Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session V52: Polymer Melts and Solutions |
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Sponsoring Units: DPOLY Chair: Bryan Beckingham Room: LACC 512 |
Thursday, March 8, 2018 2:30PM - 2:42PM |
V52.00001: Maintaining Small Molecule Supersaturation in a Diblock Micelle Corona Reservoir Ziang Li, Theodore Lenk, Letitia Yao, Frank Bates, Timothy Lodge In this study, poly(N-isopropylacrylamide-co-N,N-dimethylacrylamide) (PND) statistical copolymers and a series of poly(N-isopropylacrylamide-co-N,N-dimethylacrylamide)-b-polystyrene (PND-b-PS) diblock polymers were synthesized, in which the molecular weight of the PND corona block was kept constant while the polystyrene core block length was varied. Corona region of the micelle, rather than the core, was utilized as a reservoir to store hydrophobic small molecules. Small molecule crystallization was inhibited by partitioning to micelle corona, resulting in enhanced small molecule supersaturation. Solution-state properties of the polymers were quantified by dynamic light scattering, small angle x-ray scattering and cryogenic transmission electron microscopy. Diffusion coefficients of two hydrophobic model drug molecules were measured in presence of the polymers by pulsed field gradient NMR spectroscopy, from which the drug-polymer “binding” constants were quantified. The in vitro dissolution of both drug molecules was systematically investigated as a function of composition and solution-state assembly of the polymers. The corona loading strategy demonstrated in this study can be potentially applied to store any hydrophobic small molecules with proper design of the polymer micelle. |
Thursday, March 8, 2018 2:42PM - 2:54PM |
V52.00002: Single Polymer Relaxation Dynamics in Entangled Solutions Yuecheng Zhou, Charles Schroeder We directly observe the relaxation dynamics of single DNA molecules in semi-dilute entangled solutions using fluorescence microscopy and microfluidics. Our results reveal the emergence of multiple relaxation modes upon increasing concentration. Fluorescently labeled tracer polymers are dissolved in a background of semi-dilute entangled λ-DNA solutions (ce 〈c〈c**), and the relaxation dynamics are directly observed. Single polymer relaxation trajectories are shown to exhibit either a single-mode or double-mode exponential decay, which starkly contrasts relaxation behavior from dilute (c«c*) and semi-dilute unentangled solutions (c*〈c〈ce). As polymer concentration is increased from 3.6 c* to 19.1 c*, the fraction of molecules that exhibit single-mode exponential decay behavior decreases, while the fraction of double-mode exponential trajectories increases. We further explore the power law scalings of these relaxation times with concentration. We find the slow double-mode relaxation time is consistent with blob theory for semi-dilute entangled solutions in a good solvent, whereas the single-mode relaxation time and the fast double-mode relaxation time are found to have the same but weaker power law dependence on concentration. |
Thursday, March 8, 2018 2:54PM - 3:06PM |
V52.00003: Coil dimensions of poly(ethylene oxide) in an ionic liquid Aakriti Kharel, Timothy Lodge The combination of polymers and ionic liquids (ILs) provides a platform for designing materials with tunable properties that have a wide range of applications. However, the use of these materials would benefit from a thorough understanding of the polymer behavior in ILs, and in particular the dimensions of polymer chains in ILs. Recently, a number of simulations and experimental studies have reported conflicting findings with respect to the radius of gyration of varying chain lengths of poly(ethylene oxide) (PEO) in an IL. In order to elucidate such discrepancies, direct measurements of coil size is desirable; however, the similarity in refractive indices of PEO and ILs limits the use of techniques such as light scattering. We therefore employ small-angle neutron scattering (SANS) to assess coil dimensions of varying molecular weights of perdeuterated PEO in an IL. Our results indicate that PEO chains behave as flexible random coils that are moderately swollen in the IL of interest; the Flory exponent ν ≈ 0.55. This experimental work therefore clarifies the uncertainties surrounding the dimensions of PEO coils in IL, and can help benchmark better simulation models and relevant experiments. |
Thursday, March 8, 2018 3:06PM - 3:18PM |
