Biphenyl-bridged thermoplastic polyesterimide films with good high-temperature dimensional stability and high-frequency dielectric properties for potential applications in two-layer flexible copper clad laminates: preparation and characterization

One homo-polymerized (PEsI-a) and four copolymerized (PEsI-b ~ PEsI-e) thermoplastic polyesterimide (PEsI) were prepared from the aromatic diamine of 2-(4-aminobenzoate)-5-aminobiphenyl (ABABP) and the biphenyl-linked dianhydrides of 3,3',4,4'-biphenyltetracarboxylic acid dianhydride (BPDA) and biphenyl dibenzoate-3,3ʹ,4,4ʹ-tetracarboxylic acid dianhydride (BPTME) via the two-step thermal imidization procedure. The obtained flexible PEsI films were casted using the thermally cyclized of poly(amic acid) (PAA) precursors. The PEsI films exhibited the semi-crystalline characteristics and sharp peaks were observed in the X-ray diffraction (XRD) measurements. Meanwhile, the distinct endothermic peaks in the first scanning of the differential scanning calorimetry (DSC) tests were observed. For the copolymerized PEsI-b ~ PEsI-e films, in which the BPTME contents increased in the polymers, the optical transparency of the samples gradually deteriorated until the totally opaque PEsI-e film with a haze of 100% was obtained. The semi-crystalline feature endowed the PEsI films good dimensional stability at elevated temperature, demonstrated by the linear coefficients of thermal expansion (CTE) values from 9.3 × 10–6/K (PEsI-a) to 25.9 × 10–6/K (PEsI-e) in the range of 100 ~ 200 °C, the glass transition temperatures (Tg) exceed 216 °C, and the 5% thermal decomposition temperature (T5%) over 500 °C. In addition, the PEsI samples possess the dielectric strength (Ds) in the range of 220 ~ 230 V/μm, the dielectric constants (Dk) as low as 3.29, and the dielectric dissipation factors (Df) in the range of 0.0041 ~ 0.0050 at 10 GHz.