Isomeric biphenyl polyimides. (I) Chemical structure-property relationships

A series of polyimides (PIs) were prepared from three biphenyltetracarboxylic dianhydrides (BPDA) isomers, i.e., 3,3',4,4'-BPDA (s-BPDA), 2,3,3',4'-BPDA (a-BPDA), and 2,2",3,3"-BPDA (iBPDA) with various diamines through the conventional two step process. Polymerization reactivity, solubility, and dynamic mechanical properties (T-g, softening behavior, and storage modulus at glassy state) were compared between PIs derived from these BPDA isomers and para/meta diamine isomers. The solubility in NMP increased as follows: i-BPDA-PI > a-BPDA-PI > s-BPDA-PI. The order is attributed to semi-crystalline and fully amorphous morphologies of s-BPDA-PI and other isomeric BPDA-PIs, respectively. The use of meta-diamines lowered the T-g whereas the use of bent/distorted BPDA isomers enhanced the T-g (s-BPDA-PI < a-BPDA-PI < i-BPDA-PI). The BPDA isomer effect on T-g is based on the difficulty of obtaining internal rotation around the biphenyl linkages. The width of the DMA rubbery plateau region decreased in the order of i-BPDA < a-BPDA-PI < s-BPDA-PI, depending on the crystallinity and the extent of chain entanglement. In a-BPDA- and i-BPDA-PI systems, PIs from meta-diamines led to higher modulus than the corresponding PIs from para-diamines.