Nanoparticles in Copolymers
Polymers are large macromolecules consisting of many smaller molecules, known as monomers, that are covalently linked together to form long linear chains. In the simplest case, all the monomers are same. For example, they might be styrene molecules covalently linked together to make polystyrene as shown in the top of Figure 1. This is a homopolymer. At the next level of complexity, a polymer can consist of two different monomers, which we might call "A" and "B". The monomers can be linked together in a random sequence, for example, AABABABBABBAABABA..., resulting in a "random copolymer". Alternatively the monomers can be arranged in blocks, for example AAAAAAAAAAAABBBBBBBB, resulting in a "block copolymer". Block copolymers may have 2, 3, or more blocks. Such polymers are known, respectively, as diblock copolymers, triblock copolymers, etc.
Even in liquid form, the interactions between the different blocks tend to favor structures where similar blocks of the block copolymers cluster near each other. Thus, a polymer melt consisting of AB block copolymers will tend to organize itself in a way so that the A-blocks from different copolymers are near each other, the B-blocks are near each other, and the A and B blocks are as far away from each other as possible, consistent with the constraint that the A and B blocks of each individual copolymer are covalently linked together. This organization of the A and B blocks into different domains is referred to as micro phase separation. The chemical differences between the A and B blocks need not be large. Replacing a C-H in styrene with a N to make 2-vinylpyridine (see Figure 1) is sufficient to cause micro phase separation in the polystyrene poly(2-vinylpyridine) block copolymer system. The structures that form depend on the relative number of monomers in each block. When the number of monomers in each block is roughly the same, a lamellar (sheet-like) structure forms. Figure 2 shows a few of the structures that block copolymers can form, with the A blocks shown in red and the B blocks in blue. The typical length scales are on the order of tens of nanometers, so the structures are really quite small.
We are interested in how the nanostructures that block copolymers form can be exploited to direct the placement of small nanoparticles in order to make new functional materials. For example, we find that if gold particles are coated with a thin layer of polystyrene, then the particles tend to assemble at the center of the polystyrene layers in a lamellar phase of a diblock copolymer consisting of polystyrene (PS) blocks linked to poly(2-vinylpyridine) (PVP) blocks. A transmission electron microscope (TEM) photograph of a cross-section of PS-PVP block copolymer sample containing polystyrene-coated gold nanoparticles is shown in Figure 3(a). The gold particles appear as dark spots in the center of the polystyrene blocks, which appear as light stripes. Similarly, we find that if the gold particles are coated with PVP, they go to the center of the PVP block phase.
By contrast, if the gold particles are coated with a mixture of polystyrene and poly(2-vinyl pyridine), then they assemble at the interface between the polystyrene and poly(2-vinyl pyridine) blocks, as shown in the TEM photograph in Figure 3(b).
The localization of gold nanoparticles within the different diblocks or at the interfaces between the diblocks is driven by a competition between entropy and enthalpy. On the one hand, any localization of the nanoparticles reduces their entropy and is therefore disfavored. On the other hand, putting gold nanoparticles coated with one type of polymer (e.g. PS) in contact with a different polymer (e.g. PVP) costs enthalpy (or energy) relative to putting it in contact with the same polymer (e.g. PS). This is the primary driving force behind the localization of the nanoparticles at different places within the diblock copolymer phases. In addition, there is a contribution from the fact that block copolymer strands are distorted (which decreases the entropy of the block copolymers) by the presence of gold nanoparticles, which must be factored in to properly account for the total free energy of the system. This is why particles coated only with PS or only with PVP tend to go to the center of the PS or PVP lamellae, because the PS or PVP polymer strands stretch the least to accommodate the nanoparticles in this case.
These experiments were done as part of the Ph.D. theses of Julia Chiu and Bumjoon Kim in collaboration with Gi-Ra Yi and Ed Kramer, Professor of Materials and Chemical Engineering at UCSB.
You can obtain more information by contacting Professor David Pine at the address below or by contacting any of his collaborators listed above.
You can also read more complete accounts of our work published in the following journal articles: JACS 127, 5036-5037 (2005). and Advanced Materials 17, 2618-2622 (2005).
This work was supported by the Department of Energy, the National Science Foundation MRSEC Program at UCSB, and by the BK-21 Program at KAIST.
