Rui-Ying Bao

Polymorphism of racemic poly(L-lactide)/poly(D-lactide) blend: effect of melt and cold crystallization.

The crystallization and melting behaviors and crystalline structure of melt and cold crystallized poly(L-lactide)/poly(D-lactide) (PLLA/PDLA) blend were investigated by differential scanning calorimetry (DSC) and wide-angle X-ray diffraction (WAXD), respectively. The isothermal crystallization kinetics during the melt and cold crystallization process were analyzed using the Avrami equation. The overall crystallization rate constant (k) of cold crystallization is much higher than that of melt crystallization. Moreover, k as a function of crystallization temperature shows different trends in melt and cold crystallization, indicating different crystallization mechanisms in the melt and cold crystallization. The polymorphic crystallization of homocrystallites (the transition crystallization temperature from δ to α form) is not altered by either the equimolar blending of PLLA and PDLA or the type of crystallization procedures, while the crystallization window for exclusive stereocomplex crystallites is widened from 170 °C for melt crystallization to 170-200 °C for cold crystallization. The stereocomplex crystallites are hard to form in both melt and cold crystallization at crystallization temperatures of 90 and 100 °C, and the crystallinity of stereocomplex crystallites for cold crystallization is higher than that of melt crystallization at temperatures above 110 °C. Especially, a pure and significantly higher crystallinity of stereocomplex crystallites can be achieved at 170-200 °C by cold crystallization. The results provide a huge possibility to control stereocomplex crystallization to enlarge its applications.

Self-assembled high-strength hydroxyapatite/graphene oxide/chitosan composite hydrogel for bone tissue engineering

Graphene hydrogel has shown greatly potentials in bone tissue engineering recently, but it is relatively weak in the practical use. Here we report a facile method to synthesize high strength composite graphene hydrogel. Graphene oxide (GO), hydroxyapatite (HA) nanoparticles (NPs) and chitosan (CS) self-assemble into a 3-dimensional hydrogel with the assistance of crosslinking agent genipin (GNP) for CS and reducing agent sodium ascorbate (NaVC) for GO simultaneously. The dense and oriented microstructure of the resulted composite gel endows it with high mechanical strength, high fixing capacity of HA and high porosity. These properties together with the good biocompatibility make the ternary composite gel a promising material for bone tissue engineering. Such a simultaneous crosslinking and reduction strategy can also be applied to produce a variety of 3D graphene-polymer based nanocomposites for biomaterials, energy storage materials and adsorbent materials.

A new approach to construct segregated structures in thermoplastic polyolefin elastomers towards improved conductive and mechanical properties

Aiming at improved electrical conductive performance and simultaneously enhanced mechanical properties, a novel segregated structure was constructed for poly(ethylene-co-octene) (POE)/multi-walled carbon nanotube (MWCNT) elastomeric conductive composites with chemically cross-linked POE granules. Structural examination revealed the formation of unique phase morphologies with a stable segregated structure, in which the uncross-linked POE/MWCNT phase localized out of the cross-linked granules. With such a novel segregated structure, a percolation threshold as low as 1.5 vol% of MWCNTs was observed, which is significantly lower than the melt compounded POE/MWCNT composites; the stress at 100% and 300% stretching increased for more than 12% and 30%, respectively, and the tensile modulus inherent to the matrix elastomer was maintained. The elastic recovery of the composite with such a novel segregated structure was more than 85% and 65% after large strains up to 100% and 300%, respectively, always higher than the melt compounded POE/MWCNT composites. The Shore A hardness of the elastomeric conductive composites with cross-linked POE granules was also lower, showing better elasticity of POE/MWCNT composites with such a novel segregated structure. All these results demonstrated that the elastomeric POE/MWCNT conductive composites with such a novel segregated structure exhibited greatly reduced percolation thresholds with enhancement in mechanical properties, which provides a new way for the preparation of elastomeric conductive composites with simultaneously improved electrical performance and mechanical properties.

