Açık Erişim ve Akademik Arşiv Politikası

The purpose of this policy is to provide global open access via internet to Özyeğin University’s scientific research results within the framework of open access principles. Open Access to research publications aims to improve the recognition of the university and its researchers and also to increase the number of citation of the publications. It also ensures the rise of the prestige of the university. The content of the academic archive is composed of preprint, postprint or publisher version of the articles, conference materials, master’s theses, PhD dissertations, book chapters and other types of publications. The inputs of the projects supported by fund providers are also included in the academic archive.Bu politikanın amacı Özyeğin Üniversitesi personelinin bilimsel yayınlarının internet aracılığı ile tüm dünyaya ücretsiz olarak açık erişim prensipleri çerçevesinde erişime açık olarak sağlanmasıdır. Araştırma yayınlarının açık erişim yolu ile yayınlanması, hem üniversitenin hem de araştırmacıların tüm dünyada tanınırlığının artmasına ve araştırmaların daha fazla atıf almasına yardımcı olacak; ayrıca üniversitenin saygınlığını geliştirecektir. Kapsam içeriğinde olan yayınlar Özyeğin Üniversitesinin dış dünyaya yaptığı katkılar olan araştırma literatürü, makaleler, konferans bildirileri, yüksek lisans ve doktora tezleri, kitap bölümleri ve diğer yayınların ön baskısı, son baskısı ya da yayıncı sürümüdür. Bununla birlikte araştırma literatürü kapsamında fon sağlayıcı kuruluşlarınca desteklenen proje çıktıları da yer alır

Subsecond Morphological Changes in Nafion during Water Uptake Detected by Small-Angle X-ray Scattering

The ability of Nafion® membrane to absorb water rapidly and create a network of hydrated interconnected water domains provides this material with an unmatched ability to conduct ions through a chemically and mechanically robust membrane. The morphology and composition of these hydrated membranes significantly affects their transport properties and performance. This work demonstrates that differences in interfacial interactions between the membranes exposed to vapor or liquid water can cause significant changes in kinetics of water uptake. In-situ small-angle X-ray scattering (SAXS) experiments captured the rapid swelling of the membrane in liquid water with nanostructure rearrangement on the order of seconds. For membranes in contact with water vapor, morphological changes are four-orders-of-magnitude slower than in liquid water, suggesting that interfacial resistance limits the penetration of water into the membrane. Also, upon water absorption from liquid water, a structural rearrangement from a distribution of spherical and cylindrical domains to exclusively cylindrical-like domains is suggested. These differences in water-uptake kinetics and morphology provide a new perspective into Schroeder’s Paradox, which dictates different water contents for vaporand liquid-equilibrated ionomers at unit activity. The findings of this work provide critical insights into the fast kinetics of water absorption of Nafion membrane, which can aid in the design of energy conversion devices that operate under frequent changes in environmental conditions

Morphology of supported polymer electrolyte ultra-thin films: a numerical study

Morphology of polymer electrolytes membranes (PEM), e.g., Nafion, inside PEM fuel cell catalyst layers has significant impact on the electrochemical activity and transport phenomena that determine cell performance. In those regions, Nafion can be found as an ultra-thin film, coating the catalyst and the catalyst support surfaces. The impact of the hydrophilic/hydrophobic character of these surfaces on the structural formation of the films has not been sufficiently explored yet. Here, we report about Molecular Dynamics simulation investigation of the substrate effects on the ionomer ultra-thin film morphology at different hydration levels. We use a mean-field-like model we introduced in previous publications for the interaction of the hydrated Nafion ionomer with a substrate, characterized by a tunable degree of hydrophilicity. We show that the affinity of the substrate with water plays a crucial role in the molecular rearrangement of the ionomer film, resulting in completely different morphologies. Detailed structural description in different regions of the film shows evidences of strongly heterogeneous behavior. A qualitative discussion of the implications of our observations on the PEMFC catalyst layer performance is finally proposed

Protonation-Induced Microphase Separation in Thin Films of a Polyelectrolyte-Hydrophilic Diblock Copolymer

