Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session G31: Padden Award Symposium |
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Sponsoring Units: DPOLY Chair: Nitash Balsara, University of California at Berkeley Room: 339 |
Tuesday, March 19, 2013 11:15AM - 11:27AM |
G31.00001: Scaling Reversible Adhesion in Synthetic and Biological Systems Michael Bartlett, Duncan Irschick, Alfred Crosby High capacity, easy release polymer adhesives, as demonstrated by a gecko's toe, present unique opportunities for synthetic design. However, without a framework that connects biological and synthetic adhesives from basic nanoscopic features to macroscopic systems, synthetic mimics have failed to perform favorably at large length scales. Starting from an energy balance, we develop a scaling approach to understand unstable interfacial fracture over multiple length scales. The simple theory reveals that reversibly adhesive polymers do not rely upon fibrillar features but require contradicting attributes: maximum compliance normal to the substrate and minimum compliance in the loading direction. We use this counterintuitive criterion to create reversible, easy release adhesives at macroscopic sizes (100 cm$^{2}$) with unprecedented force capacities on the order of 3000 N. Importantly, we achieve this without fibrillar features, supporting our predictions and emphasizing the importance of subsurface anatomy in biological adhesive systems. Our theory describes adhesive force capacity as a function of material properties and geometry and is supported by over 1000 experiments, spanning both synthetic and biological adhesives, with agreement over 14 orders of magnitude in adhesive force. [Preview Abstract] |
Tuesday, March 19, 2013 11:27AM - 11:39AM |
G31.00002: Cavitation in block copolymer modified epoxy Carmelo Declet-Perez, Lorraine Francis, Frank Bates Today, brittleness in epoxy networks limits most commercial applications. Significant toughness can be imparted by adding small amounts of micelle forming block copolymers (BCP) without compromising critical properties such as high use temperature and modulus. Curing the network locks in the self-assembled BCP micellar structures formed in the monomer resin providing control of the resulting morphology. Despite significant research over the last decade, a complete description of the parameters influencing toughness in block copolymer modified epoxies is still lacking. In this presentation we compare the ultimate mechanical behavior of epoxies modified with spherical micelle forming BCP's containing rubbery and glassy cores using real-time in-situ small-angle X-ray scattering (SAXS) performed during tensile deformation. Striking differences in the 2D SAXS patterns were documented for epoxies modified with rubbery (PEP) versus glassy (PS) micelle cores. Rubbery cores dilate by 100{\%} in volume upon specimen yielding, while the glassy micelle cores deform at approximately constant volume. These results provide direct evidence of a cavitation mediated mechanism for toughness in block copolymer modified epoxies. We further interpret characteristic butterfly features in the 2D SAXS patterns in terms of epoxy network deformation. [Preview Abstract] |
Tuesday, March 19, 2013 11:39AM - 11:51AM |
G31.00003: Polymer Welding and Self-healing: Strength Through Entanglements Ting Ge, Mark O. Robbins, Dvora Perahia, Gary S. Grest Polymer interfaces are crucial in determining the mechanical strength of many systems. A common means of welding joints or self-healing cracks is to apply heat and allow polymers to interdiffuse. As the microscopic mechanism of interface strengthening is difficult to isolate experimentally, we probe the molecular origins of interfacial strength using large scale molecular simulations of welding and self-healing of cut systems. Systems are heated well above the glass temperature $T_{g} $ and then quenched below $T_{g} $ for mechanical testing. The interfacial strength is characterized by the maximum shear stress $\sigma_{\max } $ before failure. As strength grows, the dominant failure mode changes from chain pullout at the interface to chain scission, as in the bulk. In all simulations, $\sigma_{\max } $ saturates long before polymers diffuse by their own size. Bulk strength is observed for miscible welds, while strength is suppressed for cut systems due to short chain segments that remain near the interface. Entanglements are tracked using the Primitive Path Analysis. We find that the bulk response is not fully recovered until the density of entanglements at the interface reaches the bulk value. Moreover, the increase of $\sigma_{\max } $ before saturation is proportional to the number of interfacial entanglements between chains from opposite sides, which correlates linearly with the interdiffusion depth. [Preview Abstract] |
