Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session B56: Symposium Honoring William W. Graessley IIFocus
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Sponsoring Units: DPOLY Chair: Ramanan Krishnamoorti, Univ of Houston Room: LACC 515B |
Monday, March 5, 2018 11:15AM - 11:51AM |
B56.00001: Effect of Partial Saturation on Thermodynamic Interactions in Polydiene/Polyolefin Blends Invited Speaker: Megan Robertson Polymer blends exhibit properties that are highly dependent on interactions between components, typically quantified by the Flory-Huggins interaction parameter, χ. Prediction of blend phase behavior requires characterization of the temperature dependence of χ. Polyolefins and polydienes are important materials with commercial relevance as elastomers. The majority of previous studies on the thermodynamics in polyolefin and polydiene systems have focused on polymer pairs within the same class (i.e. polyolefin/polyolefin and polydiene/polydiene blends), which generally exhibit a small and weakly temperature dependent χ. There is little quantitative information on thermodynamic interactions in systems that contain both polydienes and polyolefins. In our previous work, we characterized the χ parameter in a model polydiene/polyolefin blend based on 1,2-polybutadiene (1,2-PBD) by small angle neutron scattering (SANS). 1,2-PBD was chosen as a model system as it is amorphous, allowing for characterization of χ over wide temperature range, and can be synthesized through anionic polymerization with low dispersity and high 1,2 content (>99%). SANS data were analyzed through Random Phase Approximation and Zimm analyses in order to extract χ as a function of temperature. We observed an unusually large χ parameter in blends of 1,2-PBD and saturated 1,2-PBD that exhibited a strong temperature dependence. This feature enables facile control over phase behavior during processing at elevated temperatures. We also studied the impact of partial saturation on the thermodynamics of polydiene/polyolefin blends. The χ(T) behavior in blends of fully saturated (with deuterium) 1,2-PBD with partially saturated (with hydrogen) 1,2-PBD, at varying levels of saturation (covering the full range from 0-100%), was characterized. The applicability of the random copolymer theory to predict χ(T) behavior in these blends was evaluated. |
Monday, March 5, 2018 11:51AM - 12:03PM |
B56.00002: Phase Behavior of Binary Polymer Blends Doped with Salt Shuyi Xie, Frank Bates, Timothy Lodge Binary polymer blends are predicted to demonstrate unusual phase behavior when doped with salt, after accounting for ion-induced cross-linking and self-energy effects. In this work, we present cloud point measurements on low molecular weight poly(ethylene-alt-propylene)/polyethylene oxide (PEP/PEO) binary polymer blends doped with various salts, and explore the influence of the concentration of the salt and the size of the anion on the phase diagram. The addition of salt dramatically decreases the miscibility of the binary blends and results in an asymmetric cloud point profile. The coexistence curve is found to deviate from the cloud point profile, which can be attributed to the effect of the redistribution of ions between the two coexisting phases. Experimental data are compared with the phase diagrams obtained from a recent theory of Ren, Nakamura and Wang and may serve as a benchmark for future experimental design of polyelectrolyte systems and further theoretical research. |
Monday, March 5, 2018 12:03PM - 12:15PM |
B56.00003: Chain Exchange Kinetics of Triblock Copolymer Micelles: Effect of Corona Block Asymmetry En Wang, Frank Bates, Timothy Lodge The chain exchange kinetics of block polymer micelles has been observed to be much slower than small surfactants due to the enhanced energy barrier for expulsion of polymer chains. Recently, micelles formed by a symmetric poly(ethylene-alt-propylene)-b-poly(styrene)-b-poly(ethylene-alt-propylene) (PEP-PS-PEP) triblock copolymer were reported to exhibit three orders of magnitude faster exchange kinetics than the equivalent PS-PEP diblock micelles when immersed in squalane, a selective solvent for PEP. Dissipative particle dynamics simulation also revealed faster kinetics in BAB’ triblocks than in the AB diblock, where A is the core block, and B and B’ are distinct corona blocks. This work systematically investigates the structure and chain exchange kinetics of asymmetric PEP-PS-PEP’ triblock micelles by varying the corona block asymmetry at constant core block length and total corona block length, via small-angle X-ray and time-resolved small-angle neutron scattering measurements. Both symmetric and asymmetric triblock micelles display faster exchange kinetics than the diblock. However, the asymmetric micelles exhibit non-monotonic changes in kinetics as corona block asymmetry increases, which we attribute to enhanced compatibility between the core and the shorter corona block. |
Monday, March 5, 2018 12:15PM - 12:27PM |