Block copolymers composed of poly(oligo ethylene glycol methyl ether methacrylate) and poly(2-vinylpyridine) are disordered in the neat state but can be induced to order by protonation of the P2VP block, demonstrating a tunable and responsive method for triggering assembly in thin films. Comparison of protonation with the addition of salts shows that microphase separation is due to selective protonation of the P2VP block. Increasing acid incorporation and increasing 2-vinylpyridine content for P2VP minority copolymers both promote increasingly phase-separated morphologies, consistent with protonation increasing the effective strength of segregation between the two blocks. The self-assembled nanostructures formed after casting from acidic solutions may be tuned based on the amount and type of acid incorporation as well as the annealing treatment applied after casting, where both aqueous and polar organic solvents are shown to be effective. Therefore, POEGMA-b-P2VP is a novel ion-containing block copolymer whose morphologies can be facilely tuned during casting and processing by controlling its exposure to acid.United States. Dept. of Energy. Office of Basic Energy Sciences (Award DE-SC0001088)National Science Foundation (U.S.) (Award CMMI-1246740

Fibrillar structure of Nafion: Matching fourier and real space studies of corresponding films and solutions

cited By 229International audienceUsing both scattering and microscopy techniques, we have characterized the complex Nafion structure over a large range of length scales. Analysis of experimental data from dry membrane to aqueous dispersion suggests an intrinsic fibrillar structure. The fibrils correspond to elongated polymeric aggregates surrounded with the ionic charges. In the Nafion membrane, the fibrils are entangled and collapsed with a degree of orientation at the mesoscopic scale. Upon swelling and temperature treatment, these aggregates are hydrated and dispersed in a colloidal suspension

Evidence of elongated polymeric aggregates in Nafion

cited By 326International audienceSmall-angle X-ray and neutron-scattering techniques have been used to probe the structure of swollen Nation membranes in the range 10-10000 Å. From analyzing the scattering data as a function of the polymer volume fraction and using a contrast variation method for the neutron experiments, we suggest a new structural model of Nation in the hydrated state. It is based on the aggregation of the ionomer chains into elongated polymeric bundles with a diameter on the order of 40 Å and a length larger than 1000 Å, surrounded by the electrolyte solution

Orientation of drawn Nafion at molecular and mesoscopic scales

cited By 98International audienceThe structural orientation in Nafion films in an ambient environment and under stretching is analyzed using small/wide-angle X-ray scattering. On the basis of the absolute values of Hermans' orientation factor, we characterize the structural anisotropy at different length scales (between angstroms and several hundreds of angstroms) in agreement with a previously proposed fibrillar model of the Nafion membrane. Bundles of elongated polymeric aggregates orient at small draw ratios, together with an orientation of the aggregates within such a bundle. The crystal structure of this semicrystalline polymer is analyzed, and we show that the crystallinity of the Nafion film does not change as a function of the draw ratio

Caractérisation de la structure des membranes ionomères (NAFION^{\mbox{\tiny\textregistered}}) par diffusion de rayons X aux petits angles

Ionomer membranes, like Nafion^{\mbox{\tiny\textregistered}} used in fuel cell, present a nano- phase separation between domains with different ionic concentrations. Up to now models describe the ionic domains as spheres of about 40 Å diameter. Small angles X-ray scattering studies over a large range of wave vectors, lead to a new assumption for the Nafion structure, describing the polymer aggregation as elongated objects surrounding by the ionic charges.Les membranes ionomères de type Nafion^{\mbox{\tiny\textregistered}} utilisées en pile à combustible, sont caractérisées par une nano-séparation de phases entre des domaines plus ou moins riches en sites ioniques. Les modèles proposés pour décrire ces domaines ioniques, les représentent généralement sous forme de sphères de 40 Å de diamètre. L'étude en diffusion de rayons X que nous avons menée récemment, sur une large gamme de vecteurs d'ondes, nous permet de proposer une vision différente de la structure du Nafion en considérant une agrégation de polymères sous formes d'objets très allongés, avec en surface les charges ioniques