Structural Studies of Polymers: Fun, Exciting and Useful Science!
A remarkable achievement in the last 100 years is the creation of a host of useful chemical materials which enhance every aspect of our lives. Many of these are large molecules called polymers made up of repeating units called monomers. All of us are familiar with synthetic polymers such as polyethylene, polystyrene, nylon, and polyester. My sabbatical at UCSB involved studying the structure of a number of useful materials which have interesting stories, some of which have been published (see Current Research). Many of these are man-made “designer” materials while others are protein biopolymers that under certain conditions forsake their natural structures by mis-folding. When proteins “go bad” in this way a number of diseases can result such as Alzeimer’s and “mad cow“ disease. Structure is so important to chemists because it is basic to understanding the chemical and physical properties of molecules. I will show you the structure of some of these so that you can see how they are built up out of the various monomers. These have structures much more complex than simple molecules such as O2 or CO2, so special techniques are required to study them. I will describe some of the methods we use. By clicking on the Powerpoint link below the entire Phi Kappa Phi lecture given in February, 2003, can be downloaded and viewed slide by slide which are annotated to explain the contents. An overview of the talk follows.
Polymers consisting of a single repeating monomer are called homopolymers. Examples are polyethylene, Teflon, and polypropylene. Polymers made up of two alternating units like nylon are called alternate copolymers. Products may be “straight-chain” or branched . Depending on the groups substituted to the backbone and their orientation, properties may give very flexible or very rigid materials.
The most complex class of polymers are natural biopolymers. These materials are part of the chemistry of life and are now in our everyday vocabulary. Consider DNA which encodes our genetic characteristics or the proteins which comprise our muscle or hemoglobin. DNA is composed of repeating units called nucleotides composed of a sugar/phosphate backbone part and a base. There are only 4 common bases (adenine, cytosine, thymine, and guanine) but the order in which they occur along the backbone contains all of the encoded information that makes you who you are. Proteins, on the other hand, are made up of sequences of 20 amino acids which may be ordered in innumerable ways to give chains varying in length from a few to thousands of units. These chains then fold into structures we call conformations. While there are nearly an infinite number of possible conformations, usually only one of these are “natural;” some of the unnatural ones may very well lead to disease, which we will talk about later. I will use examples currently being studied by us in the Bowers group at UCSB.
This discussion leads naturally to my work on Polyhedral Oligomeric Silsesquioxanes (POSS) compounds. They form a large class of starting monomers used to construct polymers with many useful properties. The POSS structure is based on typical silicon-oxygen cages such as the one depicted below. In this structure, silicon atoms are located at the corners of a cube with oxygen atoms on each edge of the resulting “box.” There are also other POSS “boxes” built from, 6, 10, 12 and 14 silicon atoms. The “R’s” shown are carbon-containing chains bonded to each silicon which allow us to attach these POSS units together to form POSS polymers.
The Air Force Office of Scientific Research partially funded my sabbatical because of its interest in the practical applications of these POSS materials. The polymers have low densities and are resistant to high temperatures, and therefore can be used for light-weight insulation in rocket motors, or for high- temperature lubricants under conditions where ordinary hydrocarbon-based polymers would completely decompose. Since POSS polymers are not purely carbon-containing but contain the inorganic element silicon bonded to oxygen, they are resistant to solar radiation and atomic oxygen in space, and can be used for corrosion-resistant plastics in this environment. The POSS polymers are easy to make employing existing technology for the manufacture of traditional materials such as polyethylene, PVC, and nylon. I began this project studying typical monomers and their structures so that we could apply this knowledge to the structure of the polymer. I will give examples of some of the simple structures we studied and how we were able to make use of powerful experimental techniques such as “ion mobility” mass spectrometry to get information on the shapes and sizes of molecules. A stream of ions, perhaps having different sizes and shapes, are generated from molecules of interest and passed through a “drift” cell containing helium. The ions separate themselves because larger ones will collide with more helium atoms and therefore go slower than smaller, more compact ones. We measure average cross-sectional areas of the various ions (think of a “shadow” of an object projected onto a screen) from the times it takes each ion to emerge from the cell and arrive at the detector.
Our goal is to use this information to obtain the 3-dimensional structure of the polymer we are studying. We do this by using theoretical tools such as molecular modeling and dynamics calculations to match theoretical structures with the experimentally determined shapes. So far we have had nearly a 100% success rate in assigning structures to the various peaks observed in our experimental “arrival time distributions.” This gives us confidence we can understand more complex polymers and guide the synthetic chemists as they make them. I will show you our results on the POSS polymers as well as DNA and Alzheimer’s proteins as visually as possible using some of the computer programs we work with every day. Most of the time we have been fascinated by what we have discovered. Sometimes we have even been surprised!