Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session L24: John H. Dillon Award Symposium |
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Sponsoring Units: DPOLY Chair: Tim Bunning, Wright Patterson Air Force Base Room: Colorado Convention Center 201 |
Tuesday, March 6, 2007 2:30PM - 3:06PM |
L24.00001: Swell Gels to Dumbbell Micelles: Construction of Materials and Nanostructure with Self-assembly Invited Speaker: Bionanotechnology, the emerging field of using biomolecular and biotechnological tools for nanostructure or nanotecnology development, provides exceptional opportunity in the design of new materials. Self-assembly of molecules is an attractive materials construction strategy due to its simplicity in application. By considering peptidic or charged synthetic polymer molecules in the bottom-up materials self-assembly design process, one can take advantage of inherently biomolecular attributes; intramolecular folding events, secondary structure, and electrostatic interactions; in addition to more traditional self-assembling molecular attributes such as amphiphilicty, to define hierarchical material structure and consequent properties. Several molecular systems will be discussed. Synthetic block copolymers with charged corona blocks can be assembled in dilute solution containing multivalent organic counterions to produce micelle structures such as toroids. These ring-like micelles are similar to the toroidal bundling of charged semiflexible biopolymers like DNA in the presence of multivalent counterions. Micelle structure can be tuned between toroids, cylinders, and disks simply by using different concentrations or molecular volumes of organic counterion. In addition, these charged blocks can consist of amino acids as monomers producing block copolypeptides. In addition to the above attributes, block copolypeptides provide the control of block secondary structure to further control self-assembly. Design strategies based on small (less than 24 amino acids) beta-hairpin peptides will be discussed. Self-assembly of the peptides is predicated on an intramolecular folding event caused by desired solution properties. Importantly, the intramolecular folding event impart a molecular-level mechanism for environmental responsiveness at the material level (e.g. infinite change in viscosity of a solution to a gel with changes in pH, ionic strength, temperature). [Preview Abstract] |
Tuesday, March 6, 2007 3:06PM - 3:18PM |
L24.00002: Morphological Characterization of Silicone Hydrogels Samuel Gido Silicone hydrogel materials are used in the latest generation of extended wear soft contact lenses. To ensure comfort and eye health, these materials must simultaneously exhibit high oxygen permeability and high water permeability / hydrophilicity. The materials achieve these opposing requirements based on bicontinuous composite of nanoscale domains of oxygen permeable (silicones) and hydrophilic (water soluble polymer) materials. The microphase separated morphology of silicone hydrogel contact lens materials was imaged using field emission gun scanning transmission electron microscopy (FEGSTEM), and atomic force microscopy (AFM). Additional morphological information was provided by small angle X-ray scattering (SAXS). These results all indicate a nanophase separated structure of silicone rich (oxygen permeable) and carbon rich (water soluble polymer) domains separated on a length scale of about 10 nm. [Preview Abstract] |
Tuesday, March 6, 2007 3:18PM - 3:30PM |
L24.00003: The Application of Specular X-ray Reflectivity to Characterize Patterned Surface Wen-li Wu, Hae-Jeong Lee, Christopher L. Soles Specular $x$-ray reflectivity (SXR) has been used extensively for thin film characterization and depth profiling. Recently its application has been extended to quantify nanoscale patterns made of photo lithographic polymers. SXR results complement small angle $x$-ray scattering measurements by providing details information in the cross section of features on nanoscale surface patterns. This talk is to focus on the limit of applying SXR as an effective surface pattern metrology. Polymeric line gratings with periodicities or pitch ranging from 200 nm to 16 $\mu $m were chosen as test samples and it is expected that as the pitch size reaches beyond the coherent length of x-ray SXR will no longer be applicable. The optics of the SXR instrument dictates the coherent length of the x-ray; it provides a coherent length of a few micrometers in the longitudinal direction and sub-micrometers