Youping Wu

New understanding of microstructure formation of the rubber phase in thermoplastic vulcanizates (TPV)

The breakup of the rubber phase in an ethylene-propylene-diene monomer (EPDM)/polypropylene (PP) blend at the early stage of dynamic vulcanization is similar to that in an unvulcanized EPDM/PP blend because of the low crosslink density of the EPDM phase. In this work, the minimum size of the rubber phase in the unvulcanized EPDM/PP blend was first calculated by using the critical breakup law of viscoelastic droplets in a matrix. The calculated results showed that the minimum size of the rubber phase in the unvulcanized blend was in the nanometer scale (25-46 nm), not the micrometer scale as reported in many works. Meanwhile, the actual size of the rubber phase in the thermoplastic vulcanizate (TPV) at both the early stage and the final stage of dynamic vulcanization was observed by using peak force tapping atomic force microscopy (PF-AFM). The results indicated that the EPDM phase indeed broke up into nanoparticles at the early stage of dynamic vulcanization, in good agreement with the calculated results. More interestingly, we first revealed that the micrometer-sized rubber particles commonly observed in TPV were actually the agglomerates of rubber nanoparticles with diameters between 40 and 60 nm. The mechanism for the formation of rubber nanoparticles and their agglomerates during dynamic vulcanization was then discussed. Our work provides guidance to control the microstructure of the rubber phase in TPV to prepare high performance TPV products for a wide range of applications in the automobile and electronic industries.

Preparation, fracture, and fatigue of exfoliated graphene oxide/natural rubber composites

Exfoliated graphene oxide (GO) reinforced natural rubber (NR) composites were prepared by a simple and promising latex co-coagulation method. Latex co-coagulation realized the complete exfoliation and uniform dispersion of GO in a NR matrix. The quasi-static fracture and dynamic fatigue behaviors of the composites were investigated. Mechanical properties and J-testing were used to characterize the fracture resistance, and fatigue tests were carried out under cyclic conditions of constant strain. Results revealed that with increasing GO sheet content, the mechanical properties, fracture initiation and propagation resistance were all highly improved. A much higher reinforcing efficiency of GO sheets than that of traditional fillers was realized. Fatigue crack growth resistance was remarkably enhanced with the incorporation of only 1 phr GO sheets. The relatively weak fatigue resistance of composites filled with 3 phr and 5 phr GO was attributed to their high hysteresis loss and tearing energy input. It revealed that GO sheets would be promising fillers in preparation of rubber composites with high fracture and fatigue resistance at low filler content.

Revealing the toughening mechanism of graphene-polymer nanocomposite through molecular dynamics simulation.

By employing united atom molecular dynamics simulation, we have investigated the effects of polymer-graphene interaction ε(np) volume fraction of grapheme φ thermodynamics of polymer matrix (rubbery versus glassy), interfacial interaction in the case of the same dispersion state, shape of nanoparticles (NPs) such as C60 CNT and graphene at the same loading on the toughening efficiency of polymer nanocomposites. By beginning with the pure polymer, we observe that a plateau stress occurs at long chain length because entangled polymer chains in fibrils cannot become broken. We find that the work needed to dissipate during the failure increases with the increase of ε(np) and φ and the yield point in the stress-strain behavior occurs at a smaller strain for an attractive NPs filled system compared to the pure case, attributed to the more mechanically heterogeneous environment. The thermodynamics of the polymer matrix (below and above Tg) seems to have a significant effect on the toughening efficiency of graphene sheets. In the case of the same dispersion state, stronger interfacial interaction always induces long and highly orientated polymer fibrils along the deformation direction, with graphene sheets being encapsulated in these fiber-like bundles. By characterizing the interaction energy between polymer-polymer and polymer-graphene as a function of the strain, we find that the separation of polymer chains from the graphene sheets cease immediately after the yield point, followed by the continuous propagation of the cavities by excluding surrounded polymer chains and graphene sheets together. We also find that at the same attractive interfacial interaction and same loading, the toughening efficiency exhibits the following order: graphene > CNT > C60 Generally, the toughening mechanism of graphene sheets results from the formation of long and highly orientated polymer fibrils to prevent the occurrence of the rupture, which can be greatly improved by the strong interfacial interaction and the large surface area compared to CNT and C60 This also indicates that polymer matrices with high flexibility and mobility of polymer chains tend to be better toughened. It is hoped that this simulation work will provide rational guidance for fabricating high performance of polymer nanocomposites with excellent toughness.

