Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session B24: Focus Session: Reversibly Associating Polymers: Theory & Experiments |
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Sponsoring Units: DPOLY Chair: Sanat Kumar, Columbia University Room: Colorado Convention Center 201 |
Monday, March 5, 2007 11:15AM - 11:51AM |
B24.00001: Reversible Associating Polymers as Biological Mimics Invited Speaker: |
Monday, March 5, 2007 11:51AM - 12:03PM |
B24.00002: Architectural effect on the self-assembly of supramolecular triblock copolymer melts Won Bo Lee, Richard Elliott, Kirill Katsov, Glenn H. Fredrickson Thermoreversible, supramolecular self-assembly in multi-block copolymer melts is studied within the framework of self-consistent field theory. This approach is adapted to study a system composed of two chemically distinct, but reactive homopolymer species: a linear A-homopolymer with a single reactive group at one of the ends, and a linear B-homopolymer with reactive groups at both ends. Reversible bonding occurs between the functional groups from different polymer species so that the reacting system can contain A, B, AB and ABA (co)polymer species whose overall volume fractions are controlled by the segmental incompatibility, bonding strength and homopolymer chain lengths. Architectural variations of these copolymers, arising from the differing lengths of A and B homopolymers, have a dramatic effect on not only the micro-phase separation but also on the extent of reversible bonding. Two characteristic phase diagrams are constructed to illustrate this behavior and possible technological applications are discussed. [Preview Abstract] |
Monday, March 5, 2007 12:03PM - 12:15PM |
B24.00003: Osmotic Properties of Acrylic Triblock Copolymer Gels Rafael E. Bras, Kenneth R. Shull Triblock copolymer solutions are of interest both practically and scientifically. This interest stems largely from their ability to self assemble into soft elastic solids. Acrylic triblock copolymers solvated in alcohols are of particular interest as they exhibit a rapid thermo-reversible transition from a viscous liquid at elevated temperatures (but still less than 100 C) to a strong elastic solid at room temperature. The rapid transition is a result of the increasing interaction parameter between the polymer endblock and solvent upon cooling, resulting in self assembly and the eventual stabilization of endblock aggregates below the endblock glass transition temperature. Several methods, including vapor pressure osmometry, small angle x-ray scattering, and rheometry, were used to examine the properties of semidilute PMMA-PnBA-PMMA acrylic triblock solutions in the temperature regime near the critical micelle temperature. The measured osmotic pressure is controlled by contacts between the well solvated midblocks, the solvent-endblock interaction energy, and the elasticity of the triblock network. [Preview Abstract] |
Monday, March 5, 2007 12:15PM - 12:27PM |
B24.00004: Computational study of gel transition and jamming in an ensemble of reversible associating polymers Arlette Baljon, Danny Flynn, David Krawzsenek The sol/gel transition is studied in model telechelic polymers. We find that, as in the case of glass-forming liquids, the transition range is very broad. Four characteristic temperatures for gel formation are calculated: (1) the temperature below which relaxation times no longer show Arrhenius dependence on temperature; (2) the Vogel-Fulcher temperature at which the structural relaxation time extrapolates to infinity; (3) the analogue of the crossover temperature defined by Mode Coupling Theory; (4) the ``jamming'' transition temperature at which the number of reversible bonds sharply increases and their distribution over micelles changes qualitatively. These temperatures are obtained from measurements of diffusivity, specific heat, and network topology. The effect of external shear will be discussed as well. [Preview Abstract] |
Monday, March 5, 2007 12:27PM - 12:39PM |
B24.00005: Phase Behavior of Semi-Flexible Polymer Gels Venkatram H. Padmanabhan, Sanat K. Kumar Histogram Reweighting Monte Carlo simulations were performed to obtain the sol-gel phase diagrams for a system of semi-flexible polymer chains in a cubic box with periodic boundary conditions. Our calculations stress the importance of patchiness -- in its absence the systems form standard liquid crystalline phases. In contrast, for strong patch interactions, with decreasing temperature, the chains experience a local ``arrest'' in positions due to the interchain interactions, thus slowing down its dynamics. The physical aggregation of these chains is similar to that of Flory Type -- III gels. [Preview Abstract] |
Monday, March 5, 2007 12:39PM - 12:51PM |
