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Author: Yuhui Chen
Publisher:
ISBN:
Category : Chemistry
Languages : en
Pages : 0
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Book Description
Polyelectrolytes are polymers bearing charged repeat units. Based on the charge, a polyelectrolyte can be categorized as either a polycation or a polyanion. When dissolved in water, the charges on a polyelectrolyte are balanced by small counterions. Mixing certain polycations and polyanions in an aqueous solution will result in solid precipitants due to polymer-polymer interactions and the entropy gain of releasing the small counterions. This type of material is called polyelectrolyte complexes, PECs. PECs have a wide range of applications including drug delivery systems, film coatings, and membranes. Because of their charged nature, PECs can bind with enzymes and DNA or many bio-medical applications. To further explore the applications of PECs, it is important to understand the dynamics and viscoelastic properties of this special type of polymer. Many factors such as water, temperature, and composition heavily influence the behaviors of PECs, and therefore were studied in this work.The first part of the dissertation focuses on water and the viscoelastic properties of various PECs. Thirteen different polyelectrolytes were used to produce those PECs, and radiolabeling and NMR techniques were used to determine the polycation: polyanion stoichiometry in the materials. Rheological studies were performed on those PECs to determine their glass transition temperatures, Tg. The PECs exhibit a wide range of Tgs within the working range of aqueous solutions between 0 and 100 °C. To study the correlation between Tg and water, a radiochemical method was developed to measure the pore volume within the PECs, which was used to differentiate between PEC water and pore water. Contrary to what was generally believed, the Tg correlated poorly with the water volume fraction in PECs. There was a weak correlation in a series of PECs in which one of the polyelectrolytes was held constant. On the other hand, time-temperature superposition of linear viscoelastic responses provided a classical estimate of the fractional free volume of PECs, which correlated well with Tg. The second part investigated the correlation between the stoichiometry and viscoelastic properties of a certain PEC, poly(diallydimethylammonium) (PDADMA)/poly(4-styrenesulfonic acid) (PSS). Various nonstoichiometric PECs with excess in either PDADMA or PSS were produced by a "quenching" method. A xvi maximum of 36% molar excess of PDADMA or 40% molar excess of PSS was achieved. Those nonstoichiometric PECs exhibited lower Tgs compared to stoichiometric ones, and an excess of PDADMA had a bigger effect on lowering the Tg. It was shown that the decrease in Tg was caused by the increase in water content. Due to the excess polyelectrolytes and counterions, more water was introduced to the nonstoichiometric PECs and therefore lowered the Tg via the plasticizing effect. In addition, time-temperature superposition revealed a correlation between segmental relaxation times and stoichiometry. Nonstoichiometric PECs were shown to have elevated fractional free volumes and decreased polymer volume fractions, which are responsible for changes in their mechanical properties. The third part focuses on the formation and properties of PEC blends. Four PECs with distinct Tgs were used to produce two PEC blends with varying compositions. Each blend is comprised of one polycation and two polyanions, and the molar ratio of the two polyanions was adjusted to create blends with the same components but different molar compositions. Rheological experiments were carried out to determine the Tgs of the blends. It was shown that one of the blends exhibited Tgs in between the two individual PECs, and the Tgs can be modeled using the Kwei Equation. The other blend showed two different Tgs that were similar to the individual PECs' Tgs, indicating the blend was immiscible. The immiscibility was explained by the difference in critical salt concentration between the two individual PECs.
