During a recent event in the Dean’s Seminar Series, students and faculty were delighted by "The Captivating Polymer Brush: A Structure Associated with Surfaces and Interfaces," a lecture given by Dr. Lynn Penn, Professor and Department Head of Chemistry at Drexel. People slowly filtered into the Faculty Club on the 6th floor of MacAlister Hall for this short but informative lecture.
Dr. Lynn Penn is originally from the University of Kentucky, where she researched topics like chemical adhesion and tethered polymer chains. Her lecture focused on these polymer chains and how they anchor to a complementary surface, bathed in solvent, to form what’s known as a "polymer brush." These brushes are filled with polymer chains that extend outward in order to avoid overlapping with each other. For, the basic function of a polymer chain is to repel other chains by means of an envelope that encloses it. It follows that polymer brush surfaces will repel each other and only intermingle through the application of great force.
This is how the formation of a polymer brush works: in a solution, polymer chains are tossed around and bounce against each other since they resist overlap. Each coil, essentially, has its own territory. Polymer brushes form when a surface is derivatized to cause the complementary ends of the polymer chains to "graft" or tie down to the surface. This initial tying forms a "mushroom layer," where coils are tied down but still retain their natural, spherical shape. Many scientists consider this mushroom layer to be a barrier to the addition of more polymer chains, and that brushes are formed very slowly as new chains eventually get through, but Dr. Penn’s argument is that this natural property of the coils—the repelling behavior—actually facilitates the addition of new chains to form the polymer brush. There is mutual cooperation, or "assisted tethering," between the tied chains and the incoming coils. The incoming coils cause the extension of the anchored chains as they are repelled out of the way. Experiments confirm that not only are these mushroom layers facilitators, but that the addition of new chains occurs fairly rapidly and not slowly, like theorists originally thought. Dr. Penn emphatically informed the audience that the results of her experimentation were "contrary to the idea of the brush as a barrier" and that "the more free chains in a solution, the more that go on [the surface]." Dr. Penn acknowledged that experiments do differ, and that brush formation is sometimes faster in one experiment than it is in another, depending on the situation. She says "A polymer brush is like a switch," in that it is a barrier to long, inflexible chains and other brushes, but open to shorter chains of the same kind.
So what is the importance of studying polymer chains and brushes? So far, polymer brushes have only been observed in the lab setting. However, Dr. Penn suggested a natural occurrence of a polymer brush. She presented the possibility of polymer brushes explaining the morphology of the pores of the nuclear envelope within our own living cells. These pores have been observed to selectively allow or disallow certain macromolecules into the nucleus; sometimes they allow water and ions in, and other times they keep RNA out. Biologists have been baffled by how these pores make their selections, but Dr. Penn hypothesizes a very interesting scenario. What if the pores function as gateways made up of opposing polymer brush surfaces? These pores are usually closed with tethered polymer chains stretched out on both sides to block the gateway. When letting molecules through, these chains can coil by means of an external stimulus and thereby open. But no one is certain if this hypothesis is truly the case with such nuclear pores.
Dr. Penn concluded her lecture by saying that the behaviors of polymer brushes is varied and can help illuminate how cells function. She continues to research the fascinating possibilities of the captivating polymer brush.
Charlotte Lenox is majoring in English at Drexel, and is a transfer student from the University of Alaska Southeast. She was born and raised in Juneau, Alaska.