ChBE Seminar Series: David Green
Tuesday, March 2, 2010
11:00 a.m.-12:15 p.m.
Room 2110, Chemical and Nuclear Engineering Bldg.
Professor Sheryl Ehrman
Nanoparticle Dispersion in Polymer Melts
Department of Chemical Engineering
University of Virginia
Polymer nanocomposites (PNCs), a developing class of materials, consist of nanoscale objects such as rods, particles and clays dispersed in concentrated polymer solutions and melts. When nanofillers are integrated into these bulk phases, resulting material properties such as tensile strength, hardness, heat resistance, and electrical conductivity may be dramatically different than previously accessible with traditional fillers and may be finely controlled. PNCs hold promise in applications in biotechnology, catalysis, and plastics. However, a major challenge in developing these systems is overcoming the driving forces that promote aggregation, yielding suboptimal material properties. Specifically, these driving forces are long-range attractive Van der Waals forces between particles and the thermodynamic favorability for the polymer chains of the melt to maximize available conformations by driving particles together. A common technique to promote full dispersion of nanoparticles is steric stabilization, achieved by chemically modifying the surface of the particles by covalently bonding polymer brushes. For this graft layer to be effective in stabilizing particles, its conformation must be stretched such that interpenetration by the chains of the matrix polymer may occur. This state is commonly referred to as complete wetting and is accessed when the graft chain length is longer than that of the melt at sufficiently high graft densities. However, to date, the impact of wetting on the nanoparticle dispersion has not been well quantified. Thus, a major focus of our research is to elucidate how wetting controls the optimal dispersion of nanoparticles. We accomplish this through light scattering and rheological measurements on polydimethylsiloxane (PDMS)-grafted silica nanospheres in concentrated PDMS solutions and melts. By controlling the brush graft density and the matrix chain length of these model systems, our results indicate that their wetting and flow properties can be quantifiably linked and finely controlled. Ultimately, our results lead to a better understanding of the role of the graft layer on nanoparticles dispersed in polymer solutions and melts and will aid in optimization of material properties of polymer nanocomposites in applications in biotechnology, optics, catalysis, and plastics.