Yonggang Shangguan

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.

TOUGHENING OF ETHYLENE-PROPYLENE RANDOM COPOLYMER/CLAY NANOCOMPOSITES: COMPARISON OF DIFFERENT COMPATIBILIZERS *

Ethylene/propylene-random-copolymer (PPR)/clay nanocomposites were prepared by two-stage melt blending. Four types of compatibilizers, including an ethylene-octene copolymer grafted maleic anhydride (POE-g-MA) and three maleic-anhydride-grafted polypropylenes (PP-g-MA) with different melt flow indexes (MFI), were used to improve the dispersion of organic clay in matrix. On the other hand, the effects of organic montmorillonite (OMMT) content on the nanocomposite structure in terms of clay dispersion in PPR matrix, thermal behavior and tensile properties were also studied. The X-ray diffraction (XRD) and transmission electron microscopy (TEM) results show that the organic clay layers are mainly intercalated and partially exfoliated in the nanocomposites. Moreover, a PP-g-MA compatibilizer (compatibilizer B) having high MFI can greatly increase the interlayer spacing of the clay as compared with other compatibilizers. With the introduction of compatibilizer D (POE-g-MA), most of the clays are dispersed into the POE phase, and the shape of the dispersed OMMT appears elliptic, which differs from the strip of PP-g-MA. Compared with virgin PPR, the Young’s modulus of the nanocomposite evidently increases when a compatibilizer C (PP-g-MA) with medium MFI is used. For the nanocomposites with compatibilizer B and C, their crystallinities (Xc) increase as compared with that of the virgin PPR. Furthermore, the increase of OMMT loadings presents little effect on the melt temperature (Tm) of the PPR/OMMT nanocomposites, and slight effect on their crystallization temperature (Tc). Only compatibilizer B can lead to a marked increases in crystallinity and Tc of the nanocomposite when the OMMT content is 2 wt%.

Shear induced self-thickening in chitosan-grafted polyacrylamide aqueous solution

A remarkable shear induced self-thickening of chitosan-graft-polyacrylamide aqueous solution was observed. After the polyelectrolyte solution presenting shear thinning was subjected to a high-rate shear for several minutes, their viscosities recovered and then a much higher zero shear viscosity than the original one appeared. Obviously, the self-thickening differs from conventional shear thickening or viscous recovery, as reported previously. The mechanism of self-thickening was investigated by rheological methods together with TEM, 1H NMR and DLS, etc. It was found that some aggregates exist in original chitosan-graft-polyacrylamide aqueous solution and the scale of such aggregations would become larger within several minutes after a strong shear. The thickening was proven to be the result of an enhanced scale of GPAM aggregation in aqueous solution, and the mechanism of aggregation was proven to be intermolecular hydrogen bonding effects. Besides, the shear-induced self-thickening appears to be facile, maintainable and easily controllable by changing the shear conditions.

Microstructure, morphology, crystallization and rheological behavior of impact polypropylene copolymer

AbstactImpact polypropylene copolymer (IPC), named polypropylene catalloy, not only possesses excellent impact property, but also presents good rigidity. Its superior performances result from the complicated composition and microstructure. In the present article, recent progress in the studies on microstructure, morphology, crystallization and rheological behavior of IPC is summarized, and findings of the authors and their collaborators are reported. In general, IPC is divided into three components, i.e., ethylene-propylene random copolymer (EPR), a series of different segment lengths ethylene-propylene copolymer (EbP) and propylene homopolymer. The reasonable macromolecular structures of EbP and a multilayered core-shell model of dispersed phase structure in IPC were proposed, in which the dispersed phase consists of an outer EbP shell, an inner EPR layer and an EbP core. It is found that the annealing at melt-state may lead to an abnormal phase inversion, and the phase inversion disappears when temperature cools down to room temperature. The cause of phase inversion is ascribed to the existence of EbP component, which results in the stronger activity of the dispersed phase. The crystalline structure and morphologic results confirm the formation of β-iPP in IPC. Furthermore, it is found that the ethylene content in IPC and cooling rate of the samples have an important influence on the formation of β-iPP. Based on the crystallization kinetics analyzed by Lauritzen-Hoffman theory, crystallization behavior of different IPC samples is discussed and it is proposed that the dilution effect of ethylene-propylene copolymer has a more remarkable influence on surface nucleation than on crystal growth. In addition, annealing at high temperature can result in the changes of chain structure for IPC, and this instability is ascribed to the oxidative degradation and crosslink reaction mainly in iPP component.