Tuesday, March 19, 2013 11:51AM - 12:03PM |
G31.00004: Magnetically aligned polymer-nanowire composites for solar energy harvesting Pawel Majewski, Candice Pelligra, Chinedum Osuji We present a solution-based approach of producing aligned arrays of ZnO nanowire-polythiophene composites for photovoltaic applications. We employ a two-step hierarchical self-assembly to maximize the efficiency of electron and hole transport in the system. First, we coat the wires with the polymer utilizing nanowire surface-directed crystallization and alignment of the polymer backbones along the long axes of the wires, then we employ magnetic fields to direct the assembly of the composites into the ordered arrays. We present quantitative SAXS data taken in-situ during the alignment process addressing the influence of paramagnetic doping level of ZnO and the magnetic field strength on the quality of the alignment. We compare the electrical conductivity of the aligned arrays of the composites to non-aligned ones and discuss the possible degree of conductivity enhancement upon the alignment in this and in analogous systems. [Preview Abstract] |
Tuesday, March 19, 2013 12:03PM - 12:15PM |
G31.00005: Self-similarity and energy dissipation in stepped polymer films Joshua McGraw, Thomas Salez, Oliver Baeumchen, Elie Raphael, Kari Dalnoki-Veress We have recently learned how to prepare polymer films whose only feature is a step in the height profile. In the melt, Laplace pressure drives a flow that levels the topography, with the excess energy of the height step being dissipated by viscosity. It has been observed that the profiles are self-similar in time for a variety of molecular weights and geometries. Given the surface tension, this simple observation allows a precise measurement of the viscosity by comparison with numerical solutions of the thin film equation. It is also possible to derive a master expression for the time dependence of the excess surface energy as a function of the material properties and film geometry. Thus, all geometries and molecular weights fall on a single temporal curve. The material parameter allowing this collapse is the capillary velocity -- the ratio of the surface tension to the viscosity. [Preview Abstract] |
Tuesday, March 19, 2013 12:15PM - 12:27PM |
G31.00006: The Consequence of Donor-acceptor Miscibility on Charge Transport and Photovoltaic Device Performance Kiarash Vakhshouri, Derek Kozub, Chenchen Wang, Alberto Salleo, Enrique Gomez Recent energy-filtered transmission electron microscopy studies revealed that amorphous mixed phases are ubiquitous within mesostructured polythiophene/fullerene mixtures. The role of mixing within nanophases on charge transport of organic semiconductor mixtures, however, is not fully understood. Through the combination of Flory-Huggins theory and energy-filtered transmission electron microscopy, we have estimated the miscibility limit of polythiophene/fullerene blends. We have also demonstrated the interplay between miscibility and percolation to describe field-effect mobilities as a measure of the conductive pathways present in a model organic semiconductor mixture (amorphous polythiophene/fullerene blends). Our studies reveal that the miscibility of the components strongly affects electron transport within amorphous blends. Immiscibility promotes efficient electron transport by promoting percolating pathways within organic semiconductor mixtures. However, strongly immiscible systems would readily phase separate into large domains, preventing efficient charge separation in organic photovoltaics. Consequently, an optimum degree of miscibility between donor/acceptor mixtures exists for the application of such mixtures to organic solar cells. [Preview Abstract] |
Tuesday, March 19, 2013 12:27PM - 12:39PM |
