Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session J11: Dillon Medal Symposium |
Hide Abstracts |
Sponsoring Units: DPOLY Chair: Arun Yethiraj, University of Wisconsin Room: 310 |
Tuesday, March 19, 2013 2:30PM - 3:06PM |
J11.00001: John H. Dillon Medal Lecture: Molecular Heterogeneity in Block Copolymer Self-Assembly Invited Speaker: Mahesh Mahanthappa Narrow molecular weight dispersity in block copolymers has long been considered necessary for well-defined, periodic structure formation, by analogy to various crystallization processes. Consequently, much attention has focused on narrow dispersity copolymers derived from controlled and ``living'' polymerization techniques. However, these methods restrict the palette of functional monomers amenable to block copolymerization, thus constraining the physical and chemical properties of the resulting materials. New polymer syntheses enable access to a ``Pandora's Box'' of block copolymers with unusual chemical functionalities and useful physical properties, at the expense of introducing significant segmental dispersities into the resulting copolymers. The development and use of these functional materials requires basic understanding of the physical implications of continuous segmental dispersity on block copolymer phase behavior. Our work aims to understand the physical principles underlying polydisperse ABA-type triblock copolymer self-assembly, in order to transform segmental dispersity into a predictable and useful tool for manipulating block copolymer morphology. We have systematically demonstrated that mid-segment dispersity in ABA triblock copolymers does not preclude the formation of classical, structurally periodic, microphase separated morphologies. Mid-segment dispersity instead shifts the locations of the composition-dependent phase windows, dilates the microdomains, and unexpectedly stabilizes the microphase separated ABA triblock copolymer melts. Studies of three different polydisperse copolymer systems have provided general insights into the consequences of chain length heterogeneity on block copolymer self-assembly. [Preview Abstract] |
Tuesday, March 19, 2013 3:06PM - 3:18PM |
J11.00002: Sphere Forming SISO Tetrablock Terpolymers Frank Bates, Jingwen Zhang, Sangwoo Lee A series of poly(styrene-$b$-isoprene-$b$-styrene-$b$-ethylene oxide) (SISO) tetrablock terpolymers was synthesized using anionic polymerization methods and investigated for melt morphology by synchrotron small-angle x-ray scattering (SAXS), electron microscopy and dynamic mechanical spectroscopy. Thermodynamic incompatibility between the I and O blocks, relative to that characterizing S and I and S and O leads to the formation of spherical domains containing an O core surrounded by a S rich shell embedded in a matrix of mixed S and I. Varying the composition and relative length of the internal and terminal S blocks resulted in five distinct states of ordering, including two cubic (Im3m and Pm3n symmetry), a hexagonal (p6/mm), a tetragonal (P4$_{\mathrm{2}}$/mnm) and a quasicrystalline (dodecagonal) phase. These results demonstrate the concept of decoupling domain geometry from domain packing in multiblock polymers. [Preview Abstract] |
Tuesday, March 19, 2013 3:18PM - 3:30PM |
J11.00003: What do we understand about equilibrium in block polymer micelles? Timothy Lodge The factors that dictate the choice of micellar morphology - sphere, worm, or vesicle - are generally well known. In contrast, the pathways by which micelles reach equilibrium (either in terms of morphology or aggregation number), are not. Even as simple a question as ``How does the critical micelle concentration depend on chain length?'' does not have a clear answer. The current situation will be summarized, in light of recent experimental results. [Preview Abstract] |
Tuesday, March 19, 2013 3:30PM - 3:42PM |
J11.00004: Melt and Solid-State Structures of Polydisperse Polyolefin Block Copolymers Richard Register, Sheng Li Recent developments in coordinative chain transfer polymerization have enabled the synthesis of ethylene-co-octene block copolymers, where the blocks are either crystallizable (an ethylene-co-octene random copolymer block with low octene content) or amorphous (analogous block with high octene content). With a suitable choice of catalyst type(s) and reactor train configuration, accessible chain architectures include diblock, where each block ideally has the most-probable distribution of chain lengths, and multiblock, where both the individual blocks and the number of blocks per chain follow the most-probable distribution. With a sufficiently large interblock octene differential, block copolymers of both architectures, containing roughly equal masses of the two types of block, self-assemble in the melt into well-ordered lamellar structures, despite the large polydispersity. Interblock mixing, induced by the modest Flory interaction parameter and the broad distribution of block lengths, yields an enormous domain spacing (\textgreater\ 100 nm) despite the relatively low average block molecular weights (\textless\ 50 kg/mol). Extensive interblock mixing also allows the polyethylene crystals to grow freely and nearly isotropically across the domain interfaces, while preserving the domain structure present in the melt; in the solid state, the optical and x-ray contrasts between dissimilar domains are greatly enhanced due to their different levels of crystallinity. (Work conducted in collaboration with Jeffrey Weinhold, Philip Hustad, and Brian Landes of Dow Chemical Core R{\&}D.) [Preview Abstract] |
