To design and control the biodegradability of polymer materials, it is crucial to obtain a better understanding of the aggregation states and thermal molecular motion of polymer chains in aqueous environments.  Aggregation states and thermal molecular motion of PA4 microfibers without/with a post-heating drawing treatment under dry and wet conditions were examined by attenuated total reflectance-Fourier transform infrared spectroscopy and wide-angle X-ray diffraction analysis in conjunction with dynamic mechanical analysis.  Sorbed water molecules in the microfibers induced the crystal transition from a meta-stable γ-form to a thermodynamically stable α-form via activation of the molecular motion of PA4 chains. 
THERM