Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session Y4: Assembly and Organization in Polymeric Systems |
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Sponsoring Units: DPOLY Chair: Jack Douglas, National Institute of Standards and Technology Room: Colorado Convention Center Korbel 2B-3B |
Friday, March 9, 2007 11:15AM - 11:51AM |
Y4.00001: Self Organization via Frontal Polymerization Invited Speaker: There are three modes of frontal polymerization: Isothermal, Photo and Thermal Isothermal frontal polymerization (IFP) is a directional polymerization that utilizes the Norish-Trommsdorff effect, to produce optical gradient materials. When a solution of methyl methacrylate and thermal initiator contacts a polymer seed (a small piece of poly(methyl methacrylate), a viscous region is formed in which the polymerization rate is faster than in the bulk solution. PhotoFP is driven by a continuous input of light. Thermal frontal polymerization is the propagation of a localized reaction zone through the coupling of thermal transport with the Arrhenius dependence of the kinetics of an exothermic polymerization. We will examine IFP and its use in making Gradient Optical Materials (GRIN) and our work on elucidating the mechanism. We will consider how thermal frontal polymerization can be used rapid rapid repair, making gradient materials and to study interesting nonlinear modes of thermal front propagation. [Preview Abstract] |
Friday, March 9, 2007 11:51AM - 12:27PM |
Y4.00002: Structural Analysis and Properties of Supramolecular Assemblies Invited Speaker: Supramolecular and particle assembly is controlled by the shape and interaction among the constituents, and there is great interest involving asymmetric systems. Here we explore physical rules and behavior governing the assembly of anisotropic molecules and particles. These are wedge and conical shaped amphiphilic molecules and asymmetric particles. Diffraction and microscopy are used to determine the assembled structures (such as tubes and capsules) and to establish relationships between structures in systems that undergo transitions from one ordered state to another. [Preview Abstract] |
Friday, March 9, 2007 12:27PM - 1:03PM |
Y4.00003: Controlling Self-Assembly in Thin Block Copolymer Films: From Model Systems to Applications Invited Speaker: We discuss recent experiments and computer simulations dealing with the influence of external fields (electric fields, surface fields) on the spontaneous structure formation in block copolymer melts and block copolymer solutions. We demonstrate the aligning effect of electric fields on non-cubic block copolymer microphases and discuss the microscopic processes responsible for the macroscopic effects. Recent findings include the possibility to tune the nanoscopic characteristic spacing via the strength of the external field. In thin films, on the other hand, the presence of surfaces and the geometrical constraints of finite film thickness dominate the resulting structures. We shall include some applications of these structures, e.g. in the area of responsive membranes and organic light harvesting devices. [Preview Abstract] |
Friday, March 9, 2007 1:03PM - 1:39PM |
Y4.00004: pH and Solubility Effects as Control Mechanisms for Vesicle Interfaces Invited Speaker: It was shown some time ago that in diblock copolymer vesicles the corona chains are segregated by length with short block inside and long blocks outside. More recently, it was shown that a triblock copolymer consisting of a long polystyrene segment with short segments of poly(acrylic acid) and poly(-4-vinyl pyridine) at each end could be converted into vesicles which, depending on the pH during preparation, could have either PAA or P4VP on the outside. For example a low pH, at which the 4VP is protonated, leads to P4VP on the outside because those chains become longer than the nonionic PAA. Furthermore, inversion of the vesicles from PAA outside to P4VP outside is possible by dropping the pH of the solution. It is now shown that under extreme pH conditions in dimethylformamide, the inside and outside interfaces are behaving exactly opposite to what it is expected. For example, at very low pH (3), at which the P4VP should be ionic and therefore longer than PAA, it is found that the PAA is on the outer interface of the vesicle, while the P4VP is inside. The reason for such behavior can be found in the solubilities and coil dimensions of the quaternized P4VP, which is poorly soluble in DMF at low pH and also the poor solubility of the poly(sodium acrylate) in DMF at high pH. The poor solubilities overwhelm the coil expansion accompanying ionization at moderate pH. This solubility effect thus provides us with an added mechanism for controlling the interior and exterior interfaces of vesicles, in addition to the block length and the simple pH effect. This work is based on the MSc Thesis of Renata Vyhnalkova, McGill University, 2005. [Preview Abstract] |
Friday, March 9, 2007 1:39PM - 2:15PM |
Y4.00005: Nanomechanics of Bone: Nanogranular Friction and Heterogeneity Invited Speaker: Bone is a complex nanostructured biocomposite material composed of approximately 60 wt percent carbonated apatite mineralites (10s of nanometers in length and width, 3-5 nm in thickness) that permeate in and around type I collagen fibrils. Here, we present results from studies investigating the ultrastructural origins of the strength and toughness of bone, which is critical for its proper physiological function. A combination of dual instrumented nanoindentation, three-dimensional elastic-plastic finite element analysis (FEA) using a Mohr-Coulomb cohesive-frictional strength criterion, and angle of repose measurements was employed. Our results suggest that nanogranular friction between mineral particles is responsible for increased yield resistance in compression relative to tension and that cohesion originates from within the organic matrix itself, rather than organic-mineral bonding. Nanomechanical heterogeneity is also expected to influence elasticity, damage, fracture and remodeling of bone. Here, we quantify the spatial distribution of nanomechanical properties at the length scale of individual collagen fibrils using atomic force microscope-based nanoindentation. Our results show elaborate patterns of stiffness which do not correlate with topography, and hence are attributed to organic-inorganic compositional variations and nanoscale porosity. We propose a new energy dissipation mechanism arising from nanomechanical heterogeneity which offers a graceful means for ductility enhancement, damage evolution, and toughening. This hypothesis is supported by FEA which incorporate the nanoscale experimental data and predict markedly different biomechanical properties compared to a uniform material, through nonuniform inelastic deformation over larger areas and increased energy dissipation. The fundamental concepts discovered here are applicable to a broad class of biological materials and may serve as a design consideration for biologically-inspired materials technologies. [Preview Abstract] |
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