Author: Yuhui Chen
Publisher:
ISBN:
Category : Chemistry
Languages : en
Pages : 0
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Book Description
Polyelectrolytes are polymers bearing charged repeat units. Based on the charge, a polyelectrolyte can be categorized as either a polycation or a polyanion. When dissolved in water, the charges on a polyelectrolyte are balanced by small counterions. Mixing certain polycations and polyanions in an aqueous solution will result in solid precipitants due to polymer-polymer interactions and the entropy gain of releasing the small counterions. This type of material is called polyelectrolyte complexes, PECs. PECs have a wide range of applications including drug delivery systems, film coatings, and membranes. Because of their charged nature, PECs can bind with enzymes and DNA or many bio-medical applications. To further explore the applications of PECs, it is important to understand the dynamics and viscoelastic properties of this special type of polymer. Many factors such as water, temperature, and composition heavily influence the behaviors of PECs, and therefore were studied in this work.The first part of the dissertation focuses on water and the viscoelastic properties of various PECs. Thirteen different polyelectrolytes were used to produce those PECs, and radiolabeling and NMR techniques were used to determine the polycation: polyanion stoichiometry in the materials. Rheological studies were performed on those PECs to determine their glass transition temperatures, Tg. The PECs exhibit a wide range of Tgs within the working range of aqueous solutions between 0 and 100 °C. To study the correlation between Tg and water, a radiochemical method was developed to measure the pore volume within the PECs, which was used to differentiate between PEC water and pore water. Contrary to what was generally believed, the Tg correlated poorly with the water volume fraction in PECs. There was a weak correlation in a series of PECs in which one of the polyelectrolytes was held constant. On the other hand, time-temperature superposition of linear viscoelastic responses provided a classical estimate of the fractional free volume of PECs, which correlated well with Tg. The second part investigated the correlation between the stoichiometry and viscoelastic properties of a certain PEC, poly(diallydimethylammonium) (PDADMA)/poly(4-styrenesulfonic acid) (PSS). Various nonstoichiometric PECs with excess in either PDADMA or PSS were produced by a "quenching" method. A xvi maximum of 36% molar excess of PDADMA or 40% molar excess of PSS was achieved. Those nonstoichiometric PECs exhibited lower Tgs compared to stoichiometric ones, and an excess of PDADMA had a bigger effect on lowering the Tg. It was shown that the decrease in Tg was caused by the increase in water content. Due to the excess polyelectrolytes and counterions, more water was introduced to the nonstoichiometric PECs and therefore lowered the Tg via the plasticizing effect. In addition, time-temperature superposition revealed a correlation between segmental relaxation times and stoichiometry. Nonstoichiometric PECs were shown to have elevated fractional free volumes and decreased polymer volume fractions, which are responsible for changes in their mechanical properties. The third part focuses on the formation and properties of PEC blends. Four PECs with distinct Tgs were used to produce two PEC blends with varying compositions. Each blend is comprised of one polycation and two polyanions, and the molar ratio of the two polyanions was adjusted to create blends with the same components but different molar compositions. Rheological experiments were carried out to determine the Tgs of the blends. It was shown that one of the blends exhibited Tgs in between the two individual PECs, and the Tgs can be modeled using the Kwei Equation. The other blend showed two different Tgs that were similar to the individual PECs' Tgs, indicating the blend was immiscible. The immiscibility was explained by the difference in critical salt concentration between the two individual PECs.
Author: Shy Hsien Wu
Publisher:
ISBN:
Category :
Languages : en
Pages : 634
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Author: John D. Ferry
Publisher: John Wiley & Sons
ISBN: 9780471048947
Category : Technology & Engineering
Languages : en
Pages : 676
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Book Description
Viscoelastic behavior reflects the combined viscous and elastic responses, under mechanical stress, of materials which are intermediate between liquids and solids in character. Polymers the basic materials of the rubber and plastic industries and important to the textile, petroleum, automobile, paper, and pharmaceutical industries as well exhibit viscoelasticity to a pronounced degree. Their viscoelastic properties determine the mechanical performance of the final products of these industries, and also the success of processing methods at intermediate stages of production. Viscoelastic Properties of Polymers examines, in detail, the effects of the many variables on which the basic viscoelastic properties depend. These include temperature, pressure, and time; polymer chemical composition, molecular weight and weight distribution, branching and crystallinity; dilution with solvents or plasticizers; and mixture with other materials to form composite systems. With guidance by molecular theory, the dependence of viscoelastic properties on these variables can be simplified by introducing certain ancillary concepts such as the fractional free volume, the monomeric friction coefficient, and the spacing between entanglement loci, to provide a qualitative understanding and in many cases a quantitative prediction of how to achieve desired results. The phenomenological theory of viscoelasticity which permits interrelation of the results of different types of experiments is presented first, with many useful approximation procedures for calculations given. A wide variety of experimental methods is then described, with critical evaluation of their applicability to polymeric materials of different consistencies and in different regions of the time scale (or, for oscillating deformations, the frequency scale). A review of the present state of molecular theory follows, so that viscoelasticity can be related to the motions of flexible polymer molecules and their entanglements and network junctions. The dependence of viscoestic properties on temperature and pressure, and its descriptions using reduced variables, are discussed in detail. Several chapters are then devoted to the dependence of viscoelastic properties on chemical composition, molecular weight, presence of diluents, and other features, for several characteristic classes of polymer materials. Finally, a few examples are given to illustrate the many potential applications of these principles to practical problems in the processing and use of rubbers, plastics, and fibers, and in the control of vibration and noise. The third edition has been brought up to date to reflect the important developments, in a decade of exceptionally active research, which have led to a wider use of polymers, and a wider recognition of the importance and range of application of viscoelastic properties. Additional data have been incorporated, and the book s chapters on dilute solutions, theory of undiluted polymers, plateau and terminal zones, cross-linked polymers, and concentrated solutions have been extensively rewritten to take into account new theories and new experimental results. Technical managers and research workers in the wide range of industries in which polymers play an important role will find that the book provides basic information for practical applications, and graduate students in chemistry and engineering will find, in its illustrations with real data and real numbers, an accessible introduction to the principles of viscoelasticity.
