Designer Liquids in Polymer Systems

Applied ionic liquids research is rapidly extending its reach at a number of fronts in polymer science and technology. In the past few years, these liquids have not only been employed as solvents for various types of polymerization, but they have also been used to dissolve polymers, to add functionality to them, and to create new polymer composites.

"These applications are really just the tip of the iceberg," according to Robin D. Rogers, professor of chemistry and director of the Center for Green Manufacturing at the University of Alabama, Tuscaloosa.

Rogers and Christopher S. Brazel, assistant professor of chemical and biological engineering at the university, co-organized a symposium on the topic sponsored by the Polymer Chemistry Division at last month's national meeting of the American Chemical Society in Anaheim, Calif.

"The symposium is, we think, the first international symposium dedicated to polymers in ionic liquids, and it demonstrates how the field has changed in a short period of time," Rogers said. "The aim of the symposium is to look into the range of applications and processes, as well as to try to determine where the advantages of ionic liquids outweigh any disadvantages of their use."

Many common room-temperature ionic liquids consist of nitrogen-containing organic cations and inorganic anions. Their chemical and physical properties can be tuned for a range of potential applications by varying the cations and anions. Over the past few years, the liquids have generated increasing interest as potential designer solvents for clean technologies because they generally have no detectable vapor pressure and are thermally stable, nonflammable, and relatively undemanding to manufacture.

"Ionic liquids are also being used to introduce new or modified properties into polymers, either through the ionic liquid itself or as functional additives allowed by solution processing of polymers in ionic liquids," Rogers noted. "Examples include the use of ionic liquids as plasticizers. The ionic liquid can be solvent and plasticizer, just solvent, or just plasticizer."

Brazel pointed out that more rapid and higher molecular weight polymerizations are possible in ionic liquids compared with traditional solvents. "Living polymerizations that don't require the often tedious synthesis procedures necessary with other solvents can be conducted in ionic liquids," he explained.

In living polymerizations, which can be radical, anionic, or cationic, the reactive intermediates are generated reversibly so that they are either active (when monomer is added) or dormant (until more monomer is added). Irreversible chain termination does not occur (C&EN, Sept. 9, 2002, page 36).

At Monash University, Victoria, Australia, chemistry professor Douglas R. MacFarlane and Ph.D. student Ranganathan Vijayaraghavan have been investigating the living nature of cationic polymerizations in ionic liquid solvents [Chem. Commun., 2004, 700].

"In principle, the ionic liquid should provide a long-lived 'living' state by stabilizing the carbocation in the polymer backbone," MacFarlane noted. He has carried out the cationic polymerization of styrene in the ionic liquid N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)amide using mild acid catalysts such as organoborate acids to obtain living polymers of narrow polydispersity.

"THE IONIC POLYMERIZATION reactions can be carried out under mild conditions because of the special properties of the ionic liquid," he explained. "The ionic liquid medium also has the effect of altering the reactivity ratios in copolymerization reactions--for example, in the case of styrene/methyl methacrylate copolymerization."

Kevin H. Shaughnessy, assistant professor of chemistry at the University of Alabama, Tuscaloosa, observed that ionic liquids are unique among reaction media in that they are polar yet can also be designed to be noncoordinating.

"We have hypothesized that polar, noncoordinating ionic liquid solvents would accelerate certain catalytic processes, in particular those catalyzed by cationic metal centers with open coordinate sites, by stabilizing charge-separated catalytic intermediates or transition states," he said.

Shaughnessy, Rogers, and coworkers have applied weakly coordinating ionic liquids with weakly coordinating anions to the copolymerization of styrene and carbon monoxide using palladium catalysts.

"Ionic liquids provide higher activities than commonly used organic solvents," Shaughnessy observed. "The acceleration is strongly anion dependent, with more coordinating anions decreasing polymerization activity."

Meanwhile, Jimmy W. Mays, a polymer chemist at the University of Tennessee, Knoxville, and at Oak Ridge National Laboratory, and coworkers have been comparing radical polymerizations of styrene and methyl methacrylate in various room-temperature ionic liquids.