V52.00004: Computational Calorimetry of PNIPAM Cononsolvency in Water/Methanol Mixtures Cahit Dalgicdir, Francisco Rodriguez Ropero, Nico Van der Vegt Poly(N-isopropylacrylamide) (PNIPAM) is a widely used thermoresponsive polymer which dissolves in aqueous solutions and in pure methanol, however precipitates at low methanol concentrations of water/methanol mixtures. This interesting phenomenon where mixtures of two good solvents act as a poor solvent is called cononsolvency and the molecular mechanism for PNIPAM cononsolvency in water/methanol mixtures is still debated. Using extensive computational simulations we find a fingerprint for the cononsolvency of PNIPAM in water/methanol solutions in the calorimetric enthalpy of the collapse transition.[1] The microscopic interactions reveal that polymer hydration is the determining factor for chain collapse. In particular, the hydrogen bonds between water molecules and the amide proton are frustrated by the addition of methanol leading to a collapse. [1]C.Dalgicdir, F.Rodríguez-Ropero, N.F.A. van der Vegt J. Phys. Chem. B, 2017, 121 (32) |
Thursday, March 8, 2018 3:18PM - 3:30PM |
V52.00005: Chain Exchange Kinetics of Spherical Block Copolymer Micelles: Effect of Core Size Dan Zhao, Yuanchi Ma, Timothy Lodge Numerous evidence suggests that chain exchange among discrete micelles plays a vital role in establishing the nano-scale structures resulting from self-assembly of block polymers. In the past decade, the time-resolved small angle neutron scattering (TR-SANS) technique has been utilized to reveal the critical molecular characteristics, including core block chain length, dispersity and core block/solvent interaction parameter, that can affect the kinetics of molecular exchange processes. In this work, we examine the effect of micellar structure on the chain exchange kinetics in spherical micelles, decoupled from chain and solvent characteristics. This was realized by preparing two sets of micelles using different protocols, but from the same block copolymer in the same solvent. SAXS reveals a 30% difference in the core size of these two micelles. More importantly, TR-SANS shows that the chain exchange in the micelles with a larger core size is retarded by a factor of 3.5 compared to that in the smaller ones. Possible causes for this finding will be discussed. These results clearly indicate that, in addition to the molecular characteristics of the block polymer/solvent system, the geometrical structure of the micelle plays an important role in unimer exchange. |
Thursday, March 8, 2018 3:30PM - 3:42PM |
V52.00006: Understanding electric field effects in phase-separating soft matter Jonathan Martin, Glenn Fredrickson, Kris Delaney Phase-separating fluid and polymer mixtures provide a flexible platform for the design of functional materials containing multiple segregated domains. Polarizability contrast in such systems leads to van der Waals interactions that favor immiscibility, inducing aggregation of the more polarizable molecular species. External electric fields provide a modality for biasing this behavior and tuning the miscibility of such mixtures; however, fundamental understanding of electric field effects on phase behavior has been limited by disagreement between existing theoretical approaches and experimental literature. In this talk, we report on a study of the phase separation behavior of binary mixtures of dielectric fluids, employing a recently developed field theory representation of polarizable molecular species. The role of electric fields in altering the miscibility of these systems is also discussed. Extensions to polymer blends and broader soft matter systems are briefly outlined. |
Thursday, March 8, 2018 3:42PM - 3:54PM |
V52.00007: Linear-Nonlinear Dichotomy of Rheological Responses in Particle-Filled Polymer Solutions Wentao Xiong, Xiaorong Wang In this study, we found an anomalous nonlinear behavior under large amplitude oscillatory shears, where the amplitude of stress deviates strongly from the linear dependence of strain, while the time dependence of stress remains sinusoidal. We named this phenomenon as "linear-nonlinear dichotomy", which is usually accompanied with the Payne effect in filled rubbers. In order to understand more molecular details regarding this "dichotomy", we examined a series of oil-extended polyisoprene polymers filled with carbon black. We found that the “dichotomy” emerges from a sharp transition when the mesh size of the entangled polymer network exceeds the diameter of filler particles. The topological structures imposed by the polymer chains on the movement of particles play a key role in the "dichotomy" rheology. |
Thursday, March 8, 2018 3:54PM - 4:06PM |
V52.00008: Molecular simulations study of assembly in solutions of block copolymers and nanoparticles Daniel Beltran-Villegas, Arthi Jayaraman Hybrid materials composed of inorganic nanoparticles (NPs) and amphiphilic block copolymers (BCPs) combine desirable characteristics of both materials, and are useful in a broad range of applications such as biological cargo delivery, pollutant capture, chemical sensing, and separation/purification applications. In recent years, there has been growing interest in controllably changing solvent quality for the BCPs by mixing solvents and utilizing the effective solvophobicity of the BCP block(s) to tailor the assembled structure. In this talk, we will present our recent coarse-grained molecular dynamics (MD) simulations study of assembly in solutions of BCPs and NPs as a function of BCP composition and sequence, NP size and affinity to BCP blocks at varying solvophocity, both in bulk and near surfaces. We will describe how the above design parameters and bulk/surface conditions impact the solvophobicity needed for assembly, and then as a function of solvophobicity, the shape and size of assembled structures with and without NPs, NP uptake, and the spatial arrangement within the assembled structure. |