B56.00004: Asymmetric Block Copolyelectrolyte Morphology Mapping Induced by Strong Electrostatics Sebastian Russell, Sanat Kumar, Luis Campos, Alan West A central goal in the design of polymer electrolyte membranes is to improve their integrity while maintaining high selective ion conduction. Here we develop a new class of copolymers with one block containing a cyclopropenium (CP) ion per monomer and show that they form either disordered structures or cylindrical morphologies with an ion-conducting matrix even when the charge block fraction is as low as 10 vol%. We attribute these results to the CP polyelectrolyte having a remarkably low static dielectric constant, εr = 2.2 D, resulting in very strong electrostatic interactions that dominate the microphase segregation behavior. We define a packing parameter that balances the range of ionic interactions against the size of the charge-neutral block. This treatment demonstrates the CP block has a positive spontaneous interfacial curvature for all investigated charge neutral macroinitiators and rationalizes the observed asymmetric morphology mapping. We stress that the morphology of materials in this limit of strong electrostatics are unusually beneficial for selective charge transport applications. |
Monday, March 5, 2018 12:27PM - 12:39PM |
B56.00005: Disorder-to-Order Transition upon Heating in an All-Hydrocarbon Polynorbornene Diblock Copolymer William Mulhearn, Richard Register Most block copolymer chemistries, with appropriate molecular weights, undergo order-to-disorder transitions (ODT) upon heating. ODT behavior predominates because the Flory interaction parameter χ, characterizing the repulsive strength between components, is usually positive and decreases with temperature. We identify a new diblock copolymer chemistry, composed of hydrogenated poly(n-hexyl norbornene) and poly(cyclohexyl norbornene), which instead undergoes a disorder-to-order transition (DOT) upon heating. Block copolymers exhibiting a DOT are extraordinarily rare, and to our knowledge this is the first all-hydrocarbon species identified to do so. Demixing at high temperatures is not driven by a large free volume mismatch between components, the mechanism commonly invoked to explain DOTs in block copolymers or lower critical solution temperatures in blends. We compare the DOT polymer with a family of other polyethylene and norbornene-based diblocks, and find that the thermal expansion coefficient mismatch between blocks, related to the free volume mismatch, does not dictate the type of phase behavior. Instead, we attribute DOT behavior to an enthalpic effect, where the component solubility parameters do not scale with the reciprocal of density as expected for a simple liquid. |
Monday, March 5, 2018 12:39PM - 12:51PM |
B56.00006: Mobility of Polymer-Tethered Nanoparticles in Polymer Melts Ting Ge, Michael Rubinstein A scaling theory is developed for the motion of a polymer-tethered nanoparticle (NP) in a polymer melt. Both NPs tethered with a single polymer chain (tail) and with multiple chains (tails) are studied. For a single-tail NP in a polymer melt, we identify two types of scaling regimes, particle-dominated regimes and tail-dominated regimes, that depend on the NP diameter d and the size of the tail Rtail. In a particle-dominated regime, a tethered NP moves as a bare NP, while the effects of the tethered tails on NP motion can be neglected. In a tail-dominated regime, the motion of a tethered NP is not significantly affected by the tails below a crossover time, but is dominated by the tails above the crossover time. For a multi-tail NP in an unentangled polymer melt, the boundaries separating the particle-dominated and tail-dominated regimes in the (d,Rtail) parameter space depend on the number of tails z. For a multi-tail NP in an entangled polymer melt, the motion of the entangled tails is described based on the dynamics of a star polymer entangled with the underlying polymer melt network. The mobility of such a multi-tail NP is approximated as the lower of the mobilities of the bare NP and of the branch point of the corresponding entangled star. |
Monday, March 5, 2018 12:51PM - 1:03PM |
B56.00007: Corona Driven Orientational Control of Grafted Nanoparticle Self-Assembly Thi Vo, Fang Lu, Yugang Zhang, Oleg Gang, Sanat Kumar Traditional methods in nanoparticle self-assembly utilize spherical cores as the anchoring motif. While successful in enabling directed control of interactions, spherical cores tend to restrict the range of accessible morphologies. This limitation serves as a significant hurdle for the establishment of self-assembly as a robust technique for both the design and fabrication of nanomaterials. Recently, experiments have began to utilize cores of varying shapes in order to expand on the current library of self-assembled morphologies. Using our theoretical models, we show that there exists a significant level of preferential partitioning to positions of varying curvature on the surface of anisotropic cores. This non-trivial graft distribution can be translated into different packing modalities, where the type of packing is not determined by the shape of the solid particle core, but rather by the emergent shape of the soft shell. These results highlight anisotropy as a powerful handle not only for accessing new morphologies, but also for a selective transition between different orientational packing for cores of the same shape type. |
Monday, March 5, 2018 1:03PM - 1:15PM |
