This collaborative research project proposes to support work by research teams from Case Western Reserve University (CWRU) and the University of Akron (UA) on fundamentals of nanocomposite formation by a bottom-up self-assembly approach from dispersions of polyhedral oligomeric silsesquioxane (POSS) molecules in several thermoplastic polymers. These nanocomposite materials will have the processing ease of unfilled polymers and will be suitable for manufacturing of articles with micro- and nano-scale features by high-speed injection molding, fiber spinning, and thermoforming. The fundamentals developed in this work will offer superior alternatives to most top-down polymer nanocomposites, where orders of magnitude increase in viscosity over that of the host polymers is a norm and achieving nanoscale dispersion is a challenge. The degree of nanofiller/polymer and nanofiller/nanofiller interactions will be governed by the choice of polymer systems and POSS grades. A continuous single screw chaotic mixing device with peak shear rate in the range 50-100s-1 will render POSS nanoparticles oriented in the form of spheres of ~50 nm dia, long fibrils with ~5 nm dia, and/or lamellas of ~5 nm thickness. The hierarchical structures and the nanoparticle morphology and orientation will be correlated with mechanical and thermal properties.
The research program brings together research teams with complementary expertise from CWRU and the UA and provides common platform to collaborate freely and seamlessly. Undergraduate researchers from CWRU, UA, and from the currently NSF-funded summer REU programs at these schools will participate in the proposed research. Every effort will be made to ensure
Composite materials, from the Corvette automobile, to pleasure boats and the Boeing 787 touch our lives in many ways. In composites, stiff reinforcing materials are combined with tough polymers to obtain "the best of both worlds". Nanocomposites, composites which incorporate nanometer-sized reinforcing materials, are currently being explored throughout the world as a means of further expanding the range of properties that can be obtained. One specific type of nanofiller for composites is a family of materials called polyhedral oligomeric silsesquioxanes, or "POSS"; these are 1.5 nanometer diameter cubes of silicon oxide, with organic functional groups at their corners. The organic groups are hoped to better compatibilize the stiff silicon oxide with common organic polymers. With compatibility comes the ability to produce useful materials. The working hypothesis of this project, which was a joint effort of Case Western Reserve University, the University of Akron, two universities in Spain, and the producer of POSS was that one would want POSS grades to be only moderately compatible with the polymers they hopefully would reinforce. If the POSS would be highly soluble in the polymer, then it would simply act as a plasticizer, making the plastic more flexible (such as plasticizers work on the dashboards of automotives). If the POSS was highly incompatible with the polymer, then they would not mix properly, and no property enhancements could occur. This project taught us that the reality of POSS/polymer nanocomposites is more complex that we hypothesized. It is true that moderate compatibility between POSS and polymer is necessary, but that is insufficient in of itself to produce effective reinforcement of the polymer. We demonstrated that reinforcement only occurs in situations where the polymer is highly oriented (stretched), accompanied by crystallization of the polymer, creating an ordered structure of crystals connected by oriented "amorphous" polymer chains. The common method for producing such structures is fiber spinning - that orientation and crystallinity provides the strength observed in commercial fibers. In such structures, the POSS is forced into the amorphous region of the fiber, where it is "nanoconfined" and tends to crystallize into localized fibrils which reinforce the polymer fibers. Why this is important is that these results provide a roadmap for the production of higher modulus (stiffness) fibers which retain their favorable properties at elevated temperatures. An application where this is important is the reinforcement of automobile tires, which contain large amounts of polymer tire cord material. If the stiffness and thermal properties of those reinforcing fibers can be improved, then fewer can be used, decreasing tire weight and ultimately increasing fuel economy for the U.S. automobile fleet. This work was carried out by a group of one PhD student, three undergraduates and two high school students under the mentoring of the lead engineering professor. The PhD student is now trained and ready to move into industry. One of the undergraduates has graduated and is working in new product development for a manufacturing company, while the second is preparing to go to graduate school and the third continues his undergraduate engineering education. Both of the high school students (women, one an African American student from inner city Cleveland) are now college sophomores studying chemistry.
