Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session P40: Polymer Melts & Solutions |
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Sponsoring Units: DPOLY Chair: Azar Alizadeh, GE Global Research Room: A122/123 |
Wednesday, March 23, 2011 8:00AM - 8:12AM |
P40.00001: Radical-cured block copolymer modified thermosets Erica Redline, Lorraine Francis, Frank Bates Poly(ethylene-\textit{alt}-propylene)-$b$-poly(ethylene oxide) (PEP-PEO) diblock copolymers were synthesized and added at 4 wt. {\%} to bisphenol A glycidyl methacrylate (BisGMA). The mixture was thermally cured using free radical chemistry. In separate experiments, 4 wt. {\%} PEP-PEO was added to a combination of poly(ethylene glycol) dimethacrylate (PEGDMA) and BisGMA and cured. Based on small angle X-ray studies of the modified monomers before curing, diblock copolymers self-assembled into well-dispersed spherical micelles with PEP cores and PEO coronas. TEM results showed that these micellar structures were retained during curing. Fracture resistance measurements indicate that the addition of block copolymers does not significantly toughen these thermoset materials. This finding is contrary to the large increase in fracture resistance observed in block copolymer-modified epoxies. We propose that differences in network structure, originating during polymerization, are responsible. [Preview Abstract] |
Wednesday, March 23, 2011 8:12AM - 8:24AM |
P40.00002: Cavitation in Filled Styrene-butadiene Rubber: A Real Time SAXS Observation Huan Zhang, Arthur K. Scholz, Fabien Vion-Loisel, Edward J. Kramer, Costantino Creton Cavitation of filled and unfilled elastomers under confinement at the macroscopic scale has been experimentally reported and theoretically modeled. However, cavitation occurring at the nanometer length scale has not yet been demonstrated conclusively in rubbers. Real time SAXS with synchrotron radiation was employed to probe the structure changes in carbon black filled styrene-butadiene rubber (SBR) under uniaxial loading. The scattering invariant was calculated and increased sharply at a critical extension depending on both filler content and crosslinking density around q = 0.1 nm-1, which we attributed to the formation of voids. At very large strains, a sharp and wide streak developed perpendicular to the tensile axis in reciprocal space, suggesting the deformation of the voids in elliptical voids along the tensile direction. In step cycle test, we observed that voids only appeared when the current strain exceeded the maximum historical strain (Mullins effect) and attributed the increase of the scattering invariant outside the Mullins region to the creation of new voids rather than to the reopening of old ones. [Preview Abstract] |
Wednesday, March 23, 2011 8:24AM - 8:36AM |
P40.00003: Why is the tube model inapplicable for entangled polymer dynamics at large deformation Shi-Qing Wang, Yangyang Wang Accumulating experimental revelation of the phenomenology governing dynamics of entangled linear polymers at large deformations has caused us to question the legitimacy of the tube model as an acceptable theoretical description of nonlinear polymer rheology. Upon an explicit investigation of its premise, we have come to realize that the tube model did not overcome the difficulty confronted by other theories and did not contain the basic physics required to explain why and how the entanglement network must break down during large deformations. It considered an unrealistic situation where a load-bearing chain relaxed fast in an affinely deformed tube so that only the chain segment orientation produced the shear stress for applied rates lower than the Rouse rate. A non-monotonic relation between the resulting shear stress and imposed strain for startup shear and step deformations arose from excessive chain orientation not collapse of the entanglement network. In the tube model, the nature of the overshoot is not yielding (transition from elastic deformation to flow), but an elastic instability. Accumulating experimental observations contradict this picture. This presentation will elucidate how the emerging physical picture differs from that of the unrealistic tube model. [Preview Abstract] |
Wednesday, March 23, 2011 8:36AM - 8:48AM |
P40.00004: Non-Gaussian Stretching Behavior of Entangled Polymers Yangyang Wang, Shi-Qing Wang The behavior of entangled SBR melts and solutions in rapid uniaxial extension has been studied by rheometric and rheo-optical measurements. A yield-to-rupture transition occurs around the same Rouse Weissenberg number of nine for all samples when the failure mechanism changes from chain disentanglement (yielding) to chain scission. Our results show that elastic rupture takes place only when chains between entanglements are near full extension, the strain at rupture grows with increasing entanglement spacing, and the critical stress for rupture is proportional to the polymer concentration. These characteristics validate the well-known idea to represent entangled polymers in terms of a transient network. [Preview Abstract] |
