Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session A21: Focus Session: Reversibly Associating Polymers: Theory and Experiments |
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Sponsoring Units: DPOLY DBP Chair: Ken Shull, Northwestern University Room: Morial Convention Center 213 |
Monday, March 10, 2008 8:00AM - 8:12AM |
A21.00001: Self Assembly of Mixed-Valence Ionic Amphiphiles into Faceted Vesicles Megan Greenfield, Graziano Vernizzi, Liam Palmer, Samuel Stupp, Monica Olvera de la Cruz We show that anionic and cationic amphiphiles of unequal charge can co-assemble into small faceted vesicles and we propose a theoretical model to explain the faceting behavior. The strong electrostatic interaction between the +3 and -1 head groups increases the Columbic cohesion energy of the amphiphiles and should favor the formation of a two-dimensional, flat ionic surface. The vesicle surface can form edges by breaking the ionic lattice, which can be visualized as faceted shapes. Our results demonstrate that a large charge imbalance between the cationic and anionic head groups of amphiphiles enables their coassembly into facetted vesicles. We anticipate this work to be a starting point for rationally designing new self-assembled supramolecular structures. [Preview Abstract] |
Monday, March 10, 2008 8:12AM - 8:24AM |
A21.00002: Electrospining Solutions of Associating Polymers -- the Case of Stereocomplex PMMA Matija Crne, Jung Park, Mohan Srinivasarao A mixture of isotactic and syndiotactic PMMA polymers (also called a ``stereocomplex PMMA'') forms a supramolecular helical structure, which is held together by non-covalent bonding. This association is thermoreversible and solvent-sensitive. If the concentration of stereocomplex PMMA is high enough, the solutions form thermoreversible physical gels. We have examined the influence this associating behavior has on the process of electrospinning. In our work, we have used solutions of stereocomplex PMMA to study the effect of physical gelation interaction on the fiber morphology and compared it to the solutions of atactic PMMA of similar molecular weight. We have found that the stereocomplex PMMA solutions do not follow the same empirical rules that are applicable for linear polymers in solution. Instead, the concentration necessary for the production of smooth, continous fibers is much lower. We ascribe this extraordinary behavior to the ability of these polymers to associate and form a network during the electrospinning process. The elasticity of the fluid jet thus rises and results in a more stable jet. The resulting fibers are interesting, as they have greater temperature stability than regular atactic PMMA. Therefore they have a bigger processing window for making composites with better mechanical properties. [Preview Abstract] |
Monday, March 10, 2008 8:24AM - 8:36AM |
A21.00003: ABSTRACT WITHDRAWN |
Monday, March 10, 2008 8:36AM - 9:12AM |
A21.00004: Unique Properties of Reversibly Associating Polymer Networks Invited Speaker: Reversibly associating functional groups offer the polymer physicist with a new tool to develop stimuli-responsive polymers. Our focus has been to attach reversibly associating groups onto rubbery network polymers. Free radical copolymerization was used to synthesize a series of crosslinked poly(n-butylacrylate)s containing quadruple H-bonding ureidopyrimidinone (UPy) side-groups. Resulting elastomeric networks contain both covalent and dynamic non-covalent crosslinks, and this unique architecture is shown to affect viscoelastic behavior and mass-transport properties. Shape-memory effects are studied quantitatively using thermomechanical techniques. Experiments show how reversible interactions, such as hydrogen bonding, are capable of stabilizing mechanically strained states. Unlike conventional shape-memory polymers, these dynamic networks lack a well-defined shape recovery temperature. Instead, their shape recovery rate depends on temperature. To further study the dynamics and temperature dependence of mechanical relaxation, isothermal creep experiments and dynamic mechanical analysis were performed. Creep data, acquired at several different temperatures, are fit to a simple viscoelastic model. Fit viscosities exhibit Arrhenius-like temperature dependence with activation energies of $\sim $90 kJ/mol, which is in rough agreement with H-bond dissociation barriers. Molecular transport through dynamic networks is studied using gravimetric sorption and dye-diffusion techniques. Diffusion depends on temperature, network architecture, solute size, and the interaction between the solute and the network. Membranes with high temperature-sensitive diffusion properties may be useful in applications such as transdermal drug delivery, microfluidics, or liquid chemical separation processes. [Preview Abstract] |