Thursday, March 8, 2018 4:06PM - 4:18PM |
V52.00009: Microfluidic Solvent Extraction for the Precise Formation of Polymer Particles and Capsules William Sharratt, Joao Cabral Microfluidics provides a precise platform for the generation of polymer solution droplets and their subsequent extraction to yield polymeric capsules with a range of applications. Control over particle structure and morphology is predicated, however, on polymer solution thermodynamics, composition pathways and gradients derived from the selective solvent removal, and eventual kinetic arrest. We investigate the role of Mw, degree of hydrolysis and droplet concentration in the formation of poly(vinyl alcohol) capsules. Specifically, aqueous PVA solutions are emulsified in an immiscible carrier phase to form well-defined droplets, which are then immersed into a selective extraction solvent, which is miscible with water but a non-solvent for the polymer. The progressive concentration of the polymer droplets and ingress of non-solvent leads to phase inversion and the formation of internally porous particles with a smooth, polymer-rich skin. By varying initial polymer concentration with respect to the overlap and concentrated crossovers, and separately Mw to tune the overall solution viscosity, we map a plethora of accessible internal morphologies. Further, by inducing anisotropic extraction conditions, non-spherical particles are trivially formed. |
Thursday, March 8, 2018 4:18PM - 4:30PM |
V52.00010: Pinch-off Dynamics, Extensional Rheology and Printability of Polyelectrolyte Solutions Leidy Jimenez, Jelena Dinic, Vivek Sharma Biological macromolecules like proteins, DNA and polysaccharides, and many industrial polymers, are classified together as polyelectrolytes for in solution, the repeat units in their backbone are decorated with disassociated, charge-bearing ionic groups, surrounded by counter-ions. In diverse applications like inkjet printing, sprayable cosmetics and insecticides, paints and coatings that involve formation of fluid columns or sheets that undergo progressive thinning and pinch-off into drops, the dominant flow within the necking filament is extensional in nature. The extensional rheology response of the charged macromolecular solutions are not as well understood as that of their uncharged counterparts. Here focus on the characterization of capillary thinning and pinch-off dynamics, extensional rheology and printability of two model systems: sodium (polystyrene sulfonate) and poly(acrylic acid) by using dripping-onto-substrate (DoS) rheometry technique. Due to an interplay of hydrodynamics-induced and charged-induced stretching, both the measured extensional relaxation times and the extensional viscosity values show salt- and polymer concentration-dependent behavior that is not expected or anticipated from the typical shear rheology response. |
Thursday, March 8, 2018 4:30PM - 4:42PM |
V52.00011: Melt Rheology of Cellulose Ethers Designed for Hot Melt Extrusion Tirtha Chatterjee, Kevin O'Donnell, Mark Rickard, Brian Nickless, Yongfu Li, Robert Sammler Formulation of new drug entities is increasingly challenging due to poor solubility of the drug and limited polymers that can produce amorphous solid dispersions (ASDs). One of the leading technologies to manufacture ASDs is hot melt extrusion (HME) in which the solid or melted drug is mixed with a molten polymeric carrier. The polymeric carrier must have adequate melt rheology at temperatures acceptable for pharmaceutical applications. The AFFINISOL™ HPMC HME cellulosic polymer family has been engineered to have a melt viscosity that enables their use as excipients in HME. These (AFFINISOLTM HPMC HME) cellulose ethers were found showing solid-like viscoelasticity well beyond their glass transition (Tg +70 oC). We demonstrate that in pharma-relevant HME processing temperature range these polymers behave as yield-stress fluids and flow only when the applied stress exceeds a critical stress value (yield stress) which is easily achieved at typical extrusion conditions. An infrared spectroscopic study revealed that the majority of the hydroxyl groups in cellulose ether chains remain hydrogen-bonded (either intra- or inter-molecular) between 50 and 250 oC. We hypothesize that these hydrogen bonds act as physical crosslinks that give rise to yield-stress like rheological properties. |
