Weiming Zhou

Toughening mystery of natural rubber deciphered by double network incorporating hierarchical structures

As an indispensible material for modern society, natural rubber possesses peerless mechanical properties such as strength and toughness over its artificial analogues, which remains a mystery. Intensive experimental and theoretical investigations have revealed the self-enhancement of natural rubber due to strain-induced crystallization. However a rigorous model on the self-enhancement, elucidating natural rubbers extraordinary mechanical properties, is obscured by deficient understanding of the local hierarchical structure under strain. With spatially resolved synchrotron radiation micro-beam scanning X-ray diffraction we discover weak oscillation in distributions of strain-induced crystallinity around crack tip for stretched natural rubber film, demonstrating a soft-hard double network structure. The fracture energy enhancement factor obtained by utilizing the double network model indicates an enhancement of toughness by 3 orders. Its proposed that upon stretching spontaneously developed double network structures integrating hierarchy at multi length-scale in natural rubber play an essential role in its remarkable mechanical performance.

Synthesis of recyclable, chemically cross-linked, high toughness, high conductivity ion gels by sequential triblock copolymer self-assembly and disulfide bond cross-linking

In this article, we report the synthesis of a disulfide bonded reversibly chemically cross-linked ion gel with high toughness and conductivity by sequential triblock copolymer self-assembly and the subsequent oxidation of thiol groups. Through reversible thiol-disulfide exchange, the ion gels had both high toughness of chemicals and recyclability of physical cross-linked ion gels. The triblock copolymer (SOS-SH) was prepared as follows: first, the RAFT copolymerization of styrene and 4-vinylbenzyl chloride (VBC) using CTA–PEO–CTA as a bi-functional macroRAFT agent was performed to obtain a triblock copolymer (SOS-Cl); then, the chloride group of SOS-Cl was replaced by an azido group to obtain SOS-N3; and finally, the click reaction of SOS-N3 with O-ethyl-S-prop-2-ynyl carbonodithioate and subsequent aminolysis were conducted to obtain SOS-SH. The disulfide bonded reversibly chemically cross-linked ion gel could be re-dissolved when mixed with a little amount of mild reducing agent (e.g., DTT) in CH2Cl2 with vigorous stirring, which reformed again after the removal of solvent and oxidation of thiol groups. The ion gels could undergo the reduction–oxidation cycle at least twice with a little loss of ionic conductivity and toughness (less than 25%), exhibiting good recyclability. Raman measurements were performed to confirm the existence and the key role of disulfide bond on the recyclability.

The spatial correlation between crystalline and amorphous orientations of isotactic polypropylene during plastic deformation: An in situ observation with FTIR imaging

On the basis of research method in FTIR imaging, we made a heterogeneous thin film of isotactic polypropylene (iPP) that contains a few large spherulites (∼150 μm) which are surrounded by small spherulites (∼15 μm) for tensile testing. The evolution processes of crystalline and amorphous orientations of iPP are monitored with its characteristic peaks at 998 and 973 cm−1, respectively. By introducing the correlation images, the analysis demonstrates the relationships between the orientation evolutions of crystalline and amorphous phases in a space of 250 μm × 250 μm detecting area. During the plastic deformation, crystalline orientation is higher than amorphous orientation outside the large spherulite, while that is opposite inside the region. In addition, the evolutions of crystalline and amorphous orientations almost keep a positive correlation.

Imaging the strain induced carbon black filler network structure breakage with nano X-ray tomography

Aiming to study the mechanical enhancement by the filler network in a rubber composite, three-dimensional images are acquired with in situ full field transmission X-ray microscopy (TXM), and the network structure of carbon black (CB) aggregates in a rubber matrix are studied with and without strain. Statistical analysis shows that the frequency of similar-sized aggregates decreases with the increase of aggregate size as well as the inter-aggregate distance monotonically without strain. An oscillation of the frequency-size plot is induced by strain on top of the damping trend, which is interpreted as stretch-induced breakage and re-aggregation of CB aggregates. Calculations adopting a soft-hard network model, predict a reduction of the contribution of the CB network to the mechanical property of the rubber composite by about 60%, caused by the breakage and re-aggregation of CB aggregates compared to those without strain. The experimental results directly prove the structural origin of the Payne effect and also show that TXM is a valuable tool to study the mechanical enhancement mechanism of filled rubber composites.

A recyclable disulfide bond chemically cross-linking, high toughness, high conductivity ion gel based on re-shaping and restructuring in the gel state

In this communication, we report the recyclability of a disulfide bond cross-linking ion gel with high toughness and ionic conductivity based on re-shaping and restructuring under heat and pressure in the gel state. The restructuring was realized by disulfide metathesis catalyzed by both an ionic liquid (IL) and the residual copper salt from the click reaction. The ion gels could undergo the breaking–restructuring cycles at least six times with little performance loss (less than 10%), exhibiting good recyclability.