Synthesis and linear viscoelastic behavior of poly(amic acid)–organoclay hybrid

Synthesis and rheology of poly(amic acid)–organoclay hybrids were studied in order to address the molecular chain ordering and its relationship with clay particles in the prepolymer. The poly(amic acid) was prepared from oxydianiline and pyromellitic dianhydride solution in N-methylpyrollidinone with exfoliated montmorillonite. The composite solution was mixed to the nanoscale level, and the extent of defoliation and phase separation were studied using X-ray diffraction, transmission electron microscopy, and differential scanning calorimetry. Linear viscoelasticity was used to examine the influence of increasing extent of organoclay. The gelation time of the prepolymer decreased exponentially with increasing clay content. Scaling behavior by power law frequency independence at gel point was found (G′ ∝ G″ ∝ ω″) from slopes of storage and loss moduli. The power law exponent n was found to be about 0.5. At low organoclay concentrations, the gel becomes stiffer and gel strength rises with the concentration. At higher concentrations, the gel becomes weak, gel strength decreases, and phase separation seems to take place, possibly due to organoclay aggregation. The study suggests that linear viscoelasticity can be used to understand the evolution of molecular order in prepolymers of polyimide and its clay hybrids. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 80: 592–603, 2001