Directed Assembly of Nanoparticles into Composite Materials

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National Science Foundation - Division of Chemical &Transport Systems Particulate & Multiphase Processes Program (1415)Proposal Number: 0652073Principal Investigators: Wong, MichaelAffiliation: William Marsh Rice UniversityProposal Title: Directed Assembly of Nanoparticles into Composite MaterialsIntellectual MeritCapsule materials are used in application areas as diverse as medicine, foods, cosmetics, and paints. There have been significant advances in the development of new capsule synthesis routes, but there remain issues concerning wide particle size distribution, non-scalability, and low structural stability. A newly reported method involves mixing a negatively charged nanoparticle suspension with solutions of a positively-charged polymer and a multivalent anion, leading to the apparent self-organization of nanoparticles into micron-sized capsule structures. The underlying chemistry behind this unique materials synthesis route is not well understood, however. This proposal describes a three-year research and teaching plan that focuses on understanding the relevant physical chemistry and colloid science concepts that explain this particular form of nanoparticle assembly. Addressing the question "What are the principles that govern the nanoparticle assembly synthesis of microcapsules," the new knowledge will serve to advance materials chemistry, specifically by (i) improving the control of microcapsule properties, (ii) increasing one's ability to process nanoparticle suspensions into functional advanced materials, and (iii) contributing to the field of directed nanoparticle assembly as an emerging synthesis tool. The specific research objectives of this project are to gain a stronger understanding of polymer aggregates as charged colloid species; and to ascertain the formation mechanism of the shell walls. The successful completion of the proposed tasks (1) will explain the pH effect on polymer aggregate formation in terms of acid-base chemistry and DLVO theory, (2) will explain the pH effect on microcapsule formation in terms of charged nanoparticle/polymer aggregate interactions, (3) will provide evidence for a coalescence mechanism for polymer aggregate growth, and (4) will support a diffusion-deposition model for shell formation in which nanoparticle size controls the microcapsule shell thickness.Broader ImpactsThe use of nanoparticle building blocks is an important area of materials research, and the results from this project should inspire a fresh approach to nanoparticle-based structures. This form of nanoparticle assembly is quite general, suggesting hollow sphere structures with new compositions and functionalities and entirely new nanoparticle-based architectures are possible. The mild synthesis conditions and the structural properties of microcapsules provide advantages of scalability, ease of encapsulation, and potential applications. Ultimately, a stronger understanding of the NP assembly chemistry will contribute to improved structural control over a technologically relevant material, and to a better understanding of charged-polymer/multivalentanion and charged-NP/charged-polymer interactions.Through this research project, student researchers will learn and develop cutting-edge science guided by the application of engineering concepts. They will perform interdisciplinary research; use and develop state-of-the-art synthesis and characterization techniques; and mentor undergraduate researchers. Female students and students from underrepresented groups will be supported in this project. Three new educational/outreach activities are proposed: a lab module based on NP assembly, a Houston-area high school teacher hosting program, and a high school teachers science lessons development program, with the latter two in coordination with the NSF NSEC center at Rice University.