Quaternary ammonium salt modified montmorillonite (QASMMT) nanoplatelets were well dispersed in poly(ether ketone ketone) (PEKK) polymers to make pleasing fifth generation (5G) substrates. Substantially smaller dielectric properties and linear thermal expansion coefficient (LCTE) were found for each PEKKaxQASMMTy film series filled with small and appropriate QASMMT loadings. The dielectric constant (εr) and dielectric loss (tan δ) values acquired for each PEKKaxQASMMTy film series reduced to a lowest value as QASMMT loadings approached 3 wt%. Satisfactory εr (2.59, 2.71, and 2.80 at 1 MHz), tan δ (0.0032, 0.0038, and 0.0042 at 1 MHz) and LCTE (∼36.8×10?6/°C, 38.8×10?6/°C and 40.5×10?6/°C) for 5G communication were acquired for PEKKax QASMMT3 films filled with only 3 wt% optimum loading of QASMMT nano-platelets. In the meantime, the onset degradation temperatures acquired for each PEKKax QASMMTy film series increased substantially with increasing QASMMT loadings. All free volume properties, such as, the radius of the free-volume-cavity or free-volume-cavity numbers per unit volume evaluated for every PEKKax QASMMTy film sequence enlarged to a largest value, as QASMMT loadings approached an appropriate value of 3 wt%. Substantial smaller εr and tan δ were acquired for PEKKa and PEKKax QASMMTy with larger free volume properties. Possible reasons accounting for these substantially diminished dielectric and LCTE properties of PEKKax QASMMTy films are proposed.