In an initial investigation, Mays, Brazel, Rogers, and coworkers examined free-radical polymerizations of methyl methacrylate and styrene in the ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF₆]) using conventional organic initiators.

"We showed that conventional free-radical polymerization in this ionic liquid offers unique advantages in terms of polymerization kinetics in addition to green chemistry benefits," Mays reported. "Specifically, polymerization rates are greatly increased--by nearly an order of magnitude--with a simultaneous increase in molecular weight."

The Tennessee group then questioned whether these advantages were a general phenomenon of free-radical polymerization in ionic liquids or specific to the monomers and ionic liquid used in the original investigation. At the ACS meeting, Mays summarized the results of testing radical polymerizations of styrene and methyl methacrylate in a dozen room-temperature ionic liquids.

"In nearly all cases, we observed the enhanced polymerization kinetics," he said. "Thus, the effect is a general one. We attempted to correlate molecular weight and rates with the viscosity and polarity of the ionic liquids, but no clear trends are evident."

Mays also presented results on an "ionic-liquid-assisted, free-radical polymerization." His group used tris[hexyl(tetradecyl)phosphonium] bis(2,2,4-trimeth-ylpentyl)phosphinate as the ionic liquid for the room-temperature polymerization of methyl methacrylate using benzoyl peroxide as initiator.

"NORMALLY, THIS REACTION must be heated to 60 ^oC or higher to promote decomposition of the peroxide and onset of polymerization," he noted. "We believe that this ionic liquid acts as a reducing agent, in conjunction with benzoyl peroxide--an oxidizing agent--to create free radicals at room temperature. In other words, it is a redox initiator system.

"Thus, in this case, there is the potential for green synthesis, high molecular weights, rapid rates, and no need to heat the reaction," he continued. "This is certainly a unique polymerization system."

Brazel and colleagues at the University of Alabama have been looking into the use of room-temperature ionic liquids as plasticizers. In recent work, they compared poly(methyl methacrylate) (PMMA) plasticized with [bmim][PF₆] or its hexyl relative [hmim][PF₆] with PMMA formulated with diethylhexyl phthalate (DEHP), a traditional plasticizer. Diethylhexyl phthalate is the name recommended by the International Union of Pure & Applied Chemistry. In the plastics industry, it is usually referred to as dioctyl phthalate (DOP).

DEHP is an excellent plasticizer for PMMA and poly(vinyl chloride) (PVC). However, the use of DEHP in medical plastics and other plastics that come into contact with humans has led to health concerns. Brazel pointed out that DEHP migrates from plastic and leaches into saline and other biological fluids.

"Although DEHP has been found to bioaccumulate over years of normal human exposure, its specific and chronic toxicities have not been fully evaluated," he said. "Much of our work has focused on the use of ionic liquids as plasticizers that can compete with DEHP in terms of flexibility while offering greater thermal stability, better low-temperature performance, and reduced leaching."

Last year, Brazel's group reported that room-temperature ionic liquids based on imidazolium salts are "excellent" plasticizers for PMMA [Eur. Polym. J., 39, 1947 (2003)]. They showed that the high-temperature stability of the ionic liquids tested is higher than that of DEHP. The liquids also have the ability to reduce glass-transition temperatures to near 0 ^oC.

More recently, the group has been examining the leaching and migration resistance of phosphonium-based and other ionic liquids when used as PVC plasticizers. The researchers had some success at forming flexible PVC, though some of the ionic liquids were not as successful at lowering glass-transition temperatures.

Plasticizer migration from PVC to other solids was minimal using several of the ionic liquids, Brazel noted. He added that though leaching into water was reduced using ionic liquids, it is still a significant challenge.

At the Center for Green Manufacturing, staff scientist John D. Holbrey, Rogers, and colleagues have shown that not only many common synthetic polymers but also biopolymers such as cellulose, dextran, and starches are soluble in the low-melting ionic liquid [bmim]Cl. The group is particularly interested in the use of the liquid to dissolve and derivatize cellulose, a natural polymer that is insoluble in water.

