Endeavour to balance mechanical properties and self-healing of nature rubber by increasing covalent crosslinks via a controlled vulcanization

Biobased epoxidized natural rubber/sodium carboxymethyl cellulose composites with enhanced strength and healing ability

Conventional vulcanized rubbers cause a non-negligible waste of resources due to the formation of 3D irreversible covalently cross-linked networks. The introduction of reversible covalent bonds, such as reversible disulfide bonds, into the rubber network, is an available solution to the above problem. However, the mechanical properties of rubber with only reversible disulfide bonds cannot meet most practical applications. In this paper, a strengthened bio-based epoxidized natural rubber (ENR) composite reinforced by sodium carboxymethyl cellulose (SCMC) was prepared. SCMC forms a mass of hydrogen bonds between its hydroxyl groups and the hydrophilic groups of ENR chain, which gives the ENR/2,2'-Dithiodibenzoic acid (DTSA)/SCMC composites an enhanced mechanical performance. With 20 phr SCMC, the tensile strength of the composite increases from 3.0 to 10.4 MPa, which is almost 3.5 times that of the ENR/DTSA composite without SCMC. Simultaneously, DTSA covalently cross-linked ENR with the introduction of reversible disulfide bonds, which enables the cross-linked network to rearrange its topology at low temperatures and thus endows the ENR/DTSA/SCMC composites with healing properties. The ENR/DTSA/SCMC-10 composite has a considerable healing efficiency of about 96 % after healing at 80 °C for 12 h.

Design of epoxidized natural rubber/poly(lipoic acid) elastomer with fast and efficient self-healing under a mild temperature

Most of the dynamic covalent bonds (DCBs) for self-healing rubber must be activated at relatively high temperatures due to the requirement of high energy during the exchange of dynamic bonds, which may lead to unexpected degradation or excessive crosslinking of rubber. Herein, we designed and fabricated a highly stretchable, self-healable and reprocessable rubber by introducing dynamic disulfide bonds into the crosslink network of epoxidized natural rubber (ENR). Lipoic acid (LA) was firstly uniformly dispersed into ENR via a latex film formation technique, and then underwent a dynamic covalent ring-opening self-polymerization during hot pressing process, during which the carboxyl group of poly(LA) attacked the epoxy group of ENR to form β-hydroxyl ester bond crosslinks. As a result, a revisable covalently crosslinked network without rigid steric hindrance groups was constructed, which exhibited a super self-healing efficiency of 99 % after self-healing at 80 °C for only 3 h. The elongation at break of the elastomer could reach 1115 % and the recovery rate of reprocessing was as high as 91 %.

Dual X-ray- and Neutron-Shielding Properties of Gd2O3/NR Composites with Autonomous Self-Healing Capabilities

The neutron- and X-ray-shielding, morphological, physical, mechanical, and self-healing properties were investigated for natural rubber (NR) composites containing varying gadolinium oxide (Gd2O3) contents (0, 25, 50, 75, and 100 parts per hundred parts of rubber; phr) to investigate their potential uses as self-healing and flexible neutron- and X-ray-shielding materials. Gd2O3 was selected as a radiation protective filler in this work due to its preferable properties of having relatively high neutron absorption cross-section (σabs), atomic number (Z), and density (ρ) that could potentially enhance interaction probabilities with incident radiation. The results indicated that the overall neutron-shielding and X-ray-shielding properties of the NR composites were enhanced with the addition of Gd2O3, as evidenced by considerable reductions in the half-value layer (HVL) values of the samples containing 100 phr Gd2O3 to just 1.9 mm and 1.3 mm for thermal neutrons and 60 kV X-rays, respectively. Furthermore, the results revealed that, with the increase in Gd2O3 content, the mean values (± standard deviations) of the tensile strength and elongation at break of the NR composites decreased, whereas the hardness (Shore A) increased, for which extreme values were found in the sample with 100 phr Gd2O3 (3.34 ± 0.26 MPa, 411 ± 9%, and 50 ± 1, respectively). In order to determine the self-healing properties of the NR composites, the surfaces of the cut samples were gently pressed together, and they remained in contact for 60 min; then, the self-healing properties (the recoverable strength and the %Recovery) of the self-healed samples were measured, which were in the ranges of 0.30–0.40 MPa and 3.7–9.4%, respectively, for all the samples. These findings confirmed the ability to autonomously self-heal damaged surfaces through the generation of a reversible ionic supramolecular network. In summary, the outcomes from this work suggested that the developed Gd2O3/NR composites have great potential to be utilized as effective shielding materials, with additional dual shielding and self-healing capabilities that could prolong the lifetime of the materials, reduce the associated costs of repairing or replacing damaged equipment, and enhance the safety of all users and the public.

Strengthened self-healable natural rubber composites based on carboxylated cellulose nanofibers participated in ionic supramolecular network

Cellulose, as a green reinforcing agent for rubber, has excellent improvement on the tensile strength but usually accompany with a deterioration of extensibility and self-healing property. Herein, we report an efficient method to prepare robust and self-healable natural rubber/zinc dimethacrylate/carboxylated cellulose nanofibers (NR/ZDMA/CNC) composites which are constructed by a CNC participated ionic supramolecular network. Ionic supramolecular network in NR is generated by the polymerization of ZDMA during a controlled peroxide-initiated vulcanization of NR. Interestingly, NR with massive ion clusters has strong affinity with CNC, which facilitates the uniform dispersion of CNC and the compatibility between CNC and NR. Meanwhile, CNC participates into the supramolecular network via non-covalent interaction with NR chains equipped with ionic crosslinks. This greatly reduces the adverse effect of CNC on the dynamic characteristics of supramolecular network. As a result, the tensile strength of NR/ZDMA composite with 20 phr CNC could reach 4.13 MPa, while its self-healing efficiency still maintains at >80 %. Thus, NR composites with non-covalent interaction between CNC and supramolecular network display improved strength, maintained extensibility, and excellent self-healing capability. This study thus demonstrates a feasible approach to reduce the negative effect of reinforcing fillers on a self-healing rubber based on supramolecular networks.

Role of Maleic Anhydride-Grafted Poly(lactic acid) in Improving Shape Memory Properties of Thermoresponsive Poly(ethylene glycol) and Poly(lactic acid) Blends

Generally, poly(ethylene glycol) (PEG) is added to poly(lactic acid) (PLA) to reduce brittleness and improve mechanical properties. However, shape memory properties of PEG/PLA blends suffered due to the blend’s incompatibility. To enhance shape memory abilities of the blends, 0.45% maleic anhydride-grafted poly(lactic acid) (PLA-g-MA) was used as a compatibilizer. Thermal and mechanical properties, morphologies, microstructures, and shape memory properties of the blends containing different PLA-g-MA contents were investigated. The compatibilized blend with 2 wt% PLA-g-MA exhibited enhanced tensile modulus, strength, and elongation at break, as well as a lower glass transition temperature and degree of crystallinity than the uncompatibilized blend. Results revealed that PLA-g-MA improved interfacial adhesion between phases and promoted chain entanglement. Shape fixity performance of the compatibilized blends were comparable to that of neat PLA. The compatibilized blend containing 2 wt% PLA-g-MA possessed the best shape fixity and recovery performance. Although a high recovery temperature was expected to enhance the recovery of the PEG/PLA blends, the compatibilized blends can be recovered to their original shape at a lower temperature than the PLA. This study illustrated the possibility of optimizing PLA properties to meet requirements necessary for biomedical applications.