Ionic polymers ionomers

The final section of the book deals with association of weakly ionic polymers (ionomers) in non-aqueous solvents. This area of study may bridge the gap between the fields of polyelectrolytes and ionomers. In the former case, the... 

Ionic polymers are also formulated from TDI and MDI (43). Poly(urethane urea) and polyurea ionomers are obtained from divalent metal salts of /)-aminohen2oic acid, MPA, dialkylene glycol, and 2,4-TDI (44). In the case of polyureas, the glycol extender is omitted. If TDI is used in coatings apphcations, it is usually converted to a derivative to lower the vapor pressure. A typical TDI prepolymer is the adduct of TDI with trimethyl olpropane (Desmodur L). Carbodiimide-modified MDI offers advantages in polyester-based systems because of improved hydrolytic stabihty (45). Moisture cure systems based on aromatic isocyanates are also available. 

Polymers can be modified by the introduction of ionic groups [I]. The ionic polymers, also called ionomers, offer great potential in a variety of applications. Ionic rubbers are mostly prepared by metal ion neutralization of acid functionalized rubbers, such as carboxylated styrene-butadiene rubber, carboxylated polybutadiene rubber, and carboxylated nitrile rubber 12-5]. Ionic rubbers under ambient conditions show moderate to high tensile and tear strength and high elongation. The ionic crosslinks are thermolabile and, thus, the materials can be processed just as thermoplastics are processed [6]. 

The linkage between two chains can also be ionic. Thus the copolymer between ethylene and methacrylic acid (MA) (up to 15% MA), made by free radical polymerisation, yields a polymer with pendant carboxyl groups. Neutralisation with zinc ions gives a crosslinked, thermo-reversible polymer (Surlyn ). The resulting polymer (ionomer) has limited properties, although it is the favoured material for the outer covering of golf balls. 

These results confirm the observation that polyelectrolyte aqueous solutions show two separate decay modes in the autocorrelation function and support our contention that ionic polymer systems generally behave similarly in polar solvents [23], To support this, it may be added that similar dynamic scattering behavior was recently reported for another type of ionomer, polyurethane ionomer, dissolved in a polar solvent, dimethylacetamide (e = 38) [92], Finally, it should be stressed that the explanation given above for light scattering (both static and dynamic) behavior of salt-free polyelectrolytes is based on the major role of intermolecular electrostatic interactions in causing characteristic behavior. No intramolecular interactions are explicitly included to explain the behavior. This is in accord with our contention that much of the polyelectrolyte behavior, especially structure-related aspects, is determined by intermolecular interactions [23]. 

Random ionomers having very small ion contents (e.g., 0.3 mol% for a 400,000 molecular weight PS) and telechelic ionomers show some deviations from the behavior noted herein. Although these deviations are of interest in studies of the essential features of polyelectrolyte behavior of ionic polymer solutions, we have limited our discussion to typical random ionomers (having an ion content of over 1.0 mol%). 

Since the ions in ionic polymers are held by chemical bonds within a low dielectric medium consisting of a covalent polymer backbone material with which they are incompatible, the polymer backbone is forced into conformations that allow the ions to associate with each other. Because these ionic associations involve ions from different chains they behave as crosslinks, but because they are thermally labile they reversibly break down on heating. lonomers therefore behave as cross-Unked, yet melt-processable, thermoplastic materials, or if the backbone is elastomeric, as thermoplastic rubbers. It should be noted that it is with the slightly ionic polymers, the ionomers, where the effect of ion aggregation is exploited to produce meltprocessable, specialist thermoplastic materials. With highly ionic polymers, the polyelectrolytes, the ionic cross-linking is so extreme that the polymers decompose on melting or are too viscous for use as thermoplastics. 

A number of ionic polymers exist that have a recognized elastomer as the covalent backbone and have a small ionic content, so they may be called elastomeric ionomers. The ions provide at least a part of the cross-links in these polymers. Those elastic ionomers that are cross-linked exclusively by their ions have, however, the useful feature of being thermoplastic. 

Nemat-Nasser S, Wu YX (2003) Comparative experimental study of ionic polymer-metal composites with different backbone ionomers and in various cation firnns. J Appl Phys 93 5255... 

