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 cha racterize and rationalize the crystalline transition displayed by poly amide 6,6 upon temperature and water treatment. Polyamide 6,6 material as processed by extrusion exhibits crystalline features close to thos e reported for the pseudohexagonal alpha2 phase. Upon storage, the wat er uptake induces solid state polymorphic transitions. This results in an evolution of the crystalline moiety toward the so-called alpha1 tr iclinic form. This phenomenon is reversible under temperature treatmen t. Modeling of the preferred three-dimensional organizations of polyam ide 6,6 chains shows that several of these structures are energeticall y stable. They correspond to intermediate arrangements between the alp ha1 and alpha2 phases. Further modeling of water interaction with an a ssembly of polyamide 6,6 chains indicates that water does not penetrat e the crystalline part. Instead, by reacting with the crystalline surf aces, more preferentially with the bc plane, it induces the structural evolution from alpha2 to alpha1. Suggestions about the interactions b etween water molecules and the amorphous phases are also presented.