Towards balanced strength and toughness improvement of isotactic polypropylene nanocomposites by surface functionalized graphene oxide

Balanced stiffness and toughness is always the goal of high-performance general plastics for engineering purposes and the interfacial crystalline structure control has been proved to be an effective way to approach this goal. In this work, a kind of novel β-nucleating agent (β-NA) for isotactic polypropylene (iPP), one of the most rapidly developing general plastics, was supported onto the surface of octadecylamine functionalized graphene oxide (GO-D), and the effects of functionalized graphene oxide (GO) on the crystallization behavior, crystalline structures and mechanical properties of iPP composites were studied. The presence of the octadecyl chain changes the hydrophilic GO to be hydrophobic, and further supporting of β-NA onto GO-D (GO-N) does not change its solubility in xylene. The hydrophobic nature of octadecyl chains on the GO-D and GO-N surfaces leads to improved interfacial adhesion with the non-polar iPP matrix. At the same time, GO-N exhibits high efficiency in inducing the formation of β-crystals of iPP. The relative content of β-crystals, kβ, reaches a value as high as 73.6% at a loading of 0.1 wt% GO-N, resulting in a maximum increase in impact strength by almost 100% and a simultaneous improvement of the tensile strength by about 30%. This work provides a potential industrializable technique for high-performance iPP nanocomposites.

Temperature induced gelation transition of a fumed silica/PEG shear thickening fluid

The effect of temperature on the rheological behaviors of a shear thickening fluid (STF) prepared by dispersing fumed silica (SiO2) particles into polyethylene glycol (PEG) under mechanical stirring and ultrasonication was investigated using a rotational rheometer. Under steady shear, the system showed an obvious shear thickening behavior due to the formation of “hydroclusters” of SiO2 particles driven by hydrodynamic lubrication forces. The value of the critical shear rate at which the shear thickening begins grows monotonically with temperature. Dynamic temperature sweeps show that elevating the temperature induces a gelation transition of the SiO2/PEG system when the concentration of SiO2 exceeds a critical value, which is found to be lower for the system consisting of higher average molecular weight PEG. The gelation process also becomes more remarkable at a higher concentration of SiO2 particles. It is found that the temperature induced gelation of SiO2/PEG sol is essentially related to the disappearance of the solvation layer on the surface of SiO2 particles as well as the change of hydrogen bonds.

Tuning the structure of graphene oxide and the properties of poly(vinyl alcohol)/graphene oxide nanocomposites by ultrasonication

The structure of graphene oxide (GO) was tuned by controlled ultrasonication (40 kHz, 400 W). It was found that one hour of sonication was the critical point. Under ultrasonication, GO was exfoliated gradually in one hour and further treatment led to a reverse trend. The size of GO sheets decreased sharply in the first hour and then remained almost unchanged with the extension of sonication. In other words, once the GO sheets were cut into pieces small enough, they tended to restack together. Defects on the surface of GO were easily generated even within a short period of ultrasonication and the chemical structure was also changed. GO treated with different sonication times was incorporated into poly(vinyl alcohol) (PVA), and the structure and properties of PVA/GO nanocomposites were also discussed. The hydrogen bonding between the GO and PVA matrix increased before the critical point and then decreased, which was the key factor influencing the glass transition temperature. Meanwhile, the mechanical strength of the nanocomposites was improved before the critical point. The reinforcing mechanism is believed to be the effective load transfer between the PVA matrix and GO via strong hydrogen bonding interactions caused by the exfoliation effect.

