Research

1. high strength disclination

When liquid crystal was observed under polarizing optical microscope, different textures could be obtained corresponding to their phase structure. Schlieren texture in which a core bears several dark brushes is one of the typical textures of the liquid crystals which represents the defect of disclination. Because a greater brush-number implies a greater curvature of director n and, as a result, a greater elastic energy F. The more the dark brushes emanates from a core, the more times the LC director revolution occurs around the core, therefore, the higher elastic distortional energy the core bears. Normally, only two or four extinguished brushes emanating from a core are observed. Stable singularity point with high number of brushes is rarely observed. As shown bellow is the formation process of such a high strength disclination point with the decreasing of temperature.

2. Ionic liquid crystals

The introduction of ionic moiety into liquid crystal phase could lead to some special properties, such as ion conductivity in mesophases, self assembled wires and channels. Furthermore, ionic liquid crystals with relatively low melting points could be used as ordered solvents for polymerizations and stereochemically controlled organic reactions.

The methylimidazolium and pyridinium ions are often used for the development of ionic liquid crystals as well as for ionic liquids.

Rod-like pyridinium ionic liquid crystals containing a biphenyl core were prepared, their liquid crystal properties are influenced by both the spacer length and the substitute pattern of the pyridinium group.

Discotic liquid crystals containing ionic moiety are rarely reported. In order to understand the influence of the introduction of ion on  the phase structure of discotic liquid crystal, a N-methylimidazolium ion was Asymmetrically Incorporated into a triphenylene disk, resulted in an ordered columnar lamellae phase.

3. Liquid crystal alignment

Uniform alignment of liquid crystals on treated substrate surfaces is very important for both LCD application and fundamental research of LC theory. Alignment layer is a thin film coated in the inner surface of liquid crystal cell to control the orientation or alignment of Liquid crystal. In my PhD study at the Institute of Chemistry, Chinese Academy of Science, I finished the dissertation of "study on the liquid crystal alignment layers based on ladder-like polysilsesquioxanes". Here is the abstract of my dissertation. As shown on the right side is one of our papers was selected as an cover article of Advanced Materials.

4 Liquid crystal self-assembled by hydrogen-bond

Liquid crystal molecule often has a special shape, like rod, disk, and banana shaped, which often be prepared by tedious steps of reaction. Some molecules don't have liquid crystalline properties themselves, and the other kind molecules too. but when they mixed together, liquid crystals could be obtained. These liquid crystals are formed by self-assembly, such as hydrogen bonding. Those two different kinds of molecules are the H-bond donors and acceptors. The pair of a carboxylic acid with a pyridyl group is the favorite choice in many studies.

We demonstrated an excellent example: a polymer containing azopyridine segments was prepared, when complexed with different carboxylic acids, photo-active liquid crystals could be easily obtained by tuning the commercially available acids as H-bond donor. As shown, even a simple low boiling point acetic acid, when mixed with the polymer, a smectic phase was observed.

5. Atom transfer radical polymerization

Atom transfer radical polymerization is a controlled/"living" polymerization, can be used for a variety of monomers and less sensitive to moisture. It is a convenient way to prepare block copolymer with well defined structures.

The control of the polymerization afforded by ATRP is a result of the formation of radicals that can grow, but are reversibly deactivated to form dormant species. Reactivation of the dormant species allows for the polymer chains to grow again, only to be deactivated later. Such a process results in a polymer chain that slowly, but steadily, grows and has a well-defined end group.

I prepared several new homepolymers/block copolymers by ATRP with different catalyst system:

6. Nanostructure of block copolymer

Block copolymers are those polymers that two or more different polymer chains are joined together by covalent bond. As for diblock copolymer, two different polymer chains are connected together. As a result of the connectivity and incompatibility between the chemically different chains, diblock copolymers can self-assemble into nanometer-sized structures. The resulting morphologies such as spheres, cylinders, lamellae varies according to the composition of the copolymer.

We seek to achieve fundamental understanding of novel nanostructure and morphology in supramolecular liquid crystals with immiscible polymer chains. As shown bellow is an example of the morphology of  a disk containing triblock copolymer: polyethylene-block-poly(ethylene oxide)-block-pentakis(pentyloxy)-triphenylene, a three layer lamellae structure (left) was confirmed by TEM (right).

 

7. Photo-active polymer

Azobenzene-containing polymers are potentially useful materials for optical and photonic applications. As shown bellow is the polarizing optical micrographs showing a grating recorded at room temperature via a photomask (40 µm fringes) on a film of azobenzene containing triblock copolymer elastomer (structure as above, the triblock copolymer 5) stretched to 100% deformation.