New insights from variable-temperature and variable-pressure studies into coupling and decoupling processes for ion transport in polymer electrolytes and glasses

A fresh analysis of literature data shows how the influences of temperature and pressure on ion transport and structural relaxation in glass-forming systems may be combined within the framework of ‘master plots’ based on the equation E_A = M · V_A, to reveal new insights into coupling and decoupling effects in a wide range of systems. E_A,σ and V_A,σ are, respectively, instantaneous activation energies and volumes for ionic conductivity and the parameter, M_σ, is a corresponding ‘process modulus’. For structural relaxations occurring at the glass transition, the appropriate modulus is given by M_s = T_g · dP/dT_g. We can now identify typical behaviour patterns for fragile liquids on the one hand, and typical inorganic glasses on the other. Thus, the parameters, M_σ and M_s, for fragile systems such as molten Ca(NO₃)₂:KNO₃ (CKN) or a typical polymer electrolyte such as a complex of LiCF₃SO₃ in PPG, are found to remain constant over a wide range of temperatures down to T_g, despite changes in the temperature (and pressure) dependences of the ionic conductivities, as indicated for example by a return to Arrhenius behaviour in the case of CKN, or by so-called Stickel plots and changes in the VTF parameters for the polymer electrolytes. If E* and V* are activation energies and volumes assigned to elementary steps, when again E* = M · V*, we can go further and identify the microscopic processes driving forward structural relaxation. In the case of inorganic glasses, where usually we find the decoupling index R_τ ≈ 10¹², we identify two distinct decoupling paradigms represented by strong and fragile systems respectively, where in both cases the activation volumes for ion transport are very similar to the corresponding ionic volumes. In the former case (typified by the strongly cross-linked silicate and aluminosilicate systems), the negative activation volumes for structural relaxation (negative values of dT_g/dP) are clearly indicative of a ‘water-like’ behaviour attributable to the collapse of the network under pressure. On the other hand, for the more fragile fast-ion conducting silver iodomolybdate glass, the experimental results show that M_s (at T_g) ≈ M_σ (in glass), implying some recoupling of structural relaxation to ion transport. Arguments based on the dynamic structure model lead us to predict that a similar close link should exist between M_s (at T_g) and M_σ in the relatively fragile lithium and sodium borate glasses, thus highlighting the need for more information concerning the effects of pressure on the glass transition temperatures of common inorganic glasses.