along the lateral direction on the reflection plane and a few nanometers along the lateral direction perpendicular to the reflection plane. SXR measurements were made at various azimuthal angles between the incident $x$-ray beam and the line grating, with 0\r{ } being the incident beam parallel to the line grating and 90\r{ } being perpendicular to each other. For periodicities less than 900 nm, the perpendicular and parallel measurements yield comparable SXR results which can be quantitatively analyzed using a one-dimensional model invoking effective medium approximation (EMA), i.e. SXR measures the lateral average electron density of the surface pattern. For periodicities 900 nm and greater, EMA breaks down and the SXR results can not be analyzed using any one-dimensional model. Work is on-going to determine the nature of transition between the EMA applicable region and the inapplicable region. The effect of surface patterns irregularity on this SXR application is another topic of current study. [Preview Abstract] |
Tuesday, March 6, 2007 3:30PM - 3:42PM |
L24.00004: Collaborative Investigations of Supramolecular Polymer Assembly Processes. Karen Wooley It is a pleasure to participate in this symposium, honoring Darrin J. Pochan's awarding of the John H. Dillon Medal for advancing our understanding of the physics of assembly and chain conformation of synthetic polypeptides. Assemblies of polypeptides, polysaccharides and polymers of nucleic acids are, of course, complex natural systems that form the bases of life. Over the past three years, we have worked together as a highly interdisciplinary team of investigators, to investigate the self assembly behaviors and resulting morphologies for synthetic amphiphilic block copolymer systems. This presentation will highlight the findings from these collaborative studies, including the importance of the block copolymer composition and topology and the significance of the assembly conditions. [Preview Abstract] |
Tuesday, March 6, 2007 3:42PM - 3:54PM |
L24.00005: Self-Generated Fields and Morphogenesis in Polymer Crystallization Jerold Schultz Thermal, compositional and stress fields are created during the crystallization of polymers from the melt. The roles of the thermal and compositional fields are dictated by their Peclet number for crystallization: Pe = $\Lambda $V/D, where $\Lambda $ is a characteristic length for the process, V is the velocity of the crystallization front, and D is the mass or thermal diffusivity. Pe is a measure of the ratio of the distance a molecule \textit{must} move in the process to the distance it \textit{can} move. If Pe is significantly greater than 1, the growth interface must restructure itself, to lower the Peclet number toward unity. Reviewed in this talk are numerical and analytical studies of the effects of compositional fields on morphological development during spherulite growth and the effect of thermal fields during high-speed spinning. [Preview Abstract] |
Tuesday, March 6, 2007 3:54PM - 4:06PM |
L24.00006: Propagating Waves of Self-Assembly in Organosilane Monolayers. Jack Douglas, Kirill Efimenko, Daniel Fischer, Fredrick Phelan, Jan Genzer Wavefronts associated with reaction diffusion and self-assembly processes are ubiquitous in the natural world. While it often claimed that this type of self-sustaining front propagation is well described by mean field `reaction diffusion' or `phase field' models, respectively, it has recently become appreciated from simulations that fluctuation effects can lead to appreciable deviations from the classical mean field theory (MFT) of this type of front propagation. The present work addresses the existence of fluctuation effects in the particular case of the frontal self-assembly of the organosilane self-assembled monolayers on silica-coated surfaces. By following the progress of this self-assembly process via near-edge x-ray absorption fine structure spectroscopy (NEXAFS), we find that these layers organize from the edge of the wafer as a propagating planar wavefront with a well-defined velocity, c. In accordance with two-dimensional simulations of this type of front propagation that include fluctuation effects, we find that the interfacial widths w(t) of these self-assembly fronts exhibit a power-law broadening in time rather than the constant width predicted by mean field theory. Moreover, the observed exponent values accord rather well with previous simulation estimates. This study confirms that fluctuation effects can cause interfacial broadening in autocatalytic front propagation, as found in earlier computations. [Preview Abstract] |
