Research Projects

1. PIL nanolatex
2. PIL block copolymers by controlled /”living” radical polymerization
3. PIL as carbon precursors
4. Nitrogen-doped carbon fibers and membranes derived from electrospun PILs
5. Diversified applications of chemically modified 1,2-polybutadiene
6. PIL copolymer as smart stabilizer for carbon nanotubes
7. Inner structure and self-assembly of PIL nanoparticles
8. Spherical PIL brushes (in preparation)

1. PIL nanolatex

A facile preparation of vinylimidazolium‐typed poly(ionic liquid) (PIL) latexes and nanoparticles was performed via dispersion polymerization of ionic liquid monomers in aqueous solutions at 70°C without addition of further stabilizers. A homologous series of vinylimidazolium‐typed ionic liquid monomers with different alkyl tail length (C8−C18) were synthesized via quaternization of 1‐vinylimidazole with corresponding n‐alkyl bromides. For ionic liquid monomers with sufficiently long alkyl chains (≥C12), PILs nanoparticles of 20−40 nm in diameter were found, which were self‐stabilizing in aqueous media. In the same procedure, preparation of cross‐linked PIL nanoparticles was performed in the presence of 10 mol % of divinylimidazolium‐based crosslinker. The as‐synthesized cross‐ linked PIL nanoparticles could be transferred into organic solvents, such as polar DMF, and nonpolar toluene after anion exchange with lithium bis(trifluoromethylsulfonyl)imide. This dispersion polymerization requires no dispersing agent and potentially enables a large scale synthesis of PIL nanoparticles in both aqueous and organic solutions.

2. PIL via controlled /”living” radical polymerization

Controlled synthesis of double hydrophilic block copolymers comprising a hydrophilic poly(ionic liquid) (PIL) segment was achieved via the RAFT/MADIX process. The non‐ionic segment is made up from either poly(N‐ isopropylacrylamide) (PNIPAAm) or poly(N,N‐dimethylacrylamide) (PDMA), due to their favorable controllable solubility in water at room temperature. They were employed as macro‐chain transfer agents (macro‐CTAs) for the RAFT polymerization of four different 1‐vinylimidazolium ionic liquid monomers possessing different alkyl substitutes and anions. The block copolymers of PNIPAAm‐b‐PIL are dual stimuli‐responsive copolymers that can respond to the changes in temperature and ionic strength in aqueous solution.

3. PIL as carbon precursors

A template‐free preparation of mesoporous graphitic carbon nanostructures with high electric conductivity is presented using ionic liquid monomers or poly(ionic liquid) polymers as carbon precursors. The carbonization was performed in the presence of FeCl₂ at temperatures between 900 and 1000 °C. It was found that FeCl₂ plays a key role in controlling both the chemical structure and the texture morphology of the graphitization process. A detailed investigation on the carbonization process demonstrated that 900 °C is a threshold temperature where a synergistic formation process enables the development of the superior physical properties, such as large surface area and low resistance. The as‐synthesized carbon products are graphitic, mesoporous, and highly conductive, as proven by XRD and TEM characterizations and conductivity measurements. Via an acid etching process, iron and iron carbide nanoparticles, the remainder of the primary catalyst, can be removed, leaving pure mesoporous carbon nanomaterials with a comparably well developed graphitic structure. Without demand for any template, this method is facile and easy to scale up and might contribute to the wide range of applications of carbon nanostructures.

4. Nitrogen-doped carbon fibers and membranes derived from electrospun PILs

Chemically well-­defined poly(ionic liquid)s were prepared and applied as precursors for nitrogen-­doped carbon fibers and membranes via the electrospining technique. Vinylimidazolium-­ and vinylpyridinium-­based poly(ionic liquid)s of high molecular weight and possessing an allyl group in each repeating unit were synthesized by chemical modification and subsequent anion exchange with dicyanamide. They were in turn electrospun into fibers together with a co-­crosslinking agent trimethylolpropane tris(3-­mercaptopropionate and a radical initiator 4,4ʹ‐azobis(4-cyanovaleric acid). The fibers were crosslinked at 80°C, stabilized at 280°C and carbonized at 1000°C under nitrogen atmosphere. The nitrogen contents were found to be 6.3% and 8.0% respectively. Their electric conductivity was measured to be ca 300 S/cm.

5. Diversified applications of chemically modified 1,2-­polybutadiene

Commercially available 1,2-polybutadiene (from Aldrich) was transformed into a well-defined reactive intermediate by quantitative bromination. The brominated polymer was used as a polyfunctional macroinitiator for the cationic ring-opening polymerization of 2-ethyl-2-oxazoline, which yielded a water-soluble polymer brush. Nucleophilic substitution of brominated polybutadiene by 1-methylimidazole resulted in the formation of polyelectrolyte copolymers consisting of mixed units of imidazolium, bromo, and double bond. These copolymers were unimolecular soluble in water, and were used as stabilizers in the emulsion polymerization of styrene. In addition, they were also studied for their ionic conducting properties.

6. PIL copolymer as smart stabilizer for carbon nanotubes

A novel, simple-to-make copolymer stabilizer which responds to both temperature and ionic strength in aqueous solution, was prepared and applied to generate “smart” waterborne dispersions of carbon nanotubes. The copolymer stabilizer was prepared via free radical polymerization of N-isopropylacrylamide (NIPAM) and a low fraction of an IL monomer, 1-ethyl-3-vinylimidazolium bromide (EVImBr, content: 2.2-12.8 mol %). Our experiments clearly illustrate that one can tune the aqueous stability of CNTs over a wide temperature range and can precisely control the destabilization of MWCNTs at a desired temperature. This approach can be expanded to other aromatic carbon nanostructures, like fullerenes and graphenes, as the intrinsic polarizable cation-π interaction holds true for all carbon nanostructures.

7. Inner structure and self-assembly of PIL nanoparticles

Cryogenic TEM (Cryo-TEM) characterization of uncrosslinked PIL nanoparticles (alkyl chain length C12-C18) in aqueous solution revealed a highly ordered and tunable inner structure of PIL nanoparticles, which formed spontaneously by precipitation polymerization in water. They exhibit either multilamellar (for C12 and C14) or unilamellar (for C16 and C18) vesicular morphology, depending on the tail length of the quaternizing alkyl chains. In addition, unidirectional super-assembly to a nanoworm mesostructure was found when the polymerization was conducted at a higher monomer concentration. This research demonstrates that homopolymers, in spite of the simplicity in the synthesis and composition, may exhibit highly complex and ordered nanostructure.