Nanoparticle Ordering via Functionalized Block Copolymers in Solution

Abstract

We consider nanoparticles and functionalized copolymers, block copolymers with attached end groups possessing a specific affinity for nanoparticles, in solution. Using molecular dynamics, we show that nanoparticles are able to direct the self-assembly of the polymer/ nanoparticle composite. We perform a detailed study for a wide range of nanoparticle sizes and concentrations. We show that the nanoparticles order in a number of distinct phases: simple cubic, layered hexagonal, hexagonal columnar, gyroid, and a novel square columnar. Our results show that nanoparticles ordered with functionalized block copolymers can provide a simple and efficient tool for assembling novel materials with nanometer scale resolution. B lock copolymers in solutions or melts are known to self-assemble into mesophases with one-, two-, or threedimensional periodic order with typical repeating distances in the 10Ϫ200 nm range. 1,2 In solutions, the self-assembly of the different phases can be controlled by tuning external conditions such as pH or temperature. The wide availability and relatively inexpensive synthesis of block copolymers as well as the ability to exquisitely control so many diverse phases with periodic order make them compelling candidates for the building blocks of novel materials. Thin films of block copolymers can provide "bottom-up" templates for fabrication of devices on sub-30-nm length scales that are inaccessible to standard lithography techniques, opening the door for molecular size electronic components and ultrahigh density magnetic storage media. Applications include growth of nanowires 3,4 and nanocrystals used in devices such as flash memory, 5 metal-oxidesemiconductor capacitors, 6,7 arrays of quantum dots, 8,9 and photonic crystals. 10 For most applications, the proposed fabrication process consists of forming a thin copolymer film on a substrate, which is then chemically treated to remove one of the copolymer components. The remaining component is used as a mask for growing the active component. It would be even more desirable to include the active component (e.g., metallic or silica nanoparticles) from the outset and form the desired structure in a single pass. Direct application of this method is limited mainly due to nanoparticle aggregation and incompatibility of nanoparticle surface and ionic solutions with the polymers. 11 These problems can be circumvented by the use of self-assembling functionalized block copolymers, that is, polymers with covalently attached end groups that show specific affinity for nanoparticles, as agents for assembling ordered nanoparticle structures. 12,13 The theoretical understanding of the factors that lead to a successful self-assembly of nanocomposites is relatively limited. Recent studies include investigations of the influence of nanoparticles on the selfassembly and nanostructure formation of diblock copolymer melts based on solving cell dynamical system equations, 14 Monte Carlo simulations, 15 and self-consistent mean field/density functional theory. 16,17 ...

Phases of polymer systems in solution studied via molecular dynamics

“…Peaks indicating an ordered structure are only observed in a single run, the one at φ P = 0.25, ε N = 2.0, ε F = 3.5. A lattice of micelles is present offset from a lattice of spherical aggregates of inorganic particles, similar to the cubic phase found in ref [13]. Due to the extreme amount of computer time that would be required [64], no attempt is made to ascertain the boundaries of this ordered cubic phase or it's reproducibility.…”

“…Previous studies have looked at a single polymer architecture (CA 5 B 7 A 5 C ) and investigated the phase diagram of these polymers combined with inorganic particles as a function of both polymer and inorganic particle concentration, inorganic particle size [13] and affinity [12].…”

“…A wide region of the CA 5 B 7 A 5 C phase diagram at φ P = 0.20 is covered by the square columnar phase [13]. It also appears at φ P = 0.25 with the polymer CA 13 B 7 A 13 C. Hydrophobic beads B arrange in cylindrical micelles which order on a square lattice while the inorganic particles I aggregate into cylindrical collections on an offset square lattice.…”

“…[12] by Chris Knorowski, a REU student I mentored for a summer, and the subsequent work by Rastko Snepnek in ref. [13], whom I collaborated with.…”

“…In many cases, the nanoparticles themselves are functionalized with short polymers in order to solubilize and/or prevent agglomeration. In previous papers [12,13], a strategy is proposed where the polymer is functionalized with the addition of end blocks having a high affinity for …”