Tailored Rigid−Flexible Block Copolymers. 3. Fate of the Flexible Block within the Mesophase

Diblock copolymers were prepared by condensation of benzoyl-terminated poly(p-benzamide) [PhCO(PBA)] and anilino-terminated poly(m-phenylene isophthalamide) [PhNH(MPD-I)]. The copolymers were freed from unreacted prepolymers and also partially fractionated by selective extraction techniques. The formation of the nematic phase was investigated for three copolymers having a similar length of the rigid PhCO(PBA) block and a fraction β of flexible segment covering a range from 0.5 to 0.7. In the N,N-dimethylacetamide (DMAc) containing 3% LiCl solvent the critical volume fraction v 2 i of the PhCO(PBA) prepolymer was largely unaffected by copolymerization with a similar length of the flexible PhNH(MPD-I) block. At the largest β value only a 25% increase in the critical composition of PhCO(PBA) was observed. The mesophase appears capable of admitting much longer flexible sequences than anticipated. These results are qualitatively in line with a reduced flexibility of the flexible segment due to the self-consistent orientational field of the mesophase. However, comparison of data in aggregating and nonaggregating solvents suggests that mesophase formation should be described in terms of supramolecular assemblies rather than molecularly dispersed units.