Author: Vaqar Mustafa Shah Syed
Publisher:
ISBN:
Category :
Languages : en
Pages : 71
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The addition of salt speeds up chain relaxation dynamics in polyelectrolyte complexes (PECs), and time-salt superposition (TSS) approaches to describe the linear viscoelastic response of PECs are well-established. However, TSS is carried out at fixed initial polyelectrolyte concentrations, and varying the initial polyelectrolyte concentration results in distinct TSS master curves. In this thesis, we show that accounting for the small ions that accompany the oppositely charged polyelectrolyte chains (designated as accompanying counterions) enables assimilation of these distinct TSS master curves into a single universal master curve. This approach, that we christen as time-ionic strength superposition (TISS), enables a unified description of the PEC viscoelastic response in terms of the solution ionic strength, that accounts for both the accompanying counterions and the added ions, and underlines the dynamic similarities between PECs and semi-dilute polymer solutions. The sticky electrostatic associations among the oppositely charged chains, however, contribute additional relaxation modes in the PECs. We demonstrate that the timescales of these additional relaxation modes are described quantitatively by a modified sticky Rouse model that accounts for the influence of solution ionic strength on the electrostatic screening and chain friction. We then investigate the effect of the cationic valency of the added salt on the composition and rheology of the PECs. A stronger screening of electrostatic interactions is observed with increasing cation valency, leading to higher concentrations of polyelectrolytes in the supernatant phase and lower concentrations in the complex phase. In addition, electrostatic bridging of the polyanion chains by the multivalent cation alters the chain relaxation process in PECs by competing with the electrostatic interactions between the oppositely charged chains, resulting in non-monotonic variations of PEC moduli with increasing ionic strength of the solution.
Author: Martin Müller
Publisher: Springer
ISBN: 364240734X
Category : Technology & Engineering
Languages : en
Pages : 236
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Book Description
Advances in Polymer Science enjoys a longstanding tradition and good reputation in its community. Each volume is dedicated to a current topic, and each review critically surveys one aspect of that topic, to place it within the context of the volume. The volumes typically summarize the significant developments of the last 5 to 10 years and discuss them critically, presenting selected examples, explaining and illustrating the important principles, and bringing together many important references of primary literature. On that basis, future research directions in the area can be discussed. Advances in Polymer Science volumes thus are important references for every polymer scientist, as well as for other scientists interested in polymer science - as an introduction to a neighboring field, or as a compilation of detailed information for the specialist.
Author:
Publisher:
ISBN: 9789463959247
Category :
Languages : en
Pages : 189
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Book Description
The majority of human-made materials are represented by plastics and elastomers, polymeric solids synthesized from petrochemically obtained feedstock. When compared to materials that are constructed from the polymers produced in cells, human-made synthetics are substantially less adapted to function with or within living organisms. The adaptation of materials to fit a set of requirements can be described in terms of their character in the dimensions of hydrophilicity, viscoelasticity, and toughness. Polymers used by plants and animals, such as in tissues, are always built from water-soluble compounds, and are therefore predominantly hydrophilic, unlike most synthetic materials. The viscoelasticity, a property which encompasses both the time-dependent compliant (viscous) as well as the elastic (solid) response of a material, of natural materials is diverse and can be often described as solid with a dissipating component. Finally, natural materials present a toughness, the fracture resistance of a material to high deformations, that is adapted to their function in the organisms that the materials serves. While hydrophilicity, viscoelasticity, and toughness can be tuned in synthetic materials to some extent, the majority of available materials do not offer independent control of either. The adoption of synthetic materials for use in the human body is highly attractive for the medical field: novel materials will allow to better rehabilitate tissues or even take over their function. However, chemical platforms do not offer the repertoire that billions of years of evolution allowed to perfect in biology. In this thesis, I present complex coacervates as a material class that offers untapped possibilities to bridge the property gap between natural and synthetic materials. Complex coacervates, also termed polyelectrolyte complexes or saloplastics when high stiffnesses are reached, are phases that form on reversible association of oppositely charged polyelectrolytes in water. Complex coacervates have a strongly salt-depended viscoelasticity (saloplasticity), with