The focus of this proposal is novel materials created by isotropically grafting inorganic nanoparticles with organic polymers. Since the inorganic nanoparticles and organic polymers typically dislike each other, these hybrid particles behave like nanoparticle amphiphiles. By analogy to amphiphiles, these hybrid particles can self assemble into a range of superstructures of immediate relevance to many applications in the physical and biological sciences. Preliminary calculations suggest that this self assembly reflects a balance between the energy gain when particle scores approach versus the entropy loss of distorting the grafted polymers. The PIs plan to consider this issue theoretically and begin by delineating regions of parameter space where these self assembled structures are formed, and where they are equilibrium rather than temporally evolving structures amenable to kinetic control. Looking ahead, we ask how more complicated architectures, such as particles grafted with block copolymers might behave. This question is inspired by the large zoology of structures that have been predicted and obtained from triblock copolymers. The PIs also extend these ideas to other nanoparticle shapes, e.g., nanorods and nanosheets, and examine what shapes of nanoparticle assemblies may arise. Again, inspired by a broad range of experimental activities on block copolymers, the PIs query the role of external fields (e.g., flow, electric, magnetic) in directing the superstructures that form. The overarching goal is to a prior design isotropically decorated nanoparticles that can spontaneously assemble into progressively more complex superstructures. While these questions are of import from a fundamental viewpoint, they will be of particular practical interest since they provide unique means of controlling the global nanoparticle dispersion state, and hence the macroscopic properties, of polymer nanocomposites. The PIs propose a collaborative effort between two PIs, who will combine computer simulations and mean-field theory to tackle fundamental issues underpinning our nascent understanding of self-assembly (and directed assembly) of nanoparticle amphiphiles.
The PIs have collaborated actively for over twenty years, and the PIs bring separate but complementary skill sets to the proposed research. The geographical proximity, and shared graduate students, also strongly facilitate this research and emphasizes the synergistic nature of the activities proposed.
Broader Impact:
The ability of decorated nanoparticles to self assemble into superstructures of arbitrary complexity, and the ability to direct this assembly process through the use of external fields, could fundamentally alter our ability to design nanoparticle assemblies (and hence polymer nanocomposites) with desired macroscale properties. Apart from these research activities, the PIs shall continue to develop REU programs targeting underrepresented minorities. The PIs shall utilize the fact that FAMU, a partner school, is a historically black school, and use this to recruit undergraduate students with the goal of retaining them in the sciences.
The main focus of the this project, which was active in the period 9/1/10-8/31/13, was on grafted nanoparticle and colloid-polymer systems. A key finding was that variations in core size and grafting density of nanoparticles lead to self-assembly into a variety of distinct structures. At the boundaries between different structures, the nanoparticle systems undergo thermoreversible transitions. This structural behavior, which has not been previously reported, deviates significantly from that of simple liquids. The reversible nature of these transitions in solvent-free conditions offers new ways to control self-assembly of nanoparticles at experimentally accessible conditions. For colloid-polymer systems, we found that density binodals exhibit universal characteristics along the low-colloid-density branch, but such features are not present in the corresponding high-density phase. However, pressure binodals collapse nicely under such a scaling, which allowed us to produce a binodal curve whose shape is independent of either size ratio. Finally, a remarkable discovery was made in the last year of the award, namely that essentially pure HCP crystals can be obtained from a colloid-polymer system for specific polymer chain lengths, because of the favorable chemical potential of the polymer in the void spaces of the crystal.Differentiation between crystal isomorphs via the interplay of polymers and void symmetry has many important potential applications, especially in self assembling photonic nanostructures where long-range crystalline order is crucial. In terms of development of human resources, two undergraduate students, two graduate students (one female), and two post-doctoral associates received support from the award. One graduate student received her PhD degree in September 2013 and will be joining a technology development company, while the other student will likely complete his degree in the fall of 2015. Of the postdoctoral associates, one is now in academia, and one is employed a research engineer. The following publications resulted from this award and acknowledge its financial support: A. Chremos and A. Z. Panagiotopoulos, "Structural Transitions of Solvent-Free Oligomer-Grafted Nanoparticles," Phys. Rev. Lett., 107, (2011). A. Chremos, A. Z. Panagiotopoulos, H. Y. Yu and D. L. Koch, "Structure of solvent-free grafted nanoparticles: Molecular dynamics and density-functional theory," J. Chem. Phys., 135, (2011). A. Chremos, A. Z. Panagiotopoulos and D. L. Koch, "Dynamics of solvent-free grafted nanoparticles," J. Chem. Phys., 136, (2012). B. B. Hong, A. Chremos and A. Z. Panagiotopoulos, "Dynamics in coarse-grained models for oligomer-grafted silica nanoparticles," J. Chem. Phys., 136, (2012). B. B. Hong, A. Chremos and A. Z. Panagiotopoulos, "Simulations of the structure and dynamics of nanoparticle-based ionic liquids," Faraday Discussions, 154, 29-40 (2012). N. A. Mahynski, T. Lafitte and A. Z. Panagiotopoulos, "Pressure and density scaling for colloid-polymer systems in the protein limit," Physical Review E, 85, (2012). N. A. Mahynski, B. Irick and A. Z. Panagiotopoulos, "Structure of phase-separated athermal colloid-polymer systems in the protein limit," Physical Review E, 87, (2013). N. A. Mahynski and A. Z. Panagiotopoulos, "Phase behavior of athermal colloid-star polymer mixtures," J. Chem. Phys., 139, (2013). T. Lafitte, S. K. Kumar, and A. Z. Panagiotopoulos, "Self-Assembly of Polymer-Grafted Nanoparticles in Thin Films," submitted for publication (2013). D. Meng, S. K. Kumar, G. S. Grest, N. A. Mahynski and A. Z. Panagiotopoulos, "Entropically-Driven Phase Behavior of Nanoparticle/Polymer Melts," submitted for publication (2013). N. A. Mahynski, A. Z. Panagiotopoulos, D. Meng, and S. K. Kumar, "Stabilizing a Colloidal Crystal by Leveraging Void Distributions," submitted for publication (2013). D. Meng, S. K. Kumar, G. S. Grest, N. A. Mahynski and A. Z. Panagiotopoulos, "Entropically-Driven Phase Behavior of Nanoparticle/Polymer Melts," submitted for publication (2013). A. Nikoubashman, N. A. Mahynski, A. H. Pirayandeh and A. Z. Panagiotopoulos, "Flow-Induced Demixing of Polymer-Colloid Mixtures in Microfluidic Channels," in preparation (2013).