Wednesday, March 23, 2011 8:48AM - 9:00AM |
P40.00005: Parameters of slip-springs model of polymer entanglement from the maximum likelihood principle Timothy Palmer, Alexei Likhtman, Jorge Ramirez, Mark Matsen The slip-spring model for polymer entanglements proposed by A.E.Likhtman [Macromolecules; 2005; 38(14); 6128] replaces entangling chains with slip-links, which are anchored via springs. The use of such models allows reduction of complex multi-chain problem to a simpler single chain problem. In this work we test the slip-spring model on the simplest possible situation: replacing two entangled chains by one chain with one slip-link. We demonstrate how Maximum Likelihood Estimation (MLE) can be used to generate parameters for the slip-spring model by observing a multi-chain system. The test system being considered consists of two Rouse polymer chains that are anchored by the ends in an entangled state. The effect of the entanglement is enforced by rejecting all steps that lead to topology violation. We show how the results of this MLE indicate that slip-link models with slip-links fixed in space are not satisfactory, and demonstrate analytically the dependence of the plateau modulus upon the strength of the slip-spring. Our results contradict recent calculations of Schieber and Horio [JCP; 2010; 132(7); 074905] who claimed that the plateau modulus must be independent of the slip-spring strength. [Preview Abstract] |
Wednesday, March 23, 2011 9:00AM - 9:12AM |
P40.00006: Segmental orientation dynamics in bidisperse entangled linear polymer melts Zuowei Wang, Jing Cao, Alexei Likhtman, Ronald Larson Extensive molecular dynamics simulations were performed to investigate the segmental orientation dynamics in mono- and bidisperse entangled linear polymer melts. The binary blends consist of short probe chains diluted in long chain matrices of chain length up to 30 entanglements. With increase of the chain length in monodisperse melts, the bond vector autocorrelation function was found to approach a distinctive time-dependent power law, which is compared with recent NMR experiments. When introduced into long chain matrices, the segmental orientation relaxation and monomer diffusion of short probe chains slowed down strongly due to the suppression of constraint release (CR) effects. The same trend was observed for the end-to-end vector correlation function, reflecting the CR effects on contour length fluctuations. On the other hand, the time-dependent orientation coupling parameter in the entangled systems demonstrates the similar universal behaviour as that discovered in unentangled melts. Considering the stress-optical law was recovered in all simulated systems, our simulations should clarify the connection between rheology and other experimental techniques, which are essential for progress in modeling entangled polymers. [Preview Abstract] |
Wednesday, March 23, 2011 9:12AM - 9:24AM |
P40.00007: Simulations of polymer melts modeled as chains of interacting soft-colloids Anthony Clark, Marina Guenza The range of time and length scales accessible to dynamical simulations of melts of long polymer chains is strongly limited by the computational demands of calculating large numbers of forces between monomers. Simulations modeling each polymer as a point particle interacting by an analytical soft pair potential have previously been successfully developed to extend this range. For many effects in polymer systems, however, submolecular degrees of freedom remain relevent to molecular-level behavior even at long times and large length scales. To allow for the inclusion of relevant submolecular degrees of freedom, we use analytical effective potentials based on our model of the structure of polymer melts on the level of large chain sub-blocks to simulate homopolymer melts. We demonstrate that structure on the block and center of mass level consistent with the structural model and monomer-level simulation data can be reproduced for large systems and long times at much lower computational cost than monomer-level simulations. Using this model, we also test the effects of additional short-range repulsive interactions between chain subunits on structure and dynamics. [Preview Abstract] |
Wednesday, March 23, 2011 9:24AM - 9:36AM |
