Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session Q25: Physical Properties of Melts and Solutions |
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Sponsoring Units: DPOLY Chair: Mitchell Anthamatten, University of Rochester Room: Morial Convention Center 217 |
Wednesday, March 12, 2008 11:15AM - 11:27AM |
Q25.00001: Structure and Assembly of Dense Solutions and Melts of Polymer Tethered Nanoparticles Arthi Jayaraman, Kenneth S. Schweizer We generalize the microscopic Polymer Reference Interaction Site Model theory to study intermolecular pair correlation functions and collective structure factors of dense solutions and melts of polymer-tethered spherical nanoparticles. The complex interplay of entropy (translational, conformational and packing) and enthalpy (particle-particle attraction) leads to different structural arrangements with distinctive scattering signatures. Strong concentration fluctuations indicative of aggregate formation and/or a tendency for microphase separation occur as the total packing fraction and/or particle-particle attraction strength increase. A microphase spinodal curve is estimated by extrapolation of the inverse of the amplitude of the small angle scattering peak. For nanoparticles, twice the diameter of monomers, carrying a single tether the microphase spinodal temperature grows roughly as a power law function of packing fraction with an exponent much less than unity. Increasing the nanoparticle diameter lowers the microphase spinodal temperature and results in a qualitative change of its packing fraction dependence. The effect of nanoparticle size, number and length of tethers, position of the grafting sites, total packing fraction, and particle-particle attraction strength on the structure, scattering patterns and tendency for microphase ordering has been studied in detail. [Preview Abstract] |
Wednesday, March 12, 2008 11:27AM - 11:39AM |
Q25.00002: Line shape analysis of dynamic light scattering results on polymeric microgel nanoparticles Kiril A Streletzky, Imaan Benmerzouga, John McKenna Hydroxypropylcellulose (HPC) is nontoxic polysaccharide with temperature dependent water solubility. HPC polymer chains can be chemically cross linked into stable nanoparticles- microgels. HPC microgel properties depend on polymer and salt concentration and cross linking density. One of the most important properties of HPC is its critical temperature of 41$^{o}$C, at which the polymer undergoes a reversible phase transition. HPC microgels also undergo a reversible volume phase transition in which particles shrink considerably. This property might lead to application of microgels as effective targeted drug delivery and release system. We used Dynamic Light Scattering to study microgels at different temperatures and applied line shape analysis algorithm to analyze resulting spectra. We found that the microgel initial size depends heavily on polymer concentration. We also found that varying salt concentration affects the dynamics of microgels. In addition, we were able to determine the effective cross linking density that yields relatively monodisperse microgels. We explored the structure of several microgel solutions by angular dependence analysis and found that most of them were spherical particles. In addition, we explored dynamics of the same microgels at different temperatures that enabled us to monitor their shrinking behavior. [Preview Abstract] |
Wednesday, March 12, 2008 11:39AM - 11:51AM |
Q25.00003: Neutron Flow-Mapping of Controlled-Architecture Polymer Melts Tom McLeish, Nigel Clarke, Pierre Chambon, Edoardo De Luca, John Embery, Christine Fernyhough, Tim Gough, Richard Graham, Isabelle Grillo, Lian Hutchings, Kamakshi Jagannathan, Harley Klein We report on results of a new method for probing complex flows of entangled polymer melts that is able to compare simultaneously chain configurations on different length scales and stress distribution in the flow with the predictions of molecular models. Controlled-architecture melts synthesised by anionic polymerisation and selectively deuterated are made in sufficient quantities to fill a recirculating flow device that contains a windowed processing zone. This may take the form of a constriction or a cross-slot. The whole processing rig is scanned across a narrow neutron beam before a small-angle detector that reports on the structure factor of labelled chains (this sometimes requires the subtraction of two differently-labelled experiments). The same flow is probed in birefringence, measuring the independent orientation at the bond level of the chains. [Preview Abstract] |
