What are you working on?
I work in the area of self-assembly of polymers. One the one hand, we look at two- dimensional self-assembly, in which we are putting together polymers using charge attractions to create films that are assembled from a surface and structured on the nanometer length scale. In this case, we have a number of interesting material systems that can contain different functionalities, and the applications range from energy and electrochemistry, to biomaterials and drug delivery. In this 2-D area, we have also done a range of micro-scale and nano-scale patterning of these kinds of thin films.
On the other hand, we also look at three-dimensional self-assembly. In this case, we look at polymers which undergo a thermodynamically driven assembly to create nano-scale structures in 3-D. Some of the most interesting things we’ve been looking at more recently involve the creation of materials which assemble in solution into nanoparticles which can be used as carriers for drugs or as nano-scale structures that can serve as a template for other materials.
Could you, in layman’s terms, explain how you might do that patterning, and then maybe
talk about one specific project?
Sure. Perhaps 60% of my group works with the layer-by-layer thin film assembly approach. We’re using electrostatics to build up a functional polymer thin film, one layer at a time. Each layer has dimensions ranging from molecular to tens of nanometers in scale. You can take a substrate with some initial charge, and dip it into a bath that contains a dilute solution of a polymer which contains the opposite charge. That polymer will be attracted to the charge on the surface and absorb to the substrate until, finally, the surface charge is reversed. Because the polymers in solution now have the same charge as the surface, electrostatic repulsion prevents further deposition. The amount that is absorbed depends on the charge density along the polymer chain, which can be changed to get a desired thickness. If acid or basic amine groups are present on the polymer chain, changes in pH will increase or decrease the charge along the polymer backbone. You can also add salt to the solution, which shields the charge along the backbone and
makes it act as if it has less charge. In either case, you end up either with something that is absorbed very flatly on the charged surface, or polymers that are adsorbed to the surface in thick loopy coils, akin to a shag carpet on the surface. Adsorption, by the way, is when the material just sticks to the top of the surface of a substrate. Absorption is when it goes into the bulk material, like water into a sponge. The thicknesses of those layers can range from five angstroms, which is half a nanometer, to tens of nanometers. After the polymer adsorption step, we rinse the substrate in dilute water to remove anything that isn’t truly absorbed, and then adsorb another polymer of opposite charge. In this manner, by alternating the adsorption of oppositely charged polymers, we continue to build up this material system layer by layer.
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