more liquid-like phases forming at higher concentrations of added salt. Coacervates are highly hydrophilic due to their constituents being water-soluble, yet are insoluble in water. The latter property is unique for non-crosslinked solids, and as such coacervates represent a platform of enormous interest for future biomaterials. Nonetheless, the saloplasticity of complex coacervates lacks two degrees of control to allow fabrication of a material with given specification. First, addition of salt not only reduces the terminal relaxation time, but also softens the complex. Second, salt modifies the response of the complex to large deformations, thus the fracture resistance or toughness. The central mission of this thesis is to provide chemical tools to control the viscoelasticity and toughness of complex coacervates other than the salt concentration. First, we show that the stiffness of coacervates can be effectively controlled by introducing metal-ligand bonds into the coacervate that form transient crosslinks between the polyelectrolyte chains. Choosing metals with shorter or longer relaxation times allows an additional layer of control over the viscoelasticity. Thus, hybrid metal-ligand complex coacervates can be made at the desired stiffness and terminal relaxation time. Moreover, we show that metal-ligand complexation increases the resistance of coacervate to fracture at high deformation amplitudes. Thus, key mechanical properties are disengaged from the salt concentration. Furthermore, we demonstrate a synthesis route towards polyelectrolyte with the bottlebrush architecture. A bottlebrush polymer comprises a backbone onto which a dense array of side-chains is grafted. The side-chains stretches the backbone, which results in a markedly different phase behaviour of complexes with linear, oppositely charged polyelectrolytes. Finally, we present an improved synthesis of poly(acrylic acid), a ubiquitous polyanion. We demonstrate the necessity of using a quantitative de-esterification method when polyanions are synthesized from their esterified precursors. Specifically, the self-assembly behaviour of thermoaggregating triblock copolymers is shown to be disabled when non-quantitative deprotection has left hydrophobic impurities in the polymers. Our improved methodology effectively prevents issues of non-quantitative ester cleavage. In short, this thesis provide chemical handles on the mechanical behaviour of complex coacervates. The mechanical targets of viscoelasticity and toughness are addressed with metal-ligand complexation, changes to architecture of the polyelectrolytes from linear to densely grafted, and improvements to the structural fidelity of the polyelectrolytes themselves.
Author: Huan Zhang (Polymer engineer)
Publisher:
ISBN:
Category : Photoelectrochemistry
Languages : en
Pages : 0
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Polyelectrolyte complexes (PECs) are formed based on the strong association of oppositely charged polyelectrolytes, a process driven by a combination of enthalpy and entropy, but often with entropic considerations dominating. PEC based materials including polyelectrolyte coacervates, precipitates, and multilayers (PEMs), exhibit promising application in a variety of areas. These abundant applications of PECs are made possible by their highly controllable/tailorable properties upon exposure to different stimuli like pH, salt, organic solvent, and so on. It is important to understand how these different parameters affect the complexation of polyelectrolytes and the structures of PECs. In this dissertation, the main goal is to investigate the effects of solution composition including salt content, pH of solution, and solvent quality on the properties of polyelectrolyte coacervate, precipitate and multilayers. Based on these studies, the potential of applications of PEC materials in self-healing, 3D printing under ethanol and adhesive were studied. Firstly, the interactions between various types of salt and polyelectrolytes were studied. The energetics of the binding process between salt and polyelectrolytes were investigated. Various types of cations and anions were employed in this study, including alkali metal, alkaline earth metal ions, transition metal ions and a Hofmeister series of anions. Based on the different interactions between these ions and polyelectrolytes, the salts present different effects on the properties of PECs. Salt effects on coacervate, especially coacervate stability, are summarized in Chapter III. Chapter IV shows the study of salt effects on the properties of PEMs including the growth and swelling/deswelling behaviors. In Chapter V, the effects of salt and pH on the self-healing ability of bulk PEC materials were studied. The rheological properties of hydrated, bulk PEC material are strongly influenced by salt and pH treatment, which directly controls the crosslink density in the material, chain mobility, and therefore self-healing ability. Alkali metal ions and transition metal ions show different effects on the self-healing ability of the BPEI/PAA complex. The effects of these two different types of salts on complex are reversible, which presents a possible way to control the material properties, ranging from self-healing to completely unable to do so. Due to changes in charge density of weak polyelectrolytes in response to pH, the self-healing of weak PECs can be enhanced