Phase structures and transition behaviors in polymers containing rigid rodlike backbones with flexible side chains. V. Methylene side-chain effects on structure and molecular motion in a series of polyimides

A series of organo-soluble hairy-rod polyimides was recently synthesized from 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA) and di[n-alkyl]-4,4′-diamino-6,6′-dibromo-2,2′-biphenyldicarboxylate with side chains of varying lengths (the numbers of methylene units), BACBP(n). Dynamic mechanical (DM) results reveal two cooperative relaxation processes for BACBPs(n > 10), which correspond to the two (low- and high-) transition temperatures observed in differential scanning calorimetry (DSC). For BACBPs(n 10)] and possibly the medium- [for BACBPs(n 10)] adopt monoclinic unit cells, whereas those having the short side chains [BACBPs(n ≤ 10)] possess hexagonal unit cells. The drastic temperature difference between the low- and medium-relaxation processes observed in DM experiments may be explained because of a change in the lateral packing arising from the variation of the side-chain length. The limited mobility afforded the BACBPs(n ≤ 10) is a result of their more ordered conformation and interdigitation between the neighboring side chains. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1633–1646, 1999