Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session J20: Frank J. Padden Jr. Award Symposium |
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Sponsoring Units: DPOLY Chair: Glenn Fredrickson, University of California, Santa Barbara Room: 321 |
Tuesday, March 17, 2009 11:15AM - 11:27AM |
J20.00001: Pressure Effects on Polymer Coil-Globule Transitions near an LCST David Simmons, Issac Sanchez A model for the pressure - temperature behavior of the coil-globule transition (CGT) of a polymer in dilute solution is developed without adjustable parameters. The predicted pressure-temperature conformational behavior semi-quantitatively correlates with extant experimental data. The model yields a heating induced coil-to-globule transition (HCGT) temperature that increases with pressure until it merges with a cooling induced coil-to-globule transition (CCGT). The point at which the CCGT and HCGT meet is a hypercritical point that also corresponds to a merging of lower critical solution temperature (LCST) and upper critical solution temperatures (UCST). Theoretical results are discussed in terms of a generalized polymer/solvent phase diagram that possesses two hypercritical points. [Preview Abstract] |
Tuesday, March 17, 2009 11:27AM - 11:39AM |
J20.00002: Structure-property relationships in ABA copolymer gels with A homopolymer additions Michelle Seitz, Rebecca Rottsolk, Kirt Page, Kenneth Shull ABA acrylic triblock copolymers with poly(methyl methacrylate) endblocks and poly(butyl acrylate) midblocks transition from free flowing liquids to elastic solids with decreasing temperature in alcohol solvents. Homopolymer PMMA chains can be solubilized in the micelle cores if they are shorter than the endblocks. Indentation and compression tests were used to determine gel's modulus and large strain behavior. Gels with volume fractions of PMMA less than $\sim $0.2 are highly elastic and have moduli dictated by stretching of bridging midblocks. At higher PMMA contents, gels exhibit greater permanent deformation and moduli over an order of magnitude larger than would be expected from rubber elasticity alone. Small angle X-ray and neutron scattering and mean field simulations were used to correlate changes in gel structure and micelle morphology with the addition of homopolymer. [Preview Abstract] |
Tuesday, March 17, 2009 11:39AM - 11:51AM |
J20.00003: A Versatile Method for Covalent Layer by Layer Assembly of Thin Organic Films Hernan R. Rengifo, Cristian Grigoras, Jeffrey Koberstein Layer by layer (LbL) assembly techniques construct multilayer thin films by sequential deposition of monomolecular layers of organic molecules. One of the drawbacks associated with their use is that monomolecular layers are usually held together by relatively weak forces such as Van der Waals, electrostatic and hydrogen bonding interactions, and can therefore be lacking in mechanical integrity. We demonstrate herein that heterobifunctional polymers, functionalized with one azide chain terminus and a protected alkyne group as the other chain terminus, constitute a powerful and versatile means for the \underline {covalent} layer-by-layer (CLbL) assembly of thin polymer films. Each monomolecular polymer layer is covalently bound to both the preceding and following layers to produce a robust multilayer structure. Because the coupling chemistry used, ``click'' chemistry, is highly chemoselective, the layering process is virtually independent of the chemical nature of the polymer so that the constitution of each layer can be selected at will. [Preview Abstract] |
Tuesday, March 17, 2009 11:51AM - 12:03PM |
J20.00004: How do entangled polymeric liquids flow? Sham Sundar Ravindranath, Shi-Qing Wang This work focused on investigating fundamental questions in polymer dynamics such as how entangled polymeric liquids respond to fast external deformation. By developing an effective particle tracking velocimetric (PTV) method, along with conventional rheometric measurements, new insights can be gained into the phenomenology of entangled polymers in presence of startup shear, step strain and large amplitude oscillatory shear (LAOS). During startup shear of well entangled systems, the shear field becomes inhomogeneous after the stress overshoot for a range of applied shear rates beyond the Newtonian region [1]. The emergence of shear banding after stress overshoot helped us to identify the stress overshoot as indicating yielding, whose characteristics obey some scaling laws. In step shear, contrary to the conventional perception that entangled polymers would undergo quiescent relaxation, the PTV observations reveal macroscopic motions after shear cessation [2]. The recoil-like macroscopic motions appears to reflect an elastic breakdown of the entanglement network due to sufficient build-up of retractive forces. LAOS experiments also demonstrate that entangled polymers cannot sustain a high magnitude of fast deformation without undergoing cohesive failure [3]. [1] \textit{Macromolecules }\textbf{2008}, $41$, 2663 [2] \textit{Macromolecules }\textbf{2007}, $40$, 8031 [3] \textit{J. Rheol. }\textbf{2008}, $52$, 341. [Preview Abstract] |
