Effect of monomer composition on properties of copolyimides derived from 3,3',4,4'-biphenyltetracarboxylic dianhydride/4,4'-oxydianiline/1,3-bis (4-aminophenoxy)benzene

A series of uncontrolled molecular weight homopolyimides and copolyimides based on 3,3',4,4'-biphenyltetracarboxylic dianhydride (s-BPDA)/4,4'-oxydianiline (4,4'-ODA)/1,3-bis(4-aminophenoxy)benzene (TPER) were synthesized. All the polyimides displayed excellent thermal stability and mechanical properties, as evidenced by dynamic thermogravimetric analysis and tensile properties testing. A singular glass transition temperature (T _g) was found for each composite from either differential scanning calorimetry (DSC) or dynamic mechanical analysis (DMA), but the values determined from tan δ of DMA were much different from those determined from DSC and storage modulus (E') of DMA. The Fox equation was used to estimate the random T _g values. Some composites exhibited re-crystallization after quenching from the melt; upon heating, multi-melting behavior was observed after isothermal crystallization at different temperatures. The equilibrium melting temperature was estimated using the Hoffman-Weeks method. Additionally, DMA was conducted to obtain E' and tan δ. Optical properties were strongly dependent on the monomer composition as evidenced by UV-visible spectra. X-ray diffraction was used to interpret the crystal structure. All the results indicated that composites with TPER composition ≥ 70% were dominated by the TPER/s-BPDA polyimide phase, and ≤40% by the 4,4'-ODA/s-BPDA polyimide phase. When the ratio between the two diamines was close to 1:1, the properties of the copolyimides were very irregular, which means a complicated internal structure.