B24.00006: Modeling the Crystallization of Proteins Hongjun Liu, Sanat Kumar, Shekhar Garde We have used molecular dynamics and monte carlo simulations to understand the pathway to protein crystallization. We find that models which ignore the patchy nature of protein-protein interactions only crystallize inside the metastable gas-lqiuid coexistence region. In this regime they crystallize through the formation of a critical nucleus. In contrast, when patchiness is introduced we find that there is no need to be inside this metastable gas-liquid boundary. Rather, crystallization occurs through an intermediate which is composed of disordered aggregates. These are formed by patchy interactions. Further, there appears to be no need for the formation of a critical nucleus. Thus the pathways for crystallization are strongly controlled by the nature of protein-protein interactions, in good agreement with current experiments. [Preview Abstract] |
Monday, March 5, 2007 12:51PM - 1:03PM |
B24.00007: Physical Properties of Anionic Peptide Amphiphile Fibers Grown in the Presence of Polyion Salt Megan Greenfield, Yuri Velichko, Samuel Stupp, Monica Olvera de la Cruz We analyze the structure and mechanical properties of self-assembled gels formed by anionic peptide amphiphiles (PA) in the presence of cationic peptides and polyion salt. The PA molecules, which are composed of a hydrophobic alkyl tail, a beta-sheet forming region, and a hydrophilic epitope region, self-assemble into cylindrical micelles in water with multivalent salt. The fibers grow in one dimension by forming an internal beta sheet along the middle segment; the hydrophobic tail hides inside the fiber and the epitope region is exposed on the surface. Rheology and electron microscopy are used to investigate the physical properties of the resulting PA gels. The PA-fibers form a self-supporting gel at a concentration of one weight percent. Our experimental results show a strong dependence on the nature and valency of the polyions. We will present a theoretical model that incorporates both PA self-assembly and gelation of PA-fibers in the presence of polyion salt. [Preview Abstract] |
Monday, March 5, 2007 1:03PM - 1:15PM |
B24.00008: Reversible and Tunable Network Formation of Ca$^{2+}$-Sensitive Biomaterials Shana Topp, Vikram Prasad, Gianguido C. Cianci, Eric R. Weeks, Justin P. Gallivan A major goal of polymer science is to develop ``smart'' materials that sense chemical signals in complex environments and respond with predictable changes in their mechanical properties. We present a genetic toolbox of natural and engineered protein modules that can be rationally combined in manifold ways to create reversible self-assembling materials that vary in their composition, architecture, and mechanical properties. The protein calmodulin, which binds Ca$^{2+}$ with micromolar affinity but does not bind Mg$^{2+}$, was employed to develop materials that could discriminate between the two cations. Using calmodulin and other modules in the toolbox, we produced several materials that reversibly self-assemble in the presence of Ca$^{2+}$ and characterized these materials using particle-tracking microrheology. Since the properties of these materials could be predicted from the dilute solution behavior of their component modules, an expanded toolbox may be generally useful for creating new stimuli-sensitive materials. [Preview Abstract] |
Monday, March 5, 2007 1:15PM - 1:27PM |
B24.00009: Folding and Aggregation of Mucin Domains. Brigita Urbanc, Rama Bansil, Bradley Turner Mucin glycoproteins consist of tandem repeating glycosylated regions flanked by non-repetitive protein domains with little glycosylation. These non-repetitive domains are involved in polymerization of mucin via disulfide bonds and play an important role in the pH dependent gelation of gastric mucin, which is essential to protecting the stomach from autodigestion. We have examined the folding and aggregation of the non-repetitive sequence of von Willebrand factor vWF-C1 domain (67 amino acids) and PGM 2X (242 amino acids) using Discrete Molecular Dynamics (four-bead protein model with hydrogen bonding and amino acid-specific hydrophobic/hydrophilic and electrostatic interactions of side chains). Simulations of vWF C1 show 4-6 $\beta $-strands separated by turns/loops with more loops at lower pH. A simulation of several vWF C1 proteins at low pH shows aggregates still with a high content of $\beta $-strands and enhanced turn/loop regions. For the PGM 2X simulation the contact map shows several salt bridges enclosing hairpin turns. The implications of these simulations for describing the aggregation/gelation of PGM will be discussed. [Preview Abstract] |
Monday, March 5, 2007 1:27PM - 1:39PM |