Thursday, March 8, 2018 4:42PM - 4:54PM |
V52.00012: Uniaxial Elongational Behavior of Unentangled Polymer Melts Hiroshi Watanabe, Yumi Matsumiya Uniaxial extensional behavior was examined for unentangled Polystyrene (PS; M = 27k) and Poly (tert-butyl styrene) (PtBS; M = 53k) in melt state with a commercially available filament stretching rheometer, VADER 1000. At sufficiently high stretching rates, strain hardening was observed in viscosity growth function of both PS and PtBS, in particular for PtBS. Stress relaxation just after stretch to Hencky strain of 2.5 was also examined. Relaxation time was insensitive to stretching rate when the Weissenberg number Wi defined with respect to the terminal relaxation rate in the linear regime is smaller than unity, whereas the initial relaxation (just after cessation of stretching) was considerably accelerated when Wi > 1. This result is consistent with previously reported results for entangled PS chains, where the initial Rouse relaxation was accelerated under fast stretch. Thus, the reduction of monomeric friction under stretching at high Wi plays a significant role in uniaxial extensional behavior of polymer melts irrespective of the entanglement. |
Thursday, March 8, 2018 4:54PM - 5:06PM |
V52.00013: Extensional strain-hardening induced by π-π interactions in barely entangled polymer chains: the curious case of Poly(4-vinylbiphenyl) CARLOS LOPEZ-BARRON, Huaxing Zhou Aromatic π- π interactions between phenyl groups of adjacent chains in poly(4-vinylbiphenyl) (PVBP) have profound effects on the dynamics of this polymer. We report two unexpected nonlinear viscoelastic responses of PVBP when subjected to uniaxial flow. One is the unprecedented observation of extensional strain hardening (SH) in a barely entangled polymer melt. The second (and more intriguing) finding is that SH of lightly (or even barely) entangled melts occurs at strain rates one order of magnitude below the coil-stretch transition predicted by Rouse theory (dεH/dt = 0.5/τR).We postulate that this behavior is due to a molecular rearrangement mechanism (supported by X-ray diffraction measurements) that involves flow-induced π- π stacking of the phenyl groups, which results in an enhancement of the friction coefficient between polymer chains. |
Thursday, March 8, 2018 5:06PM - 5:18PM |
V52.00014: Challenging the Chain Retraction Hypothesis of the Tube Model: Small-Angle Neutron Scattering Experiments Yangyang Wang, Zhe Wang, Christopher Lam, Wei-Ren Chen, Weiyu Wang, Christopher Stanley, Kunlun Hong, Yun Liu, Lionel Porcar Small-angle neutron scattering experiments have been performed to survey a long-standing problem in polymer physics regarding the molecular relaxation in entangled polymers after a large step deformation. The classical tube theory of Doi and Edwards envisions a fast chain retraction process immediately after the deformation, followed by a slow orientation relaxation through the reptation mechanism. This chain retraction hypothesis, which is the keystone of the tube theory for macromolecular flow and deformation, is critically examined by analyzing the fine features of the two-dimensional anisotropic spectra from small-angle neutron scattering by entangled polystyrenes. We show that the unique scattering patterns associated with the chain retraction mechanism are not experimentally observed. This result calls for a fundamental revision of the current theoretical picture for nonlinear rheological behavior of entangled polymeric liquids. |
Thursday, March 8, 2018 5:18PM - 5:30PM |
V52.00015: Challenging the Chain Retraction Hypothesis of the Tube Model: Large-Scale Molecular Dynamics Simulations Wensheng Xu, Jan-Michael Carrillo, Bobby Sumpter, Yangyang Wang The chain retraction hypothesis of the tube model for nonlinear polymer rheology has been challenged by the recent small-angle neutron scattering experiment [Phys. Rev. X 7, 031003 (2017)]. In this work, we further examine the microscopic relaxation mechanism of entangled polymers after a large step uniaxial extension by using large-scale molecular dynamics simulation. We show that the unique structural features associated with the chain retraction mechanism of the tube model are not computationally observed, confirming the previous experimental results. In particular, molecular dynamics simulation allows us to directly and unambiguously determine the evolution of the radius of gyration tensor after large step deformation. Contrary to the prediction of the tube model, the radius of gyration in the perpendicular direction to stretching increases monotonically towards its equilibrium value throughout the stress relaxation in our simulation. These findings provide important observations for improving our understanding of nonlinear rheology of entangled polymers. |
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