Ethylene/(meth)acrylic acid ionomers plasticized and reinforced by metal soaps

Metal soaps, also known as fatty acid salts, resemble oligomers of ethylene/methacrylic or ethylene/acrylic acid (E/(M)AA) ionomers, in that they contain carboxylic salt headgroups and long methylene sequences in their hydrocarbon tails. Such soaps might thus be expected to form miscible blends with E/(M)AA ionomers under suitable conditions, providing a separate route to increasing an ionomer's ion content and modifying its physical properties. We show here that the structure and property modifications induced by blending metal soaps into E/(M)AA ionomers are complex, and depend on both the neutralizing cation and on whether the hydrocarbon tails are crystallizable. In the melt at sufficiently high temperatures, all blends show a coassembled structure, where the salt groups of the soap coaggregate with the salt groups on the ionomer; despite the high ion content of these blends, they retain the melt processability characteristic of neat E/(M)AA ionomers of much lower ion content. Non-crystallizable magnesium oleate and magnesium erucate act as permanent plasticizers, lowering the matrix glass transition temperature. Magnesium stearate, whose alkyl tails easily form a rotator phase, can slowly 'cocrystallize' with ethylene sequences in the ionomers, leading to high moduli; however, primary crystallization is suppressed in these blends. Finally, while sodium stearate is miscible with the ionomers at elevated temperatures, it phase-separates on cooling, prior to crystallization of the ionomer; such blends are essentially composites of pure stearate and ionomer phases, with their associated individual properties, rather than possessing new structures or properties resulting from coassembly.