Chain packing versus chain mobility in semialiphatic BTDA-based copolyimides

A series of partially aliphatic copolyimides derived from 3,3′,4,4′-benzophenonetetracarboxylic dianhydride (BTDA), 4,4′-diaminodiphenylmethane (DDM) and 1,6-diaminohexane (DAH) was synthesized. Solubility tests and structural characteristics, evaluated by wide-angle X-ray diffraction measurements (WAXD) did not vary steadily with the content of the flexible segment. On the dynamic mechanical analysis (DMA) the evolution of the secondary relaxations (γ and β) suggested an increased chain tightness as more aliphatic component is included. The onset temperature of α-relaxation decreased regularly with the content of aliphatic sequence. During the α-relaxation the trend of E′, E″ and tan δ with temperature, and at multiple frequencies, evidenced the overlapping of phenomena. The increased mobility determines the formation of oriented structures that behave as constraints. Simultaneously, as the flexible content (DAH segment) is more important, orientation relaxations take place with increasing temperature. The equilibrium between stiffening and flowing makes the polyimides with high aliphatic content behave like elastomers.