Effect of co-solvent on the structure and dielectric properties of porous polyimide membranes

A series of porous polyimide (PI) membranes with 3,3′,4,4′-benzophenonetetracarboxylic dianhydride (BTDA) and 4,4′-diaminodiphenyl ether (ODA) in the different ratio of co-solvent (N, N-dimethylacetamide (DMAC)/1, 4-butyrolactone (GBL)) were prepared by a novel wet phase inversion method. The influence of co-solvent on the structure and dielectric properties of membranes were investigated. PI membranes changed from finger-like structure to spongy-like structure with the increasing of the GBL content since the intermolecular interaction between PI induced by GBL. The proportion and size of finger-like structure gradually decreased with the increase of GBL content in DMAC/GBL co-solvent. Although PI membranes exhibited low dielectric permittivity ranges from 1.7-2.5, the dielectric breakdown strengths were also relatively low. In particular, the PI from pure GBL yielded the highest breakdown strength (260.05 kV mm^-1) since the low proportion of macrovoids and defects. Moreover, when the ratio of GBL/DAMC was 84/16, the resultant PI membrane possessed a low dielectric constant (1.99) as well as relatively high breakdown strength (100.53 kV mm^-1). Therefore, porous PI membraness may be potential in many applications of electronics and microelectronics.