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Abstract: The intention of this work was to investigate the concentration dependence of glass transitions and ionic conductivity in a solvent-free polymer electrolyte system based on an amorphous polymer matrix. The study was carried out on an elastomeric random copolymer of butadiene and acrylonitrile (PBAN) containing LiAsF6 in the range from pure PBAN to 3.35molkg−1 (or a CN/Li ratio of 2.25:1), which covered both the homogenous (up to CN/Li=4:1) and two-phase regions of the system. The results demonstrated that both T g and ionic conductivity slightly increased with salt content up to a CN/Li ratio of 10:1, and typical behaviour for amorphous solid polymer electrolytes was observed within this concentration range. For higher salt contents, CN/Li ratios from 8:1 to 6:1, the increases in T g was much larger than expected, and rapid decrease in conductivity were also observed. Near the limit of the homogeneity region of the PBAN–LiAsF6 system (CN/Li=5.75:1 to 4:1), broad dispersion of the experimental T g and conductivity values were observed. In individual cases, abrupt decreases in T g were observed simultaneously with dramatic increase in electrolyte conductivity (up to 10−4 Scm−1). For highly concentrated samples (CN/Li=5.75:1 to 4:1), as well as for two-phase samples up to 3.25:1, glass transitions and conductivity were established to be strongly dependent on the sample pre-history, which is typical for the metastable state of a system. The complicated behaviour of glass transitions and ionic conductivity of the PBAN–LiAsF6 electrolytes must be attributed to the specific transformations of their ionic and molecular structures. Structural changes are related to diffuse phase transitions that have been proposed at moderate (2nd order) and limiting (1st order) salt concentrations. [Copyright &y& Elsevier]