Solid-state 13C NMR study of annealing effect on the crystalline phase of poly(ethylene-co-methacrylic acid) ionomers

Abstract

Annealing effect at 65 °C on the crystalline phase of Poly (ethylene-co-methacrylic acid) (EMAA) and the EMAA ionomer neutralized with Na+ cation (EMAA54Na: 54% of carboxy groups is neutralized) were mainly investigated by the solid-state 13C CP and DPMAS NMR. The 13C NMR spectra revealed the increase of Orthorhombic crystalline phase by annealing at 65 °C, while the Monoclinic crystalline phase unchanged. In addition, no appreciable changes were detected on carboxy groups. The crystallinity estimated from the 13C DPMAS NMR for EMAA did not increase (from 19% to 20%) after annealing, while that for EMAA54Na increased from 12% to 17%. The ratio of Monoclinic/Orthorhombic = 40/60 for EMAA did not change after annealing, whereas the ratio for EMAA54Na became 30/70 from 40/60. From the variable-temperature solid-state 13C CPMAS NMR measurements, it was suggested that the Monoclinic crystalline phase begins to melt beyond 55 °C, while the Orthorhombic crystal remained; the temperature was close to the Ti endothermic transition (so-called the order-disorder transition). Furthermore, the 13C spin-lattice relaxation time in the laboratory frame (T1C) of the Orthorhombic crystalline phase became significantly long after annealing, while that of the Monoclinic crystalline phase was not altered drastically. Similarly, the T1C of the amorphous phase was not affected by annealing. These observations suggested that the interaction between Na+ cations and carboxy groups in EMAA ionomers limits crystal growth of polyethylene, and the Ti is mainly caused by the melting of Monoclinic crystal.