Experiments and theory will be combined to delineate why adding nanoparticles to a polymer improves its mechanical properties. Attention is focused on two aspects: (a) Effect of Particle Size: It is now accepted that the modulus of a polymer melt can be increased by several orders of magnitude on the addition of particles. This reinforcement is critical to applications, e.g., tires or under the hood, since unfilled polymers are too "soft" to be used in these contexts. Since this modulus reinforcement is conjectured to go through a maximum as a function of particle size (in the 10 nm size scale), there is apparently an optimum nanoparticle size for this property. Proving the existence of this postulated maximum, and exploring its molecular origins, is a focus of this work. (b) The Payne Effect: While the increase in modulus achieved through the addition of nanoparticles is critical to certain quiescent properties of the material, this can also make the nanocomposites hard to process. The Payne effect, i.e., the orders-of-magnitude decrease of the mechanical strength of a material with increasing strain, circumvents this problem without compromising the materials' end use behavior. Understanding the molecular underpinnings of the Payne effect is then the second, interrelated goal of this work. In particular, the role of the bound polymer layer vs. polymer bridges between particles in this context will be critically examined by devising materials where the relative proportion of these two is varied. This work exploits the unique capabilities of the two PIs to study these interrelated aspects of the rheological behavior of nanocomposites with fully dispersed nanoparticles, using a combination of experiment and simulation.
Plastics are by now ubiquitous in many contexts, such as in packaging. Less familiar is the use of these materials in structural applications (e.g., building materials) or under the hood, with these deficiencies being attributed to the relative soft mechanical behavior of these materials. An ongoing goal has been to improve this particular aspect of polymers, and it has been conjectured that the addition of nanoparticles is one facile means of achieving this goal. This work will target this issue and systematically and critically evaluate the role of nanoparticles on the mechanical behavior of plastics. The research activities will be coupled to extensive education and outreach activities that target students at the K-12, undergraduate and graduate levels. The PIs will aim to recruit/retain underrepresented minority students into science/engineering disciplines at the graduate level and beyond. In particular, interactions have been developed with Florida A&M University and Grambling State University (both HBCUs) with the goal of recruiting undergraduates into the program. The PIs will continue to work with local city high school teachers with the goal of giving students, especially seniors, "hands-on"research experience.
This grant represents a long-term ongoing collaboration between Prof. Ralph Colby (Penn State) and my group in the area of polymer nanocomposites, i.e., physical mixtures of nanoparticles and polymers. These materials are uniquitous, for example in rubber tires, in plastic beer bottles etc and are a growing field of research in both the academic and industrial communities. In this grant, we have focused on two aspects - the role of attractive polymer-nanoparticle interactions on the thermal and mechanical properties of this class of hybrid materials and (ii) the transport (namely flow behavior and diffusivity) of these materials. A combined experimental and theoretical program was conducted. The results of our work have been published in several major journals, and we have enunciated new means to create nanocomposites materials where the dispersion state and hence the properties of these materials were varied by controlling how much polymer was adsorbed on the nanoparticle surfaces. We believe these results will have a significant impact on the automobile tire industry, as our method of dispersing in a polymer melt allows the NP-network response to be cleanly separated and studied in detail. In contrast the crosslinked automobile tire has strong modulus just from the crosslinked polyisoprene network in which the NP-network is embedded and this makes it difficult to isolate the two effects.
