Properties of Ethylene-Methacrylic Acid Copolymers and their Sodium Salts: Mechanical Relaxations

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Abstract
A study has been made of the mechanical‐relaxation behavior of an ethylene‐methacrylic acid copolymer containing 4.1 mole % acid units and its partially ionized sodium salts. Degrees of ionization, estimated from infrared analysis, ranged from 0% to 78%. The weight‐percent crystallinity of the samples, determined by differential‐scanning calorimetry, varied from about 15 for the acid copolymer to about 7 for the 78% ionized copolymer. Four relaxation regions have been observed. They are labelled α, β′, β and γ, and each has been assigned to motions within the amorphous phase of the polymer. In plots of the logarithmic decrement against temperature (at 1 cps), the α peak for the annealed acid copolymer occurs at 50°C and shifts to higher temperatures with increasing degree of ionization. This trend is consistent with the increase in melt flow viscosity with increasing ionization and, on this basis, the α process is attributed to the long‐range diffusional motions of chain segments. The β′ peak occurs at 23°C for the annealed acid copolymer and decreases in magnitude with increasing ionization. Hence the β′ mechanism is attributed to the micro‐Brownian motions of chain segments involving the breaking and reforming of intermolecular hydrogen bonds between dimerized carboxylic‐acid groups. The latter assignment is supported by infrared studies of hydrogen bonding as a function of temperature. The β peak is absent for the acid copolymer but appears at −10°C for the ionized polymers and increases in height with increasing ionization. Thus, the β process is attributed to motions of branched chain segments including the ionic (carboxylate) side groups. The onset of the γ peak is observed at −120°C and is associated with the local motions of linear methylene sequences. The observation that the β peak is located at a temperature very close to the β peak (−20°C) for ordinary branched polyethylene, must cast considerable doubt on the widely held view that the ionic side groups are associated to form strong interchain links. Alternative hypotheses are suggested to explain the increase in ultimate tensile strength and melt flow viscosity with increasing ionization.

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