Holbrey and coworkers have demonstrated that cellulose from pulp, field cotton, filter paper, or virtually any other source rapidly dissolves in the ionic liquid when heated gently. They noted that the potential of cellulose and cellulose products has not been fully exploited for three main reasons: the historical shift to petroleum-based polymers from the 1940s onward, the difficulty in modifying cellulose properties, and the limited number of common solvents that readily dissolve cellulose.

"Currently, cellulose processing and chemistry relies primarily on carbon disulfide and caustic bases as dissolving solutions," Holbrey pointed out. "The efficiency of existing methods for dissolving and derivatizing cellulose can be significantly improved by the availability of suitable solvents for refined and natural cellulose."

He explained that solutions of cellulose and ionic liquids such as [bmim]Cl are amenable to conventional processing techniques for the formation of cellulose threads, thin films, and beads.

In Anaheim, Holbrey described work on the use of [bmim]Cl to prepare functional cellulose materials. Many dyes, as well as complexants for coordination and binding of metal ions, that have been designed to be insoluble in water can be readily dissolved in this polar ionic liquid at high concentration, he noted. In this way, they can be integrated "into a processed hydrophilic cellulose matrix to obtain materials suitable for sensing and remediation in aqueous media."

Holbrey pointed out that this hydrophilicity, or wettability, is potentially useful in, for example, providing fast transport of water-soluble metal ions to active sites.

"Because the ionic liquid is able to dissolve many water-insoluble materials, this also provides a methodology to entrap or incorporate the materials into the cellulose matrix in a highly dispersed manner," he added.

The group has also shown that insoluble macromolecular particles, including enzymes, and inorganic nanoparticles can be introduced into the cellulose in ionic liquids to produce disperse particle composites.

"Such structurally modified cellulose materials are potentially useful in biocatalysis and magneto-responsive sensing materials," Holbrey said. "With additives to retard thermal and radiative degradation, they might also be useful as flame retardants and UV filters, respectively."

In England, research fellow Neil Winterton and coworkers at the Liverpool Centre for Materials & Catalysis, University of Liverpool, in collaboration with chemical engineers at the University of Newcastle, have prepared a series of composites, some with permanent porosity, that consist of linear polymers or cross-linked copolymers and imidazolium ionic liquids [Macromolecules, 36, 4549 (2003)]. One of the aims of the work is to isolate porous polymers from the composites that can be used in catalytic membrane reactors in which ionic liquids are employed as catalytic media for the reactions.

"We have prepared materials with permanent porosity by polymerizing well-known cross-linkers, such as divinylbenzene and trimethylolpropane trimethacrylate," Winterton said. "Porosity character- istics are known to be sensitively dependent on the medium in which polymerization occurs."

The team found that the porosity of poly(divinylbenzene) produced in an ionic liquid is different from that produced in a molecular solvent such as toluene, whereas the porosities of poly(trimethylolpropane trimethacrylate) are similar, whether produced in an ionic liquid or a molecular solvent.

"This phenomenon is known to be related to phase separation, processes of nucleation and growth, and particle aggregation, although in subtle and poorly understood ways," Winterton explained.

COMPLETE REMOVAL of the ionic liquids from isolated polymers is difficult, however. "Our results on the retention of ionic liquids highlight the need for care when considering the usefulness of ionic liquids as media for polymer preparation," Winterton remarked. "It seems to me that those applications, of which there are several, that exploit the novel characteristics of the polymer-ionic liquid composite, such as battery applications, are more likely to find earliest technical application, compared with those that rely on the isolation of pure polymers prepared in these media."

He noted that polymerizations were among the earliest chemical transformations that were studied in ionic liquids. Those studies "were motivated by interest in polymer electrolytes for possible use in battery, fuel cell, and related applications," Winterton observed.

In recent years, chemistry professor Masayoshi Watanabe at Yokohama National University, in Japan, has been combining ionic liquids and polymers to form ion gels for use as polymer electrolytes in fuel cells, lithium batteries, and dye-sensitized solar cells.

"Conventional polymer gels normally contain volatile liquids that sometimes limit their utility and durability at high temperatures and in the open atmosphere," he said.

Watanabe uses in situ radical polymerization of common vinyl monomers in ionic liquids to generate ion gels that exhibit high conductivities at room temperature. An example is an ion gel consisting of the ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([emim][TFSI]) and a PMMA network polymer.

