Hongji Zhang

pH-dependent and self-healing properties of mussel modified poly(vinyl alcohol) hydrogels in a metal-free environment

3,4-Dihydroxyphenylalanine (DOPA)-based polymers are well-known to form functional hydrogels with self-healing properties by chelating metal ions. However, DOPA-based hydrogels with self-healing properties are difficult to obtain in the absence of the metal ions, as previously reported. Thus, the aim of this study is to prepare a self-healable DOPA-based hydrogel in the absence of metal ions. Firstly, poly(vinyl alcohol)–DOPA (PVA–DOPA) was synthesized by modifying PVA with DOPA through an esterification reaction. The composition of the PVA–DOPA polymer was determined by proton nuclear magnetic resonance (1H NMR) spectroscopy. Then, the PVA–DOPA hydrogel in a metal-free environment could be easily prepared by dissolving the polymer in buffer solution. Rheological analyses showed that the PVA–DOPA polymers had different dynamic moduli depending on the pH of the buffer solutions. The results from the FTIR and UV-vis spectra indicated that there were hydrogen bond interactions between the PVA–DOPA polymers under low pH conditions, while there were both hydrogen bond and covalent interactions under high pH conditions. The PVA–DOPA hydrogel could be rapidly self-healed within 270 s, which was much quicker than the hydrogel prepared in the presence of Fe3+ (about 600 s). The metal-free PVA–DOPA hydrogel has the potential for application in coating and biomedical fields.

Multi-functional polydopamine coating: simultaneous enhancement of interfacial adhesion and CO2 separation performance of mixed matrix membranes

Inspired by marine mussels adhesive system and facilitated transport membranes, we proposed a novel strategy to simultaneously improve the MOF–polymer interface and CO2 separation performance by coating polydopamine (PDA) on the surface of ZIF-8. PDA@ZIF-8 played multiple roles in enhancing the membrane performance. First, the inherent high permeability of ZIF-8 with ultra-microporosity was anticipated to optimize the fractional free volume and enhance the gas permeability of MMMs. Second, the PDA coating can mitigate the interfacial defects and improve the adhesion between ZIF-8 and the polymer matrix. Finally, the abundant –OH and amino groups in PDA have strong interaction with CO2 molecules, which can effectively facilitate CO2 transport. The membrane separation performance for pure gases (CO2 and N2) of the PDA@ZIF-8 incorporated Pebax membrane was investigated and compared with that of the ZIF-8-incorporated membrane without the PDA coating. An anti-tradeoff effect was obtained by addition of PDA@ZIF-8. In particular, the Pebax/PDA@ZIF-8-15 MMM showed the best gas separation performance with a CO2 permeability of 267.74 Barrer and a CO2/N2 selectivity of 62.65, surpassing Robesons upper bound from 2008.

Near-infrared light-driven locomotion of a liquid crystal polymer trilayer actuator

A new and general design is demonstrated for a liquid crystal polymer network (LCN)-based actuator to perform near-infrared (NIR) light-guided locomotion. The actuator is a structured trilayer, comprising a thin reduced graphene oxide (RGO) top layer, an inactive polymer middle layer and an active LCN bottom layer. Exposing the RGO side to a moving NIR laser, a moving wave along the strip actuator is generated, which makes the strip an effective caterpillar walker whose light-driven locomotion has appealing attributes. On one hand, in contrast to most LCN-based actuators that move on a ratcheted substrate surface or need a supporting plastic frame, the trilayer actuator moves on either horizontal or inclined untreated surfaces. On the other hand, while known actuators using the photothermal effect are usually prepared by mixing the nanofiller used as a NIR light heater with the polymer, which may reduce the reversible deformation degree and increase the compatibility concerns, the easy trilayer fabrication method laminates directly a “sheet” of RGO on thick polymer layers, which circumvents the potential problems.

A CO2-responsive graphene oxide/polymer composite nanofiltration membrane for water purification

A “smart” graphene oxide (GO) based nanofiltration membrane was prepared through electrostatic and π–π interaction-driven complexation with poly(N,N-diethylaminoethyl methacrylate) bearing a pyrene end group (Py-PDEAEMA). The composite membranes show a number of attributes potentially appealing for water treatment. First, they display reversible, gas-tunable water permeability between the pore closure state under CO2 stimulation and the pore opening state upon Ar bubbling. The water permeability remains much higher than that of a pure GO membrane even at the pore closure state. Secondly, the separation of organic dye molecules from water using this membrane is similarly efficient as that using a pure GO membrane despite the much higher water permeability. Finally, as a result of CO2-induced reversal of the charge sign, the composite membrane shows an excellent trade-off between rejection of some salts like MgCl2 and water permeability, with both much higher rejection and permeability compared to a pure GO membrane. This study of a CO2-responsive, GO-based nanofiltration membrane, the first of its kind, demonstrates new perspectives in developing smart separation membranes for water purification.

Effects of modified nanocrystalline cellulose on the hydrophilicity, crystallization and mechanical behaviors of poly(3-hydroxybutyrate-co-3-hydroxyhexanoate)

Fine dispersion of nanocrystalline cellulose (NCC) in a bacterially synthesized poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBH) matrix is challenging due to their incompatibility. To improve their compatibility, NCC was first grafted with PHBH, marked as NCC-g-PHBH, using (3-aminopropyl)triethoxysilane (APES) as a coupling agent, leading to a grafting degree of ∼25 wt%. The grafting mechanism and the mechanical and crystallization behaviors of the PHBH/NCC-g-PHBH blends, prepared via solution casting, were systematically investigated. The NCC was uniformly dispersed in the PHBH matrix after grafting with PHBH. As a consequence, the mechanical and crystallization properties of the PHBH were greatly improved in the presence of the NCC-g-PHBH. With the addition of 1.0 wt% NCC-g-PHBH, the Youngs modulus of the PHBH/NCC-g-PHBH blend was increased by 15%, which is further evaluated by the Halpin–Tsai model, while the surface contact angles decreased by 30°. Furthermore, the presence of NCC-g-PHBH copolymers greatly increased the nuclei density and decreased the spherulite size of PHBH without changing the crystal structures. Therefore, this work provided a novel route to prepare fully biodegradable and bio-based PHBH nanocomposites, which may broaden the application range of PHBH.