Yunsheng Ye

Biocompatible reduced graphene oxide sheets with superior water dispersibility stabilized by cellulose nanocrystals and their polyethylene oxide composites

Organic molecular and polymeric stabilizers are useful for preparing individually dispersed graphene sheets, thus offering new possibilities for the production of nanomaterials. Although exfoliated graphene flakes with good dispersibility can be produced, their use in polymer composites remains limited due to their low stability and mechanical strength. In this work, stable high concentration aqueous dispersions (>10 mg mL−1) of reduced graphene oxide (RGO) sheets were prepared by exfoliation/in situ reduction of graphene oxide (GO) in the presence of cellulose nanocrystals (CNC). The sandwich-like structure formed with the hydrophilic outer surface of CNC forms CNC decorated RGO (CNC–RGO) which is easily dispersed in water with a high thermal stability (>320 °C) comparable to pristine CNC and other common stabilizers. Polyethylene oxide (PEO) based nanocomposites, using fully exfoliated CNC–RGO hybrids, were prepared with a simple procedure. The PEO/CNC–RGO composite films show superior mechanical properties compared to PEO composite films enhanced by other small molecules, polymer dispersants, stabilized RGO or pristine CNC. Not only are the elastic modulus and tensile strength of the composites significantly improved, but their thermal stability is also retained. The hydrothermal dehydration of GO to RGO, using biodegradable and renewable materials such as CNC, offers a “green approach” to large-scale preparation of highly biocompatible and easily dispersed RGO for a range of applications.

High performance composite polymer electrolytes using polymeric ionic liquid-functionalized graphene molecular brushes

A new structural design and tailored morphology of polymer-functionalized graphene (polymer-FG) are employed to optimize composite polymer electrolytes (CPEs). The ionic transfer conditions including Li salt dissociation, amorphous content and segmental mobility are significantly improved by incorporating polymer-FG, especially that having a polymeric ionic liquid (PIL) and a polymer brush structure [PIL(TFSI)-FGbrush]. Electrical shorts are eliminated due to the presence of the functionalized polymer on reduced graphene oxide (RGO) and a minimal amount of polymer-FG in the PEO/Li+ polymer electrolytes (PEs). Polymer-FG with PIL brushes increases significantly the Li ion conductivity of PEO/Li+ PE by >2 orders of magnitude and ∼20-fold at 30 °C and 60 °C with high Li salt loading (O/Li = 8/1), respectively. Furthermore, significant improvements in mechanical properties are observed where only 0.6 wt% addition of the PIL(TFSI)-FGbrush led to more than 300% increase in the tensile strength of the PEO/Li+ at an O/Li ratio of 16/1. Li-ion battery performance was evaluated with the CPE containing 0.6 wt% of PIL(TFSI)-FGbrush, resulting in superior capacity and cycle performance compared to those of the PEO/Li+ PE. Thus, we believe, embedding minimal amounts of structurally and morphologically optimized polymer-FG nano-fillers can lead to the development of a new class of SPEs with high ionic conductivity for high performance all-solid-state Li-ion batteries.

Simultaneous improvement in the flame resistance and thermal conductivity of epoxy/Al2O3 composites by incorporating polymeric flame retardant-functionalized graphene

Fire hazards related to polymer-based thermally conductive composites (PTCs) used in electronic equipment are a significant, but often neglected, risk. Here, we offer a solution by incorporating flame retardant-functionalized graphene (PFR-fRGO) into PTCs using a procedure that improves both their flame resistance and thermal conductivity. Briefly, PFR-fRGO was prepared by covalently grafting a polyphosphoramide oligomer (PDMPD) onto the surface of graphene, which was then introduced in situ into epoxy resin/Al2O3 (EP/Al2O3) composites. As expected, the incorporation of PFR-fRGO not only increased the thermal conduction paths by weakening the settlement of microparticles, but also reduced the interfacial thermal resistance by enhancing interfacial interactions, both of which resulted in an enhancement of the thermal conductivity of the ternary composites. The resultant EP/Al2O3/PFR-fRGO composite exhibited a superior flame retarding ability with dramatic decreases being seen in the high peak heat release rate (PHRR), the total heat release (THR) and the total smoke production (TSP), i.e. 53%, 37% and 57%, respectively, when compared to pure epoxy resin. Additionally, a synergistic flame retarding effect was found in the ternary composite compared to the EP/PFR-fRGO and EP/Al2O3 composites. The remarkable enhancement in flame retardancy was mainly attributed to the catalytic charring effect of PFR-fRGO and the template effect of Al2O3, both of which resulted in the formation of a high strength, thermally stable protective layer in the condensed phase that is able to retard the permeation of heat and volatile degradation products during combustion, slow down the heat release rate and protect the underlying polymer.

