Guo-ang Qi

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.

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.

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.

Hierarchical graphene foam-based phase change materials with enhanced thermal conductivity and shape stability for efficient solar-to-thermal energy conversion and storage

Recently, graphene foam (GF) with a three-dimensional (3D) interconnected network produced by template-directed chemical vapor deposition (CVD) has been used to prepare composite phase-change materials (PCMs) with enhanced thermal conductivity. However, the pore size of GF is as large as hundreds of micrometers, resulting in a remarkable thermal resistance for heat transfer from the PCM inside the large pores to the GF strut walls. In this study, a novel 3D hierarchical GF (HGF) is obtained by filling the pores of GF with hollow graphene networks. The HGF is then used to prepare a paraffin wax (PW)-based composite PCM. The thermal conductivity of the PW/HGF composite PCM is 87% and 744% higher than that of the PW/GF composite PCM and pure PW, respectively. The PW/HGF composite PCM also exhibits better shape stability than the PW/GF composite PCM, negligible change in the phase-change temperature, a high thermal energy storage density that is 95% of pure PW, good thermal reliability, and chemical stability with cycling for 100 times. More importantly, PW/HGF composite PCM allows light-driven thermal energy storage with a high light-to-thermal energy conversion and storage efficiency, indicating its great potential for applications in solar-energy utilization and storage.

An extremely uniform dispersion of MWCNTs in olefin block copolymers significantly enhances electrical and mechanical performances

Ethylene-α-octene copolymers with different chain architectures (ethylene-α-octene random copolymer (ORC) and ethylene-α-octene block copolymer (OBC)) were adopted to prepare elastomeric composites with multi-walled carbon nanotubes (MWCNTs) through melt mixing. Extremely uniform dispersions of MWCNTs in OBC and serious aggregations of MWCNTs in ORC were observed. The percolation threshold of OBC composites was 2.5 vol%, much lower than that of ORC composites. Rheological measurements and thermodynamic analysis revealed that the uniform dispersion of MWCNTs in OBC is due to the selective localization of the nanotubes in the molten state, and the stronger volume exclusion effect of OBC crystals in nanoscopic dimensions towards nanotubes also accounts for the developed MWCNT network after crystallization. Although OBC itself showed lower stress at a certain stretching and tensile modulus than that of ORC, the percentages of reinforcement in stress at a certain stretching and tensile modulus of OBC composites were almost twice as much as that of ORC composites at the same content of MWCNTs. Elongations of OBC composites were all higher than 1600% (even when the MWCNT content was as high as 7.84 vol%) and were always at least 200% larger than that of ORC composites at the same content of MWCNTs. The results provide significant guidance for the preparation of conductive elastomeric materials with both excellent electrical performance and mechanical properties by utilizing thermoplastic polyolefin copolymers with blocky chain architectures.

Effect of graphite oxide structure on the formation of stable self-assembled conductive reduced graphite oxide hydrogel

As a novel tissue engineering material and transistor, reduced graphite oxide (rGO) hydrogel is attracting more and more attention, and a stable and highly electrical conductive rGO hydrogel is the cornerstone for these applications. We controlled the structures of graphite oxides (GOs) with three different methods and the corresponding assembled rGO hydrogels were obtained using Vitamin C (VC) as the reducing agent and the stability and electrical resistance of the rGO hydrogel were studied. The results showed that the appropriate interlayer distance and grain size of GO prepared by two-step oxidation were beneficial for VC molecules to insert in the interspace between layers for the reduction. After reduction, the loose and tangled network structure was well assembled, which determines the stability and electrical conductivity of the resulted rGO hydrogel.

Temperature: a nonnegligible factor for the formation of a structurally stable, self-assembled reduced graphite oxide hydrogel

The three-dimensional (3D) architecture of reduced graphite oxide (rGO) hydrogels is of interest in applications such as supercapacitors, soft machines and regenerative medicine, etc. The structural stability of the rGO hydrogel is the foundation for these applications. However, little attention has been paid to this issue. Here, the structural and performance stabilities of rGO hydrogels prepared at different temperatures were investigated in detail. It was found that 40 °C was the most effective condition for the reduction of graphite oxide, as the reducibility of vitamin C was embodied successfully and the network of the rGO hydrogel was formed. The rGO hydrogel prepared at 40 °C showed the best structural stability with time, the lowest electrical resistance and the highest mechanical strength. These results provide guidance for the synthesis of structurally stable rGO hydrogels and their further applications in electrical devices.

Solvent-controlled formation of a reduced graphite oxide gel via hydrogen bonding

As a promising material with broad applications, reduced graphite oxide (rGO) hydrogels have attracted more and more great attention recently. However, most reports on rGO hydrogels focused on their applications, while the formation mechanism has not been paid enough attention. For the first time, we demonstrated the higher the ability of the solvents to form hydrogen bonds with the rGO sheets, the better the structural stability and properties of gel are. This study indicates that hydrogen bonding between solvent molecules and the oxygen-containing functional groups on rGO sheets is vital to achieve high-performance gels.

Injection Molding Shrinkage and Mechanical Properties of Polypropylene Blends

Polypropylene (PP) blends based on isotactic polypropylene (iPP), propylene-ethylene block copolymer (bPP), and propylene–ethylene random copolymer (rPP) were prepared by melt blending and the effects of content of bPP and rPP on the shrinkage during solidification and storage and mechanical properties of the blends were studied. It was found that the addition of polypropylene copolymer could effectively reduce the processing shrinkage of iPP and the lowest shrinkage of the blends was achieved at a loading of 2 wt% bPP or rPP. The flexural modulus and tensile strength of the blends decreased a little while the impact strength and elongation at break were improved greatly compared with those of iPP.