Tuesday, March 17, 2009 12:03PM - 12:15PM |
J20.00005: Polymer Diffusion in Carbon Nanofiller / Polymer Nanocomposites Minfang Mu, Nigel Clarke, Russell Composto, Karen Winey Polymer tracer diffusion through carbon nanofiller / polymer nanocomposites is measured using elastic recoil detection methods. Tracer diffusion through a single wall carbon nanotube nanocomposite is strongly suppressed at low concentrations ($\le $ 0.4-0.8 vol{\%}) and then increases at higher concentrations. In contrast, the typical Maxwell model predicts only a weak monotonic decrease. We propose a model for the carbon nanotube composite system wherein the SWCNTs function as cylindrical traps. Simulations of this model found that at low concentrations, the isolated traps retard polymer diffusion and at higher concentrations the percolated traps allow polymer diffusion to recover by providing continuous pathways. A comparison of our experimental and simulation results finds that (1) the strength of the trap increases with the molecular weight of the diffusing polymers and (2) the trap diameter increases with the molecular weight of the matrix polymer. Similarly, tracer diffusion through C60 / polymer nanocomposites exhibits a significant decrease at low concentrations and then slowly increases at concentrations larger than 0.7 vol{\%}. [Preview Abstract] |
Tuesday, March 17, 2009 12:15PM - 12:27PM |
J20.00006: Tunable Wetting of Polymer Nanocomposite Films Marla McConnell, Shu Yang, Russell Composto Surfaces with controlled wettability are of growing technological importance. In this study, nanoparticles (NPs) with tunable spacing were assembled on poly(styrene-\textit{ran}-acrylic acid), S-$r$-AA, films to manipulate the composite films' wetting properties. Amine-modified silica NPs (15-200 nm) were covalently grafted to the AA moieties on the surface of the S-$r$-AA films, in which the S phase imparts mechanical stability and the AA domains swell, increasing the roughness and surface area. By controlling surface roughness and reaction time, NP coverage ranged from 1{\%}-70{\%}. These films displayed NP-coverage-dependent water contact angles between 60$^{\circ}$and 120$^{\circ}$. The enhanced hydrophobicity is attributed to capillary climbing of S-$r$-AA chains to cover the previously hydrophilic NP surface. Upon increasing NP diameter, the contact angle was found to increase at a fixed total coverage. This increase in attributed to the increase in effective surface area with increasing particle size. This system is utilized as a platform to create Janus particles with unique optical properties and templates for investigating molecular motors. [Preview Abstract] |
Tuesday, March 17, 2009 12:27PM - 12:39PM |
J20.00007: Direct Measurement of Molecular Mobility in Actively Deformed PMMA Glasses Hau-Nan Lee, Keewook Paeng, Stephen Swallen, Mark Ediger To quantitatively understand the response of segmental motions to external stress, we performed optical measurements of dye reorientation in PMMA glasses during tensile creep deformation. Up to 1000-fold increases in mobility are observed during deformation, which supports the view that stress-induced mobility allows plastic flow in polymer glasses. Although the Eyring model describes this mobility enhancement well at low stress, it fails to capture the dramatic mobility enhancement after flow onset. In this regime, in addition to lowering the barriers for molecular motion, external stress apparently forces the shape of distribution of relaxation times to narrow significantly. The effect of stress on physical aging was also investigated. At low stress, physical aging and deformation-induced mobility act as two independent processes. However, after flow onset, the data are consistent with the view that aging has been erased by deformation. [Preview Abstract] |
Tuesday, March 17, 2009 12:39PM - 12:51PM |
J20.00008: Probing Surface Glass Temperature of Polymer Films via Pentacene Growth Mode, Microstructure, and Thin-Film Transistor Performance Choongik Kim, Antonio Facchetti, Tobin Marks Pentacene-based organic thin-film transistors (OTFTs) have been extensively studied in organic electronics. In this study, we report the fundamental importance of the polymeric gate dielectric glass transition temperature on pentacene film growth mode, and microstructure and corresponding OTFT performance. From the knowledge that nanoscopically-confined thin polymeric films exhibit glass-transition temperatures deviated from the corresponding bulk materials, we show here that pentacene films grown on polymeric gate dielectrics at temperatures well-below their bulk glass transition temperature ($T_{g }$(b)) exhibit morphological/microstructural transitions and dramatic OTFT performance variations at a well-defined temperature [herein defined as the polymer surface glass transition temperature, or $T_{g }$(s)] characteristic of the polymer structure and independent of the film thickness. Our results demonstrate that TFT measurements represent a new methodology to probe polymer surface viscoelastic properties. [Preview Abstract] |