B56.00008: Slow Dynamics of Glass Transformation in Polymer/Colloid Systems Charles Han, Guangcui Yuan The splitting and non-proportional slowing down in relaxation processes of structural materials will be discussed in this talk. A visible and measurable example of particles with a repulsive potential plus a short range attraction will be used to illustrate the bridging and jamming structures that could be the key structural characteristics which leads to various dynamic features during the glass transformation, such as: modes slowing down, (dynamic) free volume change, local heterogeneity, and etc. |
Monday, March 5, 2018 1:15PM - 1:27PM |
B56.00009: Driven Dynamics of Physical Polymer and Colloidal Gels Ahmad Omar, Yanze Wu, John Brady, Zhen-Gang Wang Gels assembled by the reversible association of polymers or colloids are a common and fascinating class of soft materials. Here, we explore the diffusive modes of the constituent particles of the gel, drawing analogies to activated glassy dynamics. While our theory and experiments suggest that single-particle (activated) dynamics control long-time relaxation in quiescence, relaxation in driven systems can be qualitatively distinct. For example, using computer simulation, we find that shearing a polymer gel results in a fast (non-activated) diffusive mode in the form of freely-diffusing mulitchain aggregates. The rapid emergence of this mode with applied shear is found to destabilize homogeneous flow for gels sufficiently close to the two-phase boundary. The driving force need not be externally applied. The induced collective motion in colloidal gels subject to internal driving forces (such as the presence of a small fraction of self-propelling colloids) can drive the system from a state of arrested metastablity to a state of lower free energy. Our findings suggest that by carefully tuning the magnitude of internal or external driving forces, new diffusive modes can emerge that can facilitate traversing kinetic barriers that are otherwise insurmountable in quiescence. |
Monday, March 5, 2018 1:27PM - 1:39PM |
B56.00010: Imaging Synthetic Polymer Crystals and Defects on Atomic Length-Scales Nitash Balsara, Xi Jiang, Douglas Greer, Kenneth Downing, Ronald Zuckermann, Joyjit Kundu, David Prendergast Determining atomic-scale structures in polymers is difficult because they degrade rapidly when studied by electron microscopy, and techniques such as x-ray scattering average over volumes much larger than the unit cells. We obtained cryo-electron microscopy images of crystals of a peptoid polymer in which we see a variety of crystalline motifs. A combination of crystallographic and single particle methods, developed for cryo-electron microscopy of biological macromolecules, was used to obtain high resolution images of the crystals. Individual specimens contain grains that are mirror images of each other with concomitant grain boundaries. Our approach is robust and may enable direct visualization of crystalline grains and grain boundaries on atomic length scales in a variety of polymers. |
Monday, March 5, 2018 1:39PM - 1:51PM |
B56.00011: Spatial Dependence of Molecular Relaxation in Deformed Polymers Christopher Lam, Wensheng Xu, Wei-Ren Chen, Zhe Wang, Christopher Stanley, Jan-Michael Carrillo, David Uhrig, Changwoo Do, Bobby Sumpter, Yangyang Wang We present a general framework for investigating the spatial and temporal dependence of structural anisotropy relaxation in deformed polymers by combining small-angle neutron scattering and the spherical harmonic expansion technique. Experiments on polymer melts over a wide range of molecular weights reveal that their conformational relaxation at relatively high momentum transfer (Q) and short time can be described by a simple and universal scaling law, with the relaxation rate proportional to Q. This scaling behavior, while further confirmed by coarse-grained molecular dynamics simulations, does not seem to stem from either the Rouse or reptative motions depicted in the classical models, and calls for further development in the theory of polymeric liquids under deformation and flow. |
Monday, March 5, 2018 1:51PM - 2:03PM |
B56.00012: Crystallinity Enhances the Tensile Ductility of Polyethylene Buckley Crist The crystalline fraction of polyethylene is controlled by crystallization conditions, molar mass and comonomer content. It has long been known that higher crystallinity leads to greater elastic stiffness and greater yield strength. Intrinsic flow properties beyond the yield point are often obscured by deformation (neck formation and propagation). We present tensile true stress-true strain behavior of a series of isotropic polyethlenes with crystalline fraction ranging from 0.27 to 0.74. Tests were at room temperature and corrected to a constant local strain rate of 2.15×10-3/sec. Unanticipated was the observation that the lowest crystallinity materials strain harden abruptly, failing at a true strain of ca. 1.5 because of finite extensitility of amorphous network chain segments. More crystalline polyethylenes flow to true strains approaching 3 with modest strain hardeing; here crystal plasticity dominates. The roles of morphology and entanglement are considered. |
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