Tuning conformational H‑bonding arrays in aromatic/alicyclic polythiourea toward high energy-storable dielectric material

Abstract

Polythioureas (PTUs) have been investigated as a dielectric material for an energy-storable capacitor in both experimental and computational approaches. However, the effect of dipolar polarization, closely associated with conformational H-bonding arrays, on dielectric properties under the operating frequency has never been studied in PTUs. Here, a series of PTUs with different spacers and additional dipoles are synthesized to explore the influence of
conformations in thiourea units on their dielectric properties. The additional dipolar substituent (−COOH) contributes to a high dielectric constant, while accompanying a remarkable dielectric loss. Alternatively, a random copolymer is prepared to adjust the −COOH content, which displays a high dielectric constant and a suppressed dielectric loss. However, it requires strict control of the segmental ratio to meet the energy-storage demand. A neat alicyclic PTU with a flexible cyclohexyl spacer also exhibited a high dielectric constant and a low dielectric loss because of its rich trans/trans conformation, which brings about an increasing effective dipole moment per unit volume. Meanwhile, it retains high breakdown strength, thus leading to high electric energy density (Ue ≈ 10 J/cm3). These results suggest that tuning conformational H-bonding arrays based on molecular design is a more effective way to improve the dielectric properties of PTUs toward a very high energy storable material.

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