Wednesday, March 12, 2008 11:51AM - 12:03PM |
Q25.00004: Chain dynamics in a semidilute polymer solution under steady shear Prasanth Jose, Grzegorz Szamel We performed a series of Brownian dynamics simulations of sheared semidilute solutions of relatively short polymer chains and found that incomplete cancellation of intra and inter chain contributions to the steady state structure factors leads to anisotropic patterns that resemble those observed in light scattering experiments on sheared semidilute solutions (J. Chem. Phys. 127, 114905(2007)). Here we investigate the changes in the single-chain dynamics in sheared semidilute solutions. We study the shear-rate dependence of the end-to-end vector relaxation, Rouse modes' dynamics, etc. We correlate the changes in the chain dynamics with shear rate dependence of the anisotropic scattering patterns and shear thinning of the solution viscosity. [Preview Abstract] |
Wednesday, March 12, 2008 12:03PM - 12:15PM |
Q25.00005: Investigation of Extensional Flow-induced Crystallization in Entangled Polymer Melts Jong Kahk Keum, Yimin Mao, Feng Zuo, Benjamin S. Hsiao To investigate the extensional flow-induced crystallization in polymer melts by means of synchrotron X-rays, a unique cross-slot flow apparatus has been devised and tested. In-situ wide-angle X-ray diffraction (WAXD) results of isotactic polypropylene ($i$PP) showed that the formation of crystallization precursor structure (i.e., shish-kebab structure) under extensional flow is strongly governed by applied strain as well as strain rate. A critical strain,$\varepsilon _{c }$exists in the formation of flow-induced crystallization precursor structure. Below $\varepsilon _{c}$, no shish-kebab structure can be formed even when the applied strain rate,$\dot {\varepsilon }$ is higher than the critical strain rate,$\dot {\varepsilon }_{c}$. Results also confirmed that the crystalline shish precursor structure is formed first and it induces the growth of folded-chain lamellae, i.e. kebabs. [Preview Abstract] |
Wednesday, March 12, 2008 12:15PM - 12:27PM |
Q25.00006: Transport and rheology in block copolymer mesophases Xusheng Zhang, Jorge Vi\~{n}als Free energy functionals of an order parameter field are widely used to describe ordered phases and flows in block copolymer melts. We present a systematic derivation of the transport equations governing order parameter diffusion and hydrodynamic flows by considering the order parameter $\phi$ and the local deformation $u$ as the two independent variables. By studying the static and dynamic response of the copolymer, we derive expressions for the reactive and dissipative components of the stresses. We finally use these equations to study the low frequency rheology of the copolymer under an applied oscillatory shear. [Preview Abstract] |
Wednesday, March 12, 2008 12:27PM - 12:39PM |
Q25.00007: Universal Scaling of Linear and Nonlinear Rheological Properties of Semidilute and Concentrated Polymer Solutions Ronald Larson, Youngsuk Heo We examine the validity of the de Gennes ``blob'' concept in predicting linear and nonlinear rheological properties of semidilute polystyrene solutions in tricresyl phosphate (TCP). At a fixed value of rescaled concentration c/ce where ce is the entanglement concentration, below a critical value of around 2.0 for our polystyrene/TCP solutions, linear and nonlinear rheological functions superimpose after the modulus and the frequency (or shear rate) of each solution are respectively normalized with the concentration-dependent plateau modulus and the equilibration time obtained from the de Gennes scaling relationships using the literature value of the solvent-quality exponent 0.53. However, once the polymer volume fraction exceeds the ``swelling volume fraction, above which the polymer takes on a random walk configuration on all length scales even in a good solvent, this universal scaling breaks down and the polymer conformation appears to be governed by Colby-Rubinstein's scaling laws for theta solutions. We estimate that all polybutadiene solutions in phenyl octane (a good solvent) from the work of Colby \textit{et al. }are above the swelling concentration and can be scaled using theta solvent scaling laws for concentrations ranging all the way up to the melt, showing universal behavior of melts and solutions above the swelling concentration. [Preview Abstract] |
Wednesday, March 12, 2008 12:39PM - 12:51PM |
