Polyimides Containing Trans-1,4-cyclohexane unit (III). Ordered Structure and Intermolecular Interaction in s-BPDA/CHDA Polyimide

Polyimide (PI) derived from 3,3',4,4'-biphenyltetracarboxylic dianhydride (s-BPDA) with trans-1,4-cyclohexanediamine (CHDA), i.e., s-BPDA/CHDA was investigated from the viewpoint of ordered structure and intermolecular interaction. Thermodynamic parameters of the model compounds for melting behavior suggested significantly restricted conformational changes in the trans-1,4-cyclohexylene unit and the presence of strong BPDI—BPDI interaction in s-BPDA/CHDA. The effect of diamine structure on the fluorescence yield also supported the presence of the BPDI—BPDI interaction or the BPDI dimer in s-BPDA-based semi-cycloaliphatic PIs. The results of the fluorescence depolarization measurements can be rationalized by a proposed mechanism assuming the presence of the BPDI dimer sites, where the fluorescence of s-BPDA/CHDA occurs by excitation of the lower energy trap sites consisting of the BPDI dimer via excitation energy migration or direct excitation of the dimer. The structure-sensitive infrared band around 550 cm^-1 gradually shifted toward higher frequency with simultaneous narrowing with increasing cure temperature, suggesting gradual ordered structure formation in s-BPDA/CHDA. Thermal imidization at 400 °C caused splitting of the C—H stretching band around 2940 cm^-1, corresponding to the disappearance of distinct glass transition for the s-BPDA/CHDA system.