All issues / Volume 14 (2020) / Issue 9 (September)
This is an editorial article. It has no abstract.
In this work, the polydopamine (PDA) coating layer was first applied to modify hydroxyapatite (HA) nanoparticles and then the functional PDA/HA (pHA) composite nanoparticles were obtained. Subsequently, a 3-D porous thermoplastic polyurethane (TPU) scaffold modified by two-stage pHA nanoparticles ( pHA-TPU/pHA ) was fabricated by vacuum-assisted solvent casting and ultrasonication-assisted methods. The results demonstrated that the pHA nano-particles were successfully prepared and introduced into the pHA-TPU/pHA composite scaffold both internally and externally. The pHA-TPU/pHA composite scaffold had an excellent interconnected network structure with high porosity of 85.7% and large pore sizes ranging from 100–500 µm. The hydrophilicity and comprehensive mechanical properties of the pHA-TPU/pHA composite scaffold were significantly higher than those of the TPU scaffold due to the addition of the pHA nanoparticles. The attachment and viability of mouse embryonic osteoblast cells (MC3T3-E1) cultured on the pHA-TPU/pHA composite scaffold were also higher than those on the TPU scaffold. These results suggest that the functional pHA-TPU/pHA composite scaffold has superior mechanical and biological properties, which makes it potentially useful for tissue engineering scaffolds.
In the frame of green-based chemistry, advanced shape memory polymers are designed from benzoxazine (RSBOX) and epoxy (R-EP) resins basing on potential natural raw materials, such as resorcinol and stearylamine. Thermal curing, investigated by differential scanning calorimetry, shows several overlapping peaks suggesting a complex curing mechanism. Dynamic mechanical analysis of cured RS-BOX/R-EP copolymers demonstrates an increase in the glass temperature and narrower glass transition by the increase of the RS-BOX ratio. In contrast, crosslinking density increases with higher epoxy resin content. All investigated materials possess one-way dual shape memory ability triggered by glass transition temperature with excellent shape fixity, while the shape recovery values ranged between 95 and 100%. The duration of the recovery process is significantly influenced by the RS-BOX amount. Additionally, the mechanical and shape memory properties of fully bio-based SMPs might be suitably tailored for advanced applications by merely varying the initial composition.
The vulcanization of high diene elastomers with conventional or semi-efficient vulcanization systems exhibit reversion at a higher temperature due to the breakage of the polysulfidic crosslinks established in the vulcanized network. As a result, the vulcanizate show inferior mechanical and thermal stability at elevated temperature. In this contest, the anti-reversion ability of Maleide F (75% N,N'-meta phenylene dimaleimide and a 25% blending agent: MF) was evaluated on the vulcanization behavior of cis-polybutadiene rubber (BR) with a conventional accelerated sulfur system. The anti-reversion ability of MF was also compared with Perkalink 900 (1,3-bis (citraconimidomethyl) benzene: PL). Based on the vulcanization studies using a moving die rheometer, it has been realized that MF forms carbon-carbon based bismaleimide crosslinks at the beginning of the vulcanization via Alder-Ene reaction. As a result, MF-filled systems exhibit a remarkable reversion resistance and a plateau type cure behavior even at 180 °C for a period of 1hr. On the other hand, PL shows its anti-reversion activity via Diels-Alder reaction after a short reversion. Moreover, the PL filled system exhibits a marching-modulus cure behavior at a higher vulcanization temperature. Therefore, PL cannot be considered as a promising candidate for the hightemperature vulcanization of BR with accelerated sulfur.
A new approach of mechanical preparation of photocatalyst zinc oxide (ZnO)/rubbers from four types of rubbers: styrene butadiene rubber (SBR), ethylene propylene diene monomer (EPDM), natural rubber (NR), and epoxidized natural rubber (ENR) with 50% epoxidation is presented. This technique is simple, fast and cost effective as ZnO/rubbers were mechanically mixed using conventional two-roll mill at 27 °C for 10 min and compressed into flat sheet. The characteristics of photocatalyst were studied by X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), ultraviolet-visible spectroscopy (UV-Vis) and field emission scanning electron microscope (FESEM). The photocatalytic activity of ZnO/rubbers was evaluated using methylene blue (MB) as a model pollutant under visible light for 300 min. The photocatalytic degradation efficiency of ZnO/SBR is nearly as good as that of ZnO powder (87.32%) compared to ZnO/NR, ZnO/EPDM and ZnO/ENR. This is due to the highest index of conjugated carbon-carbon bond of SBR and hydrogen bonding between ZnO and SBR. The formation of hydrogen bonding was confirmed by FTIR and reduction of energy band gap of ZnO/SBR. The photocatalytic degradation of MB with ZnO/SBR was could be fitted by pseudo-first-order kinetics of the Langmuir-Hinshelwood model.
