Long chain polyamides : influence of methylene sequence length and external forces on structural features

Abstract

Crystallographic studies on a series of even–even polyamides PA10.n (12, 14, 16, and 18) with variable methylene sequence length (n − 2) are performed. Temperature-dependent X-ray diffraction studies show that a triclinic α phase is the preferred polymorph after slow cooling to room temperature like in many other even–even polyamides. For PA10.12 and PA10.14 the pseudo-hexagonal high temperature phase γ is observed between the Brill transition temperature TB and melting point TM. For both higher members a distinct Brill transition is absent below TM. The influence of temperature on the packing state of the stems during cooling is discussed and compared to related polymers. An analysis of well oriented crystalline states in annealed fibers reveals that the crystalline state achieved under ambient conditions is influenced by external forces due to small energetic differences between different α and β polymorphs. All PA10.n samples show in the fiber-oriented state a prominent β polymorph which is dominant in PA10.12 and PA10.14 but superimposed with a significant α fraction in both higher members. The consequences of changes in the polymorphic state under the influence of external forces for crystalline states achieved under application relevant processing conditions are considered.