Ionomer multiplet

Ionic aggregate, in a polymer matrix of low polarity, formed through interactions of ionomer multiplets. 

Note 1 The mobility of the polymer segments surrounding the multiplets is reduced relative to that of bulk material. With increasing ion content, the number density of the ionomer multiplets increases, leading to overlapping of the restricted mobility regions around the multiplets and the formation of clusters. 

However, they are part of a polymer chain, so they cannot be separated into a distinct phase. As a result, small islands emerge in a sea of neutral monomers (Figure 4.11 a). Such aggregates are called ionomer multiplets. 

X-ray diffraction work (11,15) shows that there is an ionomer peak at 4°C which is absent in the acid precursor. This low, broad peak is not affected by annealing or ion type and persists up to 300°C. Since the 4°C peak corresponds to a spacing of about 2.5 nm, it is reasonable to propose a stmctural feature of this dimension in the ionomer. The concept of ionic clusters was initially suggested to explain the large effects on properties of relatively sparse ionic species (1). The exact size of the clusters has been the subject of much debate and has been discussed in a substantial body of Hterature (3,4,18—20). A theoretical treatment has shown that various models can give rise to supramoleculat stmctures containing ionic multiplets which ate about 10 nm in diameter (19). 

As the ion content of an ionomer increases, the proportion of the cluster phase to the multiplet-containing... 

Figure 1 (tan 5)max versus ion content for the multiplet phase and the cluster phase of Na-SPS ionomers. 

The mechanical properties of ionomers are generally superior to those of the homopolymer or copolymer from which the ionomer has been synthesized. This is particularly so when the ion content is near to or above the critical value at which the ionic cluster phase becomes dominant over the multiplet-containing matrix phase. The greater strength and stability of such ionomers is a result of efficient ionic-type crosslinking and an enhanced entanglement strand density. 

In this work we used polystyrene-based ionomers.-Since there is no crystallinity in this type of ionomer, only the effect of ionic interactions has been observed. Eisenberg et al. reported that for styrene-methacrylic acid ionomers, the position of the high inflection point in the stress relaxation master curve could be approximately predicted from the classical theory of rubber elasticity, assuming that each ion pah-acts as a crosslink up to ca. 6 mol %. Above 6 mol %, the deviation of data points from the calculated curve is very large. For sulfonated polystyrene ionomers, the inflection point in stress relaxation master curves and the rubbery plateau region in dynamic mechanical data seemed to follow the classical rubber theory at low ion content. Therefore, it is generally concluded that polystyrene-based ionomers with low ion content show a crosslinking effect due to multiplet formation. More... 

Here, possible micromechanisms of ionomers are discussed. At low ion content (less than 5 mol %), only crazes are noted and these appear to become unstable with increasing ion content. These ionomer samples tend to be more brittle compared with the PS precursor. This is probably due to the aosslinking effect produced by the ionic multiplets that are present. As mentioned before, it is known that small ionic aggregates (multiplets) are produced due to ionic dipole attractions and that... 

Eisenberg, A. Hird, B. Moore, R.B. A new multiplet-cluster model for the morphology of random ionomers. Macromolecules 1990,23, 4098. 

The Eisenberg theory indicates that at low ion concentration, as the distance between multiplets increases, the interaction between multiplets is expected to decrease to the point where the elastic forces become too large to be overcome below that concentration clustering would not be expected. This is consistent with the experimental findings. This theory has been applied well to both ethylene and styrene ionomers which contain carboxylate groups. Hash-imoto et have recently shown that the Eisenberg theory... 

Ionomers are polymers that are functionalized with ionic groups (usually anionic sites) attached at various points along polymeric backbones that are not extensively crosslinked (1-2). Such materials have a tendency to form ionic domains in which the anionic groups and their associated cations are microphase separated from the typically hydrophobic portions of the polymer. Thus, the ionic domains formed are isolated by a medium of low dielectric constant (i.e. the polymeric backbone) although, in some cases, hydrophilic channels have been reported to connect adjacent ionic domains (3). The size and structures of these domains vary with the nature of the cation, the stoichiometry of the polymer, the degree of solvation of the system and the method of preparation. They can be as small as ion-pairscor small multiplets, but in some cases they have been reported to be in the 20-100 A" diameter range. 

We assume that the basic relaxation strengths in the polystyrene relaxation spectrum as described by Ferry and others [5], based on a Rouse-Bueche [6] model, are maintained in the ionomer. A normal wedge-box distribution is assumed. The ions are in multiplets below 6 mole % ionic groups [2]. If a segment... 

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