Yeping Wu

Solvent effects on structure, photoresponse and speed of gelation of a dicholesterol-linked azobenzene organogel

By introducing methanol as a co-solvent into cyclopentanone, solvent effects on structure, photoresponse and speed of gelation of a dicholesterol-linked azobenzene organogel DCAZO2 have been investigated. Optical microscopy (OM) and scanning electron microscopy (SEM) images reveal that the gel fibers form macroscopic aggregates by adding methanol. Minimum gelation concentration (MGC) of the gels decreases with increasing methanol content, while the gel-to-sol transition temperature (Tgel) decreases firstly and increases afterwards. For photoisomerization (corresponding to gel-sol transition), the first stage rate constant kg1 of the gels decreases with increasing methanol content. The photoisomerization process of the gel with 5% methanol is different from the other five samples, whose first stage rate constant kg1 is greater than its second stage rate constant kg2. By increasing methanol content from 0% to 30%, gelation time (corresponding to sol–gel transition) of both heated gel and UV-visible light irradiated gel decreases from 7 h to 5 min and 6 h to 0 min (before visible light irradiation is accomplished), respectively. Solubility parameters and Teas plots of the gelator and mixed solvents are systematically calculated to estimate the gelator-solvent interaction. It is found that gelation requires that the gelator-solvent interaction be neither too strong nor too weak, otherwise solution or precipitate are formed.

Photoinduced reversible gel–sol transitions of dicholesterol-linked azobenzene derivatives through breaking and reforming of van der Waals interactions

A series of new symmetric dicholesterol-linked azobenzene gelators with different spacer lengths have been synthesized. The compounds with spacers of zero, two or six methylene units are denoted as DCAZO0, DCAZO2 and DCAZO6, respectively. A gelation test reveals that a subtle change in the length of the spacer can produce a dramatic change in the gelation behavior of the compounds. DCAZO2 obtains the minimum gelation concentration among the three gelators. For cyclopentanone gel of DCAZO2, the reversible gel–sol transitions by irradiation with UV and visible light are investigated by UV-vis absorption and circular dichroism (CD) spectra, SEM, TEM and XRD analyses. Upon UV irradiation of the gel, trans–cisphotoisomerization of the azobenzene groups occurs, the change in molecular polarity leads to the breaking of van der Waals interactions, resulting in the gel–sol transition. The gel can be recovered by the reverse cis–transphotoisomerization after the exposure to visible light. SEM, TEM and XRD studies reveal that the gelator molecules self-assemble into one-dimensional fibers with diameters 50–100 nm in an anticlockwise direction, which further crossed-linked to form three-dimensional networks.

Block copolymers of PS-b-PEO co-assembled with azobenzene-containing homopolymers and their photoresponsive properties

A new concept of the efficient fabrication and design of three layer photoresponsive core–shell–corona (CSC) nanostructures by the co-assembly of diblock copolymers with azobenzene-containing homopolymers is presented. Depending on the azobenzene-containing homopolymer content, the formation of CSC vesicles and CSC micelle-like nanostructures are observed, which are characterized by UV–vis absorption spectroscopy, transmission electron microscopy (TEM), atomic force microscopy (AFM) and dynamic light scattering (DLS). The observed nanostructures confirm the recent computer simulations of Xu et al. (J. Phys. Chem. B, 2010, 114, 1257). All co-assemblies are photoresponsive and show reversible cis–transphotoisomerization. Irradiation of the spherical CSC micelle-like nanostructures by polarized light induces their deformation to ellipsoidal-core-within-ellipsoidal-shell and ellipsoidal-core-within-spherical-shell nanostructures, respectively. The type of photoinduced deformation depends on the initial size of the spherical CSC micelle-like nanostructures. These CSC nanostructures could be used as photo-controlled templates for complex nanostructures.

