Exfoliated nylon II/montmorillonite (MMT) nanocomposites were prepared via in situ polymerization by the dispersion of organically modified MMT in 11-aminoundecanoic acid monomer. The polymorphic behavior of the nylon 11/MMT nanocomposites was investigated with X-ray diffraction, transmission electron microscopy, and Fourier transform infrared with attenuated total reflectance. MMT induced and stabilized the delta-crystalline form of nylon 11. The crystal structure of nylon 11 was transformed from a hexagonal delta-form structure to a triclinic alpha-form structure during the annealing process. Meanwhile, the hydrogen bonds in the nanocomposites also exhibited some differences from neat nylon 11 after annealing. (c) 2006 Wiley Periodicals, Inc.