Q25.00008: The linear rheological responses of cyclic polyoctenamer melt Miao Hu, Gregory McKenna, Yan Xia, Robert Grubbs, Julie Kornfield There is continuing interest in the dynamics of macrocylic polymers or polymer rings. Here we are working with novel polyoctenamer rings synthesized by a ring opening metathasis polymerization (ROMP) route that precludes linear contamination when pure catalyst is used. While the rings are polydisperse in their molecular weights, the method permits synthesis of extremely high molecular weight entitities. Here we report results on the dynamic moduli and the zero shear rate viscosities of both the cyclic polyoctenamer of Mw up to nearly 400,000 g/mol (which is nearly 50 entanglements) and the linear analogue. Comparisons will be made with prior literature results on rings made by ring closure methods in dilute solution where contamination with linear chains was problematic and where the entanglement density was less than 20. [Preview Abstract] |
Wednesday, March 12, 2008 12:51PM - 1:03PM |
Q25.00009: Rheology and birefringence of Fomblin YR at very high shear rates. Khaled Mriziq, Hank Cochran, Mark Dadmun Simultaneous measurements of the rheological and structural properties of perfluoropolyether (PFPE) lubricant films were measured at relatively low to extremely high shear rates using a rotational optical rheometer. The viscosity of various films with different thicknesses exhibit Newtonian behavior up to a shear rate 1$\times $10$^{4}$ s$^{-1}$, with a transition to shear-thinning behavior obvious at higher shear rates. Birefringence of these films was also measured, and these results indicate chain alignment with shear in the shear-thinning regime. The shear rate at which alignment occurs is similar to that of the onset of shear thinning. This correlation between chain alignment and shear thinning provides direct evidence that the ability of PFPEs to lubricate hard drives at high shear rates is a direct consequence of the ability of the applied shear field to align the molecules on a molecular level. [Preview Abstract] |
Wednesday, March 12, 2008 1:03PM - 1:15PM |
Q25.00010: Rheo-Dielectric Studies of Concentrated Polyisoprene Solutions Jai Pathak, Riccardo Casalini, C. M. Roland, Simone Capaccioli, Nikos Hadjichristidis We have performed dielectric spectroscopy on concentrated and well-entangled Cis-1,4-Polyisoprene solutions in n-tetradecane while they undergo steady step shear. The volume fraction of polymer is ~ 0.2, and the number of entanglements per chain varies between 20 and 30. Cis-1,4-Polyisoprene is a Stockmayer Type-A dipole, with one dipole moment component along the chain backbone. Hence dielectric spectroscopy on it reveals the global chain relaxation (normal mode), in addition to its segmental mode relaxation. We critically examine the normal mode in strongly non-linear flows in the cone (22.8 mm diameter)-platen (25 mm) geometry. We work with a sea of fluid surrounding the cone, and the edge remains intact deeper into the shear thinning regime. In addition to visual inspection of the sample edge, we also check for validity of the Cox-Merz rule, finding good agreement between the two. Preliminary results indicate that the dielectric strength continually decreases well into the non-linear regime, perhaps a consequence of the orthogonal directions of the dielectric measurement (velocity gradient direction) and the chain stretching (flow direction). [Preview Abstract] |
Wednesday, March 12, 2008 1:15PM - 1:27PM |
Q25.00011: Isothermal and Self-Seeding Crystallization from Polyethylene Solution Howard Wang, Narayan Ch Das, Kaikun Yang, Boualem Hammouda We have applied time-resolved small angle neutron scattering (SANS) to studying self-seeding and isothermal crystallization kinetics of low molecular weight polyethylene in solution. In one example, SANS spectra of isothermal crystallization of 2.1 kg/mol PE at 78.8 $^{\circ}$C from a solution containing 4.3 {\%} PE by mass show both the characteristic form and structure factors of stacked lamellae. The intensity hump around 0.025 {\AA}$^{-1}$ indicates the correlation among lamellae. On the other hand, SANS from the same solution after quenched from melt to the room temperature, then stored at 90 $^{\circ}$C for 5 min, followed by quenching to 78.7 $^{\circ}$C are different from those of isothermal crystallization; the first correlation peak from the structure factor is not obvious, implying that crystals are dominantly in single or few lamellae forms. A morphological model that accommodates arbitrary distributions of structures from individual lamellae to infinite stacks has been used to analyze the SANS data. A possible lyotropic transition from isolated to stacked lamellae is suggested. [Preview Abstract] |