Synthesis and characterization of thermally cured polytriazole polymers incorporating main or side chain benzoxazine crosslinking moieties

Crosslinking is an efficient and simple approach for enhancing the thermal and mechanical properties of polymers. Numerous studies have reported such enhancements by the incorporation of benzoxazine (a cross-linker) in the polymers structures. The great majority of these studies have focused on the effect of the benzoxazine content on the polymer matrix. As far as we know, there has been no discussion related to the effects arising from the position of benzoxazine incorporation. In order to investigate any such effects, we synthesized new benzoxazine monomers (SBz and MBz), containing bis-propargyl functional groups and new main chain and side chain benzoxazine functionalized polytriazole polymers, with the above benzoxazine moieties in the repeat unit, using click chemistry. The resulting thermal and mechanical properties of Cured-PTA-SBz-10 were better than those of Cured-PTA-MBz-10, and the Cured-PTA-SBz-4 and Cured-PTA-SBz-6 were close or even better than those of Cured-PTA-MBz-10. To better understand any thermal curing effects related to the positions of benzoxazine moieties in the polymer chain, we performed dynamic differential scanning calorimetric measurements by Kissinger and Ozawa methods. Significant enhancement of the thermal and mechanical properties comparing neat PTA with Cured-PTA-SBz-10 were noted: e.g. (i) a ∼110 °C improvement of Tg; (ii) a ∼205% improvement in storage modulus, a ∼232% improvement of tensile strength, and a ∼262% improvement in Youngs modulus. Therefore, when designing the polymer, by giving consideration to the position of the cross-linker, the resulting thermal and mechanical properties can be enhanced to the extent that an equivalent polymer can be formed with a reduced amount of cross-linker leading to cost reduction.

Well-structured holographic polymer dispersed liquid crystals by employing acrylamide and doping ZnS nanoparticles

High diffraction efficiency and low driving voltage are typically considered to be prerequisites for the practical applications of holographic polymer dispersed liquid crystals (HPDLCs), which are especially critical for their use in the state-of-the-art low-threshold mirrorless tunable lasers. Nevertheless, high driving voltages are usually resulted for HPDLCs upon increasing the holographic diffraction efficiency via optimizing the monomer/LC formulations. Herein, we present that doping nanoparticles into HPDLCs with controlled distribution is a facile and efficient approach to circumvent the aformentioned issues. Zinc sulfide (ZnS) nanoparticle doped HPDLCs with high diffraction efficiency (94.0 ± 2.1%), and a low threshold driving voltage of 2.5 V µm−1 that is decreased from 11.6 V µm−1 for the pristine form, are achieved by doping 8 wt% ZnS nanoparticles into the HPDLCs based on an acrylamide monomer, N,N-dimethylacrylamide, that contributes significantly to the high diffraction efficiency up to 98.2 ± 1.4%.

A flexible, self-healing and highly stretchable polymer electrolyte via quadruple hydrogen bonding for lithium-ion batteries

Polymer electrolytes are envisioned to be promising alternatives to their liquid counterparts in lithium-ion batteries as they are able to address electrolyte inflammability and leakage issues. However, polymer electrolytes usually suffer from cracks or breakage, which could lead to short circuits and bring severe safety issues. Herein, we propose a paradigm to address these problems in which the polymer electrolyte is formed by a physically cross-linked network via ureidopyrimidinone (UPy) containing brush-like poly(ethylene glycol) chains. The formed novel polymer electrolyte is flexible and able to provide fast intrinsic self-healability and high stretchability. It can heal the cut damage within 2 hours under ambient conditions without any external stimulus and can be stretched to more than 20 times its length without breaking. Furthermore, the novel polymer electrolyte exhibits excellent interfacial stability with electrodes, resulting in high performance batteries with stable reversible electrochemical reaction and cycling.

A One-Step Route to CO2-Based Block Copolymers by Simultaneous ROCOP of CO2/Epoxides and RAFT Polymerization of Vinyl Monomers

The one-step synthesis of well-defined CO2 -based diblock copolymers was achieved by simultaneous ring-opening copolymerization (ROCOP) of CO2 /epoxides and RAFT polymerization of vinyl monomers using a trithiocarbonate compound bearing a carboxylic group (TTC-COOH) as the bifunctional chain transfer agent (CTA). The double chain-transfer effect allows for independent and precise control over the molecular weight of the two blocks and ensures narrow polydispersities of the resultant block copolymers (1.09-1.14). Notably, an unusual axial group exchange reaction between the aluminum porphyrin catalyst and TTC-COOH impedes the formation of homopolycarbonates. By taking advantage of the RAFT technique, it is able to meet the stringent demand for functionality control to well expand the application scopes of CO2 -based polycarbonates.