Tuesday, March 6, 2007 4:06PM - 4:18PM |
L24.00007: Receptor-Ligand Interactions and Adsorption at the Oil Water Interface Kenneth Shull, Daniel Carvajal, Chi-Yang Chao Liquid/liquid interfaces are excellent models for studying adsorption processes, because equilibration at these interfaces occurs more readily than at liquid/solid interfaces. In addition, the interfacial tension can be measured directly, and can be used to probe adsorption processes and molecular binding events that take place at the interface. In this investigation we use drop shape analysis to study receptor/ligand interactions at the chloroform/water interface. A pendant chloroform droplet is suspended in water. When hydrophobically-modified polyethylene glycol is added to the chloroform droplet, segregation of this molecule to the interface introduces a barrier to protein adsorption from the aqueous phase. Avidin adsorbs irreversibly to the oil water interface when the terminus of the PEG molecule is functionalized with biotin. By changing the volume of the chloroform drop (and hence the interfacial area) we obtain pressure/area isotherms at fixed avidin coverage. Adhesion of these functionalized interfaces to other surfaces can be quantified by bringing the pendant drop into contact with another surface or interface, and measuring the contact angle. [Preview Abstract] |
Tuesday, March 6, 2007 4:18PM - 4:30PM |
L24.00008: Microstructure foundations of high carrier mobility in polymers. Eric Lin, Dean DeLongchamp, R. Joseph Kline, Daniel Fischer, Lee Richter, Andy Moad, Martin Heeney, Iain McCulloch, John Northrup The microstructure of organic semiconductor films can impact charge carrier mobility because it defines the persistence and quality of $\backslash $pi bond overlap in the source-drain plane. Important aspects of microstructure include the intermolecular packing arrangement within crystals, the surface-relative crystal orientation, and the overall crystal size and connectivity. We combine complementary microstructure measurements including polarized absorption spectroscopies, scanning probe techniques, and X-ray diffraction to investigate the microstructure details of polymer semiconductors for organic thin film transistors (OTFTs). Here, we demonstrate this approach by solving the packing arrangement of a polymer semiconductor, poly(2,5-bis(3-alkylthiophen-2-yl)thieno[3,2-b]thiophenes) (pBTTTs), with hole mobility of (0.2 to 0.6) cm2/Vs. NEXAFS combined with FTIR spectroscopy reveals nearly all-trans side chains that strongly tilt. With the XRD lamellar spacing, we show that vertically adjacent layers interdigitate. A general consideration of side chain configuration reveals a striking signature packing motif that sets high performance polymers such as pBTTT apart from the lower performance poly(3-alkylthiophenes). [Preview Abstract] |
Tuesday, March 6, 2007 4:30PM - 4:42PM |
L24.00009: Phase Separated Polymer Systems on Surfaces and Some Applications in Super-Hydrophobicity Charles C. Han, Yonghua Yao, Xia Dong The study of the influence of temperature and confinement on surface segregation in thin films of deuterated polybutadiene and polyisoprene near the critical point for phase separation by the neutron reflectivity measurements will be discussed. The results show that polyisoprene enriches at the air and silicon interfaces in both the 1- and 2-phase regions. A transition between in-plane and surface-directed (layered) phase separation is observed with increasing film thickness. We will then describe some application with the use of phase separation of polymer blends on the surface. We demonstrate that a superhydrophobic surface can be facilely created by a simple casting process under environmental atmosphere by exploiting the different solubility of the two common polymers in the solvent. Furthermore, we will report a very attractive procedure to prepare a super-hydrophobic polymeric surface with controllable sliding angle (SA) from a single component commodity polymer, isotactic polypropylene (iPP), without further modification with low-surface-energy component under ambient atmosphere. The process we used is simply subject the iPP/decalin solution to a shear field and than lower the temperature to form a crystallized network structure. [Preview Abstract] |
Tuesday, March 6, 2007 4:42PM - 4:54PM |
