61st Annual Report on Research 2016

The goal of this proposal is to quantify the effects of nanoparticles on the thermodynamics of block copolymers and to elucidate the interactions between nanoparticles and block copolymer defects. We will employ a novel field theoretic simulations framework that enables efficient simulations of block copolymers loaded with nanoparticles of arbitrary shape and surface functionality. First, we will quantify the shifts in the order-to-disorder transition as a function of nanoparticle size, interactions of the particles with the two blocks of the copolymer, and surface functionality. Second, will use a thin film geometry where we can design both the type and density of defects in a lamellar block copolymer to examine the interactions between nanoparticles and block copolymer defects. We have made significant progress on both of these goals, which I outline below.

In the work by that was partially supported by this grant, we began investigating the strength of segregation of nanoparticles to tilt grain boundaries in block copolymer nanocomposites (see Figure 1). We employed a model thin film system where the strength of the defect in the block copolymer pattern could be manipulated by changing a chemical pattern on the substrate supporting the film. We found that increasing the strength of the defect lead to an increase in stretching of the diblock copolymer chains at the grain boundary, which in turn decreases the entropy of the polymer chains. This serves as an attractor for nanoparticles placed in the film, which preferentially segregate to the grain boundary. We found that the strength of the segregation increased with both defect strength and nanoparticle size, and the free energy reduction for segregating a nanoparticle to a defect was proportional to the volume of the particle. We rationalize this through a simple argument where a volume of stretched chains is allowed to relax by the presence of the nanoparticles. It turns out that the scaling of the free energy reduction with volume of the additive is not limited to just nanoparticles, but homopolymer additives also exhibit similar behavior: their free energy of segregation also scales with their volume (the length of the homopolymer). This work was published in Macromolecular Chemistry and Physics.

Separately, we have also been developing methods to study how solvent annealing affects the distribution of nanoparticles in block copolymer thin films. Solvent annealing is a process through which a block copolymer thin film is exposed to the vapor of a volatile solvent, which absorbs into the film, swelling and plasticizing the block copolymer. Solvent vapor annealing is widely used to reduce the density of defects in block copolymer films, and it is finding increased usage in block copolymer nanocomposite thin films. We have shown that for lamellar films swollen with a neutral solvent, if we tune the interfacial activity of the nanoparticles by varying the grafting density of polymers on the particle surface, we can tune the equilibrium distribution of nanoparticles. At some particular surface chemistries, we find that the solvent will displace the particles from the free surface, leading to interfacially-active particles becoming mixed in the polymer film. Under these conditions, when the solvent is rapidly evaporated from the film, we can trap the particles in the distribution they adopt in the swollen state. However, if the solvent is removed slowly, then the particles have sufficient time to anneal back to their equilibrium distribution in the dry state. This work was recently published in Soft Matter.

Moving forward, the work supported by this award will be used to more thoroughly characterize the equilibrium thermodynamics and solubility of nanoparticles in block copolymers. We will use the polymer nanocomposite field theory that was recently developed by our group to generate phase diagrams for the influence of nanoparticles on block copolymers as a function of the nanoparticle interactions with the various blocks of the copolymer, explicit surface functionality on the nanoparticles' surfaces, and when there are specific interactions (e.g., hydrogen bonding) between the nanoparticles and the polymer. This latter route has been exploited recently in experiments to achieve high equilibrium loadings of particles in block copolymers and is a promising route for functional polymer nanocomposite materials.