. Two montmorillonite clays (MMT) with different cation exchange capacities (CEC) were exchanged with alkyl ammonium ions, in which one or two octadecyl chains are attached to the nitrogen atom. The role of alkyl chain numbers and CEC value on the dispersion of clay and rheology of PP nanocomposites under shear and extensional flow was evaluated by X-ray diffraction, scanning electron microscopy, and rheologic techniques. It was found that the low-CEC organoclay with one alkyl chain could only form a conventional composite. However, the low-CEC organoclay with two alkyl chains or high-CEC organoclay with one alkyl chain can disperse finely in the matrix. Nanocomposites containing these two organoclays showed typical shear rheologic properties of intercalated nanocomposites, but only the former showed a mild strain-hardening behavior in uniaxial extensional flow. When using an intercalant with two tails, the high-CEC clay would lead the organoclay to form mixed structures which further resulted in an inferior dispersion quality. It was proposed that the dispersion quality and rheologic properties of nanocomposites were related to the arrangement of modifier molecules in the clay galleries, which was determined by the CEC of clay and the structure of alkyl ammonium ions.

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.

Effect of Compatibilizer Content on the Shear and Extensional Rheology of Polypropylene/Clay Nanocomposites

The shear and extensional rheology of polypropylene (PP)/organoclay nanocomposites in the presence of various maleic anhydride grafted polypropylene (PP-g-MA) compatibilizer concentrations were investigated. The PP nanocomposites were prepared via direct melt intercalation in an internal mixer. The structures of the nanocomposites were characterized by X-ray diffraction (XRD) and scanning electron microscopy. It was found that both the compatibilized and uncompatibilized nanocomposites could form an intercalated structure. However, the organoclay particles can disperse well only in the compatibilized systems. The linear viscoelastic properties, including the storage modulus G′ and complex viscosity η* were very sensitive to the microstructure of the nanocomposites. The extensional viscosities of PP nanocomposites were enhanced under a low deformation rate with increasing compatibilizer content and displayed a lack of superposition for different strain rates. It was proposed that the lack of superposition might originate from the formation of a three-dimensional organoclay network, which decreased in its complexity and strength as the deformation rate increased.

Microstructure studies of isotactic polypropylene under natural weathering by positron annihilation lifetime spectroscopy

The microstructure evolution of isotactic polypropylene (PP) exposed to subtropical humid climate of Guangzhou of China, were investigated by gel permeation chromatography, Fourier transform infrared spectroscopy, differential scanning calorimeter, dynamic mechanical analysis and positron annihilation lifetime spectroscopy. Positron data showed that the free volume of PP matrix decreased with involving a shrinking of the free volume hole sizes as the extent of weathering degradation of PP aggravated. The shrinkage of free volume hole sizes may be traced to the loss of mobility of molecules of PP matrix. The increase of the glass transition temperature substantiated undoubtedly the decrease of molecular mobility of PP chains. The increase in crystallinity might increase the amount of rigid amorphous fraction of PP matrix, which induced the loss of molecular mobility. Furthermore, the decrease of ortho-positronium formation ought to be correlated to the increase in crystallinity and the increasing amount of scavenchers which were in this work represented by the carbonyl groups.

Rheological behaviors and electrical conductivity of long-chain branched polypropylene/carbon black composites with different methods

The isotactic polypropylene/carbon black (iPP/CB) and the long-chain branched polypropylene/carbon black (LCBPP/CB) composite melts with the melt blending method and the solution process were chosen in this paper to know the relationship between rheological and electrical percolation process and learn the evolution and the destruction of rheological network. The more rheological percolation threshold than electrical percolation threshold in iPP/CB composites and the less rheological percolation threshold than electrical percolation threshold in LCBPP/CB composites are mainly attributed to the two kinds of mechanisms governing the electrical network and the rheological network. The agglomeration of CB particles which is accelerated by annealing at elevated temperatures promotes the self-perfection of rheological network. The strong interaction between the polar long-chain branched structure and filler also led to the reduced tp. The network of LCBPP/CB composites is more difficult to be broken than the network of iPP/CB composites with the solution process.

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.

Influence of Surfactant Functional Groups on Morphology and Rheology of Polypropylene/Organoclay Nanocomposites

Three surfactants, with the same long alkyl tail but varying in functional groups, were selected to modify two pristine clays with different cation exchange capacities (CEC). Each of the modified clays was melt-mixed with polypropylene (PP) to prepare nanocomposites. The microstructure of the resultant nanocomposites was investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and rheological techniques. The results showed that the surfactant structure had remarkable effects on the morphology and shear rheology of the nanocomposites based on the high-CEC organoclay: use of benzyl functional groups led to the highest extent of intercalation and highest enhancement of shear properties, while use of 2-hydroxyethyl groups had the opposite effect. Nanocomposites based on low-CEC organoclay all exhibited poor dispersion and their shear behavior was changed only slightly in comparison to the polymer matrix. In the case of extensional rheology, strain hardening was observed only in the two nanocomposites containing surfactants with 2-hydroxyethyl groups, regardless of the dispersion state of the nanoparticles.