Hyperbranched polyurethane as a highly efficient toughener in epoxy thermosets with reaction-induced microphase separation

Hyperbranched polyurethane (HBPU), with flexible poly(tetrahydrofuran) segments between the branching points, was designed and prepared by a simple one-pot method. HBPU and the linear analog polyurethane (LPU) were used as tougheners in the diglycidyl ether of bisphenol A (DGEBA)/amine system, respectively. The HBPU was miscible with DGEBA resin without affecting the favorable processability of the epoxy blends. It was proved that a good phase separation structure with strong interface interaction was formed in the DGEBA/HBPU thermosets and the HBPU was more effective than LPU in toughening the epoxy resin. After adding 10 wt% HBPU, the impact strength of samples was 3 times as large as the neat epoxy. Meanwhile, the flexural strength and glass storage modulus of the thermosets were hardly affected with the addition of HBPU while obvious decreases were observed in the DGEBA/LPU thermosets. Especially, unlike most of the hyperbranched polymer tougheners, the introduction of HBPU also simultaneously improved the rubbery plateau modulus, glass-transition temperature and thermal stability of the epoxy thermosets.

Photoinduced liquid crystal blue phase by bent-shaped cis isomer of the azobenzene doped in chiral nematic liquid crystal

A photoresponsive azobenzene molecule DCAZO2 with two cholesteryl groups linked to both sides of the azobenzene group is doped in a mixture of nematic liquid crystal E7 and chiral dopant S811 (61.9 wt% E7, 36.1 wt% S811 and 2.0 wt% DCAZO2). Cooled from isotropic phase to 33.0°C, chiral nematic liquid crystal (N*LC) was formed in the sample and then the temperature was kept unchanged at 33.0°C. UV light irradiation induces the trans–cis photoisomerisation and thus an obvious phase transition. When the azobenzene groups isomerise to a cis-saturated state, the UV light was turned off and the white light was turned on at the same time. The bent-shaped cis isomer then turns back to the planar trans isomer gradually. A blue–green platelet texture representing cubic blue phase (BP) was observed and the size of the platelets was increased along with the cis–trans isomerisation. UV–vis absorption spectra indicate that the photoinduced BP exists when the isomerisation degree is between 79% and 18%, and further cis–trans isomerisation change BP back into N*LC. The large geometric structure of the cholesteryl groups and the large bent angle θ of the cis isomer are supposed to be responsible for the interesting result.

A conductive ternary network of a highly stretchable AgNWs/AgNPs conductor based on a polydopamine-modified polyurethane sponge

A highly stretchable silver nanowires/silver nanoparticles (AgNWs/AgNPs) conductor based on a polydopamine (PDA) modified three dimensional polyurethane (PU) sponge was fabricated. The PU sponge skeleton was first modified by PDA to improve adhesion to the conductive nanomaterials. The conductive micro-network was fabricated by dip-coating of the PDA modified PU sponge into the solution of AgNWs, and AgNPs were further sputtered onto its surface to improve conductivity, resulting in a conductive ternary network structure. The resistance of the PDA–PU–AgNW–Ag–PDMS stretchable conductor decreased obviously with bending or stretching cycles initially, and then remained stable even up to 1000 cycles. The composites exhibit high electrical conductivity (exceeding 27.78 S cm−1) and excellent electromechanical stability under high tensile strain (50%) owing to the combined effects from the ternary network. The significant electromechanical properties were also demonstrated by a simple stretchable circuit composed of a light-emitting diode (LED) and the stretchable conductor during bending, twisting or stretching.

Three-dimensional highly conductive silver nanowires sponges based on cotton-templated porous structures for stretchable conductors

A stretchable cotton–Ag nanowire–poly(dimethylsiloxane) (cotton–AgNWs–PDMS) conductor was fabricated by embedding the unique binary conductive network structure of cotton–AgNWs in PDMS through a simple dip-coating method. The binary conductive network structure was constructed based on cotton which had an interconnected and junction-free macroporous structure as the skeleton to support the two dimensional AgNWs network, leading to excellent electrical and mechanical properties. The conductivity of the stretchable conductor can be adjusted easily by varying the dip-coating cycles, which can reach a highest electrical conductivity of 56.82 S cm−1. The cotton–AgNWs–PDMS composite can retain excellent conductance under large tensile strains (120%). Moreover, the excellent conductance is maintained up to 1000 stretching or bending cycles. Because of the high conductivity and excellent stretchability, together with the facile fabrication process, the cotton–AgNWs–PDMS stretchable conductor might be widely used as interconnects and electrodes for stretchable intelligent and functional devices.

