Yajiang Yang

High modulus and low-voltage driving nematic liquid-crystalline physical gels for light-scattering displays

Liquid-crystalline (LC) physical gels with a high modulus and low driving voltage were prepared through the self-assembly of sorbitol derivatives as gelators in a nematic liquid crystal, 4-pentyl-4′-cyanobiphenyl (5CB). The structural difference among the used gelators, i.e. 1,3:2,4-di-O-benzylidene-D-sorbitol (DBS), 1,3:2,4-di-O-p-methylbenzylidene-D-sorbitol (MDBS) and 1,3:2,4-di-O-m,p-dimethylbenzylidene-D-sorbitol (DMDBS), is only the number of methyl groups on their phenyl rings. The phase transition temperature, mechanical and electro-optical properties of three LC gels were systematically investigated. Compared with DBS, MDBS and DMDBS with methyl groups on phenyl rings have higher gelation ability in 5CB. The three LC gels exhibit good self-supporting ability with storage moduli higher than 104 Pa when the gelator content is increased to 1.5 wt%. At 3.0 wt% and a gelator content less than 1.0 wt%, both moduli of MDBS and DMDBS gels are obviously higher than that of DBS gel due to the enhanced reinforcement of the more rigid, thicker nano-fibrils and the formed nano-fibrillar network texture in MDBS and DMDBS gels. Also, the driving voltages of LC gels decrease in the order of DBS, MDBS and DMDBS gels with increase of LC domain size and nano-fibril diameter. For DMDBS gel with 3.0 wt% gelators, the threshold voltage and saturation voltage are only 0.5 and 3.5 V μm−1, showing its potential application in self-supporting light-scattering electro-optical displays.

F−- and H+-triggered reversible supramolecular self-assembly/disassembly probed by a fluorescent Ru2+ complex

A new strategy was proposed to monitor transition behavior involved in F−- and H+-triggered reversible supramolecular self-assembly/disassembly by using a Ru2+ complex (Ru(Phen)32+) as a fluorescent probe. N,N-Dibenzoyl-L-cystine (DBC) was used as gelator to form supramolecular gels. Fluorescent images of the fluorescent-labelled DBC gels indicate that Ru(Phen)32+ is dispersed in the gapping place of three-dimensional networks formed by fibrillar DBC aggregates. In a certain range of F− concentrations, it was found that there is a good linear relationship (R2 = 0.999) between F− concentrations and the fluorescent intensity difference between the DBC gels and corresponding solutions. The phase transition temperature of DBC gels is increased with a decrease of F− concentration. Interestingly, the addition of H+ to disassembled systems results in reassembly of DBC. The rate of H+-triggered reassembly is increased with an increase of the H+ concentration. SEM images of reassembled aggregates show no substantial difference in comparison with that of original DBC aggregates, indicating that the F−- and H+-triggered disassembly/reassembly is completely reversible. Time-dependent fluorescent spectra indicate that monitoring self-assembly/disassembly transitions by using the Ru2+ complex as a fluorescent probe is a fast and precise strategy.

Thermodynamically controllable transition from 3D to 2D self-assembly of a hydrogelator induced by the phase behavior of triblock copolymers.

Triblock copolymer PE6200 (PEO(10.5)-PPO(30)-PEO(10.5)) in aqueous solution can undergo a transition from an isotropic micellar phase to an anisotropic lamellar phase at a specific temperature. Based on this feature, the self-assembly behavior of a benzenetetracarboxylic acid based hydrogelator in aqueous solutions of PE6200 has been investigated. The results of small angle X-ray scattering (SAXS) measurements indicated that the hydrogelator self-assembled into stable three-dimensional network structures below 50 °C. In the range of 50-60 °C, a transition from three-dimensional to two-dimensional self-assembly was observed, which can be attributed to the fact that PE6200 undergoes a transition from an isotropic micellar phase to an anisotropic lamellar phase. Differential scanning calorimetry (DSC) and varying temperature laser Raman spectroscopy further confirmed the thermodynamically controllable transition of 3D to 2D self-assembly. Controlling the self-assembly by utilizing the phase behavior of triblock copolymers is a novel strategy.