Effect of additives on the morphology evolution of EPDM/PP TPVs during dynamic vulcanization in a twin-screw extruder

Ethylene-Propylene-Diene Monomer/Polypropylene thermoplastic vulcanizates (EPDM/PP TPVs) have been widely used as a kind of typical “green” elastomer because of their excellent mechanical properties and recyclability. The industrial TPVs always contain various types of additives, which influence the viscosity ratio of EPDM and PP and the morphology of TPVs. This work studied the morphology evolution of EPDM/PP TPVs with various amounts of curing agents, fillers, and plasticizer during dynamic vulcanization in a twin-screw extruder, which provides much more complicated dynamic vulcanization process than haaker rheometer. The results show that the increased curing agents content leads to the faster morphology evolution of TPV because it enhances the cross-linking speed and the viscosity of EPDM. The increased fillers content leads to the later breakup of EPDM and the bigger size of the rubber aggregation because it enhanced the modulus of EPDM and weakens the interfacial interaction between EPDM and PP. In addition, the increase in the plasticizer content leads to the earlier breakup of EPDM and the larger size of the rubber phase in TPV. Our work firstly demonstrates the morphology evolution of industrial EPDM/PP TPV, and thus can provide a guidence for the industrial production of high-performance EPDM/PP TPVs.

Novel biobased thermoplastic elastomer consisting of synthetic polyester elastomer and polylactide by in situ dynamical crosslinking method

Owing to the sustainability and environmental friendliness of biobased polymers, we adopted synthesized biobased polyester elastomer (BPE) and polylactide (PLA) as the two components to produce a new biobased thermoplastic vulcanizate (TPV) by an in situ dynamical crosslinking and mixing method. The effect of blending ratio on the dynamic crosslinking and micromorphology of TPV was investigated by mixing torque measurements, degree of crosslinking measurements, TEM, DSC, and rheological properties. A large amount of crosslinked BPE particles were dispersed in the PLA continuous phase, with the particle sizes ranging from 1 to 4 μm, indicating the occurrence of phase inversion during the dynamical crosslinking and mixing process. The tensile strength and elongation at break of the biobased TPVs ranged from 11.4 MPa to 17.8 MPa and 154% to 184%, respectively. Reprocessing did not significantly reduce the mechanical properties, as an indication that biobased TPVs, like thermoplastics, have good reprocessability. In vitro cytotoxicity tests showed that our TPVs were nontoxic, at least towards mouse fibroblasts. Thus, these novel biobased TPVs with excellent mechanical properties and low cytotoxicity are reported for the first time in the flied of thermoplastic elastomers for engineering and biomedical applications.

Characterization of Filler-rubber Interaction, Filler Network Structure, and Their Effects on Viscoelasticity for Styrene-butadiene Rubber Filled with Different Fillers

For styrene-butadiene rubber (SBR) compounds filled with the same volume fraction of carbon black (CB), precipitated silica and carbon–silica dual phase filler (CSDPF), filler-rubber interactions were investigated thru bound rubber content (BRC) of the compounds and solid-state 1H low-field nuclear magnetic resonance (NMR) spectroscopy. The results indicated that the BRC of the compound was highly related to the amount of surface area for interaction between filler and rubber, while the solid-state 1H low-field NMR spectroscopy was an effective method to evaluate the intensity of filler-rubber interaction. The silica-filled compound showed the highest BRC, whereas the CB-filled compound had the strongest filler-rubber interfacial interaction, verified by NMR transverse relaxation. The strain sweep measurements of the compounds were conducted thru a rubber process analyzer; the results showed that the CSDPF-filled compound presented the lowest Payne effect, which is mainly related to the weakened filler network structure in polymer matrix. The temperature sweep measurement, tested by dynamic mechanical thermal analysis, indicated that the glass transition temperature did not change when SBR was filled with different fillers, whereas the storage modulus in rubbery state and the tanδ peak height were greatly affected by the filler network structure of composites.

Silica Modified by Alcohol Polyoxyethylene Ether and Silane Coupling Agent Together to Achieve High Performance Rubber Composites Using the Latex Compounding Method

The study of preparing silica/rubber composites used in tires with low rolling resistance in an energy-saving method is fast-growing. In this study, a novel strategy is proposed, in which silica was modified by combing alcohol polyoxyethylene ether (AEO) and 3-mercaptopropyltriethoxysilane (K-MEPTS) for preparing silica/natural rubber (NR) master batches. A thermal gravimetric analyzer and Raman spectroscopy results indicated that both AEO and K-MEPTS could be grafted on to the silica surface, and AEO has a chance to shield the mercaptopropyl group on K-MEPTS. Silica modified by AEO and K-MEPTS together was completely co-coagulated with the rubber in preparing silica/NR composites using the latex compounding method with the help of the interaction between AEO and K-MEPTS. The performance of composites prepared by silica/NR master batches was investigated by a rubber process analyzer (RPA), transmission electron microscopy (TEM) and a tensile tester. These results demonstrate that AEO forms a physical interface between silica and rubber, resulting in good silica dispersion in the matrix. K-MEPTS forms a chemical interface between silica and rubber, enhancing the reinforcing effect of silica and reducing the mutual friction between silica particles. In summary, using a proper combination of AEO and K-MEPTS is a user-friendly approach for preparing silica/NR composites with excellent performance.