Other group of ionomers for PEM fuel cells are based on polymers, such as poly [2,2 -(m-phenylene)-5,5 -bibenzimidazole] (m-PBI), obtained by polymerization of 3,3 -diaminobenzidine, 1,2,4,5-tetraaminobenzene and a variety of aromatic diphenyl dicarboxylates [189], whose structure is shown in Fig. 6.10. This non-ionic polymer becomes a proton conductor when doped with a strong acid such as sulphuric or phosphoric acid. 

The properties of ionic polymers in nonaqueous media have only recently become the subject of systematic studies. In solvents of low dielectric constant, salt groups resist dissociation and are poorly solvated. Thus, ionic moieties promote intra- and inter-polymer association in organic solvents. The tendency of ionic groups to aggregate or cluster resembles the coalescence of such groups in reversed micelles. Similar considerations underly the formation of ionic "cross-links" that modify the behavior of ionomers in the solid state. Solutions of polyions in nonaqueous media thus provide systems in which a powerful array of experimental techniques can be used to probe phenomena that are important to the bulk properties of a commercially important group of materials. The article by Teyssie and Varoqui in Part IV describe significant explorations in this novel field. 

Structures of surfactants suitable for insoluble films containing proteins are shown in Fig. 2. The films can be cast onto solid surfaces from aqueous vesicle dispersions prepared by sonication [24,25] or from solutions in organic solvents [11,19]. Films containing proteins have also been prepared from composites of bilayer-forming ionic surfactants with ionic polymers of opposite charge (Fig. 2). Examples include polystyrene sulfonate [24] or the ionomer Nafion with DDAB (cf. Fig. 2) [27]. 

Telechelic ionomers are a special class of ionic polymers in which the charged groups are situated exclusively at the chain ends (see Telechelic Polymers (85,86)) Accordingly, their solid-state structure is characterized by self-assembly of the chain ends into ion multiplets or ionic clusters. Because of this defined... 

An early, and widely used, commercial example of side-chain functionalities inducing interactions between polymer chains are ionomers, hydrocarbon macromolecules bearing, for example, carboxylic acid groups [e.g., poly(ethylene-co-methacrylic acid)], which are partially or fuUy neutralized with metal or quaternary ammonium ions. These ionomers are thermoplastic ionic polymers boasting unique physical properties such as enhanced impact strength, toughness, and thermal reversibility. They were developed and commercialized by DuPont, and have recently attracted attention due to their self-healing properties. ... 

The forth direction, analytical modeling for understanding the behaviors of these materials, has been popular approach. Testing and characterization have been conducted for developing the models. Such attempts have been done especially for ionic polymer metal composites (IPMCs)[58, 70, 72, 120]. Nemab Nasser and his co-workers carried out extensive experimental studies on both Nafion- and Flemion-based IPMCs consisting of a thin perfluorinated ionomer in various cation forms, seeking to imderstand the fundamental properties of these composites, to explore the mechanism of their actuation, and finally, to optimize their performance for various potential applications[121]. They also performed a systematic experimental evaluation of the mechanical response of both metal-plated and bare Nafion and Flemion in various cation forms and various water saturation levels. They attempted to identify potential micromechanisms responsible for the observed electromechanical behavior of these materials, model them, and compare the model results with experimental data[122]. A computational micromechanics model has been developed to model the initial fast electromechanical response in these ionomeric materials[123]. A number... 

Water Content and Hydration Temperature Effects. As mentioned earlier, the Dow membrane is more amorphous than Nafion 117. This allows the Dow membrane polymer matrix to adsoib more water than Nafion 117. The temperature at which the membrane is hydrated also influences the swelling of the ionomer matrix. The highest temperature used during the membrane preparation procedure controls the water content of the membrane. The water content of the membrane was calculated by dividing the weight of water absorbed by the total weight of the hydrated membrane. The water content of a membrane is primarily controlled by the inherent structure of the ionic polymer (Table I). 

Liu Y Zhao R, Ghafifari M et al (2012) Equivalent circuit modeling of ionomer and ionic polymer conductive network composite actuators containing ionic liquids. Sens Actuators A 181 70-76... 

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