Molecular Design of Highly Stretchable Ionomers

Moderated ionic bonding for water-free recyclable polyelectrolyte complex materials

While nature extensively uses electrostatic bonding between oppositely charged polymers to assemble and stabilize materials, harnessing these interactions in synthetic systems has been challenging. Synthetic materials cross-linked with a high density of ionic bonds, such as polyelectrolyte complexes, only function properly when their charge interactions are attenuated in the presence of ample amounts of water; dehydrating these materials creates such strong Coulombic bonding that they become brittle, non-thermoplastic, and virtually impossible to process. We present a strategy to intrinsically moderate the electrostatic bond strengths in apolar polymeric solids by the covalent grafting of attenuator spacers to the charge carrying moieties. This produces a class of polyelectrolyte materials that have a charge density of 100%, are processable and malleable without requiring water, are highly solvent- and water-resistant, and are fully recyclable. These materials, which we coin "compleximers," marry the properties of thermoplastics and thermosets using tailored ionic bonding alone.

Structure and Diffusion of Ionic PDMS Melts

Ionic polymers exhibit mechanical properties that can be widely tuned upon selectively charging them. However, the correlated structural and dynamical properties underlying the microscopic mechanism remain largely unexplored. Here, we investigate, for the first time, the structure and diffusion of randomly and end-functionalized ionic poly(dimethylsiloxane) (PDMS) melts with negatively charged bromide counterions, by means of atomistic molecular dynamics using a united atom model. In particular, we find that the density of the ionic PDMS melts exceeds the one of their neutral counterpart and increases as the charge density increases. The counterions are condensed to the cationic part of end-functionalized cationic PDMS chains, especially for the higher molecular weights, leading to a slow diffusion inside the melt; the counterions are also correlated more strongly to each other for the end-functionalized PDMS. Temperature has a weak effect on the counterion structure and leads to an Arrhenius type of behavior for the counterion diffusion coefficient. In addition, the charge density of PDMS chains enhances the diffusion of counterions especially at higher temperatures, but hinders PDMS chain dynamics. Neutral PDMS chains are shown to exhibit faster dynamics (diffusion) than ionic PDMS chains. These findings contribute to the theoretical description of the correlations between structure and dynamical properties of ion-containing polymers.

Distribution Cutoff for Clusters near the Gel Point

The mechanical and dynamic properties of developing networks near the gel point are susceptible to the distribution of clusters coexisting with percolating networks. The distribution of cluster numbers follows a broad power law, wrapped by a cutoff function that rapidly decays at a characteristic size. The form of the cutoff function has been speculated based on known results from lattice percolation and, in certain cases, solved. We obtained this cutoff function from simulated dynamic clusters of polymeric precursor chains using a hybrid Monte Carlo algorithm. The results obtained from three different precursor chain lengths are consistent with each other and are consistent with the expectation from lattice percolation.

Nonlinear Extensional Rheology of Poly(n-alkyl methacrylate) Melts with a Fixed Number of Kuhn Segments and Entanglements per Chain

Molecular theories for dynamics of entangled polymers are based on both the number of Kuhn segments per entanglement N_e and the number of entanglements per chain N/N_e. Extensive studies have shown that, for polymer chains in the solutions or melts, linear viscoelasticity can be properly normalized, whereas the nonlinear extensional rheological properties cannot be normalized when N/N_e is kept the same. The failure of the latter normalization has been attributed to a difference in N_e. Nevertheless, nonlinear rheological studies are lacking for a suitable model system with fixed N_e and N/N_e. In this study, we identify poly(n-alkyl methacrylate)s with the number of carbons per alkyl group below seven as a model system. We find that the degree of the transient strain hardening during extensional flow strengthens with increasing the size of the alkyl group even when N_e and N/N_e are kept the same, which is attributable to the weaker friction reduction when the main backbones are more separated.

Brittle-to-Ductile Transition of Sulfonated Polystyrene Ionomers

This study examines the brittle-to-ductile transition of sulfonated polystyrene ionomers (SPS) with different counterions. The polystyrene precursor was unentangled and had two ionic groups per chain on average. Thus, its terminal relaxation time was comparable to the lifetime of the associating ionic groups. Three types of ionomer samples were used to tune the association lifetime: (1) fully neutralized SPS with different alkali-metal counterions, (2) fully neutralized SPS with mixed sodium and cesium counterions, and (3) partially neutralized SPS with sodium or cesium counterions. For all three systems, the brittle-to-ductile transition could be represented by a diagram of two Weissenberg numbers, Wi and Wi_R, defined with respect to the terminal and Rouse relaxation times, respectively. A flowable region existed at sufficiently low Wi, independent of Wi_R. At higher Wi, a brittle-to-ductile transition of the ionomer melt occurred above a critical value of Wi_R. To achieve ductility during the application of rapid elongational flow, the Rouse-type motions should be sufficiently slow relative to the rate of ion-dissociation, so that the strain-induced breakup of the ionic cross-links would not cause very strong chain retraction that may further lead to the macroscopic fracture.

