285764 Engineering Entropy and Order in Nanomaterials Via Molecular Simulation

Wednesday, October 31, 2012: 12:30 PM
415 (Convention Center )
Fernando Escobedo, Department of Chemical and Biomolecular Engineering, Cornell University, Ithaca, NY

By elucidating structure-property relationships of polymeric and colloidal materials, molecular simulations can assist the engineering of materials of desirable or “super” properties; e.g., those that originate in the creation of special types of structural order or the control of phase transitions. While molecular engineering often focuses on tuning chemical interactions, the engineering of entropic interactions is often equally crucial but less appreciated.  In this presentation I will focus on mesophases; i.e., materials whose structural order lies in between that of solids and liquids, which often offer a combination of characteristics that makes them attractive to create materials with new or “super” properties. Three sample cases will be described involving diblock copolymers, polymer networks, and colloidal suspensions. Regarding diblock copolymers, I will quickly illustrate the idea that morphologies of seemingly higher structural order, such as the so-called “plumber’s nightmare phase”, can be stabilized by maximizing the conformational entropy of a homopolymer additive. Regarding polymer networks, I will describe a new design of elastomer having super-toughness (i.e., requiring a large amount of energy to be broken) by virtue of a modular mechanism involving a novel the strain-driven formation of successive smectic domains. This virtual material results in a stress-strain behavior that in some respects is similar to that of abalone shells (a nature-made supertough material) and spider dragline.

Colloidal particles can form ordered solid and liquid phases that possess unique optical, rheological, and mechanical properties, making them attractive components in the preparation of novel composite, photonic, plasmonic, and photovoltaic materials. I will describe our recent efforts on the use of molecular simulations to map out the phase behavior of suspensions of particles with polyhedral shapes. Our results provide both a basis to existing experimental observations and predictions of novel phases yet to be seen in the lab. In particular we predict the formation of various novel entropy-driven self-assembling liquid- and plastic-crystalline phases which are resilient to size polydispersity.

These examples illustrate that entropy can be harnessed to create new types of ordered materials. Entropy is indeed a “creative” natural force that gives molecules or particles the freedom necessary to explore and find different solutions that would be inaccessible if enthalpic interactions were to rule unchallenged.

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