Emerging multifunctional polymers often possess a micro- or nanostructure that imparts such systems with the properties of the constituent materials. Improved design of these polymers necessarily requires an understanding of the synergistic thermodynamic, kinetic and material factors governing structural evolution and stability. The primary research focus of the Polymer Morphology Group (PMG) is to employ, and in some cases develop, complementary microscopy and scattering techniques to probe structural characteristics of microstructured polymers, nanostructured polymers and polymer composites.
Microstructured polymers are multiphasic polymer systems exhibiting micron-size structural elements (e.g., polymer blends). Since most polymer mixtures are intrinsically immiscible, we are currently exploring processing methods by which to produce new blends with interesting properties. By using supercritical carbon dioxide as a nonselective diluent, the temperature at which phase separation occurs in a polymer blend can be controllably shifted, thereby opening new processing windows in an environmentally friendly manner. High-energy mechanical alloying yields uniquely intimate mixtures of highly incompatible liquid crystalline polymers, commodity thermoplastics, and elastomers. We have also addressed the kinetics of structural development during concurrent phase separation and chemical reaction in a functionalized polymer blend.
Nanostructured polymers self-organize at the (supra)molecular level. In the melt or solid state, such materials include block and graft copolymers, which behave as two or more chemically coupled homopolymers and, if sufficiently incompatible, order into the same nanoscale structures as surfactants. We have examined various copolymers and copolymer blends to identify pathways by which to control their equilibrium nanostructure, an important consideration in the design of nanoscale membranes and inorganic templates. Another nanostructured system consists of a polymer in the presence of dibenzylidene sorbitol, a small organic molecule that forms a fibrillar network and produces organic physical gels. We have likewise examined nanostructured polymer solutions, such as those composed of block copolymers in a selective solvent, hydrophobically interacting polymers with a surfactant or salt, and lyotropic polymers exhibiting a mesophase.
Polymer composites refer to those systems in which polymer chains reside near an inorganic surface. We have employed bond-fluctuation simulations to investigate the equilibrium and dynamic characteristics of such chains, as well as mixtures of chains, grafted to an interface at specific sites along the polymer backbone(s). The results from such work are of practical use in the design of polymers for adhesion, lubrication, and barrier applications. Moreover, we have examined commodity polymers modified by a plasma-deposited SiOx surface layer to improve barrier performance. Studies of these composite materials, already in use in the food packaging and biomedical industries, reveal that the permeation of oxygen and water vapor can be reduced by over 3 orders of magnitude below that of a conventional glassy or semicrystalline polymer.
These studies are all collaborative in nature, and we interact extensively with academic and industrial research groups throughout the U.S., Europe, and Japan. An energy-filtered transmission electron microscope, cryoultramicrotome, cryofracture-replication unit, small-angle x-ray diffractometer, several optical microscopes, and lots of enthusiasm are all indigenous to the PMG.