Ultralow-Carbon Nanotube-Toughened Epoxy: The Critical Role of a Double-Layer Interface

Understanding the chemistry and structure of interfaces within epoxy resins is important for studying the mechanical properties of nanofiller-filled nanocomposites as well as for developing high-performance polymer nanocomposites. Despite the intensive efforts to construct nanofiller/matrix interfaces, few studies have demonstrated an enhanced stress-transferring efficiency while avoiding unfavorable deformation due to undesirable interface fractures. Here, we report an optimized method to prepare epoxy-based nanocomposites whose interfaces are chemically modulated by poly(glycidyl methacrylate)-block-poly(hexyl methacrylate) (PGMA-b-PHMA)-functionalized multiwalled carbon nanotubes (bc@fMWNTs) and also offer a fundamental explanation of crack growth behavior and the toughening mechanism of the resulting nanocomposites. The presence of block copolymers on the surface of the MWNT results in a promising double-layered interface, in which (1) the outer-layered PGMA segment provides good dispersion in and strong interface bonding with the epoxy matrix, which enhances load transfer efficiency and debonding stress, and (2) the interlayered rubbery PHMA segment around the MWNT provides the maximum removable space for nanotubes as well as triggering cavitation while promoting local plastic matrix deformation, for example, shear banding to dissipate fracture energy. An outstanding toughening effect is achieved with only a 0.05 wt % carbon nanotube loading with the bc@fMWNT, that is, needing only a 20-times lower loading to obtain improvements in fracture toughness comparable to epoxy-based nanocomposites. The enhancements of their corresponding ultimate mode-I fracture toughnesses and fracture energies are 4 times higher than those of pristine MWNT-filled epoxy. These results demonstrate that a MWNT/epoxy interface could be optimized by changing the component structure of grafted modifiers, thereby facilitating the transfer of both mechanical load and energy dissipation across the nanofiller/matrix interface. This work provides a new route for the rational design and development of polymer nanocomposites with exceptional mechanical performance.

A Scalable Approach to Construct Self-assembled Graphene-based Films with Ordered Structure for Thermal Management

Large-area bulk oxidized cellulose nanocrystal (OCNC)/graphene nanocomposites with highly oriented structures were produced through a straightforward, cost-effective large-scale evaporation-induced self-assembly process followed by thermal curing. Well-aligned nano-sized graphene layers were evident and separated by the OCNC planar layers, which facilitate highly interconnected and continuous thermal transport parallel to the alignment. Hence, the laminated graphene-based nanocomposites possess an excellent in-plane thermal conductivity of 25.66 W/m K and a thermal conductivity enhancement (η) of 7235% with only a 4.1 vol % graphene loading. This value is the highest recorded among all laminated composite films with <70 wt % filler content reported to date. Using this design strategy, other large-area aligned composites with other functional nanomaterials, already in large-scale production, can be made for use in a wide range of applications.

Low-voltage-driven and highly-diffractive holographic polymer dispersed liquid crystals with spherical morphology

It is a constant pursuit to form highly-diffractive and low-voltage-driven holographic polymer dispersed liquid crystals (HPDLCs) for meeting the requirements of practical applications. Nevertheless, the high-voltage-driven characteristic is usually given while improving the diffraction efficiency of HPDLCs, and it remains a challenge to form HPDLCs with concurrent features of high diffraction and low driving voltage via a simple method. In this work, we synthesize a non-room-temperature LC, 4-butyloxy-4′-cyanobiphenyl (4OCB), and mix it with a room-temperature nematic LC mixture named P0616A. These new LC mixtures are then homogeneously mixed with monomers and a photoinitibitor composed of 3,3′-carbonylbis(7-diethylaminocoumarin) (KCD) and N-phenylglycine (NPG), followed by patterning via laser interference, generating well-structured HPDLCs. The introduction of 4OCB into the standard formulation is found to be able to optimize the morphology and electro-optical properties of the resulting HPDLC transmission gratings. By doping 5 wt% of 4OCB into the HPDLCs, a high diffraction efficiency of 92 ± 3% is obtained; meanwhile, the threshold and saturated voltages significantly decrease by 80.8% (i.e., from 12.0 ± 0.8 to 2.3 ± 0.9 V μm−1) and 73.2% (i.e., from 19.0 ± 0.6 to 5.1 ± 0.7 V μm−1), respectively, in comparison with the pristine. The enhanced performance is believed to be ascribed to the formed larger LC droplets (70 ± 20 nm) and lower interface anchoring strength (0.7 μN m−1) of the polymer network on LCs.