Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session V42: Focus Session: Supramolecular Self-Assembly--Controlling Network and Gel Formation II |
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Sponsoring Units: DPOLY DBP Chair: Nigel Clarke, University of Durham Room: A302/303 |
Thursday, March 24, 2011 8:00AM - 8:36AM |
V42.00001: Control of semi-flexible polymer networks by architecture and dynamic cross-linking Invited Speaker: The rigidity of elastic networks depends sensitively on their internal connectivity and the nature of interactions between constituents. Particles interacting via central forces become rigid above the isostatic connectivity threshold first identified by Maxwell. Stiff or semi-flexible polymers, such as those that form the cellular cytoskeleton, develop a finite network shear modulus $G$ at a lower threshold, although the degree to which the mechanics of such networks are governed by filament bending resistance remains a subject of considerable debate. Such networks also exhibit rich viscoelastic properties. For cytoskeletal networks, there is increasing evidence that the network response is governed by the compliance and dynamics of the cross-links, many of which are transient in nature. Here we study the effects of both local network architecture and dynamic cross-linking in disordered fibrous networks. Surprisingly, the network mechanics in both 2D and 3D are still governed by the central-force isostatic point, which acts as a zero-temperature critical point. Near this point, we find divergent strain fluctuations and an associated diverging length-scale, as well as an anomalous elastic regime that exhibits fractional power-law dependence of $G$ on both fiber bending stiffness and stretch modulus. Furthermore, dynamic cross-linking gives rise to a broad, power-law viscoelastic regime at low frequency \textit{$\omega $} in which $G\sim $\textit{$\omega $}$^{1/2}$. [Preview Abstract] |
Thursday, March 24, 2011 8:36AM - 8:48AM |
V42.00002: Macro- and microphase separation in multifunctional supramolecular polymer networks Zoltan Mester, Aruna Mohan, Glenn Fredrickson We develop a field-based model for a binary melt of multifunctional polymers that can reversibly bond to form copolymer networks. The mean-field phase separation behavior of several model networks with heterogeneous bonding is calculated via the random phase approximation (RPA). The extent of bonding between polymers is controlled by specified bond energies. The phase boundary calculated via RPA is the stability limit of the homogeneous disordered phase to coexisting homogeneous macrophases, for low bond strengths, and to microphases, for high bond strengths. An isotropic Lifshitz point separates these two regions along the spindodal boundary. It is demonstrated that higher functionality and higher bond strength suppresses macrophase separation due to greater connectivity between unlike species. Gelation first occurs at a bond strength higher than the Lifshitz point for tri- or higher functional polymer components. [Preview Abstract] |
Thursday, March 24, 2011 8:48AM - 9:00AM |
V42.00003: Equilibrium and nonequilibrium gelation in TPR protein/linker mixtures Tianqi Shen, Robert S. Hoy, Corey S. O'Hern Using simulations we model gelation in two-component systems consisting of tetratricopeptide repeat (TPR) proteins and peptide cross linkers. These have recently been shown [1] to form strong, mechanically stable gels with remarkable shape recovery - but only within narrow parameter regimes. Within our minimal, coarse grained model, we elucidate the effects of the packing fraction $\phi$, temperature $T$ and concentration ratio $r$ of TPR and cross linkers on the gel transition. Two gelation mechanisms are identified. At low $\phi$ and $T$, nonequilibrium microphase-separated gels may be formed by rapid temperature quenches. At higher $\phi$ and $T$, homogeneous equilibrium gelation occurs. At low $r$, gelation is suppressed due to depletion of linkers, while at high $r$ gelation is suppressed due to the ``coating'' of proteins by linkers. The gel transition line in the $(r,T)$ plane has an unusual, asymmetric form. We also briefly compare these results to those for a more realistic ``patchy'' model which incorporates the directional TPR-linker binding present in the experimental systems.\\[4pt] [1] T.\ Z.\ Grove \textit{et. al.}, JACS, \textbf{132}, 14024 (2010). [Preview Abstract] |
Thursday, March 24, 2011 9:00AM - 9:12AM |
V42.00004: Effects of inclusions on the dynamics of viscoelastic media Stephen Mirigian, Murugappan Muthukumar The modification of material properties due to the presence of inclusions in solutions and elastic composites are well known, modifying properties such as viscosity and elastic moduli. We calculate the mobility of such an inclusion in a viscoelastic medium as well as effective dynamic material properties due to the presence of such inclusions at various frequencies. [Preview Abstract] |
Thursday, March 24, 2011 9:12AM - 9:24AM |