B24.00010: Shape-Memory Network Polymers Containing Reversible H-Bonding Associating Groups Jiahui Li, Mitchell Anthamatten Thermally reversible secondary interactions such as H-bonding can be used to stabilize mechanically strained states at low temperatures. A new type of lightly crosslinked shape-memory polymer is reported which contains reversible H-bonding ureidopyrimidinone (UPy) side-groups. Butyl acrylate, UPy containing monomer, and crosslinkable monomers were copolymerized and cast into films. Experiments on those elastomers using thermal-mechanical analysis showed clear shape-memory effects. The shape recovery ratio of those elastomers is nearly 100{\%}, and the shape fixity ratio is typically 90{\%}. Interestingly, the materials' shape recovery rate exhibits Arrhenius-like temperature dependence. Activation energies were obtained by fitting the TMA creep experiment data, and they were compared to the dynamics of UPy H-bonding dissociation. [Preview Abstract] |
Monday, March 5, 2007 1:39PM - 1:51PM |
B24.00011: Miscibility studies on blends containing telechelic supramolecular polymers Michelle Wrue, Mitchell Anthamatten The incorporation of associating end groups influences the phase behavior of polymer blends. We are studying the effects of the incorporation of strong, site-selective, hydrogen-bonding groups in various polymer blend systems. We have synthesized several telechelic ureidopyrimidone (UPy) functionalized polymers. These self-complimentary functional groups have the ability to form four hydrogen bonds. When these groups are incorporated into a polymer blend, the miscibility is altered. Laser light scattering has been employed to study the phase behavior of these systems. Data from systems in which only one polymer was functionalized indicate a reduced miscibility when compared with the unfunctionalized parent blend. We are also investigating the effect of functionalizing both polymers of the blend with UPy end groups. We predict enhanced miscibility in such systems. [Preview Abstract] |
Monday, March 5, 2007 1:51PM - 2:03PM |
B24.00012: Design and characterization of well-defined supramolecular polymers Kathleen Schaefer, Matthew Kade, Craig Hawker, Edward Kramer Polymeric materials with well-defined and controllable temperature dependent properties are of interest both for technological applications and fundamental physical studies. Melt processing requires low viscosity, while resistance to fracture is desirable at material operating temperatures, and these two properties are often mutually exclusive. Through controlled radical polymerization (ATRP) we have synthesized tailor-made polymers with MHB groups specifically located at one or both chain ends or randomly along the backbone to provide thermal tunability, and by changing the nature of the MHB group (complementary or self-complementary) we can control the specificity and type of the polymer-polymer interaction. As a simple model system, we investigate the case of two end-functional MHB homopolymers that form a novel supramolecular diblock copolymer. Two energies are expected to be important in this system---$\chi $N, the Flory-Huggins interaction parameter times the degree of polymerization, which describes the polymer-polymer interaction, and $\varepsilon $, the binding energy of the MHB group. Using deuterium labeled polymers in various multilayer thin film structures, dynamic secondary ion mass spectrometry (dSIMS) allows each of these parameters to be measured independently and these values used to design technologically and physically interesting new materials. [Preview Abstract] |
Monday, March 5, 2007 2:03PM - 2:15PM |
B24.00013: Structure and stability of oligomer/$\alpha $-cyclodextrin inclusion complexes. Marcus Hunt, Silvia Villar, Marian Gomez, Alan Tonelli, Maury Balik Cyclomaltohexaose ($\alpha $-cyclodextrin, $\alpha $-CD) can form inclusion complexes (ICs) with polymer molecules in the columnar crystal in which $\alpha $-CD molecules stack to form a molecular tube. Complementary water vapor sorption and wide-angle X-ray diffractomery (WAXD) were performed on oligomer/$\alpha $-CD ICs to probe their structures and stabilities. To discern the effect of guest molecule hydrophobicity on water adsorption isotherms, polyethylene glycol (PEG, MW = 600 g/mol) and hexatriacontane (HTC) guests were used. Sorption isotherms for PEG/$\alpha $-CD IC are similar to those obtained for pure $\alpha $-CD and PEG, suggesting the presence of dethreaded PEG in the sample. WAXD collected before and after water vapor sorption of PEG/$\alpha $-CD IC indicated a partial conversion from columnar to cage crystal structure, the thermodynamically preferred structure for pure $\alpha $-CD, due to dethreading of PEG. This behavior does not occur for HTC/$\alpha $-CD IC. Sorption isotherms collected at 20, 30, 40 and 50\r{ } C allowed the calculation of differential heats of adsorption and integral entropies of adsorbed water, while solid-state $^{13}$C NMR suggested a dramatic increase in HTC and $\alpha $-CD mobilities upon complexation. [Preview Abstract] |
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