Dr. Monique Roerdink, a postdoctoral research associate in Professor Hillmyer's group has expanded the applicability of ABC triblock terpolymers that contain a robust A block (polystyrene), a hydrophilic B block [poly(dimethylacrylamide)], and an etchable C block (polylactide) developed in the IRG (structure depicted below). She has been able to incorporate a novel metathesis reactive monomer into the polystyrene blocks and has used the multifunctional triblocks to prepare new robust bicontinuous membranes containing hydrophilic channels. The membranes were prepared by first polymerizing dicyclopentadiene (DCPD) using a ruthenium metathesis catalyst in the presence of the ABC triblock and a solvent. Optically clear membranes were cast from this solution and then were treated with dilute aqueous base to remove the polylactide component and generate the nanoporosity. An SEM image of the new nanoporous membranes is shown below. These membranes should be quite useful in water-purification applications given their hydrophilic nature, excellent hydraulic permeability and narrow pore size distribution.

Tim Lodge (IRG 1) and Dan Frisbie (IRG 2). Industrial Collaborator: Mike Renn, Optomec, Inc. Lodge and Frisbie have developed a novel printable, high capacitance gate insulator material that facilitates low voltage operation of OFETs on plastic substrates. The insulator is a so-called ion gel composed of an ionic liquid and a gelating triblock copolymer, polystyrene-polyethyleneoxide-polystyrene. This material may be dissolved in solvents and printed. In a collaboration with an industrial manufacturer of aerosol jet printers (Optomec, Inc), Lodge, Frisbie, and their students have demonstrated successful low voltage operation of an array of ion-gel gated OFETs printed on flexible polyimide substrates. All components of the OFETs were printed—the metal electrodes (gold colloidal ink), the semiconductor (poly(3-hexylthiophene), and the gate insulator (the new ion gel material). The devices work at biases less than 3V and at speeds up to 10 kHz. Efforts to optimize the ion gel material and the transistor geometry for even better performance are underway. A patent on the new ion gel material has been filed by Frisbie and Lodge.

Xiaoyang Zhu (IRG 2). Understanding electronic energy levels in organic semiconductors is critical to their application in devices. Zhu has used 2PPE to measure the energy to excite an electron out of a layer of the organic semiconductor pentacene. This measurement is relevant to the use of pentacene in organic (plastic) solar cells. In the experiment, a pump photon (blue) was used to excite an electron from a pentacene molecule to a charge transfer (CT) state, in which the excited electron occupies a level just outside the pentacene layer, i.e., in the adjacent vacuum environment. This state is called a CT exciton state. A second photon (red) kicked the electron out of the CT exciton state up to the vacuum level. By measuring the electron kinetic energy, Zhu determined the binding energy of the CT exciton to be 0.45 eV. This is the energy that is required to completely separate the electron from the positively charged hole on pentacene. Knowing this number is important to understanding how organic solar cells function. One conclusion is that charge separation at a pentacene/acceptor interface in an organic solar cell must involve hot CT excitons (higher energy CT states) so that the 0.45 eV Coulomb penalty is avoided.

