Biwei Qiu

Balanced toughening and strengthening of ethylene–propylene rubber toughened isotactic polypropylene using a poly(styrene-b-ethylene–propylene) diblock copolymer

For ordinary rubber toughened plastics, the introduction of rubber will inevitably bring about the severe decline in mechanical strength due to the low modulus and rigidity of elastomers. To fabricate toughened polypropylene (PP) materials without significant strength degradation, the poly(styrene-b-ethylene–propylene) diblock copolymer (SEP) was used as the third component in an isotactic polypropylene/ethylene–propylene random copolymer (iPP/EPR) to prepare a series of PP/EPR/SEP blends. The phase morphology, dynamic mechanical behavior, crystallization behavior and mechanical properties of PP/EPR/SEP blends were systematically investigated, and compared with PP/EPR blends. The dynamic mechanical analysis results revealed that SEP has good compatibility with both EPR phase and amorphous PP phase, which led to an improvement of interfacial adhesion between them. The mechanical properties testing results indicated that the introduction of SEP could effectively promote the brittle–ductile transition for PP/EPR blends and that PP/EPR/SEP blends presented a good toughness without strength loss. Considering the fact that the individual EPR or SEP could not achieve good toughening, it was proposed that SEP and EPR have a synergistic effect on toughening PP and a modified PP with balanced toughness and tensile strength can be achieved by simultaneously adding EPR and SEP into iPP.

Segmental dynamics and physical aging of polystyrene/silver nanocomposites

We investigate the effects of silver (Ag) nanoparticles on the segmental and chain dynamics, physical aging and rheological behavior of polystyrene (PS) via a combination of broadband dielectric spectroscopy, calorimetry, and dynamic rheological measurement. The segmental dynamics of PS is found to be unchanged with increasing nanoparticle loading. After annealing below the glass transition temperature (Tg) for various time periods and measuring the recovered enthalpy values of PS, it is surprising that an acceleration and a suppression of the physical aging in PS/Ag-3% and 10% nanocomposites can be observed, respectively, corresponding to the decreased and increased calorimetric Tg, which can be interpreted by plasticizing and antiplasticizing effects. Furthermore, the filler reinforcement in rheological behavior is observed with increased weight fraction of Ag nanoparticles. The temperature-dependent horizontal shift factor reveals that the overall chain dynamics speed up in the presence of Ag nanoparticles. We also emphasize recent discrepancies in the prior studies of polymer nanocomposites and polymer thin films by comparing results.

Influence of annealing on chain entanglement and molecular dynamics in weak dynamic asymmetry polymer blends.

The influence of annealing above the glass transition temperature (T(g)) on chain entanglement and molecular dynamics of solution-cast poly(methyl methacrylate)/poly(styrene-co-maleic anhydride) (PMMA/SMA) blends was investigated via a combination of dynamic rheological measurement and broadband dielectric spectroscopy. Chain entanglement density increases when the annealing temperature and/or time increases, resulting from the increased efficiency of chain packing and entanglement recovery. The results of the annealing treatment without cooling revealed that the increase of the entanglement density occurred during the annealing process instead of the subsequent cooling procedure. Annealing above T(g) exerts a profound effect on segmental motion, including the transition temperature and dynamics. Namely, T(g) shifts to higher temperatures and the relaxation time (τ(max)) increases due to the increased entanglement density and decreased molecular mobility. Either T(g) or τ(max) approaches an equilibrium value gradually, corresponding to the equilibrium entanglement density that might be obtained through the theoretical predictions. However, no obvious distribution broadening is observed due to the unchanged heterogeneous dynamics. Furthermore, side group rotational motion could be freely achieved without overcoming the chain entanglement resistance. Hence, neither the dynamics nor the distribution width of the subglass relaxation (β- and γ-relaxation) processes is affected by chain entanglement resulting from annealing, indicating that the local environment of the segments is unchanged.

Simultaneously enhancing strength and toughness for impact polypropylene copolymers by regulating the dispersed phase with high density polyethylene

By introducing high density polyethylene (HDPE) into the dispersed phase of impact polypropylene copolymers (IPCs), the morphologies of IPC/HDPE blends were regularly tailored and consequently the tensile and impact properties were simultaneously improved. Morphological observations showed a series of multilayered core–shell dispersed particles when the content of HDPE was less than 40%, while the continuous network structure was observed beyond 40%. With an increase in the content of HDPE, the size of the core increased and the number of dispersed particles with incomplete encapsulated polyethylene (PE) cores rose. More valid ‘bridges’ made up of segmented ethylene–propylene copolymer (sEbP) appeared and connected the PE core and polypropylene (PP) matrix. Meanwhile, co-crystallization occurred in the core phase, between long ethylene chain segments of the joined HDPE and sEbP in multi-component IPCs. The increased HDPE in blends reduced defective co-crystals, and in turn led to a thicker average lamellar thickness and thinner amorphous thickness of PE. Partial inserted ethylene–propylene random sequences are constrained by narrowed PE amorphous layers. Hence, the connection between the PP matrix and the dispersed phase was strengthened by co-crystals, ‘bridges’ and restriction effects. The tensile strength of the blends was slightly enhanced with an increase in HDPE, while the greatly improved toughness was achieved at a HDPE content of 30 wt% and kept constant with more HDPE. Thus, the interactions rather than core–shell phase morphology are regarded as the predominate factor for the excellent properties.

