Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session P4: Structure in Solutions and Melts |
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Sponsoring Units: DPOLY Chair: Zhen-Gang Wang, California Institute of Technology Room: LACC 515A |
Wednesday, March 23, 2005 11:15AM - 11:51AM |
P4.00001: Self-Assembled Liquid Crystalline Gels Designed from the Bottom Up Invited Speaker: Block copolymers with long side-group liquid-crystalline (LC) midblocks and LC-phobic end-blocks form a physical network that swells readily in a small molecule LC to form model nematic gels with well-defined molecular weight between crosslinks. Ultralong ($>$800 kg/mol) SGLCP midblocks enable gelation at relatively low concentration ($\sim $5{\%} polymer), which preserves the fast dynamics of small molecule LCs. Similar to LC elastomers, an initially unaligned, polydomain gel aligns under applied strain. Further, the resulting monodomain is so well oriented that it generates clear conoscopic figures. Due to the coupling between nematic order and polymer elasticity, a novel stripe pattern forms when the gels are subjected to electric fields or when the order parameter of the LC solvent changes. Meyer and coworkers have described these patterns using a linear stability analysis that connects the band formation with the spontaneous anisotropy of the SGLCP backbone. Small-angle neutron scattering (SANS) on analogous SGLCP homopolymers confirms that greater chain anisotropy favors band formation and that the sense of anisotropy (prolate or oblate) dictates the initial band orientation. The physical junctions comprised of the LC-phobic endblocks perturb the director field on a nano-scale manifested in a reduction in the mean order parameter of the LC host characterized by both refractive indices (n$_{e}$, n$_{o})$ and NMR $^{2}$H quadrupole splitting. The physical principles demonstrated for nematic gels allow rational design of gels exhibiting higher-order LC phases, such as ferroelectric gels. [Preview Abstract] |
Wednesday, March 23, 2005 11:51AM - 12:27PM |
P4.00002: Telechelic amphiphilic polymers: assembly in water and at the air/water interface Invited Speaker: Associative polymers exhibit a rich rheology when dissolved or dispersed in water. The quintessential associative polymers are poly(ethylene oxides) (PEO) bearing hydrophobic groups at each end. Our research interests have focused on another water-soluble polymer, poly(N-isopropylacrylamide) (PNIPAM) and its hydrophobically-modified derivatives. Like PEO, PNIPAM is a non-ionic polymer soluble in water at room temperature. We succeeded recently in preparing telechelic PNIPAM samples of narrow molecular weight polydispersity that bear an octadecyl group at each chain end and carried out a systematic study of the assembly of this polymer as a function of concentration, from the dilute to the concentrate regimes and as a function of temperature (10 to 40 $^{o}$C) using static and dynamic light scattering measurements, microcalorimetry, and fluorescence spectroscopy for investigations of dilute solutions. In the concentrated regime, telechelic PNIPAM forms gels which were examined under oscillatory shear. [Preview Abstract] |
Wednesday, March 23, 2005 12:27PM - 1:03PM |
P4.00003: Network Phases of ABC Triblock Copolymers Invited Speaker: Fundamental exploration of the melt state phase behavior in linear ABC triblock copolymers has uncovered a fantastic array of over two dozen unique morphologies in just over a decade of limited scrutiny. These structures range from simple three-domain analogs of the classic diblock copolymer phases to exquisite ``decorated phases'' characterized by the presence of A/C interfaces not inherently required by the natural connectivity of the copolymer. In this presentation I will focus on our extensive research efforts targeting the discovery of multiply continuous network phases within the expansive ABC parameter space. Adopting a strategy involving block connectivities precluding A/C interface formation and compositions aimed at breaking symmetry between two and three domain lamellar regions, we synthesized a series of 43 poly(isoprene-$b$-styrene-$b$-ethylene oxide) (ISO) triblock copolymers (ranging from 15 to 25 kg/mol) to systematically explore network formation in ABC systems. Employing a battery of complementary analysis techniques including TEM, SAXS (static and under reciprocal shear), dynamic mechanical spectroscopy and static birefringence, coupled with mathematically generated level set models that bridge real and reciprocal space, we have identified a total of three independent network phases formed in this single triblock copolymer system. Two cubic network phases, Q$^{230}$ (core-shell double gyroid, \textit{Ia}$\bar {3}d)$ and Q$^{214}$ (alternating gyroid, $I$4$_{1}$32), and an unprecedented orthorhombic network phase, O$^{70}$ (\textit{Fddd}), were found to define a significant region of contiguous phase space, with order-order transitions (OOTs) found between network phases in some samples. Quite remarkably, the topology of each of these networks shares a common structure based on ordered arrays of connected 10-node loops, with each node trivalently joined to other nodes in the network. The universal presence of such networks in other ABC systems will be discussed. [Preview Abstract] |
Wednesday, March 23, 2005 1:03PM - 1:39PM |
P4.00004: Conformations and Structure in Aqueous Poly(ethylene oxide) Solutions Invited Speaker: Atomistic molecular dynamics (MD) simulations have resulted in important insights into the influence of water on the local conformations and chain dimensions of poly(ethylene oxide) (PEO) as well as the role of PEO-water polar interactions and PEO-water and water-water hydrogen bonding interactions on solution structure as a function of composition and temperature. Results of these simulations will be presented and discussed. In addition, results of recent simulation studies of PEO brushes and the interaction of PEO-modified nanoparticles in aqueous solution will be considered. [Preview Abstract] |
Wednesday, March 23, 2005 1:39PM - 2:15PM |
P4.00005: Control of contents and release kinetics in block copolymer vesicles Invited Speaker: Block copolymer vesicles have received considerable attention recently because of a wide range of potential applications. In our group, the thermodynamic aspects of vesicle formation, including curvature stabilization, as well as active loading and release from vesicles have been the focus of recent research. The vesicles are prepared from an amphiphilic diblock copolymer such as polystyrene-block-poly(acrylic acid) at a low pH (2.5) by adding water to a solution in a common solvent; then the extenal pH is raised to 6.5, and the compound, such as doxorubicin or another amine, is added. Since the compund inside the vesicle becomes ionized at the low pH, it can only escape at a rate very much slower than that of the loading process. The permeability of the wall can be controlled by the presence of plasticizers for the polystyrene wall; the plasticizers partition between the wall and the external aqueous solution with a known partition coefficient, and can be removed from the wall by dialysis. Release is then studied under perfect sink conditions and is diffusional. It is noteworthy that the rates of both loading and release can be varied by more than two orders of magnitude by controlling the plasticizer content. Also, between the loading and release processes, the vesicle wall can be hardened by removal of the plasticizer by dialysis. This degree of control makes block copolymer vesicles a promising delivery vehicle for a range of materials, including drugs. [Preview Abstract] |
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