Gengsheng Weng

New understanding in the influence of melt structure and β-nucleating agents on the polymorphic behavior of isotactic polypropylene

The β-nucleation behavior of isotactic polypropylene (iPP) is a fascinating and important issue in polymer physics; however, little about this phenomenon or its physical nature has been clearly understood. In the present study, by tuning the heating temperature (fusion temperature, Tf), the amount of ordered structures in iPP melt was controlled. In this way, the influence of five types of representative β-nucleating agents (β-NA) on the crystallization behavior of iPP with different melt structures (i.e. the amount of ordered structures) was comparatively studied by differential scanning calorimetry (DSC), polarized optical microscopy (PLOM), scanning electron microscopy (SEM), wide-angle X-ray diffraction (WAXD) and rheological measurement. A surprising synergetic effect was observed between β-NAs with α-/β-dual polymorphic selectivity (dual-selective β-NA) and the ordered structure of iPP, resulting in significant increases of the β-nucleation efficiency and the β-phase proportion of the sample. PLOM observation of the crystallization process confirmed that with the presence of ordered structures in iPP melt, a mass of dark, small crystal embryos derived from self-nuclei uniformly distributed in the melt and exhibited β-nucleation efficiency. This work provides the first evidence that for iPP nucleated with dual-selective β-NA, the ordered structures play a determining role in the β-nucleation of iPP. Under the influence of the dual-selective β-NA, the ordered structures exhibited β-nucleation efficiency and therefore encouraged β-nucleation. A possible mechanism was proposed.

Toughening rubbers with a hybrid filler network of graphene and carbon nanotubes

Unfilled rubbers usually show poor mechanical properties. Here, we demonstrate toughening natural rubber (NR) by designing a compact hybrid filler network composed of graphene (GE) and carbon nanotubes (CNTs). The physical interactions in this network have a bond energy lower than covalent bonds; and thus they preferentially break upon deformation and serve as sacrificial bonds that dissipate energy before failure of the materials. The high energy dissipation of the hybrid filler network not only increases the fracture toughness and tensile strength, but also suppresses the crack growth of the NR/GE/CNT nanocomposites. These properties will provide the nanocomposites with better sustainability during practical applications.

Tensile and impact behavior of polystyrene microcellular foams with bi-modal cell morphology:

Bi-modal PS foams with various volume fractions of large cells (fL), cell sizes and densities were prepared to investigate the effect of cell structures on the tensile and impact behaviors. The tensile results showed that for the similar density, the tensile strength and modulus decreased with the increase of fL, unless the cell size of large ones is smaller than 25 µm. Similarly, the impact experimental results showed that the impact strength decreased with increasing fL, unless the fL is in the range of 25–32%. It indicated that the bi-modal cell structure could lead to the better properties than that of uniform one, when the cell morphology was proper (fL in the range of 25–32% and the cell size of large ones smaller than 25 µm). The SEM images of impact-fractured surface of bi-modal foams further confirmed that the cell morphology with fL of 32% was more favorable to the absorption of impact energy during the fracture process.

Crack growth mechanism of natural rubber under fatigue loading studied by a real-time crack tip morphology monitoring method

The present paper deals with the crack growth behavior of vulcanized natural rubber under fatigue loading. Our research focuses on the crack tip morphology evolution and its relationship with the crack growth rate. For this purpose, an original real-time monitoring method is applied to capture the crack tip morphology during standard fatigue testing. This method involves the use of a high speed microscopic camera and a dynamic mechanical analyzer with a crack growth testing unit. The tear energy (T) dependence of the crack tip morphology is discussed. It is revealed that there are two characteristic cracking morphologies: at T 600 J m−2, the power law slope of dc/dn versus T is 2. A ligament break-up mechanism dominates. This work gives us new opportunities to study the crack growth mechanism of rubbers from the viewpoint of real-time crack tip morphology investigation.