In this research, poly(lactic acid) (PLA) was toughened and improved the crystallinity by rubber particles and inorganic filler. CaCO3 was employed as a filler and nucleating agent while poly(methyl methacrylate)-poly(butadiene-styrene) (MBS) core-shell impact modifier was added as a toughening agent. Overall, the enhancements of both the toughness and crystallization of PLA with CaCO3 and MBS were successfully achieved. The tensile modulus and strength of PLA increased with increasing CaCO3 content from 10 to 30wt%. However, they decreased slightly when CaCO3 loading reached 40 wt% due to particles agglomerations. With the addition of MBS rubber, the tensile modulus and strength of the PLA/CaCO3 composites became lower than those observed for PLA/CaCO3 composites due to the softening effect. Furthermore, the compositions with MBS showed superior toughness in terms of the tensile elongation at break and impact strength. CaCO3 nucleated the PLA crystal which reflected as the increase in the degree of crystallinity (Xc) by at least 2 times for all formulations studied. The crystallization half-time (t1/2) of PLA with 40 wt% CaCO3 was dramatically reduced, from 26 min, in neat PLA, to 0.9 min. With the addition of MBS, it did at 2.7 min for the same CaCO3 content. The maximum increment of heat distortion temperature (HDT), around 8 °C, was found for the PLA with 20 wt% CaCO3.
Dynamic covalent crosslinking such as disulfide bonds, Diels-Alder (DA) reactions are widely used for healing applications. Herein, we report a simple approach involving the metal-ligand reversible interactions in diverse nature, which helps in developing a robust and self-healable carboxylated nitrile (XNBR) rubber by employing low cost and the commercially obtainable materials. Self-healing performance and mechanical properties were organized by introducing the various metal-ligand domains into the XNBR rubber. The network of XNBR, in-situ cross-linked via metal-ligand complexes, consists of strong and weak coordination bonds. The strength of various metal-ligand modified coordination bonds, healing performance, and mechanical properties primarily depend on the type of metal ions. The Fourier transform infrared spectroscopy (FTIR) makes the various metal-ligand coordination bond formation into the XNBR rubber visible. The coordination crosslinked XNBR rubber with 4 phr of Zn and Co metal ion exhibits high tensile strength (4.3±0.6 and 10.3±1.1 MPa) with excellent healing efficiency (100 and 88%), which is far higher than the most reported non-covalent supramolecular modified elastomers. The various metal-ligand coordination bonds are fully reconstructed during the rebuilding process and exhibiting excellent self-healing property.
Stereolithography (SL) stands out as a relatively fast additive manufacturing method to produce thermoset components with high resolutions. The majority of SL resins consist of acrylate monomers which result in materials with curing-induced shrinkage problems and this, in addition to the incomplete and non-uniform conversions reached in the SL process, results in poor mechanical properties. To address this issue, a dual-curing formulation was developed by mixing an epoxy monomer into a commercial multi-acrylate SL resin: the first curing stage is acrylate free-radical photopolymerization at ambient temperature, and the second curing stage is cationic epoxy homopolymerization at higher temperatures. The fully dual-cured materials are macroscopically homogeneous, with nanoscale domains observed by Atomic Force Microscopy (AFM), and with unimodal tan delta peaks observed in Dynamic Mechanical Analysis (DMA). The uncured material was storage stable at ambient conditions for at least 9 weeks since the epoxy part was virtually unreactive at these temperatures. With the dual-cured materials, a nearly 10-fold increase in Young’s modulus was achieved over the neat acrylate resin. At the thermal curing stage, the presence of diperoxyketal thermal radical initiator to the liquid formulation facilitated the polymerization of unreacted acrylates that remained from the SL process simultaneously with epoxy homopolymerization and helped the material attain improved properties.