P40.00008: Renormalized dynamics of overdamped driven elastic media Jonathan Landy, Alex J. Levine We present the results of a dynamical renormalization group calculation used to explore the fluctuations of an elastic body steadily driven through a viscous background fluid. Direct applications of this work involve the study of the fluctuation spectrum of semiflexible filament networks driven through a background fluid by e.g. polymerization, but also include the motion of one-dimensional driven elastic objects (e.g. polymers, flux vortices etc.) In that case, a previous linear stability analysis suggests that, when such elastic lines are driven in a direction perpendicular to their axis, they become unstable at any non-zero driving force [1]. We discuss the affect of nonlinearities on these conclusions, showing that such terms can stabilize the system at finite drive velocities. We similarly explore the dynamics of lines driven parallel their axis showing that these systems exhibit ``weak dynamic scaling'' [2]. Turning to the case of driven elastic solids, we report on the effect of molecular motor-induced forces on the long length scale and long time scale dynamics of the driven system. [1] R. Lahiri and S. Ramaswamy, Are steadily moving crystals unstable?, Phys. Rev. Lett. 79, 1150 (1997) [2] D. Das et al., Weak and strong dynamic scaling in a one-dimensional driven coupled-field model: Effects of kinematic waves, PRE 64, 021402 (2001). [Preview Abstract] |
Wednesday, March 23, 2011 9:36AM - 9:48AM |
P40.00009: Thermodyanmic Scaling of Polymer Dynamics versus Shifting by T-Tg Jiaxi Guo, Sindee Simon A universal scaling law for the relaxation time ($\tau )$ of amorphous liquids as a function of temperature and volume has been proposed by Roland and coworkers: $\tau $(T,V) = F(TV$^{\gamma })$, where $\gamma $ is a material-dependent constant. We test this law for four materials, linear polystyrene, star polystyrene, and two polycyanurate networks using PVT data obtained in our laboratory coupled with the temperature dependent shift factors used to reduce the viscoelastic bulk modulus at different pressures and the dynamic shear properties at ambient pressure. In all cases, $\tau $ can be reduced both by the scaling law and by shifting to account for the changes in T$_{g}$ with pressure, i.e., by plotting versus T - T$_{g}$(P). In the polycyanurate case, time-crosslink density superposition holds and $\tau $ for the two materials can be reduced simply by shifting the temperature with respect to T$_{g}$ to account for the changes in T$_{g }$with crosslink density; however, the thermodynamic scaling for the two materials does not superpose unless the thermodynamic function is normalized by T$_{g}$V$_{g}^{\gamma }$. The validity of the scaling function and its relationship to T - T$_{g}$ will be further examined. In addition, the impact of errors in T, T$_{g}$, and V on the ability to satisfactorily reduce data and obtain universal scaling will be discussed. [Preview Abstract] |
Wednesday, March 23, 2011 9:48AM - 10:00AM |
P40.00010: The linear rheological responses of dense branched brush polymers with different side chain lengths and structures Miao Hu, Gregory McKenna, Yan Xia, Chris Daeffler, Andrew Boydston, Robert Grubbs, Julia Kornfield We examined the linear rheological responses of three kinds of dense and regular branched brush polymers. Brush polymers with different degree of polymerization were synthesized from the $\omega $-Norbornenyl macromolecule (as main chain) with linear, three combined short arms, and dendronized brush structures. The master curves for these brush polymers were obtained by time temperature superposition (TTS) of the dynamic moduli from the glassy plateau region to the terminal flow region. The glassy modulus and rubbery modulus for these brush polymers were greatly influenced by the side chain properties. Two different relaxation processes can be observed for those samples with the higher molecular weight, slightly entangled, side chains. The dilution effect of the side chain which is related to the side chain volume fraction doesn't follows theoretical expectations. [Preview Abstract] |
Wednesday, March 23, 2011 10:00AM - 10:12AM |
P40.00011: Analytical rheology of metallocene-catalyzed polyethylenes Sachin Shanbhag, Arsia Takeh A computational algorithm that seeks to invert the linear viscoelastic spectrum of single-site metallocene-catalyzed polyethylenes is presented. The algorithm uses a general linear rheological model of branched polymers as its underlying engine, and is based on a Bayesian formulation that transforms the inverse problem into a sampling problem. Given experimental rheological data on unknown single-site metallocene- catalyzed polyethylenes, it is able to quantitatively describe the range of values of weight-averaged molecular molecular weight, $M_W$, and average branching density, $b_{m}$, consistent with the data. The algorithm uses a Markov-chain Monte Carlo method to simulate the sampling problem. If, and when information about the molecular weight is available through supplementary experiments, such as chromatography or light scattering, it can easily be incorporated into the algorithm, as demonstrated. [Preview Abstract] |
