Ionomer aggregates aggregation behavior

The analysis of RDFs, gy (ry), provides valuable stmctural information of simulated many-particle systems. It allows rationalizing structural correlations between atoms or beads, aggregation behavior, and phase separation, as well as sizes, shapes, and coordination structure of distinct phase domains. Experimentally, the RDF is obtained from the structure factor S(q), which determines the intensity of x-ray or neutron scattering (Ashcroft and Mermin, 1976). The structure factor for scattering from phase domains formed by a single component, for example, apolar beads in fibrils of ionomer backbones, is given by... 

So far most studies of aggregation behavior have been conducted with low polarity solvents such as THF [4,9,15]. Recently there have been a few reports with nonpolar solvents such as xylene [16-18]. While these systems require low ion contents to form a solution, the ionomer electrical environment is more similar to that found in the solid state (i.e. low dielectric constant). In this work we use static and dynamic light scattering techniques to examine the aggregation behavior of sulfonated polystyrene ionomers in a nonpolar solvent, toluene. 

The combined effects of a divalent Ca counterion and thermal treatment can be seen from studies of PMMA-based ionomers [16]. In thin films of Ca-salts of this ionomer cast from methylene chloride, and having an ion content of only 0.8 mol%, the only observed deformation was a series of long, localized crazes, similar to those seen in the PMMA homopolymer. When the ionomer samples were subject to an additional heat treatment (8 h at 100°C), the induced crazes were shorter in length and shear deformation zones were present. This behavior implies that the heat treatment enhanced the formation of ionic aggregates and increased the entanglement strand density. The deformation pattern attained is rather similar to that of Na salts having an ion content of about 6 mol% hence, substitution of divalent Ca for monovalent Na permits comparable deformation modes, including some shear, to be obtained at much lower ion contents. 

Under cyclic loading, the fatigue life of S-PS ionomers decreases up to about 4 mol %, then rises rapidly between 4 and 6 mol %, and thereafter continues to rise slowly to about 10mol%. The transition in behavior near 5 mol% is attributed to the development of large scale aggregates, or ion clusters, that act as a reinforcing second phase that impedes crack propagation. 

As a final note, because of the presence of water in the ionic aggregates, we cannot state conclusively which cation would produce an "Inherently" tougher ionomer. It is possible that in the complete absence of water, a different ordering in the mechanical properties would prevail, as the relative size and cohesiveness of the aggregates composed of different cations may be altered in the dry state. However, the general relationship between tensile behavior and aggregate size and cohesion should remain. 

Several studies (6, 13) of the solution behavior of sulfonate ionomers have provided additional insight on the nature of the ion pair aggregation. The polarity of the solvent environment has been shown to have a major influence on the dilute solution behavior of these polymers. In the course of these studies it has been observed with selected systems that both melt viscosity values and solution behavior can vary according to the history of sulfonate ionomers. This study provides some data and provides one rationale for such differences. 

Solution behavior of ionomers can be divided into two types, primarily depending on the polarity of the solvent [46,47], One is polyelectrolyte behavior due to the dissociation of counterions in polar solvents (e.g., DMF), and another is association behavior due to the formation of ion pairs and even higher order aggregates in less polar solvents (e.g., THF). Table 2 shows the solvents frequently used for the study of ionomer solutions, as well as their dielectric constants. As the dielectric constant decreases, the degree of counterion binding and also ion pair formation changes (increases) gradually, and so does the solution behavior. In this chapter, only the polyelectrolyte behavior of ionomers in a polar solvent is described. Some brief... 

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. 

Block Ionomers. The block ionomers to be discussed are of AB or ABA type, in which one of the blocks is ionic (eg, sodium methacrylate) and the other consists of nonionic units (eg, polystyrene). While ionic block copolymers in a micelle form in both aqueous and nonaqueous solutions have been studied extensively (99-101,130,131), the viscoelastic properties of block ionomers in bulk have not received much attention (132-137). If the short ionic blocks formed micelle-like aggregates, which were surrounded by nonionic blocks, the viscoelastic properties of the diblock ionomers would be very similar to those of stars or polymers of low molecular weight (136). Thus, above the Tg of the nonionic blocks, as the temperature increased the modulus dropped rapidly with a very short rubbery plateau. For example, in a dynamic mechanical study, it was found that a homopolymer containing 490 styrene units showed a Tg at ca 115°C, and started to flow at ca 150°C. However, in the case of a diblock ionomer containing 490 styrene units and 40 sodium methacrylate ionic units showed a Tg at ca 116°C, and flow behavior was observed above ca 165°C, which was only 15°C higher than that of nonionic polystyrene (135). 

Molecular simulations of ionomer systems that employ classical force fields to describe interactions between atomic and molecular species are more flexible in terms of system size and simulation time but they must fulfill a number of other requirements they should account for sufficient details of the chemical ionomer architecture and accurately represent molecular interactions. Moreover, they should be consistent with basic polymer properties like persistence length, aggregation or phase separation behavior, ion distributions around fibrils or bundles of hydrophobic backbones, polymer elastic properties, and microscopic swelling. They should provide insights on transport properties at relevant time and length scales. Classical all-atom molecular dynamics methods are routinely applied to model equilibrium fluctuations in biological systems and condensed matter on length scales of tens of nanometers and timescales of 100 ns. 

Other MD simulations predicted markedly larger values of the percolation threshold (Devanathan et al., 2007b Elliott and Paddison, 2007). Discrepancies in calculated percolation thresholds could be artifacts of overly simplistic representations of ionomer chains in molecular-level simulations. Atomistic models fail in reproducing sizes and shapes of water clusters and polymer aggregates as well as in predicting percolation thresholds, swelling behavior, and related transport properties, if the monomeric sequences that they employ are too short. Notably, for the same reason, many simulations would be inept to reproduce the persistence length of the base ionomer. 

The introduction of small amounts of bonded ionic groups into a hydrophobic polymer normally produces an increase of the glass transition temperature (Tg), melt viscosity and the characteristic relaxation times of the polymer. The microstructure of these materials, i.e., ionomers, is generally agreed to be characterized by nanophase-separated aggregates, rich in the ionic species, and which behave as physical crosslinks between chains [1]. These ionic crosslinks can significantly modify the viscoelastic behavior of an ionomer [2]. 

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