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

作者：McCreight KW; Ge JJ; Guo MM; Mann I; Li FM; Shen Z; Jin XM; Harris FW; Cheng SZD^*

来源：Journal of Polymer Science Part B: Polymer Physics , 1999, 37(14): 1633-1646.

DOI：10.1002/(SICI)1099-0488(19990715)37:14<1633::AID-POLB8>3.0.CO;2-#

• phase structures
• transition behaviors
• flexible side chains

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 less than or equal to 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 less than or equal to 10) is a result of their more ordered conformation and interdigitation between the neighboring side chains.

• 出版日期1999-7-15