Tuning Conformational H-Bonding Arrays in Aromatic/Alicyclic Polythiourea toward High Energy-Storable Dielectric Material

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 (U_e ≈ 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.