By combining broadband dielectric spectroscopy (BDS) and differential scanning calorimetry (DSC), the ionic transport, microphase separation, and polymer relaxation in poly(propylene glycol) (PPG) and lithium perchlorate (LiClO4) mixtures have been systematically examined as a function of temperature, pressure, polymer molecular weight, and salt concentration. While the low molecular weight PPG LiClO4 mixtures exhibit only a single phase, microphase separation is observed in the mixtures of higher molecular weight PPGs (1000 and 4000 g/mol). In the samples with microphase separation, BDS and DSC yield consistent glass transition temperatures for ion-rich and ion-depleted domains. Our Walden plot analysis indicates that the ionic transport in PPG-LiClO4 is controlled by the (slow) segmental relaxation, and the data of all PPG-LiClO4 fall close to the "ideal" Walden line. Last, the application of pressure not only suppresses the microphase separation, but also decouples the ionic transport from the segmental relaxation.