New insight into stretch induced structural evolution of α trans-1,4-polyisoprene characterized by real time synchrotron WAXS and SAXS measurements

Real time synchrotron wide angle and small angle X-ray scattering (WAXS and SAXS, respectively) were used to characterize the stretch induced structural evolution of α trans-1,4-polyisoprene (trans-PI). 2D WAXS results indicated two ensembles of crystalline modifications with distinctive orientation modes coexisted during stretching. Stretching transformed part of the monoclinic α phase into highly oriented orthorhombic β phase at strain ε = ~0.4. The β phase had rather high orientational degree with polymer chains parallel to the stretching direction, while the orientational degree of α phase was much lower. Complemented by qualitative 2D SAXS analysis, it was found that amorphous layer deformation and intralamellar chain slip dominated at different stretching stage. The melt and recrystallization process of α phase which led to the formation of β phase was also investigated. Formation of two interpenetrating networks of crystalline skeleton (constructed by residual α and β crystals) and amorphous entanglement accounted for the stress-hardening in the late stage.

Study on the stretch induced phase transition of α trans-1,4-polyisoprene by in-situ SAXS and WAXS measurements

Trans-1,4-polyisoprene (trans-PI) is an important industrial polymer, which exhibits pronounced polymorphisms. However, understanding of the crystal form transition of trans-PI is rather limited. In the present work, the stretch-induced crystal form transition of α trans-PI at room temperature was investigated in detail. It was found that the β crystals with high molecular chain orientation appeared at strain = 0.,42 and the total crystallinity increased at the same time. Meanwhile, the average long spacing (lac), thickness of lamellae (lc), and amorphous layers (la) along the stretching direction had undergone multiple evolution processes during stretching. Results also showed that part of the amorphous phase and the recrystallization process of α crystals both contributed to the formation of the β form. Based on the combination analysis of the in-situ SAXS (small angle X-ray scattering), WAXS (wide angle X-ray scattering), differential scanning electron microscopy (DSC), and Fourier transform infrared spectroscopy (FTIR), it was proved that the β mesophase with layered structure emerged before the formation of β crystals. Finally, a possible multistage evolution mechanism was proposed to interpret the stretch-induced crystal form transition of α trans-PI.

Crack growth mechanism of styrene-butadiene rubber filled with silica nanoparticles studied by small angle X-ray scattering

The crack growth mechanism of rubber composites has not been explored clearly. Here, we focus on the microstructure evolution of styrene-butadiene rubber/silica (SBR/silica) composites during crack propagation. Two groups of SBR/silica composites are constructed. The silica nanoparticles used in the two groups have different sizes but the same surface properties. By crack propagation and small angle X-ray scattering measurements, the crack growth rate, nanocavitation and silica distribution morphology of the rubber composites are investigated. The role of damage structure nucleation and growth during crack propagation of the SBR/silica composites is discussed. It is demonstrated that the damage structure nucleation is the key factor for the crack growth at large tear energies, while the growth feature of damage structure dominates the crack growth at smaller tear energies. This work provides us with better understanding on the crack growth mechanism of rubber composites.

Effect of interface on bulk polymer: control of glass transition temperature of rubber

In current paper, we demonstrated that molecular dynamics and glass transition of rubber can be controlled by constructing attractive interface between rubber matrix and fillers. Based on a combination of experiments and molecular simulations, it was revealed that interfacial segmental mobility was reduced and glass transition temperatures (Tgs) of epoxidized natural rubber (ENR) were significantly improved due to in situ polymerization of zinc dimethacrylate (ZDMA). During curing, ZDMA polymerizes in rubber matrix, resulting in the appearance of nanodispersion phases of poly-ZDMA (PZDMA). It was demonstrated that coordination interaction exists between epoxy groups and PZDMA in interfacial regions. Furthermore, using dynamic Monte Carlo simulations, it was observed that the interfacial regions that have highest content of epoxy groups exhibit lowest segmental mobility. Then, the increase of ZDMA content leads to the rise of the fraction of absorbed interfacial segments, and thus the Tgs of filled rubbers are improved.