End-crosslinking gelation of poly(amide acid) gels studied with scanning microscopic light scattering

Network formation in the gelation process of end-crosslinked poly(amide acid) gels, which are the precursor of end-crosslinked polyimide gels, was studied by scanning dynamic light scattering. The gelation process is essentially non-reversible due to the formation of covalent bonds. The molecular structure formed in the gelation process is controlled by varying the equivalence ratio of end-crosslinker to oligomer during the preparation. It was found that a couple of relaxation modes are observed in the gelation point even for semi-rigid poly(amide acid) solutions. This is similar to flexible polymer solutions and convenient to characterize the molecular structure in the gelation processes. The fast mode is related to the cooperative diffusion process of the network structure formed by overlapped macromolecules, i.e., gel mode, which determines the averaged mesh-size of the network structure. The slow mode is related to the relaxation time of the formation of the transient network structure, which shows a slowing down close to the gelation point and disappears after gelation due to the formation of the permanent network structure.