Showing papers by "Liangbin Li published in 2019"

Ultrastiff and Tough Supramolecular Hydrogels with a Dense and Robust Hydrogen Bond Network

Abstract: Design of tough hydrogels has made great progress in the past two decades. However, the synthetic tough gels are usually much softer than some biotissues (e.g., skins with modulus up to 100 MPa). Here we report a new class of ultrastiff and tough supramolecular hydrogels facilely prepared by copolymerization of methacrylic acid and methacrylamide. The gels with water content of approximately 50–70 wt % possessed remarkable mechanical properties, with Young’s modulus of 2.3–217.3 MPa, tensile breaking stress of 1.2–8.3 MPa, breaking strain of 200–620%, and tearing fracture energy of 2.9–23.5 kJ/m2, superior to most existing hydrogels, especially in terms of modulus. Typical yielding and crazing were observed in the gel under tensile loading, indicating the forced elastic deformation of these hydrogels in a glassy state, as confirmed by dynamic mechanical analysis. The ultrahigh stiffness was attributed to the dense cross-linking and reduced segmental mobility caused by the robust intra- and interchain hydr...

The Tough Journey of Polymer Crystallization: Battling with Chain Flexibility and Connectivity

Abstract: Theoretical approaches addressing the mechanism of polymer crystallization remain the great challenge in polymer science. Numerous different, or even conflicting, models/theories have been proposed...

Frustrating Strain-Induced Crystallization of Natural Rubber with Biaxial Stretch.

Abstract: The supreme mechanical performance of natural rubber (NR) is commonly attributed to strain-induced crystallization (SIC). The SIC of NR during uniaxial stretch has been extensively investigated, wh...

In situ characterization of strain-induced crystallization of natural rubber by synchrotron radiation wide-angle X-ray diffraction: construction of a crystal network at low temperatures.

Abstract: Strain-induced crystallization (SIC) of natural rubber (NR) at descending temperatures as low as −60 °C is systematically investigated by in situ synchrotron radiation wide-angle X-ray diffraction (SR-WAXD) measurement. The detailed structural evolution of NR during SIC is studied in the strain–temperature space, where up to four regions are defined depending on the SR-WAXD results. In region I, the molecular chains begin to be oriented under tensile loading. The onset of crystallization happens in the very beginning of region II, and the NR crystal acts as a new physical cross-linking point to form a crystal network, namely the series model. The further increment of crystallinity (> ca. 8%) leads to the transition of the crystal network from the series model to the parallel model in region III. The crystal network is finally accomplished in region IV, where the crystallinity remains almost constant. Interestingly, regions III and IV exist only in the intermediate-temperature zone II (−40 °C to −10 °C), which are missing in zones I (−10 °C to 25 °C) and III (−60 °C to −40 °C). This suggests that sufficient crystallinity (χII–III > ca. 8%) is required to form the parallel model. The new crystal network provides a deep understanding of SIC of NR considering the microscopic features, i.e. oriented amorphous component, the onset of crystallization and crystallinity evolution and its correlation with the macroscopic stress–strain curve.

Synergistic and Competitive Effects of Temperature and Flow on Crystallization of Polyethylene during Film Blowing

Abstract: The influences of both temperature and external flow field on film blowing have been studied with a combination of a custom-built film blowing device and in situ synchrotron radiation small- and wide-angle X-ray scattering techniques. Polyethylene (PE) with different structural topologies, namely linear (MPE) and long-chain branched polyethylene (LPE), was used here with different responses to temperature and flow. The MPE film shows a spherulite-like superstructure with low orientation independent of take-up ratio (TUR), while the LPE has a typical row-nucleated structure at high TUR. But for LPE film obtained at low TUR, it exhibits the combination of both crystal morphologies. Further analysis of the microscopic structural evolution of PE during film blowing by synchrotron X-ray scattering reveals three different types of network evolution: (i) the temperature-induced crystallization (TIC) dominated process (MPE); (ii) the flow-induced crystallization (FIC) dominated process (LPE with high TUR); and (i...

An adaptive edge-based smoothed finite element method (ES-FEM) for phase-field modeling of fractures at large deformations

Abstract: This work presents the Griffith-type phase-field formation at large deformation in the framework of adaptive edge-based smoothed finite element method (ES-FEM) for the first time. Therein the phase-field modeling of fractures has attracted widespread interest by virtue of its outstanding performance in dealing with complex cracks. The ES-FEM is an excellent member of the S-FEM family developed in combination with meshless ideas and finite element method (FEM), which is characterized by higher accuracy, softer stiffness, and insensitive to mesh distortion. Given that, the advantages of the phase-field method (PFM) and ES-FEM are fully combined by the approach proposed in this paper. With the costly computational overhead of PFM and ES-FEM in mind, a well-designed multi-level adaptive mesh strategy was developed, which considerably improved the computational efficiency. Furthermore, the detailed numerical implementation for the coupling of PFM and ES-FEM is outlined. Several representative numerical examples were recalculated based on the proposed method, and its effectiveness is verified by comparison with the results in experiments and literature. In particular, an experiment in which cracks deflected in rubber due to impinging on a weak interface was firstly reproduced.

Preparation of Polyethylene and Ethylene/Methacrylic Acid Copolymer Blend Films with Tunable Surface Properties through Manipulating Processing Parameters during Film Blowing.

Abstract: Polymer films based on polyethylene (PE) and ionomer ethylene/methacrylic acid (EMAA) copolymer blend were prepared by film blowing, whose surface properties were tuned by varying processing parameters, i.e., take up ratio (TUR). Blends of PE/EMAA copolymer were firstly prepared by the melt-mixing method, before being further blown to films. The wettability of the film was investigated by measuring the contact angle/water-film encounter time, and optical properties, i.e., the haze and transmittance. The wettability was found to be enhanced with the increase of TUR. So too was the haze, while the transmittance was found to be almost independent of TUR. The XPS and AFM results directly show the increasing polar functional groups (–COO−) on the surface and roughness with increasing TUR. Further analysis of the 2D SAXS and WAXS unveiled the origin of the invariant transmittance, which resulted from the minor change of the crystallinity and the monotonic increase of the haze, with TUR resulting from the evolution of crystal orientation. In addition to other post-modification methods, the current study provides an alternative route to prepare large-scale PE films as the template for the advanced potential applications, i.e., covering in the layer of roof, the privacy of protective windows, and multitudes of packaging.

A hybrid mesh adaptive method for fracture phase field simulation

Abstract: The invention discloses a hybrid mesh adaptive method for fracture phase field simulation. The hybrid mesh adaptive method comprises the following steps: the crack tip region is approximately determined by the difference set of the phase field sets which reach the upper critical value in adjacent time steps; a rectangular region is designated by taking the node of crack tip region as the geometrical center, and all the elements in the rectangular region are thinned; for the non-crack tip region, the coarsening operation is carried out for the element whose element level is greater than the specified lowest level and the phase field value of the node is lower than the lower critical value. The coarsening operation is carried out for the element whose element level is greater than the specified lowest level. A hybrid adaptive mesh is obtained after the thinning operation and the coarsening operation described above. The method greatly improves computational efficiency and saves a large amount of memory; in addition, this method can be easily implemented in the existing finite element program, and can be easily extended to other solid/fluid finite element calculations.