Tuesday, March 17, 2009 12:51PM - 1:03PM |
J20.00009: Responsive Polymer Surfaces: Crumpling, Folding, and Snapping Films. Douglas Holmes, Alfred Crosby This work focuses on understanding deformation mechanisms and responsiveness associated with folding, crumpling, and snapping of thin polymer films attached to patterned and nonpatterned substrates. By studying folding and crumpling in confined regimes, we gain insight into material properties, while developing new strategies for adhesive, optical, and patterning applications. Using a novel processing technique, microarrays of freestanding polydimethylsiloxane plates are placed in equibiaxial compression and transition through crumpled morphologies that are difficult to attain through traditional patterning techniques. The microstructures also change their curvature through a snap-through instability via environmental stimuli. When triggered via osmotic pressure the snap transition time scales as the square of the plate thickness and the inverse of the plate modulus. Recently, we have transferred this knowledge into the crumpling of ultrathin polymer films. We have fabricated sharply folded films directly on elastomeric and silicon substrates. The fold width scales directly with the film thickness and applied strain. We find that normally brittle, polystyrene films can accommodate excessive compressive strains without fracture by undergoing strain-localizing fold events. [Preview Abstract] |
Tuesday, March 17, 2009 1:03PM - 1:15PM |
J20.00010: Structure and mobility of PEO/LiClO4 solid polymer electrolytes Susan Fullerton, Janna Maranas Solid polymer electrolytes [SPEs] for use in rechargeable lithium-ion batteries offer many advantages over traditional liquid electrolytes, including mechanical flexibility and environmental friendliness. The practical limitation is that room temperature conductivity remains insufficient to power a portable device. While it is well-established that ion mobility is driven by polymer dynamics, high conductivity values have also been reported through fully crystalline SPEs. PEO-based SPEs have a rich phase behavior, and can form several crystalline complexes depending on the lithium concentration, temperature, and recrystallization time. We investigate the structure, mobility, conductivity, and thermal properties of both semi-crystalline and amorphous PEO/LiClO4 SPEs. Structure is measured with small-angle neutron scattering, and PEO mobility with quasi-elastic neutron scattering. We observe a decoupling of ionic conductivity and PEO mobility in a semi-crystalline sample. We also determine that PEO hydrogen atoms undergo restricted rotation on a circle. The radius of the circle is consistent with a cylindrical, crystalline structure that persists to some extent in the amorphous phase. The results suggest that directed ion transport via ordered structures is perhaps equally important as polymer mobility for increasing conductivity, provided that the structures percolate over large spatial scales. [Preview Abstract] |
Tuesday, March 17, 2009 1:15PM - 1:27PM |
J20.00011: Architectural effects in strongly hydrogen bonded thermoplastic elastomers Kathleen Feldman, Craig Hawker, Edward Kramer In this work we demonstrate the synthesis of random copolymers of \emph{n}-butyl acrylate with a quadruple hydrogen bonding acrylate monomer based on 2-ureido-4[1\begin{math}H\end{math}]-pyrimidinone (UPy). Despite low T$_g$s and a lack of crystallinity, these materials show thermoplastic elastomer properties through the strong but thermoreversible UPy groups. Through the use of controlled radical polymerization and post-polymerization functionalization we are able to reach high UPy monomer content while maintaining low polydispersity and excellent control over the total molecular weight. It was found that the average distance between UPys along the chain was the major determiner of the overall properties including the plateau modulus, tensile modulus, and relaxation timescale. By using a difunctional initiator it is also possible to synthesize materials containing a homopolymer midblock and random copolymer end blocks, allowing us to address the question of how the MHB group distribution along the chain affects the bulk properties. In concentrating the UPy groups near the chain ends, the plateau modulus remained constant but the crossover frequency decreased dramatically, indicating that the effective lifetime of the hydrogen bonds within the supramolecular network increased, in keeping with prior theoretical predictions. [Preview Abstract] |
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