Liming Cao

Design of Self-Healing Supramolecular Rubbers by Introducing Ionic Cross-Links into Natural Rubber via a Controlled Vulcanization

Introducing ionic associations is one of the most effective approaches to realize a self-healing behavior for rubbers. However, most of commercial rubbers are nonpolar rubbers without now available functional groups to be converted into ionic groups. In this paper, our strategy was based on a controlled peroxide-induced vulcanization to generate massive ionic cross-links via polymerization of zinc dimethacrylate (ZDMA) in natural rubber (NR) and exploited it as a potential self-healable material. We controlled vulcanization process to retard the formation of covalent cross-link network, and successfully generated a reversible supramolecular network mainly constructed by ionic cross-links. Without the restriction of covalent cross-linkings, the NR chains in ionic supramolecular network had good flexibility and mobility. The nature that the ionic cross-links was easily reconstructed and rearranged facilitating the self-healing behavior, thereby enabling a fully cut sample to rejoin and retain to its original properties after a suitable self-healing process at ambient temperature. This study thus demonstrates a feasible approach to impart an ionic association induced self-healing function to commercial rubbers without ionic functional groups.

Self-Healing Natural Rubber with Tailorable Mechanical Properties Based on Ionic Supramolecular Hybrid Network

In most cases, the strength of self-healing supramolecular rubber based on noncovalent bonds is in the order of KPa, which is a challenge for their further applications. Incorporation of conventional fillers can effectively enhance the strength of rubbers, but usually accompanied by a sacrifice of self-healing capability due to that the filler system is independent of the reversible supramolecular network. In the present work, in situ reaction of methacrylic acid (MAA) and excess zinc oxide (ZnO) was realized in natural rubber (NR). Ionic cross-links in NR matrix were obtained by limiting the covalent cross-linking of NR molecules and allowing the in situ polymerization of MAA/ZnO. Because of the natural affinity between Zn2+ ion-rich domains and ZnO, the residual nano ZnO participated in formation of a reversible ionic supramolecular hybrid network, thus having little obstructions on the reconstruction of ionic cross-links. Meanwhile, the well dispersed residual ZnO could tailor the mechanical properties of NR by changing the MAA/ZnO molar ratios. The present study thus provides a simple method to fabricate a new self-healing NR with tailorable mechanical properties that may have more potential applications.

Study of the Crosslinking Evolution of Styrene-Butadiene Rubber/Zinc Dimethacrylate Based on Dissolution/Swelling Experiments

The crosslinking evolution of styrene-butadiene rubber (SBR) filled with in situ zinc dimethacrylate (ZDMA) during dicumyl peroxide (DCP) curing was studied. The crosslink density of the composites with different extent of curing was evaluated by dissolution/swelling experiments. The cure-curve was determined by rheometry measurements. The results revealed that the crosslinking of the SBR matrix was not affected by the ZDMA at 170°C. In this case, the decomposed DCP could provide sufficient free radicals to support both the crosslinking of SBR and polymerization of ZDMA simultaneously. However, when the peroxide radicals were limited, at 120°C, the ZDMA could promote the covalent crosslinking of the SBR matrix.

Green method to reinforce natural rubber with tunicate cellulose nanocrystals via one-pot reaction

Tunicate cellulose nanocrystals (t-CNs) isolated from marine biomass were mixed with natural rubber (NR) via two different approaches. The first approach was the green one-pot route, NR latex was first mixed with t-CNs suspension, followed by epoxidization of the mixture. Meanwhile, a two-step method, referring to the way that NR latex was first epoxidized and then mixed with t-CNs suspension, was also carried out for comparison. The interfacial interaction, thermal performance, morphology, mechanical properties and water swelling behavior were investigated. Hydrogen bonds formed in the both nanocomposites and mechanical properties improved with increasing t-CNs content. Moreover, better dispersion and enhanced interfacial interaction were achieved for one-pot method, which was ascribed to the etching effect of hydrogen peroxide on the t-CNs surface and the possible grafting reaction during one-pot process. Therefore, compared with two-step method, 20% increase in tensile strength and 50% increase in tensile modulus were achieved for one-pot method at 10 phr t-CNs content.