Wednesday, March 12, 2008 1:27PM - 1:39PM |
Q25.00012: Retention behavior of star-shaped polymers near the chromatographic critical condition Jesse Ziebarth, Yongmei Wang, Kyuhyun Im, Hae-Woong Park, Youngtak Kim, Sunyoung Ahn, Taihyun Chang The retention behavior of star-shaped polymers near the liquid chromatographic critical condition (LCCC) was investigated with 2-D liquid chromatography of polystyrene (PS) and lattice Monte Carlo simulations. At the LCCC for linear PS, stars with short arms elute after linear PS, while stars with long arms show a cross-over from late to early elution as branch number increases. Monte Carlo simulations show that two factors, excluded volume interactions and attractive end-effects resulting from initiator butyl groups, are required to explain the elution of star polymers. When polymers are modeled by random walk chains without excluded volume interactions, all stars are slightly more attracted to pores than corresponding linear chains. When polymers are modeled as self-avoiding walks (SAW) that include excluded volume interactions, stars with short arms elute later and stars with long arms elute earlier compared to linear chains. Incorporating more attractive-ends in SAWs results in the cross-over elution shown by long-armed stars in experiments. More attractive ends in PS star samples were confirmed through chromatographic retention of model monomers. [Preview Abstract] |
Wednesday, March 12, 2008 1:39PM - 1:51PM |
Q25.00013: Mechanical Hole Burning Spectroscopic Investigation Qian Qin, Gregory McKenna Mechanical spectral hole burning (MSHB) was previously applied to a densely entangled block copolymer and successfully distinguishes the heterogeneous from the homogeneous state. Here, we chose polystyrene (PS) solutions to further investigate the effect of entanglement density on mechanical spectral hole burning. The entanglement density was varied by changing either solution concentration or molecular weight of the PS. Dynamics in different regimes ranging from close to the Rouse regime into the terminal region were also examined.Our results are consistent with a heterogeneous dynamics over both entanglement (or plateau) and Rouse regimes. Terminal relaxation dynamics, on the other hand, were always found to be homogeneous for the PS/diethyl phthalate solutions investigated. In addition, no significant dependence of the hole burning event on entanglement density was observed. [Preview Abstract] |
Wednesday, March 12, 2008 1:51PM - 2:03PM |
Q25.00014: Linking number of linear chain in polymer solution and melts Qi Liao We present the statistical results of linking number of linear chains prepared by Monte Carlo and molecular dynamics simulations of polymer solution and melts. Simulations were performed for a wide range of chain lengths covering both non-entangled and entangled polymer dynamics. The simulation results for linking number dependence on chain length and distribution function are compared with the prediction and conjecture of topology. [Preview Abstract] |
Wednesday, March 12, 2008 2:03PM - 2:15PM |
Q25.00015: Computer simulation study on the shear-induced phase separation in semi-dilute polymer solutions by using Ianniruberto-Marrucci model Shotaro Nishitsuji, Mikihito Takenaka, Takashi Taniguchi, Hirokazu Hasegawa When shear flow is imposed to a semi-dilute polymer solution at its one phase region, the solution exhibits strong turbidity. This phenomenon is called shear-induced concentration fluctuation and/or phase separation. Theoretically, Doi and Onuki submitted a two fluid model which incorporates the gradient term of the stress tensor into Ginzburg-Landau type free energy functional, and account for the spatial heterogeneity of the stress field by dynamic asymmetry. We developed the new computer simulation scheme with Doi-Onuki theory, where the Ianniruberto-Marrucci model (IM model) is employed as the constitutive equation, to simulate the dynamics of the shear-induced concentration fluctuation and/or phase separation in semi-dilute polymer solutions. In the simulation results, the concentration fluctuations appear and coarsen with time under shear flow and the change in shear stress with time exhibits the overshoot behavior due to the relaxation of entanglement. These results agree with the experimental results. [Preview Abstract] |
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