Roles of Chain Stiffness and Segmental Rattling in Ionomer Glass Formation

摘要

Ionomers-polymers with bound ionic moieties-have received interest for their exceptional properties including high toughness and ion conductivity. One of the primary effects of introduction of covalently bound ions to a polymer is alteration of its glass transition and segmental dynamics. The standard model for these alterations attributes them to a covalent 'tethering' effect in which segments near ionic aggregates are immobilized within a range related to the chain persistence length. Here, results from molecular dynamics simulations of glass formation in model ionomers of varying chain stiffness indicate that chain persistence length does not play a central role in ionic-aggregate-induced suppression of local segmental dynamics. Instead, these alterations are found to accord with more universal interface-induced alterations in glass-forming liquids, consistent with a recent study in which the present authors found a correlation between near-aggregate mobility suppression and the scale of segmental cooperative rearrangements. In this case, results indicate that shifts in the overall segmental relaxation times of these polymers can be quantitatively rationalized in terms of alterations in the Debye-Waller factor, reflecting changes in the local picosecond-timescale segmental rattling. (C) 2015 Wiley Periodicals, Inc.