Methacrylic ionic clustering

Ethylene ionomers consist of copolymers of ethylene and an organic add, such as methacrylic acid, the acid moieties of which have been neutralized to form a metal salt. The metal salts from neighboring chains tend to form clusters, such as the one shown schematically in Fig. 18.3. The net result is the overall structure shown in Fig. 18.2 g), in which the ionic clusters form weak crosslinks between adjacent chains. Ionomers also contain short and long chain branches, which are similar to those found in low density polyethylene. 

Reference 7 reviews a number of electron microscopy studies of ionomer morphology in the period up to 1979. None of these studies makes a convincing case for the direct imaging of ionic clusters. This is because of the small size of the clusters (less than 5 nm based on scattering studies) and difficulties encountered in sample preparation. The entire problem was reexamined in 1980(21). In this study ionomers based on ethylene-methacrylic acid copolymers, sulfonated polypentenamer, sulfonated polystyrene and sulfonated ethylene-propylene-diene rubber (EPDM) were examined. The transfer theory of imaging was used to interpret the results. Solvent casting was found to produce no useful information about ionic clusters, and microtomed sections showed no distinct domain structure even in ionomers neutralized with cesium. Microtomed sections of sulfonated EPDM, however,... 

The effect of temperature on a SAXS scan is shown in Figure 11. Note that the reflection at 4 nm persists above the melting point of the polymer, 265°C. Similar behavior was observed in ethylene/methacrylic acid ionomer (21, 22) and is strong support for the existence of ionic clustering in the dry state of the polymer. Indeed, we will demonstrate shortly that this reflection is observed in dry polymers at room temperature neutralized with heavier metal ions. 

Ionomers consist of statistical copolymers of a non-polar monomer, such as ethylene, with (usually) a small proportion of ioniz-able units, like methacrylic acid. Ethylene-co-methacrylic acid copolymers (-5% methacrylic acid) are used to make cut-proof golf balls (see Fascinating Polymers opposite). The protons on the carboxylic acid groups are exchanged with metal ions to form salts. These ionic species phase-separate into microdomains or clusters which act as crosslinks, or, more accurately, junction zones (Figure 6-4). (We discuss interactions in a little more detail in Chapter 8.)... 

While ionomers of many types have been made and characterized [1,2,3], there is little work on the overall relaxation mechanisms. For polymers with low ionic concentrations, there is general agreement on the fundamental relaxation step. The stress relaxes by detachment of an ion pair from one cluster and reattachment to another. For the styrene/methacrylic acid Na salt (ST/-MAA-Na) system, there is a secondary plateau in the relaxation modulus which depends on the ionic content and can be described as a rubbery modulus [4], While a rubbery modulus with stress relaxation due to ionic interchange has been invoked earlier, it does not adequately describe the relaxation curves. A different approach is taken here. 

Styrene-Methacrylic Acid Copolymers and Their Salts. Two different types of experiments were performed with these copolymers. A possible clustering of the ionic groups was studied with copolymers containing protonated styrene monomers and deuterated methacrylic acid groups. The radius of gyration measurements were studied from mixtures of all deuterated chains in a protonated matrix. 

Ionic bonds, in which electrons are donated to, or received from other atoms, occur in a few polymers. Du Pont Surlyn ionomers are copolymers of ethylene and methacrylic acid. Part of the methacrylic acid is neutralised with zinc or sodium ions. Ionic bonds are relatively strong and ionomers contain clusters of ions, which act rather like crosslinks. Some ionomers are blended with other polymers to improve toughness. 

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