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Morphology of Polymers for Application in Ion Exchange Membranes /

Record Type:	Electronic resources : Monograph/item
Title/Author:	Morphology of Polymers for Application in Ion Exchange Membranes /Nayan Jyoti Saikia.
Author:	Saikia, Nayan Jyoti,
Description:	1 electronic resource (144 pages)
Notes:	Source: Dissertations Abstracts International, Volume: 86-05, Section: A.
Notes:	Advisors: Hickner, Michael A. Committee members: Alem, Nasim; Gomez, Enrique; Pester, Christian; Rioux, Robert.
Contained By:	Dissertations Abstracts International86-05A.
Subject:	Fuel cells.
Online resource:	http://pqdd.sinica.edu.tw/twdaoapp/servlet/advanced?query=31631356
ISBN:	9798346383505

Morphology of Polymers for Application in Ion Exchange Membranes /
Saikia, Nayan Jyoti,

Morphology of Polymers for Application in Ion Exchange Membranes /Nayan Jyoti Saikia. - 1 electronic resource (144 pages)

Source: Dissertations Abstracts International, Volume: 86-05, Section: A.

Ion exchange membrane (IEMs) are poised to play an important role in emerging energy conversion devices such as batteries, soft actuators and electrochemical cells, especially polymer-electrolyte-membrane fuel-cells (PEMFCs). IEMs are typically composed of a hydrophobic backbone which is covalently attached to an ionic group with a mobile counter-ion. The ion functionalized group dissociates in the presence of water releasing the counter ion and hence is responsible for ionic conduction. These materials are often called single ion conductors since only one charge is mobile, versus a salt-based ionic conductor where both anions and cations are mobile in the matrix. In IEMs conduction of counterions rely on a network of water-fille domains to transport the ion through the polymeric matrix. An effective network in the bulk provides for faster transport than an unorganized system, and hence high ionic conductivity of the ensuing material.Researchers have been interested in investigating these ionic domain networks (or microphase separated morphology) since the early days of small angle x-ray scattering (SAXS). SAXS is a fast technique that generates contrast based on electronic density between the phases. This allowed them to investigate the morphology of Nafion and propose representative structures that explain the superior ionic conductivity of the material. However, scattering alone cannot unambiguously elucidate the morphology since it relies on models to fit the experimental data, and hence TEM should be used in tandem with SAXS. In this dissertation, we will employ transmission electron microscopy (TEM) as a tool to understand the phase morphology of ionic polymers and directly visualize size, shape, and distribution of domains in the materials. Apart from this, we will use the different modalities such as field/dark field (BF), scanning transmission electron microscopy (STEM) and energy filtered TEM (EFTEM) to optimize contrast and resolution of the images.It has been well-established that polymeric materials undergo beam damage under irradiation. Radiation damage changes morphology due to heating, electrostatic charging, and sputtering and can produce artifacts that are not inherent to the unaltered material. To mitigate damage, we begin our experiment at low dose rates (< 500 e/nm2s) and slowly increase the beam intensity to make sure radiation damage is minimized. STEM modes were controlled using spot sizes and lenses to achieve variable current. We observed that STEM mode is a more efficient way to image polymers as the contrast can be switched based on atomic number of the elements present or the density variations between the phases. Density variations allowed us image ionic block copolymers based on two common backbone architectures: polystyrene-b-polybutadiene-b--polystyrene (SBS) and polystyrene-b-poly(ethylene-co-butylene)-b-polystyrene (SEBS). The SEBS-based polymer demonstrated long range perforated lamellar structures with average lamellar thickness of 55 nm while the morphology of SBS polymers were heterogenous without widespread long-range order. Some local ordering was observed in the SBS samples of length scales of about ~150 nm which indicates non equilibrium conditions for the bulk film, but block architecture and ion content can also play a big role in nanophase morphology. In general, these ionic block copolymers demonstrated deviation in phase morphology from traditional block copolymers, which can be attributed to the additional interaction that the ions have on the polymer system.

English

ISBN: 9798346383505Subjects--Topical Terms:

237770
Fuel cells.

Morphology of Polymers for Application in Ion Exchange Membranes /
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520 $a Ion exchange membrane (IEMs) are poised to play an important role in emerging energy conversion devices such as batteries, soft actuators and electrochemical cells, especially polymer-electrolyte-membrane fuel-cells (PEMFCs). IEMs are typically composed of a hydrophobic backbone which is covalently attached to an ionic group with a mobile counter-ion. The ion functionalized group dissociates in the presence of water releasing the counter ion and hence is responsible for ionic conduction. These materials are often called single ion conductors since only one charge is mobile, versus a salt-based ionic conductor where both anions and cations are mobile in the matrix. In IEMs conduction of counterions rely on a network of water-fille domains to transport the ion through the polymeric matrix. An effective network in the bulk provides for faster transport than an unorganized system, and hence high ionic conductivity of the ensuing material.Researchers have been interested in investigating these ionic domain networks (or microphase separated morphology) since the early days of small angle x-ray scattering (SAXS). SAXS is a fast technique that generates contrast based on electronic density between the phases. This allowed them to investigate the morphology of Nafion and propose representative structures that explain the superior ionic conductivity of the material. However, scattering alone cannot unambiguously elucidate the morphology since it relies on models to fit the experimental data, and hence TEM should be used in tandem with SAXS. In this dissertation, we will employ transmission electron microscopy (TEM) as a tool to understand the phase morphology of ionic polymers and directly visualize size, shape, and distribution of domains in the materials. Apart from this, we will use the different modalities such as field/dark field (BF), scanning transmission electron microscopy (STEM) and energy filtered TEM (EFTEM) to optimize contrast and resolution of the images.It has been well-established that polymeric materials undergo beam damage under irradiation. Radiation damage changes morphology due to heating, electrostatic charging, and sputtering and can produce artifacts that are not inherent to the unaltered material. To mitigate damage, we begin our experiment at low dose rates (< 500 e/nm2s) and slowly increase the beam intensity to make sure radiation damage is minimized. STEM modes were controlled using spot sizes and lenses to achieve variable current. We observed that STEM mode is a more efficient way to image polymers as the contrast can be switched based on atomic number of the elements present or the density variations between the phases. Density variations allowed us image ionic block copolymers based on two common backbone architectures: polystyrene-b-polybutadiene-b--polystyrene (SBS) and polystyrene-b-poly(ethylene-co-butylene)-b-polystyrene (SEBS). The SEBS-based polymer demonstrated long range perforated lamellar structures with average lamellar thickness of 55 nm while the morphology of SBS polymers were heterogenous without widespread long-range order. Some local ordering was observed in the SBS samples of length scales of about ~150 nm which indicates non equilibrium conditions for the bulk film, but block architecture and ion content can also play a big role in nanophase morphology. In general, these ionic block copolymers demonstrated deviation in phase morphology from traditional block copolymers, which can be attributed to the additional interaction that the ions have on the polymer system.
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