Magnetic storage of digital data is now possible at densities approaching 1 Terabit per square inch at a cost of only about a tenth of a cent per Megabit. To a large extent, the breathtaking progress in this area of technology is sustained by discovery of improved devices for detecting the magnetic field from increasingly miniaturized magnetic bits. The invention of “GMR” sensors based on stacks of ultra-thin films of magnetic metals (for which the Nobel Prize in Physics was awarded in 2007) is a perfect example. In these devices the ability to “pin” the magnetic orientation of one film in the stack while allowing another to respond freely to a magnetic field is a key principle. This “magnetic pinning”, or “exchange pinning”, is achieved by layering a simple magnet with a more complex magnetic material called an antiferromagnet. This pinning effect was discovered over 50 years ago, has been used in devices for over a decade, but is poorly understood. One of the major reasons for this is that the pinning effect is due to atomic defects in the antiferromagnet. Experiments are hindered by the fact that these defects are difficult to probe, while theoretical research is hampered by the fact that it is difficult to incorporate such defects in calculations. Recently, IRG 3 students Jyo Saha, Mike Lund, and Mun Chan, working with postdoc Jeff Parker and IRG 3 faculty Chris Leighton, Randy Victora, and Paul Crowell, have performed one of the most detailed studies of this exchange pinning in materials very similar to those used in hard disks. Most importantly, the experimental results were directly compared to realistic micromagnetic simulations. These supercomputer simulations break the sample up into hundreds of thousands of tiny magnetic elements (see figure), and are sufficiently powerful that the all-important defects can be properly accounted for. This approach has enabled the researchers to understand important factors such as the influence of layer thickness and grain size, complex magnetic switching, and even a mysterious effect where the exchange pinning deteriorates over time, a major problem for applications. [Saha, J.; Victora, R.H. Spontaneous Exchange Bias: Uniaxial Anisotropy in an Otherwise Isotropic System. Phys. Rev. B, 2007, 76, 100405(R). Saha, J.; Bolon, B.; Abin-Fuentes, A.; Parker, J.S.; Leighton, C.; Victora, R. Comparison Between Micromagnetic Simulation and Experiment for the Co/γ-Fe₅₀Mn₅₀ Exchange-biased System. J. Appl. Phys. 2007, 102, 073901; Chan, M.; Parker, J.S.; Crowell P.A.; Leighton, C. Identification and Separation of Two Distinct Contributions to the Training Effect in Polycrystalline Exchange Biased Co/FeMn Bilayers. Phys. Rev. B. 2008, 77, 014420; Lund, M.; Leighton C. Interplay Between Reversal Asymmetry, Training, and Induced Anisotropy in Epitaxial Exchange-Biased Bilayers. Phys. Rev. B 2007, 76, 104433.]

Spatial map of the distribution of magnetization directions in a 6.4 x 6.4 µm NiMn / NiFe sample according to a micromagnetic simulation. The sample is at the coercive field after demagnetizing from saturation. Comparison to similar images on the opposite side of the hysteresis loop reveals a magnetization reversal asymmetry also observed in experiment.

Silicon is the most widely used material in electronic devices and solar cells. At present, silicon is mainly used in the form of wafers cut from crystalline material, or amorphous material deposited using vacuum processes. Unfortunately, for applications such as solar cells the cost of these forms of silicon is still too high. Currently, efforts are underway in many industrial and academic laboratories to develop inks of silicon nanocrystals that may be used to ink-jet print silicon films. This manufacturing approach has the potential to significantly lower the cost in certain areas of silicon technology. Lorenzo Mangolini, a graduate student, and Xiaodong Pi, a post-doctoral researcher working with mechanical engineering professor Uwe Kortshagen, have developed a simple approach to produce inks of electrically doped silicon nanocrystals. Silicon nanocrystals are produced in a low-pressure plasma approach, in which silicon crystals between 2-6 nm in size are formed within a few milliseconds. By co-injecting dopant precursors the nanocrystals can be doped with donor and acceptor atoms. These doped nanocrystals are then immediately injected into a second plasma, in which organic molecules are grafted onto the silicon nanocrystal surfaces. The so functionalized nanocrystals are soluble in organic solvents and can be processed into nanocrystal inks. [Mangolini, L.; Kortshagen, U. Plasma-Assisted Synthesis of Silicon Nanocrystal Inks. Adv. Mater. 2007, 19, 2513.]

(William Tolman, Thomas Hoye, Marc Hillmyer)

Given the rising price of oil and finite nature petrochemical reserves, there has been a recent drive to use biorenewable materials as feedstocks for the synthesis of new polymers with properties competitive with more traditional petroleum-based materials. Carolyn Wanamaker, a graduate student working with Prof. Marc Hillmyer and Prof. William Tolman, has developed a new approach for the preparation of all-biorenewable thermoplastic elastomers from ABA triblock copolymers with polylactide hard (A) segments and polymenthide (B) soft segments. Polymenthide is derived from the widely available terpene menthol and can be readily prepared using simple catalysts. Wanamaker successfully prepared a range of ABA triblocks using a difunctional initiator approach by first preparing dihydroxy polymenthide of controlled molecular weight. From the terminal hydroxy end groups of this material she was able to grow polylactide segments of specified length. Small-angle x-ray scattering and mechanical properties analyses were performed on the triblocks; they formed various ordered state morphologies and showed mechanical performance comparable to traditional polystyrene-polyisoprene-polystyrene triblock thermoplastic elastomers (see the figure below). Recently Wanamaker has demonstrated that using isotactic polylactide end blocks can significantly improve the mechanical strength and modulus of these novel triblocks, thus rendering them potentially useful for a range of applications. In addition, she has shown that these materials can degrade in aqueous media, making them attractive for uses in the biomedical device arena.
[Wanamaker, C.L.; O’Leary, L.E.; Lynd, N.A.; Hillmyer, M.A.; Tolman, W.B. Renewable Resource Thermoplastic Elastomers based on Polylactide and Polymenthide. Biomacromolecules 2007, 8, 3634.] Supported by UMN MRSEC Award DMR#0212302.

