Processing and Assembly

While polymers share many characteristics with other classes of materials, one of their distinct attributes is their frequent use in non-equilibrium structures produced through advanced processing. Processed either from the melt or solution, polymers can be molded, spun into fibers, processed as sheets or films to name a few of the processing methods. Using fiber spinning as an example, one of the highest volume uses of natural and synthetic polymers is in fibers for textile production. The process of spinning fibers leads to molecular orientation and if taken to the extreme can transform polymers from soft materials familiar to us when used in simple milk bottles to polymers capable of withstanding a bullet. Conventional fibers are several hundred microns in diameter. Improved processing now enables the manufacture of microfibers, less than 100 microns in size which leads to materials that have considerably different characteristics than conventional textiles. The fact that simple processing can lead in effect to a wide range of distinct materials properties shows its importance as a critical aspect of polymer science and engineering.
Over the last decade the number of methods for producing true nanostructures, that is, fibers, films or processed forms only a few tens of nanometers in scale has grown dramatically. Such structures are beginning to approach the molecular level of detail possible in biomacromolecular systems. A significant body of work has now developed on how to precisely construct structures on greater than atomic length scales. Research on building nanoscale structures has taken two fundamentally different approaches colloquially known as: “top-down” and “bottom-up” processing. The “top-down” approach involves the fabrication of structures that are >50 nm in size using techniques such as lithography and holography that require no subsequent assembly step; while in the “bottom-up” approach, monomers and polymers are designed to form the building blocks that must self-assemble into various structures. Looking to biology as a model, a grand challenge will be to create sophisticated new polymer materials that enable assembly into directed shapes and structures. Even though great progress in molecular level control has resulted from these efforts, these processes are just in their infancy.
Most polymers that we encounter are not used alone, but rather in combination with added components. These “fillers” are used to control crystallinity, improve mechanical properties and generally enhance the properties of many polymer systems. Increasingly organic-inorganic polymer hybrids are being explored. It has recently been realized that sufficiently small nanoparticles (smaller than the radius of gyration of a polymer) can be more easily incorporated into a polymer host to form a true molecular level hybrid. Another grand challenge will be to develop a sufficient level of understanding that true hybrid materials may be processed with the individual components intermixed at the molecular level.