Gels of rigid polyamide networks

Gels were prepared comprising highly branched networks of rigid rodlike segments and a good solvent. Several networks in which some flexibility was built into the chain were prepared for comparison. All these systems show a remarkably steep approach to the gel point and large viscosity increases beyond it. Fractionation and end-group titrations show that the higher the rigidity of the network the less sol is extractable and the lower its molecular weight, indicating that due to segmental rigidity, individual highly branched macromolecules can hardly diffuse out of the "infinite" network, while more flexible macromolecules diffuse out with greater facility. The modulus of the gels increases with increased network concentration. This holds for gels equilibrated and immersed during measurements in good and in poor solvent. The modulus of the equilibrated immersed rigid networks appears to be insensitive to temperature changes over the range 23-150 deg C. In networks with more flexibility, the modulus of the immersed gels drops with temperature, and the drop appears to be associated with increased swelling of the gels. It seems that the segments between branch points, even those termed "flexible" in this study, are far too short and too rigid to conform with Gaussian statistics and for the gels to behave accordingly. A schematic description of the rigid network structures is proposed. Graphs. 69 ref.--AA