The miscibility of polyimide/polyimide (PI/PI) blends was investigated by using intermolecular charge-transfer (CT) fluorescence of aromatic PIs. Two blend systems, i.e. clear PI(BPDA/PDA)/PI(PMDA/PDA) and cloudy PI(BPDA/PDA)/PI(PMDA/ODA), were selected for fluorescence measurements (BPDA = biphenyltetracarboxylic dianhydride; PDA = p-phenylene diamine; PMDA = pyromellitic dianhydride; ODA = oxydianiline). For PI(BPDA/PDA)/PI(PMDA/PDA) blends, the fluorescence intensity rapidly decreased with the addition of a non-fluorescent PI(PMDA/PDA) into fluorescent PI(BPDA/PDA), while for PI(BPDA/PDA)/PI(PMDA/ODA) the fluorescence intensity first increased and then decreased with the addition of non-fluorescent PI(PMDA/ODA), showing that the miscibility of the blends strongly affects the fluorescence behaviour. In order to correlate the fluorescence data with the miscibility of PI/PI blends, dynamic mechanical analysis and phase-contrast microscopy were also carried out. Furthermore, binary blends of fluorescent PI(BPDA/PDA) with non-fluorescent PI(PMDA/PDA:ODA) random copolymer indicated that with the increase of PDA content in the copolymer the transparency of the blends ( 5 5) became better and the fluorescence intensity decreased. This shows that as a result the formation of intermolecular CT complexes in the homo-PI(BPDA/PDA) phase is hindered by the copolymer. The dependence of the miscibility of PI/PI blends on casting temperature (Tcast) at the stage of poly(amic acid) was also studied. It is clearly shown that the miscibility of PI/PI blends depends strongly on Tcast. As Tcastincreases from 40 to 120 or 130°C, the domain size in the phase-separated structure is reduced, and finally at 120 or 130°C the blends become perfectly transparent. Furthermore the dynamic mechanical spectra of the blends with higher Tcastshow that the blends are miscible. On the other hand, the CT fluorescence intensity of the blends decreases drastically with increasing Tcast. It was found that the decrease of CT fluorescence intensity reflects the increase of the miscibility of PI/PI blends. © 1991.
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