Multistage Crystal Transformation and Mechanism of PA1012 γ′ Phase by Uniaxial Stretch

Abstract

The pseudohexagonal γ′ phase of polyamide 1012 can be obtained by melt-quenching and gradually transformed into the γ form during heating, in which the trans methylene segments away from the amide group are further twisted. What happens during uniaxial stretching? In this work, the multistage structural evolution of the γ′ phase during uniaxial stretching has been studied by using in situ wide-angle X-ray scattering (WAXS) and Fourier transform infrared spectroscopy (FTIR). The multistage conformational evolution of the γ′ phase during stretching was studied by quantitative analysis of the infrared progression bands and was found to be closely correlated with the mechanical response during tensile deformation. In the yield stage, the twisted methylene segments in the initial γ′ phase undergo conformational ordering under stress activation, and the γ′ phase spontaneously transforms into a defective α phase. Furthermore, in the plateau stage, strain induces significant conformational disorder and the transient α phase transforms into the most conformationally disordered γ phase. Finally, at the strain hardening stage, the reversible crystal form transition from the γ phase to the regular γ′ phase is induced by stress, accompanied by an obvious conformational ordering process. The stress- and strain-induced conformational torsional motions are found to be completely opposite, with the former inducing conformational order and the latter inducing conformational disorder.