Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session W4: Dynamics of Polymers |
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Sponsoring Units: DPOLY Chair: Ralph Colby, Pennsylvania State University Room: Morial Convention Center 206 |
Thursday, March 13, 2008 2:30PM - 3:06PM |
W4.00001: Dielectric and Viscoelastic Investigation of Entanglement Relaxation Invited Speaker: For a chain (probe) entangled with surrounding chains (matrix), the entanglement is released on large-scale motion of the matrix chains. This constraint release (CR) mechanism plays a central role in the current tube model for entangled chains. Since the distance for the lateral motion of the probe (= tube diameter) increases on CR, the tube model often utilizes the molecular picture of dynamic tube dilation (DTD) to represent the CR effect on the probe dynamics. In the simplest case of full-DTD, the relaxed portion behaves as a solvent giving no constraint and the tube diameter increases to the diameter in the corresponding solution. This talk utilizes cis-polyisoprene (PI) having the type-A dipole to test the validity of the DTD picture with the following strategy. The type-A dipole allows us to dielectrically evaluate the survival fraction f(t) of the dilated tube at time t. The full-DTD picture can be unequivocally tested by comparing the normalized viscoelastic modulus deduced from this picture (2.0-2.3th power of f(t)) with the viscoelastic data. The comparison indicated that the full-DTD picture works satisfactorily for monodisperse linear PI but not for blends of linear PI as well as for monodisperse star PI. The failure of the full-DTD picture for the last two systems is related to the fast relaxation modes in these systems. These modes leads to a significant increase of the fully dilated tube diameter over which the CR motion of the probe cannot occur in time, which naturally results in the failure of the full-DTD picture. Even for this case, the tube diameter increases to a level accepted by the CR motion. The partial-DTD picture considering this consistency with the CR motion was found to be valid for blends and star chains. [Preview Abstract] |
Thursday, March 13, 2008 3:06PM - 3:42PM |
W4.00002: How does cohesive breakdown occur in entangled polymeric liquids? Invited Speaker: Entangled polymers are strongly viscoelastic materials with a characteristic relaxation rate that is a sensitive function of molecular weight and its distribution. At high Weissenberg number, i.e., when the rate of external deformation is far greater than the relaxation rate of entangled chains, a well entangled polymeric liquid yields like a solid before being forced to flow plastically. Recent particle tracking velocimetric observations (PTV, which are available for downloading at http://www3.uakron.edu/rheology/) show that the elastic yielding and subsequent flow take place inhomogeneously in both shear and extension. Most remarkably, an entangled polymer would suffer ``delayed'' cohesive structural breakdown after a step deformation. The transient cohesion provided by chain entanglement due to inter-molecular interactions is found to be higher at a higher applied rate. This talk will enumerate various PTV studies of step shear, startup shear and large-amplitude oscillatory shear to show how these crucial experiments have produced a phenomenological level understanding (\textit{J. Chem. Phys. }\textbf{2007}, $127$, 064903) of various flow phenomena in entangled polymers. [Preview Abstract] |
Thursday, March 13, 2008 3:42PM - 4:18PM |
W4.00003: Dynamics of Polymer-Nanoparticle Mixtures Invited Speaker: Mixtures of polymers and particles occur in a variety of applications. Traditional applications of polymers in such systems include their role as colloidal stabilizers, and in rheological modifiers. Many of these applications are characterized by the feature that the polymer size is much smaller than the size of the particle. However, more recent developments in nano- and biotechnology applications have moved the polymer-particle mixtures from the ``colloid limit'' to the ``nanoparticle limit'' where the polymer size is comparable to or larger than the size of the particle. At the equilibrium level, the curvature of the particle now plays an important role in determining the interactions and phase behavior. At a dynamical level, conventional ``continuum'' wisdom no longer applies, and counterintuitive property relationships have been observed. This talk will focus on recent work in our group to develop and apply novel computer simulations to address the issue, ``how does the equilibrium, dynamical and property aspects of nanoparticle-polymer mixtures differ from their colloidal counterparts?'' Applications of our findings to the context and experiments of polymer nanocomposites will also be presented. [Preview Abstract] |
Thursday, March 13, 2008 4:18PM - 4:54PM |
W4.00004: Shear Alignment and Realignment of Block Copolymer Microdomains in Thin Films Invited Speaker: Bulk block copolymers, like all liquid crystalline structures, are well-known to align under flow. In the past few years, we have shown that analogous flow alignment can be achieved in substrate-supported thin films ($<$100 nm thick) containing only one or a few layers of spherical or cylindrical nanodomains. Alignment can easily be imparted either by pulling a soft rubber pad in contact with the top surface of the film, or by flowing a nonsolvent fluid across the film. The latter geometry opens the possibility to ``write'' relatively complex patterns on the millimeter or submillimeter scale, where the nanodomain director follows the fluid streamline. Alignment can be achieved via either unidirectional or oscillatory shear, and is conveniently executed in a parallel-plate rheometer, where the substrate-supported film forms one ``plate'' and the ``gap'' is filled with the nonsolvent fluid. A threshold stress is required to achieve alignment of the microdomains, a stress which decreases steadily as the temperature is raised towards the polymer's order-disorder transition temperature. A simple melting-recrystallization model appears to capture the dynamics of overall alignment. Though no grain boundaries remain in well-aligned films, isolated dislocations persist. For sphere-formers, where two or more layers are required for alignment, the isolated dislocations are preferentially oriented in such a way as to facilitate sliding of the two layers of spheres past each other. Once a macroscopic orientation has been imparted to the film (over square-cm area), the microdomains can be reoriented by applying shear in a different direction, but a higher threshold stress is required than was needed for the initial alignment from the polygrain state. Recently, we have observed a sphere-to-cylinder transition in one particular block copolymer under shear, opening another possible mechanism for shear-induced alignment of the spheres which form when these cylinders relax. [Preview Abstract] |
Thursday, March 13, 2008 4:54PM - 5:30PM |
W4.00005: Nanoparticle Ionic Fluids Invited Speaker: Nanoparticle ionic materials (NIMS) are a new class of organic-inorganic hybrid materials comprised of a nanoparticle core functionalized with a covalently-attached organic corona. These materials manifest a remarkable transition to a ``solvent-free'' colloidal liquid state near room temperature. Physical properties of these nanoparticle ionic fluids can be manipulated over an unusually wide range by varying geometric and chemical characteristics of the inorganic core and organic corona. On one end of the spectrum are materials with a high core particle contents, which display properties similar to fragile glasses, stiff waxes, and gels. At the opposite extreme are systems that spontaneously form particle-based ionic fluids characterized by transport properties remarkably similar to simple molecular liquids, but with high dielectric constants, conductivities, and refractive index. This talk will introduce nanoparticle ionic fluids based on charged and uncharged corona species, explore their applications, and will discuss physical and mathematical models for understanding their interactions, complex relaxation dynamics, and rheology. [Preview Abstract] |
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