Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session P4: Kinetic Pathways to Assembly of Polymers, Particles and Biomolecules |
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Sponsoring Units: DPOLY Chair: Ryan Hayward, University of Massachusetts Room: Ballroom A4 |
Wednesday, March 23, 2011 8:00AM - 8:36AM |
P4.00001: Equilibration and metastability in block copolymer micelles Invited Speaker: The strong incompatibility between a solvophobic block and the surrounding medium leads naturally to extremely slow chain exchange kinetics between micelles. The steric barrier between well-solvated coronas similarly inhibits micelle fusion/fission processes. Consequently, equilibration of block copolymer micelles is typically prohibitively slow. As a result, it is possible for one system to adopt quite different micellar shapes and sizes, depending on preparation method. However, by using low volatility solvents such as ionic liquids or paraffinic oils, combined with weakly solvophobic blocks, it is possible to design model systems with accessible critical micelle temperatures. This enables the study of both the mechanisms of chain exchange, by time-resolved small-angle neutron scattering, and the evolution of non-equilibrium structures, by dynamic light scattering. Examples of both approaches will be discussed. [Preview Abstract] |
Wednesday, March 23, 2011 8:36AM - 9:12AM |
P4.00002: Uncovering the Biological Identity at the NanoScale; Fundamantal principles governing interactions between nanoparticles and living organisms Invited Speaker: This abstract not available. [Preview Abstract] |
Wednesday, March 23, 2011 9:12AM - 9:48AM |
P4.00003: Nanoparticle formation by block copolymer directed rapid precipitations---Flash NanoPrecipitaiton Invited Speaker: With widespread interest in the generic ``nano'' attention has been focused on strategies of making small particles. High-value applications that drive new process innovation include very hydrophobic pharmaceutical actives, dyes and pigments for ink jet printing, or the dispersal of highly toxic insecticides on carriers. While it is relatively easy to make inorganic nano-particles, for example CdS particles, it is much more challenging to make nanoparticles from low surface energy organic solids. Strategies for forming nano particles vary from supercritical spraying, supercritical freezing, milling, solvent exchange precipitation, and imbibing into polymeric micelles. The solute and process combine to give differences in crystalline/amorphous products, individual particles/agglomerates, and uniformity/polydispersity of sizes. We will give an overview of the techniques and the classes of products that each addresses. We have developed a new technology that has two components: (1) rapid and tailored micromxing in an impinging jet, and (2) novel block copolymer stabilizers. The impinging jet process allows the production of nano-particles by: 1) elimination of mass transfer limitations and compositional gradients within 10 ms as determined by independent measurements with competitive-parallel reactions, 2) production of high supersaturations and solute concentrations so that high production rates can be obtained, and 3) control of particle size by stabilization of the particle using block copolymer self-assembly. The process depends critically on control of three time scales: particle nucleation and growth, block copolymer micellization, and polymer adsorption on the particle to produce steric stabilization. We present data on characterization of the mixing times using competitive reactions, data on polymer micellization kinetics, and results on the successful production of $\beta $-carotene and taxol particles with control of the particle size between 40 nm to 600 nm. A range of block copolymers have been used : PS-b{\_}PEO, PBA-b-PAA, and PCL-b-PEO. Homogeneous rapid nucleation and growth produces particle size distributions that are much narrower than those obtained by alternate size-reduction or precipitation routes, and results in a decreased tendency to Ostwalt ripen. [Preview Abstract] |
Wednesday, March 23, 2011 9:48AM - 10:24AM |
P4.00004: ABSTRACT WITHDRAWN |
Wednesday, March 23, 2011 10:24AM - 11:00AM |
P4.00005: Soft matter self-assembly driven by specific and nonspecific attractions: dynamic pathways and the Ostwald rule of stages Invited Speaker: Ostwald's rule of stages is one of the few rules-of-thumb we possess that suggests the dynamical pathway a material will take when crystallizing. It states that bulk phases intermediate in free energy between the parent phase and the stable solid will emerge before establishment of the stable solid. Although widely applicable, this rule is frequently seen to break down in experiments and computer simulations, showing it to be without theoretical foundation. A first step in going beyond this rule is to understand why it breaks down. Here we test Ostwald's rule of stages in a statistical mechanical model of crystallization. Our model describes particles that are prototypical of a class of materials (such as proteins and patchy nanoparticles) able to form solid phases stabilized by directional attractions, as well as sparse and dense fluidlike phases. We find that the rule holds in certain regimes of parameter space and breaks down in others. Importantly, its breakdown can be anticipated using simple arguments. We show that the qualitative crystallization pathway of the model depends in general on both the thermodynamic landscape prescribed by inter-particle interactions and on the relative rate of particle rotations and translations. This observation emphasizes that any general rule of crystallization must account for both thermodynamic and dynamic factors. [Preview Abstract] |
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