Luminescence of Eu(DBM)3Phen-doped in azobenzene-containing copolymers--effects of absorption overlapping of two components.

Poly(N-isopropylacrylamide)-b-poly[6-[4-(4-methylphenyl-azo)phenoxy]hexylmethacrylate] (PNIPAM(32)-b-PAzoMM(8)) and poly(N-isopropylacrylamide)-b-poly[6-[methyl(4-nitrophenyldiazenyl)phenyl]aminohexylmethacrylate] (PNIPAM(32)-b-PAzoNO(10)) were prepared by reversible addition-fragmentation chain transfer (RAFT) radical polymerization. The optical properties of Eu(DBM)(3)Phen (Eu, Europium; DBM, dibenzoylmethide; Phen, 1,10-phenanthroline)-doped azobenzene-containing copolymer vesicle solutions were studied by UV-vis and fluorescence spectroscopy. When the electronic transition bands of azobenzene and Eu(DBM)(3)Phen overlapped at about 350 nm in PNIPAM(32)-b-PAzoMM(8), the fluorescent emission intensity at 612 nm of Eu(DBM)(3)Phen could be modulated by irradiation with UV and visible light. However, when the absorption of the azobenzene-moiety red-shifted to 477 nm in PNIPAM(32)-b-PAzoNO(10), the luminescence intensity of Eu(DBM)(3)Phen was not affected any longer. The difference between these two systems was possibly caused by the energy allocation in the luminescence process, which was discussed in detail.

Effect of functionalization of multi-walled carbon nanotube on mechanical and viscoelastic properties of polysulfide-modified epoxy nanocomposites

In order to obtain epoxy compounds with excellent mechanical properties without compromising other desired properties, pristine- or carboxyl-functionalized multi-walled carbon nanotube (p-MWCNT or f-MWCNT) along with polysulfide were incorporated into an amine-cured epoxy resin. Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy analyses proved the existence of carboxyl groups on the surface of f-MWCNT. Adding 0.1 wt% f-MWCNT significantly improved the tensile strength and impact strength of the epoxy nanocomposites by 104% and 47%, respectively. However, adding p-MWCNT had little effect on the mechanical properties of the nanocomposites. The glass transition temperature of the f-MWCNT/epoxy nanocomposites were also much higher than neat epoxy matrix and p-MWCNT/epoxy nanocomposites. The fracture surface morphology and dynamic mechanical analysis results indicated that the interfacial interactions between f-MWCNT and the epoxy matrix were much stronger than that of p-MWCNT, which ensured the much-improved mechanical properties.

Coherent random lasing from nano-scale aggregates of hybrid molecules by enhanced near zone scattering

Nano-scale aggregates of hybrid molecules, such as N,N′-di-[3-(isobutyl polyhedral oligomeric silsesquioxanes)propyl] perylene diimide (DPP) which is composed of perylene diimide (PDI) as gain group and polyhedral oligomeric silsesquioxanes (POSS) as scattering group at a mole ratio of 1:2, are formed in carbon disulfide (CS2) solution within a concentration range from 3 × 10−5 M to 10−3 M. The aggregation of DPP in the solution was characterized using ultraviolet visible (UV-vis) and fluorescence spectroscopy. Under pumping of a 532 nm pulse laser, coherent random laser (RL) is produced from the aggregates in CS2 solution with a threshold of 17.4 μJ and a Q value of 2440 at a DPP concentration of 10−4 M. Enhanced near zone scattering by aggregation is thought to boost coherent RL. This research not only extends the RL system to the extremely weak scattering regime but also provides a new direction for research on nano-scale materials for lasers using molecular design.