"The high conductivities are caused by the self-dissociating and ion-transporting abilities of the ionic liquids and by decoupling of the ion transport and polymer segmental motion," he explained. If properties such as lithium ion conduction and proton conduction are molecularly designed into the ionic liquids, the range of potential uses of ionic liquids and ion gels may greatly expand, he added.

Hiroyuki Ohno, a professor in the department of biotechnology at Tokyo University of Agriculture & Technology, in Japan, pointed out that the transport of the component ions of ionic liquids in polymer gel electrolytes along a potential gradient remains a crucial problem. "These ions are mostly useless as target ions," he noted. "A target ion is an ion that plays an indispensable role in a device. For example, the lithium cation is required for the lithium battery. When target ions, such as lithium ions, protons, or iodide ions, are added to an ionic liquid, the ions making up the ionic liquid also migrate along the potential gradient."

Addition of the target ions--in the form of salts, for example--also induces increases in the glass-transition temperature and viscosity of the polymer gel and, as a result, the ionic conductivity of the material is considerably reduced.

"The liquids turn to solid on addition of these salts," Ohno explained. "And the target ion transport number, which is the contribution of the target ion migration to total current, is very small."

Ohno has been attempting to solve the problem by polymerizing ionic liquids to form polymeric films in which only the target ions can migrate. Earlier this year, Ohno and coworkers reported the preparation of highly ion-conductive, transparent, and flexible films consisting of ionic liquid-type polymer brushes [Polymer, 45, 1577 (2004)].

"The polymerization of ordinary imidazolium ionic liquid monomers results in a considerable drop in ionic conductivity," Ohno said. "We prepared films with excellent ion conductivity from polymerizable ionic liquid monomers--N-vinyl-3-ethylimidazolium TFSI and derivatives--that have flexible hydrocarbon spacers between the polymerizable vinyl group and the imidazolium cation ring.

"Such polycationic systems may be useful for anion transport," he added. "We also showed that copolymerization of cationic and anionic monomers results in a polymerized ionic liquid moiety where no ions are inherently mobile. These materials are interesting for target ion transport after addition of suitable salts."

Ohno's group has also been investigating the possibility of improving the transport of target ions in the polymerized ionic liquids by adding zwitterionic liquids to them. Zwitterionic compounds mostly have melting points above 100 ^oC, which is generally higher than those of simple ionic liquids.

"Zwitterionic liquids are the next generation of ionic liquids," Ohno claims. "They are molten salts composed of covalently tethered cations and anions."

In a recent paper, the Tokyo researchers reported the results of a study of the relationship between structure and properties of zwitterionic liquids with sulfonate, carboxylate, or dicyanoethenolate anions and onium cations such as imidazolium ions [Aust. J. Chem., 57, 139 (2004)]. They showed that increasing the length of the hydrocarbon spacer between cation and anion generally lowered the melting points of the zwitterions.

The main attraction of zwitterionic liquids, according to Ohno, is that, like simple ionic liquids, they can be used as solvents, and because they contain both cations and anions, they do not migrate along a potential gradient.

"We have synthesized lots of zwitterionic liquids," Ohno noted. "They have potential applications as solvents in electrochemical applications and in organic synthesis."

In Anaheim, Ohno reported the preparation of novel solid ionic liquid polymer gel electrolytes by polymerizing a mixture that included an ionic liquid monomer, LiTFSI, and a zwitterionic liquid. He noted, however, that the ion conduction of the electrolyte decreased with increasing LiTFSI or zwitterionic liquid concentrations. The decrease can be attributed, he suggested, to an increase of the glass-transition temperature.

Even so, Ohno is optimistic about the potential of zwitterionic liquids, polymerized ionic liquids, and ionic liquids in general.

"It is fun to develop a variety of ionic liquids with specific functions because the design of organic ions has unlimited possibilities," he concluded. "In the future, we can expect wide-ranging applications of ionic liquids, not only as solvents for chemical reactions, but also for electrochemical uses. I suspect that one day we will be surrounded by functionalized ionic liquids in our daily lives."