The DSC low temperature endothermic peak shown by sodium neutralized poly(ethylene-random-methacrylic acid) (E/MAA-Na+) ionomers in the range of 50-70 ºC is attributed to the endothermic reordering of polyethylene chain segments belonging to two populations of quasi-crystals set from melt, initially showing no birefringence. Dry or wet thermal annealing improve their polyethylene chain segments packing expelling out the non-crystallizable methacrylic mers by absorbing and storing heat, converting them into better-organized crystals. Upon heating further they melt, using the stored energy. Hydrothermal annealing separates the relaxation of the PE chain segments inside the ionic clusters and their reordering: i) water plasticizes the ionic clusters, downshifting the relaxation temperature, detected either by static (DSC) or dynamic (DMTA) thermal analysis and, ii) shifts upwards the endothermic peak due to the reordering of PE chain segments, detected optically by cross-polarized light and X-Ray diffraction patterns.