Monday, March 10, 2008 9:12AM - 9:24AM |
A21.00005: Computer Simulations of Semi-flexible Polymer Chains Venkat Padmanabhan, Sanat K. Kumar, Arun Yethiraj Monte Carlo Simulations are performed to obtain the isotropic-nematic (IN) transition in systems with semi-flexible polymer chains of different lengths. The chains are modeled as spherical beads that interact via a square-well potential. Bonded beads are connected by strings chosen so that bond length varies between 1.01$\sigma $ and 1.05$\sigma $ (where $\sigma $ is the hard sphere diameter). The stiffness of the molecules is controlled via a potential between beads separated by two bonds; this potential restricts the distance between these beads to be between 2.02$\sigma $ and 2.1$\sigma $. The vapor-liquid coexistence and isotropic-nematic (IN) coexistence curves are obtained using computer simulations. An IN transition is found for N$_{b} \quad \ge $ 10. The density, at which the IN transition occurs, moves to higher values as N$_{b}$ is increased and then drops on further increase. This is analogous to the initial increase in the critical density for pure alkanes as the chain length is increased. [Preview Abstract] |
Monday, March 10, 2008 9:24AM - 9:36AM |
A21.00006: Reversible Networks by Hydrogen Bonding of ABA Triblock Copolymers in an Ionic Liquid Timothy Lodge, Atsushi Noro, Yushu Matsushita Ion gels, comprising a polymeric network solvated by an ionic liquid, are of great interest as, e.g., gate dielectrics in plastic electronics, polymer electrolytes with high ionic conductivity, actuators and artificial muscles, gas separation media, and sensors. We have explored the thermoreversible gelation of a model system containing poly(2-vinyl pyridine-b-ethyl acrylate-b-2-vinyl pyridine) ) (VEAV) triblocks dissolved in 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide) [emi][tfsi], where the EA blocks are under good solvent conditions. Addition of poly(vinyl phenol) (PVPh) creates hydrogen bonds with the V end blocks, leading to gelation. As each end block and each PVPh ``crosslinker'' has about 50 segments, up to 50 hydrogen bonds may be formed per block. Consequently, over the temperature range 30 -- 160 C the longest relaxation time of the gel, related to the ``sticker'' lifetime, varies by about 12 orders of magnitude. The thermal stability of the ionic liquid thus provides an unprecedented opportunity to study the dynamic properties of reversible networks over a wide range of timescale. [Preview Abstract] |
Monday, March 10, 2008 9:36AM - 9:48AM |
A21.00007: Examination of the Structure of Molten Hydrogen-Bonded Supramolecular Diblock Copolymers Kathleen Feldman, Matthew Kade, Craig Hawker, Edward Kramer A new strategy for synthesizing low polydispersity polymers containing multiple hydrogen bonding (MHB) groups at one chain end capable of heterodimerization in both solution and the melt has been developed. Two well-known MHB systems were chosen for initial studies---2-ureido-4[1H]-pyrimidinone (UPy) and 1,8-diamidonaphthyridine (Napy), and ATRP initiators containing either UPy or Napy were synthesized. These initiators were shown to produce well-defined (meth)acrylic polymers with the desired MHB functionality present at the chain end. To characterize the effectiveness of the MHB interaction in the melt, blends were cast into films, annealed at various temperatures above T$_{g}$, then quenched and their structure analyzed by transmission electron microscopy (TEM) and differential scanning calorimetry (DSC). It was shown that the nature of the hydrogen bonding group(s) present in the blend has a significant effect on structure and thermal behavior, and in particular blends of UPy- and Napy-functional chains contain a significant fraction of ``supramolecular diblock copolymers.'' [Preview Abstract] |
Monday, March 10, 2008 9:48AM - 10:00AM |
A21.00008: Thermoreversible bond formation in end-linking, difunctional polymer blends Richard Elliott, Glenn Fredrickson We investigate theoretically thermoreversible bond formation and phase transitions in a system composed of linear, end-linking, difunctional polymers. In this melt, two distinct species of equal-length links assemble into chains of varying sequences and polymerization depending on the affinities of binding between the blocks. Several limits, such as that of purely heterogeneous bonding which forms chains of alternating block sequences, are explored. Interactions between dissimilar segments are described with the Flory-Huggins contact potential and a mean-field approach is employed to investigate the effects of prevalent bonding and the onset of meso-scale ordering. [Preview Abstract] |
Monday, March 10, 2008 10:00AM - 10:12AM |