G31.00007: Theory of Polymers in Poor Solvent: Phase Equilibrium, Nucleation Behavior and Globule-to-Coil Transition Rui Wang, Zhen-Gang Wang We study the phase equilibrium and nucleation behavior of polymers in poor solvent by accounting for the large, localized fluctuations in the form of single-chain globules and multi-chain clusters. The density profile and free energy of the globule and clusters are obtained by self-consistent-field theory, which is then used in the dilute solution thermodynamics to investigate the cluster size distribution, solubility limit, as well as nucleation in the supersaturated state. Our results show that the solubility of the polymer in the dilute side of the solution is enhanced by several orders of magnitude relative to the prediction of the Flory-Huggins (F-H) theory, which scales with the chain length to the 2/3 power rather than a linear power as predicted from the F-H theory. In the supersaturated state, we work out an effective spinodal where the nucleation barrier to phase separation via growth of the clusters becomes comparable to the thermal energy. For a given supersaturation, we find that the nucleation barrier is quadratic in the chain length, suggesting a much slower precipitation rate for longer polymer chains. Tracking the density profile of the globule with decreasing $\chi$, we find the critical $\chi$ for the globule-to-coil transition of an infinitely long chain. [Preview Abstract] |
Tuesday, March 19, 2013 12:39PM - 12:51PM |
G31.00008: Dramatic role of fragility in determining the magnitude of T$_{\mathrm{g}}$ perturbations to ultrathin film layers and near-infinitely dilute blend components Christopher Evans, John Torkelson Using fluorescence, we measure the glass transition temperatures (T$_{\mathrm{g}})$ of ultrathin (11-14 nm) polystyrene (PS, bulk T$_{\mathrm{g}} =$ 103 $^{\circ}$C) layers which can be tuned over $\sim$ 80 $^{\circ}$C when sandwiched between two bulk neighboring layers of poly(4-vinyl pyridine) (P4VP), polycarbonate, poly(vinyl chloride) (PVC) or poly(tert-butyl acrylate). Between P4VP, an ultrathin PS layer has its dynamics slaved and reports the T$_{\mathrm{g}}$ of bulk P4VP. In contrast, an ultrathin PS layer is weakly perturbed (T$_{\mathrm{g}} =$ 97 $^{\circ}$C) when placed between PVC. These perturbations to the PS T$_{\mathrm{g}}$ become evident even for layers 10s of nanometers in thickness. Additionally, binary blends were prepared with 0.1 wt{\%} PS components surrounded by the same neighboring polymers as in the trilayers. The T$_{\mathrm{g}}$ reported by an ultrathin PS layer and a 0.1 wt{\%} PS blend component are the same for a given polymer pair indicating that the T$_{\mathrm{g}}$ perturbations in these two systems arise from a common physical origin. The strength of perturbations to PS correlate with the fragility of the neighboring domain in both blends and multilayers indicating that it is a key variable in determining the strength of T$_{\mathrm{g}}$-confinement effects. Fragility also tracks with the magnitude of T$_{\mathrm{g}}$-confinement effects observed in single layer polymer films supported on silicon wafers. [Preview Abstract] |
Tuesday, March 19, 2013 12:51PM - 1:03PM |
G31.00009: Confined Crystallization in Poly(3-alkylthiophene)-containing Diblock Copolymers Victor Ho, Rachel Segalman Organic optoelectronic device active layers require optimization of both the crystalline structure and the morphology at the nanometer length scale. These can be controlled simultaneously with a block copolymer in which one component is a crystalline conjugated polymer such as poly(3-alkylthiophene) (P3AT). While self-assembly of these systems requires balancing the driving forces of crystallization and self-assembly, in many systems, crystallinity dominates resulting in significant distortion or destruction of the melt phase structure. However, we show that judicious selection of the alkyl side chain in P3ATs results in melting transitions which can be controlled over a range of 150 C, and when incorporated into a block copolymer, these depressed melting transitions lead to regions of phase space for which the strength of segregation is sufficiently high at crystallization to allow for self-assembly. Phases such as crystalline majority-phase hexagonally-packed cylinders and lamellae are observed, and importantly the crystallinity of the conjugated polymer is retained in these confined geometries. [Preview Abstract] |
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