Praveen Agarwal publishes on Mobility of Molecules Tethered to Surfaces
Praveen Agarwal, a recent PhD graduate from Lynden Archer’s group, has published a paper titledCrowded, Confined and Frustrated: Dynamics of Molecules Tethered to Nanoparticles" in Physical Review Letters.
The article reports on the motions and mechanical properties of short polymer chains tethered to the surface of inorganic nanoparticles. Termed oligomers/telomers, these polymers are conventionally thought too short to entangle with their neighbors and are believed to form too weak secondary bonds to exhibit elasticity, formability and other beneficial mechanical characteristics associated with high-molecular weight polymers. Remarkably, Agarwal, Kim, and Archer find that tethering the low molecular-weight polymers to nanoparticles produces dynamical features similar to their much longer-chain, entangled counterparts. They propose an analytical model to explain the phenomena and show that crowding and confinement of polymers on the surface of particles produce topological, tube-like constraints (see inset) similar to those in entangled polymer systems. Their findings indicate that snake-like, reptation dynamics conventionally associated with long-range motions of entangled polymers in hypothetical tubes formed by their neighbors are likely present in molecular liquids comprised of chain molecules of all sizes. By silencing faster motions by tethering the molecules to slow-moving particles, the reptation motion is unmasked. In addition to providing insights into motions of tethered polymer chains (long and short), the findings by Agarwal, et. al. are significant in that they open up entirely new ways for manipulating mechanical properties of polymeric materials for applications.
The Figure reports the relaxation time (Tau) versus temperature for untethered (open symbols) and SiO2-nanoparticle-tethered (filled symbols) polyisoprene of molecular weights 3,000 g/mol (circles), 5,000 g/mol (diamonds) and 13,000 g/mol (squares). These results were obtained from broad band dielectric relaxation measurements in a bulk material, which were used to quantify dynamics of the monolayer of polymer chains tethered to particles; they take advantage of the fact that all polymer chains in a self-suspended suspension are tethered to particle surfaces.
To read the full article in Physical Review Letters: http://prl.aps.org/abstract/PRL/v109/i25/e258301