THE ROOM-TEMPERATURE ANNEALING PEAK IN IONOMERS - IONIC CRYSTALLITES OR WATER-ABSORPTION

A quaternized diol,3-(trimethylammonio)-1,2-propanediol neutralized wi th either bromine or iodine, was used to produce a polyurethane cation omer with a poly(tetramethylene oxide) soft segment and a 4,4'-dipheny lmethane diisocyanate hard segment. If these cationomers were annealed at room temperature for a period of approximately 1 month in a desicc ator filled with dry CaSO4, differential scanning calorimetry (DSC) st udies showed an endotherm centered near 70-degrees-C which was not pre sent in the unannealed polymer and did not reappear upon subsequent co oling and heating cycles in the DSC. Some authors have suggested that a very similar endotherm found in other ionomers, mostly notably ethyl ene-methacrylic acid (E-MAA) copolymer ionomers, was due to an order-d isorder transition within the ionic aggregates, i.e. ionic crystallite melting. In order to isolate the origin of this endotherm, the local environment around the anion in compression molded bromine neutralized samples was measured using the extended X-ray absorption fine-structu re (EXAFS) technique. By measuring the change in the local environment over the temperature range corresponding to the DSC endotherm, it has been shown that this endotherm corresponds to water leaving the bromi ne coordination shell, rather than ionic crystallite melting. Other st udies which include thoroughly drying the material in a vacuum oven be low the transition temperature to remove the water suggest that the en dotherm is due to the energetic change associated with water leaving t he coordination environment of the anion in combination with water vap orization.