L24.00010: Multicompartment micelles from ABC copolymers Tim Lodge We have prepared a variety of novel compartmentalized micelles from ABC terpolymers in water. The three blocks were chosen to be mutually incompatible, with one being water-soluble (polyethylene oxide), one lipophilic (poly ethyl ethylene), and one both hydrophobic and lipophobic (poly perfluoropropylene oxide). The resulting morphologies were characterized by cryogenic transmission electron microscopy. The various structures can be understood, at least qualitatively, in terms of the relative interfacial tensions and block lengths. Mixing copolymers and mixing solvents (THF and water) can also be used to tune micellar morphology. [Preview Abstract] |
Tuesday, March 6, 2007 4:54PM - 5:06PM |
L24.00011: Gold Nanoparticle Liquids and Dispersions: Structure and Phase Stability Richard Vaia, Stephen Diamanti, Robert MacCuspie, Hilmar Koerner, Mitra Yoonessi, Mark Pender By minimizing the volume fraction of the organic corona necessary to generate a net long-range repulsive pair-potential (and thus maximizing inorganic volume fraction), neat nanoparticle assembles may exhibit liquid-like behavior, which affords intriguing possibilities for numerous applications including solvent-less inks for micro-fabrication and compliant electrodes. For example, noble metal nanoparticle fluids have extended the life-cycle of an RF MEMS switch simulator by six-orders of magnitude relative to SAM surfaces and by one hundred times relative to uncoated gold switches by limiting switch failure by adhesion and shorting mechanisms. The thickness of the ionic-liquid corona, comprised of mercaptoethane sulfonate and a mixture of alkyl and PEG quaternary ammonium, is 1.4-1.5 nm, measured by small angle neutron scattering. Evidence of reversible de-aggregation of close-packed assembles of gold nanoparticles is observed around 1-4 wt{\%} in aromatic solvents, and at much lower concentrations in alkanes by small angle x-ray scattering. Thus, in addition to potential technological utility, these functionalized gold nanoparticles are providing experimental avenues to measure the phase stability of nanoparticle dispersions over a wide range of compositions. [Preview Abstract] |
Tuesday, March 6, 2007 5:06PM - 5:18PM |
L24.00012: Thin Film Composites of Block Copolymers and Bio-Nanoparticles Thomas Russell, Dongseok Shin, Ting Xu, Seung Hyun Kim, Qian Wang Thin film composites of block copolymer and bio-nanoparticle were fabricated through two-step process; adsorption of bio- nanoparticles on polymer film and subsequent annealing under solvent vapor. The humidity of the annealing chamber influenced the dispersion of bio-nanoparticles and the final morphology of the composites. Under high humidity condition, ferritins were dispersed and selectively localized at PEO cylinders of poly (styrene-b-ethylene oxide), P(S-b-EO), while the bio- nanoparticles were aggregated at low humidity. When one component of a block copolymer was charged positively, as in poly(styrene-b-N-methyl-4-vinylpyridinium iodide), P(S-b-4VPQ), the loading of bio-nanoparticles increased significantly. When the loading was low, the morphology was the same as P(S-b-EO) case. However, at high loading, ferritin particles were segregated and formed a continuous boundary around the grains of microphase separated block copolymers. As a result, a 2- dimensional hierarchical structure, where block copolymer chains microphase separated inside of discrete patches surrounded by bio-nanoparticles, was generated. This process was also applicable to anisotropic bio-nanoparticles (e.g. Tobacco Mosaic Virus). [Preview Abstract] |
Tuesday, March 6, 2007 5:18PM - 5:30PM |
L24.00013: Structure and Rheology of Shear-Banding Wormlike Micellar Solutions Norman Wagner Measurements of the micellar alignment, flow kinematics, and microstructure are presented for three WLM solutions. A special SANS flow cell enables the first direct measurements of the microstructure and micellar alignment in each individual band. These gap resolved 1-2 plane experiments demonstrate that the degree and orientation of segmental alignment of the micelles by the shear flow correlate with the measured shear viscosity. Combining the SANS measurements with flow-light scattering measurements shows that shear induces strong concentration fluctuations in the high shear band. These results show two distinctly different types of shear banding that is related to the underlying equilibrium phase behavior. The results help elucidate the mechanism driving shear banding in WLMs. [Preview Abstract] |
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