Modified polyimide (PI)/polyvinylidene fluoride (PVDF) composite films were synthesized by introducing the coupling agent (KH570). Results indicated that the modified composites exhibited high tensile strength, high tensile modulus and excellent thermal properties when the KH570 content increased. The dielectric constant slightly decreased to 4.82 with an increasing amount of KH570, but it was still not low. Noticeably, the dielectric loss of the composite films was small (<0.01, 1 kHz). The breakdown strength significantly increased to 169.65 kV mm−1 when 3 wt% KH570 was added, which was about 173% greater than that of unmodified composites. In addition, the largest value of the maximum energy storage density for the modified composites was 0.651 J cm−3, which was about 6.7 times higher than that of unmodified composites.
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