Tailoring surface hydrophilicity of porous electrospun nanofibers to enhance capillary and push-pull effects for moisture wicking

In this article, liquid moisture transport behaviors of dual-layer electrospun nanofibrous mats are reported for the first time. The dual-layer mats consist of a thick layer of hydrophilic polyacrylonitrile (PAN) nanofibers with a thin layer of hydrophobic polystyrene (PS) nanofibers with and without interpenetrating nanopores, respectively. The mats are coated with polydopamine (PDOPA) to different extents to tailor the water wettability of the PS layer. It is found that with a large quantity of nanochannels, the porous PS nanofibers exhibit a stronger capillary effect than the solid PS nanofibers. The capillary motion in the porous PS nanofibers can be further enhanced by slight surface modification with PDOPA while retaining the large hydrophobicity difference between the two layers, inducing a strong push–pull effect to transport water from the PS to the PAN layer

Novel electrochromic materials of PANI-TiO2 hybrid

Nowadays, intensive interest has been drawn to the development of derivatives of polyaniline (PANI) in order to enhance its spectro-electrochemical properties. In order to improve its electrochromic contrast, we has successfully prepared PANI-TiO2 hybrids in which inorganic metal oxide was covalently bonded to aniline groups on the macromolecular chains. The organic-inorganic hybrid composites are expected to provide synergetic effects on electrochromic properties of the device. The structures of the hybrid systems are characterized using FTIR, TGA, WAXD and SEM/EDX analysis. Meanwhile, the spectro-electrochemical properties, cyclic voltammetry behavior, coloration efficiency and response times of the devices are studied. The results reveal that PANI has been grafted onto TiO2 using N-[3-(trimethoxysilyl propyl)aniline] as cross-linking agent. The hybrid systems exhibit enhanced optical contrast and coloration efficiency than that of PANI. This can be attributed to electron-drawing ability of TiO2 that leads to more accessible doping sites and higher ionic conductivity as well as higher electrochromic contrast than PANI. This innovative approach can be further investigated to develop multicolored electrochromism in organic-inorganic hybrid systems.Bachelor of Engineering (Materials Engineering

Thermoplastic polymer nanocomposites based on polydopamine-coated clay : preparation, structures and properties

Polymer/clay nanocomposites have been widely investigated over past three decades due to the dramatic boost in properties at low filler content. Although organoclay (clay modified with organic surfactants) is commonly used to reinforce the polymer, yet the reinforcing extent has yet reached the optimum performance. Besides, both organic surfactants (organic compounds with long hydrophobic tails and hydrophilic heads) and polymers are susceptible to photo-induced degradation especially in outdoor environments, making polymer/clay nanocomposites vulnerable in practical applications. In order to overcome the aforementioned problems, in this research, D-clay (polydopamine-coated clay) was studied as multifunctional filler to improve not only interfacial interactions with a wide range of polymer matrices but also stabilities of the nanocomposites. D-clay was incorporated into both elastomer (polyurethane) and semi-crystalline thermoplastic (polypropylene) systems. The structure-property relationships of the resultant nanocomposites were investigated using TEM, XRD, DMA, tensile testing, FTIR, TGA and DSC. In particular, the reinforcing mechanism of D-clay in polyurethane (PU) nanocomposites was studied with respect to surface chemistry, filler loading and filler size. On the other hand, the stabilizing function of D-clay was verified using polypropylene (PP) as the polymer matrix since PP is well-known for its poor UV stability. Firstly, D-clay was incorporated into polyether-based PU via solvent mixing and good filler dispersion was obtained. The results showed pronounced improvement in mechanical properties, such as stiffness, tensile strength and strain at break, at 3wt% clay loading. The remarkable improvement can be attributed to the excessive hydrogen bonds between D-clay and the hard segments (hard segments are made of diisocyanate and the short-chain diol) of PU. This strong interfacial interaction between D-clay and hard segments not only facilitates the stress transfer across the filler and polymer matrix, but also acts as nucleating agent for hard segment crystallization, leading to higher hard segment crystallinity. Furthermore, the impact of high D-clay loading on mechanical properties and hard segment crystallization was investigated using polyester-based PU as matrix since severe phase separation was observed in the polyether-based PU. The results showed polyester-based PU nanocomposites with D-clay concentration above 5 wt% formed percolated clay network structure, this hindered the movement of both hard and soft segments to a certain extent. Consequently, polyester-based PU/D-clay nanocomposites showed drastic enhancement in tensile modulus. On the other hand, the effect of particle size was studied using polycaprolactone (PCL)-based PU as matrix. In this case, polydopamine-modified layered double hydroxides (D-LDHs) of different sizes were used as the fillers and the shape memory performance of the nanocomposites was evaluated. It was found that D-LDH interacted strongly with hard segments, enhancing phase separation and promoting crystallization of both hard and soft segments profoundly. The nanocomposite with 2 wt% of small D-LDH exhibited good shape memory properties since most small D-LDH interacted with hard domains at low filler loading. Hence, the incorporation of small D-LDH can reinforce hard domains without sacrificing the elasticity of the system. In order to verify the stabilizing capability of D-clay, D-clay was also introduced into the PP system. This is because PP is vulnerable to degradation owing to the presence of volatile tertiary hydrogens in the polymer backbone. The results showed drastic improvement in UV resistance and thermal stability of PP/D-clay owing to the effective radical scavenging ability of melanin-like PDA layer on clays. Meanwhile, the excellent UV resistance of PP/D-clay nanocomposites can be attributed to the masking effect imposed by PDA coating. Besides, the mechanical properties of PP/D-clay were better than organoclay at similar clay loading on account of the stronger interfacial interactions.DOCTOR OF PHILOSOPHY (MSE

