Deformation-induced phase transitions of polyamide 12 in its elastomer segmented copolymers

Deformation-induced phase transition behavior of polyamide 12 (PA12) in its segmented copolymers with polytetrahydrofuran (PTHF) was studied with in-situ wide angle X-ray scattering (WAXS) at different temperatures. In these segmented copolymers, which contain a high content of PTHF, a transformation from the stable γ phase to a metastable α″ phase is observed during tensile deformation at room temperature, which shows a similar diffraction behavior to that of the α phase but without an obvious melting point. The deformation-induced α″ phase is not a thermodynamic stable phase but arises from kinetic origins, which is in line with the condition for its formation. After the release of tensile force following deformation, the metastable α″ phase can partially transform back to the initial γ phase. The reversible phase nature may contribute somewhat to the elasticity of PTHF–PA12 systems as a result of this enthalpic contribution. Upon increasing the content of PTHF in the copolymer, the critical stress required to induce the new α″ phase increases. Upon increasing strain, the α″ phase will disappear in the samples that possess a particularly high content of PA12. Higher temperatures also prevent the γ phase from transforming to the α″ phase.