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The way polymer molecules organize on the nano scale in a solid has a major impact on the macroscopic properties of the material. It determines whether the solid is crystalline, semicrystalline, or amorphous, and it also influences the mechanical, optical, electronic, and other physical properties of the material.
"Controlling properties at the nano level and translating them into the macro level enables one to realize applications of these nanostructured materials," according to Alan R. Hopkins, a materials scientist at Aerospace Corp., El Segundo, Calif. "However, appreciation of this goal has been slower than anticipated for a variety of reasons, perhaps most importantly, because of the incredible complexity of controlling nanostructures under challenging physical, chemical, and mechanical boundary conditions."
Overcoming these scientific and technical challenges could lead to nanostructured polymers with a wide range of applications, including sensors, photovoltaic devices, light-emitting diodes (LEDs), organic transistors, silklike fibers, and drug delivery.
A symposium held last month at the American Chemical Society's national meeting in Atlanta highlighted some of the successes that scientists have had in controlling nanostructures to achieve new polymer properties that are not available in conventional polymeric materials. The symposium was sponsored by the Division of Polymer Chemistry and organized by Hopkins and Randy M. Villahermosa, manager of the Contamination Control Section at Aerospace.
John R. Reynolds, professor of chemistry at the University of Florida, Gainesville, opened with a keynote lecture on nanostructured optoelectronic and redox-active polymers. His group has been investigating "a series of conjugated polymers blended in host insulating polymers as a means of creating nano- and micro-sized light-emitting domains where the color of the emission is controllable by both blend composition and applied voltage," he told C&EN. The work was carried out by Ph.D. student Nisha Ananthakrishnan in collaboration with Kirk S. Schanze, another chemistry professor at the university.
The researchers used solution blending to blend two π-conjugated polymers—an orange-emitting p-phenylenevinylene polymer and a blue-emitting fluorene-carbazole copolymer—with a poly(methyl methacrylate) insulating host polymer. Light-emitting diodes prepared from the blends give off yellow, green, white, or blue light, depending on the composition of the blend and the voltage. The Florida team showed that equal concentrations of the two conjugated polymers in the matrix led to a bright-yellow-emitting LED, whereas unequal compositions of the conjugated polymers resulted in a color change from yellow to green with an increase in voltage.
The team has also been investigating the morphology of the blends by using a combination of atomic force microscopy, scanning electron microscopy, and photoluminescence microscopy to demonstrate that light-emitting domains of the order of 100 nm to 1 µm are formed. Complete understanding of the morphology of complex polymer blends and composites is necessary to develop any optoelectronic polymer-based device, including polymer LEDs, photovoltaic cells, and organic transistors, Reynolds said.
At the Atlanta meeting, Schanze also presented recent research, carried out in collaboration with Reynolds, on π-conjugated polyelectrolytes. He described the use of layer-by-layer deposition to fabricate organic photovoltaic cells. The active material in the device is a nanostructured, ultrathin, multilayer film consisting of alternate layers of anionic poly(phenylene ethynylene)-based electron donors and water-soluble cationic fullerene derivatives that accept electrons.
"In recent unpublished work, we have also developed a method to adsorb conjugated polyelectrolytes onto the surface of nanostructured titanium dioxide," Schanze told C&EN.
Solar-cell hybrid materials constructed by using two conjugated polyelectrolytes adsorbed on titania exhibit power conversion efficiencies of 1.5% under solar-simulated light. "By selecting polymers with either blue- or red-light-absorbing properties, spectral broadening is attained as desired for enhancing solar efficiencies," Schanze noted. "While molecular dye-based solar cells reach efficiencies of greater than 10%, the record for hybrid solar cells based solely on conjugated polyelectrolytes is around 2.5%."
The Schanze group has also been exploiting the highly fluorescent nature of conjugated polyelectrolytes to develop biosensors that can detect targets such as proteins, peptides, carbohydrates, and DNA at ultralow concentrations. "Several of these new conjugated polyelectrolytes have been applied as novel biosensors to probe the activity of protease and lipase enzymes," Schanze noted. For example, the team has used poly(phenylene ethynylene) sulfonate as the signaling element in a fluorescent sensor for protease activity that can detect the enzyme and the enzyme substrate at picomolar and nanomolar concentrations, respectively.
In a collaborative effort at the University of Missouri, Rolla, groups led by chemistry professor Frank D. Blum and physics professor Massimo F. Bertino have also been investigating the development of nanostructured polymers that have significant potential for novel electronic devices. Blum outlined recent work on the project in Atlanta.
"We have been studying both metal nanoparticle and nanowire formation," he told C&EN. "Sunil Pillalamarri, a Ph.D. student in our group, discovered that mixtures of metal ions and monomers, when irradiated with high-energy or UV radiation, can form nanowires decorated with metal nanoparticles. The nanocomposites can be prepared from a few simple precursors in a 'one-pot' single-step synthesis using water as a solvent."
Last year, for example, the group showed that composite materials consisting of polyaniline nanofibers decorated with silver or gold nanoparticles can be synthesized by irradiating aqueous solutions of aniline and silver or gold salts with γ-rays (Chem. Mater. 2005, 17, 5941). The electrical conductivity of the composites increases with the loading of the metal in the nanocomposites and is up to 50 times greater than that of polyaniline fibers alone, the authors noted.
"When monomers like aniline are used, the nanocomposites consist of metals connected with conducting polymer nanowires," Blum observed. "The nanofibers seem to form first, and then the metal nanoparticles form. The particles are firmly attached to the fibers and can only be removed, with difficulty, by sonication.
