Isomeric biphenyl polyimides. (II) Glass transitions and secondary relaxation processes

Dynamic mechanical analysis was performed on a series of the isomeric polymides (PIs) made from the reaction of s-, a-, and i-BPDA each with 4,4'-ODA, 3,3'-ODA, 1,3,3-APB and 1,4,4-APB. The glass transitions (T(g)s) moved towards higher temperatures in the order of s-, a-, and i-BPDA for the para diamine-based Pls whereas the T-g values showed little change for meta diamine-based PI regardless of the BPDA isomer. Conversely, the temperature of the beta relaxation process increased in the order of i-, a-, and s-BPDA with any diamine. These behaviors can be interpreted in terms of the steric hindrance effects due to the configuration of BPDA isomers and diamines.