Tuesday, March 19, 2013 3:42PM - 3:54PM |
J11.00005: Effects of molecular architecture and degree of hydration on the structure and properties of electrostatically self-assembled block copolymers Matthew Tirrell, Daniel Krogstad, Nathaniel Lynd, Jason Spruell, Soo-Hyung Choi, Craig Hawker, Edward Kramer Mixtures of water-soluble block copolymers, with one neutral block and the other blocks(s) either positively or negatively charged, are known to form micelles in water with micellar cross that are formed from polyelectrolyte complexes. At sufficiently high total polymer concentrations, such micellar suspensions undergo a disorder-order transition to a bcc structure. This typically occurs between 10 to 15{\%} polymer concentration. In this work, we present new data comparing the behavior of diblock copolymers and triblock copolymers. Initial results suggest that very similar final structures are formed in the two systems, when the diblock is just one of two symmetrical halves of the triblock. Kinetically, diblocks assemble much more rapidly and exhibit different rheological properties from triblocks. We are also investigating the structures formed and properties developed when these systems are dehydrated to less than 50{\%} water content. [Preview Abstract] |
Tuesday, March 19, 2013 3:54PM - 4:06PM |
J11.00006: Probing Nanoparticle Correlations in Filled Elastomers during Tensile Deformation by SAXS Edward J. Kramer, Arthur K. Scholz, Alexander Hexemer, Huan Zhang, Costantino Creton The 2D SAXS pattern from an unstrained 20 wt{\%} nanosilica-filled and crosslinked siloxane elastomer is isotropic and monotonically decreasing with scattering vector q, revealing a fractal aggregate structure of primary silica particles about 10 nm in radius. Under tensile strain along z, the invariant of the SAXS pattern, corrected for the change in sample thickness, is constant, demonstrating the absence of nanovoiding but the pattern itself shows a ``2 bar'' enhancement of intensity along z at q* = 2$\pi$/$<$z$>$. The distance $<$z$>$ and peak intensity Ip of the 2 bar pattern increase roughly linearly with extension ratio $\lambda$ until $\lambda$ $\sim$ 3 with $<$z$>$ saturating and Ip decreasing at higher $\lambda$s. Reverse Monte Carlo simulations of particle redistribution suggest that the silica aggregates separate into short rafts with compliant polymer in between along z; the extension ratio from $<$z$>$ of the nearly particle free polymer regions nearly matches $\lambda$ until $\lambda$ $\sim$ 3. For $\lambda$ $>$ 3 the rafts begin to break up, providing a partial explanation for the strong Mullins effect above $\lambda = 3$ for this filled elastomer. [Preview Abstract] |
Tuesday, March 19, 2013 4:06PM - 4:18PM |
J11.00007: Do thermally activated transitions influence the deformation of polymer glasses? Mark Ediger, Hau-Nan Lee, Benjamin Bending The availability of large scale computer simulations and new experiments allows fundamental questions about the influence of temperature on polymer glass deformation to be addressed from a microscopic perspective. Some recent simulations indicate that the total mobility induced during polymer glass deformation is a function of strain but independent of the strain rate. This result suggests that thermally activated transitions are not important during deformation which would be inconsistent with many models. We find that the integrated molecular mobility in polystyrene and PMMA glasses during deformation is roughly independent of strain rate. However, the relaxation time distribution narrows with increasing strain rate, indicating that thermally activated processes do play a role. [Preview Abstract] |
Tuesday, March 19, 2013 4:18PM - 4:30PM |
J11.00008: Self-Assembly of Gemini Surfactants Arun Yethiraj, Jagannath Mondal, Mahesh Mahanthappa The self-assembly behavior of Gemini (dimeric or twin-tail) dicarboxylate disodium surfactants is studied using molecular dynamics simulations. This gemini architecture, in which two single tailed surfactants are joined through a flexible hydrophobic linker, has been shown to exhibit concentration-dependent aqueous self-assembly into lyotropic phases including hexagonal, gyroid, and lamellar morphologies. Our simulations reproduce the experimentally observed phases at similar amphiphile concentrations in water, including the unusual ability of these surfactants to form gyroid phases over unprecedentedly large amphiphile concentration windows. We demonstrate quanitative agreement between the predicted and experimentally observed domain spacings of these nanostructured materials. Through careful conformation analyses of the surfactant molecules, we show that the gyroid phase is electrostatically stabilized related to the lamellar phase. By starting with a lamellar phase, we show that decreasing the charge on the surfactant headgroups by carboxylate protonation or use of a bulkier tetramethyl ammonium counterion in place of sodium drives the formation of a gyroid phase. [Preview Abstract] |
