A ionomer comprising an ethylene acid copolymer, wherein: the ethylene acid copolymer comprises the polymerized reaction product of: ethylene; from 2 to 20 wt.% of monocarboxylic acid monomer based on the total wt.% of the monomers present in the ethylene acid copolymer, wherein the term term "monocarboxylic acid monomer" means a molecule having a reactive portion, such as a vinyl or vinylene, that may bond to other monomers to form a polymer and a carboxylic acid (-C OH) moiety that is not included in the reactive portion; from 2 to 15 wt.% of unsaturated dicarboxylic acid monomer based on the total wt.% of the monomers present in the ethylene acid copolymer, wherein the term "unsaturated dicarboxylic acid monomer" means a molecule having a reactive portion, such as a vinyl or vinylene, that may bond to other monomers to form a polymer and two carboxylic acid (-C OH) moieties that are not included in the reactive portion and wherein the term unsaturated dicarboxylic acid monomer includes dicarboxylic acid derivative monomers such as half esters and anhydrides; and wherein 10 to 70 mole percent of total acid units of the ethylene acid copolymer are neutralized by a magnesium cation; wherein the ionomer exhibits a dimensional change of less than 25% under a stress of 138 kPa (20 psi) at a temperature of 100 °C over a period of 30 minutes. the ethylene acid copolymer comprises the polymerized reaction product of: ethylene; from 2 to 20 wt.% of monocarboxylic acid monomer based on the total wt.% of the monomers present in the ethylene acid copolymer, wherein the term term "monocarboxylic acid monomer" means a molecule having a reactive portion, such as a vinyl or vinylene, that may bond to other monomers to form a polymer and a carboxylic acid (-C OH) moiety that is not included in the reactive portion; from 2 to 15 wt.% of unsaturated dicarboxylic acid monomer based on the total wt.% of the monomers present in the ethylene acid copolymer, wherein the term "unsaturated dicarboxylic acid monomer" means a molecule having a reactive portion, such as a vinyl or vinylene, that may bond to other monomers to form a polymer and two carboxylic acid (-C OH) moieties that are not included in the reactive portion and wherein the term unsaturated dicarboxylic acid monomer includes dicarboxylic acid derivative monomers such as half esters and anhydrides; and ethylene; from 2 to 20 wt.% of monocarboxylic acid monomer based on the total wt.% of the monomers present in the ethylene acid copolymer, wherein the term term "monocarboxylic acid monomer" means a molecule having a reactive portion, such as a vinyl or vinylene, that may bond to other monomers to form a polymer and a carboxylic acid (-C OH) moiety that is not included in the reactive portion; from 2 to 15 wt.% of unsaturated dicarboxylic acid monomer based on the total wt.% of the monomers present in the ethylene acid copolymer, wherein the term "unsaturated dicarboxylic acid monomer" means a molecule having a reactive portion, such as a vinyl or vinylene, that may bond to other monomers to form a polymer and two carboxylic acid (-C OH) moieties that are not included in the reactive portion and wherein the term unsaturated dicarboxylic acid monomer includes dicarboxylic acid derivative monomers such as half esters and anhydrides; and wherein 10 to 70 mole percent of total acid units of the ethylene acid copolymer are neutralized by a magnesium cation; wherein the ionomer exhibits a dimensional change of less than 25% under a stress of 138 kPa (20 psi) at a temperature of 100 °C over a period of 30 minutes.