Phosphonium ion-containing acrylate triblock (ABA) copolymers were synthesized using nitroxide mediated radical polymerization. The polymerization of styrenic phosphonium-containing ionic liquid monomers using a difunctional alkoxyamine initiator, DEPN(2), afforded an ABA triblock copolymer with an n-butyl acrylate soft center block (DP similar to 400) and symmetric phosphonium-containing external reinforcing blocks (DP < 30). Two phosphonium monomers with different alkyl substituent lengths enabled an investigation of the effects of ionic aggregation of phosphonium cations on the physical properties of ABA block copolymer ionomers. Subsequently, the thermomechanical properties and morphologies of these materials were compared to a noncharged triblock copolymer analogue with neutral polystyrene external blocks. Shortening the alkyl substituents on the phosphonium cation enhanced the hydrophilicity of tributyl-4-vinylbenzyl phosphonium chloride (BPCl) relative to trioctyl-4-vinylbenzyl phosphonium chloride (OPCl). In both cases, phosphonium cations promoted microphase-separation and thermoplastic elastomer performance for the OPCl- and BPCl-containing triblock copolymers compared to a less well-defined, microphase segregated morphology for the styrene analogue. Dynamic mechanical analysis (DMA) of phosphonium-containing triblock copolymers exhibited well-defined rubbery plateau regions, whereas the plateau was shortened for the nonionic analogue. The solid state morphologies of the block copolymers were studied using small angle X-ray scattering (SAXS) and transmission electron microscopy (TEM), and both techniques revealed phase separation at the nanoscale. DMA studies indicated that phosphonium aggregation governed flow activation energies.