Molecular Modeling Studies of Dendrimers and Hyperbranched Polymers

177 p.Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 2001.The effects of molecular architecture on the conformational and rheological dynamics of dendrimers and hyperbranched polymers is investigated. Non-Equilibrium Brownian Dynamics (NEBD) simulations are utilized to investigate the single-molecule behavior in the dilute solution and the Tube Model is utilized to investigate the dynamics in the entangled state. The Fokker-Planck equation from the general bead-rod-spring phase space formalism of Bird and co-workers is converted to a Stochastic Differential Equation (SDE) utilizing Ito calculus, which is then forward integrated. Two different bead-rod-spring models are proposed for perfect dendrimers and linear-hyperbranched hybrids respectively. Various ensemble averaged conformational and rheological properties are monitored under both steady-state and transient conditions. Perfect dendrimers are found to exhibit much lower viscosities and negligible visco-elasticity, compared to linear chains of the same molecular weight. Studies on mixed linear-hyperbranched polymers, with precisely varying degrees of branching, indicate that an abrupt transition from dendrimer-like properties to linear chain-like properties occurs when the mole fraction of linear segments in the molecule exceeds 0.8. The reptation-based model proposed recently by Blackwell, Harlen and McLeish, for symmetric tree-like polymer melts is utilized to analyze the behavior of the linear-hyperbranched polymers in the entangled state. The average mass segments between branch points is found to be the most important architectural parameter that affects the rheology of these systems in the entangled state. Model studies are in very good qualitative and quantitative agreement with the experimentally observed behavior of polyamidoamine (PAMAM) dendrimers, polypropylene imine (Astramol) dendrimers and the AB/AB2 polyetherimide (PEI) hyperbranched copolymers, in the dilute and entangled states.U of I OnlyRestricted to the U of I community idenfinitely during batch ingest of legacy ETD