Effects of composition on microstructure and crystallization behavior for impact polypropylene copolymer investigated by restructuring the complex core-shell dispersed particles in ternary blends

A series of ternary blends of polypropylene/ethylene-propylene random copolymer/ethylene-propylene segmented copolymer (HPP/EPR/EbP) whose microstructures are similar to those of impact polypropylene copolymer (IPC) were prepared in order to systematically investigate the effects of composition on microstructure and crystallization behavior of IPC. The observation of primary phase morphology reveals that the dispersed phase with core-shell structure could be rebuilt in certain composition and excessive EPR leads to a bicontinuous phase structure in ternary blends. After undergoing same quiescent crystallization including isothermal and non-isothermal crystallization, these blend samples exhibit special composition-dependent melting behavior, i.e., the melting point increases markedly with the increase of EPR content until it turns down at a critical content (about 30 wt%). The crystallization behavior is mainly ascribed to the different nucleation abilities. It is suggested that although the compatibility between EPR and HPP components becomes worse with the increase of EPR content due to the increased interfacial area and the decreased concentration of EbP, higher EPR content in the blend facilitates to heterogeneous nucleation except for the appearance of obvious bicontinuous phase structure.

Destruction mechanism of core–shell particles in impact polypropylene copolymer during short molten-state annealing

Morphology evolution of the dispersed phase with a multilayered core–shell structure in impact polypropylene copolymer (IPC) during molten-state annealing was systematically studied through scanning electron microscopy (SEM), phase contrast microscopy (PCM) and dynamic rheological test. To demonstrate the evolution path of the dispersed phase comprised of ethylene-propylene random copolymer (EPR) and ethylene–propylene block copolymer (EbP) during annealing, different binary blends comprised of different fractions were prepared and their diffusion behavior during liquid–liquid phase separation was investigated. Compared with EPR, EbP presented a higher diffusion rate in propylene homopolymer (hPP) matrix, owing to its lower molecular weight and lower entanglement density. The statistical results of EbP and EPR domain sizes reveal that the coalescence of EbP is faster than that of EPR. In addition, the interaction parameters of EbP/hPP and EbP/EPR estimated using the Nishi–Wang equation show that EbP has a stronger affinity for hPP than EPR. Based on the diffusion rates, entanglement densities of components and great disparity in viscosity between EPR and hPP, a potential mechanism was proposed for the morphology evolution of core–shell dispersed particles in IPC during molten-state annealing.

Molecular relaxation and dynamic rheology of “cluster phase”-free ionomers based on lanthanum(III)-neutralized low-carboxylated poly(methyl methacrylate)

Molecular dynamics and linear dynamic rheology of La(III)-neutralized low-carboxylated poly(methyl methacrylate) (PMMA) ionomers with varying neutralization levels are investigated. While the ionomers do not form a clear cluster phase, increasing neutralization level causes notable retardation of the α relaxation and elevation of glass transition temperature. In addition, dynamic rheology of the ionomer melt follows the time–temperature superposition principle and, at neutralization levels above 80%, shows a long-term relaxation process and nonterminal relaxation ascribed to ionic species. Especially the ionomer with a neutralization level of 120% behaves like a critical gel. The long-term relaxation process is well described in terms of Cole–Cole curves, relaxation time spectra, complex viscosity and loss tangent. By analyzing the linear rheology in the framework of a “two phase” model, an interconnected multiplets network is identified as a mechanism being responsible for the fluid-to-solid transition of “cluster phase”-free ionomers with increasing neutralization level.

Effects of chain entanglement on liquid-liquid phase separation behavior of LCST-type polymer blends: Cloud point and decomposition rate

By preparing homogenous blend samples with different degrees of chain entanglement, we report an anomalous contribution of chain entanglement to phase separation temperature and rate of poly(methyl methacrylate)/poly(styrene-co-maleic anhydride) (PMMA/SMA) blends presenting a typical lower critical solution temperature (LCST) behavior. The meltmixed PMMA/SMA blends with a higher chain entanglement density present a lower cloud point (Tc) and shorter delay time, but lower phase separation rate at the given temperature than solution-cast ones, suggesting that for the polymer blends with different condensed state structure, thermodynamically more facilitation to phase separation (lower Tc) is not necessarily equivalent to faster kinetics (decomposition rate). The experimental results indicate that the lower Tc of melt-mixed sample is ascribed to smaller concentration fluctuation wavelength (Λm) induced by higher entanglement degree, while higher entanglement degree in melt-mixed sample leads to a confined segmental dynamics and consequently a slower kinetics (decomposition rate) dominated by macromolecular diffusion at a comparable quench depth. These results reveal that the chain packing in polymer blends can remarkably influence the liquid-liquid phase separation behavior, which is a significant difference from decomposition of small molecular mixtures.

Reconstruction of core-shell dispersed particles in impact polypropylene copolymer during extrusion

We reported an approach to reconstruct the complex phase morphology of impact polypropylene copolymer (IPC) with core-shell dispersed particles and to optimize its toughness in approximate shear condition. The molten-state annealing results indicate that the phase structure with core-shell dispersed particles is unstable and could be completely destroyed by static annealing, resulting in the degradation of impact strength. By using a co-rotating twin screw extruder, we found that the dispersed particle with core-shell structure could be rebuilt in appropriate condition with the recovery of excellent impact strength due to both the huge interfacial tension during solidification and the great difference in viscosity of components. Results reveal that almost all the extruded IPCs show the impact strength 60%–90% higher than that of annealed IPCs at room temperature. And the twice-extruded IPC shows the highest impact strength, 446% higher than that of IPC annealed for 30 min. As for low temperature tests, the impact strength of extruded IPCs also increases by 33%–58%. According to adjusting the processing conditions including extrusion speed, extrusion frequency and temperature, an optimization of toughness was well established.