Tailoring the properties of semi-aromatic copolyimides through structural manipulation towards energy-storage applications

Abstract

To enlarge the library of dielectric materials suitable for energy storage applications, three series of semi-aromatic copolyimides were developed by structural manipulation which enabled properties to be modulated. Various polar units such as nitrile, carbonyl, polyethylene and polypropylene oxide were integrated into the same polymer chains in different ratios to attain desirable characteristics. By WAXD, AFM, DSC and TGA the amorphous nature and high thermostability of copolyimide films were demonstrated, with no crystallization or phase separation, being shaped by the amount and nature of the soft segment. According to the mechanical and dielectric evaluation, a slight variation of copolymer composition tailored these properties in large limits, changing the material from a rigid to a flexible or even stretchable one, from a common material to one with self-sticky ability, from a low-k to a high-k material, all these being possible by an efficient chemical strategy. The electrical breakdown strength and energy storage performance were also governed by the chemistry design, the best value (326 kV mm⁻¹ and 1.36 J cm⁻³, respectively) being obtained for copolyimide containing the highest aromaticity and the lowest aliphatic chain length. Thus, the ability of the constitutive hard and soft segments to interpenetrate and to arrange in the polymer network led to films with variable and distinct properties suitable for use as flexible/stretchable dielectric materials or energy storage capacitors.