V42.00005: Conformation and mechanical properties of diblock fibers Sumanth Swaminathan, Francisco Solis, Monica Olvera de la Cruz We analyze the conformations of closed diblock fibers comprised of different bending rigidities and spontaneous curvatures. In each fiber, one block is a bare polymer while the other is an adsorbed protein-filament complex. The length fraction of each component and the total fiber length is controlled by tunable chemical potentials. We analytically calculate the shape of these two-component polymers for all values of the material parameters and chemical potentials. Our results yield: a complete analytical description of all possible two-component polymer conformations in two and three dimensions, a phase portrait detailing the parameter spaces in which these shapes occur, and the identification of spontaneous transitions between shapes driven by environmental changes. [Preview Abstract] |
Thursday, March 24, 2011 9:24AM - 9:36AM |
V42.00006: The Role of Multiple, Reformable Parallel Bonds on the Self-healing Behavior of Dual Crosslinked Nanogel Materials Isaac G. Salib, German V. Kolmakov, Chet N. Gnegy, Krzysztof Matyjaszewski, Anna C. Balazs Using computational modeling, we design novel self-healing materials composed of nanoscopic polymer gel particles, or nanogels. The particles are interconnected via both labile bonds (e.g., disulfide bonds) and stronger, less reactive bonds (e.g, C-C bonds) and therefore the nanogels form a ``dual crosslinked'' network. The stable bonds provide a rigid backbone while the labile bonds allow the material to undergo a dynamic reconfiguration in response to stress. We adapt the Hierarchical Bell Model (HBM) to describe the labile bonding interactions. The HBM effectively allows us to model cases where the ligands on neighboring nanogels interact through multiple sites. We show that the introduction of a small number of labile bonds that lie in parallel significantly increases the strength of the material relative to samples crosslinked solely by the stable bonds. We also isolate an optimal range of labile interconnections that provide high-strength, tough materials that are capable of self-repair. [Preview Abstract] |
Thursday, March 24, 2011 9:36AM - 9:48AM |
V42.00007: Forming Reversible Gels with Triblock Polyelectrolytes: a Field-theoretic Study Debra Audus, Glenn Fredrickson Recently, two research groups have formed reversible gels using triblock polyelectrolytes (Lemmers et al. 2010; Hunt et al., in preparation). This gel formation is driven by a phenomenon called complex coacervation, in which two oppositely charged homopolymers in solution phase separate into a polymer rich phase, known as a coacervate, and a solution phase. If instead, the polymers are triblocks with a neutral midblock and charged end blocks, under appropriate conditions they will microphase separate into micelles with cores of coacervated charged groups and coronas of neutral midblocks. These neutral midblocks act as bridges between the micelles, thereby creating a gel. One of the advantages of forming gels in this way is that the coacervate domains, and thus the gel, can be easily tuned by varying parameters such as pH, salt concentration and temperature. In order to understand the microstructures and solution sensitivity of these reversible gels, we have numerically simulated field-theoretic models of triblock polyelectrolyte mixtures in an implicit solvent. Because coacervation is driven by charge correlations, the usual mean-field assumption fails, and it is necessary to study the model beyond the level of SCFT. [Preview Abstract] |
Thursday, March 24, 2011 9:48AM - 10:00AM |
V42.00008: Supramolecular Arrest and Activation for the Network Formation of Acid Catalyzed Epoxy Polymerization Matthew Spencer, James Crivello, Chang Y. Ryu Epoxy resins are limited currently as they must be externally activated by radiation in combination with a photo acid generator or implemented using the more ubiquitous two-component approach. Two component systems begin to react upon mixing are while UV cured epoxies are limited to situations where light can reach the monomer. We propose a novel one-component system that is externally activated with the application of heat. A considerable room temperature lifetime is attributed to the systems ability to sequester super acids through a system of hydrogen bonding coordination. The model system utilizes an alkyl glycidal ether which is made universal by the addition of crown ethers to non-ether epoxy monomers. Our supramolecular-based approach to retard and trigger the epoxy polymerization is likely to enable more widespread applications in microelectronic packaging. [Preview Abstract] |
Thursday, March 24, 2011 10:00AM - 10:12AM |
V42.00009: Polymer Phononic Meta-material Networks Cheongyang Koh, Edwin Thomas Phononic Meta-materials (PMM) offer unique opportunities for molding the flow of phonons through artificial structuring of the material at relevant length scales; however, most structures rely on combining mechanically ``stiff'' and ``soft'' materials to create the desired phononic properties, usually focusing on resonances to stop phonon flow. Such an approach suffers from lack of scalability, placing fabrication and material compatibility constraints on technological realization. Here, we show that these constraints are unnecessary and that phonon propagation behavior relies on the fundamental requirements of avoided crossings in the frequency dispersion relations. In particular, we demonstrate 1) polymer/air PMMs possessing i) multiple complete spectral gaps (MCSG), ii) negative index bands, iii) both a complete sub-wavelength transverse gap and Bragg-type longitudinal gap and 2) waveguides of pma2 \textit{Frieze} group symmetry that possess MCGS; we verify their dispersion relation using Brillouin light scattering. This opens up the ability to develop novel integrated low-cost all-polymer phononic platforms for information processing via mesoscale polarization manipulation, filtering and superlensing. [Preview Abstract] |
