Roles of chain stiffness and segmental rattling in ionomer glass formation

Abstract

Ionomers-polymers with bound ionic moietieshave 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.

An Emerging Unified View of Dynamic Interphases in Polymers

“…Indeed, as shown in Figure , the range of the dynamic interphase in these ionomers exhibits a linear relationship with the size L of CRRs in the system, consistent with the results for thin films as discussed above . Second, we have demonstrated that large variations in persistence length, isolated from other changes in polymer properties, have little to no effect on the size of the dynamic interphase . Instead, the size of the dynamic interphase remains well‐correlated with the size of “stringlike” cooperative rearrangements as chain persistence length varies .…”

“…[ 80,86 ] Second, we have demonstrated that large variations in persistence length, isolated from other changes in polymer properties, have little to no effect on the size of the dynamic interphase. [ 95 ] Instead, the size of the dynamic interphase remains well-correlated with the size of "stringlike" cooperative rearrangements as chain persistence length varies. [ 87 ] Third, as shown in Figure 7 , comparison of these simulation results to earlier simulations of glass formation in the vicinity of ungrafted attractive nanoparticles reveals a quantitative correspondence between the behavior of the dynamic interphases in these systems, indicating that covalent grafting does not qualitatively alter the physics of the interphase.…”

“…where τ A and u A 2 are the values of τ α and ͗ u 2 ͘ at the onset temperature T A of glass formation, and where α is a parameter that is interpreted as relating to the degree of anisotropy of local segmental rattle volume. [ 95 ] In our recent study on ionomers, we have demonstrated that this functional form provides an excellent description of alterations in relaxation time in model ionomers relative to corresponding uncharged homopolymers, without the use of adjustable parameters to describe these changes.…”

“…Specifically, our recently developed “generalized localization model” of relaxation in supercooled liquids proposes a general relationship between relaxation and the scale of local segmental rattling in supercooled liquids . This model, which has recently been shown to describe glass formation in polymer thin films, suggests the relationshipwhere τ A and u A 2 are the values of τ α and ⟨ u 2 ⟩ at the onset temperature T A of glass formation, and where α is a parameter that is interpreted as relating to the degree of anisotropy of local segmental rattle volume . In our recent study on ionomers, we have demonstrated that this functional form provides an excellent description of alterations in relaxation time in model ionomers relative to corresponding uncharged homopolymers, without the use of adjustable parameters to describe these changes.…”

Modeling the Effect of Polymer Composition on Ionic Aggregation in Poly(propylene glycol)‐Based Ionenes

“…Many of these simulations were for solvated systems and typically specific atomistic systems unrelated to the ammonium ionenes of interest here were considered. However, several groups have studied dry ionomers using coarse‐grained models, so the results are rather generic and provide context for our work, particularly with respect to the significant progress that has been made in understanding the morphology of ionic aggregates of these coarse‐grained systems . Some of these studies considered dry ionomers with ions within the chain backbone similar to the ammonium ionenes of interest here .…”