(742a) Predicting Disorder-Order Transitions of Solvent-Free Nanoparticle-Organic Hybrid Materials

Nanoparticle–organic hybrid materials (NOHMs) are a new type of complex fluid composed of hard, inorganic nanocores with oligomeric chains covalently grafted to the surface of the core in the absence of other solvent molecules. These materials provide us with a framework to study disorder-order transitions of “self-suspended” particles. In this talk, we present a classical density-functional theory to address the liquid to face-centered-cubic solid transition of NOHMs. In the absence of intervening solvent, the nanoparticle cores in both disordered and ordered phases are distributed in homogeneous structures governed by the requirement that the tethered oligomers must fill the interstitial space. The phase boundary is determined by comparing the total free energies of the two phases, which consist of the hard-sphere translational entropy as well as the oligomer configurational entropy. Since there are no macroscopic variations in the core volume fraction, the phase that exhibits the lower total free energy is the thermodynamically stable state. When the particles are localized near the lattice positions, the oligomers have more difficulty filling the space and have more stretched configurations. Therefore the theory predicts that NOHMs with stiffer tethered oligomers have stronger entropic penalties in ordered solid and favor the disordered phase. This is consistent with the experimental observation that NOHMs can be disordered soft glasses even when the core volume fraction is close to or higher than the hard-sphere freezing volume fraction [1]. When van der Waals interactions among the cores are included, anisotropic structures can form with the interparticle spacing in one direction different from the other two.  Such structures have been observed in experimental studies and Monte Carlo simulations of polymer-grafted nanoparticles with solvent [2] and were recently observed in molecular dynamics of a solvent-free system [3]. Using the same theoretical approach, the transitions of isotropic fluid to string-like or sheet-like structures of NOHMs will be discussed.

[1] J. L. Nugent, S. S. Moganty, and L. A. Archer, Adv. Mater. 22, 3677 (2010).

[2] P. Akcora, H. Liu, S. K. Kumar, J. Moll, Y. Li, B. C. Benicewicz, L. S. Schadler, D. Acehan, A. Z. Panagiotopoulos, V. Pryamitsyn, V. Ganesan, J. Ilavsky, P. Thiyagarajan, R. H. Colby, and J. F. Douglas, Nature Mater. 8, 354 (2009).

[3] A. Chremos and A. Z. Panagiotopoulos, Phys. Rev. Lett. 107, 105503 (2011).