WATER-INDUCED CRYSTALLINE TRANSITION OF POLYAMIDE 6,6 - A COMBINED X-RAY AND MOLECULAR MODELING APPROACH

Combined X-ray and molecular modeling approaches have been used to characterize and rationalize the crystalline transition displayed by polyamide 6,6 upon temperature and water treatment. Polyamide 6,6 material as processed by extrusion exhibits crystalline features close to those reported for the pseudohexagonal alpha2 phase. Upon storage, the water uptake induces solid state polymorphic transitions. This results in an evolution of the crystalline moiety toward the so-called alpha1 triclinic form. This phenomenon is reversible under temperature treatment. Modeling of the preferred three-dimensional organizations of polyamide 6,6 chains shows that several of these structures are energetically stable. They correspond to intermediate arrangements between the alpha1 and alpha2 phases. Further modeling of water interaction with an assembly of polyamide 6,6 chains indicates that water does not penetrate the crystalline part. Instead, by reacting with the crystalline surfaces, more preferentially with the bc plane, it induces the structural evolution from alpha2 to alpha1. Suggestions about the interactions between water molecules and the amorphous phases are also presented.