"These nanocomposite structures can be photopatterned and show great promise for the production of high-density memory devices, sensors, and other applications," he said. "Because of their larger surface areas, polyaniline nanofibers have higher sensitivity and faster responses compared with bulk polyaniline."
The patterning of single crystals of organic semiconducting compounds, such as anthracene and rubrene, in well-ordered arrays was one of the topics of a symposium session on nanostructured polymer synthetic techniques. In a lecture titled "Organic Single-Crystal Transistors: From Nanometer to Centimeter Scales," Alejandro L. Briseno, a graduate student at the University of California, Los Angeles, described research carried out by a team led by Zhenan Bao, professor of chemical engineering at Stanford University.
Bao's team has developed a printing method for fabricating patterned transistors over large areas from solution and by vapor deposition. "By using microcontact printing, we print self-assembled monolayers that enable organic semiconductors to grow in certain regions," Briseno told C&EN.
The technique employs a polydimethylsiloxane stamp to transfer the self-assembled monolayer onto a substrate such as gold. The monolayer acts as a nucleation template for the growth of large, oriented, patterned crystals.
"Our technique results in a simple, fast, and highly reproducible method of patterning organic semiconductors," Briseno said. "We have also produced flexible transistors composed of patterned films and single crystals."
In a symposium session devoted to hybrid nanostructured polymeric materials, Frank Ko, professor of materials engineering at Philadelphia's Drexel University, reported on recent advances in carbon-nanotube-containing composite materials. The use of nanotubes to reinforce advanced lightweight polymeric materials could lead to improved strength and performance, according to Ko.
His group uses an electrospinning process to produce continuous ultrafine polymer yarns reinforced with single-wall or multiwalled carbon nanotubes. The technique employs a reservoir with a capillary tip to hold the nanotube-containing polymer fluid, a high voltage to align the nanocomposite fibrils in the yarn, and a rotating drum to collect the yarn.
"We expect the process to be scalable for the manufacture of continuous yarns of various deniers with different weight concentrations of both types of carbon nanotube," Ko noted.
The team used the method to prepare meter lengths of fiber from a dispersion of carbon nanotubes in a solution of polyacrylonitrile in dimethylformamide. Ko and coworkers have also successfully produced carbon nanotube-silk nanocomposite fibrils from recombinant spider silk. They demonstrated that both the strength and electrical conductivity of the fibrils could be increased 10-fold by addition of a small amount of nanotubes to the silk protein.
Polyhedral oligomeric silsesquioxane (POSS) nanoparticles can also improve the properties of polymers, according to Joseph M. Mabry, a group leader at the Air Force Research Laboratory at Edwards Air Force Base in California. POSS compounds are typically produced as either completely or incompletely condensed cage structures by the acid- or base-catalyzed hydrolysis of trifunctional silanes. Completely condensed compounds commonly contain cages with six to 12 silicon atoms. Incompletely condensed cages contain fewer silicon atoms and retain some reactive silanol groups.
At the Atlanta meeting, Mabry described how POSS compounds can dramatically enhance the thermal and mechanical properties of thermoplastics and thermosets. "POSS compounds are like functionalized nanosilica," he told C&EN. "They can be incorporated into polymers by blending or by copolymerization. Their organic groups can be tailored to match the polymer into which they will be incorporated."
Mabry pointed out that POSS compounds have been used to make polyimide nanocomposite protective layers for satellites. The nanocomposites are resistant to the space environment when the satellites are in low Earth orbit. "They form a protective silica layer upon attack by atomic oxygen and protect the underlying polymer," he explained. "These POSS polyimides are self-healing and can increase the lifetime of the satellite by an order of magnitude."
The polymer symposium also featured a session on copolymer structures. One of the lectures focused on research at North Carolina State University on the use of cyclodextrins to control polymer nanostructure and properties. The work was carried out by Ph.D. student Marcus A. Hunt, who presented the lecture; Alan E. Tonelli, professor of fiber and polymer chemistry; and coworkers.
"Cyclodextrin is a natural biodegradable compound that has many interesting properties, particularly when combined with polymer molecules," Hunt told C&EN. Cyclodextrins can act as hosts in the formation of crystalline inclusion complexes with a variety of polymers.
The complexes are typically prepared by mixing a solution of the polymer with an aqueous solution of cyclodextrin. The mixture is heated, cooled to room temperature, and stirred for several days. A precipitate of the complex is then separated from the mixture. The guest polymer chains in the complexes are isolated in continuous cyclodextrin channels with narrow cross-sections and therefore have highly extended conformations, Hunt noted.
When the host cyclodextrin in the polymer-cyclodextrin inclusion is removed by selective degradation by using an enzyme or an acid, the guest polymer chains coalesce, resulting in consolidated polymers with structures and morphologies significantly different from those usually obtained from disordered solutions or melts.
The team also showed that it is possible to polymerize styrene in cyclodextrin columns. The researchers first prepare a cyclodextrin inclusion complex containing styrene. The complex is then suspended in water containing a free-radical initiator that polymerizes the styrene to yield the polystyrene-cyclodextrin inclusion complex.
"Polymers coalesced from cyclodextrin inclusion complexes exhibit higher thermal transitions, higher crystallinity, and unusual polymorphs," Hunt observed.
The work on the inclusion complexes is an example of how two or more materials can be combined in symbiotic fashion to create new materials. It was one of numerous examples presented at the symposium of how researchers are learning to control the macroscale properties of polymers by manipulating the nanoscale structures of molecules.
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