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 metadiamines led to higher modulus than the corresponding PIs from para-diamines.