Electrical properties and morphology of carbon black filled PP/EPDM blends: effect of selective distribution of fillers induced by dynamic vulcanization

The roles of dynamic vulcanization process in the electrical properties, morphology, and rheology of carbon black (CB)-filled polypropylene (PP)/ethylene–propylene-diene rubber (EPDM) blends have been investigated. With the addition of CB, the uncross-linked (TPE) and dynamically vulcanized (TPV) composites showed a notable difference in the electrical properties, which is mainly caused by different distributions of CB particles resulting from the dynamic vulcanization process. Particularly, it was found that the CB particles in the TPE composites tended to distribute in EPDM phase, whereas the CB particles in the TPV composites were almost located in the PP matrix. The rheological behavior of the TPE and TPV composites was significantly changed with the incorporation of CB particles. Due to the selective distribution caused by the dynamic vulcanization process, the formation of the conductive network for the TPE composites is caused by the double percolation effect, while for the TPV composite, the formation of the conductive network is caused by the excluded volume effect.

Suppression of phase coarsening in immiscible, co-continuous polymer blends under high temperature quiescent annealing

The properties of polymer blends greatly depend on the morphologies formed during processing, and the thermodynamic non-equilibrium nature of most polymer blends makes it important to maintain the morphology stability to ensure the performance stability of structural materials. Herein, the phase coarsening of co-continuous, immiscible polyamide 6 (PA6)-acrylonitrile-butadiene-styrene (ABS) blends in the melt state was studied and the effect of introduction of nano-silica particles on the stability of the phase morphology was examined. It was found that the PA6-ABS (50/50 w) blend maintained the co-continuous morphology but coarsened severely upon annealing at 230 °C. The coarsening process could be divided into two stages: a fast coarsening process at the initial stage of annealing and a second coarsening process with a relatively slow coarsening rate later. The reduction of the coarsening rate can be explained from the reduction of the global curvature of the interface. With the introduction of nano-silica, the composites also showed two stages of coarsening. However, the coarsening rate was significantly decreased and the phase morphology was stabilized. Rheological measurements indicated that a particle network structure was formed when the concentration of nano-silica particles was beyond 2 wt%. The particle network inhibited the movement of molecular chains and thus suppressed the coarsening process.

Novel photodriven composite phase change materials with bioinspired modification of BN for solar-thermal energy conversion and storage

The development of solar energy conversion and storage materials is critical to narrow the mismatch between the supply and demand of energy and to alleviate the environmental impact related to energy consumption in the coming years. Herein, novel photodriven composite phase change materials (PCMs) based on the bioinspired modification of boron nitride (BN) with superior solar-thermal energy conversion and storage performance are reported. The obtained composite PCMs show excellent performance in UV-vis sunlight harvesting, photothermal conversion, thermal energy storage, electrical insulation, shape-stabilization and high thermal conductivity. The preparation procedure is eco-friendly, easy to handle, and suitable for the practical application of energy materials based on polyethylene glycol (PEG)/polydopamine (PDA)@BN composites with enhanced performance for energy conversion and storage.

An ice-templated assembly strategy to construct graphene oxide/boron nitride hybrid porous scaffolds in phase change materials with enhanced thermal conductivity and shape stability for light–thermal–electric energy conversion

Latent heat energy storage and release media of organic phase change materials (PCMs) are promising to utilize thermal energy coming from solar radiation for effective thermal management. However, the inherently low thermal conductivity and poor photoabsorption of organic PCMs lead to slow thermal charging/discharging rates, hindering the direct thermal energy conversion and storage. Here, we demonstrate that multifunctional PCMs with high thermal conductivity, improved shape-stability and efficient light–thermal–electric energy conversion can be fabricated by introducing polyethylene glycol (PEG) into graphene oxide (GO)/boron nitride (BN) hybrid porous scaffolds (HPSs) constructed via an ice-templated assembly strategy. Owing to the self-assembly of thermally conductive fillers during ice-growth, the obtained PCMs exhibit a high thermal conductivity (as high as 1.84 W m−1 K−1 at 19.2 wt% of BN), which is much higher than that of the composites fabricated by the solution blending method. Furthermore, the obtained composite PCMs with high energy storage density and excellent thermal stability can also be utilized to realize efficient light-to-thermal and light-to-electric energy conversion and storage, providing promising application potential in advanced energy-related devices and systems for solar energy utilization and storage.