Wednesday, March 23, 2011 10:12AM - 10:24AM |
P40.00012: Pressure-Volume-Temperature Behavior of Hyperbranched Polyols: Experiment and Modelling Mukul Kaushik, Sergei Nazarenko, Brian Olson The pressure volume temperature behavior of two generations of hyperbranched polyesters Boltorn$^{TM}$ H40 and H20 was studied by PVT measurements using high pressure dilatometer. Volumetric expansivity, and free volume parameters were determined for both generations in the melt state. The PVT data were fitted to Simha-Somcynsky (SS) equation of state (EOS) and Sanchez--Lacombe (SL) equation of state (EOS) to calculate occupied volume and fractional free volume. The values of occupied volume and fractional free volumes obtained through both the equations of states were similar. Simulated atmospheric pressure V-T data were generated by using Discover module of Accelrys$^{\mbox{{\textregistered}}}$. Quality of equilibrium was confirmed by energy stabilization and closeness of experimental and simulation densities. WAXD and temperature-volume curves obtained by molecular dynamics simulations were comparable to the experimental data. Well relaxed amorphous cell was further utilized to study hydrogen bond network and determination of O-O pair correlation function of terminal hydroxyl groups. [Preview Abstract] |
Wednesday, March 23, 2011 10:24AM - 10:36AM |
P40.00013: Influence of the Hofmeister Series on Lower Critical Solution Temperature (LCST) Polymers Ryan Toomey, Leena Patra Lower critical solution temperature (LCST) polymers can serve as model systems for probing the effect of ions on the stability of biological macromolecules. In this talk, we show how permutations in the chemical structure of poly(N-isopropylacrylamide), including n-propylacrylamide, cyclopropylacrylamide, and N-vinylisobutyramide influence the action of ions in the Hofmeister series. By using a combination of ellipsometry and FTIR, we show that ions salt out neutral polymers by enhancing the surface tension of the hydrophobic portions of the polymer. Weakly hydrated ions (known as chaotropes) can also lead to salting-in effects through interactions with amide dipoles. This salting-in effect is strongly modulated by the surrounding hydrophobic groups. The larger the hydrophobic group the weaker the salting-in effect, indicating that the specificity of the Hofmeister series results from a combination of ion-dipole interactions and hydrophobicity. [Preview Abstract] |
Wednesday, March 23, 2011 10:36AM - 10:48AM |
P40.00014: Ionic Liquids for the Imaging of Wet Polymer Morphology David Hoagland, John Harner, Malvika Bihari Unlike convention aqueous and organic solvents, ionic liquids are essentially nonvolatile and thus compatible with the high vacuum environments of electron microscopy. Here is described the room temperature imaging of wet polymer systems such as patterned gels, gel networks, polymeric vesicles, and proteins. Both TEM and SEM images will be offered, along with a discussion of difficulties in applying the two techniques. Via SEM, imprinted surface structures as small as 100-300 nm can be captured for chemically crosslinked gels (polyHEMA), and via TEM, the structure of a physical gel (PEG) is viewed at the 50-to-10-nm scale, revealing network connectivity established by PEG crystallinity. Self-assembled vesicle and micelle structures will be presented for dispersed block copolymers, and the same approach will be applied toward discerning the quality of dispersion for proteins (ferritin) and other nanoparticles. [Preview Abstract] |
Wednesday, March 23, 2011 10:48AM - 11:00AM |
P40.00015: Biaxial Strain Testing of Extremely Soft Gels Kenji Urayama, Yohsuke Bitoh, Toshikazu Takigawa We present a biaxial tensile tester to characterize the nonlinear stress--strain behavior of extremely soft polymer gels with very low shear moduli, of the order of 100 Pa, under general (equal and unequal) biaxial strain. Stretching of gel sheet specimens in a solvent bath can avoid finite self-weight bending deformations that have precluded biaxial tensile experiments with such extremely soft gels. General biaxial strain covers a wide range of physically accessible deformations in contrast to conventional uniaxial strain that is only a special one among them. The biaxial data for fully swollen chemical gels reveal that the exceptional agreement of uniaxial data with predictions of the simplest rubber elasticity model (ideal gas model), which has been known for over 60 years, is superficial because the model evidently fails to describe the biaxial data. This new biaxial tester will be a powerful tool for the full characterization of the large deformation behavior of extremely soft materials, including biological soft tissues. [Preview Abstract] |
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