Lessons from Simulation Regarding the Control of Synthetic Self-Assembly
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Lessons from Simulation Regarding the Control of Synthetic Self-Assembly Jack F. Douglas and Kevin Van Workum† National Institute of Standards and Technology, Polymers Division, Gaithersburg, Maryland 20899, † Present address: US Naval Academy, Annapolis, MD 20142 ABSTACT We investigate the role of particle potential symmetry on self-assembly by Monte Carlo simulation with the particular view towards synthetically creating structures of prescribed form and function. First, we establish a general tendency for the rotational potential symmetries of the particles to be locally preserved upon self-assembly. Specifically, we find that a dipolar particle potential, having a continuous rotational symmetry about the dipolar axis, gives rise to chain formation, while particles with multipolar potentials (e.g., square quadrupole) having discrete rotational symmetries led to the self-assembly of random surface polymers preserving the rotational symmetries of the particles within these sheet structures. Surprisingly, these changes in self-assembly geometry with the particle potential symmetry are also accompanied by significant changes in the thermodynamic character and in the kinetics of the self-assembly process. Linear chain growth involves a continuous chain growth process in which the chains break and reform readily, while the growth of the two-dimensional polymers only occurs after an ‘initiation’ or ‘nucleation’ time that fluctuates from run to run. We show that the introduction of artificial seeds provides an effective method for controlling the structure and growth kinetics of sheet-like polymers. The significance of these distinct modes of polymerization on the functional character of self-assembly growth is illustrated by constructing an artificial centrosome structure derived from particles having continuous and discrete rotational potential symmetries. INTRODUCTION Molecular self-assembly at equilibrium is central to the formation of numerous biological structures [1-3] and the emulation of this process through the creation of synthetic counterparts [4-9] offers great promise for nanofabrication [10-15]. The central problems in this field are an understanding of how the symmetry of interacting particles encodes information about the geometrical structure of the self-assembled structure and the nature of the thermodynamic transitions governing this type of organization. A first attempt at establishing principles for molecular self-assembly was made by Crick and Watson in the case of the protein shells of viruses. [16] They argued that viral shell structures assembled from equivalent protein particles must have the form of hollow tubes (helical in general) or closed shell structures belonging to the family of Platonic polyhedra (cube, tetradedron, octahedron, dodecahedron, icosahedron) where each particle has an equivalent local environment with its neighbors. The equivalence of the local environment requires that a permutation of particles within these structures preserves their local interactions wit
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