Properties and unique morphological evolution of dynamically vulcanized bromo-isobutylene-isoprene rubber/polypropylene thermoplastic elastomer

We studied the microstructure, morphological evolution and the corresponding mechanism, and the properties of bromo-isobutylene-isoprene rubber (BIIR)/polypropylene (PP) thermoplastic vulcanizates (TPVs). Interestingly, a large number of single rubber nanoparticles were observed in the crosslinked BIIR/PP blends, ascribed to the improvement of compatibility between the BIIR and PP with increasing dynamic vulcanization (DV) time, as demonstrated by the increase in interfacial phase thickness and the decrease in the interfacial tension. Most of these single nanoparticles agglomerated as the DV proceeded, leading to the deterioration of the rubber network. Another interesting observation was that the size of rubber agglomerate decreased as the DV proceeded, leading to the strengthening of the rubber network. Importantly, the as-prepared BIIR/PP TPV exhibits good processability, high elasticity and good mechanical property. The relationship between the unique morphology and properties were studied. Our study provides guidance for the preparation of high-performance BIIR/PP TPV for its industrial applications such as medical bottle stoppers.

Tuning the visco-elasticity of elastomeric polymer materials via flexible nanoparticles: insights from molecular dynamics simulation

Tuning the viscoelasticity of polymeric materials by incorporating nanoparticles (NPs) has received considerable scientific and technological interests. Contrary to increasing the energy dissipation for damping materials, here we direct our attention to study how to decrease the energy dissipation of elastomer nanocomposites (ENCs) under periodic dynamic loading–unloading cycles. Through molecular dynamics simulation, we firstly simulate the pure cis-polybutadiene (cis-PB) system, by calculating the mean-squared end-to-end distance and the radius of gyration as a function of the chain length, the diffusion coefficient of polymer chains as a function of the temperature, the glass transition temperature, the stress–strain curves at different strain rates and temperatures, the tension–recovery and compression–recovery curves at various cross-linking densities. These results validate the accuracy of the united atom model and force-field of cis-PB. Then we show that the incorporation of flexible nanoparticles (NPs) such as graphene nanoribbons and carbon nanotubes can effectively decrease the dynamic hysteresis loss, by taking advantage of the reversible mechanical deformation of the anisotropic NPs. This effect can be further strengthened by the stronger interfacial interaction, higher loading and larger size of this kind of NPs. The underlying reason stems from the synergistic motion between the NPs and their surrounding polymer chains, leading to much smaller internal friction. This work may open up potential opportunities to fabricate high-performance polymer nanocomposites, such as energy-saving ENCs tailored for tire tread.

Interface tailoring of layered silicate/styrene butadiene rubber nanocomposites

To improve the interfacial interaction in clay/SBR nanocomposites prepared by latex compounding method, a novel clay modification for the nanocomposites was introduced before latex compounding with SBR using three kinds of organic modifiers, namely, hexadecyl trimethyl ammonium bromide (C16), bis(hexadecyl) dimethyl ammonium bromide (DC16) and 3-aminopropyl triethoxy silane (KH550). On the other hand, bis(triethoxysilylpropyl)tetrasulfide (Si69) was added into the KH550 modified clay/SBR nanocomposite during later mechanical blending, and was designed to interact with both KH550 and rubber and thus improve the interface. Structure changes of the nanocomposites were followed by study of X-ray diffraction, transmission electron microscopy and rubber process analyzer. Dynamic mechanical analysis and tensile tests were carried out to obtain information about the mechanical properties of the nanocomposites. The results revealed that, with the organic modification, clay was dispersed finely in the rubber matrix with part rubber-intercalated or part modifier-intercalated structure. Compared with the unmodified nanocomposite, the tensile strength, the stress at 300% strain, and the tear strength of modified SBR–clay nanocomposites were significantly improved. Moreover, the type of modifiers and strength of interfacial interaction determined the properties of the nanocomposites. The incorporation of KH550 and Si69 brought the best modification effect among all the modification methods.