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.

Multiregion shear thinning for subsequent static self-thickening in chitosan-graft-polyacrylamide aqueous solution.

A special shear thinning phenomenon followed by static self-thickening in chitosan-graft-polyacrylamide (GPAM) aqueous solutions was investigated. This multiregion shear thinning can be defined as the first stage of the recently reported shear induced self-thickening (SIT) in our previous work. The three thinning regions (labeled as N1, N2, and N3) are considered very important, and they can reflex the complex variations of intermolecular interactions among and inside the aggregates in solution with increasing shear rate. To verify this multiregion shear thinning, a critical concentration of GPAM for this three-region shear thinning was first investigated. Shear recovery tests with the maximal shear rates located in the N1-N3 were carried out to ascertain the crucial role of shear thinning in SIT. The mechanisms of these three shear thinning regions were proposed based on the dependence of shear rheological behavior on various conditions in each region, including GPAM concentration, grafting ratio, temperature, added hydrogen bonding breaker, and salt. The above results confirm that N1 is due to the breakage of the interactions among hydrogen bonding aggregates, while N2 and N3 are attributed to the progressive destruction of the aggregates. As the first stage of SIT, shear thinning can markedly break the original aggregate and expose additional hydrogen bonding stickers to reform more aggregates with bigger size, resulting in the final higher viscosity.

Chain entanglement and molecular dynamics of solution-cast PMMA/SMA blend films affected by hydrogen bonding between casting solvents and polymer chains

The effects of intermolecular interaction between casting solvents and polymer chains on molecular entanglement and dynamics in solution-cast poly(methyl methacrylate)/poly(styrene-co-maleic anhydride) (PMMA/SMA) films were investigated by dynamic rheological measurement and broadband dielectric spectroscopy. A series of polymer blend films were cast from the mixed solvents composed of m-xylene and acetic acid with different mass ratio of acetic acid (Rac) at a solution concentration of 5 wt%, and in solutions the quantity of hydrogen bonding between PMMA and acetic acid was adjusted by Rac. FTIR results confirmed the existence of hydrogen bonding between carbonyl in PMMA and hydroxyl in acetic acid. Although the topological entanglement density of the resultant films decreased with increasing Rac, the α-relaxation peak shifted towards lower frequency and a higher glass transition temperature (Tg) appeared due to the increased cohesional entanglement in PMMA/SMA blend films induced by hydrogen bonding between PMMA and acetic acid. Furthermore, the dc conductivity decreased due to the more homogeneous structure in PMMA/SMA blend films cast from mixed solvents with higher Rac. Neither the width distribution of α-relaxation nor the dynamics of β-relaxation in these films was influenced by hydrogen bonding between PMMA and acetic acid due to the unchanged heterogeneity of the segmental dynamics and local environment of the segments. These results revealed that the hydrogen bonding between polymers/solvent during casting film can greatly influence the chain entanglement and molecular dynamics of the resultant polymer blends due to the memory effect of polymer chain.

Rheology of nitrile rubber with hybrid crosslinked network composed of covalent bonding and hydrogen bonding

A hybrid crosslinked network composed of covalent bonding and non-covalent bonding was constructed in nitrile rubber (NBR) by using the compound crosslinking agents dicumyl peroxide (DCP) and N,N-methylenebis acrylamide (MBA). DCP not only acted as a chemical crosslinking agent for NBR but also initiated MBA association onto the rubber chains. In addition to the crosslinked network of covalent bonds, the acylamino groups of MBA could associate reversibly into junctions between rubber chains to form additional hydrogen bonds. It was found that the total crosslinking density of NBR increased with the increasing MBA amount. Both dynamic mechanical analysis and dynamic rheological measurement results indicated that samples with more hydrogen bonding were more sensitive to deformation and temperature. For the vulcanized NBR samples with MBA, their glass transition temperature was higher than those of vulcanized samples only with DCP and slightly increased with increasing MBA amount. The existence of hydrogen bonding led to a shorter linear viscoelastic region in vulcanized NBR samples with MBA. In addition, compared with the samples vulcanized with only DCP, the vulcanized samples containing MBA presented a higher storage modulus in the low frequency region but a lower one in the high frequency region. These results indicated that the hydrogen bonding induced by MBA decreased both the motion ability of the chain segments between the crosslinking points and the energy dissipation generated by internal friction of the polymer chain.