title: Solvent controlled ion association in structured copolymers: Molecular dynamics simulations in dilute solutions.
abstract: Tailoring the nature of individual segments within ion containing block co-polymers is one critical design tool to achieve desired properties. The local structure including the size and distribution of the ionic blocks, as well as the long range correlations, are crucial for their transport ability. Here, we present molecular dynamics simulations on the effects of varying the concentrations of the ionizable groups on the conformations of pentablock ionomer that consist of a center block of ionic sulfonated styrene tethered to polyethylene and terminated by a bulky substituted styrene in dilute solutions. Sulfonation fractions f (0 ≤ f ≤ 0.55), spanning the range from ionomer to polyelectrolytes, were studied. Results for the equilibrium conformation of the chains in water and a 1:1 mixture of cyclohexane and heptane are compared to that in implicit poor solvents with dielectric constants ε = 1.0 and 77.73. In water, the pentablock collapses with the sulfonated groups on the outer surface. As f increases, the ionic, center block increasingly segregates from the hydrophobic regions. In the 1:1 mixture of cyclohexane and heptane, the flexible blocks swell, while the center ionic block collapses for f > 0. For f = 0, all blocks swell. In both implicit poor solvents, the pentablock collapses into a nearly spherical shape for all f. The sodium counterions disperse widely throughout the simulation cell for both water and ε = 77.73, whereas for ε = 1.0 and mixture of cyclohexane and heptane, the counterions largely condense onto the collapsed pentablock.
authors.affiliation: Department of Chemistry, Clemson University, Clemson, South Carolina 29634, USA.
authors.forename: Dipak
authors.initials: D
authors.lastname: Aryal
authors.affiliation: Department of Chemistry, Clemson University, Clemson, South Carolina 29634, USA.
authors.forename: Dvora
authors.initials: D
authors.lastname: Perahia
authors.affiliation: Sandia National Laboratories, Albuquerque, New Mexico 87185, USA.
authors.forename: Gary S
authors.initials: GS
authors.lastname: Grest
mesh_terms: D003510:Cyclohexanes / Q000737:chemistry; D006536:Heptanes / Q000737:chemistry; D057927:Hydrophobic and Hydrophilic Interactions; D007477:Ions / Q000737:chemistry*; D056004:Molecular Dynamics Simulation; D020959:Polyethylene / Q000737:chemistry*; D012964:Sodium / Q000737:chemistry; D012996:Solutions; D012997:Solvents / Q000737:chemistry*; D020058:Styrene / Q000737:chemistry*; D013696:Temperature; D014867:Water / Q000737:chemistry
chemical_list: D003510:Cyclohexanes; D006536:Heptanes; D007477:Ions; D012996:Solutions; D012997:Solvents; D014867:Water; D020058:Styrene; C506365:Cyclohexane; D020959:Polyethylene; D012964:Sodium