Thursday, March 24, 2011 10:12AM - 10:24AM |
V42.00010: Formation of Anisotropic Block Copolymer Gels Chya Yan Liaw, Kenneth Shull, Kevin Henderson, Derk Joester Anisotropic, fibrillar gels are important in a variety of processes. Biomineralization is one example, where the mineralization process often occurs within a matrix of collagen or chitin fibers that trap the mineral precursors and direct the mineralization process. We wish to replicate this type of behavior within block copolymer gels. Particularly, we are interested in employing gels composed of cylindrical micelles, which are anisotropic and closely mimic biological fibers. Micelle geometry is controlled in our system by manipulating the ratio of molecular weights of the two blocks and by controlling the detailed thermal processing history of the copolymer solutions. Small-Angle X-ray Scattering and Dynamic Light Scattering are used to determine the temperature dependence of the gel formation process. Initial experiments are based on a thermally-reversible alcohol-soluble system, that can be subsequently converted to a water soluble system by hydrolysis of a poly(t-butyl methacrylate) block to a poly (methacrylic acid) block. [Preview Abstract] |
Thursday, March 24, 2011 10:24AM - 10:36AM |
V42.00011: Microstructural Organization of Elastomeric Polyurethanes with Siloxane-Containing Soft Segments Taeyi Choi, Jadwiga Weklser, Ajay Padsalgikar, James Runt In the present study, we investigate the microstructure of two series of segmented polyurethanes (PUs) containing siloxane-based soft segments and the same hard segments, the latter synthesized from diphenylmethane diisocyanate and butanediol. The first series is synthesized using a hydroxy-terminated polydimethylsiloxane macrodiol and varying hard segment contents. The second series are derived from an oligomeric diol containing both siloxane and aliphatic carbonate species. Hard domain morphologies were characterized using tapping mode atomic force microscopy and quantitative analysis of hard/soft segment demixing was conducted using small-angle X-ray scattering. The phase transitions of all materials were investigated using DSC and dynamic mechanical analysis, and hydrogen bonding by FTIR spectroscopy. [Preview Abstract] |
Thursday, March 24, 2011 10:36AM - 10:48AM |
V42.00012: Viscoelastic Properties of Photo-crosslinked Shape Memory Elastomers Jiahui Li, Christopher Lewis, Darcy Chen, Mitchell Anthamatten Lightly crosslinked polymer networks containing self-complementary hydrogen-bonding side-groups (e.g. ureidopyrimidinones, UPy) have been shown to exhibit unique shape-memory properties. Our synthetic approach, involving photo-crosslinking, enables both the crosslink density and UPy-content to be systematically varied. To better understand how hydrogen bond dynamics impact viscoelastic properties, dynamic mechanical analysis was performed on a series of photo-crosslinked elastomers. The presence of UPy side-groups imparts a broad dynamic transition over a frequency range that depends on the UPy content. The UPy side-group dynamics result in a high level of mechanical damping, and they enable damping characteristics to be tailored. Time temperature superposition (TTS) analysis was performed, and resulting shift factors show Arrhenius behavior. Activation energies were calculated, and elastomers with higher UPy content exhibit higher activation energies. [Preview Abstract] |
Thursday, March 24, 2011 10:48AM - 11:00AM |
V42.00013: Origins and stability of the polydomain regime in isotropic-genesis nematic elastomers Bing Lu, Fangfu Ye, Xiangjun Xing, Paul Goldbart We address the physical properties of nematic elastomers that have been randomly crosslinked in the high-temperature isotropic state. We do this by constructing a replica Landau theory in terms of a coupled pair of order-parameter fields: one for vulcanization, the other for nematic order. We focus on the correlations of the trapped-in nematic fluctuations as a diagnostic of the structure of the elastomer, determining them for a range of temperatures and disorder strengths. Our analysis shows that, in fewer than four spatial dimensions, the quenched randomness associated with the crosslinking prevents the emergence of long-range order, either of the mondomain nematic or of the spatially modulated type. It also shows that, for sufficiently strong disorder and low enough temperatures, the system exhibits unusual short-range oscillatory structure in the local nematic alignment. [Preview Abstract] |
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