Polar groups and the charge-transport capability play significant roles in the dielectric properties of organic polymers, and thus influence the electric energy density upon application as a capacitor material. Here, the dielectric properties and electric conductivity of a series of polymers containing 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) radicals are investigated. The neat radical polymer poly(TEMPO methacrylate) (PTMA) has a high dielectric constant but poor breakdown strength. Poly(methyl methacrylate) (PMMA) is introduced as an insulating polymer with high resistivity on breakdown, along with molecular design of PTMA. Copolymers of TEMPO methacrylate and methyl methacrylate, P(TMA-r-MMA), exhibit high breakdown strengths but low dielectric constants. PMMA blended with TEMPO exhibits the highest electric energy density of 7.4 J cm −3 (that of PTMA is 0.48 J cm −3 as a control), with both a high dielectric constant (≈6.8) and a high breakdown strength (≈500 MV m −1 ). It benefits from long-range but not bulk charge transport in the blends, which is different from the bulk charge transport in PTMA and the short-range charge transport in P(TMA-r-MMA). These results indicate that the TEMPO moiety located in the high breakdown matrix leads to a high energy-storage density in the capacitor.
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