A21.00009: Inter- and Intra-Molecular Interactions of Associative Polymers in Solution M. Wei, R. David, Julie Kornfield Model polymers with matched backbone length are used to examine the effects of degree of functionalization and type of interaction (self-associating or donor-acceptor) on shear and extensional rheology of associative polymer solutions. Series of polymers were prepared by functionalization of 500 and 1300 kg/mol polybutadiene chains with carboxylic acid side groups (A stickers, self-associating) and tertiary amine side groups (N stickers, forming strong hydrogen donor-acceptor interactions with A stickers). We found that stickers drive phase separation at extents of functionalization as low at 0.5 mol{\%}. Intramolecular associations dominated the behavior of A-functionalized chains even at semi-dilute concentrations, leading to chain collapse and reduced shear and extentional viscosities. Finally, we found that intermolecular interactions were much more favorable for dilute mixtures of A-functionalized and N-functionalized chains (as evidenced by increased zero-shear viscosity and by the formation of large aggregates), but that associations still reduced solution elasticity and extentional viscosity in elongational flow. [Preview Abstract] |
Monday, March 10, 2008 10:12AM - 10:24AM |
A21.00010: Molecular Dynamics Simulation of Polyelectrolyte Brushes: From Hemispherical Micelles to Maze-like Aggregates. Jan-Michael Carrillo, Andrey Dobrynin We present results of the molecular dynamics simulations of the effects of solvent quality, strength of the electrostatic interactions, chain degree of polymerization and grafting density on the conformations of planar polyelectrolyte brushes in salt-free solutions. The polyelectrolyte brush could form: (1) hemispherical micelle aggregates, (2) vertically oriented cylindrical micelles, (3) maze-like aggregate structures, or (4) thin polymeric layer uniformly covering the substrate. These different brush structures appear as a result of the fine interplay between electrostatic and monomer-monomer attractive interactions. The brush thickness depends nonmonotonically on the value of the Bjerrum length. This nonmonotonic dependence is due to counterion condensing inside the brush. [Preview Abstract] |
Monday, March 10, 2008 10:24AM - 10:36AM |
A21.00011: Effect of Hydrogen-Bonding Junctions on Microphase Separation in Block Copolymers Greg Stone, Jim Hedrick, Fredrik Nederberg, Nitash Balsara The morphology of poly(styrene-block- trimethylene carbonate) (PS-PTMC) copolymers with and without thiourea groups at the junction between the blocks was studied by a combination of small angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). The thiourea groups are known to exhibit inter-molecular hydrogen bonding. We demonstrate that the presence of thiourea groups results in increased segregation between PS and PTMC blocks. We focus on symmetric systems with total molecular weights in the 5 kg/mol range. In conventional block copolymers without hydrogen bonding groups it is difficult to obtain strong segregation in low molecular weight systems because the product chi*N controls segregation (chi is the Flory-Huggins interaction parameter and N is the number of monomers per chain). The incorporation of hydrogen bonding groups may provide a route for the generation of patterns with small, sharply defined features using block copolymers. [Preview Abstract] |
Monday, March 10, 2008 10:36AM - 10:48AM |
A21.00012: Relating Chain Structure to Physical Properties of Branched Polymers Ramnath Ramachandran, Gregory Beaucage, Amit S. Kulkarni, Vassilios Galiatsatos, Douglas C. McFaddin We investigated linear and branched polyethylene (PE) using small-angle neutron scattering (SANS). The experiments were conducted on dilute solutions of PE in deuterated p-xylene. A variety of structural information$^{\dag }$ such as fractal dimension ($d_{f})$, connectivity dimension ($c)$, minimum path dimension ($d_{min})$, long chain branch fraction ($\phi _{br})$, radius of gyration ($R_{g})$ and persistence length ($l_{p})$ were obtained. Such information presents a qualitative and quantitative assessment of branching in polymers. Theoretical models such as `binary contacts per pervaded volume' model$^{\ast }$ were employed to correlate the structural information of the polymer to its entanglement molecular weight ($M_{e})$. $M_{e}$ was used to predict physical properties such as plateau modulus ($G_N^0 )$ and zero-shear viscosity (\textit{$\eta $}$_{0})$. We relate physical properties of branched polymers to their structural properties.$^{ \dag }$Beaucage G. \textit{Physical Review E} \textbf{70},031401 (2004) $^{\ast }$Colby \textit{et al}.\textit{ Macromolecules }\textbf{25,} p.996 (1992) [Preview Abstract] |
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