Fragility and low temperature excess specific heat in polymer electrolytes

The mechanical properties and the low temperature specific heat have been measured in a class of polymer electrolytes based on complexes of poly(ethylene) oxide (PEO) and sodium metal salts. These blends are semicrystalline polymers whose degree of crystallinity is affected by the salt concentration: increasing the salt molar fraction tends to produce a single crystalline phase. The "fragility" of the amorphous phase has been obtained by modelling the mechanical αa relaxation. Increasing crystallinity increase the non-exponentiality of the αa relaxation and decreases both the fragility and the excess specific heat over that predicted by the Debye theory. These effects proves that the additional low-energy vibrations causing the low temperature excess specific heat are mainly introduced by the amorphous phase with no correlation with the degree of fragility.