[PDF] The Molecular Dynamics Of Ionic Aggregation In Ampholytic Ionomers eBook

Author : Michel Pineri
Publisher : Springer Science & Business Media
Page : 608 pages
File Size : 43,46 MB
Release : 1987-05-31
Category : Science
ISBN : 9789027724588

GET BOOK

Proceedings of the NATO Advanced Research Workshop, Villard de Lans, France, June 15-21, 1986

Author : Dihui Ruan
Publisher :
Page : 61 pages
File Size : 41,45 MB
Release : 2015
Category : Ionomers
ISBN :

GET BOOK

Ionomers - polymers with bonded ionic groups - are important because of their wide applications in various fields. The glass formation behavior of ionomers has direct impact on their properties and performance, and therefore demands a deeper understanding. The ionic groups collapse into distributed ionic aggregates in the ionomers. The region surrounding the aggregates shows dynamic suppression among uncharged polymer chains, and the size of this region is correlated to chains' persistence length. This phenomenon is a consequence of the fact that there is bond connectivity between aggregates and polymers, which differs from materials with only non-bonded interfaces, like composites. Here, we perform molecular dynamics simulations of model ionomers to test this conventional view and propose a fundamental reconsideration of grafted micro-phase materials like ionomers. Based on our results, the covalent grafts are not the central factor determining linear segmental dynamics and glass formation. Instead, we find that they are equivalent to strong physical attractions, as in ungrafted nanocomposites and nanoconfined glass-formers, where near-interface mobility suppression is mediated by cooperative rearrangements intrinsic to glass-forming liquids, rather than by a unique covalent `tethering effect'. This conclusion indicates the need for a revised understanding of glass formation and segmental dynamics in materials incorporating covalent grafting.

Author : Mahdi Ghelichi Ghalacheh
Publisher :
Page : 137 pages
File Size : 39,8 MB
Release : 2016
Category :
ISBN :

GET BOOK

Different functions are expected from the polymer electrolyte membranes used in fuel cells. They work as a proton conduction medium, as a separator, and as an electronic insulator. The current membrane materials of choice are perfluorosulfonic acid (PFSA) ionomers such as Nafion. The two main challenges that PFSAs still face, after three decades of extensive research, are a limited lifetime and a lack of basic structural understanding. To investigate the chemical degradation phenomena, we devised a kinetic model of radical formation and attack to PFSA ionomers. Analytical relations are derived to obtain the content of aggressive radicals as a function of iron ion content and hydrogen peroxide. The mean-field type, coarse-grained ionomer model distinguishes ionomer headgroups, side chains, and ionomer backbone. The model is used to study the impact of different degradation mechanisms and ionomer chemistries on PEM degradation. Application of the model to degradation data of various PFSAs highlights the important role of radical attack to the ionomer headgroups. The insufficient understanding of the membrane structure thwarts further forays in degradation modeling. To this end, we undertook molecular dynamics simulations of the conformation of single chain ionomers as a function of different structural parameters. This study revealed the nonmonotonic effect of the side chain length and density on the conformational behaviour and rigidity of ionomer backbones. We discuss how the changes in these architectural parameters change the ionomer affinity to counterions and the corresponding ion mobility. Studying the aggregation of ionomer chains revealed their spontaneous aggregation in dilute solution. We explored the effect of various parameters such as ionomer hydrophobicity and side chain content on ionomer bundle formation. Minimization of the surface free energy of hydrophobic backbones is the driving force of ionomer aggregation, while the repulsion of anionic headgroups opposes the aggregation. The results rationalize the experimental studies and highlight the role ionomer bundles as the prevailing structural motif in PFSA materials.

Author : M.R. Tant
Publisher : Springer Science & Business Media
Page : 524 pages
File Size : 33,88 MB
Release : 2012-12-06
Category : Technology & Engineering
ISBN : 940091461X

GET BOOK

Polymers have achieved an enviable position as the class of materials having the highest volume of production, exceeding that of both metals and ceramics. The meteoric rise in the production and utilization of polymers has been due to advances in polymer synthesis which allow the creation of specific and well-defined molecular structures, to new knowledge concerning the relationships between polymer structure and properties, and to an improved understanding of how processing can be used as a tool to develop morphological features which result in desired properties. Polymers have truly become 'engineered materials' in every sense of the term. Polymer scientists and engineers are forever seeking to modify and improve the properties of synthetic polymeric systems for use in specific applications. Towards this end they have often looked to nature for advice on how to design molecules for specific needs. An excellent illustration of this is the use of noncovalent bonding (ionic, hydrogen, and van der Waals) in lipids, proteins, and nucleic acids, where these noncovalent bonds, acting both intra and intermolecularly, precisely control the structure and thus the function of the entire system. The utilization of ionic bonding, in particular in man-made polymers has attracted widespread interest in recent years, since ionic interactions exert a similar strong influence on the structure and properties of these synthetic systems.