by pH treatment as well. In Chapter VI, the effects of ethanol on the complexation of polyelectrolytes, rheological properties and structure of polyelectrolyte precipitates were investigated. The complexation of polyelectrolytes is inhibited by the presence of ethanol due to the weakening interactions between polycation and polyanion under low dielectric constant environment. For the polyelectrolyte precipitate formed from higher ethanol content mixture, the precipitate shows a smaller modulus and larger loss angle. In comparison, by immersing polyelectrolyte precipitate in a higher ethanol content mixture, the precipitate shows an increased modulus. This enhancement of mechanical properties is mainly due to the dehydration of the polyelectrolyte precipitate, which results in an enhanced interaction between BPEI and PAA (lowering the dielectric constant surrounding ion pairs strengthens them). The precipitate formed from high ethanol content mixture (vol% of ethanol > ~ 40%) can be dissolved in water to form a highly viscous polyelectrolyte coacervate, which shows potential applications in 3D printing under ethanol. In Chapter VII, electrophoretic deposition (EPD) was used to fabricate polyelectrolyte complex films, and it was shown to create thicker films than those created by the simple deposition of polyelectrolyte complexes in the absence of external electrical fields. A pulsed EPD with controlled pulse ON time and pulse OFF times can effectively suppress the formation of these bubbles. The mobility and zeta potential of BPEI/PAA complexes are modulated by the ratio of BPEI to PAA and pH of complexes, which will affect the thickness of deposited complex film. Regardless of different pulsed mode, by changing the electric field strength, the maximum thickness of BPEI/PAA complex film is obtained at a moderate electric field strength. High electric field strength will result in the dissolution of BPEI/PAA complexes due to formation of air bubbles and the high pH environment near the working electrode. In contrast, by using strong polyelectrolyte complex pair, PDAC/PSS, a proportional relationship between the thickness of complex film and electric field strength is observed. Therefore, the local pH change near working electrode is an important factor in the deposition of weak polyelectrolyte pairs. Moreover, ionic strength of the complex solution influences the charge density and stability of BPEI/PAA polyelectrolyte complexes. This work also shows the fabrication of polyelectrolyte complex-dye films using EPD. The loading of dye in complex films can be enhanced using proper EPD method.
Author: Haowei Jiang
Publisher:
ISBN:
Category : Polyelectrolytes
Languages : en
Pages :
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Book Description
This work examines two distinct cases where the mechanical properties are controlled by additives or structure. First, the viscoelastic properties of polymer complexes are examined as a function of salt content as salt provides a route to tune the rheological properties through disruption of the associations of the complex. These salt effects have been proposed to collapse to a master curves for the rheological properties of ionic systems through time-salt superposition (TSS) as an analog to time-temperature superposition. Here we demonstrate differences in the salt (sodium chloride and choline chloride) dependence of branched polyethylenimine-poly(acrylic acid) (BPEI-PAA) containing approximately 50 wt % water using the frequency dependence of G' and G'' and extensional measurements for complexes. The shape of the time average modulus ([E(t)]) obtained from constant strain rate extension experiments is applied to study the relaxation behavior in long timescales. However, inconsistent hydration (partially from dehydration during extension) leads to inconsistences in [E(t)], which hinders the ability to firmly draw conclusions about the applicability of TSS in these systems. Three-dimensional (3D) printing has been commonly used for rapid prototyping manufacturing. However, application of 3D printed parts to products has been limited by their inferior mechanical properties due to the printing process. Core-shell structured filaments can overcome the drawback of the weak interfaces in 3D printing, where the shell material enhance the interfacial strength due to their lower solidification temperature than the core materials. In this work, we demonstrate that PC-ABS-polyethylene core-shell filaments can improve the mechanical properties of 3D printed parts as compared to those printed from pure PC-ABS. We compare the influence of HDPE vs LDPE on the properties of the printed part. The impact resistance can be approximately increased to 3 times as that of pure PC-ABS.
Author: Tsetska Radeva
Publisher: CRC Press
ISBN: 1482270684
Category : Science
Languages : en
Pages : 936
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Book Description
An examination of the fundamental nature of polyelectrolytes, static and dynamic properties of salt-free and salt-added solutions, and interactions with other charged and neutral species at interfaces with applications to industry and medicine. It applies the Metropolis Monte Carlo simulation to calculate counterion distributions, electric potentia
Author: Martin Müller
Publisher:
ISBN:
Category :
Languages : en
Pages :
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