Zeolite Growth in Confined Spaces

(Lee Penn, Alon McCormick, Andreas Stein, Michael Tsapatsis)

Zeolites are aluminosilicate materials with pore size of molecular dimensions. The purpose of the current seed project was to investigate if zeolite particles with tightly controlled sizes and shapes can be made by hydrothermal growth under confinement in precisely defined pore spaces. Such zeolite particles are desirable for applications in adsorption, catalysis and membrane separations. Fabrication of ultra-thin molecular sieve membranes by seeded growth (1) is the application of interest for the group of co-PIs. In addition to the technological potential, synthesis of zeolites in confinement is of fundamental interest in understanding nucleation and growth phenomena (2,3). Under appropriate conditions, it was possible to produce zeolite spheres conforming precisely to the pore space of 3-DOM carbon templates (4). The SEM micrograph in the bottom shows zeolite spheres formed inside the 3DOM carbon pores. The TEM micrograph in the top left shows one zeolite sphere and the one in the top right shows a high magnification image, with the FFT in the inset, confirming the polycrystalline nature of an all-silica zeolite. Preliminary results from this seed grant led to a recently funded NIRT project (NIRT: Precise Building Blocks for Hierarchical Nanomanufacturing of Membranes with Molecular Resolution, CMMI 0707610). Work is in progress on extending the technique in order to isolate the smallest possible zeolite crystals in high yield and investigate their use in the fabrication of molecular sieve films thinner than 100nm. Supported by UMN MRSEC Award DMR#0212302.

On June 23, 2007, 100 students, ages 6-15, each with a parent, attended a day-long program Materials for Energy and Nanotechnology, put on by MRSEC faculty and students. The students were attendees at the Davidson Institute for Talent Development Young Scholars Annual Meeting, held in St Paul. Events included a plenary lecture on Chemical Energy by Frank Bates and Marc Hillmyer, visits to two or three faculty labs and characterization facilities, a barbeque on the Northrop Mall, and a show by the Physics Force.

Four faculty (Bates, Hillmyer, Lodge, Macosko) and seven graduate students (Erin Arndt, Mike Bluemle, Will Gramlich, Carlos Lopez, Louis Pitet, Zach Thompson, Ling Zhang) from IRG-1, Microstructured Polymers, led a day-long class for 23 high school students. The students were participants in a weeklong summer camp, Exploring Careers in Science & Engineering, put on by the Institute of Technology’s Center for Educational Programs (ITCEP, http://www.itcep.umn.edu/index.php). During the morning the students synthesized three different polymer materials in the Chemistry Department, including a biorenewable polymer, polylactide. In the afternoon the students went over to Chemical Engineering & Materials Science, and subjected their polymers to various physical tests.

(IRG-1)

Fundamental research conducted by IRG-1 on the self-assembly of amphiphilic block copolymers in water has led to the recent commercialization of copolymer additives as a toughness enhancer in advanced epoxy formulations. A crucial insight was the realization that the same physical principles that govern the choice of micellar morphology in water might also apply in low molar mass epoxy precursors. Then, the exciting discovery was made that the micellar morphology and dispersion could be preserved during epoxy curing, leading in some cases to a remarkably tougher thermoset material. The transmission electron microscopy image of poly(butyleneoxide)-poly(ethylene oxide) wormlike micelles formed in a cured bisphenol A epichlorohydrin / phenol novalac epoxy resin shown below is representative of the types of composites that give dramatically improved toughness over the pristine epoxy.