Tuesday, March 19, 2013 4:30PM - 4:42PM |
J11.00009: Exotic nanoparticles with block copolymer design and solution construction with kinetic control Darrin Pochan Kinetic pathways and temporal stabilities of different micelles and nanoscale aggregates have been used to construct exotic nanoparticles in solution. Due to low chain exchange dynamics between block copolymeric micelles and solvent, global thermodynamic equilibrium is extremely difficult, if not impossible, to achieve in block copolymer assembly. However, by taking advantage of this slow kinetic behavior of polymeric micelles in solution, one can purposely produce multicompartment nanoparticles and mulitgeometry nanoparticles by forcing different block copolymers to reside in the same nanoscale structure through kinetic processing. While kinetically trapped in common nanostructures, local phase separation can occur producing compartments. This compartmentalization can be used within common micelle geometries to make complex spheres and cylinders or can be used to make new nanostructures such as multigeometry aggregates (e.g. hybrid cylinder-sphere aggregates, disk-cylinder nanoparticles). [Preview Abstract] |
Tuesday, March 19, 2013 4:42PM - 4:54PM |
J11.00010: Polymer Solar Cell Device Characteristics Are Independent of Vertical Phase Separation in Active Layers Yueh-Lin Loo Preferential segregation of organic semiconductor constituents in multicomponent thin-film active layers has long been speculated to affect the characteristics of bulk-heterojunction polymer solar cells. Using soft-contact lamination and delamination schemes -- with which we have been able to remove compositionally well characterized polymer thin films, flip them over so as to reverse their composition profiles, and then transfer them onto existing device platforms -- we showed unambiguously that the device performance of P3HT:PCBM solar cells are independent of the interfacial segregation characteristics of the active layers. Temperature-dependent single-carrier diode measurements of the organic semiconductor constituents suggest that the origin of this invariance stems from the fact that P3HT comprises a high density of mid-gap states. Hole carriers in these mid-gap states can in turn recombine with electrons at the electron-collecting interface, effectively promoting electron transfer from the cathode to the active layer. [Preview Abstract] |
Tuesday, March 19, 2013 4:54PM - 5:06PM |
J11.00011: Dynamics of Magnetic Field Alignment of Block Copolymers by In-Situ SAXS Chinedum Osuji, Manesh Gopinadhan, Pawel Majewski The use of external fields to direct block copolymer self-assembly is well documented. Magnetic fields offer particular promise due to their space-pervasive nature and the ability to produce arbitrary alignments over truly macroscopic length scales in appropriate systems. We present here the results of in-situ SAXS studies performed using a custom superconducting magnet integrated with lab-scale x-ray scattering instruments. We consider the case of side-chain liquid crystalline diblock copolymers ordering under high magnetic fields. Despite the coincidence of the block copolymer order-disorder transition (ODT) and the LC clearing temperature in these weakly segregated materials, there is no measurable effect of the field on the ODT of the system, up to 6 T. This is in line with estimates based simply on the magnitudes of the relevant energy scales - the free energy of field interaction and the enthalpy of the isotropic-LC transition. We show that the alignment of the system is critically limited by the viscosity of the mesophase such that alignment can only be advanced by residence in a small temperature window near the ODT. This residence produces a weakly aligned system which thereafter transitions to a strongly aligned state on cooling even in the absence of the field. [Preview Abstract] |
Tuesday, March 19, 2013 5:06PM - 5:18PM |
J11.00012: Anion Transport in Hydrated Block Copolymers Nitash Balsara, Guillaume Sudre, Sebnem Inceoglu Polystyrene-block-polychloromethylstyrene (PS-b-PCMS) copolymers, were synthesized by nitroxide-mediated controlled radical polymerization. Separate aliquots of the PS-b-PCMS samples were quarternized to transform the PCMS block. This resulted in block copolymers with ionizable blocks. We refer to ion-containing block copolymers synthesized from the same precursor as matched pairs: SAM (containing trimethylammonium chloride) and SIM (containing n-butylimidazolium chloride). Self-assembly in these copolymers resulted in the formation of lamellar phases regardless of composition, chemical formula of the bound ion, and chain length. Chloride ion conductivity and water uptake measurements on one of the matched pairs led to similar results. The chloride ions in this matched pair were replaced by hydroxide ions and the changes in conductivity due to this are reported. [Preview Abstract] |
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