Crystalline structure and morphology in nylon-12: A small- and wide-angle X-ray scattering study

Simultaneous small- and wide-angle X-ray scattering has been employed to monitor the temperature dependence of the crystalline structure and morphology of nylon-12. Wide-angle measurements reveal that the alpha'-phase is a stable high-temperature crystalline phase. Upon cooling, a crystalline transition takes place from the alpha'-form into the hexagonal gamma-phase. The small-angle data indicate at this point a discontinuous change in the density. At low temperatures, two crystal reflections occur at wide angles. This is interpreted as a monoclinic or an orthorhombic structure, which evolves from the hexagonal structure of the gamma-form by an anisotropic thermal expansion. The evolution of the crystalline thickness during cooling and heating suggests the occurrences of partial surface crystallization and melting. This effect is superimposed on the transition alpha' <----> gamma and leads to crystal thickening and thinning during cooling and heating, respectively. The transition from gamma to alpha' with increasing temperature arises from a "one-dimensional-melting" type of rupture between the hydrogen-bonded sheets. Finally, fast quenching from the melt results in the gamma'-phase, which transforms upon annealing into either the gamma- or the alpha'-polymorph. This process proceeds most probably through a melting and recrystallization process.