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Unformatted postprint, Organic ionic plastic crystals (OIPCs) have been recognised as promising solid-state electrolyte materials for next-generation energy storage devices. Recently, the addition of polymer nanofillers to OIPCs has led to the design of OIPC-based solid-state electrolytes with enhanced mechanical stability and ion conductivity. However, the mechanisms of enhancement and the influence of different polymer surface chemistries on the ion dynamics are not yet well understood, which has hindered the further development of high-performance OIPC-based electrolytes. In this work, we selected two different polymer nanoparticles, poly(vinylidene fluoride) (PVDF) and polystyrene (PS), and investigated the effects of the polymer surfaces on the thermal behaviour and ion transport properties of the OIPC, N-ethyl N-methyl pyrrolidinium bis(fluorosulfonyl)imide ([C2mpyr][FSI]). We found significantly different thermal behaviours, as well as ion transport properties in the OIPC/nanoparticle composites. Specifically, compared with pure [C2mpyr][FSI], the addition of PVDF nanoparticles effectively enhanced the ion conductivity of the OIPC composite, with the optimum achieved near the percolation threshold of PVDF nanoparticles. In contrast, the addition of PS nanoparticles to the OIPC led to a slight enhancement at low concentrations and then a significant decrease in conductivity at higher concentrations. DSC, FTIR and EIS confirm that the interaction between the PVDF nanoparticles and the OIPC induces the formation of less ordered OIPC layers on the PVDF surfaces, leading to the conductivity enhancement. Finally, different structure models based on the results of this work are proposed, which provide principle guidelines for the design of future OIPC-based highly conductive electrolyte materials.