| Abstract: | 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), although two DM relaxation processes can be observed, the low‐temperature relaxation peak shifts into a medium temperature region that is difficult to observe in DSC experiments. Measurements indicate that the density of BACBP(n)s decreases with increasing numbers of methylene units. A discontinuity in the rate of density change can be seen at BACBP(10). Variable temperature solid‐state nuclear magnetic resonance experiments were also carried out to determine the molecular origins of each of the observed DM relaxation processes. 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 ≤ 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 |