The following text is extracted from a press release of November 21, 2007 (http://news.dow.com/dow_news/prodbus/2007/20071121a.htm): Dow Epoxy, a global business unit of The Dow Chemical Company and its affiliates ("Dow"), today introduced FORTEGRA™ Epoxy Tougheners, low viscosity materials for use in amine, DICY, anhydride and phenolic cured epoxy systems.... The FORTEGRA line is based on a specially designed self-assembling block copolymer that creates the particles needed for toughening the cured epoxy and, at the same time, does not result in big changes to other properties such as viscosity, cure speed or chemical resistance. "With FORTEGRA Epoxy Tougheners, formulators no longer have to make big compromises on key properties to improve the toughness of epoxy systems as they are forced to do today with alternative materials," said Ohnishi Hideyuki, global business director for Dow Epoxy’s Performance Resins group. And because these toughening products are effective at levels as low as 3 to 10 percent by volume, not as much is needed compared to similarly priced alternative products – making FORTEGRA Epoxy Tougheners a more reasonable choice."

Many of the properties of a nanoparticle are determined, at least in part, by the structure and composition of the surface. This explains why researchers across the globe are developing ways of studying nanoparticle surface chemistry and methods for rationally functionalizing the surfaces of synthetic nanoparticles. Consider, for example, silicon nanocrystals. Their dispersibility in a given solvent, their stability with respect to oxidation, their photophysical behavior: all of these, and other, properties are strongly influenced by bonding of the very outer layer of silicon atoms. Jason Holm, a Mechanical Engineering graduate student working with Chemistry professor Jeff Roberts, has just described a new way of studying the surfaces of pristine silicon nanocrystals in the aerosol state. The method relies on the measurement of extraordinarily small changes in particle size, smaller than have ever been reported for this type of aerosol instrumentation. It was used to monitor desorption of hydrogen from the surfaces of 6 nm-diameter silicon nanoparticles. Hydrogen thermally desorbs over the same temperature range at which it desorbs from a silicon wafer, through a complicated process involving the gradual conversion of high hydride into low hydride species. These findings may be useful in developing processes for depositing organic monolayers onto the particle surfaces. This work was supported in part by the MRSEC Program of the National Science Foundation under Award Number DMR-0212302. [Holm, J.; Roberts, J.T. Surface Chemistry of Aerosolized Silicon Nanoparticles: Evolution and Desorption of Hydrogen from 6-nm Diameter Particles. J. Am. Chem. Soc. 2007, 129, 2496]

A major goal of organic electronics is the development of new kinds of solution processable organic dielectric materials that can serve as gate insulators in organic thin film transistors (OTFTs). An important figure of merit for prospective gate dielectrics is the specific capacitance, which determines how much charge can be induced in the semiconductor channel of an OTFT for a given applied gate voltage; higher capacitance translates into higher induced charge densities and therefore both higher ON currents and lower switching voltages. In an inter-IRG collaboration, postdoctoral fellows Jiyoul Lee (IRG 2) and Yiyong He (IRG 1) and graduate student Matt Panzer (IRG 2) demonstrated that a gel electrolyte (a so-called "ion gel") based on a mixture of an ionic liquid and a block-copolymer can provide both large specific capacitance (>10 mF/cm2) and greatly improved polarization response times (~1 ms) when used as the gate dielectric in a polymer TFT (Figure 1). The improved properties allow transistor operation at frequencies greater than 500 Hz, significantly faster than what has been demonstrated previously with conventional polymer electrolytes and opening the door to a broader range of applications. Furthermore, the ion gel material is solution processable, making it potentially compatible with high throughput patterning methods (e.g., ink jet printing). A provisional patent was filed on 2/9/07. This work was supported primarily by the MRSEC Program of the National Science Foundation under Award Number DMR-0212302. [Lee, J.; Panzer, M.J.; He, Y.; Lodge, T.P.; Frisbie, C.D. Ion Gel Gated Polymer Thin Film Transistors. J. Am. Chem. Soc. 2007, in press]

Frank Bates, Marc Hillmyer and their co-advised student Zach Thompson in the Department of Chemical Engineering and Materials Science at the University of Minnesota have been exploring the self-assembly of block copolymers in thermoset resins in order to uncover the fundamental mechanisms responsible for extraordinary fracture toughness observed at very low levels of block copolymer incorporation (1-5%). This work follows the Ph.D. thesis completed by June Wu in December of 2005. The transmission electron microscopy image of poly(butyleneoxide)-poly(ethylene oxide) wormlike micelles formed in a cured bisphenol A epichlorohydrin / phenol novalac epoxy resin shown below is representative of the types of composites that give dramatically improved toughness over the pristine epoxy. This new thrust represents a collaboration with the Dow Chemical Company and scientists at Texas A&M. This was supported in part by the MRSEC Program of the National Science Foundation under Award Number DMR-0212302.

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