Paper Citing NAMD - Abstract
Pasquino, R.; Zhang, B.; Sigel, R.; Yu, H.; Ottiger, M.; Bertran, O.; Aleman, C.; Schlueter, A. D.; Vlassopoulos, D.
Linear Viscoelastic Response of Dendronized Polymers
MACROMOLECULES, 45:8813-8823, NOV 13 2012
We investigate the: linear rheology of well-defined dendronized polymers (DPs). They consist of polymethacrylate. backbones with tree-like branches (dendrons) of different generations, from zeroth to fourth, grafted at each monomer and a methyleneoxycarbonyl spacer between the polymerizable group and the dendritic substituent. The degrees of polymerization for the different generation polymers are almost constant, allowing for systematic studies as a function of generation. Because of the synthetic approach, these macromolecules possess tert-butoxycarbonyl (Boc) groups which promote hydrogen bonding, whereas the benzene groups allow for weaker bonding (pi-stacking) as well. The master curves of frequency-dependent storage and loss moduli of these macromolecular structures were obtained via time-temperature superposition of dynamic frequency sweeps at various temperatures. To access slow relaxations, creep measurements were performed. at long times and converted to frequency-dependent moduli. For the first generation, it was possible to detect relaxation processes suggesting an approach to the terminal regime (flow). On the other hand, the zeroth and second to fourth generation polymers exhibited a solid like behavior throughout a wide range of frequencies. The fast relaxations reflect the coupling of segmental friction and hydrogen bonding and render the WLF-type analysis nontrivial. On the basis of the molecular structure of these unique materials as revealed by molecular dynamics simulations and complementary studies With their linear analogues poly(methyl methacrylate) and poly(tert-butyl)methacrylate; we propose that DPs resemble Weakly interpenetrating elongated core-shell systems. As generation increases, their enhanced :rigidity, and intermolecular hydrogen bonding,,which occurs primarily toward the outer surface of the DPs, appear to dominate the dynamics PG0 is not a DP and has an open structure that promotes intermolecular bonding. These results provide design : guideline's for ultrahigh-molecular-weight responsive polymers with possibilities for multifunctional substitution and tailoring Frheological response from liquid like to solid-like.