Polymer Conformations, Entanglements and Dynamics in Ionic Nanocomposites: A Molecular Dynamics Study

We investigate nanoparticle (NP) dispersion, polymer conformations, entanglements and dynamics in ionic nanocomposites. To this end, we study nanocomposite systems with various spherical NP loadings, three different molecular weights, two different Bjerrum lengths, and two types of charge-sequenced polymers by means of molecular dynamics simulations. NP dispersion can be achieved in either oligomeric or entangled polymeric matrices due to the presence of electrostatic interactions. We show that the overall conformations of ionic oligomer chains, as characterized by their radii of gyration, are affected by the presence and the amount of charged NPs, while the dimensions of charged entangled polymers remain unperturbed. Both the dynamical behavior of polymers and NPs, and the lifetime and amount of temporary crosslinks, are found to depend on the ratio between the Bjerrum length and characteristic distance between charged monomers. Polymer-polymer entanglements start to decrease beyond a certain NP loading. The dynamics of ionic NPs and polymers is very different compared with their non-ionic counterparts. Specifically, ionic NP dynamics is getting enhanced in entangled matrices and also accelerates with the increase of NP loading.

Extremely Tough, Puncture-Resistant, Transparent, and Photoluminescent Polyurethane Elastomers for Crack Self-Diagnose and Healing Tracking

Ensuring material performance reliability and lifetime is crucial for practical operations. Small cracks on the material surface are often detrimental to its safe operation. This study describes the development of a hydrogen bond-rich puncture-resistant polyurethane elastomer with supertoughness. The as-prepared polyurethane transparent films feature high tensile break strength (57.4 MPa) and great toughness (228 MJ m^-3). Additionally, a facile, low-cost, crack self-diagnostic approach through photoluminescence using a small luminous pen is reported. The materials efficiently achieved self-healing at 90 °C after the crack formation. The change of fluorescence intensity on the crack can be used to track the self-healing process. Therefore, this work provides a guideline for the material design of supertough, puncture-resistant, transparent, and healable elastomers and a crack self-diagnosis and healing approach.

A Critical Approach to Polymer Dynamics in Supramolecular Polymers

Over the past few years, the concurrent (1) development of polymer synthesis and (2) introduction of new mathematical models for polymer dynamics have evolved the classical framework for polymer dynamics once established by Doi-Edwards/de Gennes. Although the analysis of supramolecular polymer dynamics based on linear rheology has improved a lot recently, there are a large number of insecurities behind the conclusions, which originate from the complexity of these novel systems. The interdependent effect of supramolecular entities (stickers) and chain dynamics can be overwhelming depending on the type and location of stickers as well as the architecture and chemistry of polymers. This Perspective illustrates these parameters and strives to determine what is still missing and has to be improved in the future works.

Miscibility and Nanoparticle Diffusion in Ionic Nanocomposites

We investigate the effect of various spherical nanoparticles in a polymer matrix on dispersion, chain dimensions and entanglements for ionic nanocomposites at dilute and high nanoparticle loading by means of molecular dynamics simulations. The nanoparticle dispersion can be achieved in oligomer matrices due to the presence of electrostatic interactions. We show that the overall configuration of ionic oligomer chains, as characterized by their radii of gyration, can be perturbed at dilute nanoparticle loading by the presence of charged nanoparticles. In addition, the nanoparticle's diffusivity is reduced due to the electrostatic interactions, in comparison to conventional nanocomposites where the electrostatic interaction is absent. The charged nanoparticles are found to move by a hopping mechanism.

The role of electrostatic repulsion in the gelation of poly(vinyl alcohol)/borax aqueous solutions

This study examined linear viscoelasticity (LVE) of semidilute non-entangled poly(vinyl alcohol) (PVA) and borax aqueous solutions: one borax molecule dissociates into two B(OH)3 molecules and two B(OH)4- ions, and the latter can crosslink the PVA chains. Gelation was observed with increasing concentration of borax in the solutions. Analysis of LVE with the reversible gelation model reveals that although the number density of crosslinking ions increases with increasing concentration of borax, the fraction of the crosslinking ions decreases, which is attributed to the intensified electrostatic repulsion when more sites of the PVA chains are occupied by the ionic crosslinkers.