A high throughput method for preparation of highly conductive functionalized graphene and conductive polymer nanocomposites

Highly conductive graphene sheets were prepared by coating graphene oxide with polydopamine (PDA) followed by reduction with hydrazine. Polyacrylonitrile/graphene nanocomposites prepared via solution blending exhibit high electrical conductivities at very low graphene loadings owing to the good exfoliation and relatively planar conformation of the PDA-coated graphene in the polymer matrix

Simultaneous catalyzing and reinforcing effects of imidazole-functionalized graphene in anhydride-cured epoxies

In this study, an imidazole-functionalized graphene (G-IMD) was prepared from graphene oxide by a facile one-pot method. The functionalized graphene not only showed improved organic compatibility but also could simultaneously play the roles of a cure accelerator and reinforcement for anhydride-cured epoxies. Our results showed that G-IMD could successfully catalyze the curing reaction without the addition of any routine accelerator. Thermal and mechanical properties of the epoxy–G-IMD nanocomposites were systematically studied at different filler loadings. Compared with neat epoxy resin, tensile strength and Young's modulus of the nanocomposites were enhanced by 97% and 12%, respectively, at only 0.4 wt% G-IMD loading. Dynamic mechanical analysis and electron microscopic results revealed that the drastic improvements in mechanical properties could be attributed to the homogeneous dispersion of G-IMD and covalent bonding at the interface, which effectively improved the efficiency of load transfer between the matrix and graphene.Accepted versio

Highly conductive graphene by low-temperature thermal reduction and in situ preparation of conductive polymer nanocomposites

Polydopamine-coated graphene oxide (DGO) films exhibit electrical conductivities of 11 000 S m−1 and 30 000 S m−1 upon vacuum annealing at 130 °C and 180 °C, respectively. Conductive poly(vinyl alcohol)/graphene and epoxy/graphene nanocomposites show low percolation thresholds due to the excellent dispersibility of the DGO sheets and their effective in situ reduction

Reinforcement of polyether polyurethane with dopamine-modified clay : the role of interfacial hydrogen bonding

Dopamine-modified clay (D-clay) was successfully dispersed into polyether polyurethane (PU) by solvent blending. It is found that the incorporation of D-clay into PU gives rise to significant improvements in mechanical properties, including initial modulus, tensile strength, and ultimate elongation, at a very low clay loading. The large reinforcement could be attributed to the hydrogen bonds between the hard segments of PU and stiff D-clay layers that lead to more effective interfacial stress transfer between the polymer and D-clay. Besides, the interactions between D-clay and PU are also stronger than those between Cloisite 30B organoclay and the PU chains. Consequently, at a similar clay loading, the PU/D-clay nanocomposite has much higher storage modulus than the PU/organoclay nanocomposite at elevated temperatures

Shape memory polyurethane with polydopamine-coated nanosheets: Simultaneous enhancement of recovery stress and strain recovery ratio and the underlying mechanisms

Two different sizes of polydopamine-coated layered double hydroxides (D-LDHs) are incorporated into polycaprolactone-based polyurethane (PU) to enhance the mechanical and shape memory properties of the PU. The results show that D-LDH interacts strongly with hard segments and hence enhancing phase separation between hard and soft segments. It is found that the tensile moduli of the PU/D-LDH nanocomposites are much higher than that of neat PU at 60 oC. In comparison with neat PU, the nanocomposite with 2 wt% of small D-LDH exhibits a 60% increase in recovery stress while shape fixity and strain recovery ratio are also improved simultaneously. This is because at low filler loading, most small D-LDH nanosheets interact with hard domains and they are not large enough to connect neighbor hard domains. They can hence reinforce the hard domains without sacrificing the elasticity of the system. Two-dimensional X-ray diffraction studies indicate that most small D-LDH nanosheets are able to rotate back from aligned state to original random state in shape recovery process, justifying the improved strain recovery ratio.ASTAR (Agency for Sci., Tech. and Research, S’pore)Accepted versio

Transition-metal-ion-mediated polymerization of dopamine : Mussel-inspired approach for the facile synthesis of robust transition-metal nanoparticle-graphene hybrids

Inspired by the high transition-metal-ion content in mussel glues, and the cross-linking and mechanical reinforcement effects of some transition-metal ions in mussel threads, high concentrations of nickel(II), cobalt(II), and manganese(II) ions have been purposely introduced into the reaction system for dopamine polymerization. Kinetics studies were conducted for the Ni2+–dopamine system to investigate the polymerization mechanism. The results show that the Ni2+ ions could accelerate the assembly of dopamine oligomers in the polymerization process. Spectroscopic and electron microscopic studies reveal that the Ni2+ ions are chelated with polydopamine (PDA) units, forming homogeneous Ni2+–PDA complexes. This facile one-pot approach is utilized to construct transition-metal-ion–PDA complex thin coatings on graphene oxide, which can be carbonized to produce robust hybrid nanosheets with well-dispersed metallic nickel/metallic cobalt/manganese(II) oxide nanoparticles embedded in PDA-derived thin graphitic carbon layers. The nickel–graphene hybrid prepared by using this approach shows good catalytic properties and recyclability for the reduction of p-nitrophenol

Highly electrically conductive layered carbon derived from polydopamine and its functions in SnO2-based lithium ion battery anodes

Thin carbonized polydopamine (C-PDA) coatings are found to have similar structures and electrical conductivities to those of multilayered graphene doped with heteroatoms. Greatly enhanced electrochemical properties are achieved with C-PDA-coated SnO2 nanoparticles where the coating functions as a mechanical buffer layer and conducting bridge