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 fr om 3,3',4,4'-biphenyltetracarboxylic dianhydride (BPDA) and di[n-alkyl]-4,4 '-diamino-6,6'-dibromo-2,2'-biphenyldicarboxylate with side chains of varyi ng 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 obser ved in differential scanning calorimetry (DSC). For BACBPs(n < 10), althoug h two DM relaxation processes can be observed, the low-temperature relaxati on peak shifts into a medium temperature region that is difficult to observ e in DSC experiments. Measurements indicate that the density of BACBP(n)s d ecreases with increasing numbers of methylene units. A discontinuity in the rate of density change can be seen at BACBP(10). Variable temperature soli d-state nuclear magnetic resonance experiments were also carried out to det ermine the molecular origins of each of the observed DM relaxation processe s. The low- [for BACBPs(n. > 10)] and possibly the medium- [for BACBPs(n < 10)] temperature relaxations are associated with the onset of motion in the side chains, and the high-temperature relaxation is associated with motion in the backbones. Wide-angle X-ray diffraction analysis indicates that the lateral packing periodicity of the backbones in the unoriented polyimides changes its relationship with the side-chain length at around 10 methylene units. In the oriented films of these polyimides, furthermore, those having the long side chains [BACBPs(n > 10)] adopt monoclinic unit cells, whereas those having the short side chains [BACBPs(n less than or equal to 10)] po ssess hexagonal unit cells. The drastic temperature difference between the low- and medium-relaxation processes observed in DM experiments may be expl ained because of a change in the lateral packing arising from the variation of the side-chain length. The limited mobility afforded the BACBPs(n less than or equal to 10) is a result of their more ordered conformation and int